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Q & A: Observation in particle physics.

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Most recent answer: 05/19/2012
Q:
In particle physics where scientists are trying to "view" the smallest particle, How do you get around certain problems like, any equipment used to observe are made out of the same small materials,And how do you get any kind of "resolution" Because observation in the classical sense requires photons which are very small. Or in other words to obtain any decent resolution of an object the the object has to be "scanned" by something smaller.
- Chris (age 57)
Glen Cove , NY 11542
A:
Hi Chris,

The whole idea of "observation" in a field particle physics is quite different from the classical way we think of observation, e.g. using a microscope to look at a cell.

First of all, the idea of observing something visually, with light, has a fundamental problem. The main problem is that optical instruments (lenses, microscopes, etc), even if they were built to perfection, are "diffraction limited". Since light is a wave, it will interfere with other light waves. This interference makes it impossible to use light to accurately resolve things that are very small.

To get around this issue, experiments like the Large Hadron Collider do something completely different. They generally will have an area set up that is surrounded by detectors.  They then attempt to create the particles they want to study in this area. In the early days people used cloud chambers and bubble chambers in which the paths of particles could be directly seen from the trails they left. The type of trail (curvature in magnetic and electric fields...) would tell a lot about the particles.

Now faster detectors able to keep track of far more particles quickly are used. When the particles hit a detector, they leave a kind of subatomic "fingerprint". All observations in particle physics are reliant on the interactions between the particle we want to observe, and some part of the experiment.

Physicists can analyze this fingerprint and find out all sorts of interesting things about the particle. This is how they "observe" the particle's properties. Since these detectors are incredibly sensitive, they tend to pick up a quite a lot of "noise": readings that aren't the particle's fingerprints. The best way to alleviate this problem is to measure billions of fingerprints. When you have more fingerprints, your data are more conclusive, and you can be more certain that your observations of the particle's properties are correct.

-Matt J.

(published on 03/29/2012)

Follow-Up #1: photon energy, wavelength, size, frequency

Q:
On the same note. I was able to watch a few of Leonard Suskinds lectures on you tube where it was explained that one way of observing some particles was to "accelerate " a photon to add energy to it which is supposed to make the photon smaller and shift it's frequency. Does the resonant frequency determine the size of a photon? And does the size of any particle correlate to frequency?
- Chris guggenheimer (age 57)
GLEN COVE
A:
Adding energy to a photon does raise its frequency and lower its wavelength. Of course this isn't true "acceleration" (as you know but some readers might not),  since the speed of the photon remains the speed of light.

It's not really true that reducing the photon wavelength makes the photon "smaller". In fact, there's no particular size associated with a photon. Its wave can be more or less spread out, with the less spread-out ones forced to have a big spread in momentum. This Heisenberg uncertainty relation does apply to all sorts of particles.

One difference between photons and particles such as electrons, neutrons, and protons is that the photons have no rest mass. Their wavelength, λ, has a simple relation to their frequency, f :
 λ=c/f,
where c is the speed of  light. For a particle with rest mass m0 the relation is slightly more complicated:
λ=(c/f)/sqrt(1-(m0c2/hf)2,
where "h" is Planck's constant.

Mike W.

(published on 05/19/2012)

Follow-up on this answer.