Thank you to NORD VPN for supporting PBS. What does the strong nuclear force, the fundamental
symmetries of nature, and a laundry detergent have in common? Well, they’re all important parts
of the tale of the axion - a tale which may take us beyond the standard model and
solve one of the most vexing mysteries in astrophysics. The history of the axion is a classic physics
tale: intrepid scientists delve deep into trackless mathematics in search of answers
to a mystery. And there, against all expectations, they find the hint of a completely new and
unexpected denizen of the natural world. In this case the mystery was a subtle inconsistency
in the behavior of the fundamental forces. And the unexpected discovery? A brand new
particle - the axion - which, while not yet proven to exist, may explain a much more famous conundrum. The axion may explain dark matter. To understand the origins of the axion we
need to go back and look at one of the most powerful concepts in physics: symmetry. We
expect the laws of physics to be symmetric with respect to certain properties of the
universe. If the equations describing a given physical process do not change when you transform
a particular property of the universe, we say that process is symmetric to that transformation. For example,
most of physics is symmetric to mirror reflections - the world mostly works the same when you
flip the sign of the x, y, and z axes. Another example is flipping the charges of particles
- positive to negative and vice versa - most of the equations of physics hold under that
flip. But not all. In a previous episode we talked about how the weak nuclear force is
NOT symmetric under combined charge and mirror inversion and - or in physics-speak, the weak
force is not CP symmetric - its behavior changes if you flip charges and do a mirror or parity
reflection. Okay, so what has all this got to do with
axions? Well hold on, we’ll get there. Given CP violation in the weak force it’s natural
to ask if it happens in the strong nuclear force also. The strong force is the fundamental
force that binds quarks together into protons and neutrons, and is mediated by the gluon
particle. Our best theoretical description of the strong
force is quantum chromodynamics - QCD. That’s a deep and rich subject that will get its
own episode before long. For now it’s enough to know that the equations of motion of the
strong force, derived with QCD, actually allow violation of CP symmetry. In fact they almost
demand it - and yet no such violation has ever been observed. Here’s one example - if the strong
force is CP violating, it’s predicted that the neutron should exhibit an electric field
like you’d get from a pair of positive and negative charges - an electric dipole field.
Our very sensitive measurements have found that no such field exists- or if it is there
then it’s a trillion times weaker than predicted by a CP-violating QCD. This
discrepancy between theory and experiment is known as the strong CP problem and is currently
unsolved. One of the a proposed solutions to the strong CP problem is going to give us the axion. But before we can see how that happens, we
need to understand why quantum chromodynamics predicts a CP violation in the first place.
Compared to quantum electrodynamics, which describes electromagnetism, QCD is complicated
to say the least. For one thing, the vacuum in QCD is full of
weird structure. You might ask how can a vacuum, aka “nothing” have structure? Well, in
quantum field theories, the vacuum isn’t really nothing. “Vacuum” is the word we
use to describe the lowest energy state of a field - which is what you’ll find when
there are no actual particles around, and as we saw in previous episodes, the vacuum is a very
lively place! Particularly in QCD, where there isn’t just one lowest energy state - there
are infinite lowest energy states. And the vacuum can hop between these different
states. But because they're all the same energy, quantum weirdness allows the QCD vacuum
to sort of simultaneously occupy all of those states at once. This bizarre structure for the vacuum
alters the equations of motion that come from QCD, new terms get added and the strength of those terms is governed by a new fundamental constant - theta. It’s tricky to describe what theta actually signifies
- in fact there are different physical interpretations - but one way to describe it is that it’s
a phase offset picked up by the quantum field as it moves between the different possible
minimum energy states of the vacuum. And it’s these new terms - the ones added
by the weird vacuum - that appear to violate CP symmetry. That means the strong force should
