[HN Gopher] Why can't I go faster than the speed of light?
___________________________________________________________________
Why can't I go faster than the speed of light?
Author : lanna
Score : 132 points
Date : 2021-09-12 15:34 UTC (7 hours ago)
(HTM) web link (gravityandlevity.wordpress.com)
(TXT) w3m dump (gravityandlevity.wordpress.com)
| programmer_dude wrote:
| Wow, this is the first time I have understood why nothing can go
| faster than the speed of light and why time dilation is a thing.
|
| A very easy to read article, give it a try.
|
| Also if you are curious why a moving charge creates a magnetic
| field watch this Veritasium video:
| https://www.youtube.com/watch?v=1TKSfAkWWN0
| [deleted]
| thriftwy wrote:
| Frankly though, I not sure why they couldn't just use a simpler
| correction: postulate that moving charge only creates magnetic
| field with regard to objects moving relative to it. Which is
| also basically true?
|
| This way, the stationary observer will sure notice that there's
| F_B, but will also know that the charge moving alongside the
| (also moving) rod does not experience it.
| DangitBobby wrote:
| That's the thing though, in the article the charged object
| does not move relative to the rod. The rod, object, and
| person B are stationary relative to each other. I was
| actually really surprised to read that two charged objects
| moving with the same velocity relative to each other
| generates a magnetic field from A's perspective.
| sytelus wrote:
| If you just extend time dilation beyond zero (I.e. negative time)
| then speed faster than light is possible. The constraint
| described in the article will still be maintained. Also, notice
| that negative time would be the perception of external observer.
| ySteeK wrote:
| wouldn't it go backward in time if it goes faster than light?
| Light travels in null-time from point a to point b (light-
| perspective of course) - anything slower, needs time to go from a
| to b. if i travel faster than light, i can - but i travel to b
| before i start at a. Maybe because this is not possible.
| drran wrote:
| How much you will travel back in time by using a teleport
| (instant travel, infinite speed)?
| sylr wrote:
| With gifs: https://imgur.com/gallery/BT2mG
| zfxfr wrote:
| Wow thank you ! I have read the whole article but I couldn't
| wrap my head around it. I found the "rod story" confusing then
| I followed your link (and also watched the related Youtube
| video).. This really makes things clearer. I am really not into
| maths but this version seems much easier to understand and very
| logical.
| dhosek wrote:
| My favorite general relativity paradox: A runner is running with
| a 20m pole at a sufficient velocity so that a stationary observer
| sees the pole having length 10m. The runner runs through a barn
| which is 10m long. And two people outside shut the doors
| simultaneously so that the runner is completely enclosed in the
| barn for a moment.
|
| But from the runner's perspective, the barn has length 5m long.
| What does she see happen when the doors are closed on her?
| awb wrote:
| At these high speeds and tiny distances, very little would
| probably be noticeable to a human observer. If any matter
| collided it would be a catastrophic release of energy
| evaporating the human.
|
| But I think over longer distances there would be a time delay
| for the light of the explosion to reach the individual, so they
| would probably the light of a visually warped pole colliding
| with visually warped barn doors very briefly before
| disappearing into an explosion.
| kazinator wrote:
| Since the two doors are not in the same location in space,
| there is no absolute "simultaneously".
|
| What one frame of reference sees as a simultaneous closure of
| the doors looks like non-simultaneous closure of the doors in
| another frame of reference.
|
| Simultaneity means that two events coincide _in time and
| space_.
|
| Different time and/or space means: not simultaneous!
| gridspy wrote:
| How do the two people at opposite ends of the barn agree on
| when to close the doors as a runner approaches at relativistic
| speeds?
|
| Also, relevant xkcd (what-if) https://what-if.xkcd.com/1/
|
| Most likely from the runner's perspective the world turns white
| as they dissolve to plasma prior to reaching the barn.
| n3k5 wrote:
| > _And two people outside shut the doors simultaneously_
|
| No, they don't. I'd phrase it differently, e.g. "each door is
| shut for an instant when the end of the pole is just inside the
| door". (Also, make it clear that the barn is long enough to fit
| a 10m pole between the two shut doors.)
|
| When you put it as "shut the doors simultaneously", it's more
| troll physics than a paradox.
|
| Meta-puzzle: figure out how to still imply that the runner is
| completely enclosed for an instant, without incorrectly
| insinuating that it is so from her perspective.
| [deleted]
| contravariant wrote:
| From the frame of reference of the runner the pole will first
| hit the door at the end of the barn (unless it opens in time),
| and the door at the front of the barn will only close just
| after the back of the pole has passed the door.
|
| If the doors actually don't open in time then the resolution
| for the fact that the pole doesn't physically fit is that
| nothing can be rigid at relativistic speeds. So either the door
| or the pole or both will deform (violently) to allow the pole
| to fit inside the barn.
| cletus wrote:
| The best analogy for this I've read is that your motion through
| spacetime is constant. Think of a quarter circle dial with an
| arrow that goes all the way from 0 degrees (no relative movement
| in space) to 90 degrees (relative speed of light). The faster you
| go through space, the slower you go through time. The effects of
| this are barely noticeable until you get near the speed of light.
| This effect is time dilation.
|
| This, I believe, is the general relativity view of spacetime.
|
| What I like most about this is that it highlights the importance
| of understanding the domain of a function. The above shows how
| any object with rest mass has a space velocity domain of [0, c).
|
| People really don't want this to be true so latch on to any wild
| theory that would seem to bypass this cosmic speed limit, be it
| wormholes, FTL drives or whatever. Pretty much all of these
| theories rely on taking an equation with a domain over real
| numbers and plugging in negative values for things like mass.
|
| Garbage in, garbage out.
|
| I mean if mass can be negative, why limit yourself to real
| numbers? Why not a complex number for mass?
|
| As to why this is the case and how to reconcile it with a
| quantized view of spacetime... is beyond my pay grade.
| soneil wrote:
| The simplest way I could ever wrap my head around this, is to
| understand that everything travels through spacetime at the same
| speed. There is no travelling faster or slower, just crossing the
| graph at a different angle.
| jayd16 wrote:
| What gets me about these classical ways of thinking about
| higher dimensions is collisions. Couldn't you bump into
| something going a different time rate and get deflected
| backwards if it was indeed analogous to classical movement?
