SCIENCE AT THE SHINE DOME canberra 6 - 8 may 2009
Symposium: Evolution of the universe, the planets, life and thought
Friday, 8 May 2009
Dr Tamara Davis
Department of Physics, University of Queensland, Brisbane
Tamara Davis specialises in interpreting astrophysical data in terms of their implications for fundamental physics. She completed her PhD at the University of New South Wales in 2004, and received the award for the best science PhD submitted at UNSW that year. She then worked at the Australian National University’s Research School of Astronomy and Astrophysics, helping design a space telescope for NASA, before moving to Denmark to join the Dark Cosmology Centre. In 2008 she returned to Australia and is currently a research fellow at the University of Queensland, working with the Australian-led WiggleZ dark energy survey, while maintaining a part-time associate professor position at the University of Copenhagen.
The evolution of physical law
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Thank you very much, Brian, for the great introduction, and thanks very much to the Academy for giving me the chance to talk about what is a really exciting topic for me. Professor Turner has given us a fantastic introduction to the evolution of the universe; now I am going to talk about the evolution of physical law. Basically, I want to ask: are the laws of physics set in stone, or what parts of the laws of physics might be changeable?
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In science, there is a great tradition that things that were once considered static turn out to be mutable; species are a fantastic example. The realisation that species evolve and change led to a revolution in biology that allowed a far more rational, far more coherent view of nature’s history. I think even the word ‘revolution’ – the modern use of it anyway – came from the revolution of the spheres, which was Copernicus’s seminal work, which showed us that heavenly objects are not fixed and immutable and we are not the centre of the universe.
The Copernican revolution was arguably brought to its current state of affairs by Hubble, who showed us that the universe is expanding. Before Hubble, the idea that the universe was fixed and unchanging was so set in our consciousness that no lesser a mind than Einstein’s was so horrified by the thought his equations predicted an expanding universe that he introduced the infamous cosmological constant into his equations in what was ultimately a futile attempt to stabilise the universe against expansion or contraction. In retrospect, that attempt is somewhat ironic because Einstein was the person who taught us that space is, indeed, dynamic and changeable. But he found difficulty when he tried to fit his new ideas into the preconceived notion that the universe had to be eternal or static. So that leads us to the question of: what do we still have in our preconceived notions that are ill-founded? That is what I want to talk about today.
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The famous physicist John Wheeler, who is famous for amongst other things coining the term ‘black hole’, thought of physical law in terms of law transcending previous law. He had this investigation of a staircase where, at each step of our increasing knowledge, we discover something new that overtakes the previous aspects of the universe that we thought were fundamental. Think of something as simple as density: that used to be fundamental, but we realise that that changes with pressure. So our understanding of our physical law evolves. In a way, this can be thought of as a form of natural selection, because our previous theories have tiny little mutations on them that people put forward. Most of these got get shot down immediately, some persist and occasionally we come up with ideas that are adapted to the data so much better than the previous theory that they take over and go to dominate the rest of our thought.
I am not necessarily most interested in the evolution of our understanding of the physical laws but in whether the laws of physics themselves can change and what aspects of the laws might be mutable.
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In order to talk about that, we first have to talk about exactly what are the laws that I am referring to. A few different things come to mind. The conservation laws are one – things like conservation of energy. Apart from having really firm observational status of these conservation laws, we have a pretty good reason, just based on physical principles of the logic of what we would expect a physical law to do, to surmise that these hold. It was Emily Noether who put forward the proposition that realised that conservation laws represent symmetries in nature. So conservation of energy is just the concept of time symmetry, the fact that the laws of physics should be the same then as they are now. If that was not true, it would be pretty hard to do any physical experiment and have any predictive power in physics, if it changed over time. Conservation of momentum is just spatial symmetry. If you move, the laws of physics are the same. Things like angular momentum are rotational symmetry. So the laws of physics are the same, no matter in which direction I look.
Despite the fact that these seem pretty reasonable assumptions, symmetries can be broken; in fact, our very existence demands that. Things like reflection symmetry might not hold perfectly. So even these conservation laws are things that we might need to actually question.
You might also think of other things like the dynamic laws. In physics, we have a tendency to give the name ‘law’ to things that are not really particularly fundamental. Hooke’s law of springs is one example. It says that the strength of a spring is proportional to how much you stretch it, but really the electromagnetic force holding the spring together is the more fundamental of the forces that is going on here. How about the laws and the forms of the laws that describe the most fundamental forces that we have in nature? For example, is the 1/r2 law of gravity really fundamental, or is this something that could possibly change? Think about that.
