Quantum mechanics: We indeed know how we can use it, but we still do not understand it completely.

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Neither cosmology nor quantum mechanics nor another theory, with which we work at present, has a chance to be the ultimate theory. Read more:

Extracts from an interview follow:

US physicist Daniel Greenberger, together with Michael Horne and Anton Zeilinger, has developed an important system of quantum physics.

**STANDARD:** One of your best known works came into being with Michael Horne and Anton Zeilinger. In quantum physics systems were considered for a long time in which two particles cannot be described separated from each other – entangled particles. With the Greenberger Horne Zeilinger (GHZ-) State you have been the first who studied the entanglement of more than two particles – what is special about this?

**Greenberger:** Albert Einstein, Boris Podolsky and Nathan Rosen have defined something in their examination with entangled states what they called an "element of reality": When one can say something about a particle in one situation without interacting with it, then this particle would have to be an element of reality – that is a classical concept which seemed to make a lot of sense.

**STANDARD:** What would be an example of such an "element of reality"?

**Greenberger:** Let us consider the case that I have two sheets of paper in front of me. On one I write an X, on the other a Y. These sheets I give to two persons. As soon as one person opens his paper and has an X in front of it, he knows that the other person has got the Y, without opening his sheet. Einstein would have said that in this situation the Y is a real property of the paper, already at that moment when I separate the sheets. Quantum physics does not deny this. It says when one has a system which is entangled in the sense that there is an X and a Y, but one does not know one which sheet stands what, the state of the individual sheets is undetermined for so long until I have a look at one of them. Until the measurement, only the correlation of the sheets is defined, but not the individual state of a sheet.

**STANDARD:** Niels Bohr, who represented the quantum mechanical view, and
Albert Einstein, who held on to the classical idea, have held a long debate about this – why?

**Greenberger:** We have a situation here, in which both Einstein’s argument, that the sheets have a defined state since the separation, as well as also Bohr’s argument, that they do not have this until the separation, seem to be correct – although they are completely different standpoints. For a long time it was thought that that is just a philosophical discussion. But John Bell could show that one can differentiate experimentally between classical and quantum physics. When the Bell experiments were carried out, they ended in favour of quantum physics.

**STANDARD:** The GHZ experiment is a further development of the Bell experiments. Which further cognitions can be won when one has a look at an entanglement with more than two particles?

**Greenberger:** What we have shown in the GHZ experiment was that one can produce a state with three particles, which is impossible to produce classically. And this quantum mechanical state obeys the classical idea of elements of reality: When one measures two of these particles, one can predict the third without measuring it – so as Einstein had demanded it. That is the reason why the GHZ state is so essential: One can predict, without to touch. And even so one cannot produce such a state classically.

**STANDARD:** What do we have to deduce from this result?

**Greenberger:** The idea of the classical idea of causality – that that what happens at one place, is determined through something that has happened at an earlier point in time - is no longer valid in quantum mechanics. In this respect the theory is a-causal. By the way: When we did this, we had no idea where that would lead to. When we first had found out this, we thought we had made a mistake because it seemed so impossible.

**STANDARD:** Does one have to give up causality completely with quantum mechanics?

**Greenberger:** We need a new concept of causality – and also of reality. At present we do not know what "real" or "causal" means – these words have different meaning in different contexts in quantum mechanics. The classical understanding of science, that the scientist can look at the system in a very gentle way without disturbing it. In quantum mechanics that is not possible. The results here depend on how one observes.

**STANDARD:** Would it be necessary at this point in time to also think about the philosophical understanding of physical concepts?

**Greenberger:** Indeed, I think that is very important to approach these things also from the philosophical perspective. Nevertheless there is a strong tendency in physics to say that philosophy cannot help there because experimental situations are not clear. But often they are clear, and there are many important philosophical considerations.

**STANDARD:** At what can philosophical insights be important?

**Greenberger:** For example with the attempt to quantify gravitation and to unify quantum mechanics and theory of relativity, the people deal with very many mathematical problems. But I say: Long before one reaches mathematical problems, one comes across physical, conceptual problems. When one attempts to quantify gravitation without understanding it on the simplest level, one puts the cart before the horse. In quantum mechanics it is generally very often valid: We indeed know how we can use it, but we still do not understand it completely. We can make fantastic experiments, which are very exact, but when we rush our understanding too far ahead, it falls apart. That is what makes the theory so fascinating.

**STANDARD:** Are there similar situations in other fields?

**Greenberger:** In the last 40 years we have made fantastic progress in cosmology. But we are still not very close to the ultimate truth. The problem of dark matter and dark energy remains in existence; about 90 per cent of the universe we do not know about. How can there someone look into my eyes and say: "We are close to it understanding the universe"? It appears to me that there is a tendency in physics to be too self-satisfied. We should develop a sceptical attitude. I think neither cosmology nor quantum mechanics nor another theory, with which we work at present, has a chance to be the ultimate theory.

**Daniel Greenberger** (82) is physics professor at City College New York, where he researches and teaches until today in full time.

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