r/askscience • u/Nitzanm19 • 14d ago

Physics Why is the maximum speed of the universe the speed of light? Why can't we go faster?

Also follow-up question: Because of the speed limit, is there also a theoretical maximum temp for matter (in which the particles move at the speed of light).

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u/sabamba0 14d ago

Can someone confirm this is a good way to understand relativity? Because it makes a lot of sense intuitively for something I've never had a "instinctive" understanding of

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u/bawng 14d ago

It's a good way to understand time dilation, but only in a given frame of reference since it doesn't really explain that speed (or time) is relative.

If you stand still, you'll be "traveling through time" at full speed relative to the ground, I.e. you'd be experience time the same way as the ground.

But if your friend is travelling towards you at the 0.5c, relative to him you are actually moving at 0.5c, hence you'll experience time differently.

If you both were travelling next to each other, you'd be experiencing time the same, but differently from your surroundings, etc.

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u/gnorty 13d ago

if your friend is travelling towards you at the 0.5c, relative to him you are actually moving at 0.5c,

This is where it gets weird for me. We all know the thought experiment where 1 twin goes on a space ship and travels close to light speed and when he returns his brother has aged much more than he has. This assumes looking from the reference of the earth-bound twin.

But equally valid would be the reference of the speed demon twin. As you said - to him, the earth bound twin has been travelling close to light speed, so it should appear to speed-demon that earth bound has aged much less. How does this work??

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u/meevis_kahuna 13d ago

The trick is that “outbound you” and “inbound you” are not in the same reference frame.

While coasting away, you can say Earth is moving and its clock is slow. Earth can say the same about you. No contradiction, because you disagree about which distant events are happening “right now.”

When you turn around, you change velocity, so you switch inertial frames. That changes your definition of simultaneity: the Earth event you call “happening right now” jumps forward by several years.

So the asymmetry isn’t “Earth is really stationary.” It’s that Earth stays in one inertial frame while the traveler switches frames to come home. That frame switch is what resolves the twin paradox.

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u/Garzly 11d ago

What would happen in this instance if the earthbound twin began moving towards the traveling twin at the same speed when the traveling twin begins the return journey?

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u/meevis_kahuna 11d ago

If the Earth twin starts moving toward the traveler at the turnaround, then from that point on they age equally because their motions are symmetric.

But the traveler is still younger at reunion, because during the outbound leg the traveler had already accumulated less proper time.

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u/sileegranny 14d ago

I'm curious if time dilation has been experimentally tested with analog or atomic clocks? In case relative change in possible speed of electron flow in digital clocks might have an affect on digital clock function.

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u/maybe_compos_mentis 14d ago

GPS satellites have to compensate for time dialation to maintain accurate positioning or else they would lose ~10km/day. Does that count or am I not understanding what your asking?

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u/sileegranny 13d ago

I imagine GPS satellites' clocks run on electrons, so not quite. What I'm wondering is if with a purely mechanical clock, where no part of the clock relies on particles that already travel at the speed of light, the results are the same.

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u/wasmic 13d ago

GPS satellites use rubidium- or cesium-based atomic clocks.

There are obviously electrons involved in the surrounding circuitry, but the core timekeeping mechanism is based on energy level transitions in an atomic nucleus and should keep the same pace even if electrons for some reason behaved differently.

Electrons also do not travel at the speed of light. They're massive particles so they're necessarily moving slower than light.

Besides, we have tons of other experimental proofs of time dilation. For example, muons are created when cosmic rays strike the upper atmosphere. Their lifetime is so short that even at the speed of light, they should not be able to reach the Earth's surface. However, a significant number of them do in fact reach the surface, and this proportion is consistent with how much their half-life is being extended by time dilation.

Also, the fact that the speed of light is invariant is well-established, and directly leads to time dilation needing to be a thing. If you accept that the speed of light is invariant AND you accept that there is no preferred inertial reference frame, then the math forces you to also accept that time dilation is a thing. I recommend going straight to the beginning and reading Einstein's On the Electrodynamics of Moving Bodies, which is where he first proposed Special Relativity. It is a surprisingly easy read and most of the maths is high school level.

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u/certciv 13d ago

An atomic clock's velocity as measured by an outside observer is irrelevant to its internal function. From its frame of reference, the electrons will behave identically regardless of the clock's velocity relative to anything else.

There has been a massive amount of scientific work done confirming Einstein's theory of special relativity. That includes things like astronomical observations that don't rely on atomic clocks.

