The Fine-Structure Constant
The two fundamental assumptions about modern cosmology (the study of the origins of the universe) and astrophysics are homogeneity
. The first means that matter is evenly spread out through the universe (if you average out over large enough scales), while the second means that the laws of physics are the same throughout the universe. The principle of homogeneity has always seemed arbitrary to me, and unjustified; at the local level we find matter is quite clumpy, from everyday objects to planets
, to galaxies
and clusters of galaxies, and so on. As we step back to take a larger and larger view, we see the same pattern repeating of matter concentrated into small volumes with empty space between. It is counter-intuitive that there should be some arbitrary point at which matter suddenly stops this pattern, but since the discovery of the Cosmic Microwave Background (CMB), it’s been understood that this is indeed the case. Much of our current understanding of the universe’s origins rely on this observed fact (although recent observations have opened cracks of suspicion here – the CMB isn’t as uniform as we once thought).
The second assumption, universality, feels much more solid. After all, a beam of light is a beam of light, regardless of where it happens to be. A car with a mass of one tonne when on earth should have the same mass on another planet on the other side of the universe. The battery would still provide electricity, the chassis would still be made of steel, and the wheels would still be round.
And yet since the 1930’s, physicists have been asking if this is in fact true. After all, we’ve never been to the other side of the universe – how do we know? It might seem silly to even ask, but a scientist will always be bothered by such assumptions. After all, the whole point of science is to find truth – if you’re just going to assume something is true because it “Seems obvious”, then you might as well hang up your lab coat and find a proper job!
The laws of physics are structured in such a way that everything hinges on a handful of universal constants. These are specific numbers that have been measured to a high degree of precision and are key to the operation of the universe. When doing calculations in physics, these constants show up all over the formulae. One of these numbers is ‘π’ (or ‘pi’ in english). If you divide the circumference of a circle by its diameter, the answer will always be exactly ‘pi’ (or approximately 3.141592654…..), regardless of the size of the circle. Another is ‘e’, the charge on an electron. All electrons have a charge of 0.000000000000000000160 Coulombs (or 1.60E-19, in scientific notation). Yet another value is ‘c’, the speed of light in a vacuum (299792458 meters per second). These values have all been measured, calculated and confirmed thousands of times by physicists all over the world. We know their values and according to the principal of universality, we know them to be eternal and unchanging. But what if we’re wrong about this?
One of these constants, α (alpha), is called the Fine-Structure Constant. Unlike those mentioned so far, it is a derived constant which means that it’s not a single measured attribute but a combination of other constants. If you take the square of the charge on an electron (e) divided by the speed of light (c) and planck’s constant (h), and then multiply the whole lot by 2π, you find that all those units cancel out and the answer is a pure ratio of α = 1/137.036. This rather arbitrary seeming number turns out to be pretty fundamental to the inner workings of the universe. It governs the force between electrically charged particles, including between an atom’s electrons and protons. If this value were different by as little as 4%, the universe would be a very different place. Nuclear fusion as we know it would not work, so stars would not burn. Without stars, there would be nothing to convert primeval hydrogen and helium into heavier elements like carbon or oxygen. And without carbon and oxygen, we could not exist. The Fine-Structure Constant is important.
An atom is made of a nucleus surrounded by electrons swarming around specific energy levels, or “orbits”. If a beam of light of just the right colour (which depends on the frequency of the light) strikes that atom, some of those electrons can be temporarily knocked to a higher energy level, and the light is then absorbed. If we therefore shine white light (a mix of all frequencies) through a gas and then through a prism, the atoms of that gas will tend to filter out very specific frequencies and we’ll see that those missing frequencies as dark lines in the resulting spectrum. This simple, yet ingenious, device is known as a spectrograph and by noting which colours are missing from the spectrum, astronomers and other scientists can very accurately identify the gas (or mixtures of the gas). A number of factors can distort the readings. including the famous Doppler effect; if a specific pattern is shifted slightly towards the red side of the spectrum, we know that the gas we’re measuring is moving away from us (or we’re moving away from it, not that it makes any practical difference). This is the famous “Red Shift” which astronomers refer to when they explain how they know that the universe is expanding. It is important to remember that no matter what effect these influences have, the specific patterns of lines are constant – only their position changes. The spacing of the lines within a pattern is strictly controlled by the energy levels within the atoms that caused the lines, and that in turn relies on the Fine-Structure Constant.
Enter John Webb and Julian King from the University of New South Wales, who recently submitted a paper to Physical Review Letters which suggests that the Fine-Structure Constant may not be constant after all. It began with a series of observations by Dr Webb almost ten years ago, when he used the giant ten meter Keck telescope (The biggest telescope in the world at the time, four times larger than the Hubble Space Telescope) to look at 76 quasars spectroscopically. After carefully analysing the spectral patterns and calculating backwards to determine the Fine-Structure Constant, he found something very odd: The further away each of those quasars was, the smaller α turned out to be. Because light travels at a finite speed, objects very far away are actually seen as they were in the past, so he theorised that the value of α has been slowly increasing, and that it is now 0.0006% higher than it was 9 billion years ago. Now this is not a very large change, but the size is immaterial – the problem is that it could change at all!
When a scientist announces something so fundamentally new, or strange, the correct response is scepticism. Other scientists will then run the experiment themselves (or in the case of astronomy, make their own observations) and see if they get the same answers. In this way, new information can be tested, and mistakes can be eliminated. So in 2004, another set of researchers set to work and found no measurable change in α. Unfortunately, their work was found to be flawed, leaving the question unanswered.
In an attempt to test their own work, Dr Webb and co. recently performed observations of another 60 quasars, from the same telescope (the ESO’s Very Large Telescope, or VLT, in Chile) that the second team had used. Unlike Keck, this telescope is in the southern hemisphere, so sees the opposite side of the sky. They once again found a changing value of α, but this time the value seemed to go up in time! There were enough common quasars between the two surveys that they were able to calibrate their readings and iron out any technical differences, so they were confident that the data was right. The only possible explanation for the two opposing results was that α does not vary with time at all — it varies with space! If you travel in one particular direction, α seems to become smaller, while if you travel in the opposite it seems to increase. This implies that the principal of universality is wrong.
Now, it’s unclear which of the various constants making up α are changing. Is it the charge on an electron? Is it the speed of light? Nobody knows. In fact, at this stage only one team has managed to show any variation in the Fine-Structure Constant at all, so it might still turn out to be a false alarm. Considering the importance of such a big discovery, it’s safest to not speculate at all until more teams of scientists can conduct their own independent surveys and confirm this effect. But if it is true, then we have just broken one of our most cherished assumptions: That we can predict and model the universe at extreme distances by the same laws that we use here on Earth.
Update (4 November 2011):
The paper referenced above has finally, after a year, been accepted for publishing by the peer-reviewed journal Physical Review Letters. You can access the abstract here.
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