Look, what does it mean for communication to be instantaneous? Don’t answer that it happens simultaneously/at the same time in two locations; that’s just dodging the question. What does it mean for two differently located events to happen at the same time?
For most of history, there was a simple definition which was implicitly used: Synchronize a whole lot of watches at the same location, then send them out along different journeys through spacetime. The time coordinate of an event in spacetime will be the reading on any watch which winds up there, and two events are simultaneous if they have the same time coordinate.
Only there turned out to be a problem with this definition: it’s incoherent. Watches which start out synchronized can meet up again at some event with different readings. So we can’t use this definition to assign time coordinates to events in spacetime, and therefore can’t use it to tell if different events are simultaneous.
Alright, no matter. We don’t need such a definition anyway; we can describe how things work just fine without it.
Except when someone starts asking “What if we had instantaneous communication?”. Then we need to know what “instantaneous” means, and, well, we don’t have any meaning to give it. The questioner has to tell us what they mean by that term.
Intuitively speaking, if you and I are having a conversation and responding to one another with no greater lag than if our faces were inches apart, then we are participating in communication that is virtually instantaneous. That seems easy enough to define.
This is my last comment before this branch of the conversation submerges beyond my depth, but I was thinking of it more like proposing that the sum of the angles of a triangle could be greater than 180 degrees. It does not work in the physical world, but obviously there are other geometries in which this is an unremarkable proposition.
Ah, touché. I suppose that does provide a fine account of what we mean by instantaneous communication, without providing an account of simultaneity of events more generally.
Well, I guess the thread is getting hung up on the black box of instantaneous communications. What I was thinking of (as I linked to earlier) was the idea of quantum entanglement and using some method (currently unknown) to send data (in the form of communications) across large distances essentially instantaneously. Conversely, I suppose if there was a way to create directed micro-wormholes (how? Who knows? Is it theoretically possible? If so, there you go) between two places and then feed data (in the form of instantaneous communications) through, that would also qualify. Or, some other method.
As I’ve said, if this violates some law of physics and is flat out impossible, then that’s the answer (I’d appreciate a Dummy’s Guide to why it’s impossible, if that’s ok, so I can try and grasp the reasons). No worries. If it’s even theoretically possible then stop focusing on how the communications would work (unless it affects the answer, in which case make some assumptions) and just black box it. Theoretical physicists do this all the time, after all. I remember an article by Cecil looking into the possibilities of a time travel device that was chocked full of assumptions and theoretical aspects (something like ‘assume negative energy…what’s that? Who knows, just go with it’).
ETA: Look. I’m no physicist. I have exactly 3 years of college physics, which I only took because they were required. I don’t have the math or the language to discuss this stuff…I’m merely curious. If the answer is ‘it’s impossible’, then all I ask is to try and explain the reasons why in layman’s terms.
Quantum entanglement doesn’t help instantaneous communication at all, any more than if you and I had a pair of magic dice which were guaranteed to roll the same numbers in the same sequence as each other (no matter when or how far apart we were when we rolled them) while being perfectly unpredictable considered in isolation, we could use them to help achieve instantaneous communication. It might be interesting that we’re both getting the same messages despite being very far apart, but if we don’t have any control over the messages, then we’re not communicating, just both watching the same show.
Building on the ‘twin paradox’ earlier from this thread… let’s say that your brother accelerates away from earth at a velocity that is a very large fraction of C.
You have magic instantaneous communications; you both have identical
stopwatches.
You ring him up to do an experiment to demonstrate that time is moving more slowly in his frame of reference. On a preordained signal, you’ll both start counting 60 seconds, and the first one to finish tells the other to stop counting.
You count off 60 seconds. Because time is moving slower for him, he is still drawling in a molasses-like voice “2, 3, 4…”
Assuming he’s counting at a rate of what you both previously agreed was 1 second, that means he’s now in the past (relative to you). If he can hear you tell him to stop counting, then you have just demonstrated the ability to send messages into the past. All the known laws of physics say that nothing can travel into the past, therefore the experiment as described cannot occur in reality.
I thought quantum entanglement was the basis for the research being done on quantum teleportation…which would (to my mind) say that if I rolled a 1 then you’d see 1 pip come up on your associated dice (if I’m grasping even the edges of both your point and the theory).
