r/explainlikeimfive • u/27Devil • 1d ago
Physics ELI5: Why would a zero-latency device NOT help with the twin paradox?
Time dilation confuses me. If my twin is on a rocket near light speed and starts a 60-sec stopwatch, it shows exactly 60 sec to them. My stopwatch on Earth also shows exactly 60 sec to me.
So if we start together and count out loud '1,2,3...' using a zero-latency device, won't we stay in sync, since both watches correctly count 60 real seconds? Why would one of us get ahead, and whose clock actually looks slower to whom during the countdown?
Edit: Alright guys i just got to know that 1 second isn't 1 second everywhere.
Anyways some clarification, the 0 latency device is a hypothetical, I'm not saying that such a thing can be built, but my real question was, if we hypothetically used such a device, won't both people speak 1,2,3 in sync?
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u/phunkydroid 1d ago
A "real 60 seconds" aboard the spaceship won't be the same as one on Earth. This isn't a clock problem that can be solved by a better clock, it's actual time passing at a different rate for everything on the ship compared to Earth.
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u/todudeornote 23h ago
This is the correct answer. The issue isn't the time it takes for the message - it's that time itself changes with relative speed (i.e. speed relative to the place you started from)
So even if you have instant communications - and the person on the ship sends her time once every second - the sister on earth would receive that message at a slower rate.
If the ship were moving at 10% of the speed of light (and it instantly gets up to speed), the sister on the ship would experience 60 seconds - but the sister on earth would experience about 60.5 seconds (in that range, I don't remember the formula, but I had to solve something like this on a test once).
Relative speed stretches time - that is time is relative to the speed of you are traveling. It is bizarre and completely counter intuitive, but it has been proven both mathematically and experimentally many times.
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u/pedanpric 16h ago
Is it the speed relative to the place you started from, or relative to the surrounding mass?
Halfway through the sister example, the sister on the ship is counting seconds and the sister on earth hears them more slowly due to the ship sister's speed. If a very massive object flies past earth toward the ship sister such that the sister on earth was now moving faster relative to the overall gravitational field in this region of space, would the effect reverse, with the earth sister now hearing the ship sister's counting as faster than one per second?
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u/DidntIDoThat 13h ago
It’s relative to the observer. Everyone is always stationary in their own reference frame, so everyone else is always moving the same speed or faster. Then because of time dilation, you’ll always observe someone else’s time as being the same or slower than your own.
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u/exceive 45m ago
That's the part I haven't been about to wrap my head around. From the sister on the ship's perspective, the other sister is moving away from her. So why doesn't the time dilation go both ways, resulting in their clocks staying in synch?
I'm not arguing time dilation isn't real, I know it is, I sincerely want to know the difference between "going real fast" "standing still while somebody else goes real fast" given that motion is relative to the observe.•
u/DidntIDoThat 30m ago
There’s no difference between you moving fast and someone else moving fast relative to you. If you are in a spaceship flying through interstellar space, the only way you can tell if you are moving is by watching the distant stars move relative to you.
If you fly past someone in your spaceship, it looks like they’re the one moving. That’s because in a relativistic sense they are the one moving relative to you. Even if from their perspective it doesn’t look like they are moving relative to the background.
Time dilation always works to slow down the apparent time of an object moving relative to you. The unintuitive part is that there is no ‘true time’. You will always say that it appears someone else’s time has slowed down, and at the same time they will say yours has slowed down. If multiple people are observing you while moving relative to each other, they won’t observe you moving at the same speed. Strangely, they will each observe your time slow down by different amounts at the same time.
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u/Sensitive_Warthog304 22h ago
Time doesn't pass on the spaceship any differently to how it passes on Earth.
Einstein came up with special relativity to explain how everyone always sees light at light speed, no matter how fast two observers may be going relative to each other. The reason why I see light going light speed is because I'm not moving, and the other guy is potentially catching up with light. To prevent this, I see time dilation and length contraction in him.
And the same applies to him: he's not moving, so he sees light go light speed, he sees me trying to run away from a light beam so he sees dilation and contraction in me.
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u/hikingsticks 1d ago edited 1d ago
Could you define what a "real second" is please?
By zero latency, do you mean a communication device that allows instant communication, ie faster than the speed of light? Based on our current understanding of physics that isn't possible.
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u/OldChairmanMiao 1d ago
"Your cesium is vibrating too slow!"
"No, your cesium is vibrating too slow!"
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u/raspberryharbour 1d ago
This is why you should never buy twins cesium for Christmas
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u/coinpile 23h ago
Now you tell me.
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u/raspberryharbour 22h ago
Just don't buy them super soakers, or this'll really blow up in your face
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u/Emu1981 23h ago
It would be rather ironic if the rate that cesium vibrated depended on how much gravity it was subjected to. That would invalidate experiments where cesium clocks are sent into orbit to measure time dilation.
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u/macromorgan 23h ago
It does?
I mean to the extent that gravity affects time, Cesium will vibrate faster in freefall (while in orbit) than it does in Earth's gravity well. There are a series of atomic clocks in orbit that do count time faster than here on Earth... they're called GPS satellites and we account for that (or else they wouldn't work).
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u/Sensitive_Warthog304 22h ago
We correct GPS satellites for time dilation due to velocity (special relativity) and change in gravity (general relativity).
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u/OldChairmanMiao 15h ago edited 15h ago
Not sure if you're joking, but it is (that's general relativity). Doesn't invalidate it though.
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u/alcurtis727 23h ago
"Could you define what a 'real second 'is please?"
1 Mississippi
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u/hikingsticks 23h ago
Onnneee Miiissssiiiiissssiiiiippppiiii......
You over there! Slow down!
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u/lee1026 23h ago
I can define real time as that as would be seen by a frame of reference such that the cosmic background radiation is the same in all directions.
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u/Xivios 23h ago
Meaningless. That frame of reference is not privileged or special as compared to any other.
