Do Gravitons Cause Decoherence?

Sean Carroll writes:

Quantum Hyperion | Cosmic Variance: The orbit of Hyperion around Saturn is fairly predictable; happily, even for lumpy moons, the center of mass follows a smooth path. But the orientation of Hyperion, it turns out, is chaotic — the moon tumbles unpredictably as it orbits, as measured by Voyager 2 as well as Earth-based telescopes. Its orbit is highly elliptical, and resonates with the orbit of Titan, which exerts a torque on its axis. If you knew Hyperion’s orientation fairly precisely at some time, it would be completely unpredictable within a month or so (the Lyapunov exponent is about 40 days). More poetically, if you lived there, you wouldn’t be able to predict when the Sun would next rise.

So — is Hyperion oriented when nobody looks? Zurek and Paz calculate (not recently — this is fun, not breaking news) that if Hyperion were isolated from the rest of the universe [except for the gravitational pull on it by Titan and Saturn], it would evolve into a non-localized quantum state over a period of about 20 years. It’s an impressive example of quantum uncertainty on a macroscopic scale.

Except that Hyperion is not isolated from the rest of the universe. If nothing else, it’s constantly bombarded by photons from the Sun, as well as from the rest of the universe. And those photons have their own quantum states, and when they bounce off Hyperion the states become entangled. But there’s no way to keep track of the states of all those photons after they interact and go their merry way. So when you speak about “the quantum state of Hyperion,” you really mean the state we would get by averaging over all the possible states of the photons we didn’t keep track of. And that averaging process — considering the state of a certain quantum system when we haven’t kept track of the states of the many other systems with which it is entangled — leads to decoherence. Roughly speaking, the photons bouncing off of Hyperion act like a series of many little “observations of the wavefunction,” collapsing it into a state of definite orientation.

So, in the real world, not only does this particular moon (of Saturn) exist when we’re not looking, it’s also in a pretty well-defined orientation — even if, in a simple model that excludes the rest of the universe, its wave function would be all spread out after only 20 years of evolution. As Zurek and Paz conclude, “Decoherence caused by the environment … is not a subterfuge of a theorist, but a fact of life.” (As if one could sensibly distinguish between the two.)

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?

I am confused. Not as confused as I am about the powers of the Vice President of the United States as President of the Senate, but confused.

Comments

[Colin Danby]

Sounds like I should get a little more sun.

[Bram Cohen]

Gravitons are 'special' - they don't act like other particles, and their postulated existence is highly dubious. According to relativity gravity is an inherent aspect of the universe, not something carried out by particles at all. This is part of why a Grand Unified Theory has been so elusive - all the forces except for gravity fit within a reasonably coherent schema, but gravity insists on being the odd one out, completely resisting all attempts to unify it.

[g]

John Wheeler: "If you are not completely confused by quantum mechanics, you do not understand it."

[Neil B ☼]

Pardon the rather long comment, but below I reiterate the concerns I expressed at that CV thread about glib "decoherence" sophistry:

"Decoherence" advocates forget that figures giving “probability” for a given state or outcome are based on collapses and specific events already happening, then fed into the decoherence pretended “explanation” of what Wikipedia calls “appearance” of collapse - it’s a circular argument, fallacious in the familiar way. Decoherence advocates don't explain to my satisfaction why the waves spreading around, interacting or not with each other, don’t just stay patterns of waves as they do in both classical physics and in the pure mathematics of wave evolution (as in the “deterministic evolution of the wave under the Schrodinger equation” etc.) No, waves only get connected to “probability” because something weird about the universe gets them to express localization events, it still doesn’t make any sense at the fundamental level - they are just putting out double talk that reminds me of the sophistry put out by Wittgenstein’s supporters.

BTW, how many saw the very interesting and poignant Nova show about Hugh Everett and his musician son Mark? The idea of constantly splitting parallel worlds is cool as a weird idea but I don’t buy it. One thing to consider: in the Schrodinger’s cat situation, there is an unstable nucleus that may or not have decayed after a given time. That means that the cat is superposed alive/dead, etc. But that means that not only is the wave representing say, an emitted beta (electron) spread out over all angles and not just a narrow ray (presumably - given ordinary angular uncertainty) but it can’t even be a nice crisp shell. IOW, emitted particles have to keep “leaking out” over time, and that makes everything even more difficult to sort out. REM that by contrast, usually we see the example put as, the shell expands to reach a spherical screen and must collapse (but still at a given moment!) somewhere on the shell, with impact time being a bit uncertain but not a major issue. Well?

