Posted by “A constant struggle, a ceaseless battle to bring success from inhospitable surroundings, is the price of all great achievements.” -Orison Swett Marden
One of the greatest assumptions we make in our study of the laws of nature is, well, that they’re laws of nature, not particularly special to where or when we happen to be looking at them.
Image credit: NASA & ESA.
Whether we look on our home planet, within our own galaxy, at a relatively nearby galaxy (like NGC 4522, above), or at galaxies extremely far away (faintly visible in the background), we always tend to assume that the fundamental laws that govern the Universe don’t particularly care about where or when you measure them.
But they might.
Image credit: 2-degree Field Galaxy Redshift Survey.
The reason we assume the Universe has the same laws and the same fundamental constants everywhere — at all times and in all directions — is because it looks like it’s obeying the same laws everywhere!
Image credit: ESO / P. Grosbøl.
Galaxies follow the same clustering patterns in all directions, exhibit the same gravitational phenomena, and show similar internal dynamics no matter where or when in the Universe we look. If they didn’t, we might start to question whether either the laws of gravity or maybe even the fundamental gravitational constant, G, varied in either time or space.
Similarly, if electromagnetic or quantum phenomena appeared to vary over space or time, or be significantly different in one direction over another, we might worry about our electromagnetic and quantum laws and/or constants.
Image credit: WMAP / NASA / WMAP science team.
From constraints in the cosmic microwave background, we can show that — from when the Universe was 380,000 years old until today, where it’s 13.7 billion years old — the speed of light could have varied by no more than 4%, a very impressive constraint!
There are a number of good theoretical reasons to expect that the fundamental constants, Planck’s constants, the speed of light, and Newton’s gravitational constant, really are constant. But it’s always conceivable that there is some sort of small variation — either in time or space — to these parameters. The only way to know for sure is to measure them.
So, how can we do that? In principle, it’s simple.
Image credit: Szdori, OrangeDog and WikiMedia Commons.
The most common element in the Universe is hydrogen; by number, over 85% of the atoms in the Universe are hydrogen, even after you account for 13+ billion years of stars burning their hydrogen fuel through nuclear fusion! As shown above, the fundamental atomic transitions of hydrogen are well-known. So, you’d imagine, if we can measure hydrogen’s spectral lines in every direction, and we can do it at a variety of distances (and hence a variety of lookback times), we can see whether these fundamental constants determining these frequencies have changed or not.
But in practice, there’s something we can do that’s even better.
Image credit: Fabry-Pérot interferometer/étalon fringes, by John Walton.
Because there’s not only the basic atomic structure of hydrogen, there’s also more advanced, subtle signals that comes from hydrogen atoms. In particular, I’m talking about fine and hyperfine structure in the atomic transitions of hydrogen and its heavy, stable isotope, deuterium.
Image credit: DJIndica of WikiMedia Commons.
Well, the person who helped devise the best technique to measure this fine structure at high redshift — and is arguably the world’s leading expert on it — is Australia’s John Webb. The fine-structure constant is actually a combination of a few well-known constants, including the electron charge, Planck’s constant, the speed of light, and π. If one of them were changing (presumably not π), this would be very big news! And the reason to get excited about the fine structure constant is that it’s been measured so precisely. Specifically, the fine-structure constant, α, is:
Now, what Webb does — and he’s been doing it since before I started graduate school — is he takes the best spectra of quasars I’ve ever seen. In particular, he looks at absorption lines from the intervening hydrogen gas clouds in between the quasar itself and us. Here’s an example from a 2002 paper of his.
Image credit: M. T. Murphy, J. K. Webb, V. V. Flambaum, and S. J. Curran.
From these spectra, if the fine-structure constant was truly constant, one would expect to get the same signature for it from all of these absorbing hydrogen clouds. But that is not, and has not been, what Webb’s team has been finding.
Image credit: Same team, different paper, from 2000.
What they find — and what they’ve consistently been finding for more than a decade — is that the fine structure constant appears to have been different, by a few parts in a million, in the distant past! The effect is small, but their error analysis has been checked, scrutinized, criticized, etc., and has been shown to be correct by multiple independent sources. To put it bluntly, they’re doing science the way science is supposed to work.
Image credit: Same team, different paper, this time from 2002.
Even though terrestrial measurements show that it hasn’t varied here over the past 4 billion+ years, that doesn’t mean it isn’t different elsewhere in the Universe, or wasn’t varying at an earlier time, and has been constant only recently. Their data still needs to be explained.
