Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime

[atyy]

Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime
Sean Carroll
https://www.amazon.com/dp/1524743011/?tag=pfamazon01-20

Review of the book by Matt Leifer
Does the many-worlds interpretation hold the key to spacetime?
https://physicstoday.scitation.org/doi/10.1063/PT.3.4366

 

[atyy]

@Demystifier should like this part of Leifer's review:
"The middle section ends with a chapter on alternative interpretations, such as de Broglie–Bohm theory, spontaneous collapse theories, and quantum Bayesianism. Although Carroll charitably says he is glad others are studying those approaches, it comes off as disingenuous because he gives all of those theories bad reviews. For example, he makes a big deal of the alleged difficulties coming up with a Bohmian version of quantum field theory, but he later argues that QFT should emerge from a more fundamental discrete theory, whereas stochastic Bohm-like models for such discrete theories are known. Not that I want to advocate Bohmian theories myself, but the alternatives to Everett are not as hopeless as Carroll makes them out to be."

 

[PeroK]

There are some priceless comments in the reviews:

"There is almost nothing here in the form of explaining what he is talking about to someone who is intelligent but isn't part of the quantum vocabulary club. There's nothing like telling us the "wave function collapses when it is measured" to get across absolutely zippo about what he is talking about.

Usually, in my experience, hiding behind non-explanatory lingo like this is a sign the writer doesn't really understand the subject matter."

And:

"There might be something to “Many-Worlds” theory, but Carroll completely rejects the idea that quantum physics has anything to do Consciousness and Will. It’s the massive error of this book. Experiences have everything to do with Cognition; thus Will. For a more in depth read in this, I recommend Alexander Wendt’s Quantum Mind and Social Science."

 

[AlexCaledin]

Prof. J. Hartle is arguing that QM may very well be seen as dealing with the whole superposition of all the possible spacetime variants of the universe (thus solving the quantum gravity problem) :

 

[Elias1960]

Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime
Sean Carroll
https://www.amazon.com/dp/1524743011/?tag=pfamazon01-20

writes:

It is believed by many Bohmians that a non-equilibrium initial distribution will evolve toward equilibrium, just as a gas of classical particles in a box evolves toward an equilibrium thermal state; but the status of this idea is not yet settled.

Sorry, but this is settled. It is Valentini's subquantum H-theorem.

Valentini, A. (1991). Signal-locality, uncertainty, and the subquantum H-theorem I. Phys Lett A 156(1,2), 5-11

Just as it is time for physics as a field to take quantum foundations seriously, it’s time for quantum foundations to take field theory and gravity seriously.

The first consideration of Bohmian field theory is, for the EM field, the original paper of Bohm. The standard source for dBB field theory is
Bohm.D., Hiley, B.J., Kaloyerou, P.N. (1987). An ontological basis for the quantum theory, Phys. Reports 144(6), 321-375.

 

[AlexCaledin]

Just stumbled over this:

"If you assume that gravitation arises as a sort of statistical phenomenon over a large number of elementary particles . . . "

- said to Feynman in an old discussion ( - does it sound like a sort of branching spacetime interpretation?):

 

[expos4ever]

I got about 90% through and then gave up. Let me be clear: I have heard his lectures and he certainly seems to be trying his utmost to make this exotic and abstract subject accessible to regular shmoes like me (I have an undergrad engineering degree and have read a fair bit of the material out there on QM that is directed to the general public). I have to say, though, that I found much of his material hard to follow - as if he is assuming too much about what the reader knows. I also finds he makes many statements where my reaction is "surely he must know that this requires some clarification". I wonder: am I the only one who finds this book a little impenetrable?

I am old enough, and hopefully wise enough, to recognize the possibility that, for whatever reason, this subject is beyond my ability to grasp. I also read David Deutsch's first book (which may have been only obliquely related to the subject matter of Carroll's book) and had the same general reaction - that the author seems "too close" to his subject matter to explain it to a general audience. The roadblock may well be me, but I cannot help but wonder if all these efforts to explain the subtleties of modern physics to the masses could do with a little more care in the explaining. I think of Steven Pinker's "the curse of knowledge" concept - the idea that it is very hard for an expert (who is writing something) to imagine what is like to be in the position of the reader.

 

[vanhees71]

I think your frustration originates from the fact that you restricted your study to popular-science treatments. With an engineering degree, I'm pretty sure you learned enough math to read an introductory QM textbook. QM is not as difficult as classical electrodynamics as far as the math is concerned. My favorite as an introductory textbook is J. J. Sakurai, Modern Quantum Mechanics. If the "spin-1/2-first approach" followed in this book is not to your taste, then try the Feynman lectures vol. 3.

