Welcome to this year’s Thinkers lecture!
The theme this year is ‘Small Fact, Startling Conclusions’. What do I mean by this? Well, it is a reference to the fact that many times the most dramatic and startling insights science achieves is not born from some similarly dramatic and startling observation. Quite the contrary: Sometimes an observation that seems totally unremarkable turns out to be the key to a revolutionary new idea which can sometimes be so outlandish. the scientific community itself is reluctant to accept the conclusion.
I do not wish to talk about the structure of scientific revolutions. People interested in that topic would do well to seek out the book ‘Structure Of Scientific Revolutions’ by Thomas Khun, which is probably the definitive study. Instead, I want to devote this lecture to a couple of scientific hypotheses, each one of which begins with an observation that does not seem at all dramatic but which leads step by step to a conclusion that is really quite remarkable.
Remember, though, that these are ‘hypotheses’ and you should not consider the conclusion to be ‘true’. ‘’Hypothesis’, after all, is defined as (from the Oxford English dictionary) ‘a supposition or proposed explanation made on the basis of limited evidence as a starting point for further investigation; a proposition made as a basis for reasoning, without any assumption of its truth’. Perhaps someone will one day provide an alternative explanation that obsoletes the hypotheses talked about in this lecture? Really the point is not to convince you all that the conclusions are correct, but merely to persuade that there is enough reason not to reject them out-of-hand as obviously crazy.
OK. Well, the first small fact and startling conclusion I want to talk about concerns the stars. Ever since the 1920s, astronomers have known that starlight is not as bright as it should be. The reason why not is well understood. It is because there is an awful lot of dust in space and its particles are scattering starlight, ensuring less of it reaches us than would otherwise be the case. The question astronomers would like to answer is this: What is the dust actually made of?
What do we know about it? We can be pretty sure that each particle is about a thousandth of a millimetre across. We make this supposition on the basis of a simple argument. Grains of dust that are this size are best at scattering light. If they were much bigger or smaller, you would need more dust than can possibly exist in order to achieve the observed amount of scattering.
The issue of abundance also rules out an early candidate material for the dust. We used to suspect it might be iron but the universe cannot possibly have made enough of the stuff to account for the amount of dust out there in interstellar space.
When telescopes able to see in infrared were introduced, that also ruled out another possible candidate, which was water ice. Ice absorbs infrared light at a wavelength of 3.1 micrometres but no such absorption was detected. However, it was determined that the dust absorbed infrared light at 3.4 and 10 micrometres.In 1974, Chandra Wickramashinge conducted a series of laboratory tests with the purpose of finding a material that most closely matched the observed absorption. He began by testing organic polymers, complex organic molecules that are the basic building blocks of life. It turned out that organic polymers were the closest thing to the mysterious grains that had been tested so far.
There are many kinds of polymers, and Wickramashinge needed to test each in turn in order to obtain an even closer match and narrow down the list of candidates further still. But, instead of testing polymers one by one, he decided to take a shortcut. This involved testing the absorption effect of dried bacteria. The reason why he took this course of action is simply because bacteria contain many different organic polymers, so this was a convenient way to test many kinds simultaneously.
When the tests were carried out, though, bacteria turned out to be the closest thing yet. And that was not the only way in which they matched. The size of bacteria happens to be about the same size as the interstellar dust grains. Astronomers had long compared the size of the grains to the size of bacteria, but none had ever suggested the grains really were bacteria. But Wickramashinge suspected that interstellar space might be a vast graveyard of micro-organisms. Furthermore, his laboratory tests threw up a prediction. He found that dried bacteria absorbed light in a characteristic way between 2 and 4 micrometres. Would the interstellar dust also absorb light in this way? In order to test this prediction, Wickramashinge needed a bright beacon of infrared light, far enough away for its light to stream through lots of dust on its way to Earth. Fellow astronomer Fred Hoyle recommended GC-IRS7, which is a star lying close to the centre of the Milky Way.
Now, the centre of the Milky Way is only visible in the southern hemisphere. Fortunately, Wickramashinge had a brother, Daya, who worked as an astronomer on the Anglo-Australian Telescope (AAT), down in Australia. He had been awarded observing time for some other project and he was able to squeeze in an observation of GC-IRS7 in between his scheduled observations.
