[HN Gopher] Particle seen switching between matter and antimatte...
___________________________________________________________________
Particle seen switching between matter and antimatter at CERN
Author : birriel
Score : 136 points
Date : 2021-06-12 12:20 UTC (10 hours ago)
(HTM) web link (newatlas.com)
(TXT) w3m dump (newatlas.com)
| brabel wrote:
| > if ever a matter and antimatter particle come into contact,
| they will annihilate each other in a burst of energy.
|
| > To complicate things, some particles, such as photons, are
| actually their own antiparticles.
|
| I believe "a burst of energy" means a bunch of photons? So, we
| still end up with the same amount of energy as before, but all
| particles are now photons? Like in the heat death of the
| universe?
| Filligree wrote:
| Nah, it's symmetric.
|
| An electron and a positron can annihilate, producing a pair of
| photons. Or two photons can annihilate, producing an electron
| and a positron. Or...
|
| Well, in fact the symmetry works for _any_ rotation, not just
| 180 degrees. An electron and a photon can "annihilate",
| producing an electron* and a photon going the other way. This
| is also known as "A photon bouncing off a mirror".
|
| *: The careful reader will note there's no positron in this
| one. That's because positrons are what happen when an electron
| hits a photon so hard it bounces backwards in time -- that is,
| it's an electron going backwards in time. For _one_ definition
| of 'time', at least.
|
| To make the mirror version work, the photon needs to have lower
| energy than would be involved with matter annihilation.
| Otherwise your gamma-ray photon will tear through the mirror
| instead of bouncing back, probably making a nice hole. This
| doesn't mean the rotation doesn't work, it's just that what I
| was describing wasn't really a "mirror" as commonly considered.
| But also, because momentum (or, more usefully, rapidity) is
| defined in terms of the angle the particle makes with time,
| rotations other than 180 degrees don't conserve momentum. So
| it's not a gamma-ray photon anymore.**
|
| **: Photons, obviously, have a fixed 'angle with relation to
| time'. You can take that as referring to the angle their
| _waves_ make with time, instead.
| leephillips wrote:
| I don't know about the heat death of the universe, but yes, the
| mass-energy is conserved, and the energy that comes out is in
| the form of photons.
| _Microft wrote:
| Now we know how much mass a minus-sign has ... 10^-41kg ;)
| _Microft wrote:
| 1bit of information stored at 300K has a mass of 10^-38kg
| according to [0]. That's a few orders of magnitude off but
| almost uncannily close.
|
| [0] https://aip.scitation.org/doi/10.1063/1.5123794
| zzt123 wrote:
| Hold up. The article shows a mass difference of 10^-38 for
| the matter/antimatter states. That's not a few orders of
| magnitude off your number, that's literally your exponent.
| _Microft wrote:
| The article gives the mass difference in grams, not
| kilograms. That's where the factor of 10^-3 to the number
| in my top comment comes from.
| zzt123 wrote:
| Ahhhh. Tricky units!
| throw0101a wrote:
| This reminds me of a 2004 weblog post by one of the co-
| creators of ZFS, Jeff Bonwick, on why 128 bits was chosen:
|
| > _Some customers already have datasets on the order of a
| petabyte, or 2^50 bytes. Thus the 64-bit capacity limit of
| 2^64 bytes is only 14 doublings away. Moore 's Law for
| storage predicts that capacity will continue to double every
| 9-12 months, which means we'll start to hit the 64-bit limit
| in about a decade. Storage systems tend to live for several
| decades, so it would be foolish to create a new one without
| anticipating the needs that will surely arise within its
| projected lifetime._
|
| > _If 64 bits isn 't enough, the next logical step is 128
| bits. That's enough to survive Moore's Law until I'm dead,
| and after that, it's not my problem. But it does raise the
| question: what are the theoretical limits to storage
| capacity?_
|
| > _Although we 'd all like Moore's Law to continue forever,
| quantum mechanics imposes some fundamental limits on the
| computation rate and information capacity of any physical
| device. In particular, it has been shown that 1 kilogram of
| matter confined to 1 liter of space can perform at most 10^51
| operations per second on at most 10^31 bits of information
| [see Seth Lloyd, "Ultimate physical limits to computation."