 

[PeroK]

I got about 90% through and then gave up. Let me be clear: I have heard his lectures and he certainly seems to be trying his utmost to make this exotic and abstract subject accessible to regular shmoes like me (I have an undergrad engineering degree and have read a fair bit of the material out there on QM that is directed to the general public). I have to say, though, that I found much of his material hard to follow - as if he is assuming too much about what the reader knows. I also finds he makes many statements where my reaction is "surely he must know that this requires some clarification". I wonder: am I the only one who finds this book a little impenetrable?

I am old enough, and hopefully wise enough, to recognize the possibility that, for whatever reason, this subject is beyond my ability to grasp. I also read David Deutsch's first book (which may have been only obliquely related to the subject matter of Carroll's book) and had the same general reaction - that the author seems "too close" to his subject matter to explain it to a general audience. The roadblock may well be me, but I cannot help but wonder if all these efforts to explain the subtleties of modern physics to the masses could do with a little more care in the explaining. I think of Steven Pinker's "the curse of knowledge" concept - the idea that it is very hard for an expert (who is writing something) to imagine what is like to be in the position of the reader.

"Whoever then has the effrontery to study physics while neglecting mathematics must know from the start that he will never make his entry through the portals of wisdom."

Roger Bacon (1214-84)

 

[expos4ever]

I think your frustration originates from the fact that you restricted your study to popular-science treatments. With an engineering degree, I'm pretty sure you learned enough math to read an introductory QM textbook. QM is not as difficult as classical electrodynamics as far as the math is concerned. My favorite as an introductory textbook is J. J. Sakurai, Modern Quantum Mechanics. If the "spin-1/2-first approach" followed in this book is not to your taste, then try the Feynman lectures vol. 3.

I hear you, but I doubt that math is the issue - clarity of explanation, or my own basic ability to follow the details of an explanation, is this issue, I believe. There is little math in Carroll's text (perhaps none?) and it is definitely not the math that is tripping me up. Either the author is not being as clear as he could be or I have some sort of fundamental mental block. With respect to the latter, perhaps my attention span is not what it should be - who knows? I am curious - to those who have read this book: did you find the explanations clear and well laid out?

 

[expos4ever]

"Whoever then has the effrontery to study physics while neglecting mathematics must know from the start that he will never make his entry through the portals of wisdom."

Roger Bacon (1214-84)

Math is not the issue - this text contains almost no math.

 

[PeroK]

Math is not the issue - this text contains almost no math.

There's a certain perverted, inexorable logic to that. It's like the story of the man who was massively overweight and the doctor said exercise was the problem. Exercise can't be the problem, the man replied: I don't do any exercise!

 

[expos4ever]

There's a certain perverted, inexorable logic to that. It's like the story of the man who was massively overweight and the doctor said exercise was the problem. Exercise can't be the problem, the man replied: I don't do any exercise!

I repeat: the text does not really contain any math - math is not the issue. Again, the problem may be mine but this is not a math issue.

 

[Lord Jestocost]

I hear you, but I doubt that math is the issue - clarity of explanation, or my own basic ability to follow the details of an explanation, is this issue, I believe. There is little math in Carroll's text (perhaps none?) and it is definitely not the math that is tripping me up. Either the author is not being as clear as he could be or I have some sort of fundamental mental block. With respect to the latter, perhaps my attention span is not what it should be - who knows? I am curious - to those who have read this book: did you find the explanations clear and well laid out?

To my mind, nobody can answer why we as conscious observers have the feeling of a spacetime regarding our experiential reality. In this context, it might be worth quoting Misner et al. on the ultimate breakdown of the spacetime concept (in “Gravitation” by Charles W. Misner, Kip S. Thorne, John Archibald Wheeler, 1973 pp. 1182–1183):

“The uncertainty principle thus deprives one of any way whatsoever to predict, or even to give meaning to, ‘the deterministic classical history of space evolving in time.’ No prediction of spacetime, therefore no meaning for spacetime, is the verdict of the quantum principle. That object which is central to all of classical general relativity, the four-dimensional spacetime geometry, simply does not exist, except in a classical approximation.

These considerations reveal that the concepts of spacetime and time are not primary but secondary ideas in the structure of physical theory. These concepts are valid in the classical approximation. However, they have neither meaning nor application under circumstances where quantum geometrodynamic effects become important. Then one has to forego that view of nature in which every event, past, present, or future, occupies its preordained position in a grand catalog called ‘spacetime,’ with the Einstein interval from each event to its neighbor eternally established. There is no spacetime, there is no time, there is no before, there is no after. The question of what happens ‘next’ is without meaning.”

