Time Dilation and spooky action at a distance

clear thinking · 19-09-2014 3:33pm #1

A bit of a Pop science question as this is not my field, but I can't find an answer anywhere on-line and I'm not a physicist. Apologies if it strays a bit into sci-fi too, but it's still a legit question, and as it happens I've never heard of any sci-fi comms device like this (patent pending).

Assumptions:
1) entangled particle science evolves to the point where they can be used as data bits in communication devices- instantaneous communications.

2) very fast interstellar travel is possible, so A.N. astronaut goes to Proxima centauri for a 40 year round trip, on his/her return 80 years has elapsed. On the way A.N. has such a communication device.

Question: Given the time dilation what happens to communications during the trip!?!

Morbert · 20-09-2014 3:09pm

clear thinking wrote: »

A bit of a Pop science question as this is not my field, but I can't find an answer anywhere on-line and I'm not a physicist. Apologies if it strays a bit into sci-fi too, but it's still a legit question, and as it happens I've never heard of any sci-fi comms device like this (patent pending).

Assumptions:
1) entangled particle science evolves to the point where they can be used as data bits in communication devices- instantaneous communications.

2) very fast interstellar travel is possible, so A.N. astronaut goes to Proxima centauri for a 40 year round trip, on his/her return 80 years has elapsed. On the way A.N. has such a communication device.

Question: Given the time dilation what happens to communications during the trip!?!

The geometry that describes time dilation also forbids faster-than-light communication. If the geometry of spacetime was changed to permit instantaneous communication, we would lose time dilation.

Entangled particles are just particles that have been correlated through some local interaction in the past. There is no need to invoke spooky action to explain this strong correlation, as it is natural in a QM framework, just as there is no need to invoke spooky forces to explain time dilation.

One of my favourite papers to come out in the last few years. See section 7.
http://arxiv.org/abs/1308.5290

Labarbapostiza · 21-09-2014 3:48pm

Morbert wrote: »

Entangled particles are just particles that have been correlated through some local interaction in the past.

This is how I understand entanglement. That it is a local interaction in the past. But I keep hearing a description, that the observation of the spin of one particle, has the action of determining the spine of the other particle.

My understanding is that if you entangle two electrons, and you don't know the spin, you say that electrons are in a superposition of both being up and down. If then if you measure the spin of one at a distance from the other, if it's spin up, if you measure the other, it will be spin down. And vice versa.

My understand of the spin of an electron when it is not entangled, is that its' spin is a superposition of being orientated in all directions, but only one it is entangled it assumes up or down relative to the other particle. And that there is no energy expended in rotating the electrons into this orientation. (also I believe, that when two electrons are entangled, their spins are orientated in all directions in a superposition. but when measured, one will be spin up, and the other spin down - but that is due to the initial entanglement)

But I keep hearing this faster than light communication thing, even seen it in science documentaries . Are there scientists who are confused, and think there is an action at a distance, or what's going on?

Is there an experiment using electrons to prove Bell's inequality?

Was the EPR paradox actually just saying something about interpretation, and Einstein wasn't seriously suggesting there was spooky action at a distance?

Morbert · 21-09-2014 9:25pm

Labarbapostiza wrote: »

This is how I understand entanglement. That it is a local interaction in the past. But I keep hearing a description, that the observation of the spin of one particle, has the action of determining the spine of the other particle.

My understanding is that if you entangle two electrons, and you don't know the spin, you say that electrons are in a superposition of both being up and down. If then if you measure the spin of one at a distance from the other, if it's spin up, if you measure the other, it will be spin down. And vice versa.

My understand of the spin of an electron when it is not entangled, is that its' spin is a superposition of being orientated in all directions, but only one it is entangled it assumes up or down relative to the other particle. And that there is no energy expended in rotating the electrons into this orientation. (also I believe, that when two electrons are entangled, their spins are orientated in all directions in a superposition. but when measured, one will be spin up, and the other spin down - but that is due to the initial entanglement)

But I keep hearing this faster than light communication thing, even seen it in science documentaries . Are there scientists who are confused, and think there is an action at a distance, or what's going on?

