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when you look in a mirror you might think you see a perfect reflection but
you're actually looking at a universe whose laws are fundamentally different
when we think about symmetry the first thing that comes to mind is mirror
symmetry butterflies Rorschach tests and classically attractive faces all look
the same when flipped left to right in physics a symmetry is when the laws of
physics are unchanged due to some transformation of coordinates we've seen
examples where this holds in the universe is symmetric to coordinate
shifts in space-time and even the rather abstract phase of the wave function in
quantum mechanics so it might be surprising to learn that with all the
weird ways in which the universe is symmetric it's not entirely symmetric
under the most intuitive one the laws of physics work differently in the universe
of the mirror let's talk about the physics of entering the mirror universe
the technical name for such a reflection is a parity transformation and when
something remains identical under such a transformation we say that the thing is
parity symmetric p symmetric parity is a weird type of symmetry in the case of
those other symmetries I mentioned the shift in position time or phase can be
by any amount small or large these are continuous symmetries but
there's no such thing as a partial reflection you either reflect or you
don't parity is a discrete symmetry other types of discrete symmetries
include charge conjugation flipping the sign of the electric charge and time
reversal sending the clock ticking backwards we'll come back to both of
these they are going to save us in the end
parity transformations involve the flipping of spatial axis if we define
the mirror to be in the XY plane reflection sin z2 minus said and vice
versa objects are turned back to front and front to back and it looks
like your right hand becomes your left and vice versa as though it actually
flipped in the x-direction not the z or actually a true x-axis reflection would
look like this left still goes to right but forward-facing stays the same this
is exactly the same as a z-axis reflection plus a rotation in fact a
reflection on any single axis can be reproduced by a reflection on a
different axis plus a rotation a reflection on two axes then puts things
back to normal while reflection on all three axes is
the same left rice switch as a reflection on one axis but formally a
full parity inversion means flipping all three axes now when you reflect the
universe obviously it is difference or position vectors point in the opposite
direction same with the velocity / momentum through our board the mirror
away from yourself and its reflection looks like it's coming towards you in
the language of parity we call properties that flip sign odd functions
properties that don't change under reflections are called even things like
the energy of the ball its mass the time on its clock and as we'll see angular
momentum don't change in the mirror universe now even though some signs do
get flipped the laws of physics should be unaltered in that mirror world
Newton's laws will still give the balls trajectory those laws work the same no
matter the direction of motion in fact if we rotate the whole mirror balls
trajectory by 180 degrees it's identical to the original trajectory but that's
only true if the ball itself is unchanged in the reflection but what if
we try something else let's put some spin on the ball say clockwise spin the
reflected ball is still spinning clockwise its angular momentum is
unchanged but the relationship between the ball's angular momentum
and it's regular momentum the direction of motion is flipped this is similar to
the switching of left and right hands in a mirror reflection of a person in fact
we talk about the direction of spin as handedness you can grab the ball with
your fingers pointing in the direction of its spin and your thumb pointing in
the direction of motion if you need to use your right hand to do that then the
spin is right-handed relative to the motion that's the case
of the unreflective ball in our example if you need to use your left hand as in
the case of the reflected ball then the spin is left-handed relative to motion
the handedness of a moving spinning object is called its helicity a spot on
its surface draws a helix that spirals either one way or the other
but the general term for handedness is chirality a chiral object is one that is
fundamentally changed by reflection parity transformations flip from
left-handed to right-handed chirality hands have left-to-right chirality
so does spinning flying balls so do many molecules and some quantum particles but
chirality in quantum mechanics means something quite specific quantum
chirality is connected to quantum spin but for any particle with mass motion
doesn't factor into it it's not the same as olicity it's best to think of quantum
chirality as a fundamental internal right or left-handed asymmetry and most
chiral particles have both right and left-handed versions do these versions
behave differently well you wouldn't think so would you why should the
universe treat righties and lefties differently in fact it was assumed for
the longest time that the universe did conserve parity and it appeared to be
the case in experiments at least for tests involving gravity electromagnetism
and the strong nuclear forces it was assumed that the weak nuclear force was
the same deal that's until the tower theta problem came along
Tao and fader were a pair of particles to quake maisons to be precise that
looked the same in all respects same spin same mass same electric charge but
different decay products the tower decayed into three pylons also miss ons
while the theta decayed into two pylons that was the only difference between
them so couldn't a single particle have to decay outcomes well yeah but the
problem is that those to decay outcomes have different parity the combination of
two pylons has even parity or the combination of three pylons has odd
parity a single particle should produce decay products with the same parity so
the same response to reflections that's what we mean by conservation of parity
at least one of these decay products broke conservation of parity violation
of parity conservation was the solution to the tell theta problem suggested by
Yi and Liang in 1956 and for which they later won the Nobel Prize they proposed
that tau and theta are the same particle which turns out to be the K on and that
the weak interaction which governs this decay does not concern parity but how to
test this Yi and Liang found a brilliant experimentalist to test the hypothesis
the her being Qin Qiong Wu who for some reason didn't get in on that Nobel Prize
in 1957 Wu conducted an experiment based on a simple argument if parity is
conserved then there should be no experiment you can do to determine
whether you live in the regular universe or a mirror universe that means that
left-handed and right-handed particles should decay in the same way we tested
this using the cobalt-60 atom the nucleus of this radioactive
isotope of cobalt decays via the weak interaction into nickel by emitting an
electron and some gamma ray photons and neutrinos the cobalt-60 nucleus also
happens to have an unusually high spin which means it's possible to align
the spin of the nucleus using a magnetic field so Wuan team applied a magnetic
