"Lost in Math" - Sabine Hossenfelder

Amazon link

Sabine Hossenfelder is a quantum gravity phenomenologist - she links theories to experiments and observations - and she has a popular blog, Backreaction. She is also losing her faith in physics as it is presently conducted.

Here is the problem. The two great foundations of physics, quantum theory and relativity, are completely successful to the precision of existing experiments and observations. Yet they are mutually inconsistent. Attempts at unification (there are many, including string theory) are presently untestable, their verification beyond the limits of colliders and instruments. There are other conundrums too such as the nature of dark energy and dark matter. In the absence of experimental data, ungrounded theories abound - they are cheap, after all! - driven by aesthetics and groupthink.

Hossenfelder is not a superstar physics-populist with a major TV contract and a slew of pop-sci publications. 'Lost in Math' is not intended for that market. It's an extended meditation on the consequences of the absence of meaningful experimental feedback. The author interviews the world's leading physicists (Witten excepted) and is typically unimpressed by what she hears. Her reactions are ironic, sardonic and hilarious. Her text reports back as if talking to peers. How refreshing.

As a bonus you get an insider's view of the hot issues: what's really happening at the LHC, the role of dark matter in cosmology and the evolution of the universe .. as well as the failure of all attempts to characterise it, the problems with the flavours of multiverse, the measurement problem, the black hole firewall controversy.

There's no easy solution to the great stagnation in fundamental physics. The author is keen that the community should be more aware of its cognitive biases, its herd behaviour and groupthink but it seems difficult to identity the incentive to improve.

As for Sabine Hossenfelder herself, she may believe that publication of this book has killed any chance of tenure for ever, but probably it hasn't. One niche closes, another opens.

I had my doubts about buying this book. Would I really learn anything that I hadn't already picked up from many other popular science books and innumerable blog posts? And is the fragility of  beauty, naturalness and elegance as criteria for theory acceptability really enough to sustain a whole volume? Yet Hossenfelder's work is wider and more profound than that. Her tour of the state of the art is a joy to read, discerning and intelligent insights sprinkled throughout. I was engrossed from beginning to end.

Confessions of an eternalist

Confession: I am an eternalist.

Eternalist Sean Carroll has a post up commenting on recent books by Lee Smolin and philosopher Roberto Mangabeira Unger who argue to the contrary. Reading the comments there is depressing  - people either don't understand the issue at all, or don't appreciate how to think about the subjective nature of 'now'. Physicists!

One physicist, Sabine Hossenfelder, has written an excellent and definitive analysis: here's an extract.

"The decisive ability that allows us to experience the present moment as being unlike other moments is that we have a memory. We have a memory of events in the past, an imperfect one, and we do not have memory of events in the future. Memory is not in and by itself tied to consciousness, it is tied to the increase of entropy, or the arrow of time if you wish. Many materials show memory; every system with a path dependence like eg hysteresis does. If you get a perm the molecule chains in your hair remember the bonds, not your brain.

"Memory has nothing to do with consciousness in particular which is good because it makes it much easier to find the flaw in the argument leading to the problem of now.

"If we want to describe systems with memory we need at the very least two time parameters: t to parameterize the location of the particle and τ to parameterize the strength of memory of other times depending on its present location. This means there is a function f(t,τ) that encodes how strong is the memory of time τ at moment t. You need, in other words, at the very least a two-point function, a plain particle trajectory will not do.

"That we experience a “now” means that the strength of memory peaks when both time parameters are identical, ie t-τ = 0. That we do not have any memory of the future means that the function vanishes when τ > t. For the past it must decay somehow, but the details don’t matter. This construction is already sufficient to explain why we have the subjective experience of the present moment being special. And it wasn't that difficult, was it?"

I'm not so much into perms as my preferred physical model of subjective temporal experience. If you imagine a robot with an updating model of observed-reality obtained from its internal and external sensors, it's easy to see that examining its own memory store at any instant at all the robot can persuade itself that this instant "now" is special. But all introspective moments are like that. If we saved each database-state where:

"the robot notes that it is aware of itself and its environment now"

to disk, there would be nothing fundamentally distinct about any of them.

That robot is, functionally, me and you and sits easily in the block universe.