Is there an experiment using electrons to prove Bell's inequality?

Was the EPR paradox actually just saying something about interpretation, and Einstein wasn't seriously suggesting there was spooky action at a distance?

Spooky action is what happens when you try to understand quantum mechanics in terms of classical principles, and when you refuse to fully internalise the fact that observables simply don't always commute with one another. I.e. Quantum correlations (entanglement) arise naturally in quantum physics, where observables are represented by Hermitian operators that sometimes don't commute, but are forbidden in classical physics, where observables are simple numbers (c-numbers to use Dirac's terminology) that always commute. Thus, if we want to assume it's classical physics all the way down, we must postulate some hidden, nonlocal machinery that maintains the apparent correlations between observables.

It is largely a philosophical/interpretational issue, which is why there are plenty of respectable scientists who believe in spooky action at a distance. But physics is ultimately about understanding observables, their frequencies, and how they relate to one another. Quantum mechanics tells us these things, and it is the deeper theory, which contains classical physics as a limit. And in the framework of quantum physics, entanglement is no more spooky or nonlocal than matching socks.

dlouth15 · 22-09-2014 2:49pm

Going back to the OP's question, let's take the assumption that instantaneous communication is possible and that if a signal is transmitted from Earth to the outbound moving spaceship, it is instantaneously received on the spaceship.

An interesting aspect of this is that in special relativity observers in relative motion to each other don't in general agree on what constitutes simultaneous events. If we talk about instantaneous transmission of a signal we need to specify whether it is instantaneous from the point of view of observers on the Earth or observers on the ship.

Lets assume then that it is the it is the location of the transmitting equipment that determines simultaneity. So when a signal is transmitted, it is simultaneously received from the point of view of the observers on Earth.

An interesting thing happens if, as soon as the signal is received on the ship, a reply is automatically sent back to earth using a transmitter on the outbound travelling ship. We have already decided that simultaneity is determined by the transmitting equipment. The interesting thing here is that the second signal will be received before the first signal has been sent.

This violates causality. A reply has been received on Earth before the signal that caused the reply has been sent.

So from the speculative point of view of science fiction, we run into interesting paradoxes if we make the assumption that faster than light communication is possible. In reality, as pointed out on this thread, quantum physics doesn't predict that entanglement allows for faster than light communication. The quantum state of the two particles are connected but QM doesn't predict that we can use this to communicate information faster than light.

Labarbapostiza · 22-09-2014 10:54pm

dlouth15 wrote: »

An interesting aspect of this is that in special relativity observers in relative motion to each other don't in general agree on what constitutes simultaneous events. If we talk about instantaneous transmission of a signal we need to specify whether it is instantaneous from the point of view of observers on the Earth or observers on the ship.
.

Okay, to give an example that does employ relatively.

A relativistic electron is travelling very fast (relativistic in travelling near the speed of light). It emits a photon. Depending on where the observers are position, they'll see a Doppler shift from the front, the back, or laterally, relative to the trajectory of the electron. Wherever it is measured the photon will have a different energy, and wherever its' measured, instantaneously, even across billions of light years, every other part of the wave will know that it has been measured, the photon will not accidently appear anywhere else.

The theory of using quantum entanglement for instantaneous communication is wrong, but let's just pretend for the sake of argument. If you were using entangled photons to communicate, first, if you wanted to communicate across a distance of ten light years, you'd need to begin the entanglement ten years in advance. Then if your 1's and 0's were represented by the polarity of the light, and you could re-orientate the polarity at either end, to represent your digits, the instantaneity would be in the shared time frame of the photons. As far as the photons are concerned, anything that happens to one, instantaneously happens to the other. But, the time difference between the spaceship and earth will manifest in the energy of the photon. The photon's energy will account for time dilation between observers in motion. Time between messages would be another issue. If the spaceship was accelerating relativistically. then the earthbound communicator could be sending messages at Giga bits per second, but they would be received on the space ship as slow clock ticks.

