November 3, 2011 | 7
I’ve been doing a little poking around the matter of the Italian Grand Prix (neutrino division). Plenty has been written about this already, of course, but what strikes me a few weeks into the story is how effectively the response to the announcement of a possible detection of faster-than-light neutrinos illustrates what actually goes into the making of a piece of science. That, of course, also sheds light on,what it looks like when the intention is not to create understanding, but to obscure it.
First, to the neutrinos themselves. For many of the actually knowledgeable folks I talk to (i.e., not me) the question about infamous Faster Than Light gang of neutrinos is not if they’ll be found out, but when.
That is: while the experimental technique reported in the OPERA measurement is good enough to be taken seriously, many physicists note that challenges to special relativity have a very poor track record. A number of other observations would have to be radically reinterpreted for the measurement of the travel time of neutrinos from CERN to Gran Sasso to stand up as an authentic discovery of faster than light travel. See my earlier post on this subject for a bit of background and some useful links.
An example: the OPERA result, if it holds up, would complicate (to say the least) the interpretation of the hugely wonderful detection of neutrinos emitted in the stellar collapse that produced Supernova 1987a. As the parent star of the supernova collapsed, the catastrophe produced 1058 neutrinos, give or take a couple. In what was dubbed the first triumph of neutrino astronomy, three detectors at widely separated locations detected a grand total of 24 of those (anti)neutrinos, all arriving within 13 seconds of each other.
Those neutrinos did reach planet earth before light from the supernova blast arrived. But that quirk of timing has nothing to do with faster than light travel. Rather, it turns on the details of supernova physics. Neutrinos are produced in the initial stellar collapse, and because neutrinos interact with basically nothing — they are untouched by either the strong nuclear force or electromagnetism — the supernova-neutrinos sped out from the dying star more or less at the moment of the blast. Light, by contrast is electromagnetic radiation – and readily interacts with charged particles.
That property caused the light of the supernova to crash around the interior of the dying star as photons interacted with all the extremely electromagnetically energetic matter at hand – a dance that held the photons up for a time. After a few hours, that light escaped from the interior of the supernova blast and could begin an uninterrupted journey our way. But by that time, it lagged behind the neutrino signal, which is what produced the gap between the neutrino and optical detections of the event.
Think of it as gridlock in the midst of a stellar rush hour — an obstruction 1987a’s neutrinos, riding on (highly metaphoric) rails, were able to avoid. The fact that the two signals arrived only hours apart simply means that the neutrinos travelled at or very close to the speed of light — not faster than. If the neutrinos traveled faster than light – even at the rather small excess suggested by the OPERA experiment — they should have arrived much earlier than they did – four years or so before the light from the explosion.
Now there is a way out of this seeming contradiction, because the supernova neutrinos were significantly less energetic than the ones measured in the OPERA experiment — so it’s not accurate to say that both results can’t be true. But even so, were superluminal neutrinos to prove to be real, then whatever new physics that might be invented to explain the result would have to do so in a way that still allowed Supernova 1987a’s neutrinos to behave as observed.
That’s the problem for any challenge to a fundamental pillar of knowledge: if the new observation is correct, it must be understood in a way that accommodates all the prior work consistent with the older view that is under scrutiny. As physics popularizers always note: Einstein’s account of gravity — the General Theory of Relativity — delivers results that collapse into those of Newton’s earlier theory through the range of scales for which Newtonian physics works just fine. If it didn’t, then that would be a signal that there was something wrong with the newer theory.
Hence the stakes here. Given that special relativity — the concept at risk if superluminal neutrinos turn out to exist — has been described to me by a physicist friend as more a property of the universe than a “mere” law of nature, it becomes clear, I think why this result is so fascinating. If neutrinos really do go faster than light, then there’s a huge challenge to come up with a theoretical account of what’s going on that allows OPERA’s neutrinos the ability to race whilst Supernova 1987a’s crew dawdled along at mere light speed — to name just one issue that would need resolution.
That is: facts on their own are orphans. They require a conscious act of decision on the part of their interrogator to gain meaning. In an essay published the same year Einstein proposed special relativity, the great mathematician and physicist Henri Poincare asked “who shall choose the facts which…are worthy of freedom of the city in science.” For Poincare, the answer was obvious: that choice “is the free activity of the scientist” — which is to say that it falls to a theorist to think through how one fact, placed next to another, fits into a coherent framework that can survive the test of yet more facts, those already known and those to be discovered.
All of which is to say that even before the Italian observations stand or fall on attempts to replicate the finding, theoretical analyses — thinking hard — can go a very long way in determining whether superluminal neutrinos prove “worthy” of a place in science’s city.
