# I How do we measure the EM field that a particle produces?

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1. Apr 6, 2017

### joegibs

[Moderator's note: posts from several threads on the same topic have been merged into this thread.]

When we measure the EM field of a charged particle, what exactly are we measuring? Are we measuring the position of the EM field, the strength of it, what? Does a penning trap measure the EM field of a particle?

2. Apr 6, 2017

### Staff: Mentor

The field doesn't have a position, so no.

Yes.

A Penning trap by itself doesn't measure anything about the particle. It just confines the particle in order to make it easier to use other apparatus to measure the particle's properties.

3. Apr 7, 2017

### joegibs

But I just learned that when we measure the EM field of a particle, we are measuring the changed the charged particle induces in the EM field. Is that right?

4. Apr 7, 2017

### Staff: Mentor

Where?

No. A way of measuring the strength of an EM field (without considering its source) is to measure its effect on a particle of known charge and mass. In situations like a Penning trap, the charged particle is not the only source of EM fields; the trap itself is a source of EM fields. In fact, as I understand it, the fields produced by the trap are much, much larger than the field produced by the particle in the trap, so the particle's field can basically be ignored; any field measurements (such as measurements of properties of the particle like its cyclotron frequency) would be telling you the strength of the trap's field. (Usually such measurements are actually used to give more precise values for things like the particle's charge and mass, with the trap's field being considered as exactly known, since it is controlled by the experimenters.)

5. Apr 8, 2017

### ZapperZ

Staff Emeritus
You really should know by now that you need to provide the exact source and reference whenever you ask something like this. You're asking us to explain what you've encountered, but you never give us the source. We have no way of knowing if (i) you read it correctly (ii) you misinterpret what was said, or (iii) you simply read a faulty source.

Zz.

6. Apr 9, 2017

### joegibs

Here is where I found that out.
http://physics.stackexchange.com/qu...to-measure-the-em-field-of-a-charged-particle
I asked it on physics stack exchange

7. Apr 9, 2017

### joegibs

http://physics.stackexchange.com/qu...to-measure-the-em-field-of-a-charged-particle
Here is the source

8. Apr 9, 2017

### Staff: Mentor

I assume you're referring to gautampk's statement:

"A charged particle interacts with the EM field, causing it to change in a predictable way, and this change can be measured, but you're not measuring the particle when you do this. You're just measuring the EM field and comparing it with a measurement you might have taken earlier before there was a particle."

In principle this is true, but in practice whether this kind of measurement is going to be doable will depend on the charge of the particle vs. the strength of the field that was there before the particle entered the experiment. In Penning traps, as I said before, AFAIK the strength of the field is so large, compared to the charge of the particle, that the particle's presence has no appreciable effect on the field strength, so a comparison of the field before and after the particle is put into the trap will show no measurable change.

I'm not aware of any practical case in which the kind of comparison gautampk describes is done. Which is to say I'm not aware of any practical case in which we explicitly measure the EM field due to a charged particle. We measure the charge of the particle directly, and if for some reason we need to know its field (which in practice, as I've said, I don't think we ever need to since the field is too small to matter), we can compute it from its charge.

9. Apr 9, 2017

### joegibs

What is the physical process behind measuring the EM field that a particle produces?

10. Apr 10, 2017

### BvU

11. Apr 10, 2017

### Staff: Mentor

This doesn't measure the particle's field, it measures the particle's response to an external field; a common application of this is to measure the particle's charge/mass ratio (which then, if you know one of those two things, tells you the other). I'm not aware of any experimental efforts to directly measure the fields caused by individual charged particles, because those fields are so weak; normally those fields, if for some reason you need to know them, are just computed from the particles' charges.

12. Apr 10, 2017

### joegibs

I asked it on physics stack exchange and here's the answer I got on how it is done.
"You take a test charge and measure the force that the test charge experiences in the presence of the source charge. Knowing the charge that the source charge carries, you can find out the field at any particular point by dividing the force that your test charge experienced at that point by the charge the source charge carries."
Does doing this procedure collapse the wave function of the charged particle?

13. Apr 10, 2017

### joegibs

http://www.physicsclassroom.com/class/estatics/Lesson-4/Electric-Field-Intensity
In this process how to we measure the force that the test charge experiences from the source charge? And how do we know the distance from which the test charge is from the source charge? Do we measure the position of the test charge or source charge in this experiment? Or is both of the wave functions still delocalized instead of being more localized?

14. Apr 10, 2017

### Staff: Mentor

15. Apr 10, 2017

### Staff: Mentor

This procedure appears to be describing a classical measurement, not a quantum measurement. I don't think it's useful if you are interested in quantum measurements. But a link to the actual post will help; I'm curious to see if whoever posted it gave any references, to either textbooks or papers describing actual experiments; what it's describing looks like an idealized experiment that would be used in a textbook for discussion.

16. Apr 10, 2017

### Staff: Mentor

Also, as I've pointed out to you in several other threads, you are phrasing this question in a way that presupposes a collapse interpretation of QM. There are also no collapse interpretations, and there is no way currently known of experimentally testing which one is right. So you really should re-think what you are asking about so that you can ask it in terms that don't assume a collapse interpretation.

17. Apr 10, 2017

### joegibs

Here http://physics.stackexchange.com/qu...ed-particle?noredirect=1#comment730230_325585
I also found a link to how the experiment is done in practice/reality
http://www.physicsclassroom.com/class/estatics/Lesson-4/Electric-Field-Intensity

18. Apr 10, 2017

### joegibs

When I ask "if the wave function collapses", I'm not presupposing a collapse interpretation. By asking if the wf collapses, I'm also asking in non collapse theories (such as MWI) if the wave function decoheres in the process. So when I ask if the wave function collapses, I'm also asking if it would decohere in non collapse interpretations.

19. Apr 10, 2017

### pixel

The referenced article explains how the electric field is defined. Why are you bringing in quantum mechanical concepts (wave functions)?

Keep in mind that the test charge must be very small so as not to disturb the source charge whose field is being measured. In fact, the definition of electric field involves the limit of F/q as the test charge q -> 0. Check out the section Another Electric Field Strength Formula on the web page you referenced. That shows how Coulomb's law can be used to get the field strength (without actually using a test charge as such a charge is an idealization).

Last edited: Apr 10, 2017
20. Apr 10, 2017

### Staff: Mentor

That seems very odd. Decoherence happens in all interpretations, so wouldn't it be better to ask if there is decoherence?

21. Apr 10, 2017

### Staff: Mentor

Ok. I don't see anything in the stack exchange discussion that describes an actual experiment. It's just quoting the idealized theoretical measurement.

I don't see where this page describes an actual experiment. If you are referring to the "interactive" part, that is a computer simulation using the theoretical equations; it's not an actual experiment.

Here is an example of a paper describing an actual experiment:

http://iopscience.iop.org/article/10.1088/1742-6596/653/1/012129/pdf

This is an experiment to measure charge, not an electric field, but it illustrates the kind of description you should be looking for. (It also involves a controlled electric field.)

22. Apr 10, 2017

### Staff: Mentor

@joegibs I have merged several of your threads on the same topic into this one. Please use this thread to discuss the topic instead of opening new ones.