What's Happening With Antimatter at CERN? Scientists Are Stumped Again

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Have a look around you. Everything you see,  from the skin of your hand to the screen you’re   watching this video on, is a different combination  of the same three building blocks of matter:   protons, neutrons, and electrons. Now let’s look  a little farther, say at Mars, or the Andromeda   galaxy, or even halfway across the observable  universe… and still, there is matter made of   protons, neutrons, and electrons, as far as the  eye can see. At first, this might not sound all   that surprising—but for once, the mystery here  isn’t that we’ve seen something we can’t explain,   but rather that we haven’t seen something we were  expecting: a universe just as full of antimatter. I’m Alex McColgan, and you’re watching  Astrum. Join me today as we explore the   world of antimatter and learn about its  interactions with other particles and even   with gravity. By the end of this video, you’ll  probably agree that antimatter is a bit weird,   but you’ll also see why some physicists  are frustrated that it isn’t weird enough. Let’s get one thing out of the way first: although  it might sound like something straight out of   science fiction, antimatter is very real. It  forms a critical part of the Standard Model of   particle physics, and particles of antimatter have  been observed in experiments going back nearly a   century. The very first detection of antimatter  dates back to a 1932 experiment conducted by Carl   D. Anderson at Caltech using a cloud chamber  immersed in a magnetic field. When charged   particles from outer space—broadly called cosmic  rays—intercept the Earth’s orbit and fly through   this chamber, the magnetic field curves their  paths according to the charge and mass of each   particle, and the clouds show a visible imprint  of their resulting trajectories. Anderson was   hoping this experiment would help determine  just what kinds of particles were streaming   into the Earth from the cosmos, and he may have  found just a little bit more than he bargained   for! What Anderson saw was that these cosmic rays  included both positively and negatively charged   particles. The masses of the negatively charged  particles lined up exactly with the known mass of   an electron, but some of the positively charged  particles were far too light to be protons.   Instead, they appeared to also have the mass of  an electron, despite having the opposite charge,   and so these never-before-seen particles came to  be known as anti-electrons, or later positrons   for short. In 1936, Anderson would win the  Nobel Prize in Physics for this discovery. Meanwhile, a British physicist, who  was also destined to win a Nobel,   had been developing a description of electrons  that would fit nicely within the framework of   quantum field theory. His name was Paul Dirac. By  1928, Dirac had realized that in order to describe   electrons as quantum fields in a way that was  physically consistent with special relativity,   they had to be a part of a larger mathematical  structure— later known as a Dirac spinor—that   inevitably gave rise to both positively  and negatively charged versions of the same   particle. In this way, Dirac had predicted  the existence of positrons before Anderson   had even built the cloud chamber that  would detect them four years later. What’s even more incredible is that electrons  aren’t the only fundamental particle to come   in a two-for-one "Dirac spinor" package. Other  particles of matter, like the quarks that make   up protons and neutrons, each have their own  anti-quark counterparts. These anti-quarks can   come together to form anti-protons and  anti-neutrons, which can then bond with   positrons to form anti-atoms and anti-molecules.  You could make a whole planet out of antimatter,   and from the outside, it would look quite similar  to an ordinary planet made of ordinary matter! But if anti-matter were too similar to  matter—if the only difference were the   sign of its charge—then it would  be impossible to explain why our   universe contains so much of one and so  little of the other. This cosmic mystery,   known as the “baryonic asymmetry of the universe,”  sent physicists on a decades-long quest to try and   find as many differences as they could between  matter and antimatter. That quest lives on today,   spearheaded by particle colliders at CERN  that are capable of producing, trapping,   and studying both positrons and anti-protons.  But before we talk about these experiments,   let’s try to summarize what we already  know about the properties of antimatter. When studying antiparticles in isolation,  experiments have confirmed with ever-greater   precision that their intrinsic properties -  namely their masses - are exactly the same as   for ordinary particles. And when studying how  antiparticles are affected by electromagnetic   forces, experiments have again found that they  behave the same exact way as ordinary particles,   except with the opposite electric charge -  just as Anderson had observed in his cloud   chamber. But electromagnetism is just one  of the four fundamental forces of nature,   alongside gravity and the weak  and strong nuclear forces. And as   physicists began to better understand  the weak force in the 1950s and ‘60s,   they realized that particles and antiparticles  are actually affected by it quite differently. The first surprise was that ordinary particles  could only feel the weak force if they were   "left-handed" and antiparticles could only feel  it if they were "right-handed." The concept of   handedness (or "chirality") is subtle and  difficult to conceptualize for particles   with mass. But a loose analogy can be drawn with  a particle's helicity, which describes whether   a particle is spin-up or spin-down along  its direction of motion. In this analogy,   a spin-up particle is called right-handed, while  a spin-down particle is