[HN Gopher] Why Is Light So Fast?
___________________________________________________________________
Why Is Light So Fast?
Author : paulpauper
Score : 110 points
Date : 2024-10-04 13:46 UTC (9 hours ago)
(HTM) web link (profmattstrassler.com)
(TXT) w3m dump (profmattstrassler.com)
| SomeHacker44 wrote:
| There is already a Part 2 out (read it yesterday) and a Part 3
| planned.
| ivan_ah wrote:
| Direct link to Part 2:
| https://profmattstrassler.com/2024/10/03/why-is-the-speed-of...
| nyc111 wrote:
| I was reading this article carefully. I noticed that he uses the
| word "stationary" to mean both "absolutely stationary" and
| "relatively at rest". The reasoning in this article is based on
| conflating and mixing the two meanings. So for instance when he
| writes "photons are always in motion" [1] he assumes that there
| are objects that are not always in motion, that is, they are in
| absolute rest. But he also writes "specifically its 'rest mass'
| m, which is the mass as measured by an observer who is stationary
| relative to the object." In this quote, he defines the word
| "stationary" as relative rest.
|
| This rhetorical trick is so common in physics, that's why I
| wanted to mention it. The trick is to define the same word twice
| with its opposite meanings and use the word with both meanings
| sometimes even in the same sentence. I wonder what is the name of
| this trick in logic.
|
| [1] This quote is from a different articla:
| https://profmattstrassler.com/waves-in-an-impossible-sea/wav...
| SetTheorist wrote:
| Generally, "equivocation" is the fallacy of using the same word
| with different meanings in an argument.
|
| https://en.wikipedia.org/wiki/Equivocation
| kccqzy wrote:
| I read the same article and I didn't find "absolutely
| stationary" in the article. My physics is rusty but even from
| my recollection of high school physics there is no such thing.
| It's clear that the word stationary means zero velocity, and
| first year high school physics taught that any measurement of
| velocity must be done in a reference frame. The idea that a
| photon is always in motion is true regardless of which
| reference frame you pick; it does not imply there are objects
| that are in absolute rest.
| thfuran wrote:
| There is no absolute reference frame that would distinguish
| "absolutely stationary", as you call it, from "relatively at
| rest". There is only relative motion. Photons are "always in
| motion" in that there does not and cannot exist any massive
| object with respect to which a photon is at rest.
| jerf wrote:
| I do not know where I first saw it, but I saw someone observe
| that the shocking thing about Einstein's theory of relativity
| is not that everything is relative; that was understood
| reasonably well for a long time before him. What is shocking
| is that the speed of light in a vacuum, or the speed of
| causality, is a constant. You can derive huge swathes of
| relativity from that fact alone.
|
| As a result, of all the speeds, when discussing the speed of
| light (which conventionally means "in a vacuum" unless
| otherwise mentioned), you can in fact ignore the question of
| reference frame, with the exception of you don't want to use
| a reference frame itself moving at the speed of light. But
| other than that, an article exclusively confining itself to
| the discussion of the speed of light in fact doesn't need to
| worry itself about the relativity of reference frames. For
| that speed alone, you can't level the complaint against it
| that it ignores the issue of different frames, because it
| uniquely doesn't matter.
|
| (Relatedly: It is frequently given as the reason you can't
| reach or exceed the speed of light is some stuff about masses
| rising. While mathematically true in its own way, I think
| there's a cleaner reason to explain why you can't reach or
| exceed it, which is that you can't even get _closer_ to it.
| No matter what you do, the speed of light is c. You
| accelerate to a thousand miles a second in some direction,
| and how much closer are you to c? The answer is, none. Light
| continues fleeing from you, in all directions, at c. There is
| no "get really close to c somehow and then just _push
| yourself over really hard_ " because there is no "get close
| to c" in the first place. No matter how hard you accelerate,
| in what direction, in what order, in what manner, you not
| only can't get "close" to c, you can't even get _closer_. For
| similar reasons, in this paragraph, I don 't need to qualify
| in which reference frame you go a thousand miles per second
| different than before, because it doesn't matter for c. You
| can't even get _slightly closer_ to it, let alone "exceed"
| it somehow. It is an absolute.)
| thfuran wrote:
| That motion is relative was understood. That even notions
| like simultaneity also depend on reference frame I think
| was shocking. And I'm not really sure what it would mean
| for light to not be at rest in any reference frame but for
| it to not have the same speed in all reference frames. I
| think that would be even more shocking than light having a
| constant speed.
| the_sleaze_ wrote:
| Please bear with me, but how is this true?
|
| If we're in an airplane going 250 m/s - it also takes me 2
| hours to fly to NYC. The air around the plane is windy, the
| air inside is still.
|
| Now we're in a spaceship headed from the earth to the moon
| & it's going to take us 3 seconds. We've agreed ahead of
| time it takes 1.5 seconds for light to go from the earth to
| the moon. C is the speed at which light moves, and we're
| taking 2x the amount of time light would. The air inside is
| still. But we're still going fast.
|
| How could it be that we are not approaching the speed of
| light?
|
| What do you make of time dilation at higher speeds if we
| aren't approaching the speed of light at all, ever?
| JdeBP wrote:
| It's the stumbling block that everyone has grasping this
| stuff, and it's rooted in the old faulty kind of speed
| calculations. Speeds don't add like you think.
