As you may know, on February 11 the team at LIGO announced that, using ultra-precise interferometers located in Louisiana and Washington state, they had recorded a “chirp” signal (known as GW150914), consistent with the observation of gravitational waves from a black-hole merger occurring 1.3 billion light years away. There followed a deluge of celebratory science articles throughout the mainstream press, hailing the final confirmation of Einstein’s theory, the inception of a new field of astronomy, and a triumph for publicly-funded “big science”.
I seem to be the only one out there who is saying this, but doesn’t this celebration strike anyone else as a tad premature?
To get a few things out of the way first: I have no argument with Einstein. I think that General Relativity (GR) is a remarkable and thoroughly battle-tested theory, and I am willing to consider it “true” just about as far as any scientific theory can usefully be said to be so. (Though it’s also technically “false”, since it doesn’t play well with quantum mechanics, but leave that aside.)
I also have no problem with the concept of gravitational waves; without knowing the math myself, there is clearly a long-standing consensus in physics that the waves are a pretty inevitable consequence of both Einstein’s equations and the concept that gravity travels at a finite speed. I think discovering gravity waves and learning to use and observe them would be incredibly interesting. I also freely admit that what LIGO observed on September 14, 2015 is fairly likely to be an example of such waves.
Yet just going on my own scientific intuitions, as of this moment the result seems, if not fishy, then weirdly unconvincing–for a number of reasons.
First, and biggest, there’s the as-yet one-off nature of the signal. No other “chirps” resembling GW150914 have been picked up by LIGO since September 14.
It’s almost impossible to overstate the importance of repeatability in legitimating scientific claims. The fewer the observations, and the more times similar methods fail to recover a reported result, the harder it becomes to rule out random fluctuations, unknown phenomena, or even misconduct–and the worse (or at least flukier) that first result looks. Science only works well when nature gives you enough clearly identifiable cases of your Phenomenon Of Interest that you can begin to play with them–to build up statistics, to explore their regularities and quirks, and so on. The longer LIGO goes with no further signals found, the more GW150914 will begin to look like either an instrumental hiccup or a tantalizing but unaccountable curio, rather like the WOW! signal.
Second, there’s the silence itself. While the sheer uniqueness of GW150914 may be curious, the length of time that has gone by with nothing more seen is equally puzzling. Given the newly enhanced detection range of LIGO, mergers of ultra-massive objects (pulsars, black holes etc.) were predicted to happen every week or so. What are the odds that the observatory would detect only one in five months of looking–i.e. twenty times fewer than expected? At the least, it suggests that the astronomical models that generated the predictions of merger frequency are a ways off.
Third, there’s the source. The problem with it is quite simply that no one knows where it is. Only two LIGO-quality interferometers were running on that fine September day; without that third detector, triangulation was impossible, confining the location of the signal’s source only to an arc across the sky, rather than a specific point.
Fourth, drawing on the above point, if we cannot even locate the source, then it’s impossible to apply other, more established modalities to corroborate the given story of what happened. This is pretty much a tautology: if we want to aim a telescope at the place the gravitational waves came from, to see if it actually looks like the kind of structure where pulsars or black holes have just been colliding… then we first have to know where that place is. But we don’t know for GW150914, and we have no other instances to use… so how can we say what the thing was? (On the other hand, inferring the discovery of gravity waves from a “black hole merger” whose only evidence for existing is itself based on gravity waves, seems like circular reasoning on a cosmic scale.)
Fifth, there’s the timing. LIGO had barely been switched on when GW150914 was observed, and even then the system was not formally in “observation mode“. Apparently there also was nobody in the control room at the moment of the observation, the scientists in charge having left for their hotel rooms. On the one hand, it could be pure chance that the signal just happened to arrive so promptly; on the other hand, it seems more likely that glitches could occur near the very outset of a hitherto-untested machine’s operation, and the more so with no one present to keep an eye on things.
Finally, there are the more interminable questions of interpretation, which in this case heads into the philosophical netherlands of scientific certainty. GR may now seem rather old-hat to physicists, who have gotten used to having it constantly in their field of view (no pun intended) for over a century; but in fact, it remains an extremely complex and abstract mathematical and conceptual structure, requiring a large number of very specific and counter-intuitive tools. Both the concepts of “gravitational wave” and “black hole”, for example, for all that we are now largely used to them, are so abstruse, so completely removed from any conceivable human intuition or direct observation, and in short so exceptionally theory-laden, that I worry that in the LIGO case the experts’ thorough knowledge of the theory of general relativity and gravitational waves may actually prevent them from thinking of other simpler interpretations.
Then again, attempts to apply Occam’s Razor to the LIGO result leave one’s head spinning. What warrants our saying that a single blip such as GW150914 really is most simply explained as “the record of a change in the path lengths in the detector due to a passing gravitational wave, itself the product of the collision of two black holes of such-and-such mass” …and not as anything else? We are put in the uncomfortable philosophical position of having to categorically settle on the nature and significance of a single event, on the basis of its resemblance to waves of an exceedingly hard-to-detect sort, produced by an invisible and non-localizable source–and all based purely on its similarity to a simulation derived from an immensely complex and abstract theory. In effect, the LIGO result walks on the very very edge between firm scientific fact versus rationalization run amok–and at this point, I’m still not completely sure which side it will end up on.