called left-handed. The second and even crazier surprise was that  right-handed antiparticles experienced a different   strength of the weak force, as compared to  left-handed ordinary particles. In practice,   this means that the quantum probabilities  for radioactive decay in ordinary nuclei are   somewhat different from the probabilities of the  analogous decay processes in anti-nuclei. This   fundamental asymmetry between particles  and antiparticles was first observed in   a 1963 experiment run by James Cronin  and Val Fitch of Princeton University,   who would be awarded yet another  Nobel Prize for their discovery. When this asymmetry was discovered, there was some  hope that it would explain the baryonic asymmetry   of the universe: perhaps these differences  in the weak force were responsible for the   abundance of matter and utter lack of antimatter  around us. But the maths didn't quite work out.   There simply wasn’t enough of a difference  between the strength of the weak force acting   on particles versus antiparticles. That was  when physicists began to turn their attention   to the strong nuclear force. Theoretical models  predicted that, just like in the weak interaction,   there should be some differences in how  left-handed particles and right-handed   antiparticles feel the strong force. But  antimatter just keeps surprising us! Every   experiment to date suggests that the strong force  treats particles and antiparticles just the same. This brings us to the last of  the four fundamental forces and   the subject of today's ongoing  experiments at CERN: gravity. To be honest, suggesting that gravity might  treat matter and antimatter differently is   kind of a long shot. Think back to the popular  legend of Galileo tossing stones of different   sizes and materials from the Tower of  Pisa: they all fell at the same rate,   because the gravitational acceleration on Earth  is 9.8 meters per second squared, regardless   of which object is falling. Of course, the  experiment works even better in a vacuum chamber,   where air resistance is taken out of the equation.  Newton expanded on this idea and showed in the   17th century that your gravitational acceleration  anywhere in space depends only on the mass of the   object pulling you and your distance from it,  but not on any of your personal properties—not   even your own mass. This famous result—known as  the Equivalence Principle—is the foundation of   Einstein’s theory of General Relativity, our most  accurate and successful model of gravity to date. With that in mind, physics is still  an experimental science at its core,   and we can’t know for sure whether  matter and antimatter obey the same   laws of gravity unless we check for ourselves.  The physicists at CERN set out to do just that,   motivated not only by the baryonic asymmetry  of the universe, but also by a few speculative   papers suggesting that the cosmological  mysteries of dark matter and dark energy   could be more easily explained if antimatter  were to have a negative gravitational charge—or   put more simply, if antimatter were to  fall up, rather than down. There are   several ongoing experiments at CERN testing  the gravitational properties of antimatter,   including AEgIS (“Aegis”), GBAR (“g-bar”),  and ALPHA (“Alpha”). Today we will focus   specifically on a key experiment coming out  of the ALPHA group that was published in the   journal Nature this past September. After  decades of assumptions, this experiment has   brought us real-world data on the gravitational  acceleration of antimatter on Earth’s surface.   But before we show you the results, let’s take  a moment to appreciate just how intricately   this experiment was designed in order to  isolate and measure the effects of gravity. The first step in the experiment is to secure  a beam of several million positrons per second   emitted from a radioactive isotope of  sodium (Na-22). Most of these positrons   end up colliding with ordinary matter in the  experiment, causing miniature explosions in   which positrons and electrons annihilate each  other and release place a small burst of energy   in the form of light. But a small fraction of the  positrons survive as they are guided through the   experimental apparatus [figure below], where they  are cooled by low-pressure gases and trapped by   electric and magnetic fields. But observing the  effects of gravity on these positrons would be   nearly impossible: their masses are so small, that  the tiny force of gravity felt by each particle   is overshadowed by even the smallest fluctuations  in the surrounding electromagnetic fields. That’s   why this collection of positrons is merged with a  separate container of antiprotons, where they bond   and form neutral anti-hydrogen atoms that are much  less responsive to stray electromagnetic fields.   And where did the antiprotons come from? Suffice  it to say that they were produced by firing   ordinary protons into a block of metal really,  really fast. Yeah—Physics is awesome like that.  Once the antihydrogen atoms are created, they  behave like tiny, weak magnets that can remain   trapped by a complicated arrangement of  external magnetic fields. But now, this   magnetic interaction is weak enough that it no  longer overwhelms the gravitational effects that   we’re trying to measure. The chamber containing  these antihydrogen atoms is nearly a vacuum:   there are just about 200,000 atoms of ordinary  gas per cubic centimeter, compared to a typical   atmospheric density of 20 quintillion atoms  per cubic centimeter. Under these conditions,   the trapped antihydrogen atoms almost never  collide or annihilate with atoms of ordinary   matter. Instead, they can more or less just float  around the chamber for minutes or longer. But   as the magnetic fields used to vertically  trap the antihydrogen atoms are weakened,   this random floating eventually allows the  antihydrogen atoms to escape through either the   top or bottom of the chamber [figure below], where  they can collide with a wall of the apparatus,   annihilate with some ordinary atoms, and release  a small burst of light. In the ALPHA experiment,   this happens over the course of about 20 seconds. The theory behind the experiment is that if  gravity really pulls antimatter downwards,   more of the antihydrogen atoms escape through  the bottom than the top. The stronger the   gravitational force, the more atoms escape  through the bottom. The simulations the ALPHA   team ran showed that under normal gravitational  attraction, about 85% of the antihydrogen atoms   should escape through the bottom, whereas  only 20% of them would escape through the   bottom if gravity pulled antimatter upwards.  