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Think of a sphere. If you have any particular force emanating from a point, the strength of that force needs to spread out over the surface area a sphere as it goes out. The area of the sphere is 4πr2, so the strength of any force that does this is going to have to decrease in proportion to the surface area that it has to cover. That means that the fact that the 1/r2 law works for gravity is just a manifestation of the fact that we live in three dimensions. That seems to be pretty fundamental – the same reason that it would work for electromagnetism. If we lived in a two dimensional world, the force would be stronger. We would have a 1/r law because the force would normally have to spread out over a disc instead of over a sphere. So, if the number of dimensions is different, maybe this could change a little.
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Also, when general relativity supplanted Newton’s law of gravity, we discovered that, ‘Uh-oh, space and time are dynamic and curved.’ So, although the same principle applies, the 1/r2 law does not actually hold perfectly any more in general relativity, because in curved space the area of the surface of a sphere is no longer exactly proportional to r2. So that was also something that could possibly change.
What I really want to ask for the rest of this talk is: when we ascend the next theoretical step, what is it that we are going to discover that is going to be dynamic that we have previously thought was constant? It is an important question to ask because we are really asking: what are the crucial aspects of physics that we need to include in the next generation of physical theories? Sort of guide our understanding in trying to explain things like dark energy and dark matter.
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That brings me to the last thing that I was thinking of here, and that is the constants of nature. They appear in a lot of theories at the moment, but there is really no good reason that they actually take the values that they do or even no good reason that they are actually perfectly constant. That will be left to the next theory that we have to be able to explain.
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Now I am going to give you a very brief glimpse into a couple of the interesting things that we are doing with the next theories of physics that will hopefully give us new insight into the universe. Recent history of advances in physics has basically been a story of unification, of bringing together things that once were considered separate. Something brought this back to me really strongly recently. It was when I was at a quiz night. I absolutely hate quiz nights. They always ask these really hairy astronomical questions that I just do not know how to answer – things like how many planets are there in the solar system? I guess it is nine or eight – what do I answer here? What are they looking for? But I was I was roped into this quiz night because I knew the quiz master and the quiz master has obviously come up with this question and said, ‘Oh, this one is for Tamara. Tamara is going to know the answer to this question; she’s an astrophysicist.’ He comes out with the question: what are the three forces of nature? Any self-respecting physicist knows that there are four forces of nature, at least in the way that we count them these days. I thought for a second and I got it. I managed to get this one right. I guessed that what they were looking for were gravity, electricity and magnetism. They must have been working from a slightly old quiz sheet because we have known for comfortably more than a century now that electricity and magnetism really are just the different manifestations of the effects of charged particles. Not only that, but, if you make calculations from just electricity and magnetism experiments, you can calculate the speed of light. This was one of the absolutely wow moments in my physics education – when I realised that just taking wires and magnets you could calculate how fast light went. That was when people realised that light was actually an electromagnetic wave.
Our current understanding of the unity of the laws has got to the point where we can now include electromagnetism and the strong and the weak force in one coherent set of laws.
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An interesting thing about that is that, in this grand unified picture, it has done really well in being able to explain the really huge differences in the strengths of the forces. I cannot do an easy example of how different the strengths of the forces are with the strong or the weak force, but I can do an easy example with gravity and electromagnetism – that is, if I’d remembered to bring my fridge magnet, I would have been able to show you how I can pick up a paperclip off the table using just a couple of grams of electromagnetism to counteract the entire mass of the Earth and the force of gravity. That shows you how weak the force of gravity is compared to electromagnetism.
Anyway, in the grand unification, if you take charged particles – we found, if you go to high enough energies, all of the forces actually have the same strength. It is remarkable, but the way it works – and it is sort of a vagary of virtual particles that I am not really going to go into – is that charged particles can surround themselves by virtual particles of the opposite sign. To use a Harry Potter analogy, that is a little bit like taking Harry Potter and wrapping him up in his invisibility cloak; it makes him more difficult to see. Quarks on the other hand have the opposite effect. They tend to attract like charges. So they surround themselves with more positive charges.
