Unravelling the Mysteries of Space and Light

[Carid]

Aren't we navigating up another tributary of the great river Anthropogenus?

To resume...

Physics is about interactions.
If the effects of interactions were transmitted instantly then everything would affect everything else.
In that were the case then the Universe would be a seething cauldron of chaos.
It ain't, it's rather well ordered in fact, hence we can exist to observe the locality of interactions.
Thus we require a limited speed of information transfer in order to observe a limited speed of information transfer.

There, I'm stuck on a sandbank.

 

[edpell]

In that were the case then the Universe would be a seething cauldron of chaos.

Suppose everytings interacts with everything but the strength of the interaction falls off as R^-2 would we have "a seething cauldron of chaos" then?

 

[inflector]

This could be wrong, of course, but I'm reasonably confident that whatever fundamental theory we finally arrive at, it will turn out to be a local quantum theory.

I may not have been following correctly, but didn't the EPR experiments of Aspect and others since prove that quantum theory is nonlocal?

 

[Chalnoth]

I may not have been following correctly, but didn't the EPR experiments of Aspect and others since prove that quantum theory is nonlocal?

No. You can't transmit information at faster than the speed of light through quantum entanglement. The field equations are also all entirely local.

 

[inflector]

Hmm, well I remembered reading about Bell's proof of this here:

http://plato.stanford.edu/entries/qm-bohm/

In part is says:

One of the achievements of John Bell was to replace the "arbitrary axioms" (Bell 1987, page 11) of Kochen-Specker and others by an assumption of locality, of no action-at-a-distance. It would be hard to argue against the reasonableness of such an assumption, even if one were so bold as to doubt its inevitability. Bell showed that any hidden-variables formulation of quantum mechanics must be nonlocal, as, indeed, Bohmian mechanics is. But he showed much much more.

In a celebrated paper published in 1964, Bell showed that quantum theory itself is irreducibly nonlocal. This fact about quantum mechanics, based as it is on a short and mathematically simple analysis, could have been recognized soon after the discovery of quantum theory in the 1920's. That this did not happen is no doubt due in part to the obscurity of orthodox quantum theory and to the ambiguity of its commitments.(Emphasis mine)

From the references, the John Bell paper seems to be:

Bell, J. S., 1964, "On the Einstein-Podolsky-Rosen Paradox," Physics : 195-200; reprinted in Bell 1987 and in Wheeler and Zurek 1983.

I wasn't able to read the initial paper yet, as I don't have access to the archives, and I'm not sure my understanding is advanced enough to understand Bell's proof, at this time.

So are you in disagreement with the authors here, or are they using the word in a different way than you are?

I was under the impression that pilot-wave theory, or de-Broglie-Bohm theory was a nonlocal hidden variable quantum theory using nonlocal in the same sense of the word that I believe the above cited Stanford article uses it.

 

[Chalnoth]

The apparent non-locality of QM stems from the collapse of the wave function, which never actually happens in reality. There is merely decoherence, not collapse.

 

[inflector]

So are you saying, for example, that in an EPR-type experiment like Aspect's the measurement of one entangled photon doesn't affect the outcome of the other measurement over distances in a way that must be happening faster than light speed?

I realize that there is no way to transmit information using entanglement, but I thought it was pretty clear that there was an instantaneous (or at least superluminal) effect due to the entanglement.

 

[Chalnoth]

So are you saying, for example, that in an EPR-type experiment like Aspect's the measurement of one entangled photon doesn't affect the outcome of the other measurement over distances in a way that must be happening faster than light speed?

Yes.

I realize that there is no way to transmit information using entanglement, but I thought it was pretty clear that there was an instantaneous (or at least superluminal) effect due to the entanglement.

Perhaps this will explain it:
Imagine, if you will, a emitter which emits entangled photons. Photons traveling in one direction have opposite spin to the photons traveling in the other direction. Now, the spins, when the photons are emitted, are in a superposition of the two available states. But now I go and measure the spin on one side and get a definitive answer: this photon is spin up. I must, if I go back and measure the other spin, get spin down: this is necessitated by the physics of the emitter. The fact that I must measure spin down on one photon if I've measured spin up on the other isn't a manifestation of non-locality, but merely of consistency.

 

[Carid]

Suppose everytings interacts with everything but the strength of the interaction falls off as R^-2 would we have "a seething cauldron of chaos" then?

The strength of the interaction may fall off as R^-2 and maybe the amount of interaction will rise as R^3 (on a large enough scale)
If my understanding is correct (a very dubious prospect mostly) this is the basis of why the inertia we experience is the fruit of our (non-instantaneous) interaction with the furthest observable objects in the Universe. The further we look, the more there is to interact with.