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Quantum Mechanics

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Princeton professor, Freeman Dyson,
physicist-mathematician (1923-2020)

from https://www.youtube.com/watch?v=X7VxXkW-8uA

Interviewer: Freeman, general relativity and quantum mechanics are the two elements of science that most explain what the world is; one, the whole universe, the other, the microstructure of reality. Everybody tells me that we gotta integrate these two to make sense of reality, and everybody’s trying to do that – is that really necessary?

Dyson: I don’t think so; but I’m in the minority there, as usual. I think the world, the world of physics, does have two different aspects, which, maybe, should not be unified. In some way, I like the universe to be more diverse, more subtle, than putting it all together.

Anyway, here’s what I think: that, the classical world [that can be measured and quantified, including Einstein’s general relativity] is what we know about the past, all our knowledge about the past, essentially, is classical, that is to say, we have facts, we know what happened when, we know that the Earth condensed out of a cloud of dust, we know that the continents have been shifting about, we know that a particular atom of uranium did, in fact, decay at 9 o’clock yesterday – so, those are facts.

On the other hand, we have quantum mechanics, which talks about the future, and enables us to calculate the future. If you want to know when this uranium atom is going to decay, there’s no way you can tell, you can calculate the probability that there’s one chance in a million that in will have decayed by the end of next week, something like that; so, that’s what quantum mechanics can do, it’s all it can do. It’s about the future, it’s about probabilities – so, why should you try to unify those two?

I don’t think you have to. And, in fact, the great founding father, Niels Bohr, who was more or less the founding father of quantum mechanics, believed in keeping them separate, and so I’m just following him. He always believed that there is a classical world, and your measuring apparatus is always classical, everything you can say with certainty is classical, and [then] there’s also the quantum world, which is not directly observable, which is there, but all you can do with it is to calculate probabilities.

So, I don’t find anything wrong with that. And, of course, there is a consequence when you come to look at general relativity, because general relativity is a classical theory, it tells how the universe behaves on a large scale, it’s a theory of gravitation, of space and time, it’s a geometrical theory, and it has been enormously successful, in a way, the most beautiful theory we have, in the sense of being precise and subtle and still simple in its foundations, and in some ways, it spoils it if they are brought together, I don’t see that one should. So, I would say, let’s leave gravitation theory, the theory of Einstein, as part of the classical world, don’t try to drag it into quantum mechanics.

Interviewer: The argument, though, takes us back to the very beginning, it says that in order to go to the first instant of the Big Bang that the two [theories] fight, and mathematically there are absurdities that occur, and so something [to unify] must happen if we want to push our understanding to that ultimate point.

Dyson: Well, that we don’t know [there is much conjecture concerning that first instant], it’s just a statement and not an argument. We know nothing about the Big Bang in detail – it certainly happened, but, and it’s certainly amazing how far we are able to explore into the past, but we certainly haven’t yet got there [to the beginning, in terms of absolute knowledge].

Interviewer: Well, we have these four fundamental forces, and first electricity and magnetism were united, and then the “weak force” was united with that, and then the “strong force”, and the one hold-out has been gravity – why should that remain a hold-out? in terms of being unified at some energy level or something?

Dyson: Well, [the reason why gravity might not join the unification is that] it could be something like temperature; temperature is a classical quantity, you can measure temperature, you can talk about temperature [as a fact of the world], but you can’t quantize it, you can’t talk about temperature as a quantum object, there is no…

Interviewer: … a “tempon”! [analogous to the purported elementary particle “graviton” which likely does not exist]

Dyson: Yes, there’s no such thing as a “tempon.” It’s a statistical property of matter in bulk. And the same thing could be said of gravitation.

Interviewer: Well, that’s very interesting, because that would mean that the so-called graviton, [purported to be] like the photon for electro-magnetism, the quanta of gravity, is – a fiction?

Dyson: Yes! I would say it is a fiction, in the sense that you can study it mathematically, but you can never study it in the real world. I cannot think of any thought-experiment which could tell whether a graviton is there or not…

Interviewer: So, your world view is completely comfortable if gravity is never integrated with the quantum?

Dyson: Right. It’s certainly quite plausible. I don’t say it’s true, but there’s certainly no evidence, as far as I can see. There is a very famous argument of Bohr and Rosenfeld… analyzing very very carefully the quantum measurements of electromagnetic fields, he proved that the photon, in fact, exists, that the photon really has the properties that you postulate for it, that you can’t have an electromagnetic field without having photons.

Now if you look very carefully in [paper] you’ll find something called “compensation”, which is very essential. The measuring apparatus consists of an electric charge and an electric current, and, when you use that to measure the field, the measuring apparatus itself produces additional fields, which will mess-up the measurement, unless you compensate, and so you have to have another set of charges and currents, which you don’t measure but you use them to compensate for the ones that do measure. So, that compensation is an essential part of this whole argument.

Well, try to do that with gravitation [which it's suggested should be possible if gravity is made of elementary particles]. When you measure a gravitational field, you have to have a mass to measure it with, and by looking at the mass you can tell what the gravitational field was, but there’s no way you can compensate a mass with a negative mass, so there’s a real difference there, and this argument of Bohr and Rosenfeld, which supports the quantization of the photon, fails when you come to gravity.

[They conclude with comments that gravitational force is so enormously smaller than the electromagnetic, and that if you tried to create an apparatus, Dyson said, to measure the gravitational force you’d have to increase this force, in order “to see a single graviton”, to make it detectable by apparatus, to such an extent that a black hole would be created.]

Dyson: And I have calculated that even in principle cannot be done, that if you did build such an apparatus to measure a single graviton, it would, in fact, collapse into a black hole, and so Nature would say “No – you can’t do it!” [All of which suggests to Dyson that gravitons do not exist and, by extension, there should be no effort to link general relativity to quantum theory.]