A few weeks ago there was (justified) hullabaloo following the release of results from the BICEP2 experiment, which (if correct as an experiment, and if correctly interpreted) may indicate the detection of gravitational waves that were generated at an extremely early stage in the universe (or at least in its current phase)… during a (still hypothetical but increasingly plausible) stage known as cosmic inflation. (Here’s my description of the history of the early universe as we currently understand it, and my cautionary tale on which parts of the history are well understood (and why) and which parts are not.)
During that wild day or two following the announcement, a number of scientists stated that this was “the first direct observation of gravitational waves”. Others, including me, emphasized that this was an “indirect observation of gravitational waves.” I’m sure many readers noticed this discrepancy. Who was right?
No one was wrong, not on this point anyway. It was a matter of perspective. Since I think some readers would be interested to understand this point, here’s the story, and you can make your own judgment.
- a past observation of gravitational waves that everyone agrees is indirect;
- a future observation of gravitational waves that we expect to happen fairly soon, one that I believe everyone will agree is direct;
- BICEP2, and how you can view it either way, depending on your perspective.
A Past Indirect Observation of Gravitational Waves
First, let me describe what everyone agrees was the first observation of gravitational waves, and was definitely indirect. In 1974, two scientists (Joseph Taylor and his graduate student Russell Hulse) discovered a pulsar. A pulsar is a city-sized neutron star (made entirely from neutrons and resulting from a Type IIa supernova) that spins rapidly — rotating many times per second — and, due to its powerful magnetic field, sends strong radio beams into space, which sweep past the Earth as the pulsar spins. We observe this as a pulsing radio signal from the location of the star.
Pulsars are common, but this one was special. Its frequency of pulsing (i.e. how many times per second does it pulse) varied slightly, growing and shrinking every 7 hours and 45 minutes. It quickly became clear this was due to the Doppler effect for radio waves; the pulsar was sometimes moving toward us, and sometimes away, because it was in orbit around something else. Detailed study (using the Newton/Einstein laws of gravity) allowed Hulse and Taylor to infer that what they were seeing was a pulsar orbiting a second neutron star. They could even figure out the orientation and size of the orbit!
Having figured this out, they could do one more thing. Einstein’s laws of gravity predict that the gravitational waves — waves in space itself — that are created by these two stars as they orbit one another, and these waves should be carrying energy out into space, reducing the energy available to the two stars. The effect of this loss of energy would be a very mild reduction in the time (or “period”) that it takes for the two stars to orbit each other — but not by very much! The period of the orbit, about 28,000 seconds, is predicted by Einstein’s equations to be shrinking by a bit more than one second per year.
Fortunately, pulsars are stable enough, and Hulse and Taylor’s measurements were easily accurate enough, that this change of about a second per year was relatively easy for them to measure during the ensuing decade. And they could compare their measurements of the change in the period with the predictions of Einstein’s theory of gravity. Remarkably, the agreement of the theory with the data is excellent! For this confirmation of Einstein’s theory’s prediction of gravitational waves, Hulse and Taylor received the Nobel Prize in 1993.
Hulse and Taylor had thus observed the effect of gravitational waves for the first time in human history. But they hadn’t observed the waves themselves; they’d observed the loss of energy, in the neutron star pair, due to the waves, but not the waving of space, compressing and expanding as the waves move by. Clearly, this detection of gravitational waves was indirect.
A Future, Likely Direct Detection of Gravitational Waves
A direct search for gravitational waves is underway now, at experiments known as LIGO and VIRGO. When a gravitational wave passes by the Earth, space itself grows and shrinks a little bit, and the distances between objects increases and decreases. It’s an incredibly tiny effect even for powerful gravitational waves; you and I would never notice it. But this shrinking and growing of space can potentially be observed with extremely stable, carefully designed lasers looking for the distance between two mirrors to shift by less than the radius of a proton, which itself is 100,000 times smaller than the radius of an atom! [The principles involved are not so different from those used in the famous Michelson-Morley experiment --- but the experimental requirements are vastly greater!]
When the repeated changing of the distance between mirrors due to a stretching and compression of space is actually observed, that will clearly be direct observation of waves of space itself — gravitational waves. This hasn’t happened yet, but the “Advanced” phase of LIGO is coming up very soon, starting this year. We may well see LIGO make discoveries within the decade.
BICEP2: Direct or Indirect?
I think it’s very clear that BICEP2 — IF the experiment’s results are correct (they have not been confirmed by another experiment yet) and IF they are correctly interpreted as due to gravitational waves (which is still an open question) —represents an advance over the Hulse-Taylor discovery. But it’s not as direct as LIGO, either.
BICEP2’s measurement [see here for some details] is actually of the polarization of light that was released 380,000 years after the Hot Big Bang began, at the time when the universe cooled enough to become transparent. This light has now become the “cosmic microwave background” [CMB] which we observe today coming from all directions in the sky. So really they’re directly observing light (microwaves rather than visible light), not waves in space itself — gravitational waves.
But the nature and size of the polarization effect they observe (“B-mode” polarization, across large swathes of sky) is believed to have only one possible source: gravitational waves, created in the early universe and ringing for 380,000 years, and then interacting with the light that is now the CMB. It is the squeezing and stretching of space within which the light is moving that causes the light to end up polarized in a unique way.
In this sense, you could say that the CMB is providing a sort of unusual photograph of gravitational waves, taken at 380,000 years post-Big-Bang. It gives far, far more detail about their nature than does the Hulse-Taylor measurement; it confirms more and different things that Einstein predicted, such as the fact that these gravitational waves have “spin two”, which is necessary for them to give B-mode polarization. If you think of it as a photograph, BICEP2’s measurement seems pretty direct.
But on the other hand, it’s nowhere near as direct as LIGO would be, where mirrors that humans have set up will actually move back and forth as a gravitational wave’s crests and troughs pass by. Far, far more detail will be available when that happens — and there will be little or no ambiguity about the interpretation of the data. For BICEP2, it’s still conceivable (though no one has thought of anything specific) that the B-mode polarization actually is not due to gravitational waves but is due to something else. The very fact that this is conceivable — that maybe the polarization comes from something other than waves in space itself — reflects the fact that the BICEP2 data involves looking at something that happened billions of years ago in very distant locations, and drawing inferences. BICEP2 isn’t itself seeing space shrink and expand; it’s observing polarized light created long ago, and then scientists are inferring that the pattern of its polarization is due to space shrinking and expanding. From that point of view, BICEP2’s detection is still rather indirect.
So call it what you will, it’s clearly (if correct and correctly interpreted) more direct than Hulse and Taylor’s measurement, and less direct than a detection at LIGO would be. Maybe we should call it “(…nnnn)direct”? In any case, what we call it isn’t important; what’s important is to figure out whether it’s correct, and what it means.