|
| There are many ways to grasp this, and one has to try
| several of them to find an intuitive explanation that
| works for onesself; but one way is to consider that you,
| in your spaceship (as long as you are coasting along with
| no rockets firing, and aren't performing orbit/deorbit
| burns) are, in your frame of reference, at rest. _Your
| speed is zero._ You haven 't approached anything at all,
| and light is still whizzing away from you at c. Indeed,
| it's Terra and Luna that are experiencing time dilation
| as far as you are concerned, because _they_ are the ones
| with the high speeds.
|
| * https://www.youtube.com/watch?v=Zkv8sW6y3sY
| (FloatHeadPhysics addressing this in another way: there
| are a other approaches still)
|
| Accelerate at the beginning and end of your trip over to
| Luna, and of course general relativity comes into play
| and things get more complicated. What many
| gedankenexperiments get wrong is that usually there's
| only a short period of burning the rockets, in the real
| world. So for most of your trip in the rocket you aren't
| burning propellant and are in an inertial frame of
| reference. No inertial frame of reference approaches the
| speed of light/causality, by postulate 2 of special
| relativity, and one is always at speed zero in one's own
| inertial frame of reference.
| dboreham wrote:
| iirc Maxwell had already shown that the speed of light was
| constant. Einstein's quest was to explain htf that could be
| the case. The word Maxwell appears in the first sentence of
| the 1905 paper, and the first couple of pages are about
| what today we would call "causality".
| elashri wrote:
| It is not a fallacy. In special relativity the important
| concept is the frame of reference. For photons there is not no
| inertial frame of reference where photon will be stationary
| (relative or absolute means nothing actually).
|
| The whole concept of measurement (which you will need to
| describe something as stationary) depends on an observer
| travelling at speed velocity less than speed of light. If you
| Try to assign a inertial frame of reference to a photon you
| will break laws of special relativity
|
| - photons always travel in speed of light from perspective of
| any inertial observer whether it is moving or at rest (verified
| by experiment)
|
| - relativity prohibited mass less particles (like photon) to be
| stationary.
|
| E = mc^2 doesn't apply in photon and apply only on stationary
| objects. The complete equation is
|
| E^2 = (p^2 c^2) + (m c^2)^2
|
| Where m is the rest mass. In photon case the second terms
| vanishes and the energy is merely the first term.
|
| -Time and space behave differently for photons. In relativity,
| time dilation and length contraction become extreme at light
| speed. A photon does not experience the passage of time in the
| way objects with mass do. Therefore, trying to define a
| reference frame for a photon would lead to contradictions in
| our understanding of spacetime.
|
| So in general it doesn't mean much absolute vs relative on
| photon case.
| AndrewOMartin wrote:
| This article mentions that the speed of light seems fast to
| humans living daily lives, but is not so fast on an astronomical
| scale.
|
| The best demonstration of this I've ever seen is on "If the Moon
| Were Only 1 Pixel - A tediously accurate map of the solar system"
| [1].
|
| If you've not seen it before, I recommend opening it, using your
| mouse wheel to scroll from the beginning (near the Sun) to Earth.
| It should take about a minute, but there's some commentary on the
| way. Then, to save your mouse and your finger some work, try
| clicking the icon in the bottom right hand corner to auto-scroll
| the map at the speed of light.
|
| [1]
| https://joshworth.com/dev/pixelspace/pixelspace_solarsystem....
| jzl wrote:
| On the topic of perfectly crafted depictions of scale in the
| Universe, I love this one: https://youtu.be/vcJHHU9upyE
| alex-moon wrote:
| Shameless plug: during lockdown I did a whole series of
| these, called Spacewalks: https://youtube.com/playlist?list=P
| Lul2c76M6HJySkSXYMoLXW9VC...
|
| These videos were super fun to make and kept me sane when I
| found myself with far too much free time and a bunch of world
| news to avoid. I never did the fifth (and final) walk but
| it's only about 100 meters long so I hope one day to do it in
| person (if I ever end up with that much free time again).
| steve_adams_86 wrote:
| This was awesome. It's strange to know that I already
| understood this (conceptually at least), yet seeing how slow it
| really is at this scale is confusing. Maybe that's not quite
| the right word. It makes my brain pause and go "that can't be
| right", but... It's right.
|
| Brains are bad at these scales. Maybe mine is worse than
| average. I can't fully believe how impossibly far away so many
| things truly are.
| S04dKHzrKT wrote:
| I like these kinds of visualizations for big things. Reminds me
| of "1 pixel wealth." https://mkorostoff.github.io/1-pixel-
| wealth/
| acchow wrote:
| > but is not so fast on an astronomical scale.
|
| This is something I find incredibly counter-intuitive. At the
| photon's reference frame (speed of light), time stops. In our
| reference frame, I'd expect some kind of "divide by zero" error
| in nature, resulting in the infinite speed of light. But it's
| not infinite. It's just... some constant.
| Ancalagon wrote:
| It's always crazy to me how empty most of space is, and that
| yet despite that gravity is strong enough on large timescales
| to pull a lot of stuff together. Then I get concerned as to why
| EVERYTHING wasn't pulled together and it's literally just this
| little bit of angular momentum conserved across vast distances
| and added together from all these little particles that is
| keeping everything apart just enough for us to exist.
| whamlastxmas wrote:
| Something to do with the universe expanding faster than
| gravity could ever hope to pull things together right?
| PaulHoule wrote:
| Our energy scale is set by the energy scale of chemistry. When
| you run it is powered by chemistry. Chemical rockets are powered
| by chemistry.