Of course, this is highly controversial. At this point, it really does appear that the data, despite good reasons to be cautious, is robust. The most recent paper is available here, and all of the subsequent figures are taken from it. Including this one:
As you can see, there are actually much larger apparent variations — up to about 50 parts in a million — when individual systems are looked at. But these aren’t spatially random variations. There’s enough data to suggest that, even when random variations are taken into account, that one direction of the Universe has a preferentially higher value for this constant, while the opposite direction has a preferentially lower value. With hundreds of systems now measured by two different instruments, these results have not only been confirmed and held up over the past decade, the combined data is very strong (at about 5-σ significance), and that makes these results really annoying.
Why annoying? Because there isn’t a good, simple explanation for this! So just what is it that’s going on?
Well, John Webb recently wrote an article detailing these results, which is an interesting read. In short, he developed this new absorption-line technique 11 years ago, it’s about 10 times better than any other technique we have for measuring this variation, and it sees a variation about 4-5 times larger than their errors. It varies across directions in space, it varies for both low-redshift and high-redshift gas clouds, and it cannot successfully be explained away by any known physics.
Theoretically, I don’t have a good explanation for this. The ones I can think of that would cause this effect — like segregated elements in these gas clouds — aren’t physically reasonable, much less well-motivated. From an observational point of view, there is at least one reason I can find to be concerned. The data comes from two instruments: the Keck Observatories and the VLT. Although the Keck data shows this highly statistically significant result at both low-redshift and high-redshift, the VLT data does not show it at low redshift, and shows a much less significant one (only three-σ, instead of four- or five-σ usually demanded) at high redshift.
The only thing I can say is the same thing the authors can say: maybe there’s some sort of problem or error associated with this technique that we do not understand. If we only had the VLT data, we wouldn’t think we were seeing anything of note; it’s only the Keck data on its own or the Keck + VLT data combined that show a very strong result.
Because the implications are so alarming, I would demand — just as I did for the alleged faster-than-light neutrinos — extraordinary evidence to back up this extraordinary claim. What would that extraordinary evidence look like? I give John Webb full points for identifying the exact two things I would want in his latest paper:
Future similar measurements targeting the apparent pole and anti-pole directions will maximise detection sensitivity, and further observations duplicated on 2 independent telescopes will better constrain systematics. Most importantly, an independent technique is required to check these results.
Now that they’ve identified the directionality of this variation in the fine-structure constant, you want to look, with multiple instruments, in those directions! If all the instruments you use see the same large, independent effect, you know it isn’t an artifact of your instrument and it isn’t due to the small number of sources (with large effects) you happened to measure in those directions so far.
So if you made those measurements and you still saw this big effect, you’d have every reason to be confident in what you’ve accomplished. But it could still be a bizarre artifact of this technique, even if you can’t identify what, exactly, is going on. So you want to find an independent technique to check it with.
Image credit: L. Horne.
It’s to use — instead of atomic absorption lines — an even more precise transition: the 21-cm Hydrogen spin-flip line! This has the potential to be even more accurate than Webb’s technique down the road, and can measure intervening gas clouds as far back as the Cosmic Microwave Background, at a redshift of 1089. (Making what we call “high redshift” today — a redshift of 3 or 4 — look like bupkis!) But at this point, it can only constrain the fine structure constant to a few parts in 1000; to practically get the accuracy they’d want would require a much greater financial investment than is feasible.
Which is, perhaps, why Webb gives this depressing, but perhaps realistic, ending to his piece:
As I said at the start of this article, no-one believes us yet, and we are in for a long battle. Some days I doubt I shall be living when the proof comes in.
The work is technical, laborious, very difficult, requires a great deal of data from extremely expensive scientific facilities, and the analyses take a lot of time and effort.
But on other days I’m more optimistic and remind myself that, for now, I’m alive and kicking and working on it.
After all, this is an extraordinary claim, but it isn’t a claim that’s been made because anyone’s done something crazy; it’s a claim that’s been made because someone’s done something amazing, and is seeing something crazy! So it’s definitely going to take some extraordinary evidence to put this on solid ground, but that’s exactly what everyone involved in this issue is shooting for, and I can’t wait to see how it turns out! (And a happy birthday to Carl Sagan, for those of you who caught the reference!)
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