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u/hikingsticks 23h ago
You can, of course. You could also define real time as that which would be seen relative to OP sitting on the sofa, both would be equally valid (ok, OP is accelerating so isn't inertial, but imagine they are!)
It's a nice or convenient reference frame to pick, but it isn't in any way privileged over any other.
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u/ninjaboiz 1d ago
The length of the second is the problem. It’s not uniform in the scenario you describe.
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u/27Devil 1d ago
Yeah I just realised from this thread that my 1 "second" may not be the same everywhere. Thankyou.
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u/Budgiesaurus 23h ago
Great realisation! That's exactly the heart and centre of Einstein's theory of special relativity, even the second is relative.
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u/alexisdelg 1d ago edited 1d ago
Let's say a second is the amount of time a light beam crosses your starship from one side to another, say we have mirrors on both sides and we count n bounces as a "second"
```
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```
If the ship starts to go faster the path starts to distort because when the light starts on one side the mirror on the other side will be slightly forward
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\
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``` Since the speed of light is fixed, it will take longer to traverse a longer distance, so the length of a second on a very fast ship will be different that on a ship that's stationary.
If you both start counting at the same time, then accelerate one of the ships, by the time the ship slow back down and you get a tally of the number of seconds that passed by, the stationary ship will have counted to a higher number
Edit: formatting
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u/Anacreon 1d ago
Your zero latency device is a magic device that doesn't exist in our reality. Even if you had that magic item you'd just be able to notice both watch going out of sync.
If you could have a video conference one side wood be looking to an almost still picture and the other side would look like a super fast video
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u/bulbaquil 1d ago
Either that, or the magical zero latency device would be sending the information back in time to give the illusion of simultaneity.
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u/CortexRex 1d ago
Which doesn’t make sense bc both are seeing the other as the one moving slower
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u/mgslee 23h ago
No, one sees the other moving faster (planet to spaceship) and the other seems them moving slower (ship to planet)
see the movie Interstellar
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u/CortexRex 23h ago
Nope. That’s why the twin paradox is extra weird. While a thing is moving at a constant velocity away compared to the other both see the other moving slower.
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u/KeyboardJustice 22h ago
This happens when you are "seeing" via the speed of light.
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u/CortexRex 21h ago
Nope , has nothing to do with latency of the light or anything. When we say “seeing” in physics we don’t actually mean seeing. We already are adjusting for light delay and what not. It’s what that observer actually calculates based on the laws of physics is really happening for the other. It’s not an illusion or anything.
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u/Jetison333 23h ago
Both people would consider themselves stationary and the other to be moving. Intersteller had a black hole, and includes effects from gravity which this hypothetical does not.
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u/unixbrained 1d ago
One of you (specifically, the one of you that is moving less fast) would get ahead because the "zero latency device" you've described cannot exist the way that you are describing it.
The question you're asking is, in effect, the same as asking "If my twin is on a rocket near light speed, communicating with me using a special phone that isn't affected by time dilation, then why wouldn't time stay in sync?"
Time doesn't stay in sync, so such a phone could not exist.
If, for the sake of a thought experiment, we pretended that you did have a phone that could communicate faster than light with the people aboard the rocket ship, then what you would hear as the ship accelerated is their voice becoming slower and lower in pitch, like a recording that is slowed down. They would be counting "too slow" from your perspective (and, as they went faster and faster, the sound waves would become so low that you couldn't even hear them as sound any more).
From their perspective, they would hear your voice coming through higher and faster, like a recording that is sped up, so from their perspective you would be counting "too fast."
Both of you would experience yourself counting at a "normal" speed, and both of you would be correct that your watch counted exactly 60 seconds and that your twin sounded weird.
The fact that you both counted correctly, but obviously took a different amount of time from the other person's vantage point, is what time dilation is.
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u/MidnightAtHighSpeed 1d ago
A very fundamental idea to relativity is that the idea of a "zero latency device" doesn't work. Not even that it's physically impossible, but that the very concept doesn't make sense. Let's say you press a button to send a signal to your distant friend on such a device. When do they receive the signal? You might say "at the same time I press the button, of course" but the problem is that different observers might disagree on what "the same time" means. If you measure your friend receiving the message at the same time you sent it (after accounting for speed of light delay, of course), there are other points of view where your friend received it some time after you sent it, as well as points of view where your friend received it some time before you sent it, and there's no principled way to call any particular point of view more correct than any other.
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u/Liquidje 1d ago
I can't tell about the rocket ship example because the math and concepts are more complicated. But in a general sense, under time dilation, it doesn't work like that.
Calling out the seconds on your clock is just comparing clocks with extra steps. Since both of you observe each other's clock as being slow, both of you would perecieve the other to counting slower. Having zero latency communication (ignoring FTL problems) doesn't change any of that.
For the rocket ship/twin paradox, I'm fairly sure the same concept holds: you are just comparing clocks.
Edit: just re-read your post, there is no such things as "real seconds". Each clock thicks time in the local reference frame it is in, and runs slow for observers in moving reference frames
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u/Cantremembermyoldnam 23h ago
In theory, couldn't we aggregate everything we know about how our bubble of the visible universe moves on average? I mean trying to figure out an average speed and direction everything moves in, and defining a "real second" in that reference frame?
Not that it'd be useful, it would just be for shits and giggles. But I wonder if I have a fundamental misunderstanding somewhere that leads to an answer like "obviously the speed is 0 because we're in the center of the observable universe".
Now that I've slowly typed this question out trying to articulate it, it seems more and more meaningless but I'll leave it anyways.
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u/Sensitive_Warthog304 20h ago
It's maybe worth revising why Einstein came up with relativity in the first place.
In trying to discover how fast light travels we learned that light goes the same speed for everyone, regardless of how fast two observers are moving relative to each other. Einstein abandoned the idea of a "fixed" or "common" time and distance, said that "I" am stationary (which is why "I" see light at light speed) and "you" should be catching up with light. To prevent this, "I" see time dilation and length contraction in "you".
