The new theory is that maybe gravity travels “at different speeds across the cosmos”:
General relativity doesn’t align with quantum theory, our most successful attempt at understanding reality so far. And the universe is expanding at an accelerating pace that doesn’t tally with gravity’s attractive nature. Our existing picture may allow us to predict the motion of the moon, but when it comes to explaining the wider cosmos, we’re missing something.
My attempts to find out what have led me to ask whether gravity itself has a mass. Physicists have argued about this for decades. Now my colleagues and I have stumbled on an intriguing answer that may lead us to a clearer picture of gravity. If we are right, then this most enigmatic of forces not only does have a mass, but the evidence that proves it is painted across the cosmos.Claudia de Rham, “What does gravity weigh? The surprise answer that reshapes reality” at New Scientist
Paper. (by Claudia de Rham, Andrew J. Tolley) Although the New Scientist article is pay walled, the paper is open access.
Our physics color commentator Rob Sheldon writes to say,
The paper summarizes the decade of work Claudia de Rahm has put into solving an inconsistency in the “gravity wave” discoveries. Gravity waves carry energy—enormous amounts of it enough to slow down a pulsar—and energy has mass, yet gravity waves are assumed to be massless and travel at the speed of light. This prediction funded a 10 year, $1bn effort to measure gravity waves at LIGO stations in Louisiana & Washington state. These waves are assumed to originate from black hole or neutron star mergers, and that some of these mergers have an optical signature picked up by astronomy telescopes.
Full disclosure—the LIGO observatory has a 1:10,000 signal-to-noise ratio, SNR, which is unbelievably small. Over in the military radar world, which pioneered the methods of matched-filter noise-reduction, 1:1 is still considered “hard” and 1:10 is suspect. No radar person would believe for a nanosecond that the LIGO data processing is seeing anything but artifacts. As if to confirm our suspicions, the Italian VIRGO observatory with 1/10 sensitivity of LIGO sees nothing even when LIGO claims a strong 100:1 signature. Nevertheless, for the sake of argument, let us assume that the LIGO results are valid.
Then there’s a real problem because the LIGO waves are seen before the light, so it would seem that gravity travels faster than the speed-of-light in a vacuum. We can paste ad hoc fixes, like a dense plasma shell that retards the light, but then we’d have to explain why this plasma cloud was stable in the vicinity of a black hole—you get the picture.
The author of this paper, Claudia de Rahm, addresses this problem by asking what happens when the gravity wave (which is quantized as a “graviton”) has mass. It makes the equations consistent, but now the same Higgs field that gives mass to quarks also applies to gravitons and modifies their propagation. But since mass causes gravity and gravity has mass, then we have the real problem that the gravity wave interacts with itself leading to infinities that have to be “normalized” away. This is all standard practice in particle physics, even if it sounds a bit odd, but what is odd is that the procedure, known as effective field theory, normally gives a “reasonable” answer to the sum over infinities. In the case of gravity waves, however, the normalization results in gravity waves travelling faster than the speed of light.
And if something is travelling faster than the speed of light, according to the Special Theory of Relativity, it is going backwards in time. It becomes a time-machine with all the problems of causality and Dr Who plot-lines.
Now I cannot verify but I do not doubt that Claudia has done her math correctly. So we are left with a choice—either the data are not reliable, or we have to jettison our theories of causality (or “reality” as Claudia cheerfully calls it.)
I have spoken before about Sabine Hossenfelder and the crisis in physics—that more and more theories are producing less and less results. This is another example of the crises we encounter when there is a marriage between fake news and speculative theorizing. It isn’t just epidemiology and global warming, but large swathes of the “hard” sciences are misled by junk data.
To me, the sad part of the story is that we know how to get good data, we know how to correct for our ignorance, we know how to properly propagate errors, but we ignore all of that when a Nobel Prize and $1bn is dangled before us. We leave it for some future generation to clean up the mess we have made of our specialties, and we have no one to blame but ourselves.