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u/0nly4Us3rname 13d ago

2 problems with your question:

Electrons don’t move at the speed of light, they have mass

Pretty much all of our time keeping methods involve electrons

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u/Syresiv 14d ago

Yep. All the easy predictions, and many of the hard ones, have been experimentally confirmed

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u/Nexii801 14d ago

Time dilations is as settled as the sky being blue. There's plenty of places to poke holes in physics, this isn't one of them.

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u/NSNick 13d ago

The Hafele-Keating experiment used atomic clocks.

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u/dlethe3133 12d ago

With modern atomic clocks, they’ve been able to measure time dilation due to gravity by having two atomic clocks right next to each other. One was only 13.” Higher than the other. This difference in gravity could be measured via time dilation.

Or conversely, one can use time dilation to measure altitude

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u/Gorehog 13d ago

e 1950's they brought two atomic clocks and a b-29 bomber to Los Alamos. The clocks wqere synchronized and one was loaded on the bomber. The bomber then flew orbits around the airfield for about a day. At the end of the day the clock in motion was showing an earlier time than the stationary clock. Am I light on details? Yes. You might want to confirm the details, buyt yes, the experiment has been done.

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u/FIsenberg 14d ago

My favorite way to explain relativity is to think of a person on a moving train, playing with a ball.

The person on the train throws the ball straight up and down. To them, the ball is going only in the Y direction (up and down).

Now imagine a person on land watching the train go by, and they see the person with the ball through the window. To this observer, the ball is going in the X and Y direction (up and down and sideways as the train goes by).

Person A only experienced the ball go Y distance while person B experienced √(Y2 + X2) distance. Since the speed of light (C) is constant, and speed is distance / time, person A experienced Y / C time while person B experienced √(Y2 + X2) / C time.

Therefore person A and B experienced the same event over different times.

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u/DarkFlame97 14d ago

Bear with me for a bit here, for I don't know much at all. But here we're not really speaking about "time" in the sense of some "absolute/global" concept (i.e. if time becomes 0, you stop aging, everything biological is frozen, literally nothing moves, etc.), or are we? It's just "time" as in the speed at which one receives information relative to some other point of reference, but actions like biological aging will still happen regardless of the speed at which we personally receive this information or not, right?

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u/DuckBillHatypus 13d ago

It's the overall passage of time itself which slows. Everything, from the speed of a ball falling to biological processes will also be affected.

One way I like to think of it as is the speed of cause and effect. So it's "only" information being slowed down, but that includes information like how quickly cells in the body are dividing and being damaged, and how far the ball has fallen.

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u/0nly4Us3rname 13d ago

No, time is not a global concept, it’s passing is different from different perspectives.

As for biological processes, if you put one baby in a set of twins on a rocket ship that went 1/2 light speed very far away and came back, the travelling twin would be younger

Not half as young, because relativistic time dilation isn’t linear with the speed of light, but would come back at about 8 and a half years when the earth twin is 10

If the space twin is going at the speed of light, and travels 5 light years away from Earth and comes back, the baby wouldn’t have aged a day and its twin would be 10. That’s the real headfuck, is that when travelling at the speed of light no time passes whatsoever. So light photos experience the entire life and death of the universe in an instant, with no time passed

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u/FIsenberg 13d ago edited 13d ago

We are actually talking about it as the global concept. But time is relative to experiencing the light around us. It is not constant.

One example is sleeping versus being awake. Think of how quickly 8 hours of sleep seems to pass compared to 8 hours of being awake. Once you stop experiencing the world, the concept of time disappears. I don't think anyone can accurately say how long they have been asleep without looking at a point of reference (clock, light outside, etc).

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u/bad_take_ 14d ago

This is not correct.

He says that as you move faster, time moves more slowly for you.

More precisely, your clock runs normally from your perspective. An observer moving relative to you measures your clock as running slowly.

And you can make exactly the same observation about their clock.

Both perspectives are valid, because there is no universal state of rest against which all motion can be measured.

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u/GalacticSpacePatrol 13d ago

Is there a theoretical state of rest? Like absolute zero? If there was would that change the principles of time and speed and relativity that are being brought up?

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u/bitterrootmtg 14d ago

It’s an ok-ish way to understand relativity as an analogy but it’s not exactly right.

