Kind: captions
Language: en
most processes in our universe are time
reversible in other words the physics
works the same way forwards or backwards
which is why you can't tell if I'm
playing these videos normally or in
Reverse people typically point to
entropy as the only exception to this
rule the second law of thermodynamics
states that the entropy of a system or
the amount of disorder always increases
with time but increasing entropy is an
emergent property the result of the
motions of many many particles which
raises the question can fundamental
particles themselves tell the direction
of time or in other words is there a
physical process on the scale of
individual particles that looks
different
forwards vs. backwards the answer
surprisingly is yes in particle physics
there are three major symmetries that
were always expected to hold charge
parity and time time symmetry as we've
already discussed means interactions
work the same way forwards or backwards
in time charge symmetry means
interactions are unaffected if all the
charges are swapped in other words there
is nothing special about what we call
positive charge nature treats it exactly
equal and opposite to negative charge
and parity symmetry means the laws of
physics are indifferent to left or
right-handed miss then understand what
that means imagine a giant mirror were
held up to our universe and in the
mirror the Z direction is reversed and
my right hand becomes my left hand but
the laws of physics shouldn't care I
mean they should work exactly the same
way in the mirror world as they do in
the regular one with no preference for
left or right-handed miss or to put it
another way
there should be no experiment that you
could do that would tell you whether or
not you are in the mirror world each of
these symmetries is known by its
initials CP and T in the 1950s it was
thought that all fundamental particles
obey these symmetries
but then in 1956 a paper written by Li
and yang pointed out that parity
symmetry had actually never been tested
in experiments involving the weak force
so at Christmas break that year physics
professor Chien Shang Wu of Columbia
University had planned to go on vacation
with her husband who was also a
physicist
but instead so intrigued by the
possibility that the weak force might
violate parity she decided to stay
behind and be the first person to test
it to do this she and a team of
low-temperature scientists cooled a
collection of cobalt-60 atoms to just
three thousandth of a degree above
absolute zero then they applied a strong
magnetic field to align all the nuclei
with their spin pointing in the same
direction now cobalt 60 is radioactive
and it decays via the weak nuclear force
releasing a beta particle which is just
an electron and what the experiment
measured was the direction in which
these electrons were emitted relative to
the spin of the cobalt 60 nuclei to see
how this would work under parity
symmetry let's consider the mirror image
version of this experiment in the mirror
the direction of the z axis is flipped
but the direction of nuclear spin is not
that's because an object that's rotating
clockwise is still rotating clockwise in
the mirror so this means that the spins
of the regular and mirror nuclei are
aligned the mirror experiment is
actually the same experiment as the
original now when a cobalt 60 nucleus
decays and emits an electron that
electron could go say to the left or to
the right now if parity symmetry is
respected the electrons should be
equally likely to go in any direction
that way both the normal and mirror
image experiments would give the same
results however if the electrons were
emitted in one direction preferentially
say in the positive Z direction well
then in the mirror experiment the
electrons would also have to fly off
preferentially in the mirror positive z
direction which is opposite to the
original positive z direction so in the
normal experiment the electrons would be
emitted say opposite the direction of
nuclear spin but in the mirror version
they'd be emitted in the same direction
as nuclear spin so how does this make
any sense it would allow you to
determine whether you're in the mirror
world or not it's like the spinning top
from Inception if the electrons from the
cobalt 60 nuclei go one way you're in
mirror world and if they go the other
way you know you're in normal world now
crazy as this may seem this is exactly
what professor Wu saw electrons were
emitted preferentially in one direction
and not just by a little bit they were
predominantly emitted opposite nuclear
spin so not only does the weak
violate parody it comes close to
violating parody as much as is
physically possible this destroyed a
basic assumption of theoretical physics
that had been around for decades somehow
the universe cares about left or
right-handed nests when wu announced her
results they shocked the physics world
after being told of the experiment
famous physicist and Nobel laureate
Wolfgang Pauli said that's total
nonsense and insisted the result was
mistaken when the experiment was
independently replicated theoretical
physicists had to accept that the
universe we live in is not the one they
had imagined the Nobel Prize was
actually awarded for the discovery of
parody violation in 1957 the very same
year these results were published this
required a profound shift in thinking
for physicists but before throwing
everything out and starting again from
scratch they formulated a workaround
maybe it was okay that the weak force
broke parity because it's not a real
symmetry of the universe itself just
part of a larger symmetry charge parity
or CP symmetry the idea was if the
mirror flipped not only these axes but
it also swapped the particles for anti
particles with their charges reversed
well then the symmetry would be restored
and the mathematics behind our laws of
physics would still work now this gave
physicists some comfort until in 1964 it
was found that some particles can also
violate the combined charge parity
symmetry and boom you got yourself
another Nobel Prize now two rules which
physicists once thought were fundamental
laws of nature were broken so they
retreated behind their last set of
theoretical defenses the combined
symmetry of CP T where T is time sure
they said the weak force violates parody
and charged parody but certainly not
charge parody and time together and to
this day physicists are still pretty
sure that CPT is a real symmetry of the
universe so far no experiment has found
a violation of CPT
in fact if CPT is violated we would have
to rewrite a lot of the last century's
work because it would mean that special
relativity and quantum field theory are
both wrong ok so let's say that CPT is a
true symmetry
think about the implications of this if
we know that CP can be violated but CP t
cannot then time symmetry must also be
broken otherwise there would be no way
for the combined three-way symmetry to
be maintained while two of the sub
symmetries are broken and physicists
have actually conducted experiments that
confirm that certain particles directly
break time symmetry for example when a
pair of quarks are held together by the
strong force there are sometimes two
different possible arrangements and they
can switch back and forth between these
two arrangements via the weak force but
switching in one direction takes longer
than switching back so if you could make
a recording of this event it would look
different if you played the recording
forwards than if you played it backwards
and that's exactly what it means to
break time symmetry in certain cases
then fundamental particles can tell the
difference between going forwards and
backwards in time the second law of
thermodynamics is not the only physical
process that prefers one direction in
time now is this the origin of our
perception that time only goes one way
or is it the reason for the universe's
arrow of time the truth is that we still
have no idea why time only goes in one
direction what's interesting is that
physicists once thought that parity
charge and time were these symmetries
that were unbreakable but over time each
of these symmetries was demonstrably
violated so is the ultimate symmetry CPT
also unbreakable or will it fall taking
quantum field theory and special
relativity with it these are just some
of the big basic unresolved mysteries
left in our quest to understand the
universe perhaps one day another
physicist will give up their vacation to
figure out the answer this episode of
veritasium was animated and co-written
by my friend Jorge Chang who is also the
creator of PhD comics and now he's
written a book
yeah it's called we have no idea it's a
book I wrote with physicist Daniel Weitz
in' and it's about all the things we
don't know about the universe
although the big things like dark matter
and dark energy but also all the little
things like what are all those
fundamental particles for and it has
cartoons in it yep and lots of bad puns
so if that sounds like it's up your
alley
you should check it out
what a link to it in the description and
olive poor haze other projects Thanks