The physics–verse has been abuzz this week with news of the BICEP2 experiment’s observations of B-mode polarization in the Cosmic Microwave Background.
There are lots of good sources on this, and it’s not really my field, so I’m just going to give a quick summary before talking about a few aspects I find interesting.
BICEP2 is a telescope in Antarctica that observes the Cosmic Microwave Background, light left over from the first time that the universe was clear enough for light to travel. (If you’re interested in a background on what we know about how the universe began, Of Particular Significance has an article here that should be fairly detailed, and I have a take on some more speculative aspects here.) Earlier experiments that observed the Cosmic Microwave Background discovered a surprising amount of uniformity. This led to the proposal of a concept called inflation: the idea that at some point the early universe expanded exponentially, smearing any non-uniformities across the sky and smoothing everything out. Since the rate the universe expands is a number, if that number is to vary it naturally should be a scalar field, which in this case is called the inflaton.
During inflation, distances themselves get stretched out. Think about inflation like enlarging an image. As you’ve probably noticed (maybe even in early posts on this blog), enlarging an image doesn’t always work out well. The resulting image is often pixelated or distorted. Some of the distortion comes from glitches in the program that enlarges the image, while some of it is just what happens when the pixels of the original image get enlarged to the point that you can see them.
Quantum fluctuations in the inflaton field itself are the glitches in the program, enlarging some areas more than others. The pattern they create in the Cosmic Microwave Background is called E-mode polarization, and several other experiments have been able to detect it.
Much weaker are the effect of the “pixels” of the original image. Since the original image is spacetime itself, the pixels are the quantum fluctuations of spacetime: quantum gravity waves. Inflation enlarged them to the point that they were visible on a large-distance scale, fundamental non-uniformity in the world blown up big enough to affect the distribution of light. The effect this had on light is detectably different: it’s called B-mode polarization, and this is the first experiment to detect it on the right scale for it to be caused by gravity waves.
Measuring this polarization, in particular how strong it is, tells us a lot about how inflation occurred. It’s enough to rule out several models, and lend support to several others. If the results are corroborated this will be real, useful evidence, the sort physicists love to get, and folks are happily crunching numbers on it all over the world.
All that said, this site is called four gravitons and a grad student, and I’m betting that some of you want to ask this grad student: is this evidence for gravitons, or for gravity waves?
We already had good indirect evidence for gravity waves: pairs of neutron stars release gravity waves as they orbit each other, which causes them to slow down. Since we’ve observed them slowing down at the right rates, we were already confident gravity waves exist. And if you’ve got gravity waves, gravitons follow as a natural consequence of quantum mechanics.
The data from BICEP2 is also indirect. The gravity waves “observed” by BICEP2 were present in the early universe. It is their effect on the light that would become the Cosmic Microwave Background that is being observed, not the gravity waves directly. We still have yet to directly detect gravity waves, with a gravity telescope like LIGO.
On the other hand, a “gravity telescope” isn’t exactly direct either. In order to detect gravity waves, LIGO and other gravity telescopes attempt to measure their effect on the distances between objects. How do they do that? By looking at interference patterns of light.
In both cases, we’re looking at light, present in the environment of a gravity wave, and examining its properties. Of course, in a gravity telescope the light is from a nearby environment under tight control, while the Cosmic Microwave Background is light from as far away and long ago as anything within the reach of science today. In both cases, though, it’s not nearly as simple as “observing” an effect. “Seeing” anything in high energy physics or astrophysics is always a matter of interpreting data based on science we already know.
Alright, that’s evidence for gravity waves. Does that mean evidence for gravitons?
I’ve seen a few people describe BICEP2’s results as evidence for quantum gravity/quantum gravity effects. I felt a little uncomfortable with that claim, so I asked Matt Strassler what he thought. I think his perspective on this is the right one. Quantum gravity is just what happens when gravity exists in a quantum world. As I’ve said on this site before, quantum gravity is easy. The hard part is making a theory of quantum gravity that has real predictive power, and that’s something these results don’t shed any light on at all.
That said, I’m a bit conflicted. They really are seeing a quantum effect in gravity, and as far as I’m aware this really is the first time such an effect has been observed. Gravity is so weak, and quantum gravity effects so small, that it takes inflation blowing them up across the sky for them to be visible. Now, I don’t think there was anyone out there who thought gravity didn’t have quantum fluctuations (or at least, anyone with a serious scientific case). But seeing into a new regime, even if it doesn’t tell us much…that’s important, isn’t it? (After writing this, I read Matt Strassler’s more recent post, where he has a paragraph professing similar sentiments).
On yet another hand, I’ve heard it asserted in another context that loop quantum gravity researchers don’t know how to get gravitons. I know nothing about the technical details of loop quantum gravity, so I don’t know if that actually has any relevance here…but it does amuse me.
BICEP2 does seem one more nail in a coffin I’ve been trying not to bury for a long time. It’s completely emotional and utterly unscientific (and I don’t have the background knowledge to fully appreciate why I’m wrong), but I’ve held out hope that (A) there are no such things as gravitons and (B) that Einstein’s smooth universe would somehow turn out to be right (and quantum physics wrong on that account).
We know there’s a conflict between GR and QP, and I want to bet on GR, but it’s not really looking like a smart bet. I’ve been clinging to the various mutually-exclusive dualities that pop up in physics and hoping maybe gravity might be such a case — that it’s not all one thing, but two depending on how you look at it. Quantized substance; smooth underlying reality.
Ah, well… what’s the line about facts destroying a perfectly good idea? 😀
I feel like your worry might be misplaced. While popular depictions often contrast the smoothness of GR with some sort of “chaotic quantum foam” or whatever, really quantum gravity is a pretty natural extension of GR. (If you haven’t read my post on this, you might find it of interest: http://4gravitonsandagradstudent.wordpress.com/2013/05/17/whats-a-graviton-or-how-i-learned-to-stop-worrying-and-love-quantum-gravity/ )
The key thing about GR is not merely that reality is smooth, but that reality is geometrical (in the mathematician-sense). The places where quantum mechanics and gravity seem to be opposed are ones where GR gives you infinities, like singularities. Quantum mechanical approaches to gravity actually tends to smooth out these singularities. For string theory, singularities get “blown up” in a way that’s very much the way that mathematicians who study geometry do it.
So I think that really, “Einstein’s dream” is actually pretty intact.
Well, that would be fine with me! As I understand it, string theory gets around the problem due to strings having actual (very, very tiny) size, so the process of approaching singularities is stopped around the Planck size. (I became quite enthusiastic about string theory after reading Brian Greene’s books. So elegant and simple. And then I started reading Peter Woit and Lee Smolin, and I’m not sure what to think anymore.)
I’ll be very interested in what comes out of the black hole firewall information-loss (or not) debate.