Brad DeLong, in re Quantum Hyperion, wonders whether photons are really responsible for the decoherence of Saturn’s moon:
But gravity works–presumably, at some level–by massive objects constantly bombarding each other with gravitons, so we are also averaging over all the possible states of gravitons that we are not keeping track of, aren’t we? That should cause decoherence too, shouldn’t it?
This is an annoyingly good question. In fact, I’m probably not giving anything away if I reveal that my esteemed co-blogger Daniel and I once tried to figure out whether or not dark matter, if it truly interacts with ordinary matter only through gravity, would be in a coherent quantum state. Still don’t know the answer (although I strongly suspect it is “no,” I’m just not sure how to prove it).
The force due to gravity on Hyperion is much larger than the force due to electromagnetism on Hyperion. All else being equal, gravity is a much weaker force, but it has the helpful quality of adding up rather than canceling out, which is why it tends to dominate over astrophysical distances.
However — it’s not always useful to think of the gravitational force on a planet as due to the exchange of gravitons. You can think of the static force between two objects as arising from the exchange of virtual particles, whether you are talking about gravity or electromagnetism. But it is also true that, in the limit where the bodies giving rise to the gravitational force are perfectly static, those gravitons add up to define a unique quantum state. (The Sun, Saturn, and Titan are not static, but probably good enough for these purposes.) So the state of Hyperion becomes entangled with the quantum states of the individual gravitational fields of those celestial bodies, not with a jillion separate gravitons from each source. When we ignore the quantum states of all the gravitons reflected off of Hyperion, we are ignoring a lot more than when we ignore the quantum states of the gravitational fields of the Sun, Saturn, and Titan.
So I think it’s the photons, not the gravitons, that are primarily responsible for the decoherence, by a wide margin. But I wouldn’t bet my reputation on it. Maybe Daniel’s reputation.
How does particle/field argument not apply to EM as well? A jillion separate photons vs the individual electromagnetic fields of the celestial bodies. Is it just the difference in strength? Does the limit work better when evaluating the EM interaction between Hyperion and the Andromeda Galaxy?
Speaking of coherence and the lack thereof, I have yet to read a common sense explanation of how the exchange of particles results in objects moving towards one another. Solar sails I grok. Gravity? Magnetic attraction? Not so much. Sounds to me like you physicists have all agreed to agree.
g, the difference is that the photons are individually propagating excitations of the electromagnetic field, not the overall static force field that is stuck to the originating body. The same exact logic would apply if you had an electrically charged body — the static electric field would be described as a single quantum state, not as a jillion independent photons.
Don, the crucial point is that it’s an exchange of virtual particles, not real ones. And the crucial point about virtual particles is that they can have any momentum at all — even negative! So by throwing a negative-momentum particle from one body to another, you actually draw them together.
What was the argument that the gravitational field around a static celestial body is in a coherent quantum state? I thought we usually describe it by a classical field configuration (more like an electromagnetic wave, except static, and less like a coherent collection of photons).
I kind of like this puzzle, because it always bugs me how the vast majority of literature on decoherence (and foundations of QM) uses single-particle QM, and not QFT, even when discussing relativistic objects like photons. You probably can get away with it most times, but sometimes you end up having to think about quantum states of virtual particles…
My argument (such as it was) is just that a single body in isolation, surrounded by a Newtonian gravitational field but otherwise in its ground state, would correspond to some definite (single, coherent) quantum state. Extra degrees of freedom (to be traced over to produce decoherence) would have to be real propagating gravitons, which are absent to a good approximation in the Solar System. (At least, no such independently-propagating gravitons are produced by the Sun.)
But it’s a good problem! I’d be happy to get input from a real expert.
Sean,
So is it a metaphor in the service of a tautology, or an actual description of a physical process? Hmmm…
In field theory you can work with wave functionals over classical field configurations. If you put Hyperion in some superposition, then Hyperion’s gravitational field will also be in a superposition, entangled with Hyperion’s state. You would then expect to get decoherence due to the tidal effects on Saturn that depend very slightly on Hyperion’s orientation.
There are of course real gravitational waves, very long wavelength and very weak, produced by the orbit of Saturn around the Sun, and also the orbit of Hyperion about Saturn. (I guess this is what you were referring to in the comment about the Sun, Saturn and Titan not being exactly static.) Let’s ignore the Saturn-Hyperion ones so we don’t get into the self force mess, but consider the Sun-Saturn ones: Presumably these waves are caused by jillions of real gravitons? Is this also one big coherent state? What about the very few that are actually absorbed or scattered by Hyperion?
Hold on guys – let’s not get carried away…
So far we have mentioned gravitons (never been seen), gravitational waves (never directly detected), quantun decoherence (never explained), superpositions of gravitational field configurations (we don’t have quantum gravity yet as far as I am aware), and even the common or garden perturbation theory in QFT with its virtual particles, which is on thin ice from a rigorous mathematical point of view (try Haag’s theorem).
Still, at least there has been no recourse yet to extra dimensions or supersymmetry…
Science or science fiction?
James,
I think gravitational waves are detectable, and so is decoherence in quantum mechanics. I don’t however believe (or know what to make of) the existence of
gravitons.
Gravity has not been satisfactorily quantized, in my understanding, and
so the existence of “gravitons” is suspect. From what I understand of QFT, quantum field theories of gravity are not “renormalizable” (i.e have unphysical
divergences) so I don’t really know how one can make sense of “gravitons” and
the question itself.
Re: Dons question,
I think of attraction as the two bodies emitting particles
directly away from each other.
Repulsion is the two bodies emitting particles directly toward
each other.