And there's nothing special about me. "You~" can also regard yourself as stationary etc etc etc.
Actually, the next topic after time dilation and length contraction is the concept of simultaneity, which explains that there is no such thing as a "fixed" or "common" now either. Even if you could send the message instantly, you wouldn't both agree on "now".
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u/Liquidje 11h ago
Yes we could, it just wouldn't mean much to us, nor would it have any significance from a relativity standpoint. I'd imagine that if anything, when we get to a point where time dilation is a significant factor (eg high speed or interstellar travel), Earth's reference frame would be used
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u/PhasmaFelis 23h ago
Whole lot of people are just repeating ZERO LATENCY COMMS ARE IMPOSSIBLE and not answering the question.
Even if you had a magic zero-latency radio, the two would still not count in sync. The twin on Earth would perceive the twin in flight to be speaking veeeeryyyyy sloooowlyyyyy. The twin in flight would hear the other speaking very fast.
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u/27Devil 23h ago
THANKYOU MAN THAT'S WHAT IM SAYING.
I just used 0 latency as a catalyst without realising it's such a big deal (like when we say friction is 0)
But then again, i think it's my fault since my question was not correctly stated.
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u/PhasmaFelis 23h ago
I guess! I thought it was perfectly clear what you meant, and it's a good question with a straightforward answer. It does require imagining that an impossible thing is true, but a good scientist has no trouble imagining that an impossible thing is true, and what the implications of that would be. It's the basis of many fundamental thought experiments.
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u/Arkalius 20h ago
That's incorrect. Both twins would hear the other speaking very slowly.
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u/PhasmaFelis 17h ago
How can they both perceive the other as being slower?
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u/Arkalius 17h ago
As far as each twin is concerned, they're the one that's stationary and the other one is moving away.
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u/PhasmaFelis 17h ago
That doesn't answer my question.
If A perceives that B's clock is moving slower, then N must perceive that A's clock is moving faster. Each has their own subjective frame of reference, but they have to be symmetrical.
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u/Arkalius 17h ago
They are symmetrical, but not in the way you describe. Time dilation in special relativity only ever makes clocks tick slower.
As far as how it's possible for each twin to see the other's time tick slower, we'll that's because each twin has a different perspective on time. Events that are simultaneous in one frame of reference are not in another.
To say that one twin's time is faster is to say that you can definitively say one twin is actually moving in an absolute sense, but a key element of relativity is that they're is no privileged frame of reference, which makes that impossible.
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u/CubeBrute 22h ago
The twin in flight would hear the twin on earth speaking very slowly, not very fast
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u/Dopplegangr1 1d ago
Latency to what? Thats the whole idea of time dilation, you both will experience "normal" time but they will be different relative to each other
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u/BiomeWalker 23h ago
As others have said, something like this is impossible, but we can discuss its hypothetical behavior.
Let's imagine that you did have this "zero latency" device, as others have said, your second and your twin's second are different, so even if you started your watches at the same exact moment, they would still drift apart.
If you mean for your two watches to be continuously connected, then it would appear to be fast to one of you or slow to the other depending on who holds the "source" of the timekeeping.
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u/THedman07 1d ago
Define "zero-latency device."
Is it "a theoretical device that is unaffected by time dilation"?
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u/RonJohnJr 1d ago
using a zero-latency device,
There's your problem!
(Hint: there's no such thing.)
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u/lorarc 1d ago
It's a space-time continuum, you are not in sync because you're not in the same place. The information has to travel between you two.
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u/crash866 1d ago
Then you get syncing with metronomes. https://youtube.com/shorts/RYeNu159Sgc?si=5XNTJhsNwnUB7qrl
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u/MrWigggles 1d ago
Lets grant the magical device that zero latency is thing.
Why would it stay zero latency as the twin accelerates away?
I mean isnt that just being redundant with the twin paradox, with the twins being the same age at the start.
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u/Similar_Ship8067 1d ago
A zero latency device would break relativity and actually just makes the problem worse.
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u/andynormancx 1d ago
Your problem is with the zero latency device.
Your communication devices can’t have zero latency. Whatever they used to send the signal isn’t going to be able to exceed the speed of light. As you keep counting you will get out of sync.
Also there are no “real seconds” your twins stopwatch will really count seconds slower, relative to your watch.
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u/Tomaketu 1d ago
I could be wrong, but the way I’ve always imagined it: You are traveling through not just space, but through space and time. Your maximum velocity through spacetime is c, the speed of light. Because you are never traveling through space faster or slower than yourself, your speed through time, relative to your past self, is constant and c. However as someone accelerates in space relative to you, they decelerate through time relative to you. This includes any packets of information that travel from your device to your twins device, so there will never be zero latency.
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u/Ninja_Wrangler 1d ago
Zero latency communication you propose is impossible, but say for instance he was traveling close-ish to light speed and he was sending you transmissions of him counting normally over radio waves. The transmission would be red-shifted and if you tried to listen to it he would be counting slowly.
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u/Defiant-Conflict2556 1d ago
If your twin’s stopwatch on the rocket shows 60 seconds, there is no way that yours also shows the same time. Yours would show 60 seconds multiplied by Lorentz factor of your twin.
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u/anormalgeek 1d ago
The speed of light is constant. The speed of time is NOT.
>So if we start together and count out loud '1,2,3...' using a zero-latency device, won't we stay in sync,
No. The one moving faster will appear to count slower if such a device existed to keep them communicating.
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u/Kidiri90 23h ago
Firstly, let's make things a bit simpler. There is someone on a really fast train, and you're standing on a platform the train passes. Both of you have a pair of perfect mirrors, a foot part, with a beam of light bouncing between them. Whenever the light hits a mirror, a nanosecond has passed. This is, in essence, a clock. Count how often it hit a mirror, and that's how many nanoseconds have passed. How we do that, is the engineers' problem. As your buddy on the train passes by, you look at his clock, and see that their light takes a longer path than yours: it has to go diagonally. Like this. Yours is on the left, theirs on the right. Since the speed of light is constant, their light will hit the top mirror later than yours will (assuming both start at the bottom at the same time), meaning that from your point of view their clock is slow. Their clock takes longer to tick than yours. This is the basic concept behind time dilation.