|
| Hypothetically you could go faster if you used fission or fusion
| energy but practically the chemical bonds get in the way. Even
|
| https://en.wikipedia.org/wiki/Nuclear_thermal_rocket
|
| is limited by the strength of chemical bonds of the reactor so it
| gets a factor of 2 or 3 or so on exhaust velocity compared to a
| chemical rocket.
| api wrote:
| This is why somewhat realistic high-power fusion rockets like
| those portrayed in The Expanse require magnetic nozzles. The
| fusion plasma would never come into contact with the material
| the engine is made of or it would melt.
|
| We could build a decent fusion pulse drive today if we had
| higher temperature more compact superconductors and super-
| efficient compact lasers that could fit in a spacecraft and
| ignite a strongly net-positive inertial confinement fusion
| pulse.
|
| Our superconductors are almost good enough, but our high-power
| lasers are _way_ too inefficient and bulky. We can 't even make
| economically viable ICF on Earth with current lasers.
| PaulHoule wrote:
| D + He3 and H + B are attractive precisely because the
| product is entirely charged particles that you can catch with
| such a magnetic nozzle. D + T is easier to ignite, but
| releases a lot of energy in neutron kinetic energy which you
| need to absorb in some material which lowers the temperature.
| D + D is the most common fuel on outer solar system and
| interstellar bodies but it also releases neutrons and might
| be most valuable in an energy system in that it products He3
| and T some of which could be separated from the plasma and
| used to fuel reactors that are either aneutronic or low
| ignition energy.
| api wrote:
| Yep, there was a physicist who did the math on the Epstein
| Drive (unfortunate name) in The Expanse and found that it
| was at the edge of possibility if you use _very_ efficient
| lasers, ICF, and D+He3 fuel. There is mention of helium-3
| and fuel pellets in the books, so it sounds like that was
| the idea.
|
| The biggest un-realism in The Expanse is the lack of huge
| heat-sinks, at least in the show. (They aren't mentioned in
| the books but I assumed they'd be there.) Without heat-
| sinks even if the drives were >95% efficient the ship would
| melt. Also in the show the thrust plumes from the engines
| are portrayed as looking like grill flames. In reality
| they'd look more like beams of light fading off into space.
| PaulHoule wrote:
| The path not taken in fusion research is
|
| https://en.wikipedia.org/wiki/Heavy_ion_fusion
|
| the argument is that "highly efficient laser" might be an
| oxymoron and if you were serious about commercial fusion
| you might trade lasers for much more efficient particle
| accelerators that run at a viable shot rate. Trouble is
| that you need heavy ions (lead) at 8GeV and it takes
| multiple barrels that are a km long or so... A huge
| machine that might be competitive with lasers for a
| commercial power plant but that can't be built in a
| subscale prototype. It might not be compatible with the
| magnetic nozzle though as 8GeV is not relativistic for
| lead ions.
| api wrote:
| I'd never even heard of HIF. I wonder how it could scale
| with today's superconductors, which are _far_ better than
| what we had in the 70s.
| PaulHoule wrote:
| See https://cds.cern.ch/record/2743157/files/Seeman2020_C
| hapter_... you can get better Q for superconducting
| linacs and also operate them in CW mode without fear of
| burning them up. Off the cuff I'd speculate that
| superconducting magnets that can support 2x the magnetic
| field might support 2x the accelerating gradient but we
| are talking AC operation here, not DC.
| fanf2 wrote:
| I thought laser fusion only exists for nuclear weapons
| research, and there isn't any path from the current
| experiments to a power plant.
| PaulHoule wrote:
| That's what I think. The efficiency of lasers is awful
| and they take hours to cool off after a shot whereas a
| commercial fusion power plant needs a shot rate between
| one every few seconds to several per second.
|
| A heavy ion power plant is possible in terms of the
| physics but needs to be the scale of a fission power
| plant to work at all and is projected to cost maybe 2x
| what an AP1000 costs assuming everything goes well and we
| know things usually work worse than you expect. So nobody
| is interested in funding a full-scale prototype, a
| reasonable development plan is you build several linac
| barrels and a test fusion facility and expect to rebuild
| that and add more barrels. It probably costs about what
| Musk spent on Twitter in the end.
| nick3443 wrote:
| Maybe they have some sort of thermally excited laser
| material and use hybrid electro/thermal lasers to "heat
| pump" waste heat back into the laser system.
| pfdietz wrote:
| The entropy has to go somewhere. Laser beams carry no
| entropy.
| Animats wrote:
| Part 2: [1]
|
| It's kind of an anthropic principle argument.[1] If the
| fundamental constants had substantially different values, the
| resulting universe would be boring. All the mass collapsed into
| one black hole, or evenly distributed as fundamental particles.
| Or atoms don't work. Or stars don't work.
|
| This leads to the usual problems - many-worlds theory, gods, etc.
| Strassler hasn't gone there, but others have.
|
| [1] https://profmattstrassler.com/2024/10/03/why-is-the-speed-
| of...
|
| [2] https://en.wikipedia.org/wiki/Anthropic_principle
| JdeBP wrote:
| It also kind of is not, inasmuch as it doesn't argue that
| there's the possibility of a universe where a "slower" speed of
| light (whatever that might mean) would not permit the existence
| of humans. It merely argues that the speeds of the human-
| centric world have to be small fractions of c, _whatever value
| c is_ , by dint of how the macroscopic relates to the
| subatomic.
|
| In natural units, in all such theoretical universes, c is 1;
| and all that this argument really states is that humans and
| similar atom-based things have to move at very small fractions
| of 1.
