[HN Gopher] How far could a spaceship go if we never ran out of ...
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How far could a spaceship go if we never ran out of thrust? (2020)
Author : ec109685
Score : 268 points
Date : 2021-08-10 06:52 UTC (16 hours ago)
(HTM) web link (www.forbes.com)
(TXT) w3m dump (www.forbes.com)
| wildmanx wrote:
| "If we never ran out of thrust, we could hypothetically reach
| anywhere in the Universe that a photon emitted today could
| reach."
|
| versus
|
| "I am a Ph.D. astrophysicist"
|
| I'm confused.
|
| In other words: How is he planning to catch up to a photon?
| maweki wrote:
| Not catching up. But reaching the destination. That's limited
| though, as space expands. And any place is reachable that the
| photon is able to reach as these parts* are not expanding
| faster than the speed of light.
|
| * The whole space between start and destination, of course
| dghf wrote:
| One thing I've never been quite clear on:
|
| > If it weren't for Einstein's relativity, you might think that,
| with each second that passes by, you'd simply increase your speed
| by another 9.8 m/s. If you started off at rest, it would only
| take you a little less than a year -- about 354 days -- to reach
| the speed of light: 299,792,458 m/s. Of course, that's a physical
| impossibility, as no massive object can ever reach, much less
| exceed, the speed of light.
|
| > The way this would play out, in practice, is that your speed
| would increase by 9.8 m/s with each second that goes by, at
| least, initially.
|
| From the point of view of an external observer, this seems clear
| enough: the ship's acceleration would reduce, at first gradually,
| then ever more sharply as its velocity approached _c_.
|
| But what about from the point of view of the ship's crew? Would
| they also measure a drop in acceleration -- e.g., would they
| weigh less?
|
| Or would it be that they would measure the same acceleration, but
| the final picosecond (according to their clock), which would
| without relativity push their velocity to and past _c_ , instead
| stretches out infinitely, thanks to ever-increasing time
| dilatation?
| cout wrote:
| As I understand, the crew would not notice any difference,
| since time is slowed in their frame of reference relative to
| that of an observer.
| ruszki wrote:
| The crew would feel as they accelerate by a constant 9.8 m/s^2.
| They would not feel any slow down in time. For an external
| observer, the crew would never reach speed of light, but they
| would close the gap continuously.
| JohnPrine wrote:
| so after 354 days would the crew believe they had exceeded
| the speed of light?
| dghf wrote:
| I _think_ they would never reach the end of the 354th day,
| because they 'd be caught in something akin to Zeno's
| Paradox: the closer they got to _c_ , the more time would
| slow down for them.
|
| You could draw up a table mapping shipboard times to Earth
| times: the length of time passing on Earth between each
| onboard second/millisecond/microsecond etc. would get
| longer and longer, with the final moment at which the ship
| achieves _c_ never actually arriving.
|
| ETA: basically what @ninkendo said (but they said it
| better, and first).
| ninkendo wrote:
| If you could somehow indefinitely maintain 9.8m/s^2
| acceleration, as you get really really close to c, your
| definition of a "second" becomes so long that it basically
| goes to infinity (time starts to travel infinitely slow for
| you.) Maintaining 9.8m/s^2 at such velocities starts to
| require infinite energy. If you were somehow able to reach
| c, your internal clock would be so slow (and the rest of
| the universe would be going by so quickly) that would
| experience the end of the universe essentially
| instantaneously.
|
| So the practical answer to what the crew would experience
| on the 355th day of 1G acceleration is: "That's
| impossible." The energy requirements approach infinity, and
| so does the speed of time for the rest of the universe
| they'd see out their window.
| JohnPrine wrote:
| thats so cool!
| stickfigure wrote:
| This description doesn't seem quite right to me. The laws
| of physics work the same in all reference frames, so from
| the frame of the crew, day 355 would simply be yet
| another day of 1G acceleration. At least inside the
| spacecraft, they would notice no difference from day 354
| (or any other previous day).
|
| When they look out the windows, they will see odd things
| with respect to the apparent speed of other visible
| objects (including they never get farther/closer at a
| rate faster than C). But there's nothing theoretical that
| prevents you from continuing to accelerate at 1G forever.
|
| Edit: I guess what I don't like about your description is
| that you mix reference frames. A spaceship can maintain
| 9.8m/s^2 (from its perspective) infinitely with constant
| energy expenditure. Energy expenditure rises
| asymptotically only if you require constant acceleration
| of the spaceship from Earth's perspective. But that's a
| peculiar way of framing it.
| swamp40 wrote:
| Curious what the "end of the universe" looks like from a
| photon's POV?
| stickfigure wrote:
| No. The earth (or whatever origin) would still appear to be
| moving away at something less than C.
| [deleted]
| adrianN wrote:
| The social effects of relativistic time dilation are an important
| topic in "The Forever War".
| neartheplain wrote:
| Fantastic sci-fi novel with continuing modern relevance. Often
| interpreted as metaphor for the author's experience in the
| Vietnam War, and his return to American society after what felt
| like a period of otherworldy time-dilation.
|
| The graphic novel version is great too, if you can find a copy:
|
| https://en.wikipedia.org/wiki/The_Forever_War_(comics)
| quickthrower2 wrote:
| I never knew you could get that far in that amount of time
| (experienced by passengers) with just 1g of force so it's
| thoroughly interesting and in some sense everything is quite
| close: as long as you are OK with a one way ticket.
| londons_explore wrote:
| A one way ticket with no messages back to/from home.
| mattowen_uk wrote:
| For such a long trip, the only logical ship type would be a
| self-sustaining generational colony ship full of hundreds of
| people. The concept of 'home' would quickly shift from being
| the Earth to being your ship, so staying in contact with your
| origin planet would become irrelevant.
| quickthrower2 wrote:
| Might be why the simulator is the most important technology
| in star trek, to keep people sane, connect with nature etc.
| [deleted]
| jonplackett wrote:
| The thing they never address - booking slots. That
| simulator would _never_ be available!
| Spare_account wrote:
| Presumably a bunch of people could book sessions in one
| simulator at the same time and simulate multiple
| simulators inside the simulation, massively increasing
| the capacity of the top-level simulator.
| jonplackett wrote:
| Haha! That's like wishing for more wishes. Definitely not
| allowed.
| mjevans wrote:
| We the viewers follow high ranking officers on a capitol
| starship. Surely this cushy position comes with a well
| equipped post on a vessel that has a good number of such
| units. Enough so that Voyager had the meme of holographic
| crewmembers. I don't know if RedDwarf (Rimmer) or some
| other scifi show got there first.
| jonplackett wrote:
| trust the internet to have already calculated the answer
| to this question (you can have 10.8 hours per month = 30
| minutes a day - not bad!)
|
| https://www.reddit.com/r/startrek/comments/3aajfy/how_muc
| h_h...
| nsxwolf wrote:
| If it's anything like VR, a bunch of the people on the
| ship would lose interest and rarely use it, leaving lots
| more time for the devotees.
| Dylan16807 wrote:
| Why do you need a self-sustaining generational colony ship
| to go 30 years? Especially in the space future when we
| should be able to slow aging down at least a moderate
| amount.
| DonHopkins wrote:
| In space, they just call the "space future" the "future".
| ;)
| senectus1 wrote:
| after the last year I imagine quite a few people would put
| their hand up for that.
|
| but also, what about deceleration?
|
| You gotta stop some time to smell the roses... so would you
| cut that acceleration time in half to decelerate?
| simiones wrote:
| It's important to note though that this kind of ship is still
| impossible as far as I remember - even with matter/antimatter
| reactions (which theoretically gives you E = mc^2 energy, the
| maximum possible per kg), you can't get enough energy to
| accelerate to the mass of the fuel to 1g, nevermind the rest of
| the ship, for the duration required for interstellar travel.
| grey413 wrote:
| The amount of energy required for fast interstellar travel is
| mind-boggling. People do not comprehend the scale. Even with
| ridiculous things like massless fuel it would take multiple
| times the entire yearly energy expenditure of the earth for
| an ISS sized ship to travel 10 light years before the
| passengers died of old age.
| CodeGlitch wrote:
| Also why stop at 1g? People could be trained to live in 1.2 or
| even 1.5g.
| wuliwong wrote:
| This video from the "Cool Worlds" youtube channel is a fun walk
| through this idea.
|
| https://www.youtube.com/watch?v=b_TkFhj9mgk
| rbanffy wrote:
| Interesting to note that when you are travelling close to c, even
| intergalactic vacuum requires a lot of mass ahead of you so the
| ship is not vaporised.
|
| Remember that when you are travelling to that galaxy 18 billion
| light years away, you'll hit every photon it sent our way over 18
| billion years in just about 45 years.
| remcob wrote:
| Could that radiative energy be turned into propulsion, at least
| theoretically?
| marcyb5st wrote:
| Yes, but photons carry momentum even if they are massless,
| and you can't convert more than 100% of that momentum to
| energy for propulsion. So, if you achieve perfect efficiency,
| you can avoid them from slowing you down and still have to
| rely on internal energy sources to accelerate you .
| TeMPOraL wrote:
| I'm guessing you could at least use them to:
|
| - Maneuver, like a sailboat.
|
| - Decelerate. Shut off your thrust and dump the light-
| absorbing covers to reveal mirrors, for 2x the
| deceleration. Photobraking?
| marcyb5st wrote:
| True and true, especially the decelerating part. Akin of
| what a light-sail would do.
| holoduke wrote:
| Can during deacceleration all energy be absorbed and used
| for accelerating at a later moment?
| marcyb5st wrote:
| Mmmh, I don't think so. At least efficiently. You might
| be able to decelerate emitting photons in front of you
| but they would escape to infinity unless you put them in
| orbit around a black hole. In that case perhaps.
|
| If we will ever have the technology to instead create
| proper massive particles at scale you could eject those
| in orbit around some other massive object and then re-
| steal their angular momentum to accelerate yourself. But
| that I believe would be an extremely lossy process so you
| would lose most of the energy.
| bobsmooth wrote:
| I always knew intergalactic hydrogen might be a problem but I
| never considered the effect of travelling really fast on photon
| absorption.
| rbanffy wrote:
| The hydrogen may be a problem well before the light or CBR
| become another.
| dvdkhlng wrote:
| If you travel close to C, those photons will be blue-shifted
| into (depending on velocity) very energetic photons (x-ray,
| gamma spectrum) that may be rather difficult to shield
| against.
