Thread: The physics of the "present"

Originally Posted by manasij7479

Maybe I'm missing your point, but matter is quite commonly considered as waves, in duality with particles.

Matter behaves in ways that can be predicted using equations that involve waves. This just means it can be predicted like that, it doesn't mean that particles are waves. Whether a particle "is" a wave is maybe not even a meaningful question.

The wave function of any particle is, in general, complex-valued and thus has no direct physical meaning. What the wave is "waving" is measured in units of probability amplitude, it is not a real thing which exists. It is just a mechanism to predict what happens.

Originally Posted by brewbuck

Matter behaves in ways that can be predicted using equations that involve waves. This just means it can be predicted like that, it doesn't mean that particles are waves. Whether a particle "is" a wave is maybe not even a meaningful question.

Similarly, "Matter behaves in ways that can be predicted using equations that involve" particles.
I didn't say that a particle is a wave, but 'matter' and 'particle' are definitely not the same thing, afaik.

The wave function of any particle is, in general, complex-valued and thus has no direct physical meaning. What the wave is "waving" is measured in units of probability amplitude, it is not a real thing which exists. It is just a mechanism to predict what happens.

Everything in physics is a mechanism to predict what happens, or abstractions of what is happening; physical significances are totally a human invention.
The probability function wave does not predict the probability of a 'particle' existing at a point in space but the magnitude of the behaviour exerted , for example chemistry uses this to determine interaction between atoms quite effectively.

N.B: This just my understanding of how things are, and I'd be happy if you point out where I'm wrong.

Originally Posted by manasij7479

Everything in physics is a mechanism to predict what happens, or abstractions of what is happening; physical significances are totally a human invention.
The probability function wave does not predict the probability of a 'particle' existing at a point in space but the magnitude of the behaviour exerted , for example chemistry uses this to determine interaction between atoms quite effectively.

If you are saying that the wavefunction is a statistical property of a whole ensemble of particles, that isn't the case -- it works even for a single particle, and really does denote probability, not collective tendencies. But wave/particle dualities occur in many contexts and not all of these are the same. The equations are similar because many physical relationships can be described as proportionalities between some quantity and its various derivatives. Mathematically, those sorts of systems have wave-like solutions. But the fact that waves arise everywhere doesn't signify that they are all of the same nature.

I base my own interpretation strongly on Feynman's, and by no means claim that it is the only valid one.

Originally Posted by megafiddle

The wave nature of particles (e.g. electrons) can be seen in the double slit experiment.
Electrons will produce interference patterns, much like light. These interference patterns
of alternating light and dark bands are characteristic of waves. In addition, there is a probability wave function which predicts where the electrons will
strike the target and therefore where the bands will appear.

It's not "in addition," it's the whole reason it happens. The wavefunction is not the electron. For just one particle you might convince yourself that the wavefunction is the electron, but for more than one particle it all breaks down. For instance, a system of two electrons has ONE wavefunction. You cannot write this wavefunction in terms of the sum of two wavefunctions belonging to each electron individually, it doesn't come out right. electron != wave

Originally Posted by megafiddle

I didn't mean that the wave function was the electron. I would say that the wave function
describes the electron, in this case it describes it's location. However the electron does
behave as if it were a wave; cover one slit and the interference pattern disappears.
Isn't that what is meant by the "wave nature" of a particle?

When you cover one slit the interference disappears because the wave function isn't interfering anymore, it still doesn't mean the electron is a wave. This is in contradiction to what you read in introductory quantum texts, but not contradictory with what you read in the more advanced texts. It becomes clearer when you start considering systems of more than one particle.

There is a great lecture by Feynman which was videotaped, and it's on Youtube. He addresses this exact question at the end of the lecture. When I get home I can post a link. For now, it basically goes like this (paraphrased):

Student: "You just spent all this time telling me that things are particle, but then you show me all these equations that look like the equations of waves. So, aren't you just being sneaky and trying to hide the fact that your theory of matter is really just a wave theory?"

Feynman: "It's true that for a single particle the equations are wave equations. A wave is a function of position and time. But things change when you consider systems of more than one particle. There is a wavefunction for the system, and it is a function of multiple positions and time. That is not a wave. Further, if a single wavefunction describes more than one particle, how can it be said that this wavefunction represents this particle or that particle? You can't."

Originally Posted by megafiddle

Don't you get the same results though, whether there are many electrons or just one? When the beam intensity
was reduced to the point where only a single electron at a time was passing through one of the slits, the interference
pattern persisted. Same as a dense beam.

With many electrons the pattern builds up quicker, but other than that it's not different than a single electron. The electrons in the beam for the most part do not interact with each other and so they can be described by independent wavefunctions. In other circumstances (like the two-electrons-in-a-box problem) there is an interaction and therefore only a single wavefunction for the system.

"Real waves" can interfere with other waves. Wavefunctions are not like that, they can only interfere with themselves.

I'm not arguing at all that something wave-like is happening, I just refuse to identify "electron" with "wave," a la Feynman. This isn't a settled issue!

Here's the video I was thinking of:

Feynman on Wave Particle Duality (QED Lecture in New Zealand) - YouTube

You've probably already watched it if you've watched/listened to Feynman, but I toss it out there just in case.

Originally Posted by brewbuck

Here's the video I was thinking of:

Feynman on Wave Particle Duality (QED Lecture in New Zealand) - YouTube

You've probably already watched it if you've watched/listened to Feynman, but I toss it out there just in case.

Thanks for the link. I had seen it before, but it makes somewhat more sense now.

So for a single particle, the probability wave function is mathematically the same as the the amplitude wave function for an electromagnetic wave?

Originally Posted by megafiddle

Thanks for the link. I had seen it before, but it makes somewhat more sense now.

So for a single particle, the probability wave function is mathematically the same as the the amplitude wave function for an electromagnetic wave?

Yes, which is why the wave theory of light works so well.