| ithkuil wrote:
| I.e. it's not that light it's fast but rather it's us who are
| slow
| kibwen wrote:
| _> In natural units, in all such theoretical universes, c is
| 1_
|
| I think this could be an instance of falling prey to the same
| sort of assumptions that the anthropic principle is intended
| to expose. For example, imagine theoretical universes where
| the speed of light is non-constant, or varies depending on
| where you are, or changes over time, etc.
| griffzhowl wrote:
| I'm not sure this affects the argument. Even if c were to
| change, creatures made of atoms would still have to be in
| conditions where their velocities relative to the objects
| around them are low compared to c, otherwise they would
| disintegrate on contact. There are other assumptions about
| atoms that go into this of course, but the goal of the
| article is to explain why any creatures made of atoms would
| think of the speed of light as very fast compared to the
| interations they're accustomed to.
| jzl wrote:
| I'll steal a line from a superb YouTube physics channel (Arvin
| Ash): it's not the speed of light, it's the speed of causality.
| And the universe _must_ have a finite speed of causality. Without
| even getting into math and physics, you can intuitively
| understand how infinitely fast causality would prevent time, and
| therefore everything else we know, from being possible.
| JdeBP wrote:
| It's not just Arvin Ash. That's actually fairly common
| terminology amongst physics educators nowadays. For starters:
| You'll find a lot of physics YouTube channels that say "speed
| of causality". It has even started to make its way into the
| astrophysics and physics textbooks in the last couple of years.
| Jsebast24 wrote:
| Einstein was not talking about light in his SR and GR
| theories. He was talking about the "speed" of light. As
| simple as that is, took me a long time to get it.
| jzl wrote:
| Interesting thanks. I hadn't seen it elsewhere myself but I
| could see how it's taken off. OP's article almost gets there,
| but never says that specifically. Rather it says "c is not a
| property of light, it's a property of the universe."
| griffzhowl wrote:
| Landau & Lifshitz, in their (classic) book on The Classical
| Theory of Fields, begin with a section called "Velocity of
| propagation of interaction".
| jprete wrote:
| And the other half of this is that brains are _extremely_
| complicated casual chains. Causality can traverse the diameter
| of the brain something like 10^8 times in the period it takes
| for light to be perceived.
| nick3443 wrote:
| Makes you wonder if neutron flux in a bomb during
| supercriticality becomes self aware for a short moment.
| verzali wrote:
| The question seems to me not why there is a speed of causality,
| but why the speed has this particular number. And it's not
| clear we know why that is, any more than we know why the proton
| mass is about 1836 times greater than the electron mass.
| JdeBP wrote:
| It's difficult to say that it has _any_ given number, since
| our measurement units for both time and space are derived
| from it (via a short detour to the size of Terra, in the 18th
| century). It can have any finite number you like, just adjust
| the metre and second to match.
|
| * https://www.youtube.com/watch?v=ZbGxXyqlhbU
| (FloatHeadPhysics on this)
|
| One physics convention just sets its value to 1. All of those
| Minkowsky diagrams that we see are measured in light seconds
| on the space axis, in order to make c have the value of 1
| space unit per time unit; so all of the graphical sheep,
| spaceships, cats, people, torches and stuff that are placed
| upon them are very much not to scale. (-:
| lordfrito wrote:
| > One physics convention just sets its value to 1.
|
| Interesting. So c2 would also equal 1. Which (in those
| scaled units) implies E = m.
|
| This not only greatly simplifies the relationship but
| actually makes much more sense that way.
| sroelants wrote:
| This convention of units is called "natural units", and
| it also sets a bunch of other units to be 1, depending on
| the flavor (planck's constant, boltzmann's constant,
| etc...)
|
| Not only does this clarify the relationships, but in many
| ways, these "natural constants" are an artifact from our
| past ignorance. Boltzmann's constant, in a way, is
| nothing more than a "conversion factor" that we hold over
| from the time when we believed temperature and energy to
| be two separate concepts. In the same way, the speed of
| light is an artifact from a time when we considered time
| and space to be distinct concepts, measuring them in
| distinct units, and needing a conversion factor (in units
| of distance/time) to map between them.
|
| It's as if we would all collectively agree that the "up"
| direction, from now on, would be measured in floops, and
| the slope of a hill would be measured in floops/meter.
|
| From a philosophical point of view, it's not just saying
| "measure time in units such that c = 1". It's saying
| "let's consider time to be a distance, and measure it in
| the same units as we do the other ones".
| Jsebast24 wrote:
| If the speed of causality were to suddenly change by whatever
| factor, would we notice it?
| BurningFrog wrote:
| It appears as "c" in the important equation E = mc2.
|
| So I expect it would change a lot.
| hinkley wrote:
| You know how the OceanGate sub imploded so fast that their
| brains didn't even have time to register they were dead?
|
| If the basic laws of the universe change, we'll be
| disincorporated before we have a chance to know it's
| happening, because it will happen at whatever the speed of
| light is in the new balance, and the chemical processes
| that make us tick will change to other chemical processes
| that don't actually work anymore.
|
| In the new universe silicon based life might function. Or
| stars might not work anymore.
| jzl wrote:
| Search YouTube for "universe fine tuning." Then come back
| here in a few years when you've gotten through everything. :)
| tasty_freeze wrote:
| No thanks. Every time I hear the argument brought up, the
| person putting it forward says it like it is a mic drop
| moment and they never discuss the counter arguments. It is
| effective for people who haven't heard the argument before
| or haven't thought it through.
