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QED Ask a Scientist – Jon Butterworth

During this year’s QED Conference in Manchester the Cosmic Genome team gave attendees the chance to tweet in, ask us at The CG desk, or even ask on camera, a question to a select few scientists from the Cosmic Genome App. 

Professor Jon Butterworth

Question - Hi, my name’s James Lennox-Gordon [@JLennoxG on Twitter], and my question is: at school we’re taught that a moving body has kinetic energy, but special relativity tells us that body a) that’s moving hits body b) that’s static, you can also explain it the other way round and say that body b) is moving, body a) is static, so how do you apply kinetic energy in that environment?

Jon - So, kinetic energy is the energy that bodies have because they’re moving, and the question correctly states that movement is all relative, there’s no absolute rest frame, all that’s correct, and what that means is there’s no absolute zero energy level in that sense.  So if you have a body moving in free space, its energy is exactly as relative to its velocity, its kinetic energy.  However, if you think of the context where kinetic energy usually shows up, for instance, the kinetic energy of molecules in a gas that’s hot and they're all moving around then there’s not choice of rest frame, which is what you do in special relativity, you choose a frame with which to measure relative velocity.

There’s no frame that you can choose in which all the atoms in the gas will all be stationary, so ideas like the total kinetic energy of that gas can never be zero, there’s no way of choosing a frame for that to be zero.  So for a single moving body it’s certainly true that the kinetic energy depends on the frame in which you look at it.  If you look at the way Einstein formulates special relativity, in fact, it’s formulated in what we call a coordinates transformation in which you move time and distance and change as you change speed in such a way as that the whole theory hangs together, and that’s also true of energy and momentum, they obviously change as well and they change in a way that’s dictated by the theory. So it all hangs together and you have things like invariant mass which doesn't change, but momentum and energy do change in the rest frame in which you're measuring them.  So how do we explain kinetic energy, special relativity tells us that a body is either moving or stationary; energy can be non zero or zero depending on what the body is doing in your frame, energy is not an absolute. 

Question - Hi, my name’s Michael Hales; recently I read about a thing for an explanation of the double slit experiment, so basically everything’s waves and particles are floating on top.  So the particle goes through one of the two slits but the wave goes through both and where the particle ends up is determined by both of the things and it seems reasonably straightforward.  And when I thought about it, it could also explain how particles appear from nowhere, quantum entanglement and sorts of bits and bobs.  I believe it’s related to something called Bohmian quantum mechanics, I don’t know; is explaining everything this way in terms of waves good for explaining it all to your dad?

Jon - Right, I presume this is Bohmian mechanics, not bohemian mechanics! Since this is the real life, not a fantasy! The question asked whether Bohmian mechanics is a good way of explaining physics to someone to help them understand it or be just wrong, I think the answer’s neither: I wouldn't use Bohmian mechanics to explain quantum mechanics because it’s not known, it’s a formulation that isn't widely accepted but it’s not known to be wrong, so as far as I know you can formulate Bohmian mechanics in a way that gives exactly the same results as quantum mechanics, so really you're arguing about an underlying layer of interpretation which is undecided and quantum mechanics, in general, if you have a…Bohmian mechanics introduces hidden variables to try and explain what most physicists would say are inherently probabilistic quantum mechanical predictions.  It would be too harsh to say Bohmian mechanics is wrong because I think it’s, as I say, it’s consistent with other interpretations of quantum mechanics, but I wouldn't use it as a starting point for explaining to anyone really; it’s controversial and I think more complex, actually.  Although maybe conceptually easier, although you still need…it still needs to be non-local, so I’m not even sure it’s conceptually easier either, really.  

I’d take the for all practical purposes approach: I’d say that we don’t quite know in quantum mechanics what’s going on but we can make very clear, practical predictions and they work, so I would start at least from that point of view.  Which is not to say it isn't interesting to ask further questions, but I would start with what we know, which is that these calculations work, quantum field theory works, we don’t need to have hidden variables, but it predicts inherently probabilistic events, and I think that’s what you need to know because that’s what describes the universe.  That’s not to say that thinking about ideas and interpretations of that, and is that all there is, and of course that’s very interesting, but if you want to learn about physics it’s good to start with the bit that we know and go on from there rather than the other way round. 

Question [also from Michael Hales] - Why is gravity a force and the Higgs field not?  

Jon - I guess the simple cop out answer is we don’t know, because we don’t actually understand gravity and quantum mechanics within a common framework, and really to draw comparisons between the Higgs field and gravity you would want to be able to formulate them within a common theoretical framework.  Of course we can’t do that, quantum mechanics has trouble with gravity, and gravity has trouble being quantised, and there’s lots of active areas of theoretical physics trying to deal with that and have been for a long time and not really got very far.

There are a couple of other things that you could say though, I think, and I’ve just been speculating on this because it’s an interesting question, but I think you could say that…there’s no real reason to say that the Higgs field isn't a force at some level, it’s not like it’s an interaction between particles, so it’s not a force like electromagnetism, or the weak force or the strong force in the sense that they're all based on symmetries and the Higgs field is not based on symmetries in the same sense. On the other hand, particles can exchange Higgs bosons between them, and that’s how forces occur in quantum field theory.  So I think if you could have…if you had a really strong, a really heavy fundamental particle, say, like the top quark which is 175 or more times heavier than the proton, which is quite heavy for a fundamental particle, if you had really heavy fundamental particles then you might well be able to observe the Higgs interactions between them as a kind of force.  The reason we don’t, or that we do have top quarks but they decay super quickly so we would never…they never live long enough for us to see that. But I think, say for instance if dark matter is a really a very heavy, stable,  fundamental particle that interacts via the Higgs field, then the Higgs would be effectively a force there, it would dictate how often dark matter particles scattered off each other or whether they attracted each other or repelled each other or whatever.  So I think in some sense it is a force, it’s just a different kind of force and gravity on one side…we don't understand gravity anyway, the other three forces in the model are based on symmetries and the Higgs field is not, but the Higgs field does provide an interaction, a way of particles scattering off and being either attracted or repelling each other, so in one sense it is a force, it’s just not like the others.  

Question [from @sparsjjoshi42]- What exactly is baryonic acoustic oscillation? 

Jon - Baryonic acoustic oscillations, well, it’s a little outside my comfort zone - I’m a particle physicist not a cosmologist - but my understanding is that at some point as the universe cooled down after the big bang, things got calm enough that electrons could…well, firstly that quarks could stick together and become protons and neutrons and they could stick with atoms to form baryonic matter, that’s neutral atoms, and that happened at some point as the universe cooled and obviously we see those things around us now.  The big bang was quite a violent event, so there were lots of ripples, pressure waves and things, sound waves if you like, in the universe, right from the beginning, echoes of the big bang if you like, and they would have been density fluctuations, so there are places where more atoms and baryons would have formed because they were denser there and they could combine quicker, and other areas where less would have formed because there was a low pressure area in these pressure waves; pressure waves are just sound waves really.  So you get these kind of structures, oscillations basically, where in some places more baryons were formed and others that weren’t, and they're still there in the universe as fluctuations in the density of baryonic matter, and I think that’s what baryonic acoustic fluctuations are and you can observe that with large scale structures of the universe, for instance.