 

[PeroK]

Then one has to forego that view of nature in which every event, past, present, or future, occupies its preordained position in a grand catalog called ‘spacetime,’ with the Einstein interval from each event to its neighbor eternally established. There is no spacetime, there is no time, there is no before, there is no after. The question of what happens ‘next’ is without meaning.”

Sounds like the sort of thing a physicist might write if his wife had just run off with a sociology professor!

 

[Demystifier]

I repeat: the text does not really contain any math - math is not the issue. Again, the problem may be mine but this is not a math issue.

What they are trying to tell you is that the lack of math is the issue. If you were reading a book with math, you would understand it better. Or maybe you wouldn't, but there is only one way to find out.

 

[Demystifier]

that the author seems "too close" to his subject matter to explain it to a general audience. ...I think of Steven Pinker's "the curse of knowledge" concept - the idea that it is very hard for an expert (who is writing something) to imagine what is like to be in the position of the reader.

You can test it as well. In the book "The Big Picture", Carroll also writes about things for which he is not an expert: stuff like biology, consciousness, the meaning of life, God, etc. If you can understand those parts of the book but not the physics parts, then you are probably right.

 

[Jarvis323]

"Whoever then has the effrontery to study physics while neglecting mathematics must know from the start that he will never make his entry through the portals of wisdom."

Roger Bacon (1214-84)

What they are trying to tell you is that the lack of math is the issue. If you were reading a book with math, you would understand it better. Or maybe you wouldn't, but there is only one way to find out.

Maybe the result will be that you feel you understand it better, or maybe the result will be that you feel you understand it less. I'm not sure which result means that you're getting closer to the portals of wisdom.

 

[Lord Jestocost]

You can test it as well. In the book "The Big Picture", Carroll also writes about things for which he is not an expert: stuff like biology, consciousness, the meaning of life, God, etc. If you can understand those parts of the book but not the physics parts, then you are probably right.

When reading such books, one should generally distinguish whether some content represented is “science” in the literal sense of the word or “science” in the ironic sense of the word.
https://blogs.scientificamerican.com/cross-check/what-is-8220-ironic-science-8221/

 

[vanhees71]

I hear you, but I doubt that math is the issue - clarity of explanation, or my own basic ability to follow the details of an explanation, is this issue, I believe. There is little math in Carroll's text (perhaps none?) and it is definitely not the math that is tripping me up. Either the author is not being as clear as he could be or I have some sort of fundamental mental block. With respect to the latter, perhaps my attention span is not what it should be - who knows? I am curious - to those who have read this book: did you find the explanations clear and well laid out?

It's the problem that there is no math in pop-sci treatments of quantum theory. You cannot even talk about quantum theory in an understandable way without math!

 

[martinbn]

It's the problem that there is no math in pop-sci treatments of quantum theory. You cannot even talk about quantum theory in an understandable way without math!

That's true for all areas of physics.

 

[Demystifier]

That's true for all areas of physics.

Quantitative features of physics always require math, but the idea is that at least qualitative features can sometimes be expressed without math. For many areas of physics it's quite true, but not for quantum physics. For example, you can explain qualitatively without math what is spin of a classical rigid body, but it's hard to explain qualitatively without math what is spin of electron.

 

[martinbn]

Quantitative features of physics always require math, but the idea is that at least qualitative features can sometimes be expressed without math. For many areas of physics it's quite true, but not for quantum physics. For example, you can explain qualitatively without math what is spin of a classical rigid body, but it's hard to explain qualitatively without math what is spin of electron.

How do you do it in the classical case without math? I think you don't mean without math, but with math that everyone understands. For example a point, a trajectory and so on.

 

[pinball1970]

Sounds like the sort of thing a physicist might write if his wife had just run off with a sociology professor!

One of the pop science books that bit the dust recently. Charity shop.

 

[Demystifier]

How do you do it in the classical case without math? I think you don't mean without math, but with math that everyone understands. For example a point, a trajectory and so on.

Spin is rotation of a body around an axis that goes through the center of the body. I guess you will say that axis and center are mathematical notions, but by "without math" I mean without equations and mathematical symbols.

 

[martinbn]

Spin is rotation of a body around an axis that goes through the center of the body. I guess you will say that axis and center are mathematical notions, but by "without math" I mean without equations and mathematical symbols.

So, a theorem like: Every finite group is a subgroup of a permutation group. is not math!

 

[Demystifier]

So, a theorem like: Every finite group is a subgroup of a permutation group. is not math!

In the context of speaking about popular physics books "without math", I think it makes sense to say so. Of course, if you wanted to present a proof of this theorem, math symbols and equations would probably be unavoidable.

Alternatively, if you want to call this math I'm fine with that, but then the popular physics book we talk about is not without math.