|
| Here is an example. The charge of an electron is exactly
| the opposite of the charge of a proton, to within
| measurement error (like ten digits). This is simply
| something which has been measured, but physics has no
| explanation for why they are the same. Getting to the
| point, what is more likely: that a God created it that way
| in order to achieve His goals, or that there is some reason
| connecting the two charges such that they _must_ be of
| exactly the same magnitude but we just haven 't figured it
| out?
|
| I'm putting my money on the latter. If there is an all-
| powerful creator, there is no reason to have fine tuning at
| all -- He could just force the desired behavior and
| outcome.
| GoblinSlayer wrote:
| Presumably charge is a property of electromagnetic field,
| so it's expected that they match.
| cypherpunks01 wrote:
| Yes, I was often very confused as to why the speed of light
| shows up everywhere, until it was reframed for me in this way.
| The fact that light travels at the same speed regardless of
| your frame of reference becomes a little less mystifying.
|
| It feels more intuitive to me when thinking about it as
| _causality_ always unfolding around you at the same speed, no
| matter your own frame.
|
| The constant c was not named for causality, but it is a nice
| coincidence.
| jmyeet wrote:
| So this seems like a better definition until you run into a
| problem, which you do pretty quickly: "casuality" isn't the
| easiest thing to define.
|
| The best definition I think I've seen is to view the universe
| as a partially ordered set of events, meaning that you can only
| order events (in time) if they're within each other's cones of
| causality. Outside of that you cannot say which happened first.
| That's the partially ordered part.
|
| But even that is incomplete and arguably even self-referential.
| What's a "cone of causality" (without relying on causality)?
|
| Also, there's the issue of what exactly time is and whether
| events are time-symmetric or not. Many physicists seem to view
| time as an emergent rather than fundamental property of our
| Universe.
| cypherpunks01 wrote:
| Causality is * _bangs hammer on bell*_
|
| Time is a relationship between clocks.. beyond that, yes,
| it's hard to say exactly.
|
| Time seems to be what prevents everything from happening at
| once.
| hinkley wrote:
| Without time does causality exist? Does anything happen at
| all? What is it to 'happen' except that something was one
| way and now is another?
|
| I can't even describe 'happen' without using verb tenses,
| which represent time.
| jerb wrote:
| Thanks, I've never heard this and it's quite profound. It's
| always bothered me that there even is a top speed, and further
| that mass becomes infinite as it's approached. But "speed of
| causality" makes these less strange.
| dyauspitr wrote:
| Why does speed of causality make it any better? It's still an
| arbitrary limit that's even more abstract and intangible than
| speed.
| adastra22 wrote:
| If the universe's causal mechanisms were infinitely fast,
| the entire history of the universe would play out instantly
| in zero time, and we'd skip straight to the heat death of
| the universe.
|
| The fact that time even exists is implied by / a result of
| causal actions having some finite propagation time.
| jiggawatts wrote:
| That doesn't work that way.
|
| Imagine a universe simulated in a computer with "ticks"
| where the entire state is updated.
|
| It would be different to ours, but it would work just
| fine.
| Terr_ wrote:
| > causal actions having some finite propagation time.
|
| I think I know what you're getting at, but somehow the
| phrasing bothers me, as if there is meta-time or as if
| cause and effect have time between them... for the photon
| at light speed, time isn't passing, it's emitted and then
| zero "time" later it hits something very far away.
|
| It's more like we somehow need to think of cause and
| effect chains that have orderings without time.
|
| I wonder if future generations will ever look back and
| casually quip something about "well they believed X
| existed, that was their problem, it all makes intuitive
| sense if you just..."
| kingkongjaffa wrote:
| Well because its decoupled from light itself or its
| properties.
|
| If there's some universal limit for causality itself, then
| light just happens to be the fastest thing among all the
| other things subject to causality.
| thiagotomei wrote:
| But it is not a coincidence. Light -- the EM field waves
| -- propagates at causality speed because the EM field
| respects a particular property of the universe, the so-
| called gauge symmetry. That is intimately connected to
| the fact that the photobs has no mass.
|
| Other similar particles, like the W and Z bosons, are
| manifestations of the weak field. Since that field breaks
| the symmetry, those particles have mass and move slower.
|
| BTW, that symmetry breaking is the very same one that
| physicists talk about when we discuss the Higgs boson.
| shadowgovt wrote:
| > Without even getting into math and physics, you can
| intuitively understand how infinitely fast causality would
| prevent time, and therefore everything else we know, from being
| possible.
|
| Can you unbox this a little? I think I may just have Friday
| brain, but I'm having some difficulty convincing myself in the
| moment that infinite-speed causality development would prevent
| time.
| woopsn wrote:
| A system in which information is communicated instantly will
| quickly reach equilibrium, after which there is nothing left
| for any part to communicate to another. Eg diffusion of heat
| eventually results in a temperature distribution in which
| there is no longer a flow of heat.
| bloopernova wrote:
| "Light travels at the speed of causality". Why does light have
| that behaviour?
| aezart wrote:
| Because it has no mass.
| flawn wrote:
| Exactly - Light cannot be subject to causality (in the
| philosphical sense)
| tshaddox wrote:
| That's not intuitive to me. Any old physics engine in a video
| game has infinite speed of causality and all the other
| classical physics stuff seems to work, including time. There
| must be some other unmentioned property of our Universe's
| physics that is important and which requires finite speed of
| causality.