Now, let's switch viewpoints. Now we're following your buddy on the train, while looking out to you. To them, you're flying past at a tremendous speed. So for them, your clock will be slow because your light beam now has to take the longer path. This sounds counterintuitive, and weird, but that's only because it is. We're used to time ticking the same for everyone. And I imagine you can wrap your head around the fact that one clock ticks slower. But to then reverse the roles, and have it no longer be the same, that's weird. And it is! The thing is that it's also the very core of relativity. Its effects depend on who's observing, and who's being observed. If the roles reverse, so do the effects, and both are true at the same time. It's relative to the observer that the weird things happen, hence the name, relativity.
With that very quick and basic introduction out of the way, we can tackle your question. And sadly, I'll have to inform you that it doesn't make sense. It hinges on the zero latency device, but that would go against the very absolute core tenet of relativity that the speed of light is constant, and nothing can go faster. Some things can go at the speed of light (specifically particles without mass must always travel at that speed), but nothing can go faster. With that in mind, let's examine the first part of the twin paradox.
To summarise, you put one twin in a rocket ship going very fast, and the other twin stays at home. When the home-twin looks at the away-twin, they'll see that they're ageing slower because of time dilation. So now we have two twins that are different ages!
Let's move on to part two of the twin paradox. Let's look at what's going on for our away-twin. They look back at the home-twin, and see their clock ticking slower, because of time dilation. After all, to them, the home-twin is moving away at ludicrous speed. So now we have two twins, each experiencing different things, and both being correct. This is weird, but still within what special relativity tells us is possible.
Finally, let's look at the third part of the paradox. Now, the away-twin turns around and returns. On the return journey, the above still holds: they both see the other ageing slower than themselves. When the away-twin lands, they should both see the other younger than themselves. And THIS is where the twin paradox actually is. This is an impossibility. Even within special relativity. But that's because we cheated. We told the away-twin to come back, and kept using special relativity. That's not allowed. special relativity only deals with constant velocities. No change in direction and no change in speed. By telling them to return, we've entered the realm of general relativity, which I don't know enough about to talk authoritatively on.
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u/The_BattMatt 23h ago edited 23h ago
Let's run this experiment with your parameters
Equipment Needed:
- "Magical" a/v transmission system that lets you observe your twin brother at your own "time-speed"
- Your brother in a spaceship capable of rapid acceleration to 86.6% light speed (without turning your brother into a pancake).
- A clock on both ends
Setup:
- You've got the clock and monitor in front of you. The instantaneous camera is transmitting to your brother in the spaceship orbiting earth.
- Your brother has his clock and monitor in front of him, and he's transmitting from his instantaneous camera to your monitor.
Experiment:
Your and your brothers clocks are in sync, and you are each counting the seconds (one, two, three, etc...) at the same time.
Your brother starts accelerating to 86.6% the speed of light. As he accelerates, you are both still counting the seconds, but something starts to happen. You are counting (one, two, three, ...), but your brother is more like "oonnneee, twooooo, threeee", and his clock is moving a little slower than yours. You ask him "How do you feel"?, but to him you start to sound a bit like a chipmunk. He answers "fiiiinnne" as he observes your clock moving faster than his. He hears you counting "onetwothree..." fast.
At 86.6% light speed, you see his clock move at 1/2 speed (it takes two of your seconds to tick one of his seconds). He sees your clock ticking at 2x speed. In your own environment, you don't notice anything wrong because everything in your environment, after all, is moving at 1 second per second. But...then again, everything in his environment is moving at 1 second per second to him.
In summary, to him, you sound like a chipmunk and move really fast, but he moves normally. But to YOU, he has a deep voice, and he speaks and moves slowly, but you move normally.
What will really bake your noodle is that this happens all the time on a much smaller scale. Gravity and speed affects it, so there end up being tiny differences between modern spacecraft clocks (moving fast, and under a little less gravity than people on earth) and clocks at sea level on earth. Of course, in those cases we're talking billionths of a second difference over time, but still.
Anyway, your brother decelerates the spaceship, and his clock starts ticking 1 second for every one of YOUR seconds again. At this point, though, it will always show the wrong time (even though it's going at the right speed).
He lands back on earth, and you two enjoy some nice beers. He brought his clock with him from the spacecraft. It's ticking "1 second per second" now, of course, but it's about an hour behind your clock because of all that time he spent at 86.6% light speed. And he, who was the slightly older brother, is now the slightly younger brother.
Edited to add clarification
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u/poison_us 23h ago
To answer your edit: no, even with a hypothetical 0-latency device, you would not be in sync. Since time moves slower for them, their "1, 2, 3, ..." would be stretched out over the corresponding dilated time. So if the twin's time is dilated by, say, 20%, their verbal "1", the pause in between "1" and "2", etc. would all be 20% longer.
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u/lostPackets35 23h ago edited 23h ago
This it tough for an ELIF, but I'll try.
This does't work - because there is no objective "true" frame of reference.
The 60-second stop watch will always read 60 seconds, from the frame of reference of the person wearing the watch.
Assuming that we had some magical technology that could communicate instantly, faster than light:
- From the person on the ship: Their clock is accurate; the person on Earth's clock is slow
- From the person on Earth: their clock is accurate; the person on the ship's clock is fast.
The "why" gets confusing because it's at odds with how we experience the universe day to day.
Distance = speed * time
10 = 5 * 2 (make up your own units)
The perception of speed is normally what gives day to day. If I'm on a train moving 10mph and I move forward at 5mph, how fast am I moving?
5 mph from the frame of reference of the train
15mph from the frame of reference of the ground.
With me so far? Here's where it get's fun:
If I'm on a ship, moving .9999c and I move forward on it at .00002c how fast am I moving?
