A2/Quantum FAQ

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We have seen in this course, that quantum physics is quite different from classical physics, which has lead to a different notions and interpretations over the years. Some of these notions, though possible even discarded by the scientific community, still continue to live on in parts if the more popular literature. In the following, we have compiled a list of questions on some of them.

A2

Quantum correlation
Introduction

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Q: Does the measurement influence the outcome of the experiment?

A: Yes. But this is not per se a surprising statement. Also in classical physics any measurement apparatus needs to have a finite interaction with the measured system. The difference is that in classical physics there is no fundamental lower bound, on how much information can be extracted from a single interaction. In quantum physics, one variant of the uncertainty relation tells us, that any information gain is accompanied with a finite disturbance, and the more information is extracted, the larger the disturbance will be.

Q: Does the consciousness of the observer play any particular role on the outcome of an experiment?

A: No.

Q: Does quantum physics predict that all parts of the universe are connected in some holistic way?

A: That depends, on the precise definition of the connection. Quantum physics does not allow for any superluminal signalling and allows a strictly local description of physics. Of course, parts of the universe with a common history can share correlation, both classical and quantum correlation, but also the quantum correlation cannot be used to remotely influence one part of the universe from a spatially separated one.

Q: Is quantum physics non-local?

A. This again depends on what is meant by “non-locality”. As we have seen in the discussion of the Bell theorem, quantum physics complies with criteria E (it is possible to violate a Bell inequality) and L (locality). If we consider two quantum systems at different places, any action at the first system will only have a measurable impact on system two after some time. Also, quantum correlations cannot be used to transmit information instantaneously. On the other hand, there is no way of replacing quantum physics with any local classical theory giving the same predictions, which was the consequence of the Bell theorem. So, if “non-local” means “superluminal signals possible” then the answer is no, where if it means “no local classical description possible” the answer is yes.

Q: How is quantum physics connected to other physical theories?

A: Comparing quantum physics to classical physics on the one hand and to relativity on the other hand, one can find characteristic differences, which we summarised in the following table:

Deterministic description possible Notion of Locality Violation of Bell’s inequality possible
Newtonian physics Yes No No
Relativity Yes Yes No
Quantum physics No Yes Yes

To be fair, one has to note that quantum physics has no build-in notion of the speed of light, but rather has a mathematical way define locality based on the commutation relation (or tensor product structure). This table might also hint to way it is so difficult to find a theory that encompasses both general relativity and quantum physics.