| takinola wrote:
| The explanation that unlocked the intuition for me was the
| postulate that all objects in the universe are moving at the
| same velocity. Some are moving faster through time and some
| faster through space. If you move faster in time, you move
| slower in space and vice versa but the vector sum of your speed
| through space-time is the same. Therefore a photon is moving
| really fast through space but does not experience any movement
| in time.
| chongli wrote:
| The speed of light is not fast. It's really, really slow! That
| is, slow relative to the size of the universe and the timespan of
| the universe, relative to human scales of size and timespan.
|
| Humans can travel around the world (our domain) in a matter of
| hours (on rockets, our fastest mode of travel thus far).
| Similarly, the fastest waves can cross the ocean in a matter of
| hours. Light, on the other hand, takes billions of years to cross
| the visible universe. It's downright glacial at those scales!
| sghiassy wrote:
| ^^ this
| JdeBP wrote:
| Yes, it takes forever to get anywhere at Warp 9. (-:
|
| More seriously: Your very point is already made near to the
| beginning of the headlined article, in the book quote. You
| might want to read beyond the headline question, otherwise
| you're just repeating what the article already says.
| chongli wrote:
| I did read the article and it is not making the same point
| I'm making. It takes the position that humans are extremely
| slow, relative to the speed of light. I'm taking a different
| position: that humans are extremely fast, relative to the
| domain in which we operate (the earth, our cities, our
| neighbourhoods, our households).
| JdeBP wrote:
| The paragraph beginning "And yet c is also slow." in the
| article is pretty much what you said.
|
| If you _now_ want to make the point that humans are fast on
| human-centric scales, which you _did not_ really make
| above, you enter a whole other discussion that involves
| biology, and humans not really being very fast at all
| compared to some other creatures. You have, after all, to
| introduce non-human entities, rockets, to show examples of
| humans being "fast".
|
| And really fast rockets aren't examples of humans being
| fast, as they are good examples of humans becoming dead,
| from the accelerations involved for starters. Take a human
| out of a rocket system, and it can go much faster. Human-
| ridden rockets are in fact slow, too, even on human-centric
| scales, compared to the things that are extremely fast in
| the human-centric world. So that argument falls down.
|
| Which leads to part 2, pointed out in a top-level comment
| by Animats, at
| https://profmattstrassler.com/2024/10/03/why-is-the-speed-
| of... which goes on to explain that humans are _by
| necessity_ slow.
|
| There are even biological discussions of the same idea: why
| humans are slower than, say, houseflies. We're slower than
| c for physics reasons, and we're slower than things on our
| own scale for biological reasons. We actually _are not_
| extremely fast. We aren 't as fast as our machines, nor
| even as fast as some other creatures.
| A_D_E_P_T wrote:
| > _Light, on the other hand, takes billions of years to cross
| the visible universe_
|
| Right, and that's just the visible universe. The full extent of
| the universe is _much_ larger -- I think that the most cautious
| lower bound estimate is that it 's 250x larger. It could be
| 10^10x larger, or even infinite/unbounded. In such a vastness,
| the speed limits for light and baryonic matter are perplexingly
| slow.
| Mistletoe wrote:
| Are galaxies and stuff out in that 250x or is it just empty
| space?
| JdeBP wrote:
| It's not observable by us, so we don't know. The figure is
| (to simplify) a deduction from what we _can_ see, the
| observable universe, and the fact that we don 't see any of
| the consequences that there would be in the observable bit
| if the entire universe were smaller.
|
| * https://doi.org/10.1111/j.1745-3933.2011.01040.x
| osigurdson wrote:
| If photo started a stopwatch and proceeded to travel 100T light
| years, the stopwatch would still read zero. That is pretty fast
| imo. It all depends on perspective.
| marcosdumay wrote:
| > Light, on the other hand, takes billions of years to cross
| the visible universe.
|
| Well... That's to be expected. It's right there on the
| definition.
| shiandow wrote:
| The size and age of the visible universe aren't that different
| in size though. I mean, they would have to be, the only reason
| they differ at all is because the universe expands.
|
| Meanwhile we measure time in hundreds of millions of meters and
| space in nanoseconds. Something causes humans to be _slow_.
|
| Which isn't that surprising, life is basically a diffusion
| process gone haywire and while we're more efficient than just a
| big rock being pushed by small particles we still rely on
| statistical physics to push molecules around and it takes a
| while for those statistics to average out.
| dimitrios1 wrote:
| I get what you are saying at, but viewed in a another way, you
| just said that the fastest thing in the universe is really
| slow.
| jmyeet wrote:
| So this touches on the anthropic principle, which is to say
| that if the Universe (and the constants within it) were other
| than what they were, we wouldn't be able to exist to
| contemplate it.
|
| The speed of light being "slow" in cosmic terms is almost
| necessary for our existence in that we need a relatively long
| period of relative stability in order to evolve into sentient
| life. And that becomes a whole lot harder if, say, the Milky
| Way was only one light day across.
| chongli wrote:
| I think in the past I would've readily accepted this
| explanation but now it seems to me like a just-so story.
|
| Bacteria can exhibit doubling times on the scale of tens of
| minutes. We know of trees that live for thousands of years.
|
| On the other hand, we know of chemical reactions that can
| propagate significantly faster that the speed of sound (high
| explosives) and nuclear reactions that propagate even faster
| that this. At the other end of the scale, we have mildly
| radioactive elements with half-lives measured in billions of
| years.
