How are small particles detected (or seen)?

[gursimran]

I was reading a bit about 'uncertainity principal', there was a statement "that we can't detect paticle whose size is smaller than the wavelenght of EM rays that we are using "

I can't understand this statement.

Why can't we see any particle whose size is smaller than the wavelength of light.
How do we actually see particle? I think our eyes detect only color with the help of wavelength.

How are very small particles detected in general?

 

[IllyaKuryakin]

I was reading a bit about 'uncertainity principal', there was a statement "that we can't detect paticle whose size is smaller than the wavelenght of EM rays that we are using "

I can't understand this statement.

Why can't we see any particle whose size is smaller than the wavelength of light.
How do we actually see particle? I think our eyes detect only color with the help of wavelength.

How are very small particles detected in general?

Several questions there, we can use a CCD or digital type camera to detect individual photons of light. Different particles are detected differently. If you put a phospher coated screen in front of an electron, when the electron hits the screen you see a little flash of light, as the electron energy knocks out photons from the phospher atom, which is the way a tube type TV screen works. In more complex particle physics, some particles aren't detected directly, but are infered, such as a neutrino. It's hard to detect a neutrino, but we know if certian particles are produced in a collision, they must produce a neutrino to maintain the mass-energy balance. If you know the momentum and reactions of the other particles, you can fill in the blanks to determine when the neutrino had to be produced and even which way it took off invisibly after it's production. Just about everything can be detected by one means or another, but a few particles haven't been detected yet. Billions of Euros are being spent at CERN to detect the Higgs particle. No luck yet, but if it's there, they will probably find it. If not, physics will have a wonderful new mystery to solve.

 

[IllyaKuryakin]

And on the other part of your question, generally light will just pass through things much smaller than the wave length of the light, and bounce off those much larger than the wave length of the light. So light can only see particles so small, and any smaller and you need to illuminate with a much shorter wavelength, like an electron, hence the electron microscope. So electrons and photons are the illumination beam for say a bacteria, and the thing we want to detect for say a double slit experiment in quantum physics. Hope that made some sense. I'm horrible at explaining things.

 

[gursimran]

Several questions there, we can use a CCD or digital type camera to detect individual photons of light. Different particles are detected differently. If you put a phospher coated screen in front of an electron, when the electron hits the screen you see a little flash of light, as the electron energy knocks out photons from the phospher atom, which is the way a tube type TV screen works. In more complex particle physics, some particles aren't detected directly, but are infered, such as a neutrino. It's hard to detect a neutrino, but we know if certian particles are produced in a collision, they must produce a neutrino to maintain the mass-energy balance. If you know the momentum and reactions of the other particles, you can fill in the blanks to determine when the neutrino had to be produced and even which way it took off invisibly after it's production. Just about everything can be detected by one means or another, but a few particles haven't been detected yet. Billions of Euros are being spent at CERN to detect the Higgs particle. No luck yet, but if it's there, they will probably find it. If not, physics will have a wonderful new mystery to solve.

Oh sorry, actually I was concerned with finding the position and velocity of these particles in the uncertainity principal.

 

[gursimran]

And on the other part of your question, generally light will just pass through things much smaller than the wave length of the light, and bounce off those much larger than the wave length of the light.

Why this is so, that light is absorved by larger particles and let go by smaller particles whose size is less that wavelength of light? I think this is somewhat related to diffraction .. but I'm not able to make it out how it it?

So light can only see particles so small, and any smaller and you need to illuminate with a much shorter wavelength, like an electron, hence the electron microscope. So electrons and photons are the illumination beam for say a bacteria, and the thing we want to detect for say a double slit experiment in quantum physics. Hope that made some sense. I'm horrible at explaining things.

Even if the particles scatter the light how do they find their position?

 

[IllyaKuryakin]

Oh sorry, actually I was concerned with finding the position and velocity of these particles in the uncertainity principal.

Oh, ok, yes, great question. The position and momentum of a particle is always a partially hidden variable. That doesn't mean it doesn't exist, it just means it can't be completely observed due to HUP. Steinberg managed to get just a little position information from very many particles by using calcite crystal that changes the polarization of each particle just a tiny bit differently depending on its position, then measuring the polarization of the particle.

It's important to note that if the particle did not have a definite position, that is, if it were spread out all over the apparatus in a probability wave, the experiment would not have worked at all.

 

[gursimran]

Steinberg managed to get just a little position information from very many particles by using calcite crystal that changes the polarization of each particle just a tiny bit differently depending on its position, then measuring the polarization of the particle.

Thanks for answering. Could you point me to such a source which can elaborate steinberg's experiment. I searched the net bt did not find anything related to this.

Also we observe objects by seeing them, or ifwe want to fin their position we use say some EM rays that hit the object and reflect and then we measure the time of journey and we we have the distance it travelled. Something like that.
Can we hit a particle like that and measure its position??

And also why does a particle smaller than wavelength of light does not absorb light and is transparent to it ??

 

[IllyaKuryakin]

If you bounce something off a particle to measure it, you change it's momentum. You can be very sure about it's position, but not have a clue where it will be an instant later, or very sure about where it's going, but not have a clue about where it is this instant, or a little of both, but never all of both. This is Hiesenberg's Uncertainty Principle. It's just a fundamental law of nature. Better measurement devices don't help. I'll have to leave the optics questions to someone in that field. I had a course in it, but haven't used it in so long I've forgotten most of it. You can find several free reports on Steinberg's experiment by typing Steinberg double slit into google. The reference to his original paper is there too, but it costs $15 if you are not a member.

 

[gursimran]

Thanks a lot for help ... I will try to clear it more from my prof and post it here if I got something interresting .