Physicists have failed to find disintegrating protons, throwing into limbo the beloved theory that the forces of nature were unified at the beginning of time.
For 20 years, physicists in Japan have monitored a 13-story-tall tank of pure water cloistered deep inside an abandoned zinc mine, hoping to see protons in the water spontaneously fall apart. In the meantime, a Nobel Prize has been won for a different discovery in the cathedral-esque water tank pertaining to particles called neutrinos. But the team looking for proton decays — events that would confirm that three of the four forces of nature split off from a single, fundamental force at the beginning of time — is still waiting.
"So far, we never see this proton decay evidence," said Makoto Miura of the University of Tokyo, who leads the Super-Kamiokande experiment's proton decay search team.
Different "grand unified theories" or "GUTs" tying together the strong, weak and electromagnetic forces make a range of predictions about how long protons take to decay. Super-K's latest analysis finds that the subatomic particles must live, on average, at least 16 billion trillion trillion years, an increase from the minimum proton lifetime of 13 billion trillion trillion years that the team calculated in 2012. The findings, released in October and under review for publication in Physical Review D, rule out a greater range of the predicted proton lifetimes and leave the beloved, 1970s-era grand unification hypothesis as an unproven dream. "By far the most likely way we would ever verify this idea is proton decay," said Stephen Barr, a physicist at the University of Delaware.
Without proton decay, the evidence that the forces that govern elementary particles today are actually splinters of a single "grand unified" force is purely circumstantial: The three forces seem to converge to the same strengths when extrapolated to high energies, and their mathematical structures suggest inclusion in a larger whole, much as the shape of Earth's continents hint at the ancient supercontinent Pangea.
"You have these fragments and they fit together so perfectly," Barr said. "Most people think it can't be an accident."
[Continues...]
[Ed. additions follow.] The article proceeds to explain the background on possible models that support grand-unified theories, explains symmetry groups and interchangeability of different quark charges, as well as the potential implications depending on positive or negative findings.
For those who are unfamiliar with the topic, Wikipedia helpfully summarizes:
A Grand Unified Theory (GUT) is a model in particle physics in which at high energy, the three gauge interactions of the Standard Model which define the electromagnetic, weak, and strong interactions or forces, are merged into one single force. This unified interaction is characterized by one larger gauge symmetry and thus several force carriers, but one unified coupling constant. If Grand Unification is realized in nature, there is the possibility of a grand unification epoch in the early universe in which the fundamental forces are not yet distinct.
Are there any Soylentils who could elaborate on what the impact might be should a GUT be confirmed? Where would we go from there?
(Score: 3, Informative) by khallow on Saturday December 24 2016, @08:50AM
The way this is usually studied is via the symmetry groups (a group has a binary composition operator (ab) to get new element of group, a "no change" identity (ae)=(ea)=a, each element has an inverse, and order of composition doesn't matter a(bc)=(ab)c) of the three forces. The mathematical models of electromagnetism (EM), weak force, and strong force when taken separately have internal, reversible transformations that leave the model of the force unchanged. For example, one can shift the phase of EM waves (which basically is a transformation between electrical and magnetic forces parameterized by the coordinate of a circle) without changing the math describing the theory. This one dimensional shift becomes the symmetry of the EM force. Similar models of the weak and strong forces have more elaborate symmetry groups.
The math labels for these three symmetry groups are U(1), SU(2), and SU(3) (see here for a description of the SU category [wikipedia.org]). U(1) are all complex numbers of absolute value 1. SU(N) is all square complex matrices of N rows and columns which have a matrix multiplication inverse generated by taking the conjugate of the original matrix (transpose the matrix and then complex conjugate of every element of the matrix) with determinant 1 (the "S" or "Special" designates matrices with determinant 1).
So we have these three known symmetry groups of the three non-gravitational forces. There are multiple ways they could fit together into a larger symmetry group. For example, they could be completely independent of each other. That yields a larger symmetry group of U(1) x SU(2) x SU(3) (you're just picking a different transformation from each symmetry group and multiplying them together).
However, there are other possibilities. I'll note that a commonly considered possibility is SU(5) as part of a number of grand unified theories, but there are other choices. It has a U(1), SU(2), and SU(3) subgroup (groups in larger groups are called "subgroups"), but as promised in the beginning, there are interactions now between the three subgroups that leads to proton decay [wikipedia.org] as a possible particle interaction pathway.