|
| This is all to say that everything is relative and no matter
| what constants you choose for the universe, they're going to
| seem arbitrary.
| alex_young wrote:
| It's all a matter of perspective.
|
| If you were to travel at nearly the speed of light, you could
| cross the universe in a matter of minutes. Of course an
| external viewer on say Earth would disagree and say it took
| billions of years, but who's counting?
| scientator wrote:
| Actually, you couldn't cross it in a matter of minutes. In
| fact, you would never even reach the edge of the visible
| universe. This is because the edge of the visible universe is
| expanding away from us at faster than the speed of light.
| alex_young wrote:
| Again, this is a matter of perspective. The amazing long
| lived Earth observer would see the universe expand out of
| view, and you along with it wouldn't they?
| scientator wrote:
| Yes, after billions of years you would move outside of
| the sight horizon of the long-lived observer on earth and
| disappear from view. For you, the traveler, this would
| happen in mere minutes. But you wouldn't have crossed the
| universe in that time because the edge of the visible
| universe is constantly expanding away from us faster than
| we can travel to catch up with it. Even if we travel at
| the speed of light.
| baal80spam wrote:
| I remember reading several years ago that there are
| celestial bodies that we will NEVER be able to see
| precisely because of this.
| S04dKHzrKT wrote:
| Lawrence Krauss has given a talk that mentions something
| similar. He says that we live in a good time because we
| can still see "everything" around us. At some point in
| the far future, any observers won't be able to determine
| many things about the universe because the "stuff" in it
| will be too far away to observe.
|
| https://youtube.com/watch?v=7ImvlS8PLIo @ 50:57
| jprete wrote:
| I would really like to hear from an actual physicist on
| this question since both of you seem correct for one of the
| reference frames and the only way I see to reconcile the
| two is with weird singularities like "the particle reaches
| the edge of the visible universe in infinite time according
| to the rest observer". (And I don't think that is right
| either.)
| scientator wrote:
| The edge of the visible universe functions for us like a
| cosmic event horizon. Similar to the event horizon around
| a black hole. A particle leaving earth at light speed can
| never reach or go beyond that horizon. Even in infinite
| time. That's assuming the universe continues to expand.
| If it starts to contract then, yeah, the horizon is going
| to crash in on us.
| jprete wrote:
| What does this look like from the photon's frame of
| reference, then? Or is it nonsensical to try to describe
| things from a frame of reference at C?
| bulte-rs wrote:
| This is probably a huge reasoning error, but wouldn't you
| "expand away" with the expanding away part of the universe
| at the same rate, sort of riding along with the expansion
| given that c is the same in the point of reference (the
| expanding away faster than c part)?
|
| n.b. I obviously lack the vocubulary to communicate
| properly about this, help needed!
| cpsempek wrote:
| on first glance it seems like an interesting take, but then you
| realize (as someone else already pointed out) that the fastest
| thing in the universe is not fast, and therefore nothing is
| fast? a little more thought should make you realize this is a
| poorly formed take. Also, worth repeating, please read the
| article before posting. It may be that your insight or critique
| is present and discussed in the article already.
| jacknews wrote:
| But why mv^2 ?
|
| Is the universe 2D?
|
| And it's E=mc^2 because the only 'consensus' value in the
| universe is c? Why is that? And so the 'mass' (whatever that is)
| must be moving at the speed of light for the equation to make
| sense, even though it's stationary?
|
| The blogs demonstrate great factual knowledge and 'mastery', but
| don't really explain anything IMHO
| prerok wrote:
| Well, depends on your viewpoint, I guess. When you really get
| into the details, it turns out we don't "really understand"
| anything.
|
| We are just making more and more detailed observations and then
| creating mathematical models of these behaviors. For example,
| we observe that space is curved around mass. We can model that
| and it helps us understand what's going on, so it's useful.
|
| We don't, however, understand what exactly is curved and what
| is this empty space that curves.
| af3d wrote:
| If you break up "mv^2" into its constituent dimensions you get
| m(d/t)^2 = m(d^2)(t^-2). Now the so-called kinetic energy of
| the object only "manifests itself" whenever there is a _change_
| in velocity of the object in question. Well, the derivative of
| velocity is an acceleration, so the object in acceleration
| would be represented as mdt^-2, aka "a force". Hence the
| _energy_ of the system is simply that force acting over some
| distance d.
|
| As to the internal/intrinsic energy of a given object, think of
| it as "hidden potential energy". It is essentially the energy
| that was required to turn photons into the matter that you, and
| I, and everything else are made of! The equation itself is mc^2
| simply because that is what you get when you rearrange and
| simplify the experimentally-verified equations which it was
| drawn from. Likewise, for c is nothing more than the measured
| value of the speed of light in vacuum for _any_ observer. Of
| course the choice of units is completely arbitrary. Whether you
| state it in miles per hour, kilometers per second, or whatever,
| the ratio remains constant.
| pharrington wrote:
| The mv^2 isn't a geometrical property - it's because the
| kinetic energy can also be thought of as the integral of an
| object's momentum with respect to time.
| idunnoman1222 wrote:
| The speed of light is in its Goldilocks zone. I wouldn't worry
| about it too much.
| MarkusWandel wrote:
| Light isn't that fast. It only goes about a foot per nanosecond.
| It can be fun to work out for people how many bits are _in_ a
| piece of cable at any moment, and that 's assuming electrical
| information in a cable travels as fast, which it doesn't.