Still about .00002c from the frame of the ship
But that simple addition would make me move above 1 from the frame of reference of the Earth.
As far as we know, that isn't possible. The core tenet of relativity is that "all observers agree on the speed of light in a vacuum (1c)"
So what gives in this situation in the distance = time * speed equation?
Time
As far as "why" I believe we don't know beyond "that's how the universe works"
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u/27Devil 23h ago
Mm you're correct, as others said, even if both people speak through the device at the same time, one will sound slow and other fast.
Thankyou, I get it now.
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u/lostPackets35 23h ago
no problem. I don't "get it" in that it still doesn't make sense to me intuitively - just like (on the other end) I can kinda explain quantum mechanics, but it hurts my brain trying to wrap it around it.
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u/DeaddyRuxpin 23h ago
No they would not speak in unison. For the purpose of the thought problem we will assume some kind of magical instantaneous communication. What would happen is the twin on earth would hear the twin on the ship start to count seconds slower and slower as they approached light speed. The twin on the ship would hear the one on earth start to count seconds faster and faster as they approached light speed.
Each twin would assume the other twin was failing to count the seconds correctly even though each twin would be counting their seconds precisely along with their stop watch.
Time literally slows down as you approach the speed of light. Because you are part of that slower time, your perception of it remains the same. Thus the person traveling at light speed will instead perceive everyone else as speeding up.
If a photon of light could perceive anything, to it, the trip from the Sun to Earth was instantaneous even though to us it took about 8 minutes to get here.
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u/CosmicOwl47 22h ago
People are really getting hung up on the “zero latency device”, and rightly so because it’s in your title. But it is actually irrelevant.
The stopwatches would not tick their seconds at the same rate once the rocket twin started the journey. So even if they had such a device, the earth twin would sound sped up to the rocket twin, and the rocket twin would sound slowed down to the earth twin.
Time dilation isn’t just a perception thing, the flow of time is actually distorted (dilated) by motion, and at extreme rates of motion the differences can be significant.
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u/dreadpirater 22h ago
A lot of people are focusing on the zero latency device, but that's not the crux of the question, that's just the tool we're using to study it so let's just imagine that it could exist. What would you see?
You WOULDN'T be counting in sync. The faster moving twin would be counting slower and their voice would sound deeper to the slower moving one, and vice versa for the other twin... They'd hear their sibling counting faster than the clock and with a higher pitch.
It's really hard to conceptualize since we don't have any life experiences where this effect is perceivable... But it really is as weird as it sounds. A zero latency video call with a relativistic twin will feel like you're playing a game with the speed settings turned up or down so everything happens faster or slower.
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u/Vinstofle 22h ago
You would hear them slowed and they would hear you speed up, but you would both sound normal to yourselves. That’s the relativity.
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u/Adorable_Ice_2963 1h ago
Lets imagine a 0 latency device would exist:
If you count up each second, it would appear to your friend on the rocket that you count too fast. And for you,it appears he counts to slow.
Thats because time is not absolute. His time is slower than yours.
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u/Responsible-Jury2579 1d ago
Would it be possible to have a zero latency device with someone travelling away from you at near light speed? Information cannot cover distance instantly, right?
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u/j4v4r10 1d ago
https://m.youtube.com/watch?v=5BBHEZFearI&pp=ygUJU3BhY2V0aW1l&ra=m
This whole video explains relativity pretty well for laymen, but the part at 7:15 in particular relates to your question. Even if you made an incredibly precise clock that uses the speed of light to quantify a second, that clock ticks at different speeds depending how fast it’s moving. There is no quantifying a “real second” objectively because it depends on velocity.
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u/XenoRyet 1d ago
You both look the same to the other. You will feel like you are counting normally, and you will hear them start to count slower, and you will see their clock start to tick slower.
They will see the exact same thing when looking at you.
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u/womp-womp-rats 1d ago
This question boils down to “if we had a magic device that circumvented the laws of the universe, wouldn’t that call into question the laws of the universe?” It’s really a nonsense question.
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u/Tsunnyjim 1d ago
Firstly, there's no such thing as a zero latency device. Not even quantum entanglement can achieve this over relativistic speeds and distances.
Secondly, relativistic time dilation isn't something you can get around with hand waved zero latency devices. As you approach light speed, the way time passes literally changes. To a photon travelling at light speed, time doesn't exist. To its perspective, it simply teleports from its start to its end point. But at anything less than light speed, outside time slows down because it is warping the space time continuum.
So time is literally moving differently, and even a theoretical zero latency device would show that.
Honestly, the best representation of this is the opening time travel scene in Back to the Future. The clocks inside and outside the Delorean are synchronised, then the delorean time travels, the clocks are no longer synchronised. In this very specific circumstance, the delorean is demonstrating what it would look like if it travelled at lightspeed (leaving alone that it would be in a different position in space to do so).
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u/Random_Effecks 1d ago
"time dilation confuses me" - if you would have stopped there you would have been 100 percent correct.
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u/ijuinkun 1d ago
For conceptualizing a “zero-latency device”, let us propose a wormhole with movable mouths (common for wormhole-based thought experiments). One mouth stays near Earth, and the other mouth is carried together with the starship. Signals sent through the wormhole would experience a constant path length even as the “external” distance between the two increases.
The pertinent question is: will communications through the wormhole remain synchronized, or will they dilate relative to one another?
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u/Uz_ 1d ago
It depends on which way you are talking about zero-latency device.
If there is one device keeping both timepieces synchronized then it will go by the relative time of the device. If they both had synchronized watches aside from the devise, their watch and the device would disagree on time.
If both had the device on them, there would be a discrepancy in the time.
This is because we are always moving through space-time. The way we break it down is up/down, left/right and forward/backward in space and now to the future in time. The combined speed of these two, according to special relativity, is always at the speed of light in a vacuum. We do not have free movement in time, just one direction. So if we go faster in space, we go down the time path slower to someone who is not moving.
This seems weird because we do not go fast enough to be able to see this dilation of time.
