Tuesday, March 18, 2014

Patterns on the sky

I'm busy reviewing ~200 promotion and tenure cases for my day job, so I don't have much time to post about the BICEP2 observation of primordial gravitational waves via their effect on the polarization of the cosmic microwave background (CMB).


Instead, I refer you to Sean Carroll, Lubos Motl and Liam McAllister (guest poster at Lubos' blog).

Assuming the result holds up, it strongly supports inflationary cosmology, and indicates that the inflation scale is only about 2 orders of magnitude below the Planck scale ~ 10^19 GeV (which would, presumably, turn out to be the true scale of quantum gravity).


In inflationary cosmology the gravitational waves which left the polarization signal arise from quantum fluctuations in de Sitter space. As with the CMB temperature, observers on different branches of the wavefunction of the universe see distinct polarization patterns on the sky. Since the CMB temperature fluctuations track energy density, these different observers also see distinct patterns of galaxy formation. In fact, whether or not an observer (a planet or galaxy) exists in a particular region of spacetime depends on the branch of the wavefunction (i.e., on a measurement outcome). I can't tell a Copenhagen story that makes sense of this -- there is no way to place observers like ourselves outside of the quantum state describing the CMB!

I guess I've said this all before 8-)
In fact, the interpretation of quantum mechanics is not entirely disconnected from practical issues in cosmology. The cosmic microwave background data favors inflationary cosmology, in which the density perturbations in the early universe originate in the quantum fluctuations of the inflaton field itself. It is very hard to fit this into the Copenhagen view -- what "collapses" the wavefunction of the inflaton? There are no "observers" in the early universe, and the locations of "observers" (such as humans) are determined by the density perturbations themselves: galaxies, stars and planets are found in the overdense regions, but quantum mechanics itself decides which regions are overdense; there is no classical system "outside" the universe! It seems much more natural to note that differential scattering of gravitons due to more or less energy density in a particular region separates the inflaton wavefunction into decoherent branches. (The gravitons decohere the inflaton state vector through interactions.) But this is accomplished through unitary evolution and does not require von Neumann projection or "collapse". Other observers, living on other branches of the wavefunction, see a different CMB pattern on the sky.

7 comments:

Vince Tullow said...

>> I can't tell a Copenhagen story that makes sense of this
But physics is not about telling pseudo-classical stories.


Obviously, BICEP2 is the observer in this case and the only difference to other experiments is that they cannot repeat it arbitrarily many times.
But they can observe different parts of the sky and we expect that they see a consistent pattern.
This is actually not expected for m.w.i. - we would expect to find ourselves in one of the many freak branches of the wave function (in the mwi wavefunction
there are more branches with defect BICEP2 instruments than branches where the BICEP2 result is (somewhat) real).


Only the Born probabilities connect the physics of the early universe with the BICEP2 observation - without them don't have anything.

LuboŇ° Motl said...

Dear Steve, your objection to the Copenhagen view of inflation is completely ludicrous because there is no process in the Copenhagen interpretation that could be called a "collapse". Bohr or Heisenberg would never use the word "collapse" and they talked about "reduction" which was always explicitly explained to be just a change of our *knowledge* about the physical system resulting from the latest measurement, not any "real modification of any real wave". This change of the knowledge is a subjective process that only occurs "in our brain" - it really means that we may replace the original probability distributions that depend on many variables by the conditional probabilities where the measured facts are already taken into account, and therefore economically "throw away" most of the previous probability distributions. Nothing is "objectively changing" in the world "out there" once we measure something.


The idea that the wave function is a classical wave that "really" collapses, in an objective classical sense, is a delusion spread by popular books and the Copenhagen interpretation is exactly the interpretation that refuses all such "realist" nonsensical misinterpretations of the wave function.

steve hsu said...

Lubos: I think this might be the third time we've had this discussion. I don't think you mean the same thing as I do when I refer to the Copenhagen Interpretation. In fact, when you write detailed things about QM you seem to have the same viewpoint that I have. But the conventional (at least in my opinion) version of Copenhagen is that given by Weinberg below:

Einstein's Mistakes, Steve Weinberg, Physics Today, November 2005: "... The Copenhagen interpretation describes what happens when an observer makes a measurement, but the observer and the act of measurement are themselves treated classically. This is surely wrong: Physicists and their apparatus must be governed by the same quantum mechanical rules that govern everything else in the universe. ..." (See more at link http://infoproc.blogspot.com/2011/01/e-pur-si-muove.html)

>> misinterpret the quantum mechanics in terms of a classical theory but quantum world - our world - is simply not classical in any sense << This interpretation, which you call stupid, IS what most people mean by Copenhagen!

In considering the CMB and the BICEP2 observer you cannot extract BICEP2 and treat it classically - the existence of BICEP2, the Earth, sun, our galaxy, etc. in this spacetime region (as opposed to an empty void) is due to a positive energy density fluctuation in the inflaton field. BICEP2 is not a classical observer that can be separated from the quantum state describing the fluctuations of the inflaton -- it is clearly just a part of the overall wavefunction. This makes the CMB situation different from the usual lab setup contemplated in what I (and I believe Weinberg) refer to as Copenhagen.


I suspect you agree with the above paragraph, except for the use of the term Copenhagen in the last sentence ...


PS When you listed winners and losers why did you not include ADD, Randall-Sundrum and other extra-dimensional solutions of the hierarchy problem? BICEP2 suggests that the scale of quantum gravity is indeed 10^19 GeV and I would think that disfavors extra-dimensional models as much as any of the theoretical ideas you mention in your post.

LuboŇ° Motl said...

Yup, it could be the 3rd time, Steve. ;-)

It's historically demonstrable that none of the stupidities that you or Weinberg or others sell as "Copenhagen interpreation" has ever been promoted as the official explanation of QM by the Copenhagen school.

"This interpretation, which you call stupid, IS what most people mean by Copenhagen!"

Right. That proves what I am saying that most people who discuss these things are stupid!


No, BICEP2 surely *is* a classical observer because it is making observations. Or we're making observations by reading their papers but because our perceptions may be shown to be classically identical to what BICEP2 detects, it's always legitimate to say that BICEP2 is an observer itself.


You're also wrong about the existence of many worlds. It's a derivable facts in quantum mechanics, just like any other prediction, that there only exists one version of the possible outcomes of an experiment, so there don't exist many worlds.


Good comments about the extra dimensional models and BICEP - I completely forgot that they're "in" this business, too.

Emil Kirkegaard said...

People in Denmark who study physics at University are taught the CPH interpretation as described by Hsu. In fact, they are only taught that one, and they often think it is the only one.

Source: I know lots of people who study physics in Denmark. Basically, Lubos is wrong about what the term "Copenhagen interpretation" means.

chartreuse1737 said...

"...guest poster at Lubos' [sic] blog..."

dear god i hope steve hasn't included himself in the bgi study.

BobSykes said...

The main problem with the usual Copenhagen interpretation is that it introduces and element of mysticism into physics. Prof. Motl's interpretation is to be preferred on that account alone.

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