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 Cents," the PBS digital series
about personal finance.
Financial experts Phillip Olson and Julia Lorenz-Olson
guide you through the complex world
of personal finance from the kitchen table to the New York
Stock Exchange.
You'll get practical knowledge about how
to spend, save, and earn, and insight
into how your brain is hardwired to react to economic problems.
If you have any money and 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.