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Why the LHC May Mean the End of Experimental Particle Physics
StartsWithABang writes: At the end of the 19th century, Lord Kelvin famously said, "There is nothing new to be discovered in physics now. All that remains is more and more precise measurement." He was talking about how Newtonian gravity and Maxwell's electromagnetism seemed to account for all the known phenomena in the Universe. Of course, nuclear physics, quantum mechanics, general relativity and more made that prediction look silly in hindsight. But in the 21st century, the physics of the Standard Model describes our Universe so well that there truly may be nothing else new to find not only at the LHC, but at any high-energy particle collider we could build here on Earth. If there are no new particles found below about 2–3 TeV in energy—particles that the LHC should detect if they’re present—it’s a reasonable assumption that there might not be anything new to find until energy scales of 100,000,000 TeV or more. And even if we build a particle accelerator to the fullest capacity of our technology around the equator of the Earth, we still couldn’t reach those energies.
The idea is that if we don't find anything, the next most likely place to go looking is at the energy where the strong, weak, and electrical forces unify, around 10^13 TeV. The number they give is a few orders of magnitude below that; we probably wouldn't have to get all the way to the grand unification energy to see hints of new particles. I think that's the evidence you're looking for; it's justified by our present theory.
It's a "reasonable assumption" in that those theories begin to break down at that scale. We expect our theories to hold quite well, which would mean that we wouldn't expect to find anything novel until we got close. And then we have every reason to expect to find new things, which is what you need to help drive a theory that's measurably different from our present one.
Of course we never know what we'll find, but it would be hard to build any sort of intermediate-sized collider, which would cost insane amounts of money, and theory predicts that it wouldn't find anything of value unless it were even bigger. It could be even worse; they might not find anything for a few more orders of magnitude, at which point they'd be probing not just the strong, weak, and electrical forces, but also gravity. We know for certain that the theory breaks down there, but the amount of energy required to probe the breakdown is simply ludicrous.
There is a good reason for that - there is no supporting evidence and, in fact, very strong evidence suggesting that it is completely wrong...but that's what you get with 'startswithabang', it usually ends with a whimper. The one of the most damning bits of evidence that there is something well before 10^19 GeV (no clue where he gets the 1^8 TeV figure from) is that the Higgs mass 125 GeV/c^2.
Unlike every other fundamental particle the Higgs has no spin, which means it has no intrinsic angular momentum like electrons, quarks, photons etc. This has the effect that quantum corrections very strongly affect its mass. In fact these corrections apply to the square of the Higgs mass and grow as the square of the energy scale so if the Standard Model is good up to the Planck scale at 10^19 GeV these corrections are of the order of 10^38 in size. Each Standard Model particle has its own correction to the Higgs mass with fermions and bosons providing opposite sign corrections.
Here is the problem though. In the Standard Model there is no symmetry between fermions and bosons and the coupling to the Higgs field, which determines these corrections, are all free parameters. So if we believe that there is nothing but the Standard Model before the Planck scale then we have an amazing co-incidence that a series of essentially random terms each of order 10^38 cancel so precisely that the remainder is of order 10^4.
To put that in context it would be like tossing a coin about 100 billion times and getting heads every single time. I don't know about you but personally I would start getting suspicious that something was fixing the result sometime around toss 100.
This is the issue with the Standard Model: the fact that there is a Higgs at 125 GeV is like the 100 billion coin tosses all coming up heads. The problem is that we do not yet know how nature is fixing the result but it does mean that the new physics required to fix it most likely occurs below ~10 TeV. While this is not a hard limit the higher in energy you go the less natural any accidental cancellation will be so really the energy limit where you expect new physics depends on how many times you can toss a coin and get heads before you believe that something is fixing the result.
No application we can think of. That's like someone mocking the guys making frogs' legs jump with electrical current in the 18th century. "Oh yes, very interesting, but so what?" And yet, within a half a century or so of those first gimmicky experiments with electricity, we had built the first high speed data network in history, revolutionizing, well, just about everything, and within a few decades of that we were replacing gas lights with light bulbs, people were using welding machines to build large steel structures and that changed, well, everything.
There really is no way you can stick a long term price tag on basic research. Right now, figuring out what lies beyond the Standard Model is an interesting abstraction. But in fifty years, or a hundred years of us cracking that code, who the hell knows what we'll be building? Exotic materials, new propulsion systems, new communications systems, who knows? If the last five hundred years of scientific research has taught us anything, it's that science is the field out of which technical innovation is grows, and basic research is the fertilizer.
The world's burning. Moped Jesus spotted on I50. Details at 11.
but the short lifetime of the muon has kept anyone from coming up with a workable proposal so far.
The other problem they had with the muon accelerator proposals which Fermilab looked at a while ago was the lethal amounts of neutrino radiation from muons decaying. While neutrinos rarely interact at energies below a PeV if you get enough of them there can be enough interactions to be dangerous if a human stood in the beam and unfortunately shielding really isn't an option with neutrinos.
Actually, you have that a bit backwards. The Standard Model says we're done finding new particles. The Higgs was the last one we expected to find, and it was so necessary to the theory that we could describe all of its attributes long before we actually found it. When we did, it matched the theory perfectly -- too perfectly. We knew its mass, spin, decay rate, and interaction with other particles just from the math before we even found it in the lab. Physicists were both relieved and saddened by the discovery as it meant the standard model was correct and there were no new physics to be found.
It's the idea of finding new particles that is all supposition. We know the standard model can't explain everything, but we don't know that missing particles are the solution. We also don't know how to detect those new particles if they do exist. Gravitons, sterile neutrinos, and black matter particles (whatever those may be) would be electrically neutral and barely interact with anything -- much less a particle detector. We suspect we will be able to detect them indirectly if they exist at all. There is a slim chance that there may be more than one type of Higgs, but other Higgs are not necessary for the theory to work and other Higgs would be at much higher energy levels.
You are correct that no one knows for certain -- that's the whole reason they conduct the experiments. But, the very well known math and theory strongly suggest that we're done. It's the wild supposition arguments that hope there's something more.
And that's not because they don't WANT to find new physics... it's just... quantum mechanics and particle physics are so well understood that it would be extremely surprising to find other fundamental particles -- b/c if they exist, they must be very very weakly interacting with all the known particles or at least very short lived to not cause chaos with the currently understood theory.