These observations supported the hypothesis that the dust is a vast graveyard of bacteria. The characteristic absorption of infrared light that Wickramashinge had found in his laboratory was present in the astronomical observations of infrared light absorbed by the interstellar dust. Surely, this was more than just a coincidence?
But the hypothesis that interstellar dust is made up of bacteria had to overcome one major hurdle: The sheer amount of dust in space. All told, it amounts to a total mass that exceeds the mass of the sun by 10 million times. That is a hell of a lot of bacteria. Now, in theory the exponential growth of replicating micro-organisms would have no difficulty in reaching such a vast mass. If you took one bacterium and let it replicate endlessly, within two weeks its descendents would be numerous enough to fill the volume of a sugar cube and two weeks after that their mass would be equivalent to that of an entire galaxy!
This, however, would only the case so long as the bacteria had a ready supply of two crucial ingredients: Nutrients and liquid water. If either the chemical building blocks necessary for replication or liquid water ran out, replication would cease. So where on Earth (or rather, in space) might you find enough water and organic building blocks to allow for a replication of bacteria to a point where their mass is 10 million times that of our Sun?
Radio telescopes have detected complex organic molecules in interstellar space. They are present in the clouds which are believed to condense into stars and their orbiting planets, asteroids and comets. If we assume bacteria are present in these interstellar clouds, might we also assume they would be present in forming solar systems?
In the case of stars and planets, this can be ruled out because a tremendous amount of heat is associated with their birth. If any bacteria were present during this time, they would be vaporised. Comets, on the other hand, would provide a relatively benign environment. I say ‘relatively’ because space is a very harsh environment. Or, at least, it is to us. But to certain types of bacteria, known as extremophiles because of their ability to survive unbelievably harsh environments, comets really would be fairly comfortable environments in which to grow. We have found extremophiles that can survive dehydration, incredibly low temperatures, intense ultraviolet light and radiation. Bacteria such as these would have no problem surviving in comets.
Now, most comets reside in a vast cloud outside the orbit of Pluto. We call it the Oort cloud. Astronomers assume that most, if not all, stars in the milky way have their own Oort cloud. If each one is a similar mass to our own, that would make the total mass of comets in the galaxy 10 million times the mass of our Sun. You will notice that this also happens to be equal to the mass tied up in interstellar dust, which (so the argument goes) is actually bacteria.
But if the bacteria are locked up in comets, how do they get into interstellar space, drifting along in vast clouds? Well, comets have a handy ejection system. Every now and then, a comet will fall sunwards. Once it is close enough to the sun, the heat will boil off surface material, resulting in the spectacular tail. Millions of tonnes of dust and ice is ejected in this manner. Space probes sent to investigate comets have shown that the particles boiling off a comet’s nucleus are the same size as bacteria, and absorb light in the same way. Also, space probes have found direct evidence for the existence of organic molecules.
Now, the dust ejected from comets does not stay put. The reason why not is because the grains are small enough to be blown by the solar wind. This carries them into interstellar space.
So, we can account for the necessary chemical building blocks. But what about liquid water? Comets, after all, are supposed to be made up of ice. Wickramashinge put forward the following explanation. We know, from studies of asteroids, that aluminium 26 is part of the raw material from which stars, planets and comets are made. Aluminium 26 is radioactively unstable, and the heat produced by this decay would be sufficient to melt the interior of a comet and keep it in a liquid state for several millions of years. Perhaps, bacteria in such a pool, fed with a ready supply of nutrients, would happily replicate?
Now, the heat source would not last forever. Eventually the aluminium 26 would have decayed into the more stable magnesium 26 and there would be no more heat to prevent the water from turning into ice. However, provided the heat died away slowly enough, there could be time for water to ‘diffuse’ out of bacteria, and that would prevent ice crystals from forming inside them and rupturing their membranes.