field to our layer of cobalt-60 and watched them decay they found that
almost all of the electrons produced in the decay emerged in the opposite
direction to the nucleus spin axis that right there was the smoking gun of
parity violation let's see why the important thing is that the momentum of
the ejected electrons had any correlation with the spin of the nucleus
at all why well let's look at how both momentum and spin are affected by parity
transformations to do this properly we need to reflect the cobalt nucleus in
all three directions left right forward back up down you can see that the spin
of the nucleus which we represent with the angular momentum vector along the
axis is not changed it's still spinning in the same direction so the axis of
that spin still points up but the same transformation on the electrons momentum
vector leaves it pointing in the opposite direction so if there is any
relationship between the direction that electrons are ejected and the spin of
the nucleus then that relationship must flip in a mirror universe but that gives
us an easy test to see which handed universe we live in ago parity symmetry
is violated in fact the only way for parity to be conserved is if there is
absolutely no correlation between spin direction and the direction of the
emitted electrons that way nothing would change in the reflection but that's not
what the experiment showed so how does the spin of the cobalt nucleus relate to
all of this chirality stuff well you can define the handedness by the
relationship between the direction of the electron and the direction of the
spin axis that's not the same handedness as the quantum chirality of all the
quarks comprising that nucleus but it's probably connected parity transformation
flip the handedness of those quarks and
apparently parity transformation must affect the direction of the decay
electrons also in fact we know that there is parity violation at the level
of quantum chirality it may be the ultimate source of the different rules
in the mirror universe why because the carriers of the weak interaction the W
and Z bosons only interact with left chiral particles they completely ignore
right-handed particles the universe of the mirror really does behave in a
fundamentally different way to our universe this realization has vexed
physicists ever since was experiment the resolution to this broken symmetry could
be in a greater symmetry see parity is intrinsically connected to the
symmetries of electric charge and the direction of time just as a reflection
in two directions will get you back to where you started
a parity transformation followed by a flipping of electric charge positive to
negative and vice-versa and a reversal in the direction of the flow of time
will also restore a system to its original state
this is CPT symmetry and so far it appears to hold in all experiments we'll
come back to our ongoing quest to discover whether CPT is the true
underlying symmetry of the universe deeper than the broken symmetry of
parity in a mirror reflected space-time whether it's classical mechanics or
quantum mechanics learning the ins and out of physics is tough thankfully there
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teaches you to tackle with difficult topics and think like a scientist by
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relativity quantum physics or brush up on your complex algebra and differential
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get to comments just a reminder you can support space-time by checking out our
new merch store link in the description or just visit us on patreon now last
time we talked about a wild notion for how life may have originated on earth
perhaps it came from space let's see what you had to say about the panspermia
hypothesis Scorch for 28 points out the problem with panspermia is that it
doesn't solve the problem of the initial origin of life the first abiogenesis
yeah it doesn't what it does do is open the possibility that that origin was
extremely improbable if it only had to happen once in the galaxy then we can
consider abiogenesis mechanisms that are tens of billions of times less probable
than mechanisms that could have got life started from scratch on earth because we
know that if life really started on earth then it started fast if it started
somewhere else then the chemical steps to the first true still could have taken
billions of years wherever that happened anyway I don't want a particularly
favored panspermia over earth-based abiogenesis but I do want to be clear
about what the hypothesis offers Penny Lane and Derek Carey suggest that if
panspermia is right then early earth life should have been more complex well
not necessarily the most simple cells do seem to have the most capacity for
surviving in space bacterial and fungal spores for example can triple down and
stabilize their DNA for very very long periods of time the more complex the
organism the more difficult it is to go to this sort of hibernation even
tardigrades can't survive for many millennia or even the millions of years
but spores may be able to Estelle Lea proposes that we become aliens who
originated life across the galaxy by sending lights-out probes carrying super
resistant life-forms well maybe let's get our atmosphere sniffing telescopes
up and running to see what's already out there first but your proposal does raise
on a notion where within a generation of being able to do this ourselves with the
sort of technology being developed for the breakthrough starshot program
so surely alien civilizations with any sort of technological Li on us
would have tried it in fact this has been proposed as an origin for life on
earth it's called directed panspermia it gets
around a lot of the problems with natural panspermia because presumably
organisms could be carefully selected or even genetically modified and protected
for such a journey more importantly the capsules or whatever containing these
seeds could be at least somewhat targeted to star systems or even planets
depending on assumptions evolving a technological civilization to the point
that they can initiate a directed panspermia program gives you much better
numbers than randomly scattering with the philic bacteria from asteroid
impacts with a non technological planet even if non-tech planets are vastly more
abundant anyway I feel like I'm straying into subjects that Isaac Arthur would do
better come to the of it he probably has done better
and speaking of awesomely speculative ideas samuel long very rightly complains
about our failure to mention Fred Hoyle or Chandra Wickramasinghe who are among
the earliest proponents of panspermia besides starting the modern interest in
the idea they also had some really cool fringe notions like the epidemics and
evolutionary leaps might be due to microbes arriving from space
in fact this stuff was all in our panspermia script at one point we
decided to be less historical and more current
anyway props to Hoyle no one ever said Hoyle was afraid of pursuing fringe
ideas even that new dark energy dark matter is a negative mass fluid thing
reference to seem heavily and yeah we'll be covering that after we do why string
theory is wrong because you guys haven't stopped complaining that it isn't up
here it'll be up next week I hope you aren't too disappointed if it turns out
that string theory is right I like you're all lining up to watch an
execution here