Wasting way too much time watching physics videos...
July 19, 2012 10:02 PM   Subscribe

I need time to go through this, but I'm wondering to the scientists if they have ever pointed an electron microscope at the center of a particle collision at the LHC? Do they see at that level of detail?
posted by sfts2 to Science & Nature (11 answers total) 2 users marked this as a favorite
An electron microscope works by shooting a beam of electrons at the target, and looking at how the electrons refract or reflect.

If you did that at the collision point of an accelerator, the electron beam would disrupt the event you were trying to observe.
posted by Chocolate Pickle at 10:26 PM on July 19, 2012 [1 favorite]

HowStuffWorks has an article about the detectors employed at LHC. All 6 are positioned in various advantageous spots around the collision chamber (chambers?).

An electron microscope is an active device-- like a microscope with a flashlight built in, it "shines" a beam of electrons into the subject, and interprets the scatterinig, absorption, and so on. Sending one beam of electrons into a collision chamber would probably just confuse the hell out of everything going in there.

One of the IMO most fascinating devices used to observe the particles is the hydogen bubble chamber, in which very fine hydrogen bubbles are blown into a liquid, and when a particle passes through the chamber, often in a spiral course due to the deliberate application of a magnetic field, it carves a path through the bubbles. Even watching such a chamber in a regular building (a museum, say) one can see cosmic rays flashing through it constantly.
posted by Sunburnt at 10:28 PM on July 19, 2012 [1 favorite]

Point 2: there's a basic physical principle having to do with how waves work. Below half a wavelength, you can't see something. That's why there are lower limits to what you can see with a light microscope.

The reason they use electron microscopy is that at high energy levels, the wavelength of the electron beam is far smaller, so it can see things which are much smaller. But there's still a wavelength, because electrons, like everything else, are both particles and waves when they move. You can't ignore the wavelength, which is a function of the beam energy.

Even if the electron beam didn't disrupt the event you were trying to observe, it's far too small to see with an electron beam. It's well below half a wavelength of any electron beam you could feasibly produce.
posted by Chocolate Pickle at 10:29 PM on July 19, 2012

Scanning electron microscopes work by repeatedly passing an electron beam over a fixed target. They don't work on moving or transitory phenomena. Also, the objects they are scanning are much, much larger than the particles being generated in the LHC; what you're talking about is roughly the equivalent of trying to take a picture of a NASCAR car crash by throwing boulders at the cars.
posted by KathrynT at 10:31 PM on July 19, 2012 [3 favorites]

A Tunneling Microscope cannot observe a changing event. It can only be used to look at something that isn't moving.
posted by Chocolate Pickle at 10:59 PM on July 19, 2012 [1 favorite]

A Tunneling Microscope cannot observe a changing event. It can only be used to look at something that isn't moving.

Yup. You scan them across a chunk of material.

Using a probe to image one of the beams of packets of protons that look like this:
would be like trying to image the pellets from a shotgun blast using a blind man's white cane.

Right near the interaction region they surround the beams withmicrostrip detectors, little comb-like arrays of material much like the CCD in a camera. These are at angles to one another, and that helps give the trajectory.
posted by sebastienbailard at 11:25 PM on July 19, 2012

Yup. You scan them across a chunk of material.

Across the surface, more precisely.
posted by sebastienbailard at 11:26 PM on July 19, 2012

Chocolate Pickle: A Tunneling Microscope cannot observe a changing event. It can only be used to look at something that isn't moving.
Technically not true. It could be used to record a frame BEFORE the event, DURING the event, and AFTER the event, in order to establish movement/change. But that would be like a super-choppy kinescope or animated GIF of just 3 frames, and it would happen much too slowly to observe anything interesting in the time frames we're talking about (particle velocities a reasonable fraction of the speed of light). So, as far as this question is concerned, a tunneling microscope could not detect change (even if the resolution limits weren't billions of times too large).
posted by IAmBroom at 10:20 AM on July 20, 2012

One answer to your question is that the LHC is a microscope --- a hadron microscope.

Different sorts of microscopes have different strengths and weaknesses and are used to look at different things. Optical microscopes behave basically the same way as your eyes and so require the least interpretation, but are limited by the wavelength of of the light that's used. Visible light has wavelengths from 400–700 nm (blue to red), so a blue-illuminated microscope could resolve two features which are 200 nm apart.

Massive, charged particles like electrons and protons have the feature that you can push them around and speed them up, and as they speed up, their effective wavelength changes. An electron that's been pushed across a 150 V potential has a wavelength of 0.1 nanometer, which is typical for the difference between atoms in a crystal. So by tuning your electron energy you can optimize an electron microscope to look at all sorts of things on the small scale. The most famous examples are the few handsful of atoms on a metal surface. But (depending on exactly what sort of electron microscope you have) there are limitations, too. Generally your surface must be conducting, and your sample must withstand vacuum.

That got us a factor of about 2000 in scale — from 200 nm to 0.1 nm. Nuclear physics is concerned with what's happening to the protons and the neutrons in the nucleus; protons and neutrons are 0.000 001 nm (1 femtometer) across. In order to resolve things at this length scale you need a probe with a much smaller wavelength, so you have to push your electrons or protons much harder. Electrons with energy 1 GeV (as if you pushed them across a potential of one billion volts) have wavelength of 0.2 fm, and can resolve not only the protons and neutrons in a nucleus but some the protons' and neutrons' internal structure, the quarks and gluons.

The inside of a proton or neutron is a pretty complicated place. A proton is "made" of three quarks, held together by gluons, but there is an "ocean" of quarks and antiquarks constantly popping in and out of the vacuum. These constituents of the proton are all interacting with each other strongly (by trading gluons), electromagnetically (by trading photons), and weakly (by trading W and Z bosons). If you had a microscope with enough resolving power, you could in principle take pictures of these transient interactions, the same way that an optical camera can take pictures of waves on the surface of the ocean. And that's what the LHC does. Its energy (7 TeV) corresponds to a wavelength of 0.000 02 femtometers, about the same factor smaller than the proton radius as the proton radius is below the size of an atom.

The LHC is a microscope. It's taking pictures of the inside of protons and antiprotons.

There's a nice display of length scales here.
posted by fantabulous timewaster at 4:10 PM on July 20, 2012 [3 favorites]

fantabulous timewaster, upvote a billionty!
posted by IAmBroom at 4:07 PM on July 21, 2012 [1 favorite]

They look at the shrapnel produced when two machine guns fire against each other, to find out what bullets are made of. The electron beam corresponds to a stream of hand-lobbed ping-pong balls aimed at the point of collision.
posted by springload at 5:14 PM on July 23, 2012

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