String Theory gets Competition: A New Attempt to Solve Physics' Biggest Mystery (Patreon)

 [This is a transcript with links to references.]

Every scientific discipline has its holy grail, and the holy grail of physics is definitely quantum gravity. Quantum gravity is Einstein’s unfinished revolution, the missing unification of General Relativity and Quantum Theory. String theory is the best-known contender, and this week have a strong newcomer.

You see, the issue is that Einstein’s theory of General Relativity doesn’t have quantum properties. So if we take a particle and do an experiment that tests its quantum properties, like the famous double slit experiment, then we don’t know what happens with its gravitational field. The quantum particle goes both ways at once, so you’d think the gravitational field of the particles also goes both ways, but that can’t be. Because Einstein’s theory doesn’t have quantum properties.

Most physicists think that the solution to this problem is to give quantum properties to space and time. This is what happens in string theory and in loop quantum gravity and in asymptotically safe gravity and in almost all existing approaches to quantum gravity. But this new approach works entirely different.

The idea that was pioneered by Jonathan Oppenheim is that space-time and with that gravity remains a non-quantum theory, or a “classical” theory as physicists say. Oppenheim has been working on this for a long time, and now he has published two papers at once.

In one single-authored new paper, he spells out the general mathematics for the idea. He says that leaving space-time classical only works if it fluctuates randomly because that’ll make it compatible with quantum mechanics. Models in which space time remains classical will generally increase some measurement uncertainties, and this one does too.This might sound counterintuitive – why does removing quantum properties increase uncertainty? -- but think of it this way. If you have a particle that goes two paths that the same time and the gravitational field does that that too, then gravitational pull always points to the middle. If the gravitational field is not quantum, then it goes either left or right but not both, so it must fluctuates between two. So, more uncertainty.

In a second paper, Oppenheim together with several colleagues works out a way to test the idea.  They say the best way to test it would be to very precisely track the mass of some weights because that should slightly fluctuate. This test might become possible in table-top experiments in the near future. Table-top experiments is what physicists call an experiment that doesn’t cost billions of dollars. Like, you know, particle colliders.

Another interesting thing about this idea is that it has a collapse of the wave-function built in, so the measurement problem of quantum mechanics disappears. This fits together very nicely with the idea of Roger Penrose that gravity is what causes the collapse of the wave-function. But Oppenheimer’s work is a mathematically more complete treatment and indeed I find it quite impressive. It’ll take me some time to fully digest the paper, so let me just say, this is a very strong contender that I think can plausibly claim to solve the problem.

However, there are always multiple ways to solve mathematical inconsistencies, like that between general relativity and quantum mechanics, and this is only one of them, so in the end experiment must decide.

I wonder what Einstein would have said about this.,

Hello?

Hello Albert,

Yes, I’m afraid this means God is still throwing dice.

I didn’t know that “craps” is a dice game. That… explains a lot.

Thanks for calling in.