
No Proton Decay Means Grand Unification Must Wait - aburan28
https://www.quantamagazine.org/20161215-proton-decay-grand-unification/
======
gus_massa
Small technical correction:

> _For instance, SU(5) groups quarks and antiquarks together with leptons and
> antileptons into “fiveplets,” which are like the indistinguishable sides of
> a regular pentagon._

The idea is to put the 5 particles in 5 places that are undistinguishable.

For that you need to use the vertex corners of an hyper-tetraedrum (
[https://en.wikipedia.org/wiki/5-cell](https://en.wikipedia.org/wiki/5-cell)
). Don't get confused by the bad drawings, if you have one of them in 4
dimensions, you can put each point at the same distance of all the other
points.

If you use a regular pentagon, then you must select an order for each of the
particles/vertex. If you select one, some are more close than the others.

(An alternative is to use a pentagon, but consider not only the rotations and
flip, but also the operations that mix the vertex/particles in any order. But
then the nice identification with the symmetry of the geometric figure is
gone. You can use a square with the central point.)

~~~
lambdasquirrel
The talk of simple geometry reminds me of the Ptolemaic universe for some
reason. Layman's comment, I know, but I just couldn't resist.

~~~
gus_massa
In some sense, yes. I remember the class were my physics professor explained
why the SU(5) could be The Group of the universe and why it failed (IIRC the
predictions were 1% off with the experiment, a good try, but enough to be
discarded.) You could feel how sad he was that SU(5) has to be discarded. :(

In some sense, no. The SU(5) group includes all the symmetries of the hyper-
dodecahedron, were you can rotate it in the 4-dimmensional space to exchange
one of the vertex/particles with another. But it also includes more strange
things, like half mixing two particles.

    
    
      a -> (a+b) / srqt(2)
      b -> (a-b) / sqrt(2)
    

This is more difficult to explain. Technically, it's a rotation in the plane
x-y of 45 degrees. And you can rotate another angles, for example in some
particles decays the important rotation is 13 degrees. And you can mix three
or four or five particles. All of this is more difficult to imagine, but it's
easy to write analytically.

The use of geometric shapes like the hyper-dodecahedron is more a nice
visualization technique. It's easier to explain than the details of the SU(5)
group and it provides a good intuition, even after studding the theory with
more details. So I prefer to ignore this technical detail, specially for a
popular science article.

------
goalieca
Modelling is an interesting thing. Our math is such an elegant language that
it can describe such amazingly rich abstractions. On one hand, we have
category theorists and computer scientists working on the more familiar
modelling of computational patterns. On the other, we have physicists coming
up with all kinds of equally curious patterns in particle fields. It seems
many of the patterns are just abstract nonsense but there are too many
coincidences to ignore. To me this hints of some underlying structure that we
are completely blind to still. Oh, how i wish to study math from the future.

~~~
HiroshiSan
Study math now! With reverse ageing in the future you could potentially become
one of the best in the field. There is time.

~~~
CamperBob2
Sounds like an _excellent_ excuse to not study math now.

~~~
HiroshiSan
Aaah...procrastination. "I'll just wait t'ill I'm 150."

~~~
pavel_lishin
Reminds me of some lyrics I like:

 _Throw it away, what 's left to need_

 _When you’ve got infinity_

 _So much out there left to find_

 _We 'll do that some other time_

------
hashhar
I would like to take a moment to praise Quanta Magazine. They really are the
one magazine that got me reading all forms of long form articles about science
which if it were written plainly would go high above my head and/or would seem
boring.

~~~
bvv
Quanta Magazine has also a good reputation among many theoretical physicists
including myself. They of course do suffer from the occasional misconception,
but as a whole the accuracy of their reporting is leaps and bounds above many
other popular science sites or blogs.

~~~
hashhar
Lifehacker introduced me to them and I've been a happy reader since then.

The thing I like the most is that don't play the "analogy game" too much and
instead generally teach a small concept and then build up on it.

While other popular media outlets like Wired or Verge simply dumb it down too
much or are factually incorrect. I do like WIRED's Science Blogs though even
though they are a little bit inactive.

------
jeebers
The article can be summarized by this line:

"Japan is considering building a $1 billion detector called Hyper-Kamiokande,
which would be between eight and 17 times bigger than Super-K and would be
sensitive to proton lifetimes of 10^35 years after two decades."

All this to possibly detect a single proton decay in 20 years. Now that's some
serious commitment!

~~~
sanxiyn
If it were solely to detect proton decay, yes, I agree it would be very
impressive. The reality is a bit more prosaic though, as the article suggests
in passing:

"In the meantime, a Nobel Prize has been won for a different discovery in the
cathedral-esque water tank pertaining to particles called neutrinos."

Hyper-K is a dual-purpose detector, a neutrino detector as well as a proton
decay detector. Substantial part of Hyper-K funding should be interpreted as
investment to a neutrino detector.

------
xefer
This is an question I've always had about proton decay: if virtual particles
can spontaneously appear anywhere at anytime, why couldn't some virtual quark
appear in the midst of the three that make up the proton causing it to fall
apart? What keeps The constituent components of a proton immune from this
behavior?

~~~
walrus1066
You can't have a single virtual particle appearing, it's always two, due to
conservation of momentum, charge etc

So it'd be a quark-antiquark pair popping up.

The proton is the lightest Baryon (3 quark particle), there is no lighter
Baryon it could decay into. The decay products have to be lighter than the
original proton, by at least the mass of the virtual quark pair, to repay the
energy 'borrowed' from the vacuum to create the virtual quark pair (because
energy is always conserved). So the proton remains unaffected by the virtual
particles popping in and out of existence around it. The virtual particles
have no choice but to effectively to annihilate with one another and
disappear, to pay back the energy debt.