And that’s the long way round to commend a really excellent piece by Matt Strassler, an old friend whose day job as a theoretical particle physicist at Rutgers informs his recently acquired mantle as a physics blogger. Check him out — not just this post — because, IMHO, he’s very rapidly proving himself to be in the first rank of popular translators of some really deep stuff.
In the linked piece, Matt writes about an argument put forward by Andrew Cohen and Nobel Laureate Sheldon Glashow, both theoreticians at Boston University. To gloss Matt’s explication: Cohen and Glashow have developed some earlier thinking that originally focused on the phenomenon called Cerenkov radiation. Matt discusses Cerenkov radiation here — basically it’s electromagnetic radiation emitted by energetic particles going faster than the speed of light in a medium (water, or air, for example, rather than a vacuum) — which, as Matt explains, does not violate special relativity.
Neutrinos do emit such radiation, very weakly, but that’s not the key to the argument; the effect is too small to matter for the OPERA result. Rather, Cohen and Glashow point out that superluminal neutrinos should have produced a different kind of emission that is roughly analogous to the Cerenkov effect — and that each time one of OPERA’s neutrinos did so, it would have lost a lot of energy — enough to register on OPERA instruments. Which means, as Matt puts it, that
… the claim of Cohen and Glashow is that OPERA is inconsistent with itself — that it could not have seen a speed excess without an energy distortion, the latter being easier to measure than the former, but not observed. The upshot, then, is that OPERA’s finding that its neutrinos arrived earlier than expected cannot be due to their traveling faster than the speed of light in vacuum. Something is probably wrong with OPERA’s expectation, not the neutrinos.
Now this is a theoretical argument; it could be wrong in a variety of ways. In the comment thread to Matt’s post, the very clever physicist Lee Smolin points to one possible physical case in which Cohen and Glashow’s proposition would not hold. Theory, interpretation, decides what facts are worthy of being known — but theories are subject to revision, of course, and never more so on those occasoins when one fact or another stubbornly refuses to submit to judgment.
But what I find so pleasing about this whole sequence of thought is the way it illustrates what actually happens in science, as opposed to the parody of scientific process you see in a lot of public accounts — especially when politically significant research is involved.
The OPERA team made the best measurement they could; when it refused to submit to their search for some alternative explanation, they published the result, no doubt reasonably certain that it would be subject to relentless examination — under which there was (and remains) a very good chance this work will be shown to be wrong. Cohen and Glashow have now offered a formal structure within which we can see that what we know of the way the universe actually works presents a major logical challenge to the validity of the OPERA claim of discovery. The ultimate resolution will turn both on continuing experimental work and on the kind of effort Glashow and Cohen offer: the hard work of figuring out what it would mean if the result were true — or perhaps better: what understanding do we have now that suggests the result is either real or an error.
Contrast that with the critique of climate science that comes from the Right, as I discussed briefly in my post on Eric Stieg’s rather blistering review of the recent results affirming (yet again) mainstream climate research. There, Stieg wrote, in effect, that the attacks on climate science turn on a refusal to engage one blunt fact: there is an underlying physical understanding of the basic theory of the process under study, which would be climate change driven by changes in the chemical composition of the atmosphere. That theoretical framework determines the course of empirical research, the search for facts worthy of being known:
…the reason for concern about increasing CO2 comes from the basic physics and chemistry, which was elucidated long before the warming trend was actually observable…The warming trend is something that climate physicists saw coming many decades before it was observed. [Italics in the original.] The reason for interest in the details of the observed trend is to get a better idea of the things we don’t know the magnitude of (e.g. cloud feedbacks), not as a test of the basic theory. If we didn’t know about the CO2-climate connection from physics, then no observation of a warming trend, however accurate, would by itself tell us that anthropogenic global warming is “real,” or (more importantly) that it is going to persist and probably increase.
Which is another way of saying that most of the noise from those who deny both the reality of climate change and impugn the honor of climate researchers misses the point. Not because there isn’t reason to test the reliability of any measurement — of a fast neutrino or a tree ring sequence, either one — but because the issue in either case is understanding what we do know, and then engaging the challenge of a new result in that context.
Hence the (perhaps meta-)value of the faster-than-light neutrino story. This experiment will have to overcome the hurdles thrown up by special relativity’s ubiquitous influence, by the physics of high energy phenomena and so on. That’s how the process of discovery moves from tantalizing initial impressions to settled knowledge. Understanding that process illuminates the hurdles facing climate science denialists: to advance their case, they must reconcile their criticisms of mainstream climate research with the exceptionally well understood basic physics of radiative transfer and the thermal properties of different gases — as well as streams of evidence flowing from direct observations and from the ongoing inquiry into the correlation between evolving climate models and what we can see of the climate itself.
