Saturday, July 19, 2008

Realism

I finally finished reading Walter Isaacson's biography of Albert Einstein, and it was extremely good. I highly recommend it.

Isaacson does a good job of explaining Einstein's resistance to the Copenhagen Interpretation of quantum mechanics, the idea that some or all elements of the universe are in an indeterminate state that is resolved by the act of observation.


Einstein rejected this interpretation throughout his life and to his death. He held to the tenet of realism, the idea that the world is in a definite state, even in the absence of observation. Most people have heard of Schrödinger's cat, but don't realize that the ideas for such a paradox came mostly from Einstein, whose own example was a keg of gunpowder hidden from observation which paradoxically exists in an exploded and unexploded state.

But apparently the vast majority of physicists currently hold to the Copenhagen Interpretation, with a smaller number holding to a Many Worlds explanation, with a very small minority still in Einstein's camp (I can't seem to find the stats on this right now, though).

Now Einstein never said that quantum mechanics was wrong. He just said it was incomplete.

Typically we use statistics to describe the degree of certainty we have about the state of the world. If a dealer shuffles a legal deck of cards and deals them into two piles of equal size, what is the probability that the ace of spades is in pile #1? It's 0.5, right? This is a description of the extent of our knowledge about the system before observing the relevant variables. If we pick up the piles and flip through them, then we have certainty about the location of the ace, and a statistical description is no longer needed.

What's weird about quantum mechanics is that it asserts that the statistic description is the complete one. In other words, the ace is half in one pile and half in the other. It exists in an indeterminate state, simultaneously in both places, until an observation takes place.

So what Einstein was arguing was that a statistical description was still a description of the limits of our knowledge about the system, while his opponents argued that the statistical description was complete description of reality.

I'm afraid I find it very hard to swallow the Copenhagen Interpretation. I should probably defer to the majority of experts in the field, but I'm afraid I can't...at least until I get a satisfactory explanation why the statistical description should be interpreted as complete, and not as an approximation.

There are a number of experiments which are meant to verify the Copenhagen Interpretation, including the Double Slit Experiment and the Beam Splitter Experiment. Every account I've seen describes the results as "weird", but fail to give a satisfactory account of what is actually going on (at least to me...maybe I'm just being stubborn).

One of the main problems I have with an observer-defined reality is the same problem Einstein had. What constitutes an observer, or an observation? Is a piece of recording equipment an observer? Einstein asked about a mouse. If you show a mouse the readout on a piece of machinery measuring the location of a particle, but don't look yourself, does this resolve the indeterminacy of the system?

And what about the state of the world before life arose? There was a time when there were no observers. Was the world in some kind of constant state of flux? If matter really is in a completely different state before observation, then how did such a fluctuating state give rise to life in the first place?

Another thing I haven't heard explained to my satisfaction is exactly what the act of observation is supposed to do to a system. Here's another example: active and passive sonar. When something like a submarine uses active sonar, that means they generate their own sound waves, and read the information that bounces back from objects the waves bump into. With passive sonar, the submarine doesn't generate its own waves, but relies on sounds that are already bouncing around in the water.

Now I could see how active observation would alter the state of the system being observed. You're injecting a new dynamic into the system when you're bombarding it with sound waves or photons or any other active process. But how exactly does passive observation affect a system?

Maybe there are answers to these questions, and I'm just not smart enough, or haven't read enough, to wrap my head around them. Maybe nobody knows.

For now, though, I'll remain in an indeterminate state.

4 comments:

Ian said...

I'm no quantum physicist (I could barely spell it), but the problem with your active/passive sonar example breaks down in the quantum case.

Fundamentally, both passive detection and active detection rely on something interacting with the detector (either a reflected signal in the active case, or the original signal in the passive case). In the macroscopic passive case, the original signal is not expected to return, so if it's altered by detection, not a big deal.

In the quantum case, it is a big deal. Firing a single photon at a particle and detecting the reflection angle (active case), or allowing the particle to strike the detector directly (passive case) will both cause changes to the particle under observation; the only way to detect a particle is to watch for interaction with our detector particles. This interaction causes uncertainty in the state of the particle under observation; hence, the uncertainty principle.

I'm sure a real physicist could give a _way_ better description than that, but that's the gist.

Philip said...

Most of the stuff I've read on quantum physics asserts that the "many worlds" interpretation of wave function collapse is now more widely accepted in the physics community than the Copenhagen interpretation. The whole thing still makes my brain do somersaults.

Sebastian said...

"In the quantum case, it is a big deal. Firing a single photon at a particle and detecting the reflection angle (active case), or allowing the particle to strike the detector directly (passive case) will both cause changes to the particle under observation; the only way to detect a particle is to watch for interaction with our detector particles. This interaction causes uncertainty in the state of the particle under observation; hence, the uncertainty principle."

This is correct, but I've never understood why it leads to the Copenhagen interpretation.

For me it suggests that human beings with modern instruments CAN'T KNOW the location of the particle without disturbing its location, not that the particle has no actual location.

Or more generally that the micro-phenomenon have an actual state, but we can't know what it is without observing it in such a way as to be disturbing that state.

Ian said...

Sebastian:
"For me it suggests that human beings with modern instruments CAN'T KNOW the location of the particle without disturbing its location, not that the particle has no actual location."

The double-slit experiment is the answer to that question. Particles do not have a definite location, as particles aren't really "particles" at all!

When attempting to measure particles as particles, you run into the detection problem described above. When attempting to measure particles as waves, you have the inherent uncertainty about waves (it doesn't make sense to talk about the "position" of a wave).

Either way, the fundamental nature of the quantum scale is completely contrary to our macroscopic perceptions. Much of the confusion around quantum phenomena comes from using a brain designed for macroscopic interaction; things break down when you get below a certain scale.

I.e., it's weird, but it's true.