| Koshkin wrote:
| > _close to C_
|
| As a programmer, this gave me this warm feeling...
| weregiraffe wrote:
| But when you accelerate even more, it will be C++.
| hateful wrote:
| Special Relativity states that it's time that changes,
| not the speed of light. C is constant regardless of your
| reference frame. So, it's still C, but instead, it will
| take a lot longer to complete your project. You're
| definitely going to miss your deadline.
| xwdv wrote:
| If you happened to be a civilization that lived at the very
| edge of the universe, such that every star was in one direction
| and there was only black emptiness in the other, could you the
| accelerate indefinitely into the blackness without worrying
| about having mass to shield you?
| [deleted]
| MichaelZuo wrote:
| Yes, shielding would be even more difficult at those speeds
| than 'unlimited thrust'.
| jsilence wrote:
| Nah, just put a solar panel at the front of the ship. Free
| energy!!
| nindalf wrote:
| And use that energy to sustain the acceleration. Sounds like
| a perpetual motion machine!
| anchpop wrote:
| Unfortunately the photons hitting the solar panel at the
| front of your spaceship will slow you down by at least as
| much as you could turn them into thrust by shooting them
| out behind you (conservation of momentum) :(
| FeepingCreature wrote:
| Could you collect photons from all around you and
| accelerate them out the back? There's no reason you have
| to place the solar panel at the front.
|
| Force would be negligible of course. But if you are in
| space, you are surrounded by energy in the form of
| starlight; it would be weird if that energy could not be
| used for work.
| dogma1138 wrote:
| No, "collecting" photons transfers momentum so at best if
| you have 100% conversion rate you won't slow down, so you
| need a separate source for acceleration.
|
| Whilst photons hit your ship from all direction the
| photons incoming from the direction of travel would be
| shifted towards higher frequencies so their momentum
| would be higher than the photons hitting the ship from
| the opposite direction so unless there is a sufficient
| difference in the number of photons hitting you from the
| back to compensate for it you would still not be able to
| extract energy to move.
|
| This means that even say a solar sail has a limit on
| acceleration at a certain speed the photons hitting the
| front of the sail would be blue shifted enough to
| counteract the acceleration of photons hitting you from
| the back, even if you convert them to useful energy it
| wouldn't matter since you'll reach equilibrium.
| [deleted]
| notimpotent wrote:
| Sorry for my ignorance, but if photons do not have mass,
| how do them impacting the solar panel have any effect on
| the ship's velocity?
| kibwen wrote:
| Photons don't have mass, but they do have momentum, which
| is the reason that solar sails work.
| salawat wrote:
| Radiation pressure. Remember, solar sails are a thing. At
| some point, you're blazing through spacetime so fast all
| that energy you're trying to share the same physical
| location with is impacting and ablating your hull
| material.
|
| Relative velocity is a hell of a drug.
| pja wrote:
| Photons do have mass, because energy has mass & photons
| carry energy. They have no rest mass, which is why they
| are usually described as massless, but this isn't the
| whole story!
| Diggsey wrote:
| What we call "mass" is actually "rest mass" - ie. the
| mass when the thing is not moving. Photons don't have
| rest mass because they don't rest.
|
| What photons _do_ have is momentum, and so due to
| conservation of momentum, hitting a photon will slow you
| down.
| jsilence wrote:
| To boldly go where no one has gone before!
| Koshkin wrote:
| Or, simply move your frame of reference.
| jsilence wrote:
| Why does the road cross the chicken?
| euroderf wrote:
| To boldly reframe your frame of reference where no man
| has reframed that frame of reference before! (cue
| music...)
| spaetzleesser wrote:
| I remember reading about a design that basically works like
| this. It's sucks up all the atoms in front of the ship and
| uses them for acceleration. I think there is also a book
| about this where such a ship has a problem which doesn't
| allow it to slow down. So they just keep accelerating. Wish
| I could remember the name.
| kadoban wrote:
| You're likely talking about a Bussard ramjet. The book
| should be Tau Zero.
| kanisae wrote:
| You are thinking of Tau Zero by Poul Anderson. It's well
| worth reading even though the cosmology is dated.
| ramraj07 wrote:
| You need mass to accelerate. Not just electricity.
| netjiro wrote:
| > You need mass to accelerate.
|
| * momentum , impulse , force
| cygx wrote:
| https://en.wikipedia.org/wiki/Photon_rocket
| a_t48 wrote:
| Just turn the energy into mass, duh.
| wcoenen wrote:
| Even ignoring galaxies, there's also the cosmic microwave
| background.
|
| It turns out that there is a rest frame in which this
| background looks the same in all directions. But if you keep
| accelerating in one direction, the CMB coming from there will
| keep blueshifting: from microwave to IR, to visible, and
| eventually hard gamma radiation.
| eru wrote:
| > It turns out that there is a rest frame in which this
| background looks the same in all directions.
|
| I was really surprised when I heard this for the first time a
| while ago. Because it means that our universe does have a
| _preferred_ frame.
| chii wrote:
| it's not really preferred in the sense that it's the
| correct one - just one which has some sentimental meaning
| to humans' interpretation.
| eru wrote:
| Well, 'preferred' as in 'different from all the others in
| a way even aliens would recognise' (ie in accordance with
| https://en.wikipedia.org/wiki/Copernican_principle ).
| TeMPOraL wrote:
| Aka. a Schelling point.
|
| https://en.wikipedia.org/wiki/Focal_point_(game_theory)
| eru wrote:
| Well, it can serve as a Schelling point. But it's
| interesting, even without any game theory.
| nabla9 wrote:
| CMB rest frame is important and special unique frame to use
| as a reference.
|
| Relativity means that there are no special frames where the
| laws of physics are different.
| ithkuil wrote:
| the CMB is a bit mind bending; it's related to the question
| "if the universe is expanding, in the past it was smaller
| and smaller; so where is the center?"
|
| Good read where you'll also learn about the "The Surface of
| Last Screaming":
|
| https://ned.ipac.caltech.edu/level5/March03/Lineweaver/Line
| w...
| cout wrote:
| I understand how there can be no center if the universe
| has positive curvature. But if the universe is saddle-
| shaped and non-infinite, surely there has to be a center?
| feoren wrote:
| Rather than think "surface of a balloon inflating", think
| "surface of an (infinite?) rubber sheet being uniformly
| stretched". It does intuitively seem like if there is 0
| curvature and no "center", then the universe must be
| infinite though ...
| effie wrote:
| I think not; consider pseudosphere, there is no obvious
| center. Its embedding in 3D space has infinite dimensions
| but the object itself has finite surface area.
|
| https://en.wikipedia.org/wiki/Pseudosphere
|
| If you mean object with finite dimensions and negative
| curvature and no center, there seems to be such a thing,
| see Fig. n066200e:
|
| https://encyclopediaofmath.org/wiki/Negative_curvature,_s
| urf...
|
| EDIT
|
| Rozendorn's idea seems to be still an open question:
|
| https://mathoverflow.net/questions/52851/the-geometry-of-
| nad...
| littlestymaar wrote:
| It's even worse than this:
|
| - all those photons are blue-shifted and becomes hard gamma
| rays.
|
| - once you get close enough from _c_ , all the stars in the
| universe appear to be either right in front of you, or right
| behind you, which means you get hit not only by gamma photons
| coming from the galaxy you're aiming at, but gamma photons
| coming from half the universe!
| IX-103 wrote:
| once you get close enough from c, all the stars in the
| universe appear to be either right in front of you, or right
| behind you
|
| That's not really right. There is a distortion where things
| appear more in front of you than they are at the time you see
| them, but that's only because they were at that angle, but by
| the time the light gets to you they're not where they appear.
|
| Also there's nothing that would shift light to appear behind
| you other than your movement away from objects that you have
| passed.
| yread wrote:
| the photons from stars behind you are still faster than you
| so they catch up with you
| hinkley wrote:
| I think this is why the Alcubierre drive will become critical
| even at well below light speed. Assuming I understand the
| physics correctly, the bubble of space around the ship is not
| moving (very fast) relative to the ship and so moving at
| fractional c will see an increase in flux but not the blue
| shift. You're still hitting every spec of dust in your path,
| but not turn the entire cone of the universe in front of you
| into gamma rays.
|
| At some point the warp drive might be cheaper than the
| shielding.
| marcyb5st wrote:
| If an alcubierre drive can be created,even inside the
| bubble of normal space time you would probably die. This is
| because of the Unrue radiation that is created by
| compressing the space time in front of you. Also, it seems
| that even with the best theoretical approach you still need
| the mass equivalent of Jupiter in exotic matter to make it
| work for a small ship :/ Spacetime is extremely resistant
| to compression/expansion.
| rakoo wrote:
| > once you get close enough from c, all the stars in the
| universe appear to be either right in front of you, or right
| behind you,
|
| I don't understand that one, could you please expand on how
| it works ?
| jandrese wrote:
| Think of it like walking in a rainstorm. Walk at a slow
| pace and most of the rain hits your head, but if you're
| moving very fast all of the rain will seem to be coming
| from the front because you are running into it instead of
| it running into you.
| minitoar wrote:
| I don't have an intuitive explanation but this is why
| https://en.m.wikipedia.org/wiki/Aberration_(astronomy)
| hinkley wrote:
| Blue shift (aka increased apparent frequency) is caused by
| the velocity between you and the source, not the velocity
| of the source. That is how the speed of light works in
| General Relativity. If you move toward a photon is doesn't
| get faster, it gets higher frequency. If that's in the
| visible range it shifts toward the blue end of the
| spectrum.
|
| If you move fast enough, all of the ultraviolet light
| coming from behind you becomes visible light. A little
| faster and all of the ultraviolet light in front of you
| becomes X-rays. And all of the hard x-rays become gamma
| rays.
| scotty79 wrote:
| Even if the photon is coming from the side, just slightly
| ahead of you, you are ramming into it at near lightspeed.
| You can't add up speeds in newtonian way so energy goes
| (for you) into blueshift.
| chadcmulligan wrote:
| If you travel very close to c (relative to everything else)
| then due to length contraction everything else in the
| universe is 0 metres away. Likewise time appears stopped on
| everything else moving close to c relative to you, due to
| time dilation. (Is this right? I've never thought of it
| this way before).
| kazinator wrote:
| If you use as much thrust as you can, you can get wherever you
| are going in less time in the moving frame of reference. So that
| it to say, the difference between going say, 0.99C and 0.9999C is
| not that significant from the outside frame of reference, being
| just 1%, the difference in how much time slows down inside the
| ship is large.