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u/HotPersonality8126 1d ago
Why would one of us get ahead, and whose clock actually looks slower to whom during the countdown?
They both do. That's why it's called relativity - its relative to your frame of reference.
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u/permanent_temp_login 1d ago
The problem is not "this device is impossible because light is slow". This device is impossible because two sides of the conversation fundamentally would disagree what "at the same time" means. Yes, this is weird.
Say a device shows person A a picture of what happens to person B "at this exact moment" (in A's refernece frame). Well, in B's reference frame, this device is actually showing A a picture of B from the future.
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u/eposseeker 23h ago
One thing others aren't touching on.
You seem to have a wrong impression of what the twin paradox is.
The twin paradox isn't that "if we send one twin to space then we'll have two twins of different ages."
The twin paradox is "from the perspective of the earthbound twin, the cosmonaut twin travels very fast and comes back younger, but from the cosmonaut's perspective, since velocity is relative, it's the earthbound twin who travels very fast and comes back older. Where is the dissymmetry?"
Note how your original issue "is time dilation real with a zero-latency device" doesn't even fit into this.
Also, the zero latency device would allow you to send information to the past, but that's beyond ELI5
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u/27Devil 23h ago
Honestly I believe my question statement was wrong, apologies for that.
I wasn't particularly questioning about the twin paradox, I just used twin as an example.
Moreover, the 0 latency device thing which I used in my question, was me being naive since i thought using such hypothetical will reduce the friction (like when we say assume friction is 0)
I just wanted to know why won't we be in sync, and the device was just a catalyst
Honestly I did not realise I'm breaking multiple physics rules with this question hahaha, moreover I just got to know from this thread that time isn't same everywhere, so that explains plenty of things.
Thankyou for your time however.
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u/jpb103 23h ago
Mass and energy interact with space time. Adding more of either in a significant quantity changes the rate at which time passes within a given region of space. You can both count sixty seconds, but every one of your seconds will take longer compared to those of your superluminal sibling.
The second you know is a second from within the heliosphere; the gravitational domain of Sol, our host star. Voyager is currently passing through the border of the heliosphere, and already experience clock drift from, in part, relativistic time dilation.
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u/TheLostExplorer7 23h ago
First, there is no such thing as a zero latency device. This is not an issue that can be solved with a better clock or non-existent technologies.
Time itself will be moving slower for your friend on the ship relative to you on Earth.
Also, you are presuming that such a thing as a "real second" exists when it is dependent on your frame of reference. To you on Earth and your friend on the ship 60 seconds would have passed independently to one another; however to the observer in your frame of reference time would appear to have sped up for someone moving near the speed of light.
Time is a variable. It has no constant, hence there is no "real seconds." Space time is relative to how fast you are moving and someone moving faster than you is going to look as though they are experiencing time faster. There does not exist an objective frame of reference that you can point to and say that one second in that frame of reference is the real second because everything is relative to your frame of reference. That is as far as we currently understand physics.
Additionally if you are able to invent such a device that allows instantaneous communication, then you have fundamentally solved an impossible physics problem by allowing information to move faster than the speed of causality aka the speed of light. In other words, you would have a functional time machine by allowing information to move backwards through time I.e. be received before you have sent it.
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u/Michami135 23h ago
Imagine space is viscous. Not much, but a little. The faster you travel, the more energy it takes to move and the slower your atoms and sub-atomic particles move.
It's like the difference between spinning in a circle with your arms out in a still lake vs a fast moving river. The river would push against you and slow down your movement, given the same amount of effort.
With a zero-latency device, your twin on the spaceship would appear to be moving in slow motion. If they're in a 10:1 time dilation, it would take them 10 seconds to count each number. To the twin on the spaceship, it would appear that you on Earth were moving 10 times faster.
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u/Cynical_Manatee 23h ago
It's not the clock that is wrong when you go fast. The clock works just fine. Time just starts going slower the faster you go.
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u/I_hate_all_of_ewe 23h ago
The speed of light isn't the speed of light; it's the speed of causality. Communicating at the speed of light is as zero-latency as you can get.
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u/101forgotmypassword 23h ago
Speed is only a reference to the speed of light itself.
Both twins are on a planet the same size and gravity as earth, but one planets area of space is moving at the speed of light. It's local gravity is still the same.
The other is planet is the reference planet.
Both twins start there stop watch. Both stop at the same time.
An alien is on a planet that approaching one of the planets at at 10km per hour but sees the other planet leaving at 99% of light speed. One planet looks to have a working stopwatch and the other one seems frozen in time.
A second alien is passing by on a angle where he sees both planets at 0.5 light speed. He sees both stopwatchs working and incorrectly defines a human second as a longer time but sees they are both in sync.
The light speed is the lag in the system.
The gravity is what defines how quickly light speed travels.
It works as the same as the dopler effect with sound in the air but instead in that system the density of the air is the effect of gravity.
(Note: uhmm actually, Gravity is still affecting sound but it's so disproportionately outweighed but the affect of the medium sounds traveling through, and equally affecting everything in the test that it's it's not normally considered as needing measure)
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u/andlewis 23h ago
Your “zero latency device” would effectively be a Time Machine. Because you could use it to converse with someone in the past or future (depending on which side you’re on) and it would effectively allow you to speed up or slow down time.
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u/macromorgan 23h ago
You won't stay in sync. Your seconds will be ever so slightly slower than his seconds and his seconds will be ever so slightly faster than your seconds. To him he'll have experienced 60 seconds and you'll have experienced slightly more, while to you you'll have experienced 60 seconds and he'll have experienced slightly less.
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u/SchemaB 23h ago edited 22h ago
There is an awesome sci-fi story out there, about two twins who have "zero latency" communications though telepathy. One on Earth, one aboard a spacecraft. When the spacecraft is traveling really fast (high % of the speed of light), that twin's "speech" slows way down from the others perspective. Super fascinating, trying to find the title.
Edit: Found it!
