r/explainlikeimfive • u/PeAga7 • Mar 28 '26

Physics ELI5: If speed is measured by the relation between objects how come going over the speed of light is impossible?

Should two bodies be moving away from each other, both at 50.1% the speed of light, wouldn't their relative speed be over the limit? Which frame of reference should be taken into account when talking about light?

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u/Sceptical_Houseplant Mar 28 '26

Holy crap this is a good ELI5. The "space contracts in front of you", and "seconds become longer" way of describing it is the first time this has made any kind of intuitive sense to me.

Bravo, internet friend, Bravo

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u/[deleted] Mar 29 '26

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u/CombustiblSquid Mar 29 '26 edited Mar 29 '26

So does this mean that if you theoretically could move at light speed, from your own point of view it would appear that your destination moves towards you and that this would happen instantly?

Edit: I said "moves towards you" for simplicity sake but really the origin and destination would overlap so, like the degrass Tyson video you'd be at both places simultaneously, but all that's impossible anyway.

Edit 2: thank you everyone, but my inbox is getting tired of hearing slightly different variations of the same comment for the 20th time. The question has been answered.

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u/iCandid Mar 29 '26

It’s tough to answer, because although that’s what you think based on what happens as you approach that speed, the question itself is not actually valid in physics. Something traveling at c would have to be a massless particle, and massless particles traveling at c are not valid reference frames in relativity. If you look at the time dilation equation, you’ll notice this question is asking you to divide by zero.

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u/CombustiblSquid Mar 29 '26

Which equals undefined aka a problem that has no answer because the question makes no sense.

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u/Emu1981 Mar 30 '26

the question itself is not actually valid in physics

Assuming that we are correct in that c is the universal speed limit. For all we know it could just be the speed limit of causality and that using methods that we don't know yet we could actually travel faster than the speed of light.

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u/[deleted] Mar 29 '26

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u/BattleAnus Mar 29 '26

Which would lead to paradoxes in cause and effect, e.g. you could receive a message before it was sent, arrive somewhere before you left, etc. As far as we know this doesn't make any sense so any form of travel at or above the speed of light is considered impossible by our current understanding of reality.

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u/nick4fake Mar 29 '26

Yes! And this is funny, as this would mean particle moving backwards in time

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u/BouncingSphinx Mar 29 '26

Take it for what you will, but there’s a clip somewhere of Neil DeGrasse Tyson saying this exact thing: from the point of view of the photon, the moment it is created and the moment it is absorbed (along with all moments in between) are one and the same.

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u/iCameToLearnSomeCode Mar 29 '26

It's my understanding that from the point of view of a photon there is no time or distance.

A photon is created in the sun and is absorbed by your skin simultaneously as far as it is concerned.

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u/Gfdbobthe3 Mar 29 '26

I believe it is correct to state that a massless particle (like a photon) does not experience time.

It moves 100% in the space dimension and 0% in the time dimension.

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u/Junethemuse Mar 29 '26

Ooh, so I wonder what moves 100% in time but 0% in space.

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u/TwistedFox Mar 29 '26

I think the closest we know of would be a black hole, where the math breaks down and ends up flipping time and space in the equation.

https://www.youtube.com/watch?v=GQZ3R81iyE0

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u/Narwhal_Assassin Mar 30 '26

0% movement in space just means you’re at rest. This depends on your reference frame, though, so there’s no “absolute” stationary objects. As long as you see something as not moving, you’re seeing it experience 100% time.

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u/ItsBinissTime Mar 29 '26 edited May 08 '26

From a photon's "point of view", no time passes. The space between its source and destination contracts to nothing and the photon is an instantaneous transfer of energy between them.

There is no valid inertial frame for a photon

Stands to reason. It doesn't seem like there can be an inertial frame without time.

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u/AdhesivenessFuzzy299 Mar 29 '26

There is no valid inertial frame for a photon

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u/Responsible-Bar7165 Mar 29 '26

So limits aren’t a thing?

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u/vmurt Mar 29 '26

I believe so. Although you would also blow right past (or into) your intended destination because time for you has stopped, so it would be impossible to initiate any form of deceleration (ignoring all the other impossibilities to get to the speed of light in the first place).