Could it be, then, that the clouds of interstellar dust is not, strictly speaking, a graveyard of micro-organisms? Could it be that a tiny fraction are not quite dead, but still viable and just waiting for the right conditions to come along and revive them? Perhaps this tiny minority drifts in space, metabolisms down to the incredibly minimal state that extremophiles are known to be capable of reaching, biding their time until the clouds they are part of condense into solar systems? Maybe an even smaller minority of viable micro-organisms happen to end up inside new comets, deep down in the interior where there is liquid water and nutrients aplenty? Perhaps these lucky few then replicate? It would only take the tiniest fraction of micro-organisms to remain viable in order for exponential growth to achieve astronomically vast numbers of descendents.
And this might clear up a couple of mysteries. Firstly, some extremophiles seem well adapted to life in outer space. This seems rather odd, given that evolution is supposed to adapt life to suit the environment it finds itself in. The standard explanation for this is that it is just sheer coincidence that some bacteria happen to have acquired the ability to survive in space. But maybe it is more than coincidence? Maybe the reason why bacteria is adapted to life in outer space is because that really is the environment in which they evolved?
This brings us to the second mystery, which is the speed at which life seems to have originated on our planet. Pretty much as soon as the Earth was a viable place for life, life sprung into existence. The idea of panspermia posits an extraterrestrial origin for life. According to this hypothesis, life of a kind existed before the Earth had reached a stage where it could support life. Wickramashinge’s hypothesis regarding bacteria as the stuff from which interstellar dust is made would suggest that bacterial life may even have existed before our solar system did.
For a long time, we have gazed up at the night sky, hoping to catch sight of extraterrestrial life. But, maybe it has amongst us all along? Maybe the grand ancestor of all life on Earth was indigenous to this planet, but rather a visitor from outer space? Maybe we are all the descendents of colonising alien bacteria?
The next small fact I want to talk about concerns neutrinos. Whenever such a particle is created it always corkscrews to the left. There seems to be no such thing as a neutrino that corkscrews in a right-handed manner about its direction of travel.
This is very peculiar, because the universe loves symmetry. Symmetry applies to anything that remains the same after being changed in some way. Think of a billiard ball. If you rotate it, the ball looks the same as it did before. It has ‘symmetry with respect to notation’. On the other hand, were someone to move the cue ball from its current location to somewhere else, this change would be apparent. The ball does not have symmetry with respect to location.
The laws of physics, though, does have symmetry with respect to space. That is what enables an experiment to be obtain the same results no matter where it is conducted. Of course, differing environments may produce different results. A paper airplane thrown in the vacuum of space will not fly in the same way as one thrown in the gigantic winds that buffet Jupiter. But that in no way means the physical laws have changed. As well as symmetry with respect to space, the laws of physics are symmetric with respect to time and there are a myriad other more abstract symmetries. The universe, it seems, is fanatically devoted to symmetry.
But there seems to be at least one instance where this breaks down. If you stand in front of a mirror and wave your right hand, your reflection waves its left hand. Imagine a neutrino corkscrewing to the left in front of a mirror. It’s reflection would corkscrew to the right. If the laws of physics were mirror-symmetric, every process reflected in a mirror ought to occur in nature. But no. After all, there is no such thing as a neutrino that corkscrews to the right.
Or is there? According to the physicists who first discovered this apparent violation of symmetry-Jsung Dao Lee and Chen Ning Yang- right-handed neutrinos do exist. But they are not made of ordinary matter. They are made of ’mirror’ matter. If mirror neutrinos exist, it stands to reason that other particles in the standard model must exist as well. After all, a neutrino accounts for the missing energy that seems to accompany the process in which a neutron decays into a proton and an electron. Therefore, there would have to be mirror neutrons and protons. According to Lee and Yang, in nearly every respect a mirror particle is exactly like its ’ordinary’ version. A tau has a mass of 1.9 and so does a mirror tau, for example. Mirror neutrinos, corkscrewing to the right instead of the left like ordinary neutrinos do, are rare exceptions.
There is, however, one way in which all mirror particles differ. They are invisible. More precisely, they do not interact with any of the force-carrying particles. If they did, our instruments would have detected them. But mirror particles must interact with each other and to do that they require force particles. So we need to postulate the existence of mirror photons, mirror gluons and so on. And if we insist that mirror particles behave exactly like ordinary ones, mirror protons, neutrons and electrons aught to combine to make mirror atoms, and mirror atoms aught to assemble into everything from mirror galaxies to mirror stars to mirror life forms. In short, coexisting with our universe there is the mirror universe, but the two are all but unaware of each other’s existence thanks to the fact that their force particles are mutually exclusive.
