A review of Sean Carroll's new book

[notFritzArgelander]

https://physicstoday.scitation.org/doi/ ... /PT.3.4366

It's not entirely favorable.

[helicon]

Although I am skeptical of many of the ideas described in this book, I still think it is an excellent one. A great deal of uncertainty about the foundations of quantum theory remain—more than most physicists are willing to admit—so naturally we all disagree. This is a masterful popular account of one approach, but for true balance, you are going to have to read a lot of other books alongside it.

As the reviewer states he disagrees about most of the propositions in the book. Another quote:

Noticing that two quantities are always the same and then trying to construct a theory in which they are identical has been a route to progress in the past. The fact that inertial and gravitational masses are always the same is just a coincidence in Newtonian mechanics, but their necessary identities played a part in constructing general relativity.

Correlation, however, does not always imply identity. Often, a numerical correlation between two quantities is explained by the laws of a theory rather than by a literal identity of concepts. In the case of entanglement and metric, it seems that we have a perfectly good explanation in terms of laws: The locality of the Hamiltonians of QFT, combined with the fact that the initial state is close to the vacuum state, means that entanglement will drop off with distance. If a credible quantum theory of gravity grew out of that approach, I would change my tune, but I do not see strong evidence of that.

So on balance once would say that three fourths of the book is incorrect - but that one quarter is likely on target.

[notFritzArgelander]

Although I am skeptical of many of the ideas described in this book, I still think it is an excellent one. A great deal of uncertainty about the foundations of quantum theory remain—more than most physicists are willing to admit—so naturally we all disagree. This is a masterful popular account of one approach, but for true balance, you are going to have to read a lot of other books alongside it.

As the reviewer states he disagrees about most of the propositions in the book. Another quote:

Noticing that two quantities are always the same and then trying to construct a theory in which they are identical has been a route to progress in the past. The fact that inertial and gravitational masses are always the same is just a coincidence in Newtonian mechanics, but their necessary identities played a part in constructing general relativity.

Correlation, however, does not always imply identity. Often, a numerical correlation between two quantities is explained by the laws of a theory rather than by a literal identity of concepts. In the case of entanglement and metric, it seems that we have a perfectly good explanation in terms of laws: The locality of the Hamiltonians of QFT, combined with the fact that the initial state is close to the vacuum state, means that entanglement will drop off with distance. If a credible quantum theory of gravity grew out of that approach, I would change my tune, but I do not see strong evidence of that.

So on balance once would say that three fourths of the book is incorrect - but that one quarter is likely on target.

I would put it slightly differently. Three quarters of the book is wrong and the basis of wrong propositions is well described in one quarter.

There is an implicit chicken-egg problem in the reviewer's second quote from your post. The Hamiltonians of QFT assume a flat relativistic background metric. So QFT needs spacetime. To have spacetime emerge from QFT is problematic then. Not many people are worried enough about that issue IMO.

[pakarinen]

SC strikes again. Release Dr. H.!

"Ja, I'm going to rip you a new one!"