| Nature 406, 1047-1054 (2000)]. A fully-populated 128-bit
| storage pool would contain 2^128 blocks = 2^137 bytes = 2^140
| bits; therefore the minimum mass required to hold the bits
| would be (2^140 bits) / (10^31 bits/kg) = 136 billion kg._
|
| > _That 's a lot of gear._
|
| > _To operate at the 10^31 bits /kg limit, however, the
| entire mass of the computer must be in the form of pure
| energy. By E=mc^2, the rest energy of 136 billion kg is
| 1.2x10^28 J. The mass of the oceans is about 1.4x10^21 kg. It
| takes about 4,000 J to raise the temperature of 1 kg of water
| by 1 degree Celcius, and thus about 400,000 J to heat 1 kg of
| water from freezing to boiling. The latent heat of
| vaporization adds another 2 million J/kg. Thus the energy
| required to boil the oceans is about 2.4x10^6 J/kg *
| 1.4x10^21 kg = 3.4x10^27 J. Thus, fully populating a 128-bit
| storage pool would, literally, require more energy than
| boiling the oceans._
|
| * https://blogs.oracle.com/bonwick/128-bit-storage:-are-you-
| hi...
| [deleted]
| Shank wrote:
| The official announcement may also be relevant:
| https://home.cern/news/news/physics/lhcb-measures-tiny-mass-...
|
| Edit: Official seminar: https://indico.cern.ch/event/1027299/
| dukwon wrote:
| The LHCb collaboration's own summary of the result:
|
| https://lhcb-public.web.cern.ch/Welcome.html#Deltamc
| tremon wrote:
| What fascinates me is that these oscillations appear to be
| continuous. What is it exactly that they're measuring/showing
| in the decay plot?
|
| Shouldn't the state transitions be discrete events, and
| therefore the plot show a square wave instead of a sine?
| dukwon wrote:
| The time-dependent probability of being in either flavour
| eigenstate is sinusoidal. It's not like a ticking clock.
| lamontcg wrote:
| Yeah this is much better than the title article.
|
| K-Kbar mixing was observed decades ago.
|
| The headline here should be about the first measurement of a
| mass asymmetry between matter and antimatter.
| dukwon wrote:
| > The headline here should be about the first measurement
| of a mass asymmetry between matter and antimatter.
|
| The mass difference is not between particle and
| antiparticle, but between the mass eigenstates, which are
| not antiparticles of eachother. It's also not the first Dm
| to be measured. In fact, it was the last remaining one.
| lamontcg wrote:
| So if the mass eigenstates aren't different then you
| don't get mixing between the flavor eigenstates, so this
| is expected? And by measuring the frequency of the FCNCs
| then that is what gives them the difference between the
| mass eigenstates? (bear with me, its been 25 years since
| I decided to become a programmer instead...)
| dukwon wrote:
| Dm is directly proportional to the frequency of the
| oscillation. (In natural units it is exactly the angular
| frequency: cos(Dm*t) appears in the decay rate) So indeed
| Dm=0 means no oscillation. You measure it by essentially
| counting the number of particle and antiparticle decays
| as a function of decay-time.
| lamontcg wrote:
| What about charged systems like proton-antiproton? We
| expect there to be some similarly tiny Dm there, but
| obviously you can't have FCNC because its no longer
| neutral?
| dukwon wrote:
| Protons and antiprotons must have the same mass under CPT
| symmetry
| [deleted]
| mdoms wrote:
| > This subatomic particle is normally made up of a charm quark
| and an up antiquark, while its antimatter equivalent consists of
| a charm antiquark and an up quark.
|
| Q: if these subatomic particles consist of a quark and an anti-
| quark, why don't they self-annihilate?
| mkaic wrote:
| as someone who isn't well versed in particle physics i'm also
| wondering this. can anyone with more experience weigh in?
| gus_massa wrote:
| There are six quarks: down, up, strange, charm, bottom, top.
|
| There are six anti-quarks: down, up, strange, charm, bottom,
| top.
|
| Using the strong force, a quark can annihilate only an
| antiquark of the same type, so the number of charm quark minus
| the number of charm antiquarks is "almost" conserved. If you
| start with a system with one charm quark like the particle
| here, you "allays" will have a system with one charm quark, or
| 2 charm quarks and on charm antiquark, or ... but "never" a
| system with no charm quarks like a few photons after
| annihilation.
|
| It's more complicated, because the weak force can annihilate
| one quark with a different antiquark. (The rules are somewhat
| complicated, and some annihilations are more usual than
| others.) So after some time, you can be lucky and a weak
| interaction can destroy your charm quark and a up antiquark, so
| the number of charm quark minus the number of charm antiquarks
| changes "slowly".
|
| Here "slowly" means that to see an effect of the weak
| interaction you must wait like 10E-12 seconds instead of the
| 10E-24 seconds you must wait until you see an effect of the
| strong interaction. The numbers change for each decay and are
| difficult to calculate and may be much longer. So everything in
| scare quotes means it's just an aproximation.