Now, I'm confused. Now if the space ship and earth are stationary relative to each other after a period of one accelerating. If communication via QE worked, earth is in the past, and the spaceship is in the future....If earth asked, what does earth look like in the future, the cosmonaut in the spaceship could take out his telescope and look at the earth. She would see earth as it is for the current communicator on earth. But the earth bound communicator would not be able to take out a telescope and look at the spaceship in the future...........But if they waited long enough, what they would see is the cosmonaut talking to them in the past......but never from the future.

dlouth15 · 24-09-2014 3:19pm

Labarbapostiza wrote: »

Now, I'm confused. Now if the space ship and earth are stationary relative to each other after a period of one accelerating. If communication via QE worked, earth is in the past, and the spaceship is in the future....

I think the language here is confusing. I don't think you can talk about a "thing" being in the future or the past. Things are extended in time. Talk instead about events.

If the ship is stationary relative to the earth, then we can forget about the strange effects of SR except for the fact that the ship will have aged less than the earth. If the round trip were complete then the astronauts would be younger in age than those they left behind, but there would not be communication difficulties. To the Earth-bound observers, the astronauts wouldn't be "in the future".

Labarbapostiza · 24-09-2014 9:17pm

dlouth15 wrote: »

I think the language here is confusing. I don't think you can talk about a "thing" being in the future or the past. Things are extended in time. Talk instead about events.

I was getting confused because I was trying to think of all the implications. And I was thinking about things in a way I hadn't before. I was trying to thinking if the order of cause and effect could be violated.

The future and the past are relative to each other. A point in space can be said to have a present, but all events it witnesses are coming from the past relative to its' point in space. The future away, the further away in the past. But never from the future. Time frames/cones/of spheres intersect each other, so when the photons are used for FTL communication, they begin and remain in the same time frame as each other , the communication is neither happening in the future nor the past.

If the ship is stationary relative to the earth, then we can forget about
the strange effects of SR except for the fact that the ship will have aged less
than the earth.

No, there are always SR effects. We just don't think about them. For a photon traveling at light speed, time slows to a halt relative to everything else. But, relative to us, I believe, that anything that happens in a photons probability distribution, happens instantaneously to an observer (I haven't worked this out mathematically - so I could be wrong, and should get into the habit of at least attempting calculations on paper before making such statements.) But if time doesn't exist for a photon, then it can take an infinite amount of time to be in an infinite amount of places, to us that would seem to be instantaneous. (Okay, I'm getting a little sci-fi here - but only as sci-fi as QE communication).

dlouth15 · 24-09-2014 10:11pm

Labarbapostiza wrote: »

But if time doesn't exist for a photon, then it can take an infinite amount of time to be in an infinite amount of places, to us that would seem to be instantaneous. (Okay, I'm getting a little sci-fi here - but only as sci-fi as QE communication).

Well what happens is that from the point of view of the photon, as it were, it is everywhere along its path at once. It experiences the journey from Earth to Proxima Centauri as instantaneous. However from the point of view of a normal observer, it travels at the speed of light.

[latex]\Delta\tau= \Delta t/\gamma[/latex] where [latex]\tau[/latex] is the elapsed proper time, i.e. the time experienced by the moving object, [latex]\Delta t[/latex] is the time elapsed on Earth, and [latex]\gamma=1/\sqrt{1-V^2/c^2}[/latex].

As [latex]V[/latex] approaches [latex]c[/latex], [latex]\gamma[/latex] goes to infinity and therefore the proper time [latex]\Delta\tau[/latex] goes to zero.

clear thinking · 25-09-2014 9:57am

Thanks for the replies, some light reading for me to decipher!

Time Dilation and spooky action at a distance

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