| londons_explore wrote:
| Say you take the one-way ticket to travel to a far off galaxy at
| 1g acceleration to go mine space-diamonds...
|
| What when someone else in a few hundred earth years invents the
| 1.1g rocket to go mine those same space diamonds? They will
| arrive millions of (local) years before you. By the time you
| arrive, the space diamonds will be all mined and you'll be seen
| as a cave man.
|
| That reason alone means it's never worth departing on a long
| journey, because someone else always has an incentive to overtake
| you and get there millions of years sooner.
| vidarh wrote:
| Quite a few scifi stories have explored the idea of someone
| leapfrogged that way. I think it'd be a major concern of
| sending someone that way.
|
| [For one light take on this, the original Marvel "Guardians of
| the Galaxy" (the year 3000 versions) have Vance Astro, Major
| Victory, go on a 1000 year journey to Alpha Centauri, only to
| find other humans got there centuries earlier and were waiting
| for his arrival]
| est31 wrote:
| Currently we are in a phase of rapid scientific progress, but
| there is no law that this will continue forever. One day we'll
| hit diminishing returns and instead of discovering new things,
| science will be about applying the knowledge discovered in the
| past correctly, as well as not losing it. Thankfully we are
| still far away from this.
| withinboredom wrote:
| Not only that, but whatever star the space diamonds are
| orbiting probably won't be there any more.
| mewpmewp2 wrote:
| It would be quite a fun expedition though, knowing that when
| you reach your destination there could already be many others
| who have reached that point so you can see how everything has
| advanced. Assuming your goals are oriented more towards
| curiosity rather than making money.
|
| You sacrifice 30 years to see what someone potentially achieved
| within 1 million years. I'm sure there would be people willing
| to do that.
|
| Let's say I'm an uber billionaire (trillionaire in the future),
| aged 45, I might think that I have tried already everything
| there's to try on earth and want to see the future. I would
| build a ship with more than enough entertainment for following
| 30 years and invite bunch of other people and just start with
| the journey.
|
| Maybe I could even leave some money behind for other groups to
| keep building better solutions that would reach the destination
| much sooner so they can already start building out the
| destination and have millions of years of time to do that
| before I reach there.
|
| It would be an enormous risk of course, but it could also pay
| off very well if for example you still own a large proportion
| of the business you leave behind to build the following
| spaceships and if it ever should reach the destination was
| still legally upholding the ownership you have. In fact in this
| case you could be magnitudes more richer in the new location
| when you arrive as compared to when you left 30 years ago in
| your time.
|
| Or if something like cryogenic freeze is possible, you could
| also do that for some of the years if you ever got bored in the
| ship.
| neartheplain wrote:
| That's also a common Star Trek plot point. Many episodes
| involve the Enterprise catching up to a ship launched from
| Earth in the pre-warp era, typically an early colony or
| exploration vessel. The most of famous of these might be Khan
| Noonien Singh's "Botany Bay:"
|
| https://memory-alpha.fandom.com/wiki/SS_Botany_Bay
| adrianN wrote:
| Aha, but if everyone thinks that way you will actually be the
| only one _daring enough_ to embark on that journey and have all
| the space diamonds for yourself!
| topynate wrote:
| > They will arrive millions of (local) years before you.
|
| No they won't - in the frame of reference of the galaxy, both
| you and your pursuer will spend most of the journey traveling
| at very close to the speed of light. The difference won't be
| enough to catch up.
|
| Put another way, in the frame of reference of the galaxy, you
| get to Andromeda (2.5 million light-years away) only about two
| years later than light would. Your pursuer, traveling at 1.1 g,
| would get there about a year and 9 months after light. So
| anyone chasing you would have to take off within about three
| months - and after a year, _nothing_ could catch you.
|
| This arrival time delay relative to light is pretty much
| constant (for fixed g) for any distance over about 1000 light
| years. For this reason, sci-fi authors who want ships to be
| overtaken by later models either slow the old ships down to
| much less than light-speed (i.e. generation ships) or introduce
| FTL travel.
| ehnto wrote:
| By that reasoning, no-one ever leaves. If no-one had ever left,
| you may as well have left and be there already.
|
| I think the reason to leave will have to be something other
| than space-diamonds, or other monetary needs. More like
| planetary conquest and scientific advancement.
| jusssi wrote:
| In addition, something approacing from opposite direction might
| _already be there_ when you decide to launch, digging away at
| the diamonds. Or a local star might have gone nova and
| vaporized the whole diamond field.
| RavlaAlvar wrote:
| Or someone who invented a device that can fold space time.
| Robotbeat wrote:
| Can someone calculate at which point a 100 ton spacecraft
| accelerating at a constant 1gee (local) would have a total
| kinetic energy exceeding the mass energy of the entire
| (observable... as in, within our light cone) universe?
| scotty79 wrote:
| I'm not sure but I think that with constant acceletation, your
| kinetc energy raises with time squared. And the whole c speed
| limit just affects how kinetic energy relates to speed.
| LeoPanthera wrote:
| This concept is explored in the second sequel to "2001: A Space
| Odyssey", "2061", albeit without approaching relativistic speeds.
|
| Clarke explores the idea that in a future where fusion reactors
| can be built, a spacecraft can have a fusion-powered motor for
| which the fuel is simply water. In the book, a spacecraft travels
| from Earth (or maybe Mars, it's been a while since I read it) to
| Jupiter, and does it the "brute force" way by simply pointing at
| Jupiter and thrusting at 1G until they are half way there, and
| then turning around and slowing down at 1G until they arrive.
|
| This has the bonus side effect of allowing the spacecraft to have
| normal gravity inside it.
| FartyMcFarter wrote:
| Forbes's UI is infuriating.
| dehrmann wrote:
| > When we take a look at conventional rockets that we launch from
| Earth, it surprises most people to learn that they barely
| accelerate more rapidly than gravity accelerates us here on
| Earth.
|
| This is actually pretty obvious when you look at a launch and how
| slowly it accelerates.
| bencollier49 wrote:
| "spacecrafts" - urgh! Who wrote this?
| [deleted]
| jillesvangurp wrote:
| Relativistic speeds are a plot point in Andy Weir's recent book
| "The Hail Mary Project". You might know him from the Martian a
| few years ago.
|
| Scott Manly did a Youtube episode on all sorts of weird
| propulsion mechanisms that people have come up with over the
| years a few months ago:
| https://www.youtube.com/watch?v=QEZv_OXA_NI. There are some
| interesting concepts in there. Including the anti matter based
| concept discussed in the article. Particularly fusion looks like
| it might get us some interesting amounts of delta v.
| JoachimS wrote:
| > Relativistic speeds are a plot point in Andy Weir's recent
| book "The Hail Mary Project". You might know him from the
| Martian a few years ago.
|
| Just read the book and can highly recommend it. One of the good
| thing about it is that the main protagonists are nice, curious
| and friendly. No backstabbing, suprise turns of evilness to
| create suspense. A pleasant read.
| stavros wrote:
| > No backstabbing, suprise turns of evilness to create
| suspense.
|
| Spoiler alert, kind of.
| apocalyptic0n3 wrote:
| In their defense, it turned out to be the right thing to
| do.
| pomian wrote:
| All of Andy Weir's books are really good. There is even a
| surprising one that's fun. Also recommend Saturn Run, by John
| Sandford, while we are on this topic.
| cecilpl2 wrote:
| I never miss an opportunity to plug my favorite Andy Weir
| short story - The Egg. It's very short and very good.
|
| http://www.galactanet.com/oneoff/theegg_mod.html
| stavros wrote:
| And in Kurzgesagt video form:
| https://www.youtube.com/watch?v=h6fcK_fRYaI
| [deleted]
| stavros wrote:
| > There is even a surprising one that's fun.
|
| Which one do you mean? I didn't like Artemis that much.
| spullara wrote:
| I loved the Martian and Project: Hail Mary but Artemis
| was a disappointment.
| vagrantJin wrote:
| Can confirm, it is a really good read. I especially liked
| that the protagonist really does things a normal person would
| do despite the extraordinary circumstances. I wish hollywood
| could take notes.
| bgroat wrote:
| A normal *smart* person. Which is even more rare.
|
| The chapter where he deduces where he's from based on what
| units he uses when he thinks about distance is incredible.
| UI_at_80x24 wrote:
| As a Canadian living close to the US border, that section
| was relatable. Using both units (metric and imperial)
| depending on scale has always felt unique to my
| geography/age.
| bgroat wrote:
| I'm also Canadian and thought the very same thing!
|
| I also really liked how he dialed in: - Okay, so I use X
| unit when I thing of A, and Y Unit when I think of B....
| But what kind of person am I that I know those values off
| the top of my head?
| vagrantJin wrote:
| Haha. Yeah the early chapters really set a brilliant
| tone. I thought the smart bit was implied. I would think
| if we were going to send someone to another star system
| to research how to stop our species from dying out - we'd
| send someone of the intellectual elite at the very least,
| "kicking and screaming"
|
| What a brilliant book.
| bgroat wrote:
| You're absolutely right from a narrative sense.
|
| I meant that writing a believable normal person is hard.
| Most people are normal.
|
| But writing a smart person is hard, because you often
| just have to have them solve problems faster/remember
| more/have information that the reader doesn't.
|
| Project Hail Mary did a wonderful job of showing that a
| smart person doesn't necessarily think "The same, but
| faster", but that they think differently and deliberately
| nkrisc wrote:
| It's why The Martian is one of my favorite movies (and
| book, of course). No evil, conniving, contrived antagonist
| designed to be hated. It's just everyone working together
| against the most unforgiving antagonist of all: the
| universe itself.
|
| I don't want to watch movies or read books where people are
| just awful to other people, I can open the newspaper on any
| given day to find that.
| howsaboutme wrote:
| > No evil, conniving, contrived antagonist designed to be
| hated. It's just everyone working together against the
| most unforgiving antagonist of all: the universe itself.
|
| Can I recommend then Becky Chamber's Wayfarer Series (4
| books). What a refreshing change.
| spiderice wrote:
| > It's why The Martian is one of my favorite movies (and
| book, of course)
|
| It's funny because even though the book was so loved
| because it was so rational, and human's actually acted
| how humans would act in that situation, Hollywood STILL
| couldn't resist making tweaking the end slightly to make
| it over the top and impractical. Thus we ended up with
| Mark Watney flying around like Iron man in space with a
| hole is his space suit.