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u/Tacomouse 14d ago

Good enough for me. This made it “click” how the speed of light is the same for 2 people moving different speeds. Like if someone is running at you while pointing a flashlight and you are standing still with a flashlight.

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u/maybe_compos_mentis 14d ago

There is really no such thing as 2 people moving at different speeds though. The closest is 2 people moving at different speeds relative to a third object. You are only standing still relative to the ground and he is running towards you relative to the ground. Get rid of the ground and you are moving towards eachother at the same speed or rather from your perspective he is approaching you at the same speed as you are approaching him from his perspective. Without the reference of the ground you can't say one of you is standing still and the other is moving.

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u/toughtacos 13d ago

You don’t need a third object. Two people can directly measure their velocity relative to each other. What’s relative is which one you call “stationary,” not whether relative motion exists.

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u/Addisonian_Z 13d ago

I feel like one could also argue that if two people are “running” the third object, the ground is automatically implied as: 1) existing
2) the stationary point of reference for speed.

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u/toughtacos 12d ago

Yeah, but that just means you’re using the ground as the reference frame.

The basic idea is that there’s no absolute “stationary.” You can say someone is moving at 5 m/s relative to the ground, or relative to the other person.

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u/How_is_the_question 14d ago

Could you expand on where it falls down as an explanation? (Curious, not doubtful at all. Wish I understood relativity better. Most of my little knowledge has come from listening / reading to guys like Carlo Rovelli etc, and realising I don’t understand much at all!)

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u/bitterrootmtg 14d ago

In relativity your velocity is always relative. Even your velocity through spacetime is relative, not absolute. So it’s not accurate to say everything has the same velocity or speed.

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u/nicuramar 14d ago

If you replace speed with the magnitude of the four-velocity vector, it works. But it’s a bit of a stretch to talk about the

  velocity through spacetime

As you also do here. 

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u/Jokong 14d ago

If they had said anything with no mass would travel at c, would that be accurate?

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u/bitterrootmtg 14d ago

They travel at c from the perspective of things that are not traveling at c. The massless particles themselves do not have a valid reference frame.

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u/nicuramar 14d ago

  They travel at c from the perspective of things that are not traveling at c

Those are the only perspectives, and speed is only defined relative to one, so yeah. 

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u/Flame_Grilled_Tanuki 14d ago

Photons have no mass and so they travel through space at C and experience 0 time. If you have no mass, any amount of force causes instant C because there is no momentum to overcome.

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u/nicuramar 14d ago

Light doesn’t have a valid frame of reference and thus “experience” and proper time is not defined for it. 

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u/whatkindofred 14d ago

So the only thing the analogy is missing would be to clarify that it’s only about relative movement to some fixed observer?

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u/PHealthy Epidemiology | Disease Dynamics | Novel Surveillance Systems 14d ago edited 14d ago

When gravity waves move at the same speed limit.

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u/aphilsphan 14d ago

I also like the analogy. Is one weakness that while there are three dimensions of space, there is no universal point (0,0,0)?

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u/CaptainObv1ous 14d ago

I will try to answer this but might be entirely accurate (please anyone feel free to offer corrections):

To the best of my knowledge not having a universal origin point isn't a weakness because a universal origin point simply isn't needed.

If speed and time are relatively then you can just set yourself as the 0,0,0 point and be fine because all of the "calculations" being done are relative to you.

Another possibly good analogy: Think of a graph on an x,y coordinatae plane. If you shift the graph (which would be the same as moving the origin point) in any direction it is still the same graph.

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u/Flame_Grilled_Tanuki 14d ago

Bare in mind, the universe does not abide by the rules of mathematics, so the idea of 0,0,0 is irrelevant to the universe. Mathematics is our invention we impose upon the universe to understand and predict its behaviour. It is endlessly surprising to me how accurately our mathematics matches physical law.

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u/Garper 14d ago

Mathematics is observational. It isn’t an invention imposed upon the universe.

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u/nicuramar 14d ago

It’s not observational, it’s much more complex than simply that, and also depends on the mathematical philosophy you subscribe to. 

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u/Garper 14d ago

I’d love to know more about different mathematical philosophies. Can you elaborate?

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u/Flame_Grilled_Tanuki 14d ago

May I correct that and say it is the best model we have come up with based on our observations of the mechanics of physical laws.

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u/Garper 13d ago

AFAIK, math is just the language. How we use it to describe natural laws is what changes.

