February 14, 2011


Roger Penrose: Impossibilities in the real world have to be possible (Subodh Varma, Jan 15, 2011, Times of India)

Many of these impossibilities are controversial, like where and how does the bizarre quantum world end, and our sensible 'classical' world begin?

One point I should make - what you say is a good example of the puzzles you see in quantum mechanics because some of them are genuinely mysterious things that are true of the world. they are not contradictory, they are just puzzling to us. Other things strike me as being genuinely contradictory. For example, the measurement of the Schrodinger's cat or the transition between the quantum world and the classical world seem to me to be things that current theory simply does not explain. So they are both intriguing. Quantum entanglement is a very intriguing issue but it is not impossible. It is something we have to get accustomed to, understand and see what it can do, what its limitations are, and what different picture of the world it provides us with. That's fascinating, but its not a paradox. Its what I call a puzzle mystery rather than a paradox mystery. Whereas, the measurement issue strikes me as a paradox in the sense that the cat in superposition of death and life is a direct consequence of Schrodinger equation. The linearity of the equation tells you that the superpositions between things can quite happily occur - say a neutron can take two paths at the same time or a photon. This is puzzling but not a paradox, in the sense of being a contradiction. It is something unfamiliar and strange. When you use the linearity of the Schrodinger equation or quantum unitarity, this tells us that you should be able to have a cat, which is both alive and dead at the same time, and you could easily produce one. It is not as if you need some sophisticated, impossible to construct apparatus. It's quite a straightforward experiment to do, although you wouldn't want to do it to a poor cat! But nevertheless, in principle, it could certainly be done without difficulty. You would not see a cat, which is half dead and half alive; you would see a cat, which is either dead or alive. This seems to me a genuine paradox and I agree with Tony Legget's discussion in his talk that the common view that it's the decoherence of the environment which gives us either/or for the cat being alive or dead, is not the explanation. There's got to be something else going on which we still don't understand and which to my mind must represent a departure from the standard rules of quantum mechanics at some level. It's a very subtle and physical level and we have to find and locate it but it has to be there.

The further point that I think is part of your question is that I have separate reason to believe that this level of departure from standard quantum mechanics or quantum linearity is something that occurs when gravity becomes involved in the quantum system. For several apparently independent reasons, gravity is a place where we will have to invoke a departure from standard quantum rules. I used to argue from the principle of general coherence which is the underlying principle of general relativity that you have to have a framework which is independent of the coordinates you use. But more specifically and more powerfully the conflict is with the principle of equivalence. This is the foundation of Einstein's theory of gravity where falling in a gravitational field freely is something where you eliminate the force. The equivalence between a gravitational force and an acceleration force is the foundation stone of Einstein's theory. And the exploration of how that fits in with quantum mechanics leads to a place where one expects a limitation to standard quantum rules. The sort of order of magnitude is something that, in the case of a cat (if you had a cat in a dead or alive superposed state) that state couldn't persist for more than a very, very tiny fraction of a second, a ridiculously small time. Planck time is 10-43 seconds, which is the sort of time you would expect if you had a particle which had a mass of 10-5 grams (the Planck mass) superposed in two separate locations. On that scale it would decay in 10-43 seconds - it is almost instantaneous. But for the huge cat it would be much less time. The bigger the mass displacement the smaller would be the time scale for it to decay to one or the other state. But it is very hard to make an experiment, which would actually see this effect. It is not so hard however as it would be to make an experiment that would test quantum gravity, in the sense of how quantum mechanics would affect space-time structure.

Some experiments of this nature were initiated, I believe.