By contrast: cherry-picking dishonestly-excerpted emails is not science.
Oh — and as long as we’ve come this far, let me add a note about another challenge to the faster-than-light neutrino claim that’s come up over the time I’ve been working on this post.
In one of dozens, at least, of efforts to pry apart the actual workings of the OPERA experiment, University of Groningen Ronald van Elburg, has offered his candidate for the (by-many) expected systematic error that could have tricked the OPERA researchers into believing they had observed an effect that is not there.
Elburg has zeroed in on one of the obviously critical elements of the measurement, the calibration of the clocks that timed the neutrinos on their journey. To make that observation, the team relied on the atomic clocks used to synchronize the signals from Global Positioning Satellites — GPS. The tricky part is that the satellites that house the clocks are in motion — pretty fast too — relative to the labs on the ground and the neutrinos traveling between the source and the detector.
When one object is in motion, travelling in a different reference frame than some measuring apparatus, then special relativity comes into play. As the TechReview’s Physics ArXiv blog describes the issue, this means
[that] from the point of view of a clock on board a GPS satellite, the positions of the neutrino source and detector are changing. “From the perspective of the clock, the detector is moving towards the source and consequently the distance travelled by the particles as observed from the clock is shorter,” says van Elburg.
The correction needed to account for this relativistic shrinking of the path as seen from the point of view of the measuring device in space is almost exactly the same size as the seeming excess speed of the neutrinos the OPERA team believes they’ve detected. And that would mean that…
far from breaking Einstein’s theory of relatively, the faster-than-light measurement will turn out to be another confirmation of it.
It’s not as open and shut as all that. Elburg’s argument makes the assumption that the OPERA team failed to account for the quite well-known special relativistic effects on GPS signals — and while they may have, we don’t know that yet. At the same time the original OPERA paper reports some checks on the timekeeping essential to the experiment. I understand that the group is working through the long list of necessary responses to specific suggestions like this one — while at the same time preparing for a yet higher precision measurement of the effect they think they have seen.
But the broader point remains: experimental physics is (and has always been) very, very hard to do, involving an effort to push the limits of precision beyond any current standard. Because the effects sought are at the limits of our capacity to detect them (necessarily; if it were easy, we’d have seen whatever it was already) there is an enormous amount of subtle knowledge that goes into constructing the framework of each experiment. The machines don’t just have to work; you have to understand in detail how quantum mechanics and relativity and all the increasingly subtle applications of the broad ideas play out at the speeds and energies and distances involved. Understanding what’s actually happening at the subtle edges of experiments — even seemingly simply ones — turns out to be very difficult to do.
How difficult? So much so that Albert Einstein himself made an error that is quite similar in some ways to the mistake Elburg suggests could have happend here. In 1930, in one his famous arguments with Niels Bohr, Einstein devised a thought experiment to show that it would be possible to measure a quantity to a finer level of accuracy than Heisenberg’s Uncertainty Principle permits. Einstein’s argument seemed airtight, and according to an observer at the scene,
It was a real shock for Bohr…who, at first, could not think of a solution. For the entire evening he was extremely agitated, and he continued passing from one scientist to another, seeking to persuade them that it could not be the case, that it would have been the end of physics if Einstein were right; but he couldn’t come up with any way to resolve the paradox. I will never forget the image of the two antagonists as they left the club: Einstein, with his tall and commanding figure, who walked tranquilly, with a mildly ironic smile, and Bohr who trotted along beside him, full of excitement…The morning after saw the triumph of Bohr.
It turned out that Einstein had left one crucial physical idea out of his analysis: the effects of his own discovery, the general theory of relativity, on the behavior of the experimental procedure. Once gravity was factored into the argument, the violation of quantum indeterminancy vanished.
That is simply to say that the neutrino experimentalists may well have made what seems from the sidelines like an obvious mistake. But if Albert Einstein could fall prey to a similar kind of error, that should tell us all we need to know about how hard it is for any one person, or even one group, to think through the full subtlety of experience. Which is why science works the way it does, by continuous criticism and self-criticism. As the neutrino story plays out, we’re watching how science ought to work.
Which, and finally we complete the long road home, is why science honestly done and described is vastly different as both a practical and a moral matter than the masked-as-science attacks on this mode of discovery that now dominate the thinking of one of the two major American political parties.