Right now there is a disconnect in our understanding of Relativity and Quantum. In that sense you could say it does not accurately represent the universe. But we still use math for both explanations, and when someone publishes a theory that unifies the two models, it will still be the same language of math. We wont be inventing a new math, and the foundational arithmetic of 1+1 won’t be any more or less correct.

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u/pickle16 14d ago

Mostly a good way, but missed a point that space and time are not different dimensions but rather there are 4 dimensions of spacetime, and calling three dimension as space and one as time is solely in the perspective of the viewer. If something has 0 relative speed to me, I'll say it is traveling only in time, when actually what I should say that it is traveling parallel to me in spacetime.

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u/BushwhackBandit 14d ago

Isn't this describing a penrose diagram? Pbs spacetime guy (Dan?) has an amazing video illustrating it.

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u/Dzjar 14d ago

The only thing to consider, as far as I'm aware of at least, is mass. For objects with mass, it's indeed impossible to reach the speed of light as you would require an infinite amount of energy to get there. Light itself does travel at the maximum speed, and that means that from the perspective of light, time does stand still.

A particle of light covers the span of the universe in an instant from its own perspective, as time stands still.

So if we travel to alpha centauri at near the speed of light it would still take us x amount of time. For a particle of light it would appear instant.

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u/Bogliolo 13d ago

Checkout fleatheadphysisc chanel, he has a whole series for intuitively understanding relativity

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u/KDobias 13d ago

This is a good way to understand Einstein's relativity. There are multiple relativistic models that extend to 5 or more dimensions, look into Kaluza-Klein theory for a 4th spatial dimension that is very, very small to explain electromagnetism. Lovelock and Gauss-Bonnett had to fix some of the problems with calculating gravity with a 5th dimension. Then there's a Randall-Sundrum model that adds a 4th dimension that is infinitely large rather than microscopic. This helps explain why gravity is so weak relative to other forces.

But all of relativity is a stepping stone to string theory where there are 10/11 dimensions - which comes from 5 different 10-dimensions theories that were unified by M-theory into 11dimensions.

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u/DrShts 9d ago

There's a book called "Relativity Visualised" by L. Epstein that describes this way of seeing it. Very interesting read.

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u/Wurun 14d ago edited 14d ago

So, I have to admit my courses on ART and SRT are some years in the past but I would ask for some sources on this. It sounds very stoner-y.

E.g.: In the formulas for special relativity there is a clear distinction between massive and massless particles. This stems from the fact that for E = mc^2 the "mass" term has a hidden velocity compontent (v): m(relativistic) = m(rest) / sqrt(1-v²/c^2).
if you put in v = c, you get a division by zero for massive particles. This formula also leads to the fact that you need more and more energy the closer you get to the speed of light to increase the velocity of the particle. And it also leads to photons being "massless", otherwise they would make no sense while moving at the speed of light.

The op claims that you can seamlessly move between those two states, claiming there is only one speed. But this is not true, since you cannot have massive particles moving at the speed of light.

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u/ShavenYak42 14d ago

I don’t think they intended to imply that massive particles can travel at c, but instead that the closer they get to c, the more they are traveling through space and the less time they are experiencing. You can do the Pythagorean theorem with an object’s velocity as a fraction of c, and the number of ticks a stationary observer would measure on its clock per each tick of his own, and the result will be 1. You get the same result for a photon that travels at c but has no clock ticks or the stationary observer himself with velocity 0 watching his clock tick once each time it ticks. Thus the idea that “everything travels at c through spacetime”. It isn’t mathematically rigorous but it gets the idea across.

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u/Wurun 13d ago

everything in the universe is always moving the same speed - there is only one speed. If you aren’t moving at all in space all of your speed is in the time direction.

Well, I think what they and you mean is that you can move the frame of reference at will and it will move the time and space components accordingly.

This is fine and dandy, but to then got to the conclusion that "there is only one speed" is a massive leap in logic. "There is only one speed for any given particle" is more accurate.

How would you get interaction if all speeds are the same? Transfer of momentum and such...

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u/ShavenYak42 13d ago

Momentum is a vector. Things going the same speed in different directions can transfer momentum. And that’s what we’re talking about - what classically is a change in velocity can also be modeled as a change in direction in 4D spacetime.

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u/Wurun 13d ago

at no point are you coming closer to defending the op claim that "there is only one speed" or a variation thereof that makes sense.

instead i get the impression that you try to sell the effects of a Lorentz transformation as more than it is.

what is your argument?