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Q & A: quantum mysteries

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Quantum Mechanics
Q:
how can light have properties of both waves and particles?
- Ben Gunn (age 15)
SLC, UT, USA
A:
There's nothing special about light. On a small scale everything (light, atoms, electrons, gluons...) has the same sort of very strange behavior. Actually, on a small scale the whole collection behaves in a wave-like fashion, described by a wave equation. What's weird is that on a large scale what we see is not the whole output of that wave equation, but only one part. If we read backwards from what we see to the wave that would have led to what we see, it's only a little part of the whole wave that we started with. That little part sometimes has the light (or atom, etc.) in a little region, with a definite energy, almost like a classical particle.
So in modern physics we've really ditched the old 'wave versus particle' issue, at least in most interpretations of quantum mechanics. But we're still left with the question of why we see only part of the wave. I'll save the long answer about that for when somebody asks.

Mike W.

(published on 10/22/2007)

Follow-Up #1: quantum measurement

Q:
OK. Why do we see only part of the wave?
- Scott
Denver, CO
A:
I knew the other shoe was bound to drop. There are a number of different ideas about why we see only part of the wave. I'll give you a very brief description of some of the main ones.

1. It may be that the entire wave continues to exist but different portions of it 'decohere' and become quite separate. There would be a version of you in each decoherent part. Each version of you only sees its own part of reality. This is called the Many Worlds interpretation, and it is in some ways the most natural way to read quantum mechanics.

2. There may be a particular value for the coordinates of all things, but guided around in a very strange way by the wave. This is called the Bohm interpretation of quantum mechanics. It is 'non-local', a term which I can explain if you're interested.

3. There may be some as yet not described 'collapse' process, by which most parts of the wave disappear, following a non-linear random equation. This is also severely non-local, and not yet fully formulated. Such ideas are sometimes called 'macro-realist'.

4. There may be some way of denying the existence of the wave except as something which helps predict human observations. various ideas like this are known as 'the Copenhagen interpretation'.

I hope that gets you started.

Mike W.

(published on 06/22/08)

Follow-Up #2: quantum measurement

Q:
My question is about the double slit experiment. As a layman, my understanding of the ramifications of this experiment are limited, but a few things about it puzzle me. As I understand it, the experiment demonstrates that, when not measured, photons (electrons, etc.) travel as waves. When measured, however, they travel as particles. That's fine. A little confusing, but fine. What really gets me is when I read that if you take the measurement (using a computer, presumably), but program the computer to immediately erase the results before anyone can see them, this doesn't collapse the quantum waveform. So, to sum up. Not observed-wave. Observed-particle. Observed(but not by a person)-wave. Is this not an example of information in the present changing the past? When the experimenter has access to the recorded information about the particles in the present, the particle in the past was a wave, but when the experimenter does not have access to the information in the present, the particle in the past behaved as a particle. If the above is true, it seems like it should be a really, really big deal. Thank you for your time.
- Theodore
U.S.
A:
You're right that understanding quantum measurement is hard and a really big deal.
Here's one way to start thinking about it. Forget about 'particles' and just think about more or less spread-out waves. When different parts of a wave lead to different large-scale results (meter readings, computer memories, thoughts, etc.) then those different parts cease to have anything to do with each other.

A device designed to measure position is one which gives different large-scale results for very slightly different parts of some wave representing a very small object. So if different parts of a wave lead to the same state of the computer (or whatever) the position was not really measured, and any prediction of what will happen requires keeping all the parts of the wave.

One interesting question concerns whether all the parts of the wave continue to exist, as our basic equations would imply. If so there are versions of each observer seeing each possible outcome of an experiment. It's very hard to devise experiments to see if the parts of the wave which we don't see really disappear or continue to propagate with other versions of us. Answering that question would be a very big deal, but perhaps it's not a question people will ever be able to answer with confidence.

Mike W

(published on 08/06/08)

Follow-up on this answer.