particle accelerator questions

Factual Questions
1

I have some questions about particle accelerators:

  1. Why does larger circumference of the accelerator lead to collisions that spit out higher energy particles?
  2. Why are higher energy particles more fundamental building blocks?
  3. Is a big enough accelerator guaranteed to get down to as fundamental a level as we want?
  4. Why do they say that it “creates conditions like shortly after the big bang”? When people complain the LHC might create a black hole that will swallow the world, the response is something like “cosmic rays collide like this all the time”. So aren’t conditions like they are NOW? What does it have to do with “shortly after the big bang”?
  5. Why do they say things like “if we get high enough energy we will get close to the planck length”?
2
  1. A larger accelerator gives you more room to accelerate particles so you can make them go faster.
  2. They’re not any more or less fundamental. But it takes a lot of energy to make them. A lot of particle accelerator experiments involve looking for things that are predicted which can only be created at very high energies and are therefore difficult to find lying around in nature.
  3. This is not really a question that makes sense. There aren’t levels. The models we have make predictions; experiments allow us to test those predictions.
  4. At the time of the Big Bang, there was a tremendous amount of energy in a relatively small space. Particle accelerators also put a tremendous amount of energy in a small space. That helps us understand things about what it’s like when you have a tremendous amount of energy in a small space. It’s correct that cosmic rays collide all the time. But it’s difficult to predict where that will happen ahead of time and put your observation apparatus there.
  5. I don’t know who is saying that, but it is not really coherent.
3

This is an indirect consequence of something called synchrotron radiation. Because when you accelerate any charged particle it radiates light, so it loses some energy even as you’re pumping energy in to speed it up. The snag with using a circular accelerator is that, while the shape allows you to keep speeding it up and up, bending it into a circle means that you’re always accelerating the beam even just to keep it going at the same speed. So there’s a trade-off in the design - the circle allows you to efficiently accelerate, but at the same time you’re always losing energy.
But the latter effect depends on the size of the circle. The tighter the bend, the more energy you lose. So it’s all better if the circle is as large as you can (in practice afford to) make it.

Hence larger circumference accelerators allow you to accelerate particles to higher energies. Hence higher energy collisions.

4

Thanks for the replies. As far as what I mean by “more fundamental”. I mean combining to create the previous thing. I get the impression (perhaps wrong please correct me if so) that scientists say what makes up a proton? Then they say well a quark. How do we see a quark? Get a high enough energy collision. What makes up a quark? Well a Higgs boson. How do we see that? Get an even higher energy collision. Etc. I may have the names wrong, but is this what is going on? And if so how do we know " higher energy collision" is definitely going to allow us to see the next building block? As far as plank length, I may be as confused as you are by my question. Google “Planck length particle accelerator”. My guess is the more fundamental building blocks being spit out of the collisions are smaller in length, and they think the particle lengths (or radius) are approaching the “Planck length”. Are they getting smaller, and are they indeed approaching the Planck length and not something smaller or larger? And why?

5

So far as anyone can tell, electrons and quarks are both fundamental particles, that is, not made up of anything else. And everything we’re familiar with is made up of electrons and quarks. So as far as the stuff we’re made of is concerned, there’s nothing left to find there.

There are other fundamental particles, some of them much harder to produce, including the Higgs Boson. And there might be many more, which we just haven’t built energetic enough accelerators to find yet (theoretical particle physicists always have plenty of hypothetical fundamental particles around). But they’re not components of what we think of as matter.

6

Let me try to rephrase question 3. E = M C squared, so higher energies will give you more massive particles. In fact, there are particles we have only just produced in recent decades, because we needed the higher energies to see them. On the other hand, there are only a certain number of different particles in the standard model, so once we have generated the complete set, is there any point to a bigger accelerator? Have already reached that size?

7

Quarks, as well as electrons, are fundamental particles. They are not made up of anything else. The Higgs boson is not inside them; it serves to transfer mass in a way analogous to the way the photon transfers the electromagnetic force. The Higgs creates the Higgs field; the photon the electromagnetic field. That’s all the levels there are: fields and fundamental particles. (There are more of each, BTW.) We’re not going to find anything else, and nothing is getting smaller. Fundamental particles don’t really have a size in the normal sense; they are point sized, which is no size at all so nothing can be smaller. The planck length is not a measurement I’ve heard applied to particles.

