Naively, I would have guessed that you could merely accelerate particles for longer periods of time before colliding them, but why doesn’t that work? A long time ago, I heard that you would need an accelerator aa big as Saturn’s orbit to achieve ToE energies. Is that right/currently believed?
It has as much to do with the curvature as anything. Charged particles lose energy by radiation (bremsstrahlung) simply by being accelerated in a curved path. The bigger the accelerator, the lower the losses, and thus the higher energies you can achieve.
Linear accelerators exist, but they have the downside of only getting one pass to both accelerate and collide. Ring accelerators like the LHC have multiple opportunities, and can choose multiple locations for where the collision happens (making it easier to support multiple instruments).
As @Dr.Strangelove alluded to, having a ring allows for multiple experiments that can be run simultaneously (or at least able to be set up without breaking another machine down):
https://cds.cern.ch/record/1125888/files/bul-pho-2008-078.jpg
A lot has been made of the LHC ‘just’ being used to confirm the Higgs boson (which completed the verification of all of the fundamental particles of the Standard Model of particle physics), but it has also produced dozens of new exotic hadrons.
This documentary is kind of out of date but still shows a lot of the practical challenges and enthusiasm about the LHC:
Stranger
Would it also be akin to taking sharp corners at too high a speed?
As you add speed and energy to the particles it seems it would take ever more powerful magnetic fields to direct them and contain them in a smaller circular path. Make the circle wider and that makes it easier?
Only in a very loose sense. If you imagine a marble circling around a bowl, it can actually do that forever, assuming the bowl is perfectly frictionless and so on. There’s no inherent energy loss with classical, uncharged particles.
But charged particles are different; if you accelerate them, they will emit electromagnetic waves. Bremsstrahlung is a special case of this when the particles have a much larger energy than their rest mass (i.e., they’re going a large fraction of the speed of light). To keep particles going in a circle, you have to continuously accelerate them (plain old centripetal force). Which means they’re constantly leaking energy.
The tighter the curve, and the higher the energies, the faster they leak. Which puts a limit on the maximum energy of your particle accelerator. Make the circle bigger and the leakage goes down, so you can run them faster before you hit the limit.
Do they spin the particles during acceleration?
This isn’t quite right. Bremsstrahlung radiation is produced by the ‘drag’ of a charged particle (usually an electron because they have the lowest mass and thus achieve the highest velocities, but it could be a muon or proton) interacting with the field of another opposite charged particle and radiating in response to change of the momentum vector. Bremsstrahlung is more of an issue with dense charged plasmas and specifically with magnetic confinement fusion, where it can radiate away enough energy to cool the plasma, preventing it from achieving or sustaining the temperature threshold for fusion reactions to occur in sufficient quantity for net power production.
Charged particle colliders like the LHC generally send beams of particles of the same charge (generally protons but they can also accelerate anti-protons and ‘heavy’ ions) toward each other or a stationary target by accelerating them with alternating magnetic fields. This interaction produces cyclotron radiation (or more properly in the relativistic case, synchrotron radiation) which is qualitatively similar to bremsstrahlung radiation but has different cause and distinct emission sprectra. Some bremsstrahlung radiation can occur when opposing beams of like charged particles are collided but that is separate from synchrotron radiation from being accelerated around the loop. Making the loop a larger diameter helps to mitigate the latter to an extent, but as energies scale up the benefit of larger ring diameter becomes less in proportion.
Quantum mechanical particle spin is different from classical angular momentum of self-rotation of a body (even though it confusingly has the same units) and is a conserved property of the particle which is generally stated in terms of being normalized to the reduced Planck constant. Spin, in the context of quantum mechanics, has to do with interactions between particles and specifically with the Pauli exclusion principle and not classical mechanics. It would be better if QM used a distinct term to distinguish but we’re stuck with a lot of unfortunate and confusing nomenclature that physicists just learn to accept.
Stranger
So, the maximum energy they can achieve happens when the loss rate exceeds the energy which can be added? Is that fair to say?