|
| Still a great movie though. But a better book.
| vagrantJin wrote:
| The martian was a fun read and undoubtedly a classic. I
| personally dont mind a well thought out antogonist who is
| very human.
| XorNot wrote:
| Engineers with a lot of resources tend to be my favorite
| villains. Anyone who genuinely, earnestly believes
| they're going to solve a big problem and has just become
| laser focused on that to the exclusion of all else:
| that's a touchpoint I can relate to for a sympathetic
| villain.
| nkrisc wrote:
| For that I just have to think back to every new-CTO lead
| replatforming effort at any previous job. Ok, they
| weren't evil, but the outcome was the same.
| munificent wrote:
| _> I don't want to watch movies or read books where
| people are just awful to other people, I can open the
| newspaper on any given day to find that._
|
| I am 1000% with you on this. The success of The Sopranos,
| Breaking Bad, and Game of Thrones are mystifying to me. I
| get enough misery and awfulness from the news.
| philipswood wrote:
| Tau Zero by Poul Anderson also has a plot built around time
| dilation.
| jamesdmiller wrote:
| Does the expansion of the universe reduce how much you have to
| slow down if you wish to land on a very distant planet?
| platz wrote:
| no because expansion only happens on the largest scales
| jamesdmiller wrote:
| But if I travel for 45 years at 1-g doesn't the largest scale
| of the universe become significant to my travels?
| irrational wrote:
| So, if you constantly accelerate for 45 years you will travel 18
| billion light years to the edge of the observable universe. But,
| if you continue for another 45 years you won't move beyond that
| point? Or do you move beyond that point but don't reach anywhere,
| it's all darkness? Or do you seem to stand still while the
| observable universe behind you expands past you to where you can
| no longer see it?
| Koshkin wrote:
| Sure, with the comfortable acceleration of _g_ during the first
| half of the journey and decelerating during the rest of it, I
| could arrive to, say, Andromeda galaxy in a few dozen years. The
| biggest question, though, for me is: OK, I'm here, _now what_?
| (Having a vivid imagination, as a child I once imagined myself
| standing on the Moon's surface, looking at the distant Earth
| shining just above the horizon and the eerie landscape below. The
| first and the only thought I had at that moment was: "What the
| hell am I doing here?")
| praptak wrote:
| Surely astronomers have a list of objects which are expected to
| be interesting from a closer distance?
| cmenge wrote:
| I guess the point is: you can't really tell anyone about it,
| because they have died tens of thousands of years ago. So
| it's "bring your friends"?!
| Koshkin wrote:
| I doubt it; besides, astronomers love their telescopes!
| scotty79 wrote:
| > "What the hell am I doing here?"
|
| I ask myself the same question whenever I leave my city.
|
| I guess I'm not really a traveller person.
| Torkel wrote:
| The year is 1492. You're imagining standing on the shore of a
| vast continent, unexplored by your people. The only
| feeling/thought you have at that imagined moment is "What the
| hell am I doing here?"
|
| That is perfectly reasonable! But I hope you can appreciate
| that the imagined feeling/thought is not shared by everyone.
| stavros wrote:
| That's not very comparable, because you could easily live a
| very fulfilling life in a new continent (well, except
| Australia, I guess?), but being on the surface of the moon
| would be a constant struggle for survival.
| rswail wrote:
| You could live a very fulfilling life next to Sydney
| Harbor, or Melbourne's Port Phillip Bay, or... well,
| anywhere that people have lived in Australia for the past
| 50,000+ years.
| stavros wrote:
| Yes, I realize there are people happily living in
| Australia, that was the joke :(
| Torkel wrote:
| Explorers do not become explorers from a wish for "easily
| living a very fulfilling life". There was nothing
| guaranteed about the quality of life offered by the great
| women and men who traveled the seas in ages past, and
| indeed many of them perished.
| jfrunyon wrote:
| No, colonizing a new continent was not "easy". It, too, was
| a constant struggle for survival. New pests, new climate,
| new soil, new flora and fauna, communication (or battle)
| with natives, etc.
| stavros wrote:
| At least it had air, though.
| pavel_lishin wrote:
| > _being on the surface of the moon would be a constant
| struggle_
|
| With today's technology, yes.
|
| And as others pointed out, life on a new continent wasn't
| all roses, either.
| jasonwatkinspdx wrote:
| These two situations are vastly different.
|
| This is also a very eurocentric point of view that ignores
| that people were already living in the Americas. What
| happened in 1492 was not grand explorers finding undiscovered
| country in the star trek sense. It was conquerers finding a
| new land they were ignorant of rich with resources they could
| take by force via superior military power.
| sudhirj wrote:
| The implications of time dilation when moving near c are also
| explored in the Three Body Problem trilogy, and pushed to the
| extreme as well. It's quite a mindfuck if you still have ties to
| the place you're leaving, but interesting if you don't.
|
| There's all the interesting idea of just accelerating forever,
| till the end of the universe (which comes faster than you think
| if you're going that fast).
| simonh wrote:
| We're really spoiled by modern rapid global travel. In the
| 1930s half of my maternal grandmother's family emigrated to New
| Zealand. I've only ever met two of those relatives once in the
| 1990s, but before jet travel such contact was impractical for
| most people. That's why whole family groups, or even
| communities would move together.
| edna314 wrote:
| The time dilation aspect in space travel is the most neglected in
| popular sci-fi. In many of these, there are intricate
| installations of some sort cryogenic sleep devices because the
| script authors assume that it takes you 150 years of travel at
| light speed to get to a place that is 150 light years away.
| jobigoud wrote:
| > the script authors assume that it takes you 150 years of
| travel at light speed to get to a place that is 150 light years
| away
|
| Or maybe they know you wouldn't travel at light speed in the
| first place.
| [deleted]
| simonh wrote:
| That's a very basic error, but I can't offhand remember coming
| across a story where the author made this mistake. After all,
| if the ship's top speed is even modestly below light speed the
| journey really could take many decades. Can you give an
| example?
| edna314 wrote:
| I wasn't referring to the author in the article, but rather
| an unspecified collection of 'popular' SciFi, where this
| aspect is not explained. It is true that, even if you reach
| speeds of 0.5c you only get a ~15% time dilation.
| TeMPOraL wrote:
| Which is why most popular sci-fi franchises end up with
| velocity caps, justified either in-universe or out-of-
| universe. E.g. Star Trek sublight propulsion (impulse
| engine) has been established indirectly and through "word
| of God" to work with speeds up to ~1/3 c, which is not
| enough for relativistic effects to cause a big impact. The
| issue is conveniently omitted, because why travel fast
| through "normal space" when you have easy access to warp
| drive / jump drive / hyperdrive / wormholes, that gives you
| FTL _and_ happens to drop you out into a convenient
| reference frame? And all the interesting action happens
| around planets anyway.
|
| This omission makes it easy for the writers to introduce
| relativity as a nerdy plot point. For example, StarGate:
| Atlantis had an episode with a ship traveling absurdly
| close to the speed of light, which was devised as a
| convenient way to introduce characters that should've been
| dead for many thousands of years.
| CodeGlitch wrote:
| Interstellar explores this idea really well. Also see Andy
| Wiers' new book "Hail Mary", which I can thoroughly recommend.
| In Hail Mary he doesn't use cryogenics but a different
| mechanism.
| nahuel0x wrote:
| Interstellar merged relativity effects with emotional ones,
| that's a brilliant literary stroke analogous to the ones from
| the s.f. masters.
| eesmith wrote:
| ... what? It's been part of SF for decades.
|
| The first three that come to mind are: Heinlein's "Time for the
| Stars", Haldeman's "The Forever War", and Anderson's "Tau
| Zero".
|
| From those titles, a DDG search finds http://sf-
| encyclopedia.com/entry/relativity listing more, also containing
| the line "Very many sf stories use relativistic time dilation
| for one-way Time Travel into the future."
| akvadrako wrote:
| There are very few sci-fi movies or books where relativity is
| important and they treat it seriously. _Tau Zero_ is one, I
| 'm not sure about _Time for the Stars_ , but _Forever War_
| does have FTL.
|
| I created a list of all these works I could find a few years
| ago. Here is one link:
|
| https://tvtropes.org/pmwiki/discussion.php?id=l6q90mvssgscsj.
| ..
|
| I think this is a complete list of the films and tv series:
| - The Star Lost - Pandorum - Passengers (2016)
| NateEag wrote:
| Spider Robinson's "Variable Star" is not hard sci-fi, but
| relativistic speeds and time dilation are relevant to the
| plot.
| eesmith wrote:
| _Time for the Stars_ has FTL at the end. (Physical FTL,
| that is. Telepathic FTL exists at the beginning.) But it 's
| otherwise it's deliberately structured around the twin
| paradox - the main character is even a twin.
|
| Indeed, while I remembered the importance of time dialation
| in _Forever War_ , I didn 't remember the FTL travel
| between collapsars.
| rsynnott wrote:
| Virtually all of Larry Niven's novels that don't have FTL
| touch on this. Also shows up in most of Alastair Reynolds'
| novels, and in the Three Body Problem series.
| akvadrako wrote:
| I know of Niven's "The Legacy of Heorot", but it only
| takes place on one planet.
|
| Most of Alastair Reynold's novels have FTL - I only know
| of one that counts, which is Pushing Ice.
|
| The 3-body problem has FTL also.
| grkvlt wrote:
| alastair reynold's revelation space series certainly
| doesn't have ftl, apart from maybe [jumper clowns spoiler
| deleted]; the conjoiner drives use [science] to achieve
| 1g acceleration to get to near light speed, and time
| dilation is part of many of the plots.
| rsynnott wrote:
| > the conjoiner drives use [science] to achieve 1g
| acceleration to get to near light speed
|
| For [science] read [magic], more or less, but yeah. In
| the very first stories they were Bussard ramjets, but
| this got retconned out (and a Bussard ramjet actually
| shows up in a later book as a failed experiment). In most
| of the books they're more or less applied magic, though.
|
| This, incidentally, seems to be a common theme, as later
| discoveries tended to fall down on the side of Bussard
| ramjets not working (due to insufficient density etc).
| The last of Niven's Known Space books have some special
| pleading for how the pilot has to carefully direct the
| ramjet to get sufficient combustion volume, a detail that
| was never present in the old ones.
| eesmith wrote:
| Niven's "A World Out of Time" is sublight, with time-
| dilation.