| yetihehe wrote:
| Yeah, but if something goes at differrent time rate, it means
| it will have (typically very vastly) different space
| velocity. You can't have objects very near going at low space
| velocity and high time velocity, so this effect will not be
| noticeable.
|
| Xkcd style explainer: When you have two things going at
| noticeably different time rates, you typically prepend
| "relativistic" [0] to all interactions. "Relativistic
| collision" sounds almost like "changes into huge amounts of
| plasma escaping from contact point".
|
| [0] https://what-if.xkcd.com/1/
| [deleted]
| minitoar wrote:
| Right, this is like the 2d projection of it which makes it
| easier for our 3-space brains to comprehend.
| SnowProblem wrote:
| And the reason for this, as we all know, is because the
| universe runs on distributed computers with no global state,
| where each computer simulates its own local physics and
| connects only to other nearby computers, and thus the so-called
| speed of light is simply the emergent rate that data may travel
| across the network.
| teawrecks wrote:
| The thing that I really have trouble wrapping my head around is
| where each "thing" begins and ends, ex. for astronomically
| large bodies that take light years to travel across, how does
| it move and how does information propagate through it? I assume
| the simplest way to think of it is for every atom to have its
| own bar in the graph, and every "thing" is kind of "wiggly" as
| information travels through it.
|
| But then that sounds like I'm describing the speed of sound,
| no? Maybe I'm confusing two concepts.
| gizmo686 wrote:
| You have to be careful when talking about 'speed' when you make
| time a dimension in your geometry. Traditionally, speed is a
| measure of how much of your space-time curve is in along the
| time axis.
|
| For any space-time path, you can consider the coordinate system
| as an observer traveling that path would see it, in which that
| observer would see itself traveling through time at a rate of
| one second per second. Geometrically, if you were to draw where
| on the path the observer's clock ticks, the distance between
| ticks as measured along that path is constant regardless of the
| path.
|
| The speed of light limit says something different. It limits
| what paths a physical observer can take.
|
| Condsider a 1+1 dimensional universe (or our 3+1 universe with
| a test particle moving along a single spatial dimension).
|
| Pick a non accelerating observer to construct the 'stationary'
| coordinate system. Plot spatial coordinates along the
| horizontal axis, and the time coordinate as the vertical axis.
| Pick units such that the speed of light is 1.
|
| A particle moving at a constant velocity will follow a straight
| line. If the line is vertical the particle is stationary. If
| the line is at an angle, the speed of the particle is the
| inverse of the slope of the line. The speed of light limitation
| says that this line cannot be shallower than 45 degrees.
|
| In more analytic terms, the distance metric for our 2
| dimensional spacetime is given by ds^2 = dt^2 - dx^2. The speed
| of light limitation says that ds^2 cannot be negative for any
| path a particle actually takes.
|
| In other words all particles must must have at least half of
| their travel be along the time dimension.
| moron4hire wrote:
| To me, this was why the statement that "B would see the
| objects growing further apart while A would see them getting
| closer together", if FTL was allowed, was not a very
| satisfying answer.
|
| So given what you've described, that means that forces
| applied to bodies need to have a time component equal in
| magnitude to the spatial components. Forces must always exist
| along that 45deg line. The limit of the force required to
| continue to rotate that spacetime velocity out of the time
| component and into the spatial components goes to infinity as
| the vector approaches that 45deg line.
|
| The fact that forces are unidirectional is the unexplained
| part. If they weren't, then we _could_ rotate that vector
| further, and start traveling backwards in time. Then wouldn
| 't B expect to see the objects moving apart, while A sees
| them moving closer together?
|
| To me, the impossibility of the disparity in observations is
| a consequence of, dependant on, no-FTL, not an explanation
| thereof.
|
| Addendum: I don't understand why you said 45deg instead of
| 90deg. I thought objects traveling at the speed of light
| would experience infinite time dilation, and thus be observed
| as having 0 passage of time.
| formerly_proven wrote:
| > Addendum: I don't understand why you said 45deg instead
| of 90deg. I thought objects traveling at the speed of light
| would experience infinite time dilation, and thus be
| observed as having 0 passage of time.
|
| Think about what the diagram shows: Every (s[pace], t[ime])
| coordinate pair on the spacetime diagram shows an
| observation of a particle. So in natural units, a photon's
| wordline is given by s=t or s=-t (traveling in one or the
| other direction). If you draw that, it's a 45deg line. It
| also gives you the light cone of the observer at (0, 0).
|
| A horizontal wordline would be something moving at infinite
| speed, not the speed of light, as it is observed at every
| place at the same time.
| kevinventullo wrote:
| Hm, but if I can make my angle orthogonal to the time axis,
| isn't that like instantaneous movement?
| [deleted]
| bollu wrote:
| Yes, which is why a photon experiences zero proper time.
| RHSman2 wrote:
| They should make a movie called 'Photon' and track its
| life. A parody based on science.
| bostonpete wrote:
| That's correct. As you approach the speed of light, the
| elapsed time in your frame of reference approaches zero.
| [deleted]
| chacham15 wrote:
| If you could move in space but not in time, then you would be
| in two places at the same time which would violate
| conservation of matter, right?
| gizmo686 wrote:
| That depends on how you define 'conservation of energy'.
| Consider an arbitrary bounded volume of spacetime.
| Conservation of energy says that the net flow across the
| boundary of any such volume is 0. Under this definition
| moving in space but not time is not a violation, as the
| same amount of mass enters the volume as exits it. The only
| odity is that both events happen at the same time
| 8note wrote:
| That definition sounds broken, since energy is still
| conserved if I move something into or out of the bounds.
|
| It stops being conserved if there's suddenly more or less
| energy inside the volume without the same amount crossing
| the boundary
| gizmo686 wrote:
| The idea is that the volume we are talking about is a 4
| dimensional volume of space-time and is bounded; not a
| three dimensional volume of space that extends to
| infinity along time.
|
| Conservation of energy says that it is impossible for
| energy to enter this volume with that same amount of
| energy exiting the volume.