If there were no gravitational force at all,   the simulations showed a more even distribution  of 55% escape through the bottom (probably only   differing from 50% due to asymmetries  in the experimental apparatus itself). What did the actual experiment find? Well,  roughly 75% of antihydrogen atoms escaped   through the bottom of the chamber, showing a  clear preference for downward-pulling gravity. As any thorough scientist would, the ALPHA team  repeated this experiment to collect a variety   of data points that tell a more complete story.  They redid the procedure under various levels of   magnetic field bias, which applied external  upward or downward magnetic forces on the   antihydrogen atoms. On this graph, a bias of -1g  means that just enough magnetic force is applied   to counteract normal gravity, while a bias of +1g  means an extra “g” of magnetic force is applied to   push the antihydrogen atoms downward, and so on.  The team made predictions through simulations for   each bias and for various possible gravitational  interactions, which produced the orange, green,   and purple curves shown here [figure below]. As you can see, the experimental data points,   shown in blue, best match the orange  curve, which represents the “normal”   simulation where gravity pulls antimatter  downwards. But because the data falls   just a bit below this curve, the best-fit  gravitational acceleration was only 0.75g:   three-quarters of the strength of  gravity acting on ordinary matter.   Does this mean that gravity affects matter and  antimatter particles differently after all? Not necessarily. Let’s have a quick look  at the error bars. They indicate that there   are two major sources of uncertainty in  the results, including an uncertainty in   the applied bias (highlight horizontal error  bars on screen), possible errors in alignment,   and other systematic and statistical uncertainties  (highlight vertical error bars for these last 3). When accounting for these uncertainties, the  best-fit gravitational acceleration is actually   reported as 0.75g±0.13g±0.16g. This means that  a full 1g of gravitational acceleration is still   fairly consistent with the collected data. Future experiments will be able to determine   more precisely how strongly gravity acts  on antimatter. But … we can already rule   out speculative theories that rely on  antimatter falling up instead of down. In the end, despite how weird and backwards the  world of antimatter is, it seems that only the   weak force actually applies differently to  particles and antiparticles. But explaining   the baryonic asymmetry of the universe would  require much more drastic differences between   the two— so scientists aren’t done looking for  them. Could there be new forces and particles   that interact even more weirdly with antimatter?  Or would you be willing to accept that having so   much more matter than antimatter around  us is a mere coincidence? In any case,   let us know if you’ve learned something new about  antimatter from watching this video, and whether   this is a topic you’d like to hear more on! We generally applaud it when hundreds of eyes turn   their telescopes to the Stars to learn everything  they can about the cosmos however it feels a   little less celebratory when similar number of  eyes look at you learning everything they can   about you from your intim traffic habits to your  passwords and sensitive data but thankfully the   sponsor of today's video NordVPN can keep unwanted  eyes off you while you browse provided they're not   using a telescope to do it you're on your own  there for online threats NordVPN is the fastest   VPN on the planet and stops what you're doing  online from being tracked and shared giving you   a feeling of safety and privacy again. with just a click you can jump between regions allowing   you to access content outside of where you are one  account can can protect up to six devices and even   comes with threat protection to keep viruses  and malware out of your computer why not give   nordvpn a try by scanning my QR code or using my  link nordvpn.com/astrum in the description below   to get 4 months free on your 2-year subscription  it's free risk with nordvpn's 30-day money back guarantee thanks for watching if you like this  imagery based video you may like my others in   this playlist a big thanks to my patrons and  members if you want to support and have your   name added to the end of every ashon video check  the links below all the best and see you next time
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Channel: Astrum
Views: 1,274,205
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Keywords: astrum, astronomy, space, physics, antimatter, particle physics, antiparticle, dirac, standard model of particle physics, cloud chamber, magnetic field, subatomic particles, paul dirac, quantum, quantum mathematics, carl anderson, anti atom, baryonic asymmetry, electromagnetism, spin-up, spin down, weak force, CERN, antimatter factory, Galileo, vacuum chamber, antihydrogen, anti-hydrogen, ALPHA experiment
Id: z6bybdozsjY
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Length: 19min 39sec (1179 seconds)
Published: Fri Feb 09 2024
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