With the Large Hadron
Collider running out of places to look for clues to a
deeper theory of physics, we're going to need a
bigger particle accelerator. And we have one, the galaxy. And the particles the
galaxy flings at us may have finally
revealed particles beyond the standard model. Physics is currently
in a weird place. Historically, no matter
how crazy our theories got, there were always new
ways to test them. Your theory predicts
a new particle. Build a particle accelerator
big enough to see it. But once your collider
spans entire countries, like the Large Hadron
Collider in Switzerland, there's only so much
larger you can go, at least on the surface of the Earth. The LHC has thoroughly
tested the standard model of particle physics. The last component of that
model, the Higgs boson, was verified in 2013. But the standard model
isn't the end of the story. There must be a more
fundamental theory that explains the origin of
this rich family of particles. Proposals for such
grand unified theories proliferate unconstrained
by even the tiniest hint of new physics from the LHC. One potentially very
important ingredient for grand unification
theories is supersymmetry. This is one that
physics had really hoped to nail down with
the Large Hadron Collider. SUSY is a proposed extension
to the standard model, designed to fix certain issues with the
theory, the most serious issue being that the standard
model can't explain the minuscule
weakness of gravity compared to the
other three forces. This is the hierarchy problem. SUSY provides a very natural
explanation for the discrepancy by introducing a new symmetry
between the fermions, which comprise matter, and the
bosons, which communicate the fundamental forces. As well as fixing the
hierarchy problem, this connection between
fermions and bosons is, in general, a step
towards unifying the particles of the standard model. It's a key feature in some
grand unified theories as well as modern string
theory, leveling it up to superstring theory. Supersymmetry predicts that
every single standard model of particle has a
supersymmetric partner particle of the opposite type. The partners of
fermions are bosons and the partners of
bosons are fermions. We'll come back to all of
this in detail another time. But the one to one
property that's relevant for today's
episode is that these supersymmetric
particles are all expected to be way more
massive than their known partners in the standard model. To solve the hierarchy
problem perfectly, those particles would need
to have masses at around what we call the electroweak energy. That's the energy at which
the electromagnetic and weak nuclear forces merge
into the same force. Physicists had hoped that,
by smashing particles together hard enough in
the Large Hadron Collider, there'd be enough energy
in those collisions to produce a
supersymmetric particle. And in fact, there
should have been, at least for the
versions of SUSY that most neatly solved
the hierarchy problem. But the LHC has seen nothing. This doesn't necessarily
kill supersymmetry. It may just be that
these new particles are way more massive than expected. If so, they can still help
with the hierarchy problem, though not as neatly
as we had hoped. To detect more massive
supersymmetric particles, you need higher energy
particle collisions. So what? Build an accelerator the size
of the planet, the solar system, give up and let theorists
just tell their stories? Actually, there is a
way to probe energies far higher than is possible
with the Large Hadron Collider. The universe itself is a pretty
good particle accelerator. Supernova explosions,
gamma ray bursts, black hole magnetic
fields are all expected to blast
high energy particles like electrons and atomic
nuclei into the universe. These are cosmic rays. The highest energy
cosmic rays can have energies around a
billion times that of the LHC. Unfortunately, for particle
physics experiments cosmic rays at these
energies are extremely rare. So it's not surprising
that we haven't seen supersymmetric particles
in our cosmic ray observations yet, or have we? Let's talk about this thing. No, it's not a downed
imperial probe droid. Your rebel base is safe. This is ANITA, the Antarctic
impulsive transient antenna. It's a cosmic ray experiment
of a very special sort. In fact, it's a
cosmic ray detector disguised as a neutrino detector
disguised as a radio antenna disguised as a hot air balloon. Probably I should explain that. When ultra high energy cosmic
rays travel through space, they bump into the photons
of the cosmic microwave background. That's the leftover heat glow
of the very early universe. Those cosmic rays lose
energy to the CMB, which is partly why the most
energetic cosmic rays are so rare here on earth. But in those
interactions, cosmic rays can create extremely
high energy neutrinos. Neutrinos are almost ghost
like particles that travel through the CMB unimpeded. So detecting the
highest energy neutrinos allows us to learn about the
cosmic rays that produced them. These neutrinos don't
just ignore the CMB. They can pass