Kris,
I was kinda playing devil’s advocate (I don’t know the HTML to insert a “Sarah Palin Wink” at this point, but please consider it done 🙂 )
Yes, I’m sure gravitational waves exist, I only said they hadn’t been “directly” detected – and experiemnts such as LIGO will hopefully rectify this before too long.
Like most, I expect quantum gravity to exist (but I doubt I’ll be the one to find it!) and, if so, we will have quantised exitations of the field, which already have been given the name “gravitons” whatever they turn out to be like (much like kids really, you don’t get a choice in this).
However, quantum decoherence remains just a practical fix with no explanation. It works well, but that is surely not enough.
-James
Here’s a stupid experiment one can do.
Have a friend place a chair somewhere in an empty room (a room that allows no light in when the door is shut) and have them turn off the lights when the leave the room.
Have another friend, who hasn’t communicated with the first friend, lead you to the door of the room with your eyes closed. After you open the door, enter and close the door, then you can open your eyes.
Where’s the chair?
The experiment is over when you find the chair and sit down.
I think you need to put the Saturnian system into a very large insulated box. If cooling the moons and gas giant to absolute zero seems tricky, just delegate the task to a technician.
Moshe said,
You’d think that if we took relativity seriously, we’d bring it into foundational discussions. I remember being told that, all things considered, relativity was significantly less weird than QM — that’s basically Feynman’s famous quote about “nobody understands quantum mechanics”, though the fact that in The Character of Physical Law he was making a comparison to relativity is sometimes forgotten. So, relativistic concerns should belong in foundational studies, and in studying the emergence of classical physics from the quantum variety, which implies using QFT (even if relativistic corrections aren’t relevant for studying decoherence in specific experimental situations).
If somebody died and made you Curriculum King, which topic would you teach first: decoherence or QFT?
for people fond of the “arrow of time”
http://www.youtube.com/watch?v=b13rc6DY74A
I think that this debate about the applicability of quantum mechanics to the motion of a moon leaves the realm of science and enters the realm of metaphysics, or quite possibly philosophy. The idea that the moon’s orientation will stop being classical due to quantum effects is inherently not testable. The orientation will stop being predicable due to the motions of dust, radiation, gravity from other planets in the solar system, and probably the motion of flies in the jungles of Russia, long before quantum mechanics plays its part.
I don’t think it’s exactly metaphysics. Macroscopic quantum objects is known to exists, like Bose–Einstein condensate, and it have some unusual mechanical properties. So why not entangled orientations ?
Gravity could be a coherent source if the exciton of the field interacts with the moon. A classical analogue of this would be a gravity wave. This would cause the moon to exhibit quadrupole oscillations and some displacement of its bulk material. This would then heat the moon up slightly and that thermalization would have a decoherent effect.
Lawrence B. Crowell
I meant to start that out by saying not a “coherent source,” but as a source of decoherence. — L. C.
Gravitational waves have been inferred indirectly. They are detectable and there is an experiment underway to try and do that, although as far as I know they have not found one. Gravitons are way more hypothetical though. Physics would take an interesting turn if quantum theory didn’t apply to gravity at all. Although there are strong reasons to believe there is a quantum theory of gravity and some unified theory of physics (even if its not string theory) it is not out of the realm of possibility that gravity just doesn’t fit into that situation.
James-
Actually I think your original post is appropriate.
How do you know there are gravitational waves?
Are you aware of the difficulty of measuring G (the constant in Einstein’s formulation of gravity) If G isn’t constant, then the prediction of gravitational waves is ???
Where do the findings of Gröblacher et al. fit into this discussion?
I’d like to ask the same question but from different prospective,
Do we really need Photons or Gravitons (or any external agent) to cause de-coherency in such a huge macroscopic system?!
More precisely my question is why we should believe that all the constituents particles of Hyperion are going to interact whit each other all at once? under what assumptions they came up with that 20 years criterion?!
suppose we have two entangled particles after an interaction the second one and a new third particle, all of them are entangled (am I right?!) then lets assume miraculously the third one happens to interact with the first one. I’m not sure but I think this scenario results in de-coherency (am I right or I should take QM course again?!)
The static gravity field will not act to decohere anything. If we think of electromagnetism the electronic states of an atom are held by a 1/r^2 force and there is no decoherence of any electron wave functions. Now if one of these electrons is in an excited state that state will decay with the emission of a photon. If that photon is in a high-Q cavity there will be an entanglement between the atom and the photon. There will then be a Rabi oscillation as a result, where the photon is reabsorbed by the atom and the electron restored to its excited state. The process will then repeat, as there is some periodic oscillation between the probability of a photon and the probability of the electron in an excited state oscillate with some frequency. However, if that photon leaves the system this is a spontaneous emission. The engtanglement between the atom and the photon is taken away by the environment or the vacuum occupation states the photon “fills up.” The first order theory for this is the Fermi Golden rule.
The point is that with any field there is no change in entanglement phases unless there is the emission of the gauge boson of that field. The same hold for gravity, though the quantum mechanics of gravity is less certain. Yet we can use some classical ideas. The quandrupole moment of a classical gravity wave will cause the body to periodically distend — squash along one direction and stretch along another, and then the opposite occurs. This is the basis for the Weber cylinders meant to detect gravity waves. This will then result in some bulk friction in the body. The temperature of will then increase and the phase space volume of that body’s dynamics. This is a measure for the removal of coherent phase which might be present in the body.
Now realistically a gravity wave will not do much. The decoherent activity of solar photons is far greater than what any gravity wave might produce. At least this is hoped! Any gravity wave the significantly heat up Hyperion would be devistating in general. For the gravity wave to have any detectable decoherence it must act on some body very very near absolute zero.
Lawrence B. Crowell
Just a shout out to all of you above for making this thread so approachable and educational for the layperson to comprehend. It is one of the reasons that i think Cosmic Variance is one of the very best blogs on the web.