Watching Andrei Linde and Renata Kallosh hear that BICEP-2 found gravitational waves is heartwarming. But with a bit of translating of everything left unsaid, it becomes even more gut-wrenchingly awesome. Allow me to translate for you to see this through a physicist's eyes.


The first thing Dr. Chao-Lin Kuo says when the doctors open their door is, "5 sigma, r of .2." He starts by stating the statistical confidence in the discovery — 5 standard deviations. He then lists the particular parameter relating a tensor to a scalar — a 0.2 ratio between gravitational wave perturbations and density perturbations.

This is the point where Dr. Kallosh hugs him, and Dr. Linde asks him to repeat himself. Dr Linde then collapses in a gleeful puddle of physicist by the front door, and the video cuts to a champagne toast.

Context? Who needs context?

How did Dr. Kuo need to say so little for everyone to instantly know what was going on?


  1. Cosmology is a small field. As a major contributor to inflationary theory, Dr. Linde knows the major experimental projects that could confirm (or destroy) his ideas. His wife, Dr. Kallosh, is a theoretical physicist herself, specializing in gravity and string theory. Her recent work focuses on low-energy modes of inflation, so she, too, knows every major experimental project that could provide data to constrain those theories.
  2. All three doctors are working at Stanford. Of course they know each other; departmental BBQs are handy like that. Even if they weren't all from Stanford, you can bet that Dr. Kuo would have introduced himself and the BICEP-2 project to Dr. Linde during a conference in the past few years. Physicists may not be famous for their social skills, but that type of networking is fundamental to creating research collaborations, and getting out of a string of endless post-doc positions and into professorships.
  3. Dr. Kuo showed up at the door with a camera crew. A camera crew is pretty solid hint that big news is coming. Since it's Dr. Kuo, it had to be about BICEP-2 results related to inflation.


So, just by answering the door, doctors Linde and Kallosh were primed to hear exactly what Dr. Kuo was going to tell them. No need for an introduction, no need for a preamble sentence, and certainly no need for artificially drawing-out tension with a rambling greeting. Nope, Dr. Kuo just got right to the point: "5 sigma, r of .2."

Decoding "5 sigma, r of .2."


But what, exactly, does that mean? The odds of getting a result with 5 sigma confidence by pure chance is 1 in 3.5 million. That's like flipping a coin 28 times and having it come up heads every time by a fluke of luck. It can happen, but it's rare.

Among physicists, 3 sigma confidence is "evidence" for something, but hitting 5 sigma is an "observation" or a "discovery." The same held true with the Higgs Boson announcement: it took 5 sigma to be an observed particle, a true discovery.


But, statistical confidence isn't perfect. Back in 2011, faster-than-light neutrinos were announced with better than 5-sigma confidence. But they were later disproved due to systematic errors that impacted all the data. No matter how many times the experiments were run, faulty cables would ensure the results were faster-than-light travel. So, even when the statistics are clean, the input data can still have errors that only come out in the vigorous debate that is scientific peer review (or, "Let me crush your momentous discovery by pointing out a small but crucial flaw.").

The evidence for gravitational waves looks good — it looks really, really good — but maybe in a few months or a year we'll be running another story on some tiny overlooked hiccup that changes everything.


If you want more discussion of statistical confidence, Evelyn Lamb and John Timmer did nice write-ups of 5-sigma during the Higgs Boson announcements.

Moving on, the ratio of gravitational wave perturbations to density perturbations (r) is calculated after the foreground noise sources are removed. The foreground removal is one of the parts of these results that is going to get a lot of scrutiny in the next stage of research (or, "Everyone pokes at your process, scrutinizes for any mistakes, and tries to rip your data to shreds"). Did the BICEP-2 team effectively remove all foreground noise, and nothing but noise?


The r=0.2 result is a bit higher than what cosmologists were expecting, but it's still consistent with theory. What doesn't make the video, and what is downright impressive, is that BICEP-2's team has 7-sigma confidence in r=0. Maybe the press release should have been titled, "See how to win a Nobel Prize with this one weird little parameter!" Picking which ratio-relationship is most likely to be the best representation in reality is complicated and not entirely dependent on sigma confidence levels, and is probably in the "TBA after we get more data..." category. For now, the real point is that with this much range with high sigmas, it's highly likely that the data represents a real observation of gravitational waves of some variety.

Beyond the headlines

This video is going viral as the moment the Father of Inflation learns he was right. Like most viral phrases and catchy headlines, that's not exactly accurate.


Andrei Linde is certainly one of the major contributors to inflationary theory, and it's been the focus of his academic career. He's staked all his intellectual hopes on it being right — I love the moment where he confesses his fear that he's tricked himself into believing his own theories because they're mathematically beautiful. But, he's not first person to think of inflation theory. That'd be Alan Guth and his notes, with "SPECTACULAR REALIZATION!" in a double-outlined box. Nor is he the only one to have worked on inflation. His contributions are magnificent and vital, but calling him the Father of Inflation ignores all the theorists whose doors Dr. Kuo didn't knock on.

As for saying it's the moment he learns he was right... Did you catch that "if" at 1:45?

If this is true, this is a moment of understanding nature of such a magnitude that it just overwhelms. Let us hope that is not [a] trick.


Why would Dr. Linde hedge such an incredible set of data supporting the ideas he wants to be true? Because shit happens. Yes, the BICEP-2 team knew that every cosmologist in the world was going to try to rip their work to pieces. (It's not malicious, it's part of why research is such an amazing, robust process!). As humans with ego, pride, and dignity, they very much don't want to go down in history for fucking up. They must have gone through their equipment, data collection, processing, and statistics so many times they went blurry-eyed and hallucinated tiny purple elephants dancing in the numbers, went to bed, and checked it all again the next morning. Then kept on doing those checks for weeks upon weeks, doing their best to prove themselves wrong before daring to believe their own results. But even with all their care, maybe they made a mistake.

This is why, in a few month's time, or maybe a year, we're going to come back to this story saying, "Gravitational waves confirmed!" as another research project comes up with consistent results. It won't be as sexy of a headline, and the press conference won't draw the traffic that crashed live-streaming servers this week. In the comments, eye-rolling and sighs of, "But didn't we already know this?" will be rampant. But it will be then that Dr. Linde, all the other founders of inflationary theory, and the BICEP-2 team, will breathe easy and drop that tell-tale, cautious, hopeful "if."

Now go on, watch that video again. Watch it through a physicist's eyes. Watch as Dr. Kallosh picks up what's happening first, and her husband takes longer for the words to sink in. Watch Dr. Kuo proudly click through the screens of data, and hear Dr. Kallosh's excitement that the results show up in three different experiments. And then finally, hear all that wonder and hope tied up into Dr. Linde's tiny, loaded "if."


This article is inspired by Jim DeLaHunt, who asked what that "5 sigma, r .2" meant, and Chris McKenny, who answered. Thank you.