Understanding the Randomness of Quantum Processes and the Arrow of Time

[ejproducts]

I am not a physics student - I am a science fiction writer. But an idea is puzzling me, and I will attempt to convey it. However, I apologise if my terminology or an incorrect understanding of things makes my question unclear.

Firstly, I am under the impression that the laws of physics do not care which way the arrow of time is going - they make sense either way. We set the arrow of time when we use equations, but in theory it would not matter if we set it forwards or backwards. In other words, it is my understanding that there is no basic process that is not the same forwards as backwards.

Secondly, I am under the impression that some quantum processes have an objectively random outcome (sometimes I hear this called a stochastic process). It is not that we do not know enough information about the situation to predict the outcome, but that until the outcome occurs only the probability of the outcome is determined - whether by us or by LaPlace's demon.

It is my understanding therefore that an objectively random process has a state where the next state is not definitively predictable (but has a predictable probability) but the next state, when it occurs, can be known. The example I was given was that in the double-slit experiment in which a detector is placed at one slit, it cannot be determined beforehand through which slit a photon will travel, but afterwards it is known which slit it went through.

If this is the case, is this process the same under time-reversal? And if it is, does that entail that there is a state which can be clearly known one moment, and then the next moment that previous state becomes only known as a probability? That is, if an event with a random outcome has a knowable outcome, can a knowable state become random in respect to future observers?

Thank you for any replies, and I understand if, instead of answering my question, you correct my understanding on any topic raised.

 

[ZapperZ]

Please note that even in classical physics, there are many phenomena that have broken time-reversal symmetry. A lot of systems undergoing phase transition would prime examples. You don't have to invoke quantum mechanics for such a thing.

Zz.

 

[ejproducts]

Thank you for your reply.

I was specifically curious about time reversal and objective randomness, which I understand is a phenomenon of quantum physics and not classical physics. I did not really know that other systems were not time-invariant: I thought that they just appeared that way on a macroscopic level, but were not on a quantum level.

 

[chronon]

It seems that you're looking for a process that is the reverse of quantum measurement, where instead of a measurement generating a (random) value, the information about a value is destroyed. Two possibilities spring to mind

1: Quantum eraser (http://grad.physics.sunysb.edu/~amarch/ ) - a double slit experiment where the slit the particle goes through is measured, but then that measurement is 'erased'.

2:Black holes. According to Roger Penrose, information is destroyed in black holes, and this compensates for the information created in quantum measurements.

 

[A. Neumaier]

Firstly, I am under the impression that the laws of physics do not care which way the arrow of time is going - they make sense either way. We set the arrow of time when we use equations, but in theory it would not matter if we set it forwards or backwards. In other words, it is my understanding that there is no basic process that is not the same forwards as backwards.

This only holds for microscopic (and basically unobservable) dynamics. The visible dynamics has always a preferred time direction expressed by the second law of thermodynamics. That a glas falls on the floor and breaks into many pieces can be often experienced. That scattered pieces of glass assemble to a glass from which you can drink was never heard of.

Secondly, I am under the impression that some quantum processes have an objectively random outcome (sometimes I hear this called a stochastic process). [...]

If this is the case, is this process the same under time-reversal?

Yes, since this is a microscopic process.

And if it is, does that entail that there is a state which can be clearly known one moment, and then the next moment that previous state becomes only known as a probability? .

In the standard interpretations, a quantum state (even when completely known) _only_ expresses probabilistic information about the system. The quantum state is the quantum analogue of the probability distribution of a classical random variable.

On the other hand, we often apply probabilistic reasoning informally to past (and hence already determined) events, such as when discussing the probability whether dinosaurs got extinct because of an impact of an asteroid.

This is because probability theory is a tool for getting numerical insight into all sorts of uncertainty,
whether in the past of in the future. Strictly speaking, these probabilities are associated to a stochastic model of the situation, but such models are often silently identified with the situation itself.

 

[ejproducts]

Thank you Chronon, I think I understand a bit more because of your post. To fully try and grasp it, I am going to try and rephrase the question as best I can (or ask a new but related question, rather).

If we take a sequence in time in which it is impossible to determine the next state of a photon or electron (the electron might have spin-up or spin-down but we it is random what the outcome will be), and then in the next moment the electron resolves this into a particular state, it is my understanding that if we could somehow rerun that same sequence from the same initial conditions, we might not get the same result (spin-up result in experiment one and spin-down result in experiment two, for example).

So what I am really curious about is: is there a sequence that when run forwards is basically deterministic - it would turn out the same way each time - but if run backwards the result would be random?

(In a similar vein, I have heard that kaon decay breaks T-symmetry, and I was wondering, How do we know if something breaks T-symmetry? Aren't we limited by our forward direction in time?)

 

[ejproducts]

2:Black holes. According to Roger Penrose, information is destroyed in black holes, and this compensates for the information created in quantum measurements.

Actually, one more thing, is it coincidental that the information creation and destruction even out, or is there an actual relationship there?

 

[cragar]

2:Black holes. According to Roger Penrose, information is destroyed in black holes, and this compensates for the information created in quantum measurements.

I thought people like Susskind showed this was wrong or maybe I am mistaken.

 

[chronon]

I thought people like Susskind showed this was wrong or maybe I am mistaken.

You're right in as much as Susskind's view that black holes conserve information is pretty much the mainstream view, but Penrose doesn't subscribe to that view.

Actually, one more thing, is it coincidental that the information creation and destruction even out, or is there an actual relationship there?

I think that Penrose's view is that the universe is structured such that in the long term the two effects balance out

 

[Bill_K]

Hawking and Hertog have proposed such an idea. See http://physicsworld.com/cws/article/news/25247