Why did Einstein have a hard time accepting quantum physics - by Nirmalya Kajuri - Quora Question Review

This document contains a review of the answer by Nirmalya Kajuri on the question in Quora: "Why did Einstein have a hard time accepting quantum physics "
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Contents

• 1. Answer Review by Nirmalya Kajuri

Reflection

• Reflection 1 - Question Review

1. Answer Review by Nirmalya Kajuri

I disagree with most of the answers here, including Victor Toth's, whom I usually agree with. Einstein did have issues with quantum theory, not just one particular interpretation of it. He did not, however, question the correctness of quantum theory. He was quite convinced that quantum theory was correct. His problem was with accepting that quantum theory as the complete description of nature. Einstein believed there had to be a deeper description of nature, of which quantum theory was a rough approximation. This belief stemmed from Einstein's conviction that there are two principles nature must follow. These principles we now call Locality and Realism. Let me explain them briefly.

You can not claim that Nature must follow certain principles.
The way a process evolves in due time, is random.
A different way is to study a group of identical physical processes and compare them with a different group. The way these groups differ you can call the principles of these groups.

Realism: Is an object (any object — could be an electron, atom or the Moon) somewhere even when you are not looking at it?

The fact of you looking or not looking at an object is a wrong approach to study physics.
Common sense assumes that when an object once is observed, it exists (for example: The Berlin Wall). That does not mean at the time when you want to see the Berlin Wall it actual exists. The Wall can be destroyed in between.

The principle of realism is basically the claim that yes, it is. Realism insists that the physical properties of an object (like position of an electron, or its spin) are objectively real.

The only thing that is objectivily real, is that an object exist.

God (or Nature) knows where the electron is, even when we are not looking.

This is not important in physics

Then Einstein said ‘God does not play dice’, he had this principle in mind.

God has nothing to do with this.

Locality: Think of two objects very very far away with many many galaxies in between. If I conduct an experiment on one of them and measure some property of it (say, its location or how fast it is moving) can the other somehow come to know what I measured instantaneously?

What is the definition of instantaneously?

The principle of locality says no, this kind of ‘telepathy’ should be impossible. Only if some signal has traveled from one to the other can the other object know. More generally, the statement is that any objective property of a system can’t change depending on what happens elsewhere, till a signal has had time to come.

Both of these principles sound very reasonable. Almost obvious probably.

Now in quantum theory, physical properties of the system are described in terms of probabilities.

To calculate probabilities is difficult.

The answer to the question ‘where is the electron/atom/Moon?’ is always given as a set of probabilities for all possible places it might be. The one exception to this is when we explicitly measure that property — at that point the object is of course entirely where we have found it to be. In other words, we can speak of an object being somewhere only when we are ‘looking’ at it. As soon as we take our eyes off, the best quantum theory can do is give you probabilities for where it might be.

Further, quantum theory does allow for a kind of telepathy. It says that if we do have two well-separated objects as before and measure some physical property of one, it can instantaneously influence the probabilities of measuring the same property on the other object. This happens when the two objects are ‘entangled’, which is a property of quantum systems we need not go into.

This does not yet outright contradict Einstein’s principles, but it does make things confusing by introducing probabilities in the story.

Einstein believed that quantum theory could not be all there was to nature. He argued that probabilities arise all the time in nature, when we have some missing information. When tossing a coin, if we knew exactly how the coin started out, the laws of classical mechanics would tell us just how the coin will land. But because we don’t have that information, we use probabilities to describe coin tosses. Einstein was convinced that something similar must be happening with Quantum Mechanics.

There must be a deeper description of nature, he believed, one that does not use probabilities. But because this description involves objects we can’t access, we must use the probability based Quantum Mechanics. He said “It seems hard to sneak a look at God's cards. But that He plays dice and uses "telepathic" methods... is something that I cannot believe for a single moment.”

In this, Einstein had been wrong.

A decade after Einstein’s death, a physicist called J.S. Bell showed that quantum theory couldn’t be compatible with both Einstein’s principles. He outlined an experiment that could test whether nature followed quantum mechanics or a theory that satsified Local Realism. Experiments by Aspect and others have now proved that quantum theory is correct. Nature could be follow either Locality or Realism, but not both.

I hope you appreciate how bizarre this is: all Einstein insisted on was an objective reality where something happening at the other end of the universe cannot immediately influence something here.

But Nature is weirder than he thought.

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