|
| Consider what it would mean for this to be violated. For
| the sake of argument, assume that all particles must move
| forward in time by a non zero amount at all points along
| there path. Since the volume is bounded, any particle
| with an infinite path must eventually have a time
| coordinate beyond the largest time coordinated in the
| volume. Therefore, the particle must eventually exit the
| volume. If you were to work out the geometry more
| carefully, you could show with relative ease that the
| particle must exit the volume an equal number of times as
| it enters. If a particle were to enter the volume without
| exiting the volume, it would mean that said particle was
| destroyed within the volume. Similarly, if a particle
| were to exit the volume without entering, it would have
| to have been created within the volume. Both of these
| situations are possible if an interaction occurs within
| the volume, but the net energy of the particles leaving
| such an interaction, must be the same as the net energy
| of the particles entering the interaction.
|
| Put another way, assume that all interactions obey the
| conservation of energy. If our original volume was V, we
| can construct a new volume V' from V by carving out sub
| volumes in which an interaction occurs. Since all such
| sub volumes obey the conservation of energy (by
| assumption), the net energy flow into and out of V' must
| be the same as for V. However, since no interactions
| occur withing V', all particles entering V' must exit V'
| an equal number of times, so the net energy flow of V'
| must be 0. Therefore the net flow of V must also be 0.
| [deleted]
| soneil wrote:
| Exactly. At one end of the scale, you're not travelling
| through space (or practically-zero on a relativistic scale),
| and you're experiencing the full affects of time.
|
| At the other end of the scale, you're travelling through
| space (or practically-C on a relativistic scale), and you're
| not experiencing time.
|
| So the whole theory of zipping around space and coming home
| to find you've barely aged, is just spending more time at a
| higher "angle" than everyone else.
| dcolkitt wrote:
| It is instantaneous _from the perspective of the object's
| internal clocks_.
|
| Traveling at the speed of light results in infinite time
| dilation. Which means that from the perspective of an outside
| observer, no time at all is passing inside the spaceship.
| jcims wrote:
| What happens to fields (magnetic, gravitational,
| electrostatic, etc) in that situation? E.g. can a photon
| 'feel' the surfaces it would ultimately interact with?
|
| One way I like to think of this is the term 'sun-kissed'.
| From the perspective of the photon, the sun is actually
| giving you a kiss on a summer day.
| wruza wrote:
| _' feel' the surfaces it would ultimately interact with_
|
| I would like to see the map of the universe at different
| potential speeds (or thrusts). E.g. you choose a point in
| space nearby the sun. At thrust zero you only see hot sun
| everywhere, because there you go anyway. But at greater
| thrusts the sun turns into a circle and you start to see
| sections of the "sky" where you could land, given the
| thrust is constant. Some areas would be still black
| because of blackholes, orbits and event horizon. I always
| wanted that simulation but never found it. It would be
| much more interesting than just looking around via
| reversed photons flying into your eyes.
| ghego1 wrote:
| On traveling at the speed of light I also highly recommend to
| watch this Veritasium video https://youtu.be/vVKFBaaL4uM
| holoduke wrote:
| Interesting to know that if we would have a 1g acceleration
| spaceshil with almost unlimited energy onboard we can cross a
| large part of our galaxy in less than 70 years.
| wizardforhire wrote:
| You can go faster than the speed of light. Its slowing thats the
| hard part!
| subroutine wrote:
| Q1. Does the future already exist? I realize this might be purely
| philosophical, but if I leave Earth, and fly through space near c
| kph, and return home a few hours later to find everyone aged 50
| years... are they the "same" people, or are they a future-
| instance of the people left? To clarify, theoretically I could
| leave earth, and return home in exactly 1 hour (from my ref
| frame), and basically make people whatever age I want by varying
| my speed. Since the same 1 hour passes for me no matter the age
| for them I chose, it seems like I'm selecting an already existent
| future from a stack, not fast-forwarding the present I left.
|
| Q2. How do black holes exist if time stops inside a black hole?
| That is, they continue to move through spacetime, even though
| spacetime is not moving within the hole? I accept that a black
| hole can form, and can stop time, I'm just curious how something
| that stops time continues to persist in the present. The way I'm
| visualizing this, is like a lava lamp about to bleb off some goo
| from the top [1]. As the object gets more and more dense, it
| curves and drags spacetime more and more, until eventually its
| density passes the Schwarzschild radius and blebs off.
|
| [1] https://m.media-
| amazon.com/images/I/615deDvfDkL._AC_SS450_.j...
| ww520 wrote:
| Regarding Q2, spacetime curvature is the grid itself. Think of
| it as "space" in the simple term where objects including black
| hole travel through. Black hole's gravity warps the surrounding
| spacetime. Think of it as the "space" is stretched out by the
| black hole's gravity.
|
| Regarding time, there's no universal global coordinate for time
| in General Relativity. Time is based on each observer's time
| coordinate. When time stops inside a black hole, it means time
| appears to stop in an outside observer's time coordinate.
|
| This is because the spacetime curvature inside a black hole has
| been stretched so much that the stretched curvature approaching
| infinite long. See the pulled down funnel [1]. An object
| traveling in no faster than light speed in the infinitely long
| curvature takes forever and its time appears frozen to an
| outside observer.
|
| But it's an illusion to the outside observer. The observer is
| seeing the light imprint emitted from the object, not the real
| object itself. See the diagram below. BH
| << o. EH. . . . .......... >> P
|
| BH is the black hole singularity and o is the falling object
| toward BH. EH is the event horizon. P is the observer. The dots
| are the light photons emitted from the object along the way.
| The photons are traveling away from o and BH toward P. When the
| photons reach P, P can measure o's movement and time.
|
| Imagine the object o has a blinking light beacon at its tail,
| blinking every second by its clock, i.e. on for half of a
| second and off for half of a second. You can see the light
| every second and measure the object's time.
|
| Since the speed of the light is constant, the photon traveling
| away from BH will take longer and longer to move across the
| stretched out space, as shown by the spaced out dots between EH
| and P. As o approaching EH, P will see the blinking light slows
| down because the photons take longer to move across the
| stretched space. P would conclude that o's time is slowing
| down. Passed EH, the photon is not coming out because the space
| is stretched longer than it can cover in its constant speed. At
| EH, the photon can still come out but at a very slow pace
| because the space is stretched matching the speed of light. The
| beacon is not blinking because the light stream been stretched
| to infinitely long. Object o appears frozen as its time has
| stopped by P's measurement of the blinking rate. But what P
| sees is the long stream of photons stretched out when o
| approaching EH. The object o has long gone in its forever
| falling across the infinitely stretched spacetime. Only its
| light imprint before EH is being observed as frozen.