show CP violation. So why doesn’t it? One possible explanation is that this theta value
- the constant in front of the CP violating terms - is just equal to zero. That would cause
those terms to vanish. But there’s no good reason why theta should be zero - at least
not within the standard model of particle physics. This fundamental constant may have
ended up very close to zero just by chance, but physicists hate using random chance to
explain the precise refinement of a value - what they call fine tuning. In 1977 Roberto
Peccei and Helen Quinn proposed another solution: what if theta isn’t a constant, but can
change in value, both over space and over time. In other words, make theta a new type
of field - a dynamic field rather than a fundamental constant. Theta will then naturally fall to
zero - because that reduces the overall energy of the vacuum, and the universe always seeks
the lowest energy configuration. By the way, there is actually another solution
to the strong CP problem its's that if any of the quarks are massless, CP
symmetry is automatically conserved. However, as far as we can tell, none of the quarks
are massless and so this solution is not generally accepted - turning theta into a quantum field
is the most promising solution. So you might recall that in quantum field
theory a particle is just an oscillation in a quantum field. So with a new field - this
theta field - we have the potential for new particles. Theta can oscillate very slightly
around its value of zero - and that oscillation gives us the axion. It was actually that of
two titans of physics - Frank Wilczek and Steven Weinberg - who first realised that
this new field could be quantised to give rise to a new particle. Wilczeck once explained how they chose the name for their new particle. They named it after axion detergent
because it seemed to ‘clean up’ the CP problem quite well. This hypothetical axion particle
would have no electric charge, no quantum spin, be extreme ly light - a tiny fraction
of the mass of the already tiny electron. It would interact very weakly via the strong
and weak nuclear forces, and via gravity. So how can we expect to detect such an elusive particles? Well, even though axions have no electric charge, they can still interact with the electromagnetic
field and produce photons via the strong force. They would do this by generating pairs of
virtual quarks which then decay into photons - the so-called Primakoff effect. This would
look like an axion turning into a photon - typically in the presence of a strong magnetic field.
And photons can turn into axions in a similar way. And this actually gives us an experiment. It should be possible to shine a light through a solid, opaque wall. It goes like this: a light is
passed through a strong magnetic field and then blocked by a metal wall. But some photons
get converted to axions in the field, and so pass directly through the wall before turning
into photons again, where they can be detected. At least in theory - so far several experiments
have not confirmed axions this way, at least so far. One issue may be that we just can’t make
sufficiently strong artificial magnetic fields. So why not let nature do at least half of
the heavy lifting? Well that's precisely what the CERN Axion Solar Telescope (CAST) in Switzerland
does. If axions exist then they should be produced in reasonable quantities in the core
of the sun. There, X-rays are constantly bouncing off electrons and protons in the presence
of strong electromagnetic fields. Perfect conditions for producing axions, among other
things. So the Sun’s core may spew out countless axions. CAST forms the detector part of the
apparatus and uses strong magnetic fields of its own to try to turn those axions back
into detectable photons. No luck yet, but the range of possible properties of axions
is being narrowed down. There are other spacey tests for axions. Magnetars
- highly magnetic pulsars - and quasars may convert some of their own gamma ray output
into axions - and that dip in gamma ray output may be measurable. In a separate phenomenon,
there does appear to be a slight overabundance of gamma rays from very distant astrophysical
sources like blazars. A lot of those gamma rays should be absorbed traveling through
the vast, not-quite-empty tracks of intergalactic space on their way to us. It appears that
too few of those gamma rays are absorbed, and so some astrophysicists have hypothesized that some gamma rays get
converted back and forth between axions and photons by the magnetic fields of entire galaxies.
