We understand gravity just fine, thank you.

Yesterday I came across a Q&A on the website of Discover magazine, titled “The Root of Gravity - Does recent research bring us any closer to understanding it?” Jeff Lepler from Michigan has the following question:

“Q: Are we any closer to understanding the root cause of gravity between objects with mass? Can we use our newly discovered knowledge of the Higgs boson or gravitational waves to perhaps negate mass or create/negate gravity?”

A person by name Bill Andrews (unknown to me) gives the following answer:

“A: Sorry, Jeff, but scientists still don’t really know why gravity works. In a way, they’ve just barely figured out how it works.”

The answer continues, but let’s stop right there where the nonsense begins. What’s that even mean scientists don’t know “why” gravity works? And did the Bill person really think he could get away with swapping “why” for a “how” and nobody would notice?

The purpose of science is to explain observations. We have a theory by name General Relativity that explains literally all data of gravitational effects. Indeed, that General Relativity is so dramatically successful is a great frustration for all those people who would like to revolutionize science a la Einstein. So in which sense, please, do scientists barely know how it works?

For all we can presently tell gravity is a fundamental force, which means we have no evidence for an underlying theory from which gravity could be derived. Sure, theoretical physicists are investigating whether there is such an underlying theory that would give rise to gravity as well as the other interactions, a “theory of everything”. (Please submit nomenclature complaints to your local language police, not to me.) Would such a theory of everything explain “why” gravity works? No, because that’s not a meaningful scientific question. A theory of everything could potentially explain how gravity can arise from more fundamental principles similar to, say, the ideal gas law can arise from statistical properties of many atoms in motion. But that still wouldn’t explain why there should be something like gravity, or anything, in the first place.

Either way, even if gravity arises within a larger framework like, say, string theory, the effects of what we call gravity today would still come about because energy-densities (and related quantities like pressure and momentum flux and so on) curve space-time, and fields move in that space-time. Just that these quantities might no longer be fundamental. We’ve known since 101 years how this works.

After a few words on Newtonian gravity, the answer continues:

“Because the other forces use “force carrier particles” to impart the force onto other particles, for gravity to fit the model, all matter must emit gravitons, which physically embody gravity. Note, however, that gravitons are still theoretical. Trying to reconcile these different interpretations of gravity, and understand its true nature, are among the biggest unsolved problems of physics.”

Reconciling which different interpretations of gravity? These are all the same “interpretation.” It is correct that we don’t know how to quantize gravity so that the resulting theory remains viable also when gravity becomes strong. It’s also correct that the force-carrying particle associated to the quantization – the graviton – hasn’t been detected. But the question was about gravity, not quantum gravity. Reconciling the graviton with unquantized gravity is straight-forward – it’s called perturbative quantum gravity –  and exactly the reason most theoretical physicists are convinced the graviton exists. It’s just that this reconciliation breaks down when gravity becomes strong, which means it’s only an approximation.

“But, alas, what we do know does suggest antigravity is impossible.”

That’s correct on a superficial level, but it depends on what you mean by antigravity. If you mean by antigravity that you can let any of the matter which surrounds us “fall up” it’s correct. But there are modifications of general relativity that have effects one can plausibly call anti-gravitational. That’s a longer story though and shall be told another time.

A sensible answer to this question would have been:

“Dear Jeff,

The recent detection of gravitational waves has been another confirmation of Einstein’s theory of General Relativity, which still explains all the gravitational effects that physicists know of. According to General Relativity the root cause of gravity is that all types of energy curve space-time and all matter moves in this curved space-time. Near planets, such as our own, this can be approximated to good accuracy by Newtonian gravity.

There isn’t presently any observation which suggests that gravity itself emergens from another theory, though it is certainly a speculation that many theoretical physicists have pursued. There thus isn’t any deeper root for gravity because it’s presently part of the foundations of physics. The foundations are the roots of everything else.

The discovery of the Higgs boson doesn’t tell us anything about the gravitational interaction. The Higgs boson is merely there to make sure particles have mass in addition to energy, but gravity works the same either way. The detection of gravitational waves is exciting because it allows us to learn a lot about the astrophysical sources of these waves. But the waves themselves have proved to be as expected from General Relativity, so from the perspective of fundamental physics they didn’t bring news.

Within the incredibly well confirmed framework of General Relativity, you cannot negate mass or its gravitational pull. 

You might also enjoy hearing what Richard Feynman had to say when he was asked a similar question about the origin of the magnetic force:

This answer really annoyed me because it’s a lost opportunity to explain how well physicists understand the fundamental laws of nature.