Is quantum mechanics messing with your memory? | Hacker News Reader

Is quantum mechanics messing with your memory?

guardian.co.uk

12 pointsjsankey11 comments

Here's my understanding of the Maccone paper. Edit: I only skimmed it.

Super-condensed QM intro: In the formalism of quantum mechanics, the dynamics of physical processes are represented by unitary operators. If Y(0) is the state of the system at t=0, then the dynamics (change, movement) of the system is represented by a unitary operator U(t), such that the state of the system at time t is Y(t)=U(t)Y(0). The technical term unitary roughly means that it is time-reversible: there exists the inverse of U(t) meaning that the inverse physical process can also occur (the hidden assumption is that there is a 1-1 correspondence between operators in the formalism and actual physical processes). The problem is, if everything is time-reversible, why don't we see glasses unbreaking or gas that came in to fill the room through a little hole finding its way out again?

Measurements in QM are not unitary. If you measure something, you can't undo it. Measurements are in this sense an "add-on" to QM.

The following argument probably pre-dates the Maccone paper: Take a room which contains a quantum system Q and an observer O. The observer makes measurements on Q, which are not unitary, so he sees time irreversible processes. Now take a superobserver O', who is outside the room. To him, the room (Q and O together) make up a quantum mechanical system Q', which may be described in a unitary way. This means that the superobserver O' can argue that even though the observer O only sees time irreversible processes, the inverse process can in fact occur (glass unbreaking), but they reverse the observer O itself (who is part of Q'), meaning that he "forgets" what he measured, his ink "disappears" from his notebook, and so on.

Maccone's contribution seems to be that he identifies O measuring Q with O and Q becoming quantum entangled and provides some formalism.

Substitute "time-irreversible" for "entropy increasing" for trendiness (eg. to pick up girls in a bar =)).

I skimmed the paper the other day, but still didn't quite understand what he was on about.

In particular, I didn't understand why he feels the need to invoke quantum mechanics at all. Thermodynamic systems don't flow backwards from high-entropy to low-entropy states simply because there are far more high-entopy states than there are low-entropy states.

Or to put it another way, what does Maccone think would happen in a purely classical universe? Would not entropy still increase there?

I find that discussing thermodynamics works terribly for picking up girls in bars, but maybe I just go to the wrong bars.

Susskind answered a similar question during his stat. phys. lectures which you can watch on iTunes.

Somebody asked the following question: given that entropy is the volume of the system in phase space and Liouville's Theorem (of classical physics) tells us that the volume in phase space is conserved, how come entropy increases? His answer: Liouville's Theorem holds, the volume stays the same, but the blob representing the system (the system's microstates) spreads out in a fractal way in phase space, increasing the "coarse-grained" phase volume / entropy. Then, a "you can't set the initial conditions infinitely precisely" / "you can't measure inifinitally presicely" type argument is invoked to say that in terms of real-life reversibility the course-grained volume is what matters, so entropy has increased despite Liouville.

Explained here: http://www.google.com/url?sa=t&source=web&ct=res&...

I don't buy it for a simple reason. It is easy to look around the macro environment and see plenty of reasons why entropy has a long way to increase. For instance for several billion more years the Earth should be receiving energy from the Sun and radiating it off into outer space, which creates a sustained low entropy situation enabling something we call "life" to exist. Therefore the question of the arrow of time seems to me to be explaining why the macro environment looks that way.

I'm not sure where I heard it, but one idea that I like is that it ties to the Big Bang and the expansion of the universe. Most people don't realize how complicated conservation laws get in general relativity. For instance it is a matter of opinion whether or not energy is conserved! (See http://www.math.ucr.edu/home/baez/physics/Relativity/GR/ener... for details.) In particular suppose a photon sets out on a long journey, gets red shifted by the expansion of the universe, and consequently loses energy. What happened to the energy of the photon?

This question is highly relevant to the issue of entropy. Current theory says that we had a very hot and almost uniform universe, which then expanded rapidly. As it expanded any clumps that formed stayed hot, while the space between them cooled off, thereby leading to an increased thermodynamic non-equilibrium. Worse yet, local gravity pulled the clumps together resulting in interesting things like stars, which further increased the amount of non-equilibrium stuff going on.

Of course this pushes the question of the arrow of the time down to cosmology and general relativity. But given that physicists debate whether conservation of energy makes sense in general relativity, I'm fairly comfortable with accepting that the laws of thermodynamics are on even shakier ground there. Which makes the physical asymmetry seem not so bad to me.

I'm not convinced that this is a really great article. But it may be as good as it's reasonable to expect a one-page popular article on this subject to be.

It is incredibly difficult to write an article like this. You can't assume that your audience knows what entanglement is. You can't assume that they know what entropy is. And even if you thoroughly understand those two things it isn't exactly clear what this research is trying to say. It reads like something that is being badly translated out of the original mathematics.

Isn't memory just a couple of changed atoms? So erasing memory would be equivalent to making atom configurations unchanged. But if atom configurations don't change, entropy doesn't change. So the whole theory seems absurd.

(Disclaimer: I don't really know QM very well)

You mean ARE quantum mechanics messing with your memory, and those quantum mechanics should go back to fixing quantum cars!

When you observe any system, according to Maccone, you enter into a "quantum entanglement" with it.

That's a nice theoretical claim, but in practice it is just plain false. Two macroscopic objects, like a human and a cup of coffee, at room temperature, entangled? Give me a break. We have to go trough a hell of a lot of trouble to create entangled pairs of photons to do experiments with. Interactions with the environment break the entanglement, which is exactly what we are doing all the time.

Maccone's solution is to suggest that in fact entropy-decreasing events occur all the time

Of course they do. The point is those fluctuation in the 'wrong' direction last an extremely short period of time. We have no trouble observing temporary decreases in entropy: the 2nd law of thermodynamics holds over macroscopic periods of time and that has been verified experimentally. Laplace's daemon doesn't exist, but he can deceive you for a nanosecond.

The problem with these damned brilliant mathematician-physicists is that they really loose touch with the boundary conditions imposed by physical reality.

That's a nice theoretical claim, but in practice it is just plain false. Two macroscopic objects, like a human and a cup of coffee, at room temperature, entangled? Give me a break.

That idea is pretty common in (so-called) many-worlds interpretations. In such interpretations, the wavefunction never collapses: the "collapse" of the wavefunction is just the observer becoming entangled with the system. This, of course, has all sorts of disturbing implications, but arguably still makes a lot more sense than any other interpretation.

(I realize this article is very light but its is an interesting point of departure...)

So are all the 'positive thinkers' correct? Is life but a giant breadth-first search where we strive for some optimum to only remember the last path we took?

I'd be willing to give up some memories to bring Dad back.

[Pathos apparently gets downmodded - interesting].

The answer is no.

The downmodding is, I think, because people are sick of metaphysical misinterpretations of QM.

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