Andrei Lebed, equivalence principle

[DrDu]

What I didn't see in your comment, but what I thought to be one of the most important points having been brought up in the discussion, is the fact that while the ground state of hydrogen + gravitational field has a formal expansion in terms of the field free eigenstates, this does not mean that hydrogen gets into instable excited states as all realistic changes of the field will be so slow that one ground state will be adiabatically transported into the other.

 

[bcrowell]

What I didn't see in your comment, but what I thought to be one of the most important points having been brought up in the discussion, is the fact that while the ground state of hydrogen + gravitational field has a formal expansion in terms of the field free eigenstates, this does not mean that hydrogen gets into instable excited states as all realistic changes of the field will be so slow that one ground state will be adiabatically transported into the other.

Lebed explicitly assumes that it's adiabatic. Yes, I think it's pretty clear that there's something wrong with his interpretation, since his predictions are contrary to the results of experiments.

I haven't been able to convince myself that there is anything trivially wrong in his arguments. His calculations show the atom changing its state adiabatically, but the locally observable properties of spacetime haven't changed, so it's no longer in the ground state. This is different from a normal case where, e.g., you adiabatically apply an electric field, so that the observable properties of the space become different.

My paper focuses on the comparison with experiment, but what I would criticize about Lebed's theoretical work in general is that his papers are silent on a lot of issues that obviously beg to be addressed. His calculations are coordinate-dependent, but the reader is left to guess what coordinates are required. He doesn't address what happens to a system when it undergoes a collision, and this makes it hard to believe that his experiment, as originally proposed, would have worked, even if his calculations and interpretation had been correct. He never seems to have considered the effect from solar gravity, which is trivial to calculate and nearly the same size as the effect he proposes for a space-based mission. He describes the calculation of probabilities of excitation, but not radiation rates; the only reasonable thing to do in order to calculate a rate seems to be to take the time derivative of the excitation probability, and presumably there is no radiation when this derivative has the wrong sign. When you calculate this rate, it depends on the dot product $\textbf{g}\cdot\textbf{v}$, which clearly violates the equivalence principle (OK, he claims that anyway) but also seems likely to violate Lorentz invariance.

Some of the things that seem to go wrong in his work, such as likely difficulties with conservation of energy-momentum, are not really difficulties that are specific to his idea. They are present more generally in semiclassical gravity. There are a lot of problems with semiclassical gravity:

http://arxiv.org/abs/1304.0471
http://motls.blogspot.com/2012/01/why-semiclassical-gravity-isnt-self.html
http://backreaction.blogspot.com/2012/01/real-thought-experiment-that-shows.html
http://relativity.livingreviews.org/open?pubNo=lrr-2008-3&page=articlesu4.html

See Lubos Motl's blog post for a nice discussion of energy nonconservation.

 

[DrDu]

Ok, but what I still don't understand: According to Lebed, the eigenstates of the hamiltonian in the field are superpositions of mass eigenstates. But to observe transitions one would have to measure mass so as to prepare a superposition of mass eigenstates and watch how they decay back into energy eigenstates, wouldn't one?

 

[bcrowell]

Ok, but what I still don't understand: According to Lebed, the eigenstates of the hamiltonian in the field are superpositions of mass eigenstates. But to observe transitions one would have to measure mass so as to prepare a superposition of mass eigenstates and watch how they decay back into energy eigenstates, wouldn't one?

Sorry, I don't understand what you mean by the second sentence. Could you explain in more detail?

 

[bcrowell]

I exchanged emails with Lebed. His initial reaction to my preprint was that he agrees that there should be nuclear excitations, but he says there shouldn't be any effect for a free-falling object. He says it matters that the system be transported at constant velocity.

 

[greswd]

Thanks for clarifying.

I thought you were going to answer my question haha

 

[Dr. Courtney]

I thought you were going to answer my question haha

The question of whether a pop science article accurately represents the views of the scientist and paper it is trying to explain to a general audience is simply much less interesting than how the assertions of the underlying paper can be experimentally tested.

At some level, all pop science articles misrepresent or overly simplify the issues they are attempting to convey to a more general readership. I long stopped worrying about that. Since my observation is that most lay readers have lost the essential connection between theory and experiment, I tend to focus on the basic point that theoretical predictions need to be tested before we assert whether we know they are true or not.

Delving more deeply into the subtleties of where and how the pop science article accurately represents the underlying text and where things are represented inaccurately requires a lot of time and effort to explain to lay readers and is not a particular strength in areas where I have not done it before. Sorry to disappoint.

 

[greswd]

If I'm understanding him correctly, then there is no reason that it has to be in free fall. It could be in a moving elevator or contained in a spacecraft whose rockets were thrusting. I think the reason he talks about a spacecraft is that he wants to make a very large change in gravitational potential.

Do the thrusters have to be thrusting? I thought the purpose of the spacecraft was to take the hydrogen to a location of microgravity only.

 

[greswd]

The question of whether a pop science article accurately represents the views of the scientist and paper it is trying to explain to a general audience is simply much less interesting than how the assertions of the underlying paper can be experimentally tested.

At some level, all pop science articles misrepresent or overly simplify the issues they are attempting to convey to a more general readership. I long stopped worrying about that. Since my observation is that most lay readers have lost the essential connection between theory and experiment, I tend to focus on the basic point that theoretical predictions need to be tested before we assert whether we know they are true or not.

Delving more deeply into the subtleties of where and how the pop science article accurately represents the underlying text and where things are represented inaccurately requires a lot of time and effort to explain to lay readers and is not a particular strength in areas where I have not done it before. Sorry to disappoint.

Alright. Then I'll ask, does the article make any crucial mistakes or errors?

 

[Dr. Courtney]

Alright. Then I'll ask, does the article make any crucial mistakes or errors?

Possibly, but these have been discussed by others above in this very thread, so I won't repeat them. However, since there are clear predictions, an experimental test would be a better arbiter than expert opinions.

 

[greswd]

Possibly, but these have been discussed by others above in this very thread, so I won't repeat them. However, since there are clear predictions, an experimental test would be a better arbiter than expert opinions.

Sorry, by errors, I mean: is there anything from Lebed's paper that the UA article reports incorrectly?

 

[DrDu]

This is a longer article by Lebed: http://www.hindawi.com/journals/ahep/2014/678087/ref/ (open access, does not appear to be on arxiv).

I don't buy a word of this!
Look at his equation 28 at t=0:
$\psi'(r)=\sum a_n \psi_n(r)=\psi_1+\sum_{i>1} \frac{-\Phi/c^2 V_{n,1} }{E_n-E_1}\psi_n(r)$ where $V_{n,1}$ is given by eq. 32.
This is nothing else as the first order time independent perturbation series for the ground state of H_0 eq 26 as it should be. Namely, on adiabatic switching, we have transformed the ground state of H in the absence of the field into the ground state of H with potential $\Phi$. Clearly, the new ground state will be stable (as any ground state is) and can't decay into anything else.

 

[greswd]

I don't buy a word of this!
Look at his equation 28 at t=0:
$\psi'(r)=\sum a_n \psi_n(r)=\psi_1+\sum_{i>1} \frac{-\Phi/c^2 V_{n,1} }{E_n-E_1}\psi_n(r)$ where $V_{n,1}$ is given by eq. 32.
This is nothing else as the first order time independent perturbation series for the ground state of H_0 eq 26 as it should be. Namely, on adiabatic switching, we have transformed the ground state of H in the absence of the field into the ground state of H with potential $\Phi$. Clearly, the new ground state will be stable (as any ground state is) and can't decay into anything else.

But doesn't that mean Lebed is contradicting his own assertion?