The foundations of quantum theory rest on its symmetries for example it should
be impossible to distinguish our universe from one that is a perfect
mirror opposite in charge handedness and the direction of time but one by one
these symmetries were found to be broken threatening to break all of physics along with them.
In his famous lectures on physics Richard Feynman talks about
what it means to expect the universe to be identical in the mirror
for it to be parity symmetric he invites us to imagine a clock in a mirror reflection.
Numbers of backwards components are all flipped left to right
and it ticks counterclockwise and then he asks us to imagine building that same
mirror clock in reality everything is constructed as though reflected -
numbers get painted backwards every screw with right-handed thread or right spiraling
coil is replaced with a left-handed version our intuition tells us that the
mirror clock should tick in exactly the same way except counterclockwise -
our intuition would be wrong. The laws of physics and so the laws of clocks are
not symmetric to this sort of parity transformation as we saw in our recent
episode the experiment that first proved this found that cobalt-60 nuclei decay
by splitting an electron out in the opposite direction to their nucleus
speed axis but in a mirror reflected universe the same decay should be in the
opposite direction so with that spin axis which is fundamentally different
physical behavior. So, what does this have to do with Feynman's clock? well, he
proposes a clock whose ticks are governed by the decay of cobalt-60.
Imagine an array of cobalt-60 atoms in a magnetic field, the cobalt nuclei have
angular momenta that will align with a magnetic field
let's say upwards so the decay electrons travel down.
A detector is placed to intercept those electrons and the clock ticks with every
captured electron. In our reflected clock we need to replace the cobalt atoms with
their parity inverted counterparts but now the decay electrons travel upwards
with the nuclear spin and away from the detector such a clock wouldn't tick at
all taken to its literal extreme a perfectly constructed mirror reflected
clock behaves differently so what's the big deal
well the violation of parity symmetry poses a threat to an even deeper
symmetry cpt symmetry the combined flipping of charge parity and time and
this symmetry lies at the foundations of quantum field theory physics must work
the same if we flip all of these properties if not physics as we know it
goes out the window which does seem like a big deal. To save physics,
Richard Feynman proposes that we build a copy of our clock out of antimatter.
Uh... sure, Feynman - why not! electrons become positronsquarks become anti quarks and vice-versa
sending protons and neutrons to their anti versions in the nuclei of our now
anti cobalt-60 and other anti-atoms sending matter to
antimatter is the C part of cpt charge conjugation all charges switch side
electric charge but also quark color charge weak hypercharge etc that's what
a switch to antimatter means how does this work in our Antion on a clock well
antimatter atoms have negatively charged nuclei which means their nuclear
magnetic fields point in the opposite direction to regular matter relative to
their angular momentum the magnetic field in our clock will align antimatter
nuclei in the opposite direction to matter nuclei so in our mirror reflected
antimatter clock the direction of the decay electrons are flipped once due to
the mirror reflection and once due to the switch to antimatter that leaves the
electrons traveling in the original direction down and the clock ticks as
normal so even though the universe isn't parity symmetric maybe it is
under a charge parity a CP transformation said right to left and
send matter to antimatter at first glance this CP symmetry appears to hold
not just in an imaginary clocks but also in the particles of the standard model
the great parity violating process is the weak interaction which only affects
left chiral fermions right chiral fermions don't feel the weak force at all but the
opposite is true of antimatter right chiral anti fermions fill the weak force
while left chiral anti fermions don't so a charged parity flip leaves you in the
same situation regarding the weak interaction if CP is what we call a good
symmetry then you shouldn't be able to do any experiment to tell whether you're
in this universe or in a CP transformed universe or physics should work the same
theoretically but don't get too comfortable we haven't looked at what
the experimentalists have to say about this you may recall from the parity
episode that the first hint of parity violation was the so-called tau theta
problem we've turned out to result from the fact that the positively charged
Kaon meson decayed in ways that violated parity conservation it turns out that