Now, this is all starting to sound like Bertrand Russell’s teapot. Russell, you may recall, challenged belief in God by positing the existence of a teapot, orbiting somewhere in space, but which was invisible to any kind of instrument. Can you prove it does not exist? Of course not, but that hardly counts as evidence that it does! Similarly, all this talk of mirror matter and mirror energy, conveniently undetectable by any imaginable means, sounds like a theory that is not even wrong.
However, according to Robert Foot of the University of Melbourne, Australia and Sergi Gninenko of CERN, there may in fact be laboratory evidence for the existence of mirror matter.This evidence concerns a measurement of the lifespan of ‘ortho positronium’. Positronium is the simplest atom-like system and it consists of an electron and a positron. If the electron and positron ‘spin’ in opposite directions to each other, that makes ‘para positronium’. If they spin in the same direction, that is ortho positronium.
According to quantum electrodynamics or QED, ortho positronium exists for a mere 142 billionths of a second before decaying into three photons. At least, that is what the theory says. But experiments using ultra-sensitive instruments have shown ortho positronium decaying faster than QED predicts. 0.1% faster in fact.
Now, 0.1% might not sound like a large discrepancy, but quantum electrodynamics is normally so fantastically accurate that this is actually a significant error. Can the mirror universe account for this result? A physicist called Bob Holdom thought so, and his explanation brings us back to symmetry.
Physicists believe that, underneath all the complexity we see around us, nature is quite simple. This leads some to argue that having apparently different forces of gravity, electromagnetism and the weak and strong nuclear forces is overly complex. They must be facets of a single ‘superforce’ that ties all particles together. Holdem took things a step further and posited the existence of force that ties both ordinary and mirror matter into one unified framework. If such a force exists, it would have a force particle associated with it. We might as well call it the ‘H-particle’.
Now consider something called conservation of electric charge- the idea that, like energy, the amount of electric charge at the end of any process is always the same as the amount at the beginning. What would happen if an electron spat out an ‘H-particle’ and became a mirror electron? If that happened, it would have no charge because mirror electrons have ‘mirror’ charge not ordinary charge. In effect, then, this process destroys electric charge. The opposite process- a mirror electron spitting out an H and turning into an electron- CREATES electric charge. Both events are forbidden.
Interactions can only arise if both ordinary and mirror particles have zero electric charge. The most common neutral particle is the photon, so Holdom’s force would be most noticeable between photons and mirror photons. The electron and positron of ortho-positronium would ‘like’ to change into a single photon, but they do not have enough momentum to do so. However, the Heinsenberg Uncertainty Principle states that ortho positronium can briefly turn into a virtual photon.
Before it turns back into an electron and positron, the virtual photon has a small chance of spitting out an H-particle and becoming a virtual mirror photon. And this virtual mirror photon can turn into a mirror electron and mirror positron. So, when we create ortho positronium it is not ‘pure’ but rather a mixture of ‘ordinary’ and ‘mirror’ ortho positronium. More precisely, the mixture constantly oscillates back and forth between the two.
Mirror ortho positronium can decay into three mirror photons. When this happens, there is no longer any ortho positronium to oscillate back to the ordinary world. If you recall the measurement of ortho peritoneum’s lifespan, in which more of it vanished in a given time than QED predicted, that could be because some of it had disappeared into the mirror universe?
This, then, suggests a way to test the hypothesis that our universe exists along with a mirror universe. Observe the decay of ortho positronium in an empty vacuum. The vacuum is necessary because if a single particle bumps into ortho positronium, that is enough to destroy the oscillations. Sergi Gninenko has proposed an experiment that encloses ortho positronium in a container whose energy content can be precisely measured. What the experimenters would be looking out for is missing energy. “Missing energy”, says Gninenko “is the unmistakable signature of the mirror universe”.
So, I hope these two examples- the dimness of starlight leading us to an extraterrestrial origin of life and the behaviour of neutrinos leading us to the existence of a mirror universe- demonstrates how the most remarkable of discoveries often begin with the most unremarkable of observations. And that concludes this year’s lecture!
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