It is a bit like surrounding Harry with Dumbledore’s army. It makes him look stronger than he is intrinsically. If you are a weak wizard, you are put off by this. But, if you are a strong wizard, you can see straight through Harry’s invisibility cloak and you can just blast away Dumbledore’s army and get straight to the core and see what the energy or the strength is of Harry in the middle. Similarly with the forces of nature: if you are an energetic particle, you can get through all the crazy virtual particles on the outside and see what the raw force is from the particle at the centre. That is basically what you get at high energies in this theory: all of the forces appear to be the same when you get right down to the core of it.
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The next step that we would like to take is to incorporate into the grand unified picture not only those forces but also gravity; string theory is one of the leading proponents that is attempting to do that. One of the things that string theory has attempted to do is to derive the constants of nature, the values of the constants from first principles. Despite some early promise, it looks as though that is not going to work. Apart from finding one beautiful mathematical law that could derive all of the laws of nature from first principles, it looks as though there’s a huge number of solutions to string theory. It causes some people to just throw up their hands in despair – well, the whole motivation for string theory has gone out the window. But other people thought: ‘Hold on, this is actually an opportunity.’ Basically it relates to Einstein’s famous question. He said he wanted to know whether God had any choice in the laws of nature or in the way he made the world. If the stringy landscape is true, then the answer is yes. So there is a whole bunch of possible constants of nature that we could have had and we just happen to have got the ones that we do by chance. It means that the laws of measuring the constants of nature could be very different in different parts of the universe, and perhaps there are different universes with different constants. So it is an interesting possibility.
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If you were at dinner last night, you would have seen Victor Flambaum give his talk on possible detections of what already seem to be potential variations in the constants of nature in different locations on Earth. So this is a thought that is experimentally or empirically testable.
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The last thing that I will mention is a really, really amazing, interesting concept. It comes to the heart of what we were trying to do when trying to merge quantum physics and gravity – and that is to talk about the possibility of quantised space time.
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I will try to do this really quickly. The quantum theory gives us the uncertainty principle: there is a fundamental lower limit to how accurately you can simultaneously measure both distance and momentum or simultaneously measure both energy and time. You see this when you see the blue ray discs, for example. They use a blue laser, which is a higher energy than the red laser that is used in normal CDs and, therefore, they can imprint information on your blue ray disk at a higher density than you could with normal CDs. Similarly, with electron microscopes: the electrons have higher energy than the optical light we use in our normal microscopes and therefore we can see the more detail. But there is a fundamental limit. Once you add gravity into the picture, as you increase the energy of your detecting device, that detecting device eventually has to turn into a black hole, at which point it is not quite as useful as a detecting device any more. This mean that there is a limit to the smallest size we can possibly measure and also the smallest time that we can possibly measure. It has led to the concept that maybe the thing that we need is a quantised space time. Maybe the continuous nature of space that we see and the continuous nature of time is as much of an illusion as the continuous nature of that chair that you are sitting on, which we know is made of particles. Perhaps space and time are emergent quantities that only make sense when you get a lot of the quanta of space together, in the same way that temperature is an emergent quantity that only makes sense once you have a certain number of molecules. So that is an exciting idea that we are still working on.
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To summarise, we have talked about what the crucial aspects are that we need to include in the next generation of physical theories – things like space and time are dynamic, force strengths do change, the constants of the forces do change at high energies; space time might be quantised, and there is even observational evidence that the constants might change. Then the interesting question of: are the laws of nature in our universe necessary or contingent; what is the mind of God, in a sense? If the stringy landscape is true, there could indeed be many universes with basically the same laws but with very, very different conditions. That is an exciting thing. Is there an ecology to the laws of the universe? Is there any way in which we can say that these constants might evolve, rather than just change, and actually have some direction in how they move?
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I will leave that for coffee and conclude that, yes, perhaps the laws of nature really are as fragile as stone. Thank you.
Discussion
Brian Schmidt: We have a little bit of time for a couple of questions from the audience, although we are going to have to keep this session fairly short so that we can move on to the next talk.
Question: Where does biology come into your laws? Is it too trivial to come into your laws?
Tamara Davis: The question is: where does biology come into the laws? I think we have to look at the whole continuum. In order to explain biology, we cannot really start with the evolution of life on Earth; we have to start with the evolution of the universe itself – you know, we are all star dust. We have to talk about the evolution of the star dust, before we can talk about evolution of life. If we are going to do that, we should really talk about the evolution of how the stars got there. That is where the evolution of physical law comes from: at the end of the spectrum of what we need to explain, if we are going to explain life.
