Entagled particle time machine, possible?

[Four Winds]

What happens if we trap entangled particles right now in a space that will exist for some time into the future. Will those entangled particles in the future be entangled to those now? We would never see back before the particles were trapped, but could we communicate with the future?

[SilentHunter]

What happens if we trap entangled particles right now in a space that will exist for some time into the future. Will those entangled particles in the future be entangled to those now? We would never see back before the particles were trapped, but could we communicate with the future?

If you entangle a microscopic object here on Earth with one on Mars then you could quantum teleport a state across. Its instantaneous so you would indeed be changing the past on Mars.

[sepulchrave]

What happens if we trap entangled particles right now in a space that will exist for some time into the future. Will those entangled particles in the future be entangled to those now? We would never see back before the particles were trapped, but could we communicate with the future?

``Entanglement'' is not all-encompassing.

You can entangle the spin of two particles, for example, and then freely move each particle around in space without affecting the entanglement.

As SilentHunter points out, affecting the spin of one of these particles will instantly affect the spin of the other.

We could also entangle the position of two particles, and then freely change the spin of each particle without affecting the entanglement.

To do what you suggest, we would need to entangle the particle right now with some other particle in the future.

I do not think that is possible.

It is also not possible to send a message using entanglement. Since the ``real'' properties of both particles are unknown, measuring one would instantly define the other particle - but the person on the other end would never know unless we told them by some other (conventional) means.

If you entangle a microscopic object here on Earth with one on Mars then you could quantum teleport a state across. Its instantaneous so you would indeed be changing the past on Mars.

No you wouldn't.

First, you aren't ``quantum teleporting a state''. Quantum teleportation is not instantaneous, it occurs at the speed of light (or slower).

Secondly, whether or not ``breaking entanglement'' occurs on Mars in the future, present, or past depends entirely on how you construct your coordinate set. Since the coordinates are arbitrary, ``breaking entanglement'' cannot have any consequences for Mars' actual past.

Fortunately it doesn't, the entanglement is only maintained as long as nothing on Mars interacts with that aspect of the quantum object.

[SilentHunter]

To do what you suggest, we would need to entangle the particle right now with some other particle in the future. I do not think that is possible.

It is also not possible to send a message using entanglement. Since the ``real'' properties of both particles are unknown, measuring one would instantly define the other particle - but the person on the other end would never know unless we told them by some other (conventional) means.

No you wouldn't.

First, you aren't ``quantum teleporting a state''. Quantum teleportation is not instantaneous, it occurs at the speed of light (or slower). Secondly, whether or not ``breaking entanglement'' occurs on Mars in the future, present, or past depends entirely on how you construct your coordinate set. Since the coordinates are arbitrary, ``breaking entanglement'' cannot have any consequences for Mars' actual past.

Fortunately it doesn't, the entanglement is only maintained as long as nothing on Mars interacts with that aspect of the quantum object.

We could setup the entangled state between a microscopic object here on Earth and one on Mars. We could entangle both objects here on Earth then rocket ship one to the red planet. You then have two entangled objects, one here and one there.

You can communicate using entanglement. Quantum cryptography allows two microscopic objects to be entangled and provides a key to detect if the entangled state has collapsed due to measurement. A measurement here and an examination of the key there can form the basis of a signal. If collapse = no then...... and if collapse = yes then ......

Measurement of one object collapses the quantum state of the other instantanously. As Mars is several light hours away it amounts to affecting the past. If examination of the key reveals that collapse = yes then the Marsian turns a light on. In essence you could conduct a measurement here and the Marsian will then turn on the light several hours ago on Mars.

[sepulchrave]

We could setup the entangled state between a microscopic object here on Earth and one on Mars. We could entangle both objects here on Earth then rocket ship one to the red planet. You then have two entangled objects, one here and one there.

Yes, I am not denying that.

You can communicate using entanglement. Quantum cryptography allows two microscopic objects to be entangled and provides a key to detect if the entangled state has collapsed due to measurement. A measurement here and an examination of the key there can form the basis of a signal. If collapse = no then...... and if collapse = yes then ......

No, you can't communicate using entanglement; precisely because you have no way of knowing whether or not a state has collapsed except by measuring it - which causes it to collapse.

In other words, you have no way of knowing whether you broke the entanglement or whether the other guy did.

Quantum cryptography, quantum teleportation, etc. use entanglement to ensure secure, uncorrupted data transfer. They do not allow the transfer of data. Data transfer must still occur using ``normal'' channels - i.e. at speed-of-light or slower.

Measurement of one object collapses the quantum state of the other instantanously. As Mars is several light hours away it amounts to affecting the past.

I will grant you that point but only in a trivial sense. The entangled object can have no influence on anything on Mars, because the aspect that is entangled must be shielded from everything on Mars (as you yourself have pointed out several times over the last few days).

So the only way you are changing the past on Mars is in the trivial sense that an observer here on Earth measures their side of the entangled pair, then informs their friend who just happened to fly by the Earth just then at relativistic speeds, and who happens to be going in such a direction and a such a speed that maps the instantaneous-to-the-Earthling-observer's coordinates of the Martian entangled object into the relativistic-traveller's non-causal past.

If examination of the key reveals that collapse = yes then the Marsian turns a light on. In essence you could conduct a measurement here and the Marsian will then turn on the light several hours ago on Mars.

No, for the reasons outlined above. Entanglement does not allow you to communicate at all (FTL or otherwise).

[SilentHunter]

Quantum cryptography, quantum teleportation, etc. use entanglement to ensure secure, uncorrupted data transfer. They do not allow the transfer of data. Data transfer must still occur using ``normal'' channels - i.e. at speed-of-light or slower.

The cryptography key which tells you if the data has been spyed upon or not can be used to send a message. If the key says no its not been spied on you could call that a 0. And if it has been spied upon you could call that a 1.

The data transfer part is irrelevant although you are right that part is limited to light speed. Its the key that we're using to send a 0 or 1 signal. That signal is instantaneous.

You could have a bundle of 1,000,000 laser beams aimed at mars. You could have 1,000,000 cryptography keys for them. That allows you to send a digital signal 1,000,000 bits long instantaneulsy - 0011011110110100101010101 etc until its a million long.

[sepulchrave]

The cryptography key which tells you if the data has been spyed upon or not can be used to send a message. If the key says no its not been spied on you could call that a 0. And if it has been spied upon you could call that a 1.

The data transfer part is irrelevant although you are right that part is limited to light speed. Its the key that we're using to send a 0 or 1 signal. That signal is instantaneous.

No it isn't. The entanglement is in the photons (or electrons or whatever you are using to relay the message) that are being transmitted at conventional speeds.

You could have a bundle of 1,000,000 laser beams aimed at mars. You could have 1,000,000 cryptography keys for them. That allows you to send a digital signal 1,000,000 bits long instantaneulsy - 0011011110110100101010101 etc until its a million long.

That is not instantaneous! Light takes a finite time to reach Mars.

In quantum cryptography the entanglement is not between the transmitter and the receiver. The entanglement is within the transmission (usually the spin of photons).