Another issue I have with decoherence: Consider the classic case of the photon split by a beamsplitter. The waves travel at right angles towards distant detectors, separated from the BS and each other by empty space, and perhaps many kilometers away. We can have the photon coherence length much less than distance to detectors. The photon must absorb in one or the other detector, not both. Consider the split wave function as it reaches the detectors. One or the other detector will ping, and then the other one is barred from also pinging. There is no way for any interaction or interference of any actual waves, to reach from the pinged detector to the other one for collapsing the wave that "was there" as we imagine it while it was just propagating. Nothing actually crosses the spatial separation, and the forbidding of the double ping happens immediately despite the distance. (Sure, there's some "connection" in entanglement but that isn't the actual influence of one wave on another, in the environmental "decoherence" sense.) There's no way to make that work out rationally. In any case, I get suspicious when apologists start talking of how something makes X "appear" to happen, that is a danger sign of BS (not a beam splitter!) at work.

[Ken Muldrew]

It's probably wise to wait until there is a quantum theory of gravity, maybe even wait until said theory has some unique empirical support, before thinking too deeply about gravitons. Of course if you are actually trying to come up with a theory of quantum gravity, then by all means, go forth and think, but otherwise, there be dragons...

[nathan]

alcohol may cause a lack of coherence

[MattF]

Well, talking through my hat here... Since individual gravitons have never been observed-- and presumably never will be observed-- they are completely delocalized. Consequently, the multi-graviton wave function is always just a product of single-graviton plane waves-- no entanglement!

[max]

"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?"

Well, assuming we have gravitons and assuming they work the way photons do, yes. Of course, they probably don't exist and if they do, they probably don't work the way photons do. So, problem solved!

"I am confused. Not as confused as I am about the powers of the Vice President of the United States as President of the Senate, but confused."

The Vice Presidency is a quantum office: it is only defined when someone is observing it.

max
['Joke. Serious joke.']

[capitalistimperialistpig]

So, according to the decoherence interpretation, which is as good a one as any, Hyperion wouldn't be there if not observed for a few years - or at least it wouldn't be at any specific where. It's not clear that these ideas have testable and calculable consequences at any scale but the submicroscopic.

[NE1]

Gravity (in the sense of potential energy, at least) does affect quantum calculations (wiki link with no useful details: http://en.wikipedia.org/wiki/Neutron_interferometry). I wouldn't know the effects of the particle conception of gravity. Hugh Everett's documentary was sort of interesting in a Son meets Father way, but I've never met anyone who really cared about that interpretation, and the kid's indignance at his father's snubbing is totally out of line. There's not a whole lot you can do by asserting something that sounds more than usual like a like a line from drunk undergrads.

[GW]

As you can gather from some of the comments, gravitons are like Supply Side Economics - they sound like a reasonable idea but it just doesn't work that way. Now, if this is so, what do you think about String Theory (I guess we'll find out with the new Swiss mega-accelerator - it might turn out to be the equivalent of a bubble burst!)

[GW]

As you can gather from some of the comments, gravitons are like Supply Side Economics - they sound like a reasonable idea but it just doesn't work that way. Now, if this is so, what do you think about String Theory (I guess we'll find out with the new Swiss mega-accelerator - it might turn out to be the equivalent of a bubble burst!)

[GW]

As you can gather from some of the comments, gravitons are like Supply Side Economics - they sound like a reasonable idea but it just doesn't work that way. Now, if this is so, what do you think about String Theory (I guess we'll find out with the new Swiss mega-accelerator - it might turn out to be the equivalent of a bubble burst!)

[Mitchell]

Brad, a five-minute survey of the literature tells me that the answer is yes, gravitons cause decoherence, but it is extremely weak owing to the weakness of the gravitational interaction. It should be happening on the scale of planets, but it should also be negligible on the scale of microscopic and perhaps mesoscopic systems, or else there would be no distinctively quantum phenomena - everything would be decohered all the time.

[capitalistimperialistpig]

Brad,

I missed your point on the first go-around, but I think that the gravitational effect of Saturn and Titan doesn't cause decoherence because we can effectively integrate over all those quantum states - it's just the classical gravitational pull. Effects of the rest of the universe can't be integrated out in this way because they aren't mutually coherent in the way that the gravitational effects of Saturn and Titan are.

[Zephir]

I can't see nothing problematic on the graviton idea, as it corresponds the classical Duillier - Le Sage gravity theory.

http://en.wikipedia.org/wiki/Le_Sage's_theory_of_gravitation

By this model the gravity is the shielding effect of many particle action, i.e. phenomena analogous to Casimir force in three dimensions. It's just certain view to subject and it's fully consistent with Aether Theory approach. And it enables the derive the ISL well - with compare to other theories.