Well, that’s a definitional statement. Synchrotron radiation isn’t the only factor in the limits that the LHC can achieve, and it has already been upgraded twice, most recently achieving a maximum kinetic energy of 6.8 TeV per beam, almost doubling the original specification of 3.5 TeV.
Stranger
Sources vary as to whether cyclotron radiation is considered a subset of bremsstrahlung or not. For example:
https://www.sciencedirect.com/topics/chemistry/cyclotron-radiation
then the electrons also emit cyclotron radiation (also called magnetic Bremsstrahlung)
Wikipedia at least also mentions both usages. I’m not sure which usage is more common in practice, or if it even comes up–physicists are likely to just use the most specialized term that fits their needs.
Of course, it’s all definitional. It’s all just electromagnetism in the end. All charged particles emit radiation under acceleration, regardless of how that acceleration took place. All these terms just draw a loose boundary on some interesting subset of accelerating behavior.
The names really just reflect the history of where the effects were discovered: plasma physics, particle accelerators, and so on.
For electron/positron colliders, radiative losses are the limiting factor.
For proton/(anti)proton colliders, magnet strength is the limiting factor.
Both limitations point to a larger radius for the circle. And this is why a given circle can achieve higher energies with protons than with electrons. However, electron/positron collisions are very clean and can also have well-defined center-of-mass-energy, whereas proton/(anti)proton collisions are a huge mess.
The 1,232 main LHC bending magnets are giant beasts: superconducting dipole magnets providing an 8.3 tesla field along a 50-ft length each. Max center-of-mass energy: around 14,000 GeV.
In the exact same tunnel (same bending radius), the previous LEP collider (“Large Electron/Position” collider) used magnets with fields only around 0.1 tesla yet, due to radiative losses, could only reach around 100 GeV center-of-mass energy.
There are discussions of future colliders with four times the radius so that the Higgs boson can be produced and studied in further detail with clean electron/positron collisions. And if one did that, then a distance future could see proton/proton collisions in that same larger tunnel without needing significant magnet tech advancement.
Some miscellaneous comments:
The deflecting field needn’t be due to an oppositely charged particle.
While this is one application of the concept, it is a very common and routine consideration in particle physics generally. Electrons above a paltry 30 MeV or so lose energy primarily through bremsstrahlung radiation when passing through matter. For muons, it’s dominant above a few hundred GeV, which isn’t too high in many contexts (e.g., cosmic ray muons), but even when it isn’t the dominant energy loss mechanism, it’s an important interaction channel for muons passing through matter.
I guess “like the LHC” here implies hadron colliders? More generally, charged particle colliders would include lepton colliders, which have been the most numerous historically, and those have always used oppositely charged counter-rotating beams (electrons and positrons). For hadron colliders, the LHC’s predecessor (Tevatron) also used oppositely charged beams (protons colliding with antiprotons). Because the magnetic field configuration is fixed, this of course can’t be changed on the fly. The LHC uses proton-proton primarily because high luminosity is much easier to achieve. It’s hard to make a lot of antiprotons.
(Not disagreeing with anything you said here; just clarifying the broader context for folks.)
That example is talking about “magnetic bremsstrahlung” as an alternative term to mean cyclotron radiation. But, yeah, nobody really says that. A quick Googling shows some usage from a half-century ago, but it’s not in regular use in modern parlance. As for regular bremsstrahlung, cyclo/synchrotron radiation would not be part of the usual definition.
As @Stranger_On_A_Train implied, you would need to be a little more specific in what you mean here. The particles are not “spun” up in any way aside from the fact that they are travelling around in a big circle, but the particles do have intrinsic quantum mechanical “spin”, and that is relevant in the collisions. Some colliders polarize the beams to enhance one spin direction over the other to measure the spin-dependent aspects of the particle interactions.
I’m going to be needlessly nitpicky - if we consider classical physics to include General Relativity, a marble circling a bowl will radiate gravitational waves in a way analogous to the EM radiation that results from an accelerating charged particle; it’s just that gravity is so incredibly weak compared to electromagnetism that decay of the marble’s movement would take more time that the universe probably has.
(please correct me if I’m wrong!)