I’m asking this because there is a scifi book I’m reading, and in the book there’s a scene where someone is communicating with a person in a spacecraft moving at lightspeed. I know their ability to communicate would probably not be possible, but let’s just put that aside for a second. Hypothetically, if you could communicate with someone moving lightspeed, would the time dilation make it so that they would appear to be moving and speaking very slowly relative to you?
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.
Something like this perhaps. Maybe not quite lightspeed, but surely close
Bad news: If a person was moving at the actual speed of light, from their own perspective they would arrive at their destination instantly. This means they wouldn’t have time to send or receive a message at all!
Assuming a velocity close but not quite as fast as C, yes, you would see severe differences in the speed of the communication. One party would be super slowed down and the other would be super sped up.
Bad news: If a person was moving at the actual speed of light, from their own perspective they would arrive at their destination instantly.
Another commenter here asked about an interesting set-up where the person moving lightspeed is circling around the stationary person. This is of course super impractical but it might allow them to communicate without catching up to one another instantly
Assuming a velocity close but not quite as fast as C, yes, you would see severe differences in the speed of the communication. One party would be super slowed down and the other would be super sped up.
Okay I guess that answers my question then
the angular momentum you would feel from circling someone at light speed would probably tear your limb from limb.
Assume a spherical cow in a vacuum
An infinite, frictionless vacuum?
Is there any other kind for spherical cows to populate?
Now I’m trying to image how spherical cows, or really anyone, can reproduce in a frictionless vacuum.
the angular momentum you would feel from circling someone at light speed would probably tear your limb from limb.
Limb from limb? Either those limbs are massless, or you’ve just given yourself infinite mass traveling at c with an infinite energy. You’d probably tear the fabric of reality limb from limb.
I’d probably just assume the traveler and receiver aren’t experiencing any maleffects in this situation. Else the traveler would be cooked by all of the radiation that they would be receiving all at once.
Not with a big enough circle. But I’m assuming we’re talking cosmic-scale circles here, with the circle covering an appreciable portion of the observable universe in order to make the g-forces bearable.
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.
Thanks for clearing that up
It’s simply not possible for massive objects to travel at the speed of light in any valid (inertial) frame of reference.
I know there are a lot of impossibilities baked into how this scenario is set up. But hypothetically, if you could have a spaceship travelling at the speed of light, and hypothetically if they could communicate with someone who is stationary (ignoring Doppler effect etc), what would it be like to talk to them? Would the time dilation make it seem like the person travelling at lightspeed is speaking very slow?
This hypothetical is of the type “immovable object versus unstoppable force.” The question becomes: which of the axioms of relativity do you want to discard? Yet, once you do, you are leaving the realm of physics and entering the realm of sci-fi, in which anything may be possible.
If you want to maintain any link to our current understanding of physics, there are no hypotheticals, no ifs or buts. It’s simply not possible to have a set of laws of physics consistent with relativity as we understand it, wherein massive objects can travel at the speed of light in vacuum.
Okay well let’s just stipulate that the object is travelling close to enough to the speed of light for there to be time dilation of some sort. Or maybe the object is stationary but near a black hole or something so there is time dilation from the gravity
Okay well let’s just stipulate that the object is travelling close to enough to the speed of light for there to be time dilation of some sort.
There is always time dilation between any two frames of references moving at nonzero speed with respect to each other. It’s generally negligible for everyday velocities, but it’s still there. You can find the degree of time dilation (and length contraction for that matter) in special relativity (i.e. ignoring gravity) by computing the gamma/Lorentz factor. For example, for 90% of the speed of light, the Lorentz factor is about 2.29.
Or maybe the object is stationary but near a black hole or something so there is time dilation from the gravity
In that case, it depends on how strong the gravitational effect is. The mathematics is a bit more complicated though. I would recommend to stick to special relativity if you’re learning about relativity as an interested layman.
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
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)
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.
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.
That’s incredibly cool. From a point of view of a being near that supernova, would we be moving a lot faster?