|
| [1] https://www.sciencenews.org/wp-
| content/uploads/2017/05/05121...
| the_gipsy wrote:
| Q1b. Are you the same person you were yesterday?
| subroutine wrote:
| I think yes, and the answer is more concrete for "oneself",
| since the molecules that make-up oneself are all accelerating
| and decelerating with you/as you in your own reference frame.
| awb wrote:
| Q1.
|
| > basically make people whatever age I want by varying my
| speed.
|
| No more so than making the Earth change shape by moving your
| physical location.
|
| > Since the same 1 hour passes for me no matter the age for
| them I chose, it seems like I'm selecting an already existent
| future from a stack, not fast-forwarding the present I left.
|
| Yeah these are more philosophical questions rather than a
| testable hypothesis. It's an interesting way of viewing the
| world, but if you're talking about a future "existing"
| scientifically and materially in concert with the present, then
| you'd need to devise an experiment to be able to enter or
| interact with that future beyond just waiting for that future
| to exist.
|
| Otherwise time travel isn't even necessary for this thought
| experiment. You could say the morning already exists and fall
| asleep and wake up and boom it's morning time. But in reality
| all of the ticks of time happened between when you fell asleep
| and when you woke up, you just weren't able to observe all of
| them at the same speed as someone who stayed awake all night.
| bambam24 wrote:
| Do you have zero mass? Otherwise just to a 150lbs person will
| have mass of a galaxy at 99% of the speed of light. And to go
| remaining %1 you will need to have a power to push entire galaxy
| 0.1% of speed of light. At 99.999998% of speed of light the man
| will need to burn all the elements in the universe. Basically a
| credit card or bank loan cannot buy you such energy.
| thom wrote:
| You can't go faster than light for the simple reason that it
| would defeat the very many performance optimisations necessary to
| keep the universe running.
| noobermin wrote:
| The beginning is the best line, that no one really knows "why"
| and what I have said to even other physicists who don't really
| understand it. A lot of the counterintuitive consequences of the
| paradox of the moving charges detailed here are a fact of nature
| but we don't really know why it must be that way. For example, in
| a modern particle physics starting point for qed, we force the
| equations to be lorentz invariant from the start but that is a
| starting assumption. But that's it, there's no real deeper answer
| to "why" beyond it being a experimentally observable fact, and
| the only resolution of these observations is that lorentz
| transformation and thus things like time dilation and length
| contraction must happen.
|
| Small relativity related tidbit: I hate when people say the
| phrase "the faster you go, time slows down for you." This is a
| problem because it implies that the moving observer notices their
| own time dilation which is reverse of the case. Of course, every
| non-accelerating observer is in their own rest frame, so it
| doesn't make sense to say "time slows down for you," because you
| are your own reference and there is no other frame to base your
| measurements on (I mean that was the whole point of relativity,
| there is no universal rest frame). Instead, when you measure the
| rate of change for _other_ reference frames moving relative to
| you, their clocks move slower when measured by your clock. So the
| actual phrase should be something like "the faster others go,
| the slower their time appears to you."
| StephenAmar wrote:
| I highly recommend watching
| https://youtube.com/playlist?list=PLoaVOjvkzQtyjhV55wZcdicAz...
|
| They do such a good job at using video to explain Special
| Relativity.
| dejongh wrote:
| Thanks. Fun to know that the speed of light prevents the universe
| from splitting up. Cool. Also I was not aware that movement
| create magnetism.
| OneEyedRobot wrote:
| Can a thing with no mass ever go less than the speed of light?
| Does it have a concept of time?
| PaulDavisThe1st wrote:
| In the 70s, I remember reading "The Quincunx of Time", which made
| great use of a key distinction (for the purposes of some classic
| 70s scifi):
|
| It's not that nothing can move faster than the speed of light,
| it's that nothing can accelerate to a speed faster than the speed
| of light.
|
| If you could bring particles into being that already moved faster
| than light, they would not violate our understanding of
| relativity or the rest of physics. Hence ... the tachyon.
| https://en.wikipedia.org/wiki/Tachyon
| masa331 wrote:
| Of course you can go faster than light, way faster. When it comes
| to physics there are no limits how fast, far or deep you can go.
| People constraint themselves with articifial limits and it's not
| necessary a bad thing. But real limits doesn't exist.
|
| I'v heard someone saying space is a big ball and outside of it is
| nothing. I imagine nothing as a black space but that actually is
| something. So there is something after all. And however weird or
| normal these other places are, it goes like this infinitely.
| chrisoverzero wrote:
| > Of course you can go faster than light, way faster.
|
| OK. How?
| masa331 wrote:
| I don't know
| holoduke wrote:
| What would happen if you had a very large rotor of let's say
| 50.000km in diameter. And let it rotate at 1r/s. The inner part
| would only move at very low speed and the outer part at near
| lightspeed. What kind of time dilation effects would be seen? If
| you would sit on the tips for a few days and then stop the rotor,
| travel back to the center, you would see a very old one right?
| midrus wrote:
| I have a probably very dumb question. If speed is relative, when
| we say something travels at... I don't know... 50% the speed of
| light, that speed is relative to what? how do you know it is 50%
| and not 53%?
|
| How do we know we're not already moving at 99% the speed of light
| (like our observable universe as a whole having that speed )?
|
| I love this stuff, but it is so counter intuitive for the average
| human.
| drran wrote:
| Higgs field is present everywhere and mediated by Higgs boson,
| so just measure speed relative to Higgs field. Or you can
| measure speed relative to CMB.
| leonardp wrote:
| I always feel like this questions is really asking "What is
| time?". This book gives a new perspective on time which I think
| is crucial for understanding the nature of it:
| https://www.harvard.com/book/the_janus_point/
| webwielder2 wrote:
| I prefer the question, "why can't light go faster?"