these Kaon particles are great at catching the universe doing weird stuff
in 1964 James Cronin and Val Fitch looked at the
outcomes of the decay of neutral Kaons these things are extra weird and usual k
on is a quantum mix of its own particle and antiparticle there are two ways to
do this mixing yielding two types of mutual Kaon one time let's call it KS
he is short-lived and has what we call an even CP state
which just means it doesn't change under a combined charge parity transformation
the other type KL is long-lived and has an odd CV state it's wave function gets
multiplied by -1 on a CP transformation and that means well it's
different to the KS state if CP symmetry is conserved KS and KL should
never transform into each other because they have different CP symmetries
Cronin and Fitch tested this by sending a bunch of both types of neutral Kaon
down a tube with a detector at the far end. The KS particles should never have
made the journey given their short lifetimes and yet a small but
significant number of decay products from KS particles were found at the far
end the only explanation is that KL particles oscillated into KS's, violating
charge parity conservation well that sucks our mirror reflected antimatter
clock doesn't work right after all nice one Fineman and that's not the worst of
it the violation of CP symmetry has much more dire consequences than a broken
imaginary clock although it does have a lot to do with time this also suggests
that time reversal symmetry is broken and to understand this we do need to
reverse time a bit and go back to the 1950s which coincidentally saw the
breaking of another imaginary clock in Hill Valley California the 1950s was
also the decade of the foundations of quantum field theory an SQFT emerged it
became clear that there is a certain symmetry that's not just intuitively
expected but also theoretically required. Starting with Julian Schwinger's
"Spin statistics theorem" in 1951 it became increasingly clear that quantum field
theory demands symmetry under the combined action of charge conjugation
parity inversion and time reversal the very axiomatic foundations of QFT state
that an antimatter mirror reflected time reversed version of our universe should
have exactly the same laws of physics this is the CPT theorem. Quantum field
theory should be CPT-invariant and we know that quantum field theory is right
at least as far as it goes it's just about the motor right theory we've ever
come up with I'll get back to what this new time reversal symmetry the T and CPT
actually means but for now let's just accept that the laws of physics must
work the same under a simultaneous flipping of charge parity and the
direction of time that was certainly the view of physicists in the late 1950s so
then come the experiments that show violation
first of P, then of CP symmetry - big deal, we still have T, CPT can be conserved.
But here's the issue if CP symmetry is violated and CPT symmetry holds then T
symmetry must also be violated why because that time reversal operation
needs to bring us from a broken CP reflected universe into a fixed CP T
universe but that means a T transformation from our working CPT universe
sends us to a broken CP universe. Burgo time reversal
transformation changes the way the universe behaves. Time symmetry is out the
window, theoretically. That sounds bad, isn't
physics supposed to work the same whether we go forwards or backwards in
time? well as we talked about in this episode don't we absolutely require time
reversal symmetry in order to conserve quantum information which itself is
required for all of quantum mechanics to make sense - well to get at this we're
going to need to talk about what we mean by reversing time the most obvious
interpretation of time reversal is literally reversing the arrow of time
and causing the universe to travel backwards in time. That is not what we
mean by the T in CPT which I'll explain in a minute but first let's think about
this simple type of T transformation as a literal rewind. Rewind the universe and
you get back to where you started pretty much by definition so presumably quantum
information is conserved in this type of time rehearsal. Mathematically, the particles
in a rewinding universe actually look like they underwent a charge parity inversion.
Matter that was going forward in time
looks like parity flipped antimatter going backwards in time.
This interpretation of antimatter as time reverse matter was first proposed by
Ernst Stueckelberg in 1941 but is now largely associated with Richard Feynman - it's
essential to his path integral approach to quantum mechanics and to Feynman
diagrams maybe that's why he was so into building antimatter clocks. So, yeah the
universe is not symmetric under this simple version of
T reversal. It's the precise inverse of a CP transformation, the two undo each other.
