Far out cosmology: string theory redux, dead (Introduction)

by David Turell , Saturday, March 09, 2024, 18:01 (257 days ago) @ David Turell

Another nail in the coffin:

https://mail.google.com/mail/u/0/#inbox/FMfcgzGxSHcKDFJSCffvfBrCJgWTFtFC

"It was a beautiful idea, no doubt, and thousands of physicists spent decades on it. But it didn’t quite go according to plan. String theory became extremely controversial about 20 years ago during a phase that’s been dubbed the “String Wars”. Then it kind of fizzled out. What happened? What were the string wars? And what are string theorists doing now?

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"After the completion of the standard model, string theory swiftly became the hottest contender for this theory of everything.

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'This version of string theory still exists today as a bottom-up approach to understand what gluons do, but the string theory that we are concerned with is a different one. You see, when physicists studied what those strings do, they found that some of them behave like a graviton, that’s a quantum of the gravitational interaction.

"It’s a particle which has never been observed, not then and not now, but that could be the start of giving quantum properties to gravity. At the same time, strings were so versatile that they could also behave like particles that make up matter and those who make up light. Strings could do all of it. It was what they’d been looking for – a theory of everything.

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"Plausible enough you might say, but even so, supersymmetry predicted something else, a type of process that can’t happen in the standard model. It involves what’s known flavour-changing neutral currents. Evidence for those processes should have shown up in the early 1990s at the Large Electron Positron collider at CERN. It did not. And so, string theorists added another fix to their theory, R-symmetry, to make their equations agree with observations again.

"Then there was the problem that string theory required a total of 10 dimensions of space to properly work. Unfortunately, it seems that the space we find ourselves in has merely 3 dimensions. String theorists explained away the extra dimensions of space by saying they are rolled up to sizes so small that we can’t see them. Again this works because measuring something small requires a lot of energy, and we might not have noticed these small dimensions because we haven’t been able to achieve particle collisions with sufficiently high energy.

"Now when I say they rolled up those extra dimensions that might suggest each of them is like a straw and there’s only one way to do it. But actually there are many different ways to do it. Already in two dimensions you have a torus or a sphere as example. And if you have 6 dimensions there’s a huge number of ways to roll these dimensions up, some hundred thousand or so. At this point, the idea that string theory was unique and would just spit out the standard model went out of the window.

"The problems did not stop there. String theory works best in a universe with a negative cosmological constant and string theorists originally just assumed that’d be so. The cosmological constant, if you remember, is a constant of nature that determines how the expansion of the universe changes, whether it gets faster or slower. The expansion of our universe gets faster, and that means the cosmological constant is positive, exactly the opposite of what string theory requires.

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"But all this paper writing didn’t help string theorists find that theory of everything they’d been looking for. There were just too many versions of string theory now, an estimated 10 to the 500. And since they couldn’t find one that actually reproduced the standard model and general relativity, they postulated all of them exist. This is the so-called string theory landscape.

I’ve always found that to be a particularly idiotic move. Just because you can’t figure out which theory describes reality doesn’t mean all of them are real. And in any case, it didn’t solve any problem because they did, as a matter of fact, not have the theory they were looking for.

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"I wasn’t the only one back then who had the feeling that something was going badly wrong in the foundations of physics. In 2006, two books appeared almost simultaneously, taking issue with string theory. One was Lee Smolin’s “The trouble with physics”, the other Peter Woit’s “Not Even Wrong”.

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"in 2010 the LHC turned on, didn’t find any evidence for supersymmetry or extra dimensions or string balls or gravitons or what have you, and the bubble in which string theory had been testable burst.

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"In summary. String theory had a really good motivation, and it was pursued as a theory of everything for good reasons. However, when that didn’t work out, string theorists were slow to get the message and a lot of time and effort was wasted on it. It’s not that string theory turned out to be completely useless. Some techniques have survived and are being used today in related areas of physics."

Comment: Long ago I quoted Peter Woit here with the same conclusions. Shows we stay up to date.