| rsynnott wrote:
| > I know of Niven's "The Legacy of Heorot", but it only
| takes place on one planet.
|
| The chronologically earlier novels in his Known Space
| universe (the Ringworld one) are mostly around near-c
| Bussard ramjets.
|
| > Most of Alastair Reynold's novels have FTL
|
| Hmm, wait, which ones? Only one I can think of is House
| of Suns, in a very restricted manner.
|
| > The 3-body problem has FTL also.
|
| It has FTL communications, but not travel.
| akvadrako wrote:
| FTL communications is not less problematic than travel.
| edna314 wrote:
| I'm sure it has been, but I'm mainly talking about popular
| sci-fi movies. I'm sure you have at least one in mind where
| they have these 'sleep' devices.
| simonh wrote:
| Having sleep pods and such isn't silly if the travel time
| really is many years.
| philipswood wrote:
| Sleep pods are also not silly if you assume your speeds
| aren't going to be a significant fraction of the speed of
| light.
| edna314 wrote:
| I mean yes, but there is a rather specific window where
| they make sense: at ~1-80% of c. Lower speeds it doesn't
| make sense to engage in interstellar travel at all,
| because even travelling 100s of years won't get you
| anywhere. At higher speeds it doesn't make sense because
| of time dilation.
| rsynnott wrote:
| But that's a... pretty important window. There are not-
| completely-bonkers hypothetical spacecraft designs that
| could get to, say, 0.2c. There's nothing particularly
| plausible that could get to 0.99c.
| eesmith wrote:
| I'm not much into visual media, so I can only come up with
| a few examples of 'sleep' devices:
|
| Star Trek's "Space Seed" - 200 years in sublight stasis, no
| mention of speed or distance. Could be 0.5c for all we
| know. 100 ly at Enterprise's warp 6 cruising speed would
| take about 4 months, which seems not unreasonable.
|
| The movie "Alien" - stasis, but seemingly with FTL given
| the times and distances involved.
| (https://avp.fandom.com/wiki/LV-223 says it was a 2 year
| voyage to the moon LV-223 in Prometheus.)
|
| The TV series "Red Dwarf" - sublight, though it did break
| the light barrier once. Science in general is only relevant
| for comedic effect. https://www.ganymede.tv/2004/04/the-
| science-of-red-dwarf/ .
|
| None of these seem to be characterized as "it takes you 150
| years of travel at light speed to get to a place that is
| 150 light years away".
|
| Which one am I missing?
| edna314 wrote:
| What came to my mind immediately was 'Passengers'. But,
| reading up the plot on Wikipedia, it is possible that it
| never actually was mentioned how far away the destination
| was or how fast they were going. So, maybe it wasn't a
| misconception on the sci-fi side, but on my side. For
| sure, any light speed travel shouldn't take any time and
| infinite amount of time should pass for the rest of the
| universe.
| eesmith wrote:
| From Wikipedia, 60 light years and "a journey lasting 120
| years".
|
| It doesn't say if that's 120 ship years or Earth years.
| I'll assume ship years as that makes the most sense in
| context.
|
| Assuming constant acceleration to the 1/2-way point,
| flip, deceleration, and using http://www.projectrho.com/p
| ublic_html/rocket/slowerlight3.ph... : T
| = (c/a) * ArcCosh[a*d/(c^2) + 1] (given acceleration and
| distance) 60 years = (c / a) * arccos(a * 30
| ly / (c^2) + 1) 1893456000.0 = (3E8/a) * arccos(a *
| 3.15576 + 1)
|
| Using Wolfram Alpha to solve - https://www.wolframalpha.c
| om/input/?i=1893456000.0+%3D+%283E... a =
| 0.14748 m/s^2 or about 1.5% g
|
| Time elapsed on Earth is only a bit longer than ship
| time: t = (c/a) * Sinh[a*T/c] = 1.15 *
| ship time ("proper time")
|
| Final velocity at flip is: v = c *
| Tanh[a*T/c] = 0.73 * speed of light
| dctoedt wrote:
| One of Heinlein's juvenile novels ( _Have Spacesuit, Will
| Travel_?) describes TFA 's concept of accelerating at 1g until
| the halfway mark, then doing a "skew flip maneuver" (I think that
| was the phrase Heinlein used) to decelerate at 1g. [0]
|
| [0] https://en-academic.com/dic.nsf/enwiki/2241027
| hutzlibu wrote:
| Slightly related:
|
| I just discovered Andymon.
|
| A 1982 sci-fi novel, where a interstellar spaceship reaches the
| destination star, after travelling a very long, but unknown time
| (part of the plot).
|
| I do not want to spoiler too much, but it was written by a
| academic physics/philosopher and his wife a mathematician. Deep
| shit. Should be standard read in schools.
| drittich wrote:
| Do you know if there's an English translation?
| hutzlibu wrote:
| I assumed there is, but also could not find one right now.
| Would be a shame, if there isn't.
| vagrantJin wrote:
| You over-estimate teenagers. But worst of all, you over-
| estimate the department of education.
| andrewflnr wrote:
| Teenagers are idiots in many ways, but also hunger for real,
| deep ideas. Maybe if they weren't so consistently
| underestimated they would take things more seriously.
| vagrantJin wrote:
| I apologize, ofcourse they have a hunger for real,
| meaningful and deep ideas but, they are still pretty dumb.
| Not dumb as in lacking intellectual ability but dumb as in
| willingly stupid. Thats the key. No shade but pop culture
| is basically a "teen spirit" transformed into profit by BS
| Inc.
|
| And ofcourse the small issue of sex being 80% of what's on
| their mind. Perfectly natural and healthy.
| hutzlibu wrote:
| I am not a fan of mandatory books, but I know I would have
| loved to read and talk about this book in school. It covers
| everything scientific, as well as philosophical existential
| questions.
| air7 wrote:
| > "In fact, so long as you kept adhering to this plan, you could
| choose any destination at all that's presently within 18 billion
| light-years of us, and reach it after merely 45 years, max, had
| passed. That ~18 billion light-year figure is the limit of the
| reachable Universe, set by the expansion of the Universe and the
| effects of dark energy. Everything beyond that point is currently
| unreachable with our present understanding of physics"
|
| What happens if I just keep going?
| DarkWiiPlayer wrote:
| The way I understand it is that it's not impossible to cover
| the distance, but that by the time you do, the distance between
| your starting point and your destination will have (at least)
| doubled due to the expansion of space.
|
| So you're still as far away from your destination as in the
| beginning (or even further), but now you're the same distance
| from your starting position as well and are effectively
| stranded.
| perihelions wrote:
| Well, an accelerating mass emits gravitational waves; and your
| spaceship is asymptotically infinite-mass; so at some point you
| destroy the universe. (Or at least the parts that are causally
| connected to you -- I guess cosmic expansion would save parts
| of it?)
|
| I don't know whether you observe this or not.
| Gunax wrote:
| There is a misconception that expanding space prevents us from
| ever reaching a destination. It's counter-intuitive
| (https://en.wikipedia.org/wiki/Ant_on_a_rubber_rope) but falls
| out of the math. If space expands at a constant rate, you could
| eventually reach _any_ destination at _any_ velocity.
|
| But if the expansion of space is accelerating (which we believe
| currently), this is not true. There really would be
| destinations that are unreachable.
|
| You would continue to travel further from your origin, but also
| witness your destination accelerate away.
| secondcoming wrote:
| Can scientists count the number of far distant stars that are
| no longer visible due to space accelerating?
| stavros wrote:
| But what if you accelerate faster than it does?
| Armisael16 wrote:
| Your top speed is bounded by the speed of light. The thing
| you're trying to reach is not, since that acceleration is
| mostly actually space expanding between you and it. It's
| going faster that you can ever go, and as the distance
| between you and it increases it accelerates even more.
| saiya-jin wrote:
| You can't go faster than light. Space in universe is
| expanding faster than light.
| cygx wrote:
| No matter how fast you accelerate, you'll chew up distances
| at a local(!) rate of less than c, so it's enough to
| consider the limiting case of light and skip the
| complications introduced by acceleration. The analogous
| classical problem is the "ant on a rubber rope". If the
| endpoint of the rope moves at a constant speed, the ant
| will be able to reach it, no matter the rope's length. If
| the endpoint accelerates, the ant might not, and only be
| able to asymptotically approach some point somewhere on the
| rope.
| stavros wrote:
| Hmm, but if we treat the end of the rope as an object
| which accelerates at x m/s^2, and I accelerate at 2x
| m/s^2, wouldn't I eventually reach that object? I don't
| know what I'm not getting...
| JZumun wrote:
| You can never go faster than c, so you can never reach
| anything that's being moved faster away.
| cygx wrote:
| > and I accelerate at 2x m/s^2
|
| A constant lab frame acceleration is incompatible with
| special relativity.
| Thiez wrote:
| Let's say the universe is turn-based, and consists of a
| single dimension. At the end of each turn the distance
| between two points in this universe doubles (because new
| empty space is created between them). Let the maximum speed
| in this universe be 300.000.000 distance-units per turn.
| When two points are, say, 5 units apart, we can easily
| travel between them (for a couple of turns, at least). But
| when two points are more than 600.000.000 distance-units
| apart, we can never again travel between them. Let's try.
|
| We're at turn 0, at point A. Point B is 600.000.000
| distance-units apart. We have infinite! acceleration and
| accelerate to 300.000.000 units/turn, and start our travel
| to point B. We move 300.000.000 units. Distances double.
| We're at turn 1. Point A is now 600.000.000 units behind
| us, and point B is still 600.000.000 units ahead of us. We
| travel 300.000.000 units! Distances double. We're at turn
| 2. Point A is now 1.800.000.000 units behind us, and point
| B is still 600.000.000 ahead of us. We're starting to
| wonder if we should have stayed at point A... hopefully
| there is a point C that was between point A and point B
| that we can still get to, because we don't seem to be
| making much progress.
|
| The universe is a bit like that, except that it looks more
| continuous, it has some more dimensions, and it's not
| doubling quite so quickly.
| toss1 wrote:
| So, Zeno finally gets his paradox to work!
| stavros wrote:
| Hmm, I see, thank you. So it's _space itself_ that
| expands, not just the objects in it that accelerate apart
| from one another. But then doesn 't that mean that some
| of that space is expanding _inside_ the objects? Do we
| know what happens there? How do objects just get more
| space inside them?
| cygx wrote:
| Thinking of spacetime as some sort of expanding fabric
| permeating every-day objects is not a good analogy in my
| opinion.