|
| A real demo is to talk via VOIP with someone on a satellite
| internet connection. The old, geostationary satellite kind. It
| takes so long for the audio to get there and back that you have
| to practically say "over" when you're done talking.
| epohs wrote:
| Here's a 5 and a half hour long video of a photon travelling
| across the solar system.. spoiler the photon in this video
| doesn't move quickly
|
| https://youtu.be/_qKOpvDa82M?si=HyO5TGSvxw0DN5yF
| greenhearth wrote:
| We are trying to speed everything up all the time and speed is
| seen as a virtue and something desirable. Could it be that we are
| trying subconsciously on a collective scale to get close to the
| natural state of the universe?
| whatshisface wrote:
| The argument given in part 2 (that the strength of nuclear forces
| compared to electric forces make nuclei very heavy relative to
| the energies that would rip their chemistry apart) does not make
| a lot of sense in the context of the fact that binding energy
| _reduces_ the mass of bound states. For example, 56Fe is lighter
| than 26 protons and 30 neutrons.
|
| The ratio of the Hydrogen atom's ground state electron binding
| energy to the electron's mass-energy is one half the fine
| structure constant squared. That implies the nuclear forces don't
| have much to do with it - electromagnetism is simply, and
| dimensionlessly (i.e. independently of any arbitrary units or
| scales), a weak force.
| goatmanbah wrote:
| Better to ask why light is so slow...
| jcd000 wrote:
| That was a nice read.
| p4bl0 wrote:
| EDIT: _Sorry I came back from the article to say this before
| reading the comments here... I should have, the top comment is
| already saying the exact same thing!_
|
| The speed of light isn't that fast. The website "the moon is one
| pixel" is a webpage where our solar system is represented at
| scale if our moon were 1px in diameter :
| https://joshworth.com/dev/pixelspace/pixelspace_solarsystem....
|
| You can scroll through it. It's so long to go from one planet to
| another. So much empty space.
|
| At some point you're tempted to click on the C button which you
| see on the bottom right of the page. Speed of light! Surely that
| will autoscroll fast! ... Nope, to scale, the speed of light is
| waaaay slower than your scrolling was! And then you realize, at
| the size of the universe, how even light isn't that fast.
| FredPret wrote:
| If I had to write a "game of life" simulation that
| simultaneously calculated the effects of all events at each
| point in a 3D matrix over time, I would:
|
| - make the matrix as rough as possible while still enabling
| interesting events (ie, try and maximize the Planck length)
|
| - make the maximum speed at which events propagate across the
| matrix as slow as possible to save the CPU (ie, try to minimize
| the speed of light)
|
| - limit the size of the simulated universe
|
| But our Planck length is tiny and the universe is probably
| humonguous unless we're being deliberately deceived by This
| Simulators.
|
| So despite the suspicions aroused by the slow speed of light,
| we might live in the mother / "real" universe after all.
| hoerensagen wrote:
| I think you might have the same misconception about the
| planck length that I had:
|
| "The Planck length does not have any precise physical
| significance, and it is a common misconception that it is the
| inherent "pixel size" or smallest possible length of the
| universe.[1] If a length smaller than this is used in any
| measurement, then it has a chance of being wrong due to
| quantum uncertainty.[2]"
|
| https://simple.m.wikipedia.org/wiki/Planck_length#:~:text=Th.
| ...
| fluoridation wrote:
| I think that sentence as written is itself based on a
| misconception. The idea is not that the Planck length is
| physically meaningful, but rather that that quantum
| uncertainty is caused by the structure of the universe
| (discrete spacetime). In other words, that uncertainty is
| analogous to aliasing in a signal.
| fluoridation wrote:
| >make the maximum speed at which events propagate across the
| matrix as slow as possible to save the CPU
|
| How would that save CPU time?
|
| The Game of Life does have a maximum speed of propagation of
| causality, but it's not designed in, it's just a consequence
| of the basic rules that define the simulation.
| FredPret wrote:
| Well, you could just slow down the execution rate of the
| whole simulation.
|
| But if you limit the speed at which events propagate, my
| feeling is the rate of events occurring will be lower
| overall, since one event triggers another and each event
| will trigger fewer secondary events per second if it
| propagates less distance per second.
|
| You could now also have islands of stability so that a
| cataclysm on one end of the simulation will take a long
| time to spread to the rest of it.
|
| IE, Andromeda can explode and we won't even know for a long
| time. In that time, we will continue doing interesting /
| entertaining things, or continue calculating the answer to
| life, the universe, and everything.
| shadowgovt wrote:
| This is a very good author. "What's a Proton, Anyway?" is also
| both educational and entertaining
| [https://profmattstrassler.com/articles-and-
| posts/largehadron...].
|
| "Ok, then, what's a hydrogen atom?
|
| It is the simplest example of what physicists call a "bound
| state" -- the word "state" basically just meaning a thing that
| hangs around for a while, and the word "bound" meaning that it
| has components that are bound to each other, as spouses are bound
| in marriage. In fact, the image of a married couple, especially
| one with one spouse weighing a lot more than the other, is
| probably the one you want."
| dboreham wrote:
| As fast as the GPUs running the simulation.
| darkhorn wrote:
| I didn't find why.
|
| When you make a wave with a rope or on a water the speed of the
| wave is formed by the speed of the interaction of the molecules
| in it. I believe similar thing happens with the light in
| subatomic level. There, may be the spped of light is limited on
| how fast an higs boson interacts with the neighbpring higs
| bosons.
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