| franciscop wrote:
| A bit all over the place, the author assumes "It is even less
| well-known that the rule "nothing moves faster than the speed of
| light" is a consequence of the laws of electricity and magnetism"
| but at the same time "This is also a well-known property of
| electricity and magnetism" and then drops some equations.
|
| Most people for whom this property of electricity and magnetism
| is well known, also know that the speed limit from general
| relativity comes from Maxwell's equations (which are on
| themselves a compilation of other previous rules).
|
| I also dug deep into this problem a while back, but without
| proper physics background couldn't get too far. The speed of
| light comes directly from the Vacuum permeability and Vacuum
| permittivity, since light is an electromagnetic wave. If these
| were different, then the speed of light would be different. Both
| of these seem to be values of the behavior of vacuum in our
| universe (and possibly electron's [1]).
|
| It is Maxwell's equations gives us that the speed of
| electromagnetic radiation needs to be c ^ 2 = 1 / (e * u) [2].
|
| [1] https://en.wikipedia.org/wiki/Fine-structure_constant
|
| [2]
| https://en.wikipedia.org/wiki/Electromagnetic_wave_equation#...
| DangitBobby wrote:
| The author's assumptions were spot on for me. I am familiar
| enough with E&M to know the equations and roughly understand
| relatively but did not know where the intuition for a universal
| speed limit comes from.
| unyttigfjelltol wrote:
| Looks to me like the short answer to the question posed in the
| headline is that matter is composed at an atomic or subatomic
| level of energies similar to light and so it would be quite
| inconvenient to try to propel the mass as a whole faster that its
| constituent parts can go. A similar principle may well be true
| for straight-line inertia, which might be caused by the same as
| the force that makes gyros stay on course. In general the
| scientific discussions seem to touch these ideas but approach
| theories in reverse of intuition so it's hard to know if they
| align.
| crdrost wrote:
| That you can't go faster than the speed of light is a direct,
| simple consequence of everybody agreeing on the speed of light.
| It's a real-life Zeno paradox.
|
| Alice and Bob are in some spaceships by a long racetrack in
| space. Alice fires a very brief pulse of light down the
| racetrack, maybe we see the rays that don't go straight through
| successively illuminate some rings around the track. Alice
| challenges Bob to race another light pulse, Bob revs his engine.
|
| As the countdown hits zero, Bob accelerates to speed c/2 relative
| to Alice, then checks the reflected light from these rings only
| to find out that the light pulse is still traveling at speed _c_
| away from him. So he drops a beacon at his current speed then
| accelerates to speed c /2 relative to that, but no dude: the
| light is still moving at _c_ away from him. Bob realizes that he
| can never win, so tries to instead measure the speed of Alice,
| who he expects to be moving at speed _c_ away from him, only to
| find that she is instead moving at speed 0.8 _c_ away from him.
|
| Now the question is, how can this be? Consider a much slower
| spaceship. When Alice fires the light pulse and Bob starts moving
| forward at a slow speed, Alice sees this bubble of light
| expanding in kind of a uniform sphere centered on her. (Say it
| reflects off of space dust instead of a track.) Since they were
| at the same position when the light Bob also sees an expanding
| bubble of light, with himself at the center. The weird stuff
| about time dilation and length contraction does not apply at low
| speeds, if Bob goes at c/1,000 say, then these are only one part
| per million.
|
| They both basically agree on how far this bubble of light is from
| Bob, in the directions perpendicular to the motion. The motion is
| parallel to the bubble in those directions, and to first order
| those parallel lines will not get any closer or further away.
| (This is why I want Bob to move at a slow speed!) They only
| disagree along the motion. Alice thinks the light is receding
| from Bob at speed c-v ahead of him, at speed c+v behind him: Bob
| sees the light recede at speed c in both directions.
|
| So they come back together to repeat the experiment and Alice
| decides to force the contradiction. Alice puts a clock ticking
| out every millisecond out at distance 1 light-second, but it will
| start at the moment she fires the pulse, stopping when the pulse
| hits it, showing 1000. She puts one of these in the direction Bob
| will travel, and in the opposite direction. Surely he must agree
| that the light started from here and that it intersected those
| two clocks when they both said 1,000.
|
| Bob agrees that the clocks look synced up and films all of this
| with a high-speed camera to make sure that there is no funny
| business, and they repeat the experiment.
|
| Right when the light hits, Bob accelerates at his usual 1,000,000
| gees for 0.03 s, to get his final speed of c/1,000. (I need to
| rewrite this to make the numbers more reasonable LOL.)
|
| Here's where something weird happens, and it is entirely
| contained to those first 30 ticks of both cameras, as Bob looks
| at them in his high-speed camera footage. Bob corrects for
| Doppler shift like you do, and agrees that these clocks appear to
| be ticking during the other 970 ms at one tick per ms, his camera
| has maybe microsecond resolution and not the nanosecond
| resolution you need to see time dilation.
|
| But during those first 30 milliseconds when Bob was accelerating,
| even after correcting for the Doppler shift, his best guess is
| that Alice artificially slowed down the clock behind him and
| artificially sped up the clock ahead of him. Because the clock
| ahead of him definitely ticked 31 times in those 30 ms, while the
| one behind definitely only ticked 29 times. So Bob says the light
| did hit these clocks when they _said_ 1000 ms, but the clock
| ahead of him _should have said_ 999ms at that time, and the light
| should have gone past it a bit by 1000ms, while the clock behind
| should have said 1001ms, and the light was actually not yet there
| at 1000 ms.
|
| This anomalous Doppler shift is proportional to both the distance
| of the clock you're looking at, and your acceleration. It is also
| called the relativity of simultaneity, and it is the only new
| prediction of relativity, in that length contraction and time
| dilation are second-order consequences of it.
| ergocoder wrote:
| "Nothing can go faster than light" is just a convention and is
| not experimentally confirmed.
|
| IIRC there are a few things that go faster than speed of light
| (e.g. universe expanding).
|
| "Spooky action at a distance" is also not known very well. That
| could also break the speed of causality law. While Bell's theorem
| hints that this is not the case, there are some exceptions to
| Bell's theorem.
|
| Right now a lot in physics are just convention like energy
| conservation and symmetry.
|
| We need another paradigm upgrade to understand these things.