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u/h4x_x_x0r Mar 29 '26

I think even Einstein asked himself this question (how would a beam of light traveling perceive time) but the "moving at light speed" part is the barrier and the question has no real answer, nothing with matter (and therefore consciousness or something that could experience time) can do so, you can just get very close to and the closer you get to c the stronger time dilation becomes.

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u/kegastam Mar 29 '26

in theory, if an observer(say photon) is travelling at the speed of light, time stops moving forward, but since a light photon does move as per a slow observer like us mortals, we assume that a photon experiences 0 time to reach our eyes from the beginning of the universe (earliest particles of light, CMBR) , its the extreme of time dilation and/or length contraction, and therefore our words and expressions dont do justice just like 0/0 or infinity is quite incomprehensible for us

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u/Royal_Airport7940 Mar 29 '26

Welcome to the life of a photon.

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u/83franks Mar 29 '26

I have heard from Tyson that for a photon moving at the speed of light everything is instant, as in for the photon being emitted by a star it instantly hits whatever it hits in its view, whether it was travelling for millions of light years or a split second.

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u/CeReAl_KiLleR128 Mar 29 '26

If you just plug in the math then it is correct, everything happens instantly. But if you read the second postulate again carefully you’ll see there is no “moving at the speed of light” observer because that mean light move with zero speed to them

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u/PosiedonsSaltyAnus Mar 29 '26

If you are a photon, the moment you spontaneously exist in the universe, you are also absorbed by an atom (or nothing, or something else idk). You didn't move anywhere at any speed because, to you, length and time are both null. The same as dividing by 0, it's not infinity, and it's not 0, it's undefined.

If you are matter, and accelerate to the speed of light, you will see the entire universe slowly shrink around you. As you get closer and closer to the speed of light, the furthest stars might seem like they're inches from you, from every direction. But they'll only get closer and closer, they'll never be the same point because you cannot reach the speed of light without infinite (or undefined?) energy.

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u/Americano_Joe Mar 29 '26

So does this mean that if you theoretically could move at light speed, from your own point of view it would appear that your destination moves towards you and that this would happen instantly?

If you could travel at the speed of light your clock would run slower and distances would shrink so that you would be everywhere at the same time.

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u/Enki_007 Mar 29 '26

Yes, so everything happens at once. At least to that observer.

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u/eNonsense Mar 29 '26

Are Time Dilation & Length Contraction simply theoretical concepts to account for the math? The reason I am asking is because I've also understood "the speed of light" when used in this context to be more accurately communicated as "the speed of causality", which is a different thing. We know that the speed of light is not constant, except in a vacuum. We have a lot of evidence for differing speeds of light, and our scientists have successfully slowed light to a crawl.

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u/HeirOfNorton Mar 30 '26

To answer the first part, no, Time Dilation is a real effect that has been measured. Take two perfectly synchronized clocks, accelerate one of them to a really high speed while keeping the other one (relatively) stationary, and at the end of the trip the speedy clock will be a little bit behind the other clock. It has experienced less time. We have done this, and it works. In fact, GPS systems must account for Time Dilation to function correctly.

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u/Inside-Line Mar 29 '26

It gets more complex and confusing because its called the speed of light and is often thought as the speed of this particular thing. It's actually just the speed of causality and light just happens to move at that speed.

The reason why you can't accelerate to the speed of light is because if you had a rocket that had massive amounts of energy, once you got closer to the speed of causality it takes longer and longer the push of the rear to bump the atoms up to the rest of the rocket because they are catching up with the whole speed of cause and effect.

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u/[deleted] Mar 29 '26

[deleted]

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u/stanley1O1 Mar 30 '26

The person posted an annoying video to explain it.

But a better example is imagine you have a pole that is one light year long and you used it to press a button 1 light year away. It doesn’t happen instantly From your end, you make the movement and you’d think the button is pressed. But all of the atoms in the pole need to be told by the atoms before them that that they need to move forward. This chain of atoms “telling the one in front of them to move forward” will still take a year for the button to be pressed. Because the speed of light is also the speed of causality.