through solid matter. Lower energy neutrinos can
flow right through the Earth as though it isn't there. We detect neutrinos
because very, very rarely one will interact
with an atomic nucleus and produce a
shower of particles. For example, the
IceCube Observatory is a one kilometer cube
of the Antarctic glacier laced with photon detectors. It spots neutrinos when
they're decayed or electrons, muons, or tau particles, which
in turn produce visible light as they streak through the ice. This is Cherenkov radiation. And IceCube's photo detectors
track these flashes. ANITA works in a
similar way, but it's focused on catching
the very highest energy neutrinos, the ones that
are produced by cosmic ray interactions with the CMB. In order to see those
extremely rare neutrinos, ANITA scans not one
cubic kilometer of ice but 15 million square kilometers
of Antarctic ice sheet. That's where the
balloon comes in. ANITA is a cluster
of radio antennae that hovers 37 kilometers
above Antarctica. If an ultra high
energy neutrino decays in the ice anywhere within
700 kilometers of ANITA, the resulting radio
frequency Cherenkov can be seen by ANITA'S antennae. ANITA is designed
to detect neutrinos that are coming from below,
passing through the earth into the ice sheet. That allows it to sort
out neutrino radio flashes from the flashes produced by
other cosmic rays coming in from above. In fact, ANITA expects to see
its most interesting, most energetic neutrinos
coming in at an angle, skimming the arc of the earth
on a shallow trajectory. They should not come
from directly below, which would require them to
pass through the entire planet. That's because the most
energetic neutrinos actually do lose energy passing
through the earth. They aren't expected to
make it all the way through without slowing
down significantly. So you can imagine that
ANITA scientists were a little confused
when they spotted two extremely high energy
radio bursts that could only have been produced by a
high energy particle passing all the way through the
middle of the planet. That's several
thousand kilometers of rock, magma, and iron. Ignoring the ridiculous
distance they traveled, these events look
like what you'd expect when a particular flavor
of neutrino, tau neutrino, interacts with the ice and
transforms into a tau lepton. And that's the heavier
cousin to the electron. The tau is cool because
it's so short lived. It produces a
Cherenkov burst when it's created and then
a second burst when it decays into a shower
of secondary particles. But seeing these very
high energy tau events from directly below
doesn't make sense. Based on our understanding
of the normal background rate of high energy
neutrinos, it's estimated that there is around
a 1 in 3 trillion chance that two tau
neutrinos could have been seen in the amount of
time ANITA has been looking. Physicists are having
trouble accounting for these events with
any known standard model particle, which brings
us back to supersymmetry. Astrophysicist Derek Fox,
Steinn Sigurdsson, and team point out that there's a
version of supersymmetry that predicts exactly the
right particle to do this job. It's the supersymmetric
partner of the tau lepton, the stau particle. Yeah. You put an S in front to
get the SUSY particle. Selectron, squark, stau. Supersymmetry is super easy. Here's the scenario. A stau particle was produced on
the opposite side of the planet by an incoming ultra
high energy neutrino plowing into the earth. The stau is theoretically
capable of zipping straight through the earth before
decaying into a regular tau lepton on the other side. This then causes a
high energy radio flash coming from directly below. That's quite a story. But it fits the
observations pretty well. There are, of course,
other possibilities. It could be a so-called
sterile neutrino. Now we talked about that before. This particle is also
not in the standard model but has nothing to do
with supersymmetry. Hints of its existence have been
found in the Fermilab particle accelerator experiments. It may also be that there
were some gigantic bursts of regular neutrinos at the
time of the observed events. Hit the Earth with enough
high energy neutrinos, for example from a
supernova explosion, and at least some of the
ultra high energy ones could make it through. In fact, one of
the two events may have been associated with
a distant supernova that was observed around the
same time and location. The probability of a chance
association with a supernova is around 3%, so it's
unlikely but it does happen. On the other hand, the
supernova in question wasn't nearly bright enough
to make even one earth penetrating ultra high
energy neutrino likely. And remember, there were two
events at different times. The other event wasn't
associated with any supernova or gamma ray burst. The final possibility is just
that we're missing something. Perhaps our understanding
of neutrino propagation through the earth is
flawed, or perhaps penguins use cell phones now. This is going to require more
observation and confirmation. The first step would be to
