I know what your asking, but im picturing someone zooming by at lightspeed, screaming their message. The person on the ground/stationary just hears the faintest dopler effect as LS person speeds by.
this post was submitted on 27 Apr 2026
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Anything moving at the speed of light in one reference frame is moving at the speed of light in every reference frame—including its own.
Which is to say, it’s not a real reference frame at all—the experience of moving at the speed of light would be instant teleportation with no subjective elapsed time. So trying to talk to someone moving at light speed would be like talking to a still image.
the experience of moving at the speed of light would be instant teleportation with no subjective elapsed time
I thought time slows down when you approach the speed of light though
Only in comparison to (relative to) others. A photon from the sun experiences no time at all between leaving the sun and landing in your eye but we perceive it as eight minutes.
Oh I see. That makes sense. The fact that it's experiencing less time is why, relative to us, it's time seems to be running slower
Physicist here. Many common misconceptions in the comments.
- No, someone traveling at light speed won't arrive "instantly" or anything of the sort. It's simply not possible for massive objects to travel at the speed of light in any valid (inertial) frame of reference. Any system that does travel at the speed of light (e.g. a photon) does not have a frame of reference in which it is at rest - instead, it moves at the speed of light in all frames of reference.
If the other person travels at some speed (just) below the speed of light, the signal they send will be Doppler shifted/time dilated according to their relative velocity.
- No, quantum entanglement cannot and never has been used to communicate faster than light. See: no-communication theorem.
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Yes. Distant galaxies that are moving away from us at relativistic speeds exhibit measurable time dilation in their inner workings.
How would you even measure time dilation in a distant galaxy? Consider standard candles like 1a supernova, which explode with near uniform power. These supernova can be observed from intergalactic distances. Gather data and record the times for various supernova explosions. You'll find that the same types of explosions take longer in more distant galacies, and that the extra time is exactly what relativity predicts.
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So, I would assume that you would be communicating through radio waves. If an object broadcasting a signal moving at the speed light away from you, I would further assume you experience a severe Doppler effect. To the point that I don't think you would experience anything coherent. You would receive small packets of information at a time, scattered across several million years.
This is just my initial impression on the fly, do not take this as any sort of gospel. I also did some communcations work for a time. So, this is tickling my brain and I might spend the rest of my evening in my books.
If we assume that the person moving at light speed is going in circles about the stationary person instead of linearly away. Would the radio waves be doppler shifted if transmitted orthogonally?
Ooh, interesting point.
I suspect all EM would be shifted according to the angle relative to the target - so at exactly 90° It would be "half shifted" - or zero. (Assumption based on blue/red shift of light).
I'm assuming the traveler is at a percentage of C, not at C (I think being at C is a completely different scenario, like would any EM escape the traveler?).
But I'm only an armchair quantum physicist (I've read a few books over the years). Look forward to what someone who understands Quantum Weirdness has to say.
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If an object broadcasting a signal moving at the speed light away from you, I would further assume you experience a severe Doppler effect.
In principle you could have equipment that cancels out any doppler effect, no?
I also did some communcations work for a time. So, this is tickling my brain and I might spend the rest of my evening in my books.
That's awesome, let me know if you find anything interesting
To cancel the effect of someone moving away from you, the equipment would simply hold the transmission until you received the whole thing.
Like waiting for someone to finish leaving a message on an amswerimg machine before hitting play.
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I remember in the game series Mass Effect they spoke of being able to break the EM communication barrier problems. They used a quantum entangled pair. Wiggle one, and the other instantaneously adopts the same position anywhere in the universe.
FTL travel needs FTL coms and radio ain't that. Star Trek handwavium called it subspace. Both of science fiction, but hey, isn't that what all this is about.
Another physicist here. I see that the issue of traveling at the speed of light has already been addressed. So I’ll ignore that bit. Otherwise, yes, the time dilation would make it appear to an observer that the traveler is speaking slowly. It would also make it appear to the traveler that the observer is speaking slowly.
So both sides of the conversation would view the other participant in the conversation to be speaking slowly?
Yep.
That's interesting. How would this work with length contraction? Would both sides view the other participant in the conversation to be experiencing length contraction?
Yep. Relativistic effects are generally not what we would intuitively expect.
Okay, so I guess a takeaway here is that each person only observes relativistic effects in the other conversation participant, but not in themselves.
I'm still about confused though with how this would work with time dilation. Like, imagine a scenario where I go in a spaceship and approach lightspeed for a while and then come back (for me, subjectively) a short while later, only to find that I had grandkids that were all senior citizens. It makes sense in that scenario that, if I were to view Earth, time would seem to be moving slowly over there. But I don't understand why, if people on Earth were observing me, they would also observe my time to be going very slowly. Intuitively it would seem that they should observe my time as moving very fast, since relative to them it is
Correct. Everyone thinks their second and meter are unchanged. Everyone else’s second is slower and their rulers are compressed.
Hard to explain the details without using math. Relativity is not intuitive as we don’t encounter relativistic effects in everyday human life.
Relativity build upon the fact that there are no absolute reference frames. If time was absolute then sure, one person would appear slow while the other appears fast. But it isn’t absolute, it is relative. This means outcomes need to be symmetric. So a stationary observer checking out a spacecraft going fast is the same as going fast while observing a stationary spacecraft.
That's interesting, thanks for explaining.
This means outcomes need to be symmetric.