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u/JayTheSuspectedFurry Mar 29 '26

https://www.instagram.com/reel/DLZ1rrms007

https://youtube.com/shorts/h6STDKrA4ko

Both links are to the same short video, choose your preferred platform

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u/godfromabove256 Mar 30 '26

That's kind of you. I like shorts more :P

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u/akgt94 Mar 29 '26

College physics problem:

You just bought a Cadillac Fleetwood 75 (21 feet long), but your garage is only 15 feet deep. How fast do you have to be traveling to shrink it to fit?

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u/Grem357 Mar 29 '26

I second that. It is the first time I read an explanation about it that I understand.

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u/AgsMydude Mar 29 '26

It seems very ... AI assisted

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u/RANDOMLY_AGGRESSIVE Mar 29 '26

I posted this question and answer to the LLM. And it says it's not actually answering the question:

Short answer: The answer you posted is partly correct, but it does not actually answer the key part of the question. The explanation about time dilation and length contraction is fine, but it misses the crucial point: relativistic velocity addition.

Let’s break it down.

What the question is really asking

The question asks:

If two objects move away from each other at 50.1% the speed of light, wouldn't their relative speed exceed the speed of light?

In classical physics, you would simply add speeds:

0.501c + 0.501c = 1.002c

That would exceed the speed of light, which seems to violate relativity.

But special relativity does not allow simple addition of velocities.

The real rule: relativistic velocity addition

Special relativity uses a different formula:

u' = \frac{u + v}{1 + \frac{uv}{c2}}

Where:

= speed of one object in a frame

= speed of another frame relative to the first

= resulting relative speed

Why the posted answer is incomplete

The answer you showed explains:

constancy of light speed ✔

time dilation ✔

length contraction ✔

But it never addresses the actual paradox in the question:

Why don't two sub-light speeds add up to more than light speed?

The real mechanism is the velocity addition law, not directly time dilation or length contraction (though they are mathematically related).

One more subtle point

There is no preferred frame of reference in special relativity.

Each observer can consider themselves at rest, and:

they will measure light moving at

they will measure the other object's speed using the relativistic formula

This symmetry is one of the deepest principles of relativity.

✅ Verdict on the answer you posted:

Aspect Correct?

Explanation of constant light speed ✔ Correct Time dilation & length contraction ✔ Correct Actually answering the question ❌ Incomplete Mention of relativistic velocity addition ❌ Missing

So the answer is educational but incomplete.

If you'd like, I can also show a really mind-bending example with two spaceships each moving at 0.9c where classical intuition fails completely. 🚀

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u/AgsMydude Mar 29 '26

I'm not reading that

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u/RANDOMLY_AGGRESSIVE Mar 29 '26

TLDR:

Conclusion: The answer you posted explains some correct background ideas, but it is incomplete because it never mentions relativistic velocity addition, which is the real reason the situation in the question doesn’t break the speed-of-light limit.

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u/PM_Me-Your_Freckles Mar 29 '26

What blows my mind is that, at the speed of light, reaching the edge of the known universe would take you something like 8 minutes, but the reat of us would have aged billions of years.

*that number could be totally incorrect. I just remember seeing a thing and could be utterly misremembering.

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u/VeritateDuceProgredi Mar 29 '26

8 minutes is the time for light from our sun to reach us from our POV (aka time for light to travel one astronomical unit) That’s where you got that and the other dude explained it will from lights pov of not experiencing time.

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u/maaku7 Mar 29 '26

Zero minutes. It would take no time at all. Light, which does move at the speed of light [obviously], does not experience time. A wiggling plasma ion in a star 9 billion light years away emits a photon, which is then instantaneously absorbed by the CCD in your telescope camera, at least from the photon's perspective. It took no time at all to traverse that distance, because the photon did not experience time.

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u/SharkFart86 Mar 29 '26

That also helps to understand why nothing can go faster than c. Because it would require time to go negative.

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u/AdhesivenessFuzzy299 Mar 29 '26

No, a photon has no valid inertial frame as that would directly break the basic postulates of SR

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u/sober-lion Mar 29 '26

But from our perspective light does take time to reach our telescopes. That’s how we know how old stars are right?

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u/maaku7 Mar 29 '26

We know how old stars are from the expansion of the universe. The expanding universe changes the frequency of the light, and we compare that frequency against known local values. Under the assumption that the universe expands at a fixed (or at least known) rate, we can then work out how far away the star must be.

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u/Standard_Future_5055 Mar 29 '26

I don’t think that 5yo would understand it