Wednesday, August 19, 2009

The Quantum Mechanics Mystery Conspiracy

There is a long-standing problem in physics concerning the flow of time. All known physical laws -- except one: the second law of thermodynamics -- are "temporally symmetric" that is, they operate equally well whether time is flowing forwards or backwards. Take a movie of any physical phenomenon and run it backwards and everything you see will be consistent with physics. But the Second Law is not really a fundamental law, it is just the statement of the empirical observation that time seems to flow in one direction and not the other, so the deeper question of why this should be so has remained a mystery.

Actually, it's not mysterious at all once you get past the quantum-physics mystery conspiracy, which insists on presenting QM in false and misleading terms. In particular, the QMMC focuses on measurement and entanglement as if they were two distinct physical phenomena, one simple and familiar, the other alien and inscrutable. In fact, measurement and entanglement are one and the same physical process. Once you realize this, the answer to the arrow-of-time "dilemma" becomes obvious: time appears to flow in one direction because the perception of the flow of time relies on the formation of records of "past" events, which is to say, the formation of classical information, which requires "measurements", which requires particles to become entangled with each other. It is possible to reverse this process, but only by bringing all the particles in an entangled ensemble physically together, which is possible in principle but not in practice. (See section 5.4 in the link above for more details.)

This result has recently been (re)discovered by a card-carrying physicist.

It's really starting to annoy me how many physicists get press by making these kinds of "breakthroughs". All this stuff was figured out in the mid-90's. You'd think that now nearly fifteen years on the word would have gotten out, but it hasn't. It's really kind of weird. I seriously doubt that there's a conscious QM Mystery Conspiracy, and yet here we are. Why people should still be scratching their heads about the arrow of time and "spooky action at a distance" in 2009 is the biggest mystery of all.

9 comments:

Don Geddis said...

What you say is not wrong, but ... it seems to me that you've taken two different "mysteries" (1. arrow of time; 2. quantum mechanics), and built an explanation of them together.

But you can resolve the arrow of time just using entropy, even in a classical world. Unlike some physics mysteries, you don't need QM to understand the arrow of time. (C.f. chapter 3 in Drescher's Good and Real.)

That said, I agree with you that the arrow of time, and spooky action at a distance, are no longer physics mysteries, even though the popular (science) press continues to write about them.

Ron said...

It's not just me, it's MacCone too. His paper is entitled, "Quantum Solution to the Arrow-of-Time Dilemma."

> But you can resolve the arrow of time just using entropy, even in a classical world.

No, you can't. It's actually not possible to reconstruct the information-theoretic argument in classical terms. The difference is that it is in principle possible to restore a classical system back to an initial state. All you have to do is restore all the constituent particles back to their original positions and velocities.

This is not possible in QM even in principle. For starters, you can't even make a *record* of the initial condition of an arbitrary quantum state. (This is called the quantum non-cloning theorem. You can prepare multiple copies of a predetermined quantum state all of which will be identical, but this is not the same thing as taking an arbitrary quantum state and making a copy.) So even if you could restore an arbitrary quantum state there's no way to *know* that you've actually restored it.

It gets worse. To restore a quantum state you have to "undo" its entanglements -- all of them. (I put "undo" in scare quotes because it's not just impossible to "undo" an entanglement, it's actually *non-sensical*. Entanglements don't *happen*, they just *are* -- in a four-dimensional quantum-mechanical sense.) To "undo" an entanglement you would have to bring the entangled "particles" back together again. (If this were not so it would lead to superluminal communication, as I describe in my paper.) But it's not possible to bring "particles" back together again because there are no "particles" -- only quantum systems that have decohered to various extents.

It is possible for two systems to "become" entangled and "then" to "become" disentangled again -- this "happens" all the "time" with quantum vacuum fluctuations for example. Note the scare quotes. This phenomenon cannot ever be "observed" because observing is by definition to become entangled with, so *by definition* "observing" makes "undoing" entanglement impossible. *That* is what produces the arrow of time. The universe isn't "really" "moving" "forward in time." What is "really" "happening" is that (classical) information can only exist in the face of -- indeed is constructed out of -- persistent entanglement. And that can only "happen" "forwards" in "time."

quantamos said...

Various fields have quite a bit of folklore, stuff that's known but nobody bothers to publish it... I've heard stories about people getting a publication just because nobody else published a formal derivation of some result.

Don Geddis said...

I understand that your information-theoretic argument requires quantum phenomena.

My only point is that there are simpler arguments (such as those suggested by Drescher) which imply an arrow of time even if the real world was completely classical.

You don't need QM to make an arrow of time. You'd still have an arrow of time, even without QM.

QM said...

**********To "undo" an entanglement you would have to bring the entangled "particles" back together again. (If this were not so it would lead to superluminal communication, as I describe in my paper.)*********

What about your patent on a device for performing super luminal communication (U.S. Pat. No. 7,126,691)? Did you get that patent as a joke or something?

Ron said...

> Did you get that patent as a joke...?

I like to think of it more as an "exercise" or an "experiment" than a "joke". I believe that the U.S. patent system is badly broken, and I thought I could demonstrate how broken it was by getting a patent on a device that violates the laws of physics. Unfortunately, the system is so badly broken that no one noticed.

QM said...

But is it really a problem that it got allowed? All it does is give you the right to prevent someone else from making, using, or selling the invention. Good luck identifying an infringer. Meanwhile, the USPTO gets funded by the fees you pay. Why wouldn't they allow it and get the maintenance fees?

Ron said...

> But is it really a problem that it got allowed?

It's not really a problem because i don't intend to sue anyone. But it's a potential problem, and illustrative of an actual problem, namely, that a lot of patents get granted that should be for one reason or another. Usually the problem is obviousness. But whatever the reason, every patent, whether or not it should have been granted, is a license to sue. The sheer number of bogus patents, each one a potential land mine for anyone wanting to bring an innovative product to market, is a serious drag on the economy. The problem is particularly acute in software. I'll write an article about it if my house doesn't burn down in the next couple of days.

Than Tin said...

Ron:
I chanced upon your Youtube video of “Quantum Mysteries: Disentangled” last week. I must say that your essay is a serious and tightly argued version than that of my own somewhat whimsical essay on the same topic. (See http://fqxi.org/community/forum/topic/1865 ):
“Mention quantum theory, the two words that follow it are great and weird: great as in the greatest scientific theory ever, and weird as in sci-fi Star Trek, Captain Kirk being teletransported across galaxies to Uranus and Alpha Centauri with stopovers in Boston and Bangkok, but with speed much much faster than you can say “Sharzam”!
There is no question that quantum theory is one of the greats, perhaps even the greatest scientific theory ever invented by Mankind, but the purity of its achievements could do very well without calling attention to stories of its weirdness.
A quantum theory without the sobriquet of weirdness can be imagined. …”