Tracks in particle detectors and quantum paths

[TrickyDicky]

What is a "quantum microparticle" - or what is not one?

This is a very good question that probably is behind the OP in part. More specifically, is the concept itself helpful or is it hampering further developements of a more fundamental theory?

You can neglect QM in the same way you can neglect the influence of gravity on particles in the bubble chamber - it is there, but you just don't (have to) care.

I'm not sure what you mean by "you can neglect QM" here. What is observed in particle detectors must be in principle compatible with the Standard model if one is using it to explain it. And ultimately with QFT and QM as that's what the SM is based on.
Or are you making a point about the difference between QFT and QM?

Nonrelativistic QM itself is just an approximation of QFT, and that might be an approximation of some more fundamental theory. So what? Does that change our view on the bubble chamber in any way?

See above.

 

[WannabeNewton]

So it is impossible both theoretically(taking Planck scale as a theoretic limit) or in practice(much bigger limit with present technology) to realize or probe a true classical trajectory.

That's because "true classical trajectories" don't exist. But how is that at arms with the situation observed in the bubble chamber? Therein the ostensible classical trajectory arises thanks to the time scales and length scales involved; certainly the particle in question is not traversing a classical trajectory but given the characteristic scales we are interested in, with regards to the bubble chamber, we can use a coarse-grained picture to describe the particle as moving on such a trajectory. Obviously if you probed the system on time scales and length scales comparable to the characteristic scales between measurements made by the "environment" on the system then the story would be quite different. We do this kind of coarse-grain analysis all the time, not just in QM, but also in e.g. statistical mechanics; of course in QM one must be quite careful conceptually when making such coarse-grained approximations.

You yourself explained this very well in post #76 and post #94 by vanhees is also spot on. Given that you've already understood the situation at hand, what exactly do you find is at arms with what mfb is saying?

 

[TrickyDicky]

I think we are all basically on the same page on this, I was simply stressing that instead of giving preference to particles over trajectories when one is given that choice one could also consider the existence of trajectories therefore letting go of particles as ontic objects(like is done to a certain extent in QFT).

 

[mfb]

This is a very good question that probably is behind the OP in part. More specifically, is the concept itself helpful or is it hampering further developements of a more fundamental theory?

"Quantum microparticle" is not a usual expression, so how can we tell what you mean by that?

I'm not sure what you mean by "you can neglect QM" here. What is observed in particle detectors must be in principle compatible with the Standard model if one is using it to explain it.

See the comparison to gravity.
Our observations must be compatible with gravity as well, but we still don't have to care about it as its influence is negligible. The same applies to QM for tracks in a bubble chamber.

Or are you making a point about the difference between QFT and QM?

No.

 

[TrickyDicky]

"Quantum microparticle" is not a usual expression, so how can we tell what you mean with that?

Sorry, I thought the meaning was obvious by the thread's context(electrons, alpha particles), just the particles that particle physicists deal with.

 

[DavidLloydJones]

How are the track leftt say by an electron in a cloud chamber and its wave function undefined trajectory related exactly?

The exact difference is that wave functions exist in our minds, and are therefore as smooth as we feel like making them. We can dream up functions with almost no limit, and do collaborative work around any of them we have symbols to convey. Bubbles, on the other hand, go bub-bub-bub in the "real world," with a size and speed of formation crucially related to the temperature and viscosity of their medium. The individuality of the bubbles strongly reinforces, I would guess, any bias our minds have to an epistemology of distinct little round objects.

You remind me, incidentally of a fond moment now fifty years ago. Frank Oppenheimer was a friend, and my wife and I helped him and Jackie, as best we could, with the earliest stages of putting the Exploratorium together. In those fine pre-clean-and-polished-itude days, one of my favourite parts of the whole thing was a cloud chamber Frank had made out of a cardboard box, a bottle of carbon tet, and a cellophane covering to watch through.

-dlj.

 

[Bruno81]

Why is it surprising that a quantum system behaves like a particle when its position can be known in the bubble chamber? When have we seen anything to the contrary?

 

[AlexCaledin]

The quantum system's "job" seems to be Event Coordinator...

Events must be simulating particles and things...