Dialogue : Roger PENROSE and SATO Humitaka
Focus on Gravity
SH: Another point that surprised many of us is that usually quantum mechanics is not directly related to general relativity, but you think that fundamentally gravity or space-time properties are always important.
RP: At some level they're intimately connected. At least I've certainly found it useful to apply quantum mechanical ideas very basically in relativity. This has to do with clocks, because relativity is fundamentally to do with time. It's the metric that basically determines time through clocks. The length of the world-line is the time measured along the world-line. And if one wants a good clock, one turns to quantum mechanics, because it's basically the relationship between mass and frequency, which is what one has in clocks. The most accurate clocks are fundamentally quantum mechanical objects. So somehow maybe there is a connection between quantum mechanics and general relativity. Without a precise measure of time, one couldn't have a clear notion of what a space-time is.
SH: Many people think the relation between space-time and quantum mechanics only matters for extreme states, such as Black Holes or the Big Bang. But you're saying that even in ordinary processes the connection between space-time and quantum mechanics is important. That's a unique point.
RP: That's right. I don't think I remember hearing anybody else saying that. State reduction is fundamentally important. The way in which the micro-world and the macro-world relate is all through state reduction. Otherwise, you would never have any correspondence between classical entities and atoms, molecules and so on-our quantum mechanical entities-and yet quantum entities fit together and produce classical entities. There's a paradox there. Niels BOHR more or less gave up and said, well, you have a classical world and a quantum world, and your measuring apparatus is supposedly classical, only somehow it doesn't explain how, on the one hand, nature has these quantum mechanical ingredients and, on the other hand, things which behave in a classical way. So this bridge from one to the other is a fundamental part of the way the world operates. If state reduction actually happens in the world, the most likely place for it to happen is in connection with gravity. There are people who think about these issues and worry about quantum mechanical rules being modified at some level, yet many of them don't use gravitational schemes. But at some stage, they eventually relate their ideas to gravity.
In my talk today, I mentioned John BELL because-although he never specifically used gravitational ideas until very late in his life-he had intuited that here was the most promising place to look for something different. There were other reasons, too: gravity is the only field that directly affects space-time structure. Other things do so indirectly, via energy momentum, but gravity is different; it directly affects space-time structure. It is space-time structure, in a sense. There's something quite different about gravity, so it seems reasonable that when quantum mechanics addresses gravity, different rules may well apply.
SH: I think that most physicists-myself included (laughs)-feel somewhat disconcerted that if the ordinary atom's gravitational effect on space-time is negligible, then space-time can be considered fixed and unaltered by atoms. Absolute space-time, essentially. But you don't think so.
RP: Quantum mechanics works fine if one is prepared to consider the SCHROINGER equation as describing everything one needs. But at a certain level, one doesn't use the SCHROINGER equation; one takes measurements to do something different. What I'm saying is, when one moves from one description to the other, that's when gravitational effects really do need to be brought in. People think they are very small-fair enough-I can understand how they think gravity is just a force, an extremely weak force. And since it is so weak, why should it affect anything ? But I'm thinking about it a different way around.
SH: Classically, the effect is small. But we still don't know about quantum action in space-time.
RP: Yes. We've been thinking about how quantum mechanics might apply to just another force, not in terms of the very basis of quantum mechanics and how that's affected by space-time structure. The very way one uses quantum mechanics depends upon knowing what time displacement is, and one starts to get into a problem with that when one looks at gravitational fields that must be dealt with in superposition according to quantum mechanics.
Yes, I'm looking at things a different way around, because the way people have been thinking about the combination of quantum mechanics and gravity has been in terms of two other things: one is cosmology and the Big Bang-also the Big Crunch, maybe, if there is one-and singularities in Black Holes, where one is forced to look for some connection between quantum mechanics and general relativity.
SH: I think most people would agree to that.
RP: It's important. But as I was saying in my talk today, there's a great puzzle here. The asymmetry of time, the difference between the beginning and the end, is huge. Yet if gravity is just another physical field where one applies ordinary quantum mechanics, why do we get this gross asymmetry in time? There's nothing else which does that.
So what I'm saying is, this has to be completely different from what one has seen in other physical theories, but people think these effects are negligible because they're looking at it the wrong way around. It's the effect of gravity on quantum mechanics, not the effect of quantum mechanics on gravity. The other place where I would expect that quantum gravity is likely to be important is much less esoteric. It's happening all the time in biology. One can't make sense of quantum mechanics without state reduction, as a physical process. One can make sense of it as just a piece of mathematics. SCHROINGER equation is evolved, but then it gives us things we don't believe, like cats being alive and dead at the same time.
SH: I feel that your image of space-time is very rich. Sometimes, when people talk about space-time, space means nothing. An empty structure, nothing there. But it seems your image of space is more structured.
RP: There's a real thing there. But this is how general relativity works, as a very objective picture of space-time as a thing, which satisfies very clear equations.
SH: A physical entity.
RP: Not just the absence of things..
SH: Which makes a crucial difference! (laughs)
RP: But I think this is what EINSTEIN was saying. Well, he was driven into this position, I suppose. It was actually MINKOWSKI who said it first-that one has to combine space and time. But to make sense of general relativity, or even to make sense of special relativity, one needs to have some very objective picture of space-time as something out there. Otherwise, it's hard to make any sense of it.
I always thought "relativity" was a very bad word somehow. Because relativity creates the wrong impression; it implies that it somehow doesn't matter, that everything is relative. And yet there are these absolute notions. Space-time is an absolute notion; even if space and time individually are relative notions, space-time is objective. I'd imagine this view is common among relativists, but I may be wrong.
SH: But then there's ZENO's paradox, which we usually take as absolute. The paradox that the criterion of existence is to mentally picture something in space. So in order to try to think about real space-time, we have to situate this space-time in space. (laughs)
RP: From the point of view of visualizing it?
SH: People always try to see the existence of something.
RP: But with relativity, most often people just calculate and write down equations, without asking for pictures of what's happening.
SH: That requires some training. It's probably too abstract for ordinary people, who think that the existence is all that we image that something in space, that always becomes very difficult, that space-time itself is a physical entity, or space. For example, it's very difficult to imagine that space-time doesn't exist, so the creation of space-time is always a trouble. (laughs)
RP: If one works with relativity, one can easily reveal the real structures.
SH: Yes, of course, but the image is very difficult!
RP: True. It's hard to think about four dimensions in a very accurate way.
SH: Why would human consciousness have been created this way ?
RP: Well, that comes back to your "sudden" questions-is it something which came about suddenly? Again, I don't think it's a specifically human quality. I don't know how far down in animals, but I would say that anyone who has owned a dog would find it hard to imagine that there isn't some kind of consciousness there. And I have very little doubt that apes, for instance, must have awareness. Elephants I'm sure do too, from things I've seen. But I'm sure it goes much more down below. I suspect there are some qualities of consciousness that go very far down in the animal kingdom. Whether fish have much consciousness, I don't know, but I don't see why not. It could be much less-there's a quantitative aspect to it-but it must be valuable to the way animals behave. Having some awareness of what's happening out there makes them behave more effectively than if they had no such awareness. It's selectively advantageous to them, so it gets developed. But the potential must have been there all the time, otherwise they couldn't draw upon it. Like in much of natural selection, something offers an advantage at one point, which enables the development of certain structures, and then these are found useful for something else. I imagine it must be like that with consciousness itself. Maybe one-celled animals don't have any (laughs), nevertheless they have structures with potential to develop it. Just maybe, and this is a pure guess, they might take advantage of some kind of quantum coherence, which even without consciousness can be valuable to these creatures. But that's all very much guesswork.