Quantum entanglement and measurements

[Baby Astronaut]

From an article that swansont had linked to.

 

According to a common interpretation of quantum mechanics, both photons are in indeterminate states until you measure them. It is important to make this distinction: it’s not simply that you don’t

know

what the polarization of each photon is until you measure it; instead, the polarization

does not take on a definite value

until you measure it.

 

So photons are value-less if not measured. 1) How do we know or prove that without measuring?

 

2) And if a photon is measured and therefore it's polarization is set, does it keep the same value regardless of how many observations are made on it from then on?

 

3) If every particle in the universe somehow were measured by instruments tomorrow, does it mean every particle in the universe would have a definite value? If so, 4) is is there an "expiration date" where the particle returns to an indefinite value again? Or would the new definite value remain forever imprinted upon the particle?

 

5) Once its value is set by having measured it, can the value of a photon's orientation be changed to different value?

[derek w]

polarized photons?which article?

[swansont]

So photons are value-less if not measured. 1) How do we know or prove that without measuring?

 

There are experiments you can do to show that the system was in an indeterminate state prior to measurement. These are the hidden-variable tests based on Bell's inequality.

http://en.wikipedia.org/wiki/Bell's_theorem

 

2) And if a photon is measured and therefore it's polarization is set, does it keep the same value regardless of how many observations are made on it from then on?

 

Yes, if measured in the same basis — you can't rotate the polarizer or beamsplitter — and there is no other interaction.

 

3) If every particle in the universe somehow were measured by instruments tomorrow, does it mean every particle in the universe would have a definite value? If so, 4) is is there an "expiration date" where the particle returns to an indefinite value again? Or would the new definite value remain forever imprinted upon the particle?

 

Any interaction can change the value.

 

5) Once its value is set by having measured it, can the value of a photon's orientation be changed to different value?

 

Yes, via an interaction. e.g. if you send it through a polarizer at 45º, it has a 50% chance of passing through, and that will be its new polarization. If you then sent it through a 50-50 beamsplitter in the original orientation, it would have a 50% chance of going in either direction. (before the rotation, it would go 100% of the time in the orientation that corresponded to its polarization.)

 

polarized photons?which article?

 

Linked to in the OP

[Baby Astronaut]

Thank you swansont. Missed two questions however.

 

3) If every particle in the universe somehow were measured by instruments tomorrow, does it mean every particle in the universe would have a definite value?

 

Meaning, now that every single particle has been measured and touched by instruments everywhere, they're no longer indefinite. So if we somehow accomplished this in a hundred years, scientists elsewhere in a million years would never know of an indefinite value because every particle was already observed long ago by Earth scientists, unless...

 

4) is is there an "expiration date" where the particle returns to an indefinite value again?

 

You observe a particle with an indefinite value, thus it has gained a definite value, but after sitting there untouched for a very long time its value returns to an indefinite state (would that actually occur)?

Edited November 29, 2011 by Baby Astronaut

[swansont]

Thank you swansont. Missed two questions however.

 

3) If every particle in the universe somehow were measured by instruments tomorrow, does it mean every particle in the universe would have a definite value?

 

Meaning, now that every single particle has been measured and touched by instruments everywhere, they're no longer indefinite. So if we somehow accomplished this in a hundred years, scientists elsewhere in a million years would never know of an indefinite value because every particle was already observed long ago by Earth scientists, unless...

 

4) is is there an "expiration date" where the particle returns to an indefinite value again?

 

You observe a particle with an indefinite value, thus it has gained a definite value, but after sitting there untouched for a very long time its value returns to an indefinite state (would that actually occur)?

 

 

Some measurements cause other states to become indeterminate. The "good" measurements (operations, mathematically) commute with each other, meaning you get the same answer no matter what order you do them in. But others do not commute, so the measurement causes some other state to be unknown.

 

And interactions are probabilistic. So you don't have a definite outcome from an interaction.

 

However, if you have made "good" measurements (i.e. the operators commute) and it does not subsequently interact, it will remain in that set of states.