The instability of AdS has gotten a few minutes of fame, receiving an article in *Quanta* magazine. It looks like a decent write-up, except for a part that confuses the geometry of AdS spacetime with ordinary special relativity. The main technical paper they point to is arXiv:1812.04268.

On the one hand, these various instability results seem natural, in a fashion. If you want thermalization to happen in the boundary field theory, then you want black-hole formation to happen in the bulk gravitational theory. But on the other hand, the radical difference in stability properties makes me question just how much AdS quantum gravity can tell us about quantum gravity in *our* universe. The line all along was, “You can put whatever local physics you want into an AdS box!” Yet the very property of AdS that makes it a good background for quantum gravity — it’s a convenient box to put gravity in — means that the answer to questions like, “If I do this in my lab, will a black hole happen?” might be different. To me, this calls into question the idea that well-defined measurements are the same in AdS and in dS or Minkowski, which in turn leads me to speculate that the meaning of the verb “to quantize” cannot be the same.

But that’s freewheeling chatter on my part; mostly I’m just here to leave a couple links in case anybody is curious.

]]>When you smack electrons against each other harder, they act like their charge is bigger. I don’t believe *that* will go away with a different formalism. Renormalization relates their charge at high momenta to their charge at low momenta. It does so very accurately. I don’t think we want that to go away.

When we extrapolate this to arbitrarily high momenta, things go crazy. I think we *do* want *that* to go away: that’s an artifact of assuming spacetime can be arbitrarily subdivided. (In relativistic quantum physics, particles are waves and their momentum is inversely proportional to their wavelength.)

All decent physicists know this; getting a workable theory with spacetime not a continuum is the hard part. A bunch of very smart people have worked on that for many years, and so have I. As you can probably tell, it irks me when people say “Hey! Why don’t you just stop assuming spacetime is a continuum?”—as if we’re obstinately refusing to consider an easy solution to our problems. I’m not accusing you of doing that, but lots of people do that.

]]>Good point about QED renormalization being more complex than just a continuum problem.. but yet I wonder.. if we had a totally different formalism from the outset would it still exist?

]]>Go for it! I spent 10 years trying to find that deeper and simpler thing, and I came up with spin foam models. I quit when I realized it would take a lot more work to see if they successfully deal with problems in physics.

]]>The problems I’m describing are mathematical: they exist in the math whether or not that math accurately models reality. So while you say

In this case that space is a continuum. Then if it’s not you pay the price.

in fact even if spacetime *is* a continuum we pay the price of having to work with this.

For example: renormalization of QED.

Did you read these parts?

• Part 4: quantum electrodynamics.

• Part 5: renormalization in quantum electrodynamics.

One that’s clear by now is that renormalization is a real physical phenomenon, regardless of whether spacetime is ultimately a continuum or not. It arises because each particle is surrounded by a cloud of virtual particles, so that its observed properties change as one penetrates deeper into that cloud. This is observed experimentally. The problem with the continuum is simply that in some cases this effect would go on forever.

]]>I don’t know what you have in mind by saying that particles in quantum field theory are “singularities”.

]]>Thanks!

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