Yes. Let me just put this another way. According to what people refer to as quantum gravity, normally people mean that quantum mechanics is not affected; it is only gravity that is affected or space-time structure. And so experiment to detect the effect on gravity in space-time structure would require a ridiculously huge accelerator or something, that is way beyond the techniques we have available now. But if we are looking at how gravity might affect quantum mechanics, its much more optimistic. We are only looking at experiments that are sort of at the cutting edge of present day technology. You would have to improve experiments, but its not so far. In fact, a colleague of mine Dirk Bauwmeester in Santa Barbara has been involved in an experiment in which you try to put a little mirror - this mirror being 10 micron cube or about a tenth of the thickness of a human hair, just a little too small to be able to see with the naked eye - and this would be put into a superposition of two locations which don't differ too much, only by about the distance of the width of an atomic nucleus. You wouldn't be able to spot the difference with your eye even if you could see it. But the difference would be comparable with the diameter of an atomic nucleus. And then it would be a matter of seconds according to this scheme before it would become one or the other. If the scheme is correct, that is. Based on general principles its not a detailed proposal, its just an estimate of the scale based on general principles and so it would be very exciting if it could be detected whether you could hold this superposition of the mirror for a few seconds. I haven't been in touch with Dirk for a couple of years now, so I don't really know how far it has gone. I know that the main obstruction to doing this experiment was not in any of conditions needed - for example you need to keep it at very near to absolute zero (apparently it works very well then), you need a good vacuum, you need isolation from vibration, you need the suspension holding the mirror to be very perfect and all these things look as though they can be achieved. The main problem as I understood at the time was having a mirror which was perfect enough and still as small as this. I think you can have mirrors that are perfect enough. But what you need to do is you have to split a photon into two beams so that its in a superposition of being in one beam and in another beam. Then, you have to make a cavity to preserve the photon by reflecting backwards and forwards some millions of times. This requires a mirror that is very, very perfect. Now, mirrors do exist that are that perfect but to make them small enough like a speck of dust is a tricky business. I don't know whether they can do this. I don't know the present status.

Why can't we get a handle on these mysterious, paradoxical things - is there an inherent limitation to human mind or are we just waiting for something more to be discovered?

I don't see any limitation of human understanding at this level. There are two puzzling aspects of quantum mechanics. One is the puzzling feature and the other is the contradictory feature. It is confusing and it is not something that we directly experience as we do at the classical level. We are used to them. We find it strange to imagine particles at two places at the same time. But nevertheless we can use our visual imaginations to understand wave functions and the things that are the ingredients here and as long as they make logical sense and are not too fantastically complicated they can be strange and that takes some getting used to, but they are not beyond human comprehension.

Perhaps some of their predictions can be experimentally established?

Oh yes and this happens all the time and if you see that certain experiments give you answers - you get used to seeing what sort of things come about from physical situations.

What about Einstein-Podolsky-Rosen? Is it a puzzle or paradox?

That's a puzzle. Sometimes referred to as a paradox but I regard it as a puzzle. It's a puzzle because it is unfamiliar and it requires a certain understanding of mathematical ideas which are not trivial. But you get used to it. And people who are good at quantum mechanics and make good experiments can make good predictions about what is going to happen. Ok, it's different from the experiences we have of the classical world but they are not incomprehensible. And so that's why I call them a puzzle. If I think about it for long enough, getting used to the ideas, you get to know what to expect and you get an understanding. And, this understanding is not beyond human comprehension. Where I would say something might be beyond human comprehension would be something that's actually inconsistent. So you have a description as you have in quantum mechanics - two descriptions that are in contradiction with each other. One is the evolution according to Schrodinger's equation and the other is the description of what happens when a measurement is performed. That is in contradiction with Schrodinger's equation. And that I believe requires something new so we need a theory that is within human comprehension. We are at the gap between discovering the scheme and that will require some deep insights which may not be available to all of us. But once that theory is made clear enough and explained by people who are good at explaining things I would hope that it would be within human comprehension and I don't have any expectation that it will be non-comprehensible.

Some people believe that detection itself is responsible for entanglement. What is your view?

Depends on how you are thinking about quantum mechanics. Sometimes people refer to detection in the sense of some person perceiving it. It doesn't usually mean that. It means that you invoke the measurement part of quantum mechanics. You have a device that is sensitive to one thing or another and before that device is activated, one or another thing may occur in superposition. This is well part of quantum mechanics It is not that the two things - one or the other - happens. It is the superposition of the two things. But when the detector receives it, there's something in its mechanism that says - no, it is one or another, not the two together. Somehow, there is a magnification of the quantum level process to a classical level scale and in that magnification there is something mysterious that is going on that we do not understand. So, I say that we need that understanding and that a superior, improved quantum mechanics' scheme will give us that understanding. But we don't have it now.

...why/how is there a detective/Detective?

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Posted by Orrin Judd at February 14, 2011 5:11 AM
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