| Choc13 wrote:
| Hmm not sure if troll, but energy conservation is definitely
| not just some convention. It's a fundamental thermodynamic law.
| Spooky action at a distance does not violate speed of light
| information propagation either. The particles have to be
| entangled before they're set off in opposite directions. Only
| once one is observed does the other also collapse, but it
| doesn't mean you can communicate faster than the speed of light
| because you had to prepare the information when the particles
| were together IIRC, it's been a decade since I studied quantum
| information theory.
| ergocoder wrote:
| Why do you feel the need to say somebody is a troll?
|
| Here: https://phys.org/news/2017-01-violations-energy-early-
| univer...
|
| Here: https://phys.org/news/2015-02-space-faster.html - we
| are trying hard to reconcile this and categorize universe
| expanding as something else (e.g. not a movement that has
| speed because time itself is a dimension or something). But
| this is still up for debate, tbh.
|
| Another example was one where we said CP symmetry was true
| (it was a law like a lot of things in physics) until it was
| violated by a weak nuclear force experiment.
|
| And now we are holding the fort at CPT symmetry as the law.
|
| In Physics, the evidence of anything is kinda light. A lot of
| reasonable extrapolations has been made. Still they are
| extrapolations (e.g. intelligent guess).
|
| Even the big bang itself is just an extrapolation from the
| "theory" that the universe expanding.
|
| To be fair, it is difficult to find good evidence because we
| can't dial back time, can't go observe things on Neptune,
| can't measure gravity at the subatomic scale, and etc. So, we
| have to work with what we can experimentally observe.
|
| Our tools are getting better, and this is where the physics
| paradigm shift will come from.
|
| You say like these are 100%. It is just a theory that we
| currently hold according to the little evidence that we have.
|
| Failing to recognize that is straight up unscientific.
| Choc13 wrote:
| Apologies, I wasn't trying to call you a troll, more that
| your first comment seemed to me like it might have been
| made in jest, clearly it wasn't.
|
| The articles you've linked to are interesting and there are
| clearly many scientific discoveries to be made by studying
| the early formation of the universe which will test our
| current models. However I don't feel like convention is the
| right word for laws like the conservation of energy, even
| if there are some difficulties with tying up these theories
| and new experimental evidence from events at the scale of
| the Planck length.
|
| Convention to me would mean something that has been
| accepted just because it's always been done that way and
| people didn't really bother to question why, but I don't
| think that's the case here. But we're verging on pedantry
| now so no point going down that route any further.
| hyperman1 wrote:
| AFAIK 'nothing goes faster than c' is wrong in the 'lies to
| children category'. The devil is in the details. In fact,
| plenty of things go faster than c.
|
| As a standard example, take a wall at 1m distance and a
| flashlight. move the light spot at 1 m/s. If you put the wall
| at 2m, the same spot will now go 2 m/s. If you put it at 300
| 000 km, the spot will go at 300 000 km/s, slightly over light
| speed.
|
| The problem is more one of information: For any action you
| take, no consequence can happen outside your light cone. All
| information you generate travels at c at max.
|
| None of the examples you gave violates this: Even if the
| universe expands at more than c, nothing you do will have
| influence on things too far from you.
| ergocoder wrote:
| That is a reasonable argument.
|
| And what you point out is actually the speed limit of
| causality, instead of light. It just happens that light
| travel really fast.
|
| Though the article wants to discuss the speed of light as you
| mentioned, so I focused on speed of light.
| otterley wrote:
| My favorite way to think of it is that the speed of anything is
| measured in distance per unit time. You can't change the constant
| that is the speed of light, but what if you could change what
| "time" means? The constant holds, but time doesn't; it varies
| among observers. A second from the perspective of observer A is
| not identical to a second from the perspective of observer B.
|
| Freaky, but true.
| stareatgoats wrote:
| I still don't get this. I simply can not accept that the speed
| of light is that same for all observers, _irrespective of their
| respective speeds vs the light source_. It is obviously a
| cornerstone of Einsteins theory (I forget which; either the
| special or general relativity), but to me this is simply not
| logical. To state that the speed of light is the same only
| because the relative time is variable for respective observers
| is unconvincing, which requires that things like blueshift
| /redshift to be explained by logical gymnastics, and not the
| straightforward reason that the speed of light for all
| observers is a direct function of the relative speed vs the
| light source. But what do I know.
| DangitBobby wrote:
| It's a consequence of the time dilation. You are not
| stationary towards the photon to an outside observer, so to
| them it looks like the photon moves away from them faster
| than it moves away from you. And from the obserer's
| perspective, your clock is ticking slower. That's the key.
| The amount that your clock is ticking slower is such that
| you, the one moving, would calculate the speed of the photon
| the same as the stationary observer, because you would do so
| from your own slower clock.
| stareatgoats wrote:
| Thanks, that sounds like the explanations I've seen too,
| and that I hope to understand some day. Atm I'm at the
| point where I gladly accept that time may _seem_ to dilate,
| but not that it actually does. Maybe I just need to dive
| deeper into it than I 've been willing to do so far.
| l332mn wrote:
| > To state that the speed of light is the same only because
| the relative time is variable for respective observers is
| unconvincing
|
| Not sure what you mean by relative time?
| ncallaway wrote:
| > but to me this is simply not logical.
|
| It is not logical if you really believe that speeds add
| linearly (that is, if you are going 5mph past an observer and
| throw a ball 5mph, that the result is the ball moving 10mph).
|
| Speeds don't actually add linearly like that, but they come
| very very very very close to doing so for all speeds humans
| are used to dealing with.
|
| So, we all have very deeply held gut feelings that speeds
| should add linearly. Once you let go of that, it becomes much
| easier to understand many of the things that don't feel
| logical about relativity.
|
| Ultimately, it is logical, but from a different set of axioms
| than most humans tend to have.
| wruza wrote:
| If anyone wants to _play_ with a speed of light, there [still] is
| a fun web game called "velocity raptor".
| [deleted]
| [deleted]
| [deleted]
| dvh wrote:
| This example has a flaw. Skateboard man sees object stationary,
| but the cable is moving. There should be magnetic force too.