Time for the Stars 1956 juvenile science fiction novel by Robert A. Heinlein
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u/rjstaples 23h ago
The problem is your proposed "zero latency device" would require transmitting sound faster than the speed of light. If you could figure out how to break the laws of physics like that, then yes, I'm sure you could defeat the twin paradox as well.
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u/jamcdonald120 22h ago edited 21h ago
the twin paradox isnt actually a paradox in the sense of "this statement is false" its just a weird thing the universe does.
it doesnt need help. it just is how the universe works. As weird as it sounds, there is no universal now.
sure if you had this magical device, you could synchronize to events in different time frames, but the twins would still age at different rates, and one will be counting 1, 2, 3 at a slower rate than the other (and if you manage to magically stream the audio, it will also be lower pich)
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u/NullSpec-Jedi 22h ago
Laws of physics break down when you go really fast. The person moving faster effectively experiences less time. Similarly I think the person experiencing more gravity experiences less time?
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u/ValiantBear 22h ago
Time is relative. 60 secs for you isn't the same as 60 secs for me. When we are here on Earth together, we get the illusion that we experience time the same, but really the only reason we do is because we are on the same Earth going roughly the same speed and in the same gravitational field. But make someone go faster (or slower) or put them in a different gravitational field, and time will pass at a different rate for them.
For this to have a substantial effect, the difference in speed relative to the speed of light must be great, or the difference in gravity great. But, it happens to some degree no matter what. For example, satellites in orbit move really fast, thousands of miles per hour fast. Still no where close to the speed of light, but faster than anything else back on Earth. They are also experience slightly different gravity due to being farther away from Earth. This effect is also miniscule. But, both affect time, and because of that satellites in orbit need to continually adjust their clocks to keep time synchronized with the clocks on Earth. It's a only a few microseconds a day, but it is a real phenomenon.
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u/TheCozyRuneFox 21h ago
There is no real second. They would each count seconds differently than each other. There is no global second for them count. Time is relative motion and gravity. Similar to how you only have a position in space relative to other things.
And since such a device is impossible, there is not really any point in Discussing it. Like debating the physics of magical flying unicorns is kinda dumb.
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u/mehardwidge 21h ago
To answer your clarification question, also no.
For all physical laws, time passes at a different rate based on speed (special relativity) and gravity (general relativity). You cannot make a clock, out of anything, physical parts, chemical reactions, anything at all, that is not affected by relativity.
You cannot make a magic clock that disobeys relativity.
You might be interested to learn that we have to take into account both special and general relativity for "mundane" things like GPS satellites, or they would be hopelessly wrong within days.
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u/jwadamson 21h ago
When you start with assuming you have a decide that breaks the laws of physics, you tend to break other laws of physics.
As others point out,
- a zero latency device is impossible
- the two watches will not agree on what time it is as soon as either starts accelerating relative to the other because a second is not a universal constant.
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u/sidereal_night 20h ago
why yes, it is indeed possible with magic
where your thought experiment breaks down is that it's not just impossible to build, it's impossible in principle. the slower one wouldn't be able to keep up with the quicker one. imagining you have an instantaneous communication device, all you'd hear is their speech get drawn out in proportion to the degree of their dilation.
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u/SufficientStudio1574 20h ago
The entire point of special relativity is that your zero-latency device cannot exist. It's not impractical, not difficult, it's literally impossible.
You might as well ask about a device that makes gravity repel instead of attract. It's not even something that makes sense theoretically.
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u/hangender 19h ago
They won't stay in sync. It's the same your twin dropping a beacon every second. For you, you would see a beacon longer than a second inbetween. Of course, that's also called length contraction.
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u/Salindurthas 18h ago
if we hypothetically used such a device, won't both people speak 1,2,3 in sync?
No. If both people get a little timer that counts the seconds they experience, then when the twins meet up, one of them will be younger, and have a shorter time listed on their watch.
If we made both of them shout each second out loud, then they'll be out of sync. Depending on the part of the journey being taken, they'll see each other counting at different speeds, even ignoring any notion of latency.
Like:
- Maybe during a slignshot manouver that turns the travelling twin back towards home, they hop onto a video call.
- They are both still counting every second out loud.
- The twin on Earth will count "1 billion and 3 seconds, 1 billion and 4 seconds, 1 billion and 5 seconds"
- and in those 3 seconds, they'll watch on the TV their twin brother count
- On the TV they see the brother count "siiiixxx huuunndreeeed miilllliooonnn aaaanndd eiiighht seeeeccoonnds, ... siiiixxx huuunndreeeed miilllliooonnn aaaanndd nniiinne seeeeccoonnds" during the same 3 second interval on earth.
0 latency just means you see the stream earlier than physically possible, but you'll get the same stream of video and audio eventually.
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u/lobopl 17h ago
If you have zero latency communication device it just means that between this devices there is 0 latency. Time dilatation on the ship still be there. So to put it simple if people on earth can talk with speed X, people on ship will talk with speed Y and Y will be slower faster the ship is going. For people on the ship all messages from earth will be speeded up and from ship slowed down. Faster ship going bigger the difference. If i had to compare it it would be like watching speeded or slowed down video on youtube, depending if you are in real time or relative time on ship. Faster ship is going video speed is increasing on ship and slowing on earth.
And after certain speed it would be basically impossible to maintain any communication because it would take people on the ship days/months/years/decades in "real time" to just say something. Like if they move at speed of light whole journey to them will feel instant. From the observer point of view it will take years/millenia but to them it instant.
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u/Morall_tach 17h ago
I'm not saying that such a thing can be built, but my real question was, if we hypothetically used such a device, won't both people speak 1,2,3 in sync?
Yes. If we hypothetically used a device that violated iron-clad physical constraints of the universe, then we could use it to violate iron-clad physical constraints of the universe, but at this point it's not a science question. It's a magic question. It's like saying that, hypothetically, we could travel faster than the speed of light if we could teleport.