All this is part of what is known as the Standard Model. That is a bunch of equations that predict all possible particle types and what their energies should be. They already exist. The problem is finding them, finding enough of them, and keeping them around long enough to measure their properties. That’s where the accelerators come in. Unlike neutrinos, a different kind of fundamental particle, quarks and Higgs don’t fly around freely. However, you can make them directly out of pure energy, because E=MC[sup]2[/sup]. Supply enough energy and particles coalesce into existence. Nothing to do with their size. Energy is the only thing that matters.

Are there more particles to be found? Most physicists say that the Standard Model is incomplete or inadequate. A variety of other models exist that predict their own sets of particles, none of which have been seen yet. If we do see them eventually, they won’t be any more fundamental that what we have now (or smaller, for that matter). They’ll be parallel to the known particles. But we’ll need more powerful accelerators to get there.

While I’ve been slowly composing this, others have said similar stuff. Hope this expands on them in a useful way.

8

See (no I am not in this class, or any class, and it is the lecture focus, not a homework):
https://www.calpoly.edu/~rechols/phys111/lecturefocus/Lecture24Focus.html
It asks:
2. In particle physics experiments how are probes of small wavelength created?

  1. Using today’s particle accelerators what length scale can be probed? How does this length compare with the Planck length? Will a manmade particle accelerator ever be able to probe the Planck length?

  2. Why can a string not be used to probe sub-planck-length distances?

What are they talking about here?

9

This is a different subject from particles. The short version is that to combine relativity and quantum mechanics into a theory of quantum gravity some sort of understanding of what happens in really, really small spaces is needed. Strings are a postulated substitute for fundamental particles. All matter would be built up out of strings, but they can’t be thought of as particles. There’s also loop quantum gravity which builds up everything from, yep, loops.

Chronos can supply a better explanation, I’m sure.

10

OK, the easy parts first: In quantum mechanics, everything is a wave of sorts, including the things we think of as particles. The wavelength associated with a particle depends on its energy: Higher energies mean shorter wavelengths. And waves mostly just ignore objects that are significantly smaller than the wavelength. So if you want to explore things at very small length scales, you need to do so using very high energies. For instance, if you bang on a proton at low energy, the proton looks like a single lump of stuff, because it’s too small to see the details. But if you bang on a proton with high energy (corresponding to the size of a proton), then you can tell that there are actually three lumps of stuff in a proton, the three quarks. You can try to bang on protons with even higher energies, to see if the quarks are themselves made up of something smaller (we think they aren’t, but hey, we might be wrong), but so far, this has not revealed anything.

Now, on to the String Model. It’s best summarized by xkcd. The problem is twofold. First, the strings (if they exist) are extremely tiny, many orders of magnitude smaller than what we can currently detect with our particle accelerators, so it’s very difficult to come up with any experimental tests of the String Model at all, with any reasonably-foreseeable technology. Second, there are about a gazillion different versions of the String Model, all of which make different predictions, so even if you do somehow come up with some clever way of testing a String Model, you only end up testing one very specific version, and if you rule it out, there are always still plenty more that you haven’t ruled out. For both of these reasons, the String Model is, in practice, completely untestable (which, incidentally, is why I keep calling it “model”, not “theory”, because a theory by definition must be testable).

Loop quantum gravity is the most popular alternative to the String Model. Notably, however, it does not try to build up everything from loops: It just seeks to explain gravity and nothing else (the String Model would encompass both gravity and all of what’s currently known of particle physics). To the best of my knowledge (this is getting a bit outside my range of expertise), it also does not treat the titular loops as any sort of physical object, the way string theory does with strings: It’s just a way of doing some calculations. Loop quantum gravity doesn’t have the gazillion different versions that the string model does, so it’s in principle at least a little more testable, but it still has the problem that most tests of it would require ludicrously high energies, so it’s not really practically testable yet either.

11

This distinction doesn’t seem meaningful to me. Quantum mechanics was also originally just considered a mathematical formalism. And even today QM can really only be understood mathematically; no one really understands its “true” nature. Whether or not the wave function is “real” (and certainly whether or not one believes the wave function is real), the predictions are the same.

Even atomic theory spent some time as a useful model and not necessarily a true description of the world. But what really changed? More experimental results came in, so after a while it was thought of as not just a formalism but a true description, but there’s not really an obvious point where one changes into the other.

If LQG really turns into a fruitful, predictive description of gravity, then I’d say that spin networks are just as real as atoms or photons.