I definitely don’t want to get into time travel (though my, again perhaps incorrect, understanding is that there is nothing in modern physics that negates the possibility of time travel), though I guess I can see what your point is here. If we are talking via an instantaneous link and you are moving slower to me, then this would constitute a form of time travel (of data at least)…yes?
Which sort of answers the question right there. Even if you COULD have instantaneous communications, it wouldn’t do you much good if your communications would still be lagged due to relativistic effects (I know folks up thread were saying that the effect between .1c and .2c wouldn’t be great, but say between .1c and .9c). Is that what you are saying here?
It explains Bell’s theorem. I don’t even have the 3 years of college physics, so it took a while to see what was happening. Quantum teleportation is a sort of a misnomer. It needs conventional communication at light speed or less to work. The factoids about this tend to leave out the details. Looking at entangled particles individually give you any information. You need to compare measurements of 2 entangled particles to derive information, and the comparison has to be done at light speed or less. Would have cleared up a lot of confusion if I heard that first.
Of course the Futurama episode ‘Roswell That Ends Well’ seems to show there may be exceptions:)
Sure, if I roll a 1 then you see a 1. But if neither of us has any influence on how the die rolls come out, then we’re watching the same show, but we’re not communicating with each other. Communications requires you to be able to influence the message.
If we both tune in to The Daily Show, we see the same thing. But we can’t use this to learn anything about each other unless we can somehow influence what The Daily Show broadcasts. Jon Stewart is talking to us, sure, but we’re not communicating with each other. That’s how quantum entanglement works, with the added bit that, as opposed to befriending Jon Stewart or sending a message to his writers in sufficient time, no one has ever observed any ability to influence or even accurately predict the quantum entanglement show to look like anything other other than random snow.
Relativity is based on the notion that space and time are one single thing called “spacetime”, and that two people in relative motion will disagree on how much of this stuff is space and how much of it is time. The problem Relativity has with instantaneous communication is that if you can connect two distant points in space with zero lag time in their communication, then you can just as easily connect two distant points in time with zero lag time in their communication. Indeed it predicts that for any communication faster than light, there exists a relative motion in which someone will see the communication as taking place between future and past. There’s just no way around it. Wormholes are easily shown to be just as valid as shortcuts through time as they are through space.
The only way I see to fullfill the scenerio of the OP is to postulate that Relativity is wrong in one or more of it’s founding premises, and yet in a way that does not contradict the empirically observed behavior of the universe. That’s beyond tough, it’s more a question of “is there any chance, any at all, we could be dead wrong?”
No, it is not. You need instantaneous communication in two different reference frames.
Please list them.
Fair enough.
Does not follow.
We can choose any reference frame, and speak about instantaneous in that frame.
That just demonstrates that, in the frame in which the communication is instantaneous, time dilation is different for the two twins. Looking at it from a different frame, it’s no longer instantaneous, but the amount back in time for communication in one direction is balanced by the amount forward in time in the other. What matters is if you can send information backwards in time to yourself. With communication that is instantaneous in one, privileged frame, that is not possible.
I would not call it time travel or communication into the past unless you can communicate to yourself in the past (or at least to somewhere in your past light cone). If you stick with instantaneous in a privileged reference frame (I’d make it be more-or-less the frame where the cosmic microwave background radiation is uniform), communication into the past isn’t possible.
All of our physics is based on the assumption that there is no privileged frame of reference, that assumption took Einstein far, and there are no indications to the contrary. But that is not proof that there is no privileged frame in some physics not yet discovered.
I’m saying that the demonstration that the timeframes are different is also a demonstration that the communications aren’t instantaneous. If you wanted to pretend that wasn’t the case, then the usual treatment in sci-fi is that one party’s speech is sped up chipmunk-style, the other unbearably slow. But it just makes no sense that a supposedly identical message transmitted in one direction would be perceived as 1 second, but transmitted in the other direction would take 60 seconds. You really do have to pretend like physics doesn’t exist in order to make this work.
Had to go back and revisit relativity again. The ‘third observer’ demonstrates the problem with simultaneity. Any definition of simutaneity between two time frames won’t be observed in a third. If a third time frame is used to define simultaneity for the other two, it won’t seem simultaneous in those two. If I have it right anyway.