I guess I'm just confused as to how you can have situations where someone ages faster than another due to time dilation. That doesn't seem to be a symmetrical outcome. Like this scene in Interstellar (which, as I understand it, is scientifically accurate). In this scene Matthew McConaughey goes away for 3 hours and finds that, due to time dilation, his kids have aged 23 years. There doesn't seem to be a symmetry here. Because Matthew McConaughey aged slower than his children, it would be weird if, from his point of view, his kid's clocks were ticking slower than his (his kids can't also think that he has aged 23 years in 3 hours). So how do you resolve this lack of symmetry with the requirement that outcomes need to be symmetrical from both these reference frames?
Granted, in this scene Matthew McConaughey ages due to gravitational time dilation. Is that somehow make things different? Would a similar scenario not be possible with time dilation solely caused by travelling at very fast speeds?
(Please let me know if I'm not making sense with these questions, and I'll try to reword them)
Well, parts of interstellar are accurate. :) That being said, time dilation due to gravity is real. Go someplace heavy for awhile and then leave and you will travel far into the future. The spaceship-observer example is special relativity. The gravity thing is general relativity. I’m not sure I have a non math explanation here so, simply put, time dilation due to gravity is different.
You can get a similar outcome by going somewhere real fast, then turning around, and going real fast again back towards the start. In the rocket frame that may take, say 10 years, but more years will have passed by on Earth.
You may think this breaks the symmetry I brought up earlier, and it does, but that symmetry breaking occurs when the rocket accelerates a whole bunch turning around and heading back home. On the outbound journey though the rocket will think the earth clock is slow, and vice versa. Similarly, on the return journey the same thing occurs. During the acceleration phase though things gets real weird. Or weirder I should say.
Well, parts of interstellar are accurate. :)
Wait, you're telling me it's not scientifically accurate to say that love is the one thing we're capable of perceiving that transcends the dimensions of time and space? /s
You may think this breaks the symmetry I brought up earlier, and it does, but that symmetry breaking occurs when the rocket accelerates a whole bunch turning around and heading back home.
Ah, okay that answers my question then. It's the acceleration and deceleration that changes things. Thanks for bearing with me so far and answering all my questions!
You are welcome. Thanks for the interest in physics.
Where is Hank Green when you need him?
Think Star Trek uses a stability feild so the time dilation is void. I'm trying to remember but they actually turned it off in one episode and jumped to like Jupiter and back to close the time gap. So, they could be using a similar idea of technology.
Exactly at the speed of light, the γ-coefficient would be infinite and so would be the time dialation. The eigen time of the moving person would thus be infinitely slower than the non-moving person. From the perspective of the stationary person, the time of the moving person would stand still and thus the person would never say anything. Very close to the light speed, when the coefficients are large, this problem eases but persists. The stationary person would have to wait for very long (and use a massive Doppler shift of the moving signal) to perceive something. At the end of the conversation, it will have lasted much longer for the stationary person, spending years on this. The twin paradox would basically kick in as well. If the moving person is at a speed too close to the speed of light, the stationary person might die before the conversation is over—assuming the stationary person is not immortal. That is kind of a very slow motion, yes. What a dedication, spending a lifetime on a person who can’t slow down ;-) Funny enough, from the perspective of the moving person, the effect is reversed.
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Huh?
What manner of communication moves faster than light?
It would have to be some sort of entanglement, And I think the entanglement would also normalize any time dilation. There's not exactly a way to test that yet, it's all hypothetical.
But it should just cancel out and be like you're talking to someone in the same room.
Like, there's no way for the communication (in any form) to go faster than light continuously. If it was two stationary points than wormholes or other stuff could work.
But moving at light speed, it has to be entanglement
What manner of communication moves faster than light?
It would have to be some sort of entanglement, And I think the entanglement would also normalize any time dilation. There’s not exactly a way to test that yet, it’s all hypothetical.
Yeah, in the book I'm reading this conversation is facilitated through quantum entanglement. Though I know in real life that wouldn't be possible because entanglement still can't be used to transmit information faster than light.
And I think the entanglement would also normalize any time dilation.
This is interesting. How does that work?
Though I know in real life that wouldn’t be possible because entanglement still can’t be used to transmit information faster than light.
The entire point of it, is that it can...
How does that work?
It's still entangled...
Everything about the two still has to be exactly the same, even adjusted for time dilation.
So if one is experiencing 10x the speed, 1 second of its "input" takes 10 seconds on the slow side to "output". The 1/10th speed replies and 1 second there is "output" as 10 seconds, just like it's relatively spinning 10x the speed as the reference particle.
Again, technically hypothetical because we can't build anything like that yet to actually see if it wouldnhappen. It's just one of those things where it just has to work out like that logically even though we can't see it.
Though I know in real life that wouldn’t be possible because entanglement still can’t be used to transmit information faster than light.
The entire point of it, is that it can…
Intuitively it seems like it should, but apparently that doesn't work.
So if one is experiencing 10x the speed, 1 second of its “input” takes 10 seconds on the slow side to “output”. The 1/10th speed replies and 1 second there is “output” as 10 seconds, just like it’s relatively spinning 10x the speed as the reference particle.
Ah okay I see what you mean. So I guess, for one participant in the conversation would seem like they were talking very slowly, and the other would seem to be talking very fast. But they could still be in sync, they would just perceive the time differently
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