Heavier Baryons (Sigmas, Lambdas) are indeed destabilized by virtual quark
pairs, that is the mechanism by which they decay, almost instantaneously, on
their own.

You could have an up-anti up quark pair that pops up close to the up quark of
the proton, the up quarks could 'swap places', and then the up of the proton
annihilates with the anti up of the virtual quark pair, but the the result is
still a proton.

~~~
walrus1066
r.e. Heavy baryon decay, here is a feynmann diagram showing a delta baryon
decaying into a proton and pion. The down + anti-down quark pair that appears
in the middle of the diagram are virtual particles:

[https://ned.ipac.caltech.edu/level5/Cottingham/Figures/figur...](https://ned.ipac.caltech.edu/level5/Cottingham/Figures/figure1_3.jpeg)

------
maxander
The thing that pop-science stories about GUT always seem to fail to explain
is- what does it _mean_ for a symmetry to break at a certain point in time? In
the present day physical symmetries (isotopy of space, say, or conservation
laws) are just static laws of nature; what was going on in the first
microseconds that could mess with the laws of physics itself?

...I'm sure my question contains within it at least several misconceptions,
but let that just be an illustration of how confused this kind of article
leaves laypeople.

~~~
cohomologo
The symmetry breaking should be thought of as a phase transition that occurs
as the temperature of the universe changes, like liquid freezing and becoming
ice. The universe was initially very hot, but rapidly cooled down as it
expanded and went through phase transitions when it passed the "freezing
temperature", i.e the temperature at which the laws of physics prefer to
spontaneously break the symmetry.

~~~
rwallace
I've always been curious about that as well. Suppose you took a small region
of the universe and reheated it to be unification temperature today,
presumably you would restore the symmetry, but presumably when you let it cool
down, the symmetry would _not_ randomly break in a different way, but
deterministically in the same way; isn't that a way the analogy does not hold?

To get random breaking in a different way, presumably no amount of mere
heating of matter would suffice; you would have to somehow restore the high-
energy false vacuum of the Big Bang itself? I don't suppose there's any way to
do that in today's universe, even in principle?

~~~
JumpCrisscross
> _Suppose you took a small region of the universe and reheated it to be
> unification temperature today...I don 't suppose there's any way to do
> that...?_

You're describing a particle accelerator. When we talk about the LHC accessing
"higher energies" than the Tevatron [1] we are saying it is "baking" small
parts of the universe to higher and higher temperatures.

[1]
[http://blogs.discovermagazine.com/cosmicvariance/2009/11/30/...](http://blogs.discovermagazine.com/cosmicvariance/2009/11/30/the-
lhc-surpasses-the-tevatron-in-energy-but-what-about-power/)

~~~
rwallace
Yep. But even if we could build a particle accelerator capable of reaching GUT
energy level, it wouldn't spawn new universes, would it?

------
mirimir
> ... researchers have found a variety of other symmetry groups that the
> existing particles might fit into, with extra features and variables that
> can make protons decay much more slowly.

Well, "extra ... variables that can make ..." is often a sign of EOL
desperation for theories. What I got from Kuhn, anyway.

------
Manglano
Assume the physical universe takes the form of a set of axiomatic systems
whose statements are elementary particles and whose interactions are logical
operations; i.e., metals in the presence of ions will form salts. Matter
creates the gravitational interaction as a function of mass.

Why do we assume the theories are unifiable?

~~~
just2n
I think in an effort to understand exactly how the mechanics of the big bang
worked and how you can go from an infinitesimal point of incredible energy to
an enormous and expansive universe with the diverse laws we observe today.

From the article:

> If the forces were indeed one during the “grand unification epoch” of the
> universe’s first trillionth of a trillionth of a trillionth of a second,
> then particles that now have distinct responses to the three forces would
> then have been symmetric and interchangeable, like facets of a crystal. As
> the universe cooled, these symmetries would have broken, like a crystal
> shattering, introducing distinct particles and the complexity seen in the
> universe today.

~~~
Manglano
We assume the precondition that the Universe we experience and observe
originated in a vacuum of...existences, for lack of a better term. This seems
myopic, why should it be so?

What if the universe we experience is the product of interactions between
distinct universes, which circumscribed by different physical laws, and the
only universe we can sufficiently experience and observe is that which is
governed by the Strong Nuclear Force?

~~~
bmh100
It could very well be that way, but what is the evidence for that origin? Why
not any number of other plausible explanations? It is not myopic to accept the
explanation best supported by available evidence, until sufficient new
evidence supports a different explanation. Right now, the Universe coming into
existence as a solitary singularity is the best available theory.

------
peter303
Cross out the Third (of five) Age of the universe in the 1999. Book of that
name. That was era dominated by degerate stars untils all bayrons decay. Then
the universe would be dominated by evaporating black holes.
[https://en.m.wikipedia.org/wiki/The_Five_Ages_of_the_Univers...](https://en.m.wikipedia.org/wiki/The_Five_Ages_of_the_Universe)

------
donretag
Can someone please explain the role of the tank in the detection of proton
decay? Why is water the best solution to use?

~~~
gshubert17
A neutrino interaction with the electrons or nuclei of water can produce a
charged particle that moves faster than the speed of light in water (not to be
confused with exceeding the speed of light in a vacuum). This creates a cone
of light known as Cherenkov radiation, which is the optical equivalent to a
sonic boom. The Cherenkov light is projected as a ring on the wall of the
detector and recorded . . .

From: [https://en.wikipedia.org/wiki/Super-
Kamiokande#Description](https://en.wikipedia.org/wiki/Super-
Kamiokande#Description)

~~~
donretag
Excellent. So the role of the tank is detection and not a means to accelerate
proton decay?

