thanks to brilliant org for supporting PBS Digital Studios 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 are online tools that can help like Brillion a problem solving website that teaches you to tackle with difficult topics and think like a scientist by breaking up complexities into understandable pieces and instead of just passively listening to lectures you get to master concepts by solving interesting and challenging problems so whether you want to learn about special relativity quantum physics or brush up on your complex algebra and differential equations you can learn more at brilliant org slash spacetime before we 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