# Causality in QM

Tags: causality
 PF Patron Emeritus P: 8,100 Here is a very interesting discussion of a proposal to allow backward causation in quantum mechanics. The basic idea is that the time asymmetry of QM ("collapse of the wave function") violates our expectations of a fundamental theory; all previous candidates from Newton on have been time symmetric and time asymmetry then arises from statistical probabilities. The ideas are discussed back and forth in a refreshing way, and I strongly recommend it.
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 Quote by selfAdjoint Here is a very interesting discussion of a proposal to allow backward causation in quantum mechanics. The basic idea is that the time asymmetry of QM ("collapse of the wave function") violates our expectations of a fundamental theory; all previous candidates from Newton on have been time symmetric and time asymmetry then arises from statistical probabilities. The ideas are discussed back and forth in a refreshing way, and I strongly recommend it.
Shoot, I've had Price's book for a couple of years, read it a while ago, and must have skipped over this bit. Will have to go back and read it again.

Certainly fits very well with my current philosophy (time-symmetric, realistic, deterministic, "free-will skeptical" physicalist that I am)

Thanks for this.
 PF Patron P: 742 Wonderful, thanks.
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## Causality in QM

Here is a site that gives an explanation of backward causation from an analysis of the consequences of the Minkowski metric.
 P: 2,043 Also there are several essays related to this on Mathpages
 P: 244 It seems to me that the very concept that the future can affect the past is incredibley absurd. It is in the same catagory as saying 0=1. The only way that 0=1 is to change one or both of thier definitions. IF you assert that the future has affected the past, you have changed one or both of their definitions. Why do we not accept the proposition that there is a particle that is superluminal (probably a form of a graviton) that is connecting the photons. If this particle had less mass than the photon, it would be possible for it to be much faster than the photon, but it's combination of less mass and greater speed would together equal that of the the photon. Only particles of less mass than the photon can exceed the speed of the photon. This is speculation but it is at least possible. The future affecting the past is semantically, logically and conceptually impossible.
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 Quote by sd01g It seems to me that the very concept that the future can affect the past is incredibley absurd. It is in the same catagory as saying 0=1. The only way that 0=1 is to change one or both of thier definitions. IF you assert that the future has affected the past, you have changed one or both of their definitions.
Not necessarily. Think of time as the fourth dimension, and think of spacetime being laid out in a 4-dimensional spacetime - then past, present and future "co-exist" in this four-dimensional spacetime. The only constraint on the configuration at any position on the time axis is that it must be consistent (according to the "laws of nature") with all points both past and future from that position. In this sense, past determines future just as much as future determines past - all co-exist as a single 4-dimensional self-consistent solution to the "laws of nature"

 Quote by sd01g Why do we not accept the proposition that there is a particle that is superluminal (probably a form of a graviton) that is connecting the photons. If this particle had less mass than the photon, it would be possible for it to be much faster than the photon, but it's combination of less mass and greater speed would together equal that of the the photon. Only particles of less mass than the photon can exceed the speed of the photon.
A photon has zero rest-mass. Are you postulating the existence of a particle with negative rest-mass?

 Quote by sd01g This is speculation but it is at least possible. The future affecting the past is semantically, logically and conceptually impossible.
Not at all, as I have shown above. You just need to get your head around it

MF
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 Quote by moving finger Not necessarily. Think of time as the fourth dimension, and think of spacetime being laid out in a 4-dimensional spacetime - then past, present and future "co-exist" in this four-dimensional spacetime. The only constraint on the configuration at any position on the time axis is that it must be consistent (according to the "laws of nature") with all points both past and future from that position. In this sense, past determines future just as much as future determines past - all co-exist as a single 4-dimensional self-consistent solution to the "laws of nature" A photon has zero rest-mass. Are you postulating the existence of a particle with negative rest-mass? Not at all, as I have shown above. You just need to get your head around it MF
In the interest of not having this thread locked, I will not respond and retract my prior post. Thanks.
PF Patron
P: 2,216
 Quote by sd01g It seems to me that the very concept that the future can affect the past is incredibley absurd. It is in the same catagory as saying 0=1. The only way that 0=1 is to change one or both of thier definitions. IF you assert that the future has affected the past, you have changed one or both of their definitions. Why do we not accept the proposition that there is a particle that is superluminal (probably a form of a graviton) that is connecting the photons. If this particle had less mass than the photon, it would be possible for it to be much faster than the photon, but it's combination of less mass and greater speed would together equal that of the the photon. Only particles of less mass than the photon can exceed the speed of the photon. This is speculation but it is at least possible. The future affecting the past is semantically, logically and conceptually impossible.
I had no idea that either the future or the past existed in Quantum Mechanics. My initial impression was that conditions effect other conditions synergistically and that's it. Do the terms future and past actually apply in QM?
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 Quote by sd01g It seems to me that the very concept that the future can affect the past is incredibley absurd. It is in the same catagory as saying 0=1. The only way that 0=1 is to change one or both of thier definitions. IF you assert that the future has affected the past, you have changed one or both of their definitions.
That is not what happens with the notion that lightlike paths may not just be future directed.

The distance between the emission and absorbtion of a photon in space-time is exactly zero, even if that space-time is curved.

How do we know for sure that absorbtion is not causally connected to the emission as is the emission to the absorbtion?
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 Quote by MeJennifer How do we know for sure that absorbtion is not causally connected to the emission as is the emission to the absorbtion?
sounds like Cramer's transactional interpretation to me - I like it
 P: 10 Maybe both are just one.

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