|
| First, like any other gravitational effect, locally,
| spatial expansion would manifest as pseudo-forces that
| can be counter-acted by all the other forces that are far
| more relevant at human scale.
|
| Second, the local effect of spatial expansion should be
| an indirect one: Friedmann cosmology - which is how we
| describe an isotropic, expanding universe - is a large-
| scale approximation. More realistic would be 'swiss-
| cheese' models, where spacetime in our neighbourhood can
| look vastly different from the Friedmann one, except that
| the spacetime patches need to properly fit together to
| yield the correct large-scale behaviour.
| stavros wrote:
| Ah I see, so this all happens at a quantum level that
| doesn't translate well to our intuition? That makes a
| certain amount of sense, thank you.
| cygx wrote:
| This is still relativity, not quantum mechanics.
|
| Point is, illustrative models and analogies are limited.
| Ideas like 'space itself expanding' or 'space flowing
| like a river and falling into a black hole like a
| waterfall' might help visualize some things, but can also
| lead to wrong ideas if taken too seriously.
| stavros wrote:
| > This is still relativity, not quantum mechanics.
|
| Oh hmm, I see.
|
| > but can also lead to wrong ideas if taken too
| seriously.
|
| Yeah, and it's hard to know how far to take the analogy
| unless you're already familiar with the concept it's
| trying to describe, unfortunately.
| JZumun wrote:
| (Assuming space expands evenly) we don't notice the
| expansion of space inside objects because the
| electromagnetism and gravity are vastly more powerful on
| the scales we care about and nullifies the effect. That's
| why atoms and galaxies in the Local Group stay together,
| but everything farther away is flying further away.
|
| If expansion never stops accelerating, one day in the far
| future it will overcome all the other fundamental forces
| and everything will be torn apart in a Big Rip.
| grkvlt wrote:
| wait - does this mean that some super capable
| civilisation could link the various galaxies in their
| inter-galactic empire with a space-railway (i.e. using
| actual physical tracks between them in space) assuming
| they have the staggeringly vast amount of matter and/or
| energy needed, then this would gravitationally bind them
| together and space between them could not expand?
| lalaithion wrote:
| No, "gravitationally binding" is not a thing that you can
| do by connecting things via railways. Over the long
| distances, the railways would be torn apart by expansion.
|
| If you could drastically increase the mass of the local
| group, then you could increase the range at which
| gravitational attraction to it was dominating, but it
| would require seriously increasing the mass of the local
| group.
| hoseja wrote:
| Notice the "presently".
| Lariscus wrote:
| You would never reach your destination and at some point you
| would be stranded in dark space. This kurzgesagt video has some
| info on this topic.[1]
|
| [1] <https://www.youtube.com/watch?v=uzkD5SeuwzM>
| stouset wrote:
| I don't quite understand this. I think I'd maybe understand
| if space dilated to the point where you couldn't quite hit
| that edge. Though now that I say it that "solution" seems to
| come with a whole bunch of problematic consequences.
|
| But "not getting to anything past the furthest thing we can
| see from earth today" seems fundamentally incompatible with
| the idea of reaching those farthest points in a reasonably
| bounded timeframe. There _is_ stuff beyond the limits we can
| see, it's just further than light has had a chance to travel
| so far (and may ever travel).
|
| My intuition is that as you approached those far objects,
| you'd observe them rapidly evolving forward in time until
| they reached their "present day" situation, able to see
| billions in light years in all directions with Earth right at
| the edge.
|
| But then what happens when you look at Earth? Surely you
| don't see it's billions of years old past. You'd have to see
| it at least as old as it was when you left. But something
| doesn't seem quite right about that to me. Surely I'm missing
| something.
| est31 wrote:
| Space is expanding. Which means stuff is getting further
| away from each other.
| https://en.wikipedia.org/wiki/Expansion_of_the_universe
|
| If you go far enough, stuff moves fast enough away from us
| than the speed of light. Thus, some places can't be reached
| from someone starting at our present location, ever, even
| though we might still see their light. That light is closer
| to us than the object that sent it out.
| edf13 wrote:
| Can you (Or stuff) go faster than the speed of light?
| simiones wrote:
| No, the "speed of light", c, is actually the speed of
| energy and matter. Everything is always moving at this
| speed in 4-dimensional space-time. If you are not moving
| in space, you are moving through "time" at this speed.
| Since this is just the speed things move at, it makes no
| sense to discuss moving faster or slower than this speed.
| ccozan wrote:
| Correct explanation!
| eru wrote:
| Well, if you take a really strong laser pointer and point
| it at the moon, than a flick of the wrist can make that
| point race faster along the moon's surface than the speed
| of light.
| MaxikCZ wrote:
| But its not really a point thats traveling faster than
| speed of light. The individual photons that makes the
| point travel always at the speed of light from your laser
| pointer to the moon. Its not point on the moon thats
| traveling, the travel is just ilusion because it looks
| like the point that was there a split second ago.
| eru wrote:
| Well, the 'point' is traveling faster than the speed of
| light.
|
| You are quite right that no matter nor energy nor
| information is traveling faster than light here.
| MaxikCZ wrote:
| I am not sure how to define the point, but my way of
| thinking is that its not the same point. The point never
| moves, instead it continuously vanishes and a new point
| is being continuously made next to it.
|
| That way, there really isnt a point that is traveling or
| moving, its just our perception mistaking it for a point
| because it looks and move like one.
| jasonwatkinspdx wrote:
| Yeah, exactly this. If you think of it like waving a
| garden hose back and forth it's very clear why this is
| flawed logic. Also because of quantization the further
| away you put the target surface, to try to raise the
| "speed" of the not actually existing point, the more
| discontinuous it becomes.
| gilbetron wrote:
| If I installed a really bright light source on two
| opposite ends of the observable universe and timed
| everything correctly, I could make it seems like the
| point traveled across the universe in a fraction of a
| second. Same idea, same flawed logic. Nothing actually
| moved.
| cygx wrote:
| Nothing can outpace photons (in vacuum, that is - in a
| medium, cf Cherenkov radiation). However, velocity is a
| surprisingly tricky concept. While the relative velocity
| of two objects passing each other by is well-defined,
| velocity at a distance tends to require a clock
| synchronization convention, which is to some degree
| arbitrary. In consequence, in special relativity, the
| speed of light is only isotropic by convention (cf
| various discussions about the one-way speed of light,
| which became a topic of online conversations due to
| Veritasium posting a video about it), and in general
| relativity, recession velocities (ie change in proper
| distance at constant cosmological time) of far away, but
| still observable galaxy can in fact formally be greater
| than c.
| marcyb5st wrote:
| Not with our current understand of physics. Even reaching
| the speed of light takes infinite amount of energy for
| things with mass. This is because the faster you go the
| more massive you become. Furthermore, even before
| reaching the speed of light you will become a black hole
| as more energy is added.
|
| Things without mass, instead, can only travel at the
| speed of light (photons for instance).
|
| Finally, there is a quirk in the math that would allow
| for the appearance of faster than light travel. If you
| compress the spacetime in front of you and expand it
| behind you you could can move faster than light without
| turning into a black hole or needing infinite energy
| (Google Alcubierre drive for more information). It is
| only an appearance than faster than light travel because
| you would still move at sub-luminal speed in your bubble
| of "normal" space time, but the compression/expansion
| effect would drag you through space-time at faster than
| light speed. This, however, require so called "exotic
| matter", ie matter with negative energy density (this is
| not anti-matter, but matter that has a repulsive
| gravitational field) and it is probably only a quirk of
| the math and nothing more.
| ajconway wrote:
| So, when you reach the "edge" you were going for, you
| arrive at the point when everything has already moved
| past your horizon (including Earth), there isn't a single
| hydrogen atom around, so you're stuck in the darkness
| forever?
| est31 wrote:
| Yeah from current understanding that's your fate.
| Everyone's fate actually, if you see gravitationally
| bound structures as one unit. You'd be alone waiting for
| the heat death. Therefore, I recommend decelerating to
| somewhere interesting during the time window which allows
| intergalactic travel.
| stouset wrote:
| Ah, I think I understand! Something 18bnly away from us
| today has a light bubble that includes objects outside
| our light cone.
|
| If you can get there in 45y, that doesn't mean 45y has
| passed at that location. In fact if it's 18bnly away,
| that means you'd arrive in 18bn years from their
| perspective. And not 18bn years from what you saw when
| you left, but 18bn years from when _they_ saw you leave.
| By that time, space will have expanded enough that it's
| no longer possible to see past.
| drran wrote:
| Expansion of the (visible) Universe is hypothesis, not a
| fact. Nobody pointed to an incredibly powerful energy
| source required to expand the whole visible Universe. A
| non-zero curvature of space is not confirmed also.
|
| We are able to see gravitational waves at this point.
| Where is a gravitational turbulence, created by the
| massive expansion?
|
| (Non-native speaker, but you got the idea).
| gambiting wrote:
| As far as I understand - you'll come back to where you started.
| simonh wrote:
| I'm afraid not, the accelerating expansion of the universe
| will stretch out space ahead of you faster than you can
| traverse it. From your perspective the galaxies will become
| sparser and sparser over time until you're in an ever colder
| eternal black void.
| daxfohl wrote:
| So you witness the heat death of the universe at ~50 years.
| Yet you and your spaceship are still intact then, and at
| times exponentially beyond it. Weird.
|
| Though perhaps the amount of energy it would take to
| accelerate that much exceeds the available energy in the
| universe? So maybe that's what balances it.
| jcims wrote:
| The 'specific impulse' of a rocket engine is one measure of its
| ability to convert the energy in its fuel into forward thrust.
| Its unit is 'seconds', and its measure could be interpreted as
| 'how long could one pound of fuel produce one pound of thrust'.
|
| If your rocket starts off 2/3rds fuel by mass, this will result
| in an average acceleration of 1g as the mass of the fuel is
| consumed.
|
| The most efficient chemical rockets we have today have a specific
| impulse in the ~470 second range. The highest specific impulse
| thrusters of any sort that we have in wide use today are 'ion
| thrusters', which have specific impulses in the 50,000 second
| range (but can't generate much thrust at all).
|
| So 10-12 hours at 1g is about as much as we can do with existing
| hardware.
| Gravityloss wrote:
| One very common misconception that higher specific impulse
| (ISP) is always good. Electric propulsion is different from
| chemical.