Matters aren’t helped by the size of the actual oscillation: 1/1,000 the diameter of a proton. No known microscope can probe distances anywhere close to this. My reaction to this is hard to describe. Of course, science has routinely probed domains far outside our perception or common sense, with resounding success–but these ventures have always proven massively repeatable, and repeatable without billion-dollar instruments that take a thousand people to operate. I don’t want to say that an effect so tiny can’t even exist or be measured or matter, but here again I get that impression that we are closing in on that maddening edge between “arriving at a truth” and living on stories and rationalizations.
In “The Myth of Sisyphus”, Albert Camus–faced with a well-meaning attempt to teach him about the structure of atoms, and so allay his puzzlement about his place in the world–mused:
“…all the knowledge on earth will give me nothing to assure me that this world is mine. You describe it to me and you teach me to classify it. You enumerate its laws and in my thirst for knowledge I admit that they are true. You take apart its mechanism and my hope increases. At the final stage you teach me that this wondrous and multicolored universe can be reduced to the atom and that the atom itself can be reduced to the electron. All this is good and I wait for you to continue. But you tell me of an invisible planetary system in which electrons gravitate around a nucleus. You explain this world to me with an image. I realize then that you have been reduced to poetry: I shall never know. Have I the time to become indignant? You have already changed theories. So that science that was to teach me everything ends up in a hypothesis, that lucidity founders in metaphor, that uncertainty is resolved in a work of art. What need had I of so many efforts?”
This is my reaction when I look at the LIGO paper, or read the peans about it. I can’t process it. It seems like poetry. We are dealing with distances so tiny, with phenomena so completely un-relatable to any meaningful aspect of reality as we know it, and with events which seem so certain to be swamped by billions of other conceivable phenomena, that the only question that comes is: is it any of it real? Is it an instrumental triumph or a poetic exuberance? Without more information, I can’t tell the difference. I find myself shrugging at the unfathomable, and wanting to say to the scientists: “maybe you know, and maybe you don’t”. I think that a lot of people reading about this story and trying to understand its significance have reacted the same way.
In fairness, assuming it isn’t some kind of glitch, the work of an “evil genius”, or a test “injection”, GW150914 clearly can’t be sheer coincidence. The fact that two detectors in different states found the signal within milliseconds of each other–shifted and inverted, just as would be expected based on their differing orientations–and that the two waveforms’ shapes closely match each other, is a strong indication that something “real” happened. It’s also impressive that these two waveforms closely resemble that modeled (I assume straightforwardly and definitively) by theory, even if I struggle with how much significance to assign to that resemblance.
To sum up, except for the philosophical puzzlement (which will haunt me no matter what), most of my uneasiness with the LIGO findings could be dispelled by two things happening. First, the signal must be replicated, with the new instances all closely matching the most straightforward GR predictions available; and second, for these new signals, the source must be localized and preferably corroborated with observations of some other kind (for instance, X-ray/radio astronomy) to show that the origin of the gravitational radiation does in fact coincide with a plausible pulsar or black-hole source. For all I know, these things could be revealed next week. If they are, I will join the chorus of commentators confidently celebrating the discovery.
Until then, my view is that the response to the LIGO announcement is more interesting than the announcement itself, though in a way that has almost nothing to do with gravitational waves. In decades past, a single, non-repeatable result, from an essentially unidentified physical source, would be received by the scientific community perhaps with interest but still with an attitude of intense skepticism, pending further observations. Yet here in 2016, there seems to be no trace of such skepticism or caution (excepting the ever-cantankerous John Horgan). This is all the more amazing given the very recent scientific fiascoes of BICEP2 and the Gran Sasso “faster than light” neutrinos. What is going on?
It has certainly helped in this case that almost no one really doubts the theoretical basis of gravitational waves, whereas there are doubts aplenty about cosmic inflation (BICEP2) and faster-than-light travel. Partly this is how “normal” science works: results that fit the expected theory perfectly are embraced, while those that don’t fit must clear a far higher bar. The search for gravitational waves is “normal science” par excellence, as was the search for the Higgs boson.
But it’s also true that there have been no experimental novelties in fundamental physics in the last 50 years, creating such a drought of juicy headlines and proposals that increasing numbers of physicists are tempted to abandon the criterion of experimental verification and falsifiability altogether. There seems to be a feeling of growing unease spreading in this part of the scientific community, as it seeks to follow its beautiful abstractions and constructs–epitomized by string theory–away from the albatross of experiment. Even as physicists seek “ultimate reality” in such abstractions, they are simultaneously being led into a place ever more remote from reality and ever more ambiguous in its real-world implications.
What the premature release and hype of the LIGO signal therefore tells me is fundamental physics is quietly desperate. Credibility is increasingly strained by the failure to discover particles beyond the 1970’s-vintage Standard Model, or anything else of comparable surprise, and by the recent fiascoes I mentioned. It may well be that in such a time, the laurels of “having proved Einstein right” vis-à-vis gravitational waves were just too tempting: physics needed a new grand experimental result too desperately to wait for, or demand, the proper amount of reproducibility and corroboration. Let’s hope, for science’s sake, that this desperation and haste does not lead to yet another humiliation.