Unfortunately, I don't understand the models at this point well enough to know why they're so confident that decay times in the range 10^31 to 10^39 years must be consequences of these models.
(Score: 2) by Scruffy Beard 2 on Saturday December 24 2016, @10:18AM
I don't claim to understand the models either, but I think it is kind of obvious how they got those numbers:
Given the huge number of protons they are observing, they should have seen at least one decay event by now if protons were in any way unstable.
(Score: 1) by khallow on Sunday December 25 2016, @06:24AM
(Score: 2, Insightful) by Ramze on Saturday December 24 2016, @09:09PM
I don't understand it either. I get what they're looking for, but I have no idea how these theories predict this sort of decay and on what time scale.
In the standard model, protons are as stable as a baryon can be and likely won't decay unless the big rip at the end of the universe tears them apart... or the Higgs field value changes significantly.
I can't wrap my head around this type of decay. They're saying a quark should decay into a lepton by means of one of 12 force carriers no one has ever seen. How? Why? Let's disregard the phantom force carriers for a second. What would happen to the color charge of the quark if it converted into a lepton? Would a gluon appear as a biproduct as well? If not, what happens to the other quarks in the proton that require color balance? They may have dumbed this down a bit, but their diagram just shows the quark converting into a lepton and the remaining unstable proton shattering and converting to 2 gamma rays (I assume the remaining quarks become the gamma rays.) This... does not sit well with me as the interior of a proton is much more complex than this. It's a soup of virtual quarks and gluons. There's so much energy in the virtual particles, that it's hard to define whether or not they become "real" within the proton. One could think of a proton as 3 quarks and a cloud of quark-antiquark pairs and gluons surrounding them. Considering that if you try to pull quarks apart, the energy input will create new quarks of the correct type out of the vacuum, I have a a hard time believing that if some random unknown force carrier altered a quark in a proton that there wouldn't be an immediate counter-reaction to self-correct to return the proton to its proper ground state. Things decay because they have a more stable ground state to go to. The proton is as low as it can go. The quarks are bound in a ground state. This idea that one would simply flip -- into a lepton no less -- seems absurd as it's outside the standard model, but let's go with it... sure. Maybe this GUT theory is on to something.... but... are we to believe there is no counter-reaction? Consider all the atoms in the universe. Wouldn't we see some serious lepton radiation from... everywhere if this were happening? Forget the tank of water -- look at Jupiter. Is it emitting gamma rays? If this theory were true, it should be! So should every planet in the solar system -- even the Moon and the Earth. The more mass, the more likely we should be detecting this radiation. If we aim a dish at Jupiter and pick up gamma rays, that should do it... unless there's a nuclear reactor at the heart of Jupiter, gamma rays shouldn't be present. I believe Jupiter gives off infrared, visible light, ultraviolet, and some very low energy x-rays, but not gamma rays. Why not?
Assuming this decay happens within the interior of a proton, how do they know a nearby virtual quark wouldn't absorb the energy released from the decay and itself become a real quark to replace the decayed one, essentially reversing the decay before it could be detected?
There's something seriously wrong or missing from this GUT theory. (and several others!) I'd go as far as to say I'd believe a GUT theory would be more likely to be true if it agreed with the standard model on the half-life of the proton.
I love the elegance of superstring theory, and I get that it is very compelling to make the math look beautiful and symmetric... but... 12 new force carriers and strange predictions beyond the standard model that don't seem to have any basis in reality... ugh... No matter how pretty the math may be, either the interpretation is wrong or the assumption is.
(Score: 3, Funny) by linkdude64 on Saturday December 24 2016, @09:55AM
Third Impact. Just leave it up to the kid in the robot, I'm sure we'll be fine.
(Score: 2) by Unixnut on Saturday December 24 2016, @12:27PM
> For 20 years, physicists in Japan have monitored a 13-story-tall tank of pure water cloistered deep inside an abandoned zinc mine, hoping to see protons in the water spontaneously fall apart.
I never thought I could imagine something more tedious to do than sit and watch paint dry, but physicists in Japan have managed. Thank you!
I cannot imagine how it must be when you have to explain to non-physicists what you do for a living...
(Score: 0) by Anonymous Coward on Sunday December 25 2016, @07:30AM
But they got robots to watch it dry for them.
(Score: 0) by Anonymous Coward on Saturday December 24 2016, @03:29PM
no proton decay?
maybe the universe as a whole time-travels back when a proton decays so to avoid the situation that lead to the proton decay?
thus to observer a proton decay watch for stuff repeating?