|
| In chemical propulsion, you have the energy in the propellants.
| There higher ISP is better.
|
| In electric propulsion, you use an external power source.
| Usually your propellant or reaction mass is inert, like Xenon.
| There, if you up the specific impulse but keep acceleration and
| delta vee the same, your power source mass increases. At some
| point your power source mass is a lot bigger than your
| propellant mass. Hence, your total mass for the same misaion
| would be less with lower specific impulse.
|
| So in electric propulsion there can be too high ISP for a given
| mission and power source technology.
| sdenton4 wrote:
| I've heard this before, but it seems to me that the 'problem'
| isn't the too-high ISP, but rather the heavy power source.
| For rockets, we've got the rocket equation which gives the
| exact fuel trade-off (carrying more takes you further, but
| also means carrying more weight). Is there a similar 'hard'
| relationship between ISP and power supply mass for electric
| drive systems?
| zamadatix wrote:
| It would still just be the rocket equation no? The power
| supply mass that isn't ejected as energy in propulsion
| would just be counted as part of the rocket's dry mass (mf
| in the rocket equation).
| Gravityloss wrote:
| Yes. Thrust is related to momentum while power to energy.
| So if you double the exhaust velocity, you double the
| thrust but quadruple the power needed. P
| = 0.5 * mdot * v_ex^2 T = mdot * v_ex
|
| A normal ion thruster might have ISP 4000 or 40 km/s
| exhaust velocity.
|
| From the rocket equation, if our mission requires 10 km/s
| delta vee, mass ratio is exp(10/40)=1.28. So out of every
| 1280 kg, 280 kg is propellant.
|
| If we have mass flow of 0.1 gram per second, thrust is
| 40000 m/s * 0.0001 kg/s = 4 N and power is 80 kW.
| Acceleration is about 0.27 km/s per day. Not so good for
| humans through Van Allen belts. But fine for deep space
| propulsion.
|
| 80 kW of solar cells with 200 W/kg weight efficiency would
| weigh 400 kg. This is ISS level power.
|
| So the spacecraft might have 280 kg of propellant, 400 kg
| of solar arrays, 600 kg of useful things.
| sdenton4 wrote:
| Yeah, so my question comes down to 'how much can we play
| around with weight efficiency?' For example, maybe I can
| drop a lot of solar cells in exchange for a small battery
| or capacitor capable of providing the needed power for
| the higher exhaust velocity, but in bursts. Then we
| change the firing pattern from a continuous burn to
| alternating between burning and recharging the batteries,
| and save a lot of solar panel weight.
|
| [on edit: the weight efficiency of the solar panels will
| also depend on where you are in the solar system...]
| andyxor wrote:
| nuclear pulse propulsion such as in the 1950s project Orion
| designs was estimated to be around 6000sec (theoretically up to
| 100k sec)
| https://en.wikipedia.org/wiki/Nuclear_pulse_propulsion
| blkhawk wrote:
| Hours? You mean Minutes.
| ben_w wrote:
| 50,000 seconds would be 13.89 hours, though I'll grant you
| "with existing technology" would preclude getting enough
| electrical power into an ion drive for that to do one whole
| gee.
| jcims wrote:
| Yeah 100kW for 5N ramps to ~15-20MW for each person lol
| whimsicalism wrote:
| Where are you getting that from?
| euroderf wrote:
| > So 10-12 hours at 1g is about as much as we can do with
| existing hardware.
|
| So 9.8m/s2 times 36ks yields about 360km/s. Not bad. Altho only
| a thousandth of the speed of light :-(
| Taek wrote:
| I've always pretended to myself that I understand what impulse
| means but today you've helped me actually understand it. Thank
| you!
| fouronnes3 wrote:
| GP is a nice reverse engineered explanation, but the actual
| origin of using seconds as a unit is to avoid having to
| compare metric and imperial units of speed. Isp is just exit
| velocity divided by g and it's conveniently in seconds, a
| pretty universal unit. And "one pound of thrust" only makes
| sense when you're in Earth's gravity field with gravity
| losses. Otherwise it's more precise to use Newtons for
| thrust.
| kbenson wrote:
| > conveniently in seconds, a pretty universal unit.
|
| Thankfully! Imagine if systems/cultures commonly differed
| in fundamental measures of time, just as they sometimes do
| for volume, distance, weight, etc.
| jcims wrote:
| I use it because even after being an Estes nerd as a kid it
| was the first explanation that actually made sense to me.
| [deleted]
| HPsquared wrote:
| For this kind of thing it's helpful to think about the
| dimensions and units involved.
|
| In mechanics, "impulse" is a word for a unit of momentum.
| Momentum has dimensions (Mass * Speed), or (force * time),
| and can be stated in units of Newton-seconds.
|
| "Specific" impulse is simply Impulse per unit mass of fuel
| (i.e. Newton-seconds per kilogram). The convention of stating
| it in units of seconds is based on Earth gravity (Newtons per
| kilogram).
|
| If a system has specific impulse of 100 seconds, 1 kilogram
| of fuel would be able to accelerate a mass of 1kg at an
| acceleration of 1 G, for 100 seconds.
| Someone wrote:
| Nitpick: I think that should be _1 g_
| (https://en.wikipedia.org/wiki/Gravity_of_Earth), not _1 G_
| (https://en.wikipedia.org/wiki/Gravitational_constant)
| [deleted]
| jcims wrote:
| If I'm thinking about this correctly, the potential energy
| in the fuel isn't changing the net kinetic energy of the
| total mass, it's just re-distributing it between the
| vehicle and the exhaust.
| HPsquared wrote:
| Total momentum is conserved, but distributed: the
| individual momentum of the vehicle and the exhaust jet
| will be of equal magnitude and opposite direction
| (assuming a straight line), so if the rocket started in
| free space at rest (zero momentum), after it's run for a
| while the exhaust jet and vehicle will have equal and
| opposite momentum vectors.
|
| Energy is a different thing, but related to specific
| impulse. In a perfectly efficient system (not what really
| happens), the decrease in stored energy within the fuel
| tanks would be equal to the increase in _total_ kinetic
| energy (i.e. the sum of vehicle and exhaust kinetic
| energy).
| jcims wrote:
| Thank you, momentum is definitely the term I should have
| used.
| wombatmobile wrote:
| American singer Elvis Presley explored this theme in 1968.
|
| https://youtube.com/watch?v=7Skrq8unUlE
| philiplu wrote:
| For a hard science fictional version of this, check out Poul
| Anderson's "Tau Zero", from 1970, according to Wikipedia. I read
| it a few years after that, and it's stuck with me since. A ship
| with a Bussard ramjet runs into troubles and has to keep
| accelerating, into the far future.
| tomxor wrote:
| And all at 1g... I wonder what forces the human body is capable
| of adapting to over a long period.
|
| What happens if say we double the thrust, presumably halving the
| timescales and doubling the experienced "gravity"... would a
| human body be able to adapt and get stronger? or would we
| experience adverse side effects? (not necessarily mutually
| exclusive). It could even be a way to acclimatise to a different
| planet's mass before arriving.
|
| [EDIT]
|
| Looks like 2g is reasonable. The limit is higher but it sounds a
| bit ridiculous, astronauts would all have to compete in the
| strongman competitions for 4g. Any higher and we wouldn't be able
| to walk without breaking our bones.
|
| https://www.discovermagazine.com/the-sciences/whats-the-maxi...
| P4u1 wrote:
| Thanks, I was wondering the exact same thing!
| rtkwe wrote:
| That number would probably also change over time especially as
| people grew up in that gravity, putting more stress on their
| bones which should cause them to grow thicker (see ancient
| skeletons of longbowmen for a small example and imagine that
| the extra forces are there all the time).
| api wrote:
| The theoretical limit is "to the edge of the Hubble horizon, as
| long as you don't mind it being a one way trip."
|
| As you approach 'c' time dilation shrinks time for you. Keep
| accelerating and eventually a million years outside your
| reference frame is mere days or hours inside the spacecraft. You
| could travel to the Andromeda galaxy or beyond as long as you
| were okay never returning to Earth or returning to a different
| geological epoch than the one you left. Go far enough and the sun
| might be in its red giant phase when you get back. You may have
| aged a few years or decades.
|
| This of course requires stupid amounts of energy and theoretical
| near maximum specific impulse, what I once heard called a
| "physicists' nightmare propulsion system." Something like a
| "photon rocket" or a relativistic velocity ion drive (propellant
| exit velocity near 'c') plus a very efficient fusion or
| antimatter reactor might be able to get you there. It was called
| a physicists' nightmare because if containment fails you become a
| flash of gamma rays in less than a nanosecond.
|
| One of my favorite wild speculations is that hypervelocity
| massive particles like the "OMG particle" are the jet wash from
| someone's engine.
| littlestymaar wrote:
| Alastair Reynolds' novel _Pushing Ice_ (2006)[1] covers exactly
| this topic: a bunch of humans are trapped on an alien spaceship
| accelerating indefinitely, and all the involved relativistic
| effects are covered in the book. While it 's not Reynolds best
| book, it's definitely worth reading just for the clarity and
| accuracy of the description of the relativistic effects (Reynolds
| is a former ESA astrophysicist).
|
| [1]: https://www.goodreads.com/book/show/89186.Pushing_Ice
| milansm wrote:
| Which book would you consider his best? I've only read House of
| Suns, and it was great.
| gilbetron wrote:
| Definitely the Revelation Space series is awesome. For
| something a bit different, Revenger was really cool - it has
| two sequels now that are on my queue to read so I can't speak
| to them.
| aaronblohowiak wrote:
| They are great!
| JamesBrooks wrote:
| House of Suns left me sad that it was a standalone novel, I
| was itching for more!
| lreeves wrote:
| There is another novella in the universe called Thousandth
| Night! I thought it was excellent.
| andrewflnr wrote:
| It's a slightly different universe, really, feels like an
| earlier revision. Not timeline-compatible either, with
| what happens to certain characters...
| robotomir wrote:
| I think Chasm City is his best book, but it requires context
| from other Revelation Space novels to be enjoyed fully.
| gpderetta wrote:
| Agree on Chasm City. The rest of RS has a lot of good and
| bad. I think the Prefect is his worst.
|
| Outside of RS, I also liked House of Sun a lot.
| CobaltFire wrote:
| That's an interesting take, as I rather enjoy The Prefect
| and rate Chasm City as one of his more "eh" books.
|
| I guess I'm weird.