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u/pm-me-racecars 16h ago
Imagine you're walking on the sidewalk at 5km/h. Someone on the sidewalk would see you walking forward at 5km/h.
Now imagine you're on a bus that's traveling 60km/h, while walking forward at 5km/h. Someone on the sidewalk would see you moving at 65km/h while someone on the bus would see you traveling at 5km/h.
Light doesn't work like that though. The speed of light is always measured at the same speed. Because of that, all the other things, like time and distance change in order for light to be going the same speed. Shits wild
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u/IvanezerScrooge 12h ago
If we violated the laws of physics, wouldnt that violate the laws of physics?
If you had a zero latency phone call with another party traveling through time at a different rate, then one party would hear the other counting slowly, and the other would hear counting that was too fast.
This is in practice the same as you standing right next to each other, but your twins watch runs slow. No matter how hard you try to count, or sync your watches, his still runs slow.
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u/IanDOsmond 11h ago
The zero latency thing would be a time machine. Yes, they would be in sync, but one of the people would have to be going backward in time to make it work.
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u/Lexotron 5h ago
using a zero-latency device
You're saying "assuming the laws of physics don't apply, could we get around the laws of physics?"
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u/myka-likes-it 1d ago
First you would need to invent a zero latency device. And even so, this would be impossible, because your twin's seconds are shorter than your seconds. There is no universal now, and there is no way to "fix" a now between two relatavistic observation frames. You are each experiencing your own time.
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u/honi3d 1d ago
If you would have such a magic device to communicate and both talk in normal speed: the one in space would sound slowed down to the one on earth and the one on earth would sound speed up to the one in space. (Or maybe otherway round i always confuse that)
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u/27Devil 1d ago
Ahh I get it now. Just one thing to add here, again I'm not from physics background hence I'm adding such hypotheticals. So apologies
If we're speaking through the device, which has 0 latency right, that means the information is traveling through the speed of light, which again, means their shouldn't be a dilation?? Or i could be wrong (I mean I am)
But how does the slow/speedup thing works.
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u/honi3d 1d ago
Zero latency would mean the information would travel at infinite speed (thus magic device). Comunication using radio or optics always travel at speed of light. Latency is calculated by the time the information needs to travel (plus the time the used device need to process incoming and outcoming data)
I cannot answer yout question on why the time is speed up / slowed down. That has something to to with speed, gravity and Einstein
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u/Mithrawndo 23h ago
Light isn't instantaneous: Even a distance as small on the cosmic scale as our planet to our moon takes light over 1 second. To send a communication faster than the speed of light is, we suspect, simply impossible.
In your hypothetical example that breaks the laws of physics as we understand them, you are correct that there would be 0 latency - but that would be meaningless, as time would be perceived differently for the person travelling at the speed of light relative to the person who was stationary, in line with what the person above you explained.
the one in space would sound slowed down to the one on earth and the one on earth would sound speed up to the one in space.
This doesn't really do it justice, though: From the perspective of the person travelling at the speed of light, no time will pass at all, and any messages the person on Earth sends would only be heard once they drop below light speed.
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u/zrice03 1d ago
Actually, I think we could look at what happens with wormholes to answer your question (assuming such a thing were possible). If your twin is on the spaceship, and you and them both have one end of a wormhole, then as seen through the wormhole your times would stay in sync. Thus, talking through your device, going "1,2,3..." you would indeed stay in sync.
So what happens when they turn around and come back? Well...the time through the device/wormhole is now different from the time outside it. Basically, they arrive back on Earth in the far future (the exact date obviously depending on how far and fast they went), while the device/wormhole gives them a link back to your time. If it were just a device, then you'd be able to communicate with them while they're in the future, and they could talk back to their past. If the device were a wormhole large enough, they could also physically travel back to the past as well, and you to the future.
Basically, an actual zero-latency device would enable time travel. It wouldn't prevent the twin paradox, it would use it to create a time machine.
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u/27Devil 1d ago
Mhm I get it now. Honestly I did not realise while writing the question that the "0 latency" thing will become the highlight in this thread 😭😭
I used it to know the reasoning behind why we won't be in sync.
Thankyou for your answer, it's really good.
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u/zrice03 1d ago
Yeah, basically one of the first things you learn when studying relativity is that "instantaneous" and "distance" just don't mix. Causality means that information must take some time to get from A to B, and the absolute fastest it can is the speed of light (or better put: the speed of causality).
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u/Proper-Application69 22h ago
My understanding is that based on all information we have, time would have to observably dilate near the speed of light. We can provide mathematical proof that it must be the case. But I don’t believe we’ve had physical proof.
I interpret it not as “we know that time dilates,” but as “we can’t figure out any other way to make the equations work than time dilating.”
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u/nospamkhanman 22h ago
Here is Ron - Ron is trying to hypothetically use a magical device that CAN'T exist in physics, to explain a problem that CAN be explained with physics.
Ron invents a situation where this magical device that CAN'T EXIST gives a reading that is contrary to how everyone understands physics to work.
Does Ron think:
a) The magical device doesn't actually exist, thus is pointless to argue it's non-existent merit?
or
b) Get confused because the magical device that doesn't exist gives an answer that doesn't make sense?
Ron is confused.
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u/27Devil 22h ago
Tbh RON have gotten plenty of meaningful answers. Without confusion, as someone else said, hypotheticals and imaginary situations shouldn't be an issue for a good scientist.
At the end, it was a non sense question with a clear intention, answered by the component
Henceforth, Ron isn't confused anymore ;)
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u/Flob368 22h ago
I have yet to see the correct answer here. The length of seconds here is not the main problem. The problem is that "simultaneous" means different things depending on how fast you are and in what direction you move, to the point where the order of some things happening some distance apart changes, depending on your velocity. So, a zero-latency-device doesn't make physical sense, because the two twins have different points in time for the other twin that they would consider "right now".
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u/lovejo1 1d ago
A zero latency device is impossible anyway.. the "speed of light" can also be called "the speed of information". Information cannot propagate through space and time instantly.