Do a CP transformation and then a simple T transformation and you get back to
where you started. If CP is violated then this simple time reversal is also
violated and we see this violation in the asymmetry between matter and
antimatter then there's the whole entropy business
although it's connection to quantum mechanics is still not well understood
like I said this simple interpretation of T as rewinding the universe is not
what we usually mean by the T in CPT - that T is more accurately thought of as
flipping the direction of the evolution of a physical system - an explosion
becomes an implosion and particle decay becomes particle creation. You're not
rewinding time, you're not converting matter to antimatter - you're just
reversing all momentum and spin. Essentially you're taking all particles
in the universe and pointing them back in the direction they came from. If the T
in CPT is conserved then after reversing all particle motion those
particles should perfectly retrace their steps and perfectly reverse all
reactions in their histories. As this motion reverse universe evolves forward
in time it should end up back in its starting configuration. On the other hand
broken T symmetry says that if you do this reversal the future won't perfectly
mirror the past. One prediction of this T symmetry is that all processes should
take the same amount of time going forwards as backwards
for example a quantum transition between one particle type and another should
take the same time in either direction and that gives us a test: in 2012
physicists from the BaBar collaboration at the Stanford Linear Accelerator
Center tested the speed with which B maisons transition between two types in
a T symmetric universe the speeds should be the same in either direction they
were not the same reversing the direction of the
interaction changed something fundamental about the physics indicating a violation
of T symmetry. But remember, killing T symmetry is a good thing because it may
save CPT symmetry. The last 70 years have been a true roller coaster
ride for the symmetries of nature as experiments found broken symmetries one,
by one the first parity then charge parity the hallow'd CPT symmetry looked
in danger unless we gave up on the symmetry of time itself but now with
time reversal symmetry also looking like it's broken the CPT theorem looks safe
Feynman's mirror reflected antimatter clock will work just fine but as well as
ticking backwards every atom every subatomic particle needs to tick
backwards also so we've discovered our perfect mirror universe it's a
reflection of all three: charge, space, time.
Hey everyone, I'm back from
the event horizon, aka. Australia, which as you know is the
CPT inverted version of Canada. The last episode before the break was, "Why string
theory is Wrong". Today, I want to address the comments to that episode or catch up
on last week's comments next time - in fact, I really want to address a few
points made by FieldStrength on the PBS Space Time subreddit: they covered all
the most important points, now first I want to clarify that despite the title,
"Why String Theory is Wrong", the point of the video was not to argue that string
theory is wrong instead it was a playful follow-up to why string theory is right
I'll reiterate my real opinion on the matter after addressing fill strengths
specific than very reasonable criticisms. In the video we note that string theory
pretty much requires supersymmetry in order to be right and that supersymmetry
hasn't yet been detected at the expected energy scale that's the energy scale
expected if supersymmetry is to also provide a neat resolution to the
standard models hierarchy problem but string theory doesn't require
supersymmetry to be at that scale at all for the purpose of string
theory supersymmetric particles could be far beyond the energies detected by the
Large Hadron Collider - so FieldStrength argues that our current non detection of
supersymmetric particles is no falsification of string theory and, of
course, that's correct but I didn't say otherwise - apologies if it seemed implied.
However, I do argue that this non detection is a bit of a hiccup
supersymmetry is highly compelling if it provides an answer to the hierarchy
problem and gives us super string theory it will be odd if supersymmetry existed
as part of super string theory but did nothing to help with the hierarchy
problem because it was at far too high an energy scale. But then as I did
mention in the video the current Large Hadron Collider results do not yet
completely rule out all energy scales useful for the hierarchy problem only at
the energy scales that solved the problem most neatly.
FieldStrength's other point is that the large undefined parameter space of string theory -
the so called 'String landscape' is no more of a problem for string theory than it is for
the standard model - I totally agree. In fact it's less of a problem for string
theory because in the standard model many parameters are hopelessly
unconstrained by the theory itself while in string theory it's arguable that only
one parameter is hopelessly unconstrained. My point is not that
string theory is more wrong in the standard model it's just that it's not
yet "right" - in this sense. To quote me it was never
really right and to quote Peter Weitz quoting Pauli, "it's not even wrong".
The point being that while not wrong there's not yet a way to properly test his
rightness or wrongness. FieldStrength's final point is that the untestability
of string theory is connected to the extreme energy scale of quantum gravity
and that problem is not unique to string theory. Now we do talk about that in our
first episode on general quantum gravity so apologies for not repeating it here.
So yeah string theory is apparently not yet testable,
but I disagree with those who say that this type of untestability means that
the field is not science. String theory may be currently untestable due to the
energy scales involved but the universe has no obligation to make itself
currently testable to any particular particle collider building technology
level. It's under no obligation to make every next layer of reality accessible
to the next generation of physicists. We blasted through several reality layers
from atoms to quantum fields in the past hundred years or so but maybe the next
layer will take another thousand years or more.
Randy Copeland brilliantly notes that physicists are a bit like 'House' M.D.
trying to find a diagnosis that fits the symptoms with the symptoms being all of
reality itself. Nice observation also makes me feel
better when I put my title down when I buy airline tickets in the hope of
getting an upgrade. I'm still dreading the day when the crew come to me because
there's some medical emergency on board and I have to tell them what type of
doctor I am.