|
| For example the neutron->proton+electron+antineutrino decay use
| the weak force and you must wait like 10 minutes that is a lot
| for an unstable particle. The particle discussed in this
| article has a life of almost a millisecond, that is very long
| for a particle but short for us.
|
| (Electromagnetism can annihilate only an antiquark of the same
| type, so it's not important here and it's much fainter.)
|
| ---
|
| On the other hand, you are right. This article is about the
| decay of a particle D0 into a Ks0 and pions. After some time
| (probably a few milliseconds after the experiment finalized)
| the Ks0 decays in more pions. And the new pions and the other
| pions after some time decay into photons and muons and
| neutrinos, and after some time the muons decay into electrons
| and neutrinos.
|
| The number of electrons and antielectrons (positrons) are
| equal, so they annihilate each other and you get more photons.
| Also, the number of neutrinos is equal to the number of
| antineutrinos, so they theoretically can annihilate each other,
| but in practice they just escape.
|
| So the global reaction is an annhihilation:
|
| real: D0 ---long-story---> photons + neutrinos + antineutrinos
|
| optimistic theoretical possibility: D0 ---longer-difficult-
| story---> photons
| dukwon wrote:
| Mesons have 'annihilation' decay modes, certainly, but they can
| also decay in other ways.
| ganzuul wrote:
| Charm, Up and Top are all different. They share some properties
| but not all and therefore annihilation can not take place.
| dukwon wrote:
| Sure it can, but it has to proceed via the weak interaction,
| and it's suppressed by the GIM mechanism. For example, D0-gg
| (cu or cu annihilation) is rare and not yet observed, but
| it's allowed.
| [deleted]
| ephimetheus wrote:
| I'd love it if they could stop trying to assign these results to
| individual institutes (Oxford in this case). This is an LHCb
| publication and that's that c
| gnufx wrote:
| Did the Oxford group actually not do the relevant analysis as
| the university suggested?
|
| (I conclude from the lack of mention in my current and previous
| universities' news -- wot, no Tara Shears? -- that at least
| their LHCb groups weren't significantly involved!)
| 867-5309 wrote:
| >according to Oxford physicists analyzing data from the Large
| Hadron Collider
|
| >data gathered during the Large Hadron Collider's second run,
| by physicists at Oxford University
|
| >Sources: Oxford University, CERN
| smlss_sftwr wrote:
| Aha, sneaky little particle thought it could make a clean escape
| without being noticed, did it?
| azinman2 wrote:
| > That means that if ever a matter and antimatter particle come
| into contact, they will annihilate each other in a burst of
| energy.
|
| Dumb q: if you could get a bunch of anti-matter, could it be
| harvested to generate electricity?
| eigenket wrote:
| Yes, but the only ways we know to generate antimatter require a
| lot of energy. Theoretically it could be an incredibly energy
| dense "battery" for spaceships or whatever though.
| User23 wrote:
| It's expensive too[1]. "Right now, antimatter is the most
| expensive substance on Earth, about $62.5 trillion a gram
| ($1.75 quadrillion an ounce)."
|
| [1] https://science.nasa.gov/science-news/science-at-
| nasa/1999/p...
| opportune wrote:
| Humans use positrons all the time though, it just turns out
| (anti)leptons have extremely low mass relative to their
| usefulness.
| h2odragon wrote:
| Of course they _really_ nail you on the shipping and
| handling fees.
| dukwon wrote:
| They're genuinely working on being able to deliver it in
| the back of a van:
| https://home.cern/news/news/physics/cern-approves-two-
| new-ex...
| ergocoder wrote:
| Yes. With small amount of matter/antimatter, It actually can
| power an interstellar travel where it accelerates 1g for years.
| Filligree wrote:
| You'd need some way to capture the gamma-ray photons and pions,
| but yes. I'd recommend a wall of plasma.
|
| ...then use that to run a steam engine. _Sigh_. We 'll never
| escape those.
| akmarinov wrote:
| Nice!
|
| ELI5 what implications does that have?
| jerf wrote:
| The article really already has that in it, better than you're
| going to get from anyone else.
| echelon wrote:
| That simply is not true. HN is filled with subject matter
| experts that can break down complex topics better than popsci
| journalists, and provide commentary on how this fits within
| the framework of current and future research.
| krapp wrote:
| HN is filled with people who _believe_ themselves to be
| subject matter experts, and who refuse to engage with
| posted articles as they consider leaving the sanctum
| sanