After a longer break than planned, here’s more stream of consciousness. Firstly, I should say a big thank you to those who left comments or emailed me about my blog on quantum biology. Actually since writing it I have heard that one of my recently graduated students at Surrey has been successful in being awarded a Doctoral Training Centre studentship, which basically means he has funding for a PhD and gets to choose from a pool of research projects available in the Faculty. He has expressed interest in my project to study genetic mutations by modelling them as quantum systems undergoing quantum tunnelling.I have been asked whether I think that quantum mechanics might be important in genetic mutations (that lead to the proliferation of cancer cells) because of an idea called quantum Darwinism, whereby a microscopic biological system (say the genome) can evolve quantum mechanically into a superposition of different states that all co-exist and where some states are more successful at replicating than others. Well, that’s one possibility, but you might think that within the warm, ‘noisy’ confines of the living cell nothing can behave quantum mechanically for long enough for such superpositions to persist, and that decoherence takes place too quickly – that is, the genome couples to its external environment and so the quantum weirdness leaks out a bit like the way heat leaks away from a warm object in a colder environment. But maybe, as Schroedinger suggested over 60 years ago, this happens more slowly than we might think – something to do with the special order brought about the low entropy state that is life.
Another possibility is that this decoherence, which can essentially be thought of as the environment ‘measuring’ the quantum system by interacting with it (coupling to it) can bring about what is known as a Zeno effect. That is, slowing down the quantum tunnelling process, or more speculatively, an anti-Zeno effect, which speeds up the quantum tunnelling process. My colleague in the Department, Paul Stevenson, and I published a paper a few years ago in which we show that this does indeed take place for the case of a one-dimensional wavepacket tunnelling through a square potential barrier (see left). Well, in certain genetic mutation what we have is a hydrogen bond breaking across one site and reforming somewhere else. In other words, a proton sitting in a potential well, quantum tunnels to a neighbouring one. This stuff is not new and there have been a number of papers in the past decade or two that have studied this.Anyway, all very speculative at the moment, but the possibilities are so exciting that it is well worth the effort to investigate, and an ideal PhD project.
But, at the risk of making this a very long blog, what I really planned to do was transcribe my audio podcasts (“Jim Al-Khalili’s Sci-Pods” on iTunes) as blogs for those who would rather not listen to my voice droning on and prefer the written word. So, here goes. The following is the first of my ‘Series 2′ Sci-Pods and is a discussion of Olbers’ paradox and how it connects to a proof of the Big Bang itself. Enjoy.
Why is it dark at night?
I am often asked about what proof we have that the Big Bang actually happened; that 13.7 billion years ago the whole Universe suddenly came into existence out of absolutely nothing. In fact space and time themselves didn’t exist before the Big Bang. Well there are several pieces of compelling evidence that tell us this idea is correct. The first is the most convincing: that when we look out through our telescopes at distant galaxies, far beyond our own Milky Way, we see they are all rushing apart. And the further we look out the faster they seem to be moving away from us in every direction. This expansion of the Universe suggests it must have all started when everything was much closer together, in fact all squeezed into a single point.
The second piece of evidence is that outer space has a very specific temperature of around minus 270 degrees Centigrade. This is exactly the temperature it should be by now if it began with a Big Bang 13.7 billion years ago and has been cooling down ever since.
The third piece of evidence is proportion of the different chemical elements. Most of atoms in the Universe are hydrogen (the simplest atom), followed by helium. Between them they make up about 98% of all the stuff we can see. All other elements make up the remaining couple of percent. This can only really be explained with the Big Bang idea that in the early universe these two simplest elements were cooked but once it expanded and cooled the temperature dropped below what was needed for nuclear fusion to take place and all the other 90 elements in the periodic table had to wait to be synthesised inside stars.
But there is another often overlooked proof of the Big Bang that relies on simple logic – OK and a few calculations you will have to trust me on. It is sometimes referred to as Olbers’ paradox. Put simply: just ask the question: ‘why does it get dark at night?’