| andrewflnr wrote:
| Interesting, I thought it would have been as good an intro
| as RS itself to the universe. It certainly puts Ana Khouri
| in perspective.
| KineticLensman wrote:
| I liked Century Rain for the well-done mix of Casablanca-
| style film detective noir finding himself in hard SF story
| littlestymaar wrote:
| Not an individual book, but the entire _Revelation Space_
| series is one of my favorite SF piece ever.
| soylentcola wrote:
| Also one of mine. This thread got me looking it back up and
| it appears there's a 4th novel coming out later this year
| (I didn't know about this, so perhaps you'll be equally
| glad to learn of it!)
| hinkley wrote:
| There's a short story in the Expanse Universe where Solomon
| Epstein is killed by his experimental propulsion system when it
| accelerates so fast that he can't reach the controls to turn it
| off and then blacks out.
|
| The drive is named posthumously after him based on the design
| notes found on his computer by his widow.
| austincheney wrote:
| I remember watching that episode. Such a great series.
|
| I think this is a slightly different phenomenon though. The
| problem with Epstein is rapid acceleration resulting in an
| increased gravitational force relative to the speed of
| acceleration well beyond 1g and continuing to accelerate
| faster until fuel is exhausted.
|
| The scenario in the article is about indefinite acceleration.
| Indefinite acceleration doesn't have to be fast to eventually
| reach near light speed provided enough time.
| hinkley wrote:
| In the story his miscalculation results in 37 hours of
| fuel. With no dead man's switch he dies from lack of oxygen
| to the brain, and the ship just keeps happily accelerating
| until the fuel is exhausted.
|
| For a long enough trip, anything over 1G results in your
| passengers and payload having to deal with excess stresses,
| up to and including ship damage or passenger death. In the
| Expanse they are quick to point out the increased chance of
| stroke due to prolonged and excessive acceleration.
|
| 1G is nice because you can move like you would on the
| surface.
| kbelder wrote:
| 5g acceleration for 37 hours will get you up to 1% of the
| speed of light. Constant acceleration with no fuel limits
| gets you into the realm of relativity very quickly.
| snowwrestler wrote:
| In _The Forever War_ by Joe Haldeman, soldiers are sent out on
| relativistic journeys to fight a war with aliens. Each time
| they come back, Earth culture has changed, and eventually it
| becomes unrecognizable to the soldiers, and Earthlings forget
| or stop caring about the war.
|
| It was an allegory for the U.S. involvement in the war in
| Vietnam, in which Haldeman fought.
| scotty79 wrote:
| There's a pretty cool comic book based on Forever War
| dharmab wrote:
| [spoilers]
|
| The main character is spacially and temporally separated from
| his love interest/comrade during the war. At the end, he
| receives a letter saying she's been in cryo sleep aboard a
| ship experiencing time dilation to wait for his return.
| KineticLensman wrote:
| > ... and Earthlings forget or stop caring about the war.
|
| [spoilers]
|
| Not quite. Humans develop (via cloning and other tech) to the
| point where they can finally communicate with the hive-mind
| aliens, and realise that the whole 1000-year war has
| essentially been a mistake from the outset.
|
| > It was an allegory for the U.S. involvement in the war in
| Vietnam
|
| Yes. The obvious allegory is that to the very few baseline
| humans who survive the entire war due to time dilation
| effects the whole thing was a complete waste of time.
|
| A key subplot in the book is that humanity adopts
| homosexuality such that in the latter stages of the war, the
| baseline heterosexual veterans are perceived as weird sexual
| deviants. This seems to me allegory for the difficulty
| Vietnam vets had in readjusting to civilian life.
| killingtime74 wrote:
| (I've read the book) no spoilers!
| mlinhares wrote:
| This was one of the best war movies I've ever read, almost
| ever other one I read felt bland in comparison.
| gpderetta wrote:
| It is an amazing book indeed
| ChrisMarshallNY wrote:
| Agreed.
| dhimes wrote:
| I actually had it assigned to me to read in college.
| Outstanding book.
| mattsoldo wrote:
| Project Hail Mary by Andy Weir (author of The Martian) goes
| into the relativistic effects as well, and it includes a crazy
| (made-up) fuel that could make it possible. Great easy read:
| https://www.goodreads.com/book/show/54493401-project-hail-ma...
| ftkftk wrote:
| Excellent book. The audio book is really well done as well -
| narrated by Ray Porter.
| joshstrange wrote:
| Ray Porter is an excellent audio book voice actor. He
| narrates the Bobiverse books (as well as "The Singularity
| Trap" by the same author) which I enjoy and "Project Hail
| Mary" is next on my reading list after I finish out the
| ExForce series.
| sonograph wrote:
| (spoiler warning) I gave up on the ExForce series after a
| half dozen books; it was good, just repetitive. But I
| couldn't force myself to purchase the Bobiverse #2. While
| the Bobiverse author's style was irritating to me, I
| thought the alien interactions in Bobiverse were
| laughable compared to ExForce and Hail Mary (spoiler
| sorry). The antagonist also left much to be desired.
|
| But Ray Porter's voice needs to be applied to many more
| books. I generally only stop reading audiobooks if I
| can't listen to the reader any longer
| joshstrange wrote:
| I totally understand abandoning ExForce. It got better
| once the writer went full time but it still leaves a lot
| to be desired. I put up with it because I'm a
| completionist and the author has a plan to end the series
| in just a few more books so I'll stick it out but I get
| it.
|
| I think the core ideas in Bobiverse keep me hooked as I
| just enjoy them but I see your point and don't begrudge
| anyone who gave up on it.
|
| This is a totally random question but have you ever read
| the Honor Harrington series? They aren't related at all
| (other than "sci-fi") but I'm always interested in other
| series people have read and enjoyed (or not enjoyed) and
| Honorverse is one of my favorites.
| sonograph wrote:
| I actually haven't heard of the Honor Harrington series;
| I'll look it up. Do you recommend paperback or audio for
| it?
| joshstrange wrote:
| I recommend the audio because the narrator does a very
| good job IMHO. I tried to read the books after listening
| to them first but felt it was lacking compared to the
| audio.
|
| I will say that this series is pretty technical (in that
| I talks about the speeds of things and how fast things
| are moving) which I greatly enjoy but I know isn't
| everyone's cup of tea.
| toiletaccount wrote:
| i listened to a preview of that book and couldn't even get
| to the end.
|
| i kind of hate audiobooks for this reason, my internal
| voice is just so much easier to listen to.
| pierrebai wrote:
| I would not classify it as great, but somewhat below par. I
| made a list of things I did not like about the book, but it
| would spoil many things. Not spoiling bits:
| - All characters are two-dimensional. The further from the
| main protagonist, the more 2D. - Dialogues go from
| wooden to cringe-worthy, with few exceptions. -
| Protagonist swings from super over- cautious to careless and
| back. - Alien with deus-ex-machina syndrome, always
| having the perfect tech, material or skill to fill in the
| lacks of the protagonist. - Tired trope of amnesia to
| make the protagonist remember stuff progrssively for the
| reader. - Extreme over-usage of flash-back, linked to
| previous point, but oh boy! Half the book is flashback. Waaay
| over-used story-telling tech.
|
| The science, outside of the magic fuel and magic alien
| material is mostly correct. That's the main strong point of
| the book.
| handrous wrote:
| Assessments of what gets a sci-fi or a fantasy book classed
| as "great" vary widely, it seems. I find recommendations by
| people I don't know well, for those genres especially, to
| be entirely useless. May as well throw a dart.
| bestest wrote:
| My thoughts exactly. I'd even call it quite shallow. The
| only thing saving this book is science. I wouldn't say it's
| a disappointment. But it definitely is very "Martian".
| CabSauce wrote:
| All fair points. Still an absolute page-turner. Sort of a
| trashy romance sci-fi novel.
| rsynnott wrote:
| Tau Zero by Poul Anderson is another one on the same line; a
| Bussard ramjet damaged such that it couldn't turn around.
| Because the Bussard ramjet provides both propulsion and
| radiation/particle shielding, they can't turn it off, so just
| get faster and faster...
| javajosh wrote:
| Another one: The World at the End of Time, Frederik Pohl. A
| star being ("Wan-To") accelerates another star indefinitely,
| dragging its (inhabited) system with it.
|
| https://en.wikipedia.org/wiki/The_World_at_the_End_of_Time
| lazyweb wrote:
| Read the book recently, was entertaining enough.
| sgtnoodle wrote:
| It was an interesting book, but the interpersonal dialog was
| rather tough to get through. It really picked up some
| momentum toward the end though!
| anothermoron wrote:
| > It really picked up some momentum toward the end though!
|
| Ah!
| teekert wrote:
| Since mass increases with speed, will this spaceship collapse and
| become a black hole? Or is it just massive from the observer
| reference frame?
| nuccy wrote:
| Mass is not increasing in the frame of reference of the ship,
| so no, it will not collapse into a black hole regardless of how
| much energy it received externally (e.g. laser beam from Earth)
| to accelerate closer to the speed of light.
| eesmith wrote:
| Those interested in this topic may also like the extensive
| background materials and guidelines at Project Rho, eg,
| http://www.projectrho.com/public_html/rocket/slowerlight3.ph... .
| btbuildem wrote:
| So in a way, this is time travel. If you wanted to see what the
| world looks like in five thousand years, you could get on a
| spaceship, accelerate way out to wherever and back, and voila!
| you've fast-forwarded five millennia.
|
| A fascinating thought. The ship and crew would surely be long
| forgotten. You may be greeted as an alien visitor, until the
| people of that time recognize your relatively ancient technology
| from fragments of historical records. Too bad you speak a
| language nobody else even remembers.
|
| Or perhaps you emerge as gods on a pillar of fire from the sky,
| witnessed by the primitive-again survivors of whatever has played
| out since you left.
|
| I'm strangely tempted. If you offer me a ticket on such a ship, I
| might just come along.
| justsid wrote:
| Don't forget to become a bubble boy to avoid catching all of
| the new diseases that you have 0 immune system protection
| against.
| deepsun wrote:
| At relativistic speeds, you'd be vaporized, or at least ionized
| by interstellar gas, e.g. there are about 1 atom of hydrogen in
| one cubic cm.
|
| There was a paper considering making a shield against it, either
| lead, or electric field, or magnetic field, and resources
| required for it.
| cookieswumchorr wrote:
| shit, looks like i started packing too early
| wavefunction wrote:
| Forever? Seems self-evident.
| [deleted]
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