look at the other big neutrino observatories. IceCube is really
the only one that could have potentially
spotted similar events. Actually, given
the amount of time IceCube has been in operation,
it probably should have. Fox and team looked back
into the IceCube archive and actually did find some
possible high energy tau lepton events that may have
come from directly below. The data is a little ambiguous. They may have been boring or
muon neutrinos that can easily pass through the planet. But you can bet
people will be paying a lot more attention to these
sorts of events from now on. So have we gone beyond
the standard model and proved supersymmetry? Hell no. What we have is that
tantalizing hint. Given the painful absence of new
particles from the Large Hadron Collider, any hint
of something new is bound to get
physicists excited. ANITA will keep flying. IceCube will start carefully
scrutinizing its data. And if these mysterious events
from below keep arriving, you can be sure that physicists
will find an ingenious way to confirm their nature. Perhaps we will
verify the existence of the stau and with it confirm
the supersymmetric nature of space time. Before we get to
comments, I just want to let you know about "Two
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might like to have some more, check the link in
the description below to subscribe to "Two Cents." Last week, we talked
about why string theory is so compelling to
so many physicists. Now before I get to
specific comments, let me address a
common objection. Many of you have some
interestingly passionate hate for string theory on
the grounds that it's so far proved unfalsifiable. Some of you say that
means it's not science. Now I'm not sure
I agree with you. String theory may be
wrong, but it's not testable due to the limits
of current facilities. Does that mean it's not science? Well, the universe
has no obligation to operate in a way
that is currently testable by any particular
size particle accelerator, nor does it have any
obligation to be simple enough to be deduced by even the
smartest primates in one generation, or in 100. The thing that makes
string theory less sciencey is that its modern
version, M-theory, is not particularly well-defined. But again, that doesn't
mean it's wrong. Wolfgang Pauli's "not
even wrong" might be a better description. String theory is not
precise enough yet to be confirmed wrong, which
means it might be right or not. Eddie Galtech asks why you
can't get meaningful energy from the Casimir effect. Well, it's the same reason you
can't get continuous energy for a ball falling off a roof. You get the kinetic
energy once, but then you have to expend at least as much
energy raising the ball back to the top of the roof. Same with the Casimir effect. The Casimir plates
pull together, giving you a very
tiny amount of energy. But to get more energy,
you have to pull them apart again, which takes
just as much energy as you got when
they fell together. Don C asks a good one. To quote, "I thought one of
the fundamental properties of the strings was that
the maths only works if they are one dimensional. So how can you
get world sheets?" Yes, it is a pain keeping
the number of dimensions straight in string theory. While invariance only works
for the 2D world sheet, which has one dimension of
space and one of time, this is the shape traced out by
a string moving through time. A 2D world sheet has to be
traced by a 1D physical object. So yeah, the math
of string theory only works for 1D
objects because these trace 2D world sheets. Some of you recalled
a recent episode in which we talked about a
study of gravitational waves that appears to refute the
idea of extra dimensions. Good memory. Actually, this study
was specifically evidence against the idea of
extra extended dimensions. One explanation for
the unusual weakness of the gravitational
force is that there's an extra special
dimension that has the same scale as
the familiar three, so a 4d space in which we live
on an embedded 3D manifold called a brain. Gravity then leaks into
the extra fourth dimension, causing it to weaken. But this has no bearing on the
compactified extra dimensions of string theory. Those dimensions
are tiny in extent and they're coiled on themselves
so there is nowhere for gravity to escape into. It may have implications for the
single extra extended spatial dimension of M-theory, but I
need to research that more. Olivier Westerhide
asked if we can make an update
video about Oumuamua given the new findings. Well, yes, actually. In fact, we're just
about to shoot it after these comment responses. To prove it, I won't
even change this shirt. And to end on a
funny, Dash to the Max points out that string
theory is literally like playing a sad song on
the world's smallest violin. This is so poetic. And if it proves
wrong in the end, at least we have the
perfect instrument for a requiem to string theory.
does matt read comments on here too?
Any comment on this article?
Infinite-dimensional symmetry opens up possibility of a new physics—and new particles
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