| redwood wrote:
| I think a lot of people assume that this means you couldn't go
| more than a few (~100) light-years in your lifetime... But this
| is not actually correct. Counter-intuitively you can theorically
| go any number of light-years (essentially) in your lifetime, as
| long as you are able to approach the speed of light because when
| you do so the distance is dilated and hence you're covering far
| more ground within your reference frame (of course you'd be in
| the deep future from the perspective of anyone in our normal
| reference frame).
| xyzzyz wrote:
| To put it another way, time travel machines are very much
| theoretically possible, and in fact are "only" an engineering
| problem (an extremely hard one, however). But, only forward
| travel is allowed: time travel machine could take you as far
| forward as its engineering would allow it, but there is no
| going back.
| zamadatix wrote:
| At 1G constant acceleration (both speeding up and slowing
| down) you can make it on a short vacation to the Andromeda
| galaxy and back in a lifetime (or 5 million years from the
| perspective or Earth).
| eesmith wrote:
| It looks like about 57 years.
|
| Assuming constant acceleration to the 1/2-way point, flip,
| deceleration, and using http://www.projectrho.com/public_ht
| ml/rocket/slowerlight3.ph... : Time elapsed
| (in starship's frame of reference, "Proper time") T =
| (c/a) * ArcCosh[a*d/(c^2) + 1] (given acceleration and
| distance) year = 365.25*24*3600; c = 3E8; a=9.8;
| d=1.25*1_000_000*(c * year) from math import acosh
| T = (c/a) * acosh(a*d/(c*c) + 1) print(T/year)
| => 14.3 years each quarter => 57 years round trip
|
| The speed at flip would be 99.99999999993978% c - good
| thing intergalactic space is mostly empty.
| wruza wrote:
| Bad news CMB shifts into infrared, visible, uv, xray and
| then hard gamma.
| nimish wrote:
| They are only theoretically possible if you allow for
| negative mass and energy--not an engineering problem so much
| as a "need to find exotic matter"
|
| Basically people ran the EFE "backwards" to see what matter
| distribution makes the wanted curvature. You get either
| negative mass-energy or the bubble doesn't travel ftl iirc.
| kadoban wrote:
| Gp is talking about forward-only time travel though, you
| don't need anything exotic for that at all, just lots of
| energy and an efficient way to turn it into thrust.
| xyzzyz wrote:
| No, for forward time travel you don't need any sort of
| exotic matter. You just need a really fast rocket.
| xondono wrote:
| The Alcubierre drive "beats" the speed of light, for the
| effects described 0.7-0.9c are more than good enough (if
| slightly unpractical).
|
| We can't do that either for now, but is way easier on the
| feasibility scale.
| lovegoblin wrote:
| > The Alcubierre drive "beats" the speed of light
|
| The Alcubierre drive is only a thought experiment that
| requires "exotic matter" (aka fairy dust) to work.
| godelski wrote:
| I'm pretty sure this doesn't work out because you have to slow
| down.
|
| So thinking about it, if you are traveling at that speed
| because of your inertial reference frame it is equivalent that
| everyone else around you is moving at (or near) the speed of
| light and they are moving slowly through time. This is the
| classic twin paradox and there is a resolution to it, which is
| that you can't instantaneously turn around.[0] Or in our case,
| we have to turn around to slow down.
|
| [0] https://www.youtube.com/watch?v=0iJZ_QGMLD0
| firebaze wrote:
| It does work out, in fact you could go arbitrarily far in
| space (and, mandatory, in external time) if you had infinite
| energy to spend. The acceleration and deceleration phase is
| almost negligible.
|
| Edit: the faster you go, the slower your own (inertial) time
| passes. That means the external time passes faster, and the
| factor grows to infinity the closer you get to C.
|
| In fact, subjectively there is no speed limit. As you go
| faster, anything around you ages faster, but you yourself
| won't encounter any speed limit.
| godelski wrote:
| But in your inertial reference frame the people on Earth
| are moving at (near) the speed of light. So they are the
| ones that should be staying young. Or similarly the planet
| you are traveling to is actually speeding towards you and
| you are staying still. This is why the twin paradox is a
| paradox, because of the reference frames.
| phkahler wrote:
| But motion is all relative. Who is considered to be moving
| faster?
| yawaworht1978 wrote:
| I know this is true, but I never got my head around this, maybe
| someone can help. So if a photon is emitted from the sun, it's
| passing earth immediately? Then why does sunlight need 7 or 8
| minutes to get to earth? And let's say, i travel one light-year
| at the speed of light, that should be instantaneous, right? The
| odometer would show 1 light-year, my watch would should 0
| seconds and some decimals. How much would I have aged by the
| end of the journey?
|
| If I travelled at one percent of speed of light, same distance,
| i suppose 100 years would elapse for me, how much would elapse
| on earth? Odometer would still show one light year?
|
| And what if I left earth at 50percent of light speed , traveled
| 1 light-year away and did a turn and came back to earth at same
| speed. For me, it would be 1 year, but if I had a twin brother
| who was waiting on earth, would i now be a year younger than
| him? And how is this possible?
| chrischen wrote:
| > And let's say, i travel one light-year at the speed of
| light, that should be instantaneous, right?
|
| Time slows down for the faster moving particles. And by time
| slowing down we mean all the particles in your body equally
| all start to move slower and more sluggishly in sync.
|
| This is because it takes more energy to accelerate a particle
| as it approaches the speed of light. So if you had a pendulum
| clock moving almost at the speed of slight, that velocity of
| the pendulum at rest would be at X m/s, but if the whole
| system is already moving super fast the extra X m/s would
| take too much energy. So since the energy is constant the
| relative speed of the pendulum just becomes much slower.
| thoughtstheseus wrote:
| This is a superb description.
| [deleted]
| nycdatasci wrote:
| Einstein can help: https://books.google.com/books/download/Re
| lativity.pdf?id=3H...
|
| His book was intended to help people understand these exact
| questions, without getting into any complex math.
|
| Regarding "how is this possible?": This was experimentally
| verified in the 70s in the Hafele-Keating Experiment. Read
| more here: https://en.wikipedia.org/wiki/Hafele%E2%80%93Keati
| ng_experim...
| hollander wrote:
| That Google link doesn't work.
|
| https://www.gutenberg.org/ebooks/5001
| asdfge4drg wrote:
| because light doesn't experience time, and the closer you get
| to the speed of light, the less time you experience. So what
| OP didn't say was that, while YOU can travel many lightyears
| in your lifetime, everyone you know will long be dead.
