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Macroscopic Quantum Mechanics

Leggett Nobel Laureate

I went to a fascinating talk on Friday by Leggett, who received the 2003 Nobel Prize in physics, for his work on superfluids. (Notice that in physics, most people are referred to simply by their last names – “Smith did this, and Jones did that.” It helps to have a distnctive, but easily pronouncible last name!)

Leggett was speaking about macroscopic quantum mechanics – places where you can see quantum mechanics in “large” objects. Quantum mechanics is the theory which describes matter on the small scale where the everyday laws of physics break down. Motion of electrons, how atoms form and how photosynthesis harvest sunlight are all examples of where quantum mechanics knows exactly what’s going one. But why don’t we see any of these strange predictions in everyday life? Why can’t I be in two places at once, or tunnel through walls? The most famous example is Schroedinger’s cat, which we force to be, at least according to quantum mechanics, both alive and dead at the same time. Leggett asked us about this in the talk, and while most people were willing to believe that an electron could be in two places at once, most had a hard time buying that those same principles could apply to a full sized cat.

He then went on to discuss the possible implications of this. The first option is that quantum mechanics isn’t completely correct. Much in the same way that Newton’s laws of motion had to be corrected by Einstein’s theory of relativity when you’re talking about things moving very fast, maybe there is some new law of physics which comes into play with large objects. Roger Penrose has suggested that gravity might be the key. It’s the most troublesome force for a physicist, since it has such different properties to the other forces (like the electromagnetic force) – it’s much much weaker, but can span the whole universe. Perhaps for “large” objects, gravity somehow destroys quantum effects, and leaves us with the classical universe we see. Although no-one’s entirely convinced (except maybe Penrose?!), I’m certainly open to a possibility like that. Leggett then brought up a really interesting point which was whether this might challenge our typical view of physics, that by understanding the laws of small things, you can explain the big things. He speculated that the whole might be greater than the sum of the parts – that you can’t consider the little bits and combine them but instead have to consider the whole lot in one hit. I think what he was getting at here was more than the idea of emergent phenomena, which is where simple rules create complex behaviour (as seen in ants or superconductivity!) There, in principle you can explain the results from the basic rules, but it’s just easier to look at the whole system at once. What he was suggesting was that the basic rules couldn’t explain or predict the large scale behaviour – very interesting! I personally don’t like the idea, but that doesn’t mean it’s not true. I’m just more inclined however to believe that there is physics which exists on the small scale, but just isn’t important until you get to large objects – a bit like gravity!

The other possibility is that quantum physics is correct, but we just don’t know how to interpret it right. There’s the many-world hypothesis, about parallel universes, or there’s the usual “collapse hypothesis”, where “looking” at something destroys the quantum mechanics (which most people accept, but are uncomfortable with.) Perhaps, quantum mechanics is just maths – it doesn’t represent reality, there aren’t really wave functions, and all we’ve managed to do is come up with some clever maths which describes reality. I was surprised to hear that Leggett is tempted to believe this. Again, that really doesn’t appeal to me – quantum mechanics predicts so many things correctly that it’s hard to believe it doesn’t represent the real world. Of course, it’s possible that there’s more to the theory than we know (a bit like the discovery of special relativity) but I think that we’ve got to have at least an approximation of reality. Guess we’ll just have to see! The final section of the talk was about possible experiments which might be used to test these ideas, within the next 5-10 years, looking for large scale quantum effects in rings of superconducting materials, and in biological molecules (which is of great interest to me!). I couldn’t follow this bit so well, but it sounds like they’re doable, which would be very exciting.

So all in all, it was an impressive talk by a great speaker, who is not only brilliant but able to communicate his research well. In fact, I’m about to listen to another talk of his even as I type this – let’s hope it’s as good as the first!

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