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?
If you were stationary, and talking to someone moving at lightspeed, would they appear to be speaking in slow motion due to time dilation?
5 months ago by a_gee_dizzle to c/askscience
MilkToast 4 points 5 months ago
Something like this perhaps. Maybe not quite lightspeed, but surely close
a_gee_dizzle 4 points 5 months ago
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
TheTechnician27 10 points 5 months ago
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.
OwOarchist 2 points 5 months ago
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.
shoomemer 1 point 5 months ago
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.
AbouBenAdhem 18 points 5 months ago
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.
a_gee_dizzle 3 points 5 months ago
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
a_gee_dizzle 3 points 5 months ago
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
Hapankaali 13 points 5 months ago
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.
a_gee_dizzle 1 point 5 months ago
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?
Hapankaali 5 points 5 months ago
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.
a_gee_dizzle 1 point 5 months ago
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
Hapankaali 3 points 5 months ago
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.
WoodScientist 6 points 5 months ago
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.
a_gee_dizzle 1 point 5 months ago
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?
tburkhol 2 points 5 months ago
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.
a_gee_dizzle 2 points 5 months ago
I'm struggling to wrap my brain around this
frongt 0 points 5 months ago
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.
imahappyguy 5 points 5 months ago
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.
shoomemer 5 points 5 months ago
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?
Onomatopoeia 3 points 5 months ago
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.
frongt 0 points 5 months ago
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.
a_gee_dizzle 3 points 5 months ago
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
Onomatopoeia 6 points 5 months ago
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.
a_gee_dizzle 1 point 5 months ago
I see
imahappyguy 1 point 5 months ago
Oh boy, I went down a rabbit hole. This has been bothering me to the point where I called in some favors and found some public info for what the current hypothesis on "near" light speed communications is. Because light speed is impossible...for now haha. But let's use some assumptions.
First, light speed travel is possible. Second, that we have superior tracking technology. To the point where we can find the exact location of a traveling at light speed object in space.
The best option we have now is using photonics for communications. This bypasses using radio waves by using an array of lasers that are pointed toward a receiver, which is in turn pointed to the transmitter. We can see why space and light speed travel makes this difficult. It's hard to track objects in space at very high speeds with the precision needed for this level of communication.
The reason photonics is the best option is, it opens up significantly more frequencies to work with. And some frequencies travel farther and faster than others. If we ever master light speed and light speed communications, I would bet money now it's photonics. Because what's faster than light?
Let me know if you have any questions, I'd be happy to answer. Please do not ask about FTL, crazy things start happening then. <- in jest, if you wanna ask you can. Engineers I know argue about it to the point of almost fighting. But that's engineers for you.
a_gee_dizzle 2 points 5 months ago
Oh boy, I went down a rabbit hole. This has been bothering me to the point where I called in some favors and found some public info
I appreciate the dedication!
The lasers idea is interesting.
And some frequencies travel farther and faster than others.
Don't all frequencies of light travel at the same speed?
imahappyguy 1 point 5 months ago
You are correct, my sincerest apologies. Light at different frequencies can carry more/less data. I will leave my mistake, as failures are the price of progress. I will not make that mistake again and be more careful. Thank you for your kindness.
Edit: Clarification; different frequencies affect data transfer rate
CapuccinoCoretto 2 points 5 months ago
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.
musicalphysics 4 points 5 months ago
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.
a_gee_dizzle 2 points 5 months ago
So both sides of the conversation would view the other participant in the conversation to be speaking slowly?
musicalphysics 2 points 5 months ago
Yep.
a_gee_dizzle 2 points 5 months ago
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?
musicalphysics 2 points 5 months ago
Yep. Relativistic effects are generally not what we would intuitively expect.
givesomefucks 2 points 5 months ago
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
a_gee_dizzle 3 points 5 months ago
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?
givesomefucks 1 point 5 months ago
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.
a_gee_dizzle 3 points 5 months ago
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
givesomefucks 1 point 5 months ago
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.
count_of_monte_carlo 1 point 5 months ago
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...
I want to push back on this - quantum entanglement cannot be used to transmit information faster than the speed of light. The entanglement effect does allow for instantaneous correlations, but you can’t use those correlations to transmit information on their own.
For example, imagine Alice and Bob have a pair of entangled particles. The spins are anti-correlated, so if one is spin up the other is spin down. Alice measures the spin of her particle and sees its spin up. She now knows that Bob’s is spin down. She can learn this even if he is many light years away, without needing to communicate with him.
But what information was sent? If Alice or Bob wanted to communicate a simple “yes or no” to each other, how does knowing the spin of each others particles help?
What could happen is Alice and Bob could agree that spin up is yes, down is no. And Alice could measure her particles spin, then call Bob and say “my answer is the spin of your particle” to communicate “no”. This would be physically secure encryption of her answer which is a big advantage, but it is still communicated at classical speeds.
Actually methods of quantum cryptography are more complicated and involve measuring many entangled particles to prove that no one is intercepting the information.
Entertainmeonly 2 points 5 months ago
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.
gsv 2 points 5 months ago
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.
a_gee_dizzle 1 point 5 months ago
Interesting. That makes sense, thanks for explaining
Alvaro 1 point 5 months ago
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...
tristynalxander 0 points 5 months ago path: 0 23442546, hotness: undefined, score: 0, children: 0
a_gee_dizzle 1 point 5 months ago
How so?
ChonkyOwlbear 2 points 5 months ago
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.
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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.
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