| justusthane wrote:
| > So if a photon is emitted from the sun, it's passing earth
| immediately? Then why does sunlight need 7 or 8 minutes to
| get to earth?
|
| From the photon's perspective, it passes earth immediately.
| From our perspective, it takes 7 or 8 minutes.
|
| > And let's say, i travel one light-year at the speed of
| light, that should be instantaneous, right? The odometer
| would show 1 light-year, my watch would should 0 seconds and
| some decimals. How much would I have aged by the end of the
| journey?
|
| You would have aged as much as your watch says you would have
| aged. Zero seconds.
|
| >If I travelled at one percent of speed of light, same
| distance, i suppose 100 years would elapse for me, how much
| would elapse on earth?
|
| I don't know how to do the math, but a very, very long time
| would have passed on earth.
|
| > And what if I left earth at 50percent of light speed ,
| traveled 1 light-year away and did a turn and came back to
| earth at same speed. For me, it would be 1 year, but if I had
| a twin brother who was waiting on earth, would i now be a
| year younger than him? And how is this possible?
|
| Yes, you would be younger than him, and it's possible because
| that's just how relativity and time dilation work. It's even
| practically measurable in "real" life:
| http://www.leapsecond.com/great2005/tour/
| throw_away wrote:
| > > If I travelled at one percent of speed of light, same
| distance, i suppose 100 years would elapse for me, how much
| would elapse on earth?
|
| > I don't know how to do the math, but a very, very long
| time would have passed on earth.
|
| Eh, at .01c, not so much:
| https://www.omnicalculator.com/physics/time-
| dilation?c=USD&v...
|
| (=100.005y, not even two days extra)
| wruza wrote:
| Yeah, all interesting things only begin at roughly 0.5c
| and go crazy at 0.9c. Google images on "lorentz factor"
| to get the quick idea.
|
| https://www.google.ru/search?q=lorentz+factor&tbm=isch
| [deleted]
| danielsju6 wrote:
| If you were watching their lives out the window of your
| spaceship you'd see them in fast forward vv. they'd see you
| in slow motion. Everything is relative.
|
| The main "paradox" found by experimention is that the speed
| of light is a constant, regardless of your velocity. The only
| way this could be true, if you do a thought experiment, is if
| time was dialating for you. As for the physical and
| mathematical "why is this happening", that's where Einstein
| comes in.
|
| It's an exponential. At 0.5c 100 years to you would be 115
| years elasped to an observer, 0.9c 229 years, 0.999c 2,236
| years, etc.
|
| Here's an online calculator for the dilation effect
| https://www.omnicalculator.com/physics/time-dilation
| lurquer wrote:
| > If you were watching their lives out the window of your
| spaceship you'd see them in fast forward vv. they'd see you
| in slow motion. Everything is relative.
|
| Both observers, looking at one another, would see the other
| moving near c. Neither would know who was 'actually'
| moving. Yet, you assume there would not be a symmetry in
| their respective views of the other's passage of time.
|
| Explain why.
|
| In simpler terms, a twin in a c-speed rocket could very
| well assume he was still and the earth was moving away. He
| should expect to find a younger twin when the earth
| 'returned.' Yet the examples only have the earth twin age,
| so to speak, and not the rocket twin.
| jeffwass wrote:
| The symmetry is broken in your twin example because the
| travelling twin had to accelerate to depart, accelerate
| to turn around, and accelerate to stop again at earth.
| wruza wrote:
| It's still a hard topic related to the rotational
| "symmetry".
|
| https://m.youtube.com/watch?v=cPEwkMHRjZU (7 minutes)
|
| It doesn't answer your question directly, but the tricky
| part of a twin paradox is not that the earth twin
| observes his brother in a slow motion (the space twin
| also does that, thus a paradox). It's because a space
| twin actually changes direction by acceleration at some
| point B, and at _that_ time he skips over a big part of
| earth's timeline. The video above only addresses why it's
| NOT the earth twin who changes direction by acceleration,
| which you're reasonably questioning. The universe somehow
| knows who is really "steering" and what remains more or
| less inertial. The pendulum example at the end may give a
| hint on why.
|
| Edit: also, the space twin doesn't have to experience any
| additional acceleration from the "engines" - looping
| around some gravity well would work too. E.g. an entire
| trip could be that the space twin goes to the orbit
| around the earth, gets slung away by a quickly passing
| blackhole, loops around a distant blackhole and returns,
| all in a complete free fall.
| [deleted]
| wruza wrote:
| Also, it doesn't take too much acceleration to make that trip.
| Comfortable earthlike 1g (~ 10m/s/s) is enough to build up a
| decent speed in a very reasonable time. Energy is the issue
| though.
| mjburgess wrote:
| If distances are dilating, why isn't that /less/ ground?
| shrimp_emoji wrote:
| The distance doesn't dilate; time does. The distance
| _contracts_.
|
| E.g., to a photon moving at 1c, the whole universe has a
| contracted length of 0 meters, and it crosses the whole
| universe instantly. To us, observers at <1c, the whole
| universe has a non-contracted length of <a lot> and the
| photon takes <a long time> to cross the whole universe. By
| the time the photon's 0-second journey across the entire
| universe has finished (whatever that means), we're all
| extremely old. :D This is the time dilation meme of slowly-
| aging space travelers but taken to the extreme.
| redwood wrote:
| Thanks for the clarification
| 3pt14159 wrote:
| It basically depends on your perspective. To someone on earth
| watching the spaceship speed up to the speed of light, the
| spaceship looks like its contracting. To the person on the
| spaceship, the rest of the universe looks like it is
| approaching the speed of light, and hence, contracting.
| [deleted]
| rkagerer wrote:
| It just means you can't within the lifetimes of the people you
| left behind. For that you need more exotic contraptions
| (wormholes, warp drive, etc).
| ghayes wrote:
| If they were to wait a year and then come find you far away,
| would it net out and you'd be the same age?
| Guillaume86 wrote:
| Yes
| aero-glide2 wrote:
| I've been trying to understand this since high school. No Eureka
| moment but I think every year I understand just a little bit
| more.
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