From the point of view being near the distant supernova, we are moving away from them at relativistic speed, so as much slower as they appear to us, we should appear that much slower to them.
I’m struggling to wrap my brain around this
It’s just two objects moving apart. If I drive my car away from yours, it looks exactly the same as if you had driven away from me (once you delete all the frames of reference like the ground, because there is no ground in space). The other person gets smaller at the same rate.
Have you ever been in a car or train where you couldn’t see the ground, only another car or train next to you? And you see the other vehicle moving, but you can’t tell if it’s actually them or you moving? Same thing. It’s all relative.
I’m just confused as to how we could both experience time dilation at the same time. Isn’t our time only dilated in relation to each other? So if both our times were dilated there would be no relative difference and it would look like our clocks were in sync, no?
If you sit in an ambulance with the siren blaring, and you encounter another ambulance with its siren blaring in the opposite lane, you will hear the pitch of its siren at a lower frequency as it drives away from you, from the doppler effect.
The people in the other ambulance will hear your siren’s pitch at a lower frequency as well, for the same reason.I might be missing something here. Is the doppler effect related to time dilation?
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.
One would have to be orbiting the other. I don’t think you’d get any doppler shift in that signal, because the distance between transmitter and receiver remain constant.
The fun part of that scenario is that emitted photons carry momentum from your motion. If you’re moving at a significant fraction of the speed of light, hopefully you’re using an omnidirectional transmitter.
Edit: hmm. I wonder if Terrell rotation comes into play here. Probably not in terms of the actual transmission, but it certainly would in terms of placement of a transmitter on a spacecraft.
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.
I see
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.
Where is Hank Green when you need him?
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.
Interesting. That makes sense, thanks for explaining
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.
I don’t actually know, but to my understanding…
Firstly it would be impossible because of the doppler effect and the fact that at these speeds you would go around the earth in about the same time it takes to say “Hello” , but if you ignore that:
At a difference of around 260,000 KM/S you could actually have a time dilation difference that would be perceived by the observer (the slow one) as 0.5x and anything said by the traveler would be heard by the observer in slow motion.
Again, I have no physics background, so…
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
From the link:
Though scientists still debate how the seemingly bizarre phenomenon of entanglement arises, they know it is a real principle that passes test after test. In fact, while Einstein famously described entanglement as “spooky action at a distance,” today’s quantum scientists say there is nothing spooky about it.
“It may be tempting to think that the particles are somehow communicating with each other across these great distances, but that is not the case,” says Thomas Vidick, a professor of computing and mathematical sciences at Caltech. “There can be correlation without communication,” and the particles “can be thought of as one object.”
I mean…
The alternative to them communicating but still somehow staying in sync…
Is that they have to be the same particle, that’s the alternative.
That the one electron theory isnt just real, it works on subatomic particles sometimes which in this context is orders of magnitude large…
You think that simplifies but it makes it way crazier.
for one participant in the conversation would seem like they were talking very slowly, and the other would seem to be talking very fast.
The exact opposite…
I don’t know how to put it any simpler.
Think of it as writing text on a string of ticker tape. The wider the tape, the bigger text, the faster the tape comes out by inches of length.
To relay the same info on a skinner piece of ticker tap, the text shrinks, and the tape comes out slower by length.
Regardless of the size of the text, or the speed the tape, the information is coming out at the same speed consistent speed.
But like,instead of a ticket tape, think of it as x seconds of speech.
The exact opposite…
I see
Wouldn’t it depend on whether they are moving toward you or away from you? 🤔
How so?
Like the doppler effect? Sounds coming from a thing moving toward you change differently than sounds moving away fron you.
That only works if you are speaking through the same medium. The sound waves compress or elongate. If you are talking to someone on a spaceship, the conversation would presumably be transferred from analog, to digital, back to analog, like a phone call. It’s not like if you call someone on an airplane you hear the Doppler Effect on their voice.
That gave me a hilarious image in my head thinking about a Doppler shifted phone call from a plane.
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