 

[PeroK]

Spin is rotation of a body around an axis that goes through the center of the body. I guess you will say that axis and center are mathematical notions, but by "without math" I mean without equations and mathematical symbols.

Let's take the example of popular treatments of quantum spin. The reader or student may believe they have understood it, but generally (in my experience) the reader is unable to reconcile the 2D (spin-up or spin-down) structure of the spin states with the 3D structure of spin in the spatial dimensions. I would bet that if you asked 100 people who said they understood quantum spin from popular sources that 99% (at least) would not be able to explain how these concepts are reconciled.

That's where the mathematics becomes essential and why Roger Bacon was right. Quantum spin cannot be adequately understood without sufficient mathematics.

 

[martinbn]

In the context of speaking about popular physics books "without math", I think it makes sense to say so. Of course, if you wanted to present a proof of this theorem, math symbols and equations would probably be unavoidable.

Alternatively, if you want to call this math I'm fine with that, but then the popular physics book we talk about is not without math.

I think I wasn't clear. My point was that one need maths for all physics, even classical physics. The difference is that a lot of the maths needed for classical physics is something that is intuitive enough and people can get a good idea about it from the popular texts. The problem is that then, they think that you don't need maths and try to sudy QM without it. But the maths for QM is not like the one for classical physics.

 

[vanhees71]

The difference is that with "classical physics" we have a lot of experience from everyday life, and you can just argue with the corresponding picture you have in mind about the behavior of classical objects including "matter" (solids, liquids, gases) and "radiation" (electromagnetic fields).

In quantum theory it's impossible to use such "intuitive pictures" but you need a pretty abstract mathematical machinery to even talk about quantum phenomena adequately. E.g., it's impossible to talk about spin 1/2 without the idea of the rotation group and its covering group and representations of groups on a Hilbert space. If it comes to superposition and entanglement it's even worse.

 

[Demystifier]

E.g., it's impossible to talk about spin 1/2 without the idea of the rotation group and its covering group and representations of groups on a Hilbert space.

Historically, both Pauli and Dirac described spin 1/2 without groups and their reps, so I wouldn't call it impossible. But of course, their descriptions are mathematically abstract too.

 

[vanhees71]

Interesting. Which papers by Pauli and Dirac are you referring to? I guess they used the Lie algebra (aka angular-mometum matrices/operators) of the rotation group and didn't bother much with finite rotations? That's of course indeed as abstract as the use of the groups.

BTW: Pauli was the first who really gave a valid argument, why orbital angular momenta have no half-integer-spin representations. This is missing in almost all modern textbooks, which simply make a hand-waving "uniqueness argument" for the wave function, which is flawed, because a pure state is not represented by a wave function but a wave function modulo a phase factor, which is the reason why half-integer spin representations make physical sense in the first place ;-)).

 

[Demystifier]

Interesting. Which papers by Pauli and Dirac are you referring to? I guess they used the Lie algebra (aka angular-mometum matrices/operators) of the rotation group and didn't bother much with finite rotations? That's of course indeed as abstract as the use of the groups.

My point is that they didn't use the language of Lie groups and algebras. They worked with some matrices, but they were probably not aware that those matrices are reps of Lie algebras. Similarly, someone working with position and momentum operators in elementary QM does not need to know what is Heisenberg algebra.

 

[Demystifier]

In quantum theory it's impossible to use such "intuitive pictures" but you need a pretty abstract mathematical machinery to even talk about quantum phenomena adequately.

It's impossible in minimal interpretation of QM. But that's exactly why some physicists find useful to think about non-minimal interpretations, to create some intuitive pictures associated with the abstract mathematical machinery. The many-world interpretation, which is the main subject of the book we are supposed to discuss here, is one such mental picture that some physicists find intuitive. Perhaps a popular science book is not a good place to learn something about quantum formalism, but it can be a good place to learn something about quantum interpretations.

 

[vanhees71]

My point is that they didn't use the language of Lie groups and algebras. They worked with some matrices, but they were probably not aware that those matrices are reps of Lie algebras. Similarly, someone working with position and momentum operators in elementary QM does not need to know what is Heisenberg algebra.

I'm pretty sure that Pauli knew very well, what angular-momentum operators have to do with rotations and representations of the rotation group. He obviously also knew about dynamical symmetries, i.e., the additional dynamical symmetries of the Kepler problem enabling him to solve the hydrogen energy eigenvalue problem in matrix mechanics (before Schrödinger with his wave-mechanics approach!).

The Heisenberg algebra is of course a very unjust misnomer, i.e., it should be named "Born algebra", because indeed Born was the first to write down the commutation relations for position and momentum and recognized the algebraic scheme behind Heisenberg's Helgoland paper ;-)).