That makes them invisible for part of their journey, so less likely to be blocked. Experiments thus far have not given a reliable
positive results, but it may be that axions are just lighter or more weakly interacting
than we think and so not detectable by current experiments. But new experiments and upgrades
of existing experiments will whittle away the parameter space of possible axion properties
- and eventually we’ll either spot it or decide it’s a lost cause. This all seems like a lot of work for a hypothetical
particle predicted from speculative math. But there’s a good reason for the effort:
axions may be ... dark matter. They have all the right properties - no direction interaction
with light, and only weak interactions via the other forces. And although these particles
are extremely light, axions, if they exist, are likely to have been produced in prodigious
numbers in the Big Bang. That means there could be enough of them to explain the invisible
source of gravity that seems to dominate the universe - what we call dark matter. If axions
do turn out to be real they may clean up two of the most vexing problems in modern physics
- the strong CP problem AND the nature of dark matter. Not bad for one of the tiniest
and most elusive potential particles in all of spacetime. We’d like to thank NORD VPN for supporting
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To learn more about NordVPN, just check out the link in the description. Last week we tried to figure out whether a
universe that is infinite in size should contain infinite repetitions of everything it contains,
including you. Infinite comments ensued, of which I'll now answer a finite number. Revolver Ocelot pointed out a Medium article
by Ethan Seigel that claimed that even if inflation proceded for the entire age of the
universe, the universe would still not be large enough to get duplicate regions. I want
to thank you for giving me the opportunite to pick a fight with Ethan Seigel, who is
almost always right. Almost. Ethan calculates that if the universe was inflating for its entire current age, than after those 13.5 billion years it would be around 10^10^50 times the size of our observable universe. And that sounds about right. He then compares that
to an estimate of the number of possible configurations of particles in the universe - 10^90 factorial.
Now, I assume Ethan got that number by imagining swapping around every particle in the universe in all
possible combinations. Well, many of those particles are indistinguishable from each
other - swapping two electrons or two photons doesn't change anything, so that number might be an over estimate. But at any rate, 10^90 factorial is around 10^10^90 - within a few orders of
magnitude. And I actually gave a larger number - the universe's Bekenstein bound - which is around 10^124
- giving something like 10^10^123 configurations. At any rate, it would therefore take way longer than the age of the universe for inflation to make a volume 10^10^90 or 123 times our obserable universe. So Eathan is right there. But inflation would get you there eventually as long as it never ended - and that's pretty
much the definition of eternal inflation. Ethan also points out that eternal inflation
must have had a beginning, so it hasn't yet lasted for infinite time - well also true, but
there's no reason to imagine we are near that beginning, and actually 13.5 billion years would be pretty close the beginning of an infinitely-long-lasting eternal inflation. Of course all of this is assuming that the universe is spatially finite. The simplest interpetation of the cosmological
equations derived from general relativity tell us that it it may be truly spatially infinite - in which case 10^10^90 is a piece of cake. Paul Gaither asks how far away are these duplicate
universes. Very, very far away. Roughly speaking you'd need to travel all the way across a
greater universe that is itself large enough to fit all possible configurations of observable universe sized regions. The
volume would want to be 10^10^123 (or 90) times our universe's volume, so the distance is the square root of that.
Let's say 10^10^60-ish times the diameter of our universe until things start to
repeat again. In last week's comment responses I corrected
my pronundiation for Newton's book - and then lots of people corrected my correction. Principia
Mathematica, Prinkipia mathematica, princhipia mathematica - many people swearing by each
of these pronunciations. So from now on it's "that thing Newton wrote". Anyway, it seems like some of you
are, let's say... quite detail-oriented. And so I was terrified you'd be mad that the infinite
typewriter monkey we showed was actually a chimpanzee - so, a great ape ... totally different
thing to a monkey. But no one even commented on it. C'mon guys, if we're gonna nerd, why
not nerd all the way?
a potential issue with (or test for) axion=dark matter
if dark matter could turn into photons in the presence of a strong magnetic field I would expect to observe a faint glow without an apparent source from any strong magnetic field, including for example a point in empty space beside a magnetar.
another potential method to detect axions occurs to me.
they are described as having very low mass, but as this isn't zero mass they must travel slower than the speed of light. so when a photon turns into an axion and then back into a photon it will arrive late. this could be detected as an afterglow from distant objects that would be expected to end abruptly.
I kinda think axions might turn out to be the time particle. And as such may not be able to be detected by our instruments looking for energy/matter