You might think that this is a rather trivial, even silly question to ask. After all, even a child ‘knows’ that this is because the Sun sets below the horizon, and since there is nothing else in the sky anywhere near as bright as the Sun we have to make do with the feeble reflected light from the Moon and even more feeble light from the distant stars. Well, guess what? It’s not as simple as that!
Olbers
We have good reason to believe that even if the Universe is not infinite in size (and it might be), it is so enormous that, for all intents and purposes, it does go on for ever. And so we come up against Olbers’ paradox. This states that the night sky has no right being dark at all. It should be even brighter than it normally gets during the day. In fact, the sky should be so bright, all the time, that it should not even matter whether the Sun is up in the sky or not.
Imagine you are standing in the middle of a very large forest. So large in fact that you can assume it is infinite in extent. Now try shooting an arrow horizontally such that it does not hit a tree trunk. In this idealised situation the arrow must be allowed to keep on going in a straight line without ever dipping down to the ground. You find, of course that it is impossible. Even if the arrow misses all the closer trees, it will eventually always hit one. Since the forest is infinite, there will always be a tree in the flight path of the arrow, however far away that tree is. It doesn’t matter how dense the forest is either. If you were to chop down ninety percent of all the trees, this would simply mean that the arrow will, on average, travel ten times as far before it encounters a tree trunk.
Now consider a simple model universe that is infinite, that is static (by which I mean not expanding) and with stars evenly spread out. The light that reaches us from the stars is like the example of the arrow. It does not matter where we look in the sky, if the Universe is infinite we should always see a star in our line of sight. So there would not be any gaps in the sky where we do not see a star and the whole sky should be as bright as the surface of the Sun, all the time!The real universe may also be infinite, but in other respects it is not quite like the above simple model. First of all, the stars are not spread out evenly but clumped together in galaxies. This doesn’t matter. It just means that the night sky should be as bright as an average galaxy, which is not quite as bright as the surface of an average star but still blinding. Secondly, our Universe is expanding. Does this make a difference? Physicists have carried out detailed calculations that have shown that this does not solve the problem; it just reduces it.
It was thought that maybe space is filled with interstellar dust and gas that would block the light from the more distant galaxies. But if the Universe has been around for long enough, then even this material would slowly heat up, due to the light it has absorbed, and will eventually shine with the same brightness as the galaxies it obscures.
The true answer, the one which finally lays Olbers’ paradox to rest, is that the Universe has not been around forever, so light from very distant galaxies has simply not had enough time to reach us. If the Big Bang happened 13.7 years ago, then galaxies that are further away from us than 13.7 billion lightyears (remember a lightyear is the distance covered by light in a year) are invisible to us because their light is still in transit and has yet to reach us. Admittedly, the discussion is complicated a little due to the expansion of the Universe – the very furthest galaxies we can see, because their light that has been travelling towards us for 13.7 billion years is only just reaching us today, are in fact over 40 billion lightyears away due to the expansion – but what we can see in the sky is just a tiny fraction of the whole Universe. We call this the ‘visible universe’ and we cannot, even with the most powerful telescopes, see beyond this horizon in space. So the amount of light reaching us from space, and hence the brightness (or darkness) of the night sky, depends on how far out we can see, and this tells us how old the Universe is.
Finally, we can turn Olbers’ paradox on its head and say that the real proof that the Big Bang happened is that it gets dark at night. Now isn’t that a cool argument to use when confronted by someone who is sceptical about evidence for the big bang!
Super Cool
Great work .
Brilliant blog. I’ve got a physics degree and am ashamed not to know this stuff, but it’s clear now. You explain it so well!
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Nicely explained. Fascinating stuff.
Just the ” Universe suddenly came into existence out of absolutely nothing.” – “don’t know” would be more accurate.
And I watched the program about random short-lived spontaneous matter or anti-matter in vacuums -not convinced about that either.
Getting to a clear explanation of the Big Bang but not it’s initiation may be as far as it is possible to go with observable data and logical deduction. A wealth of discovery.
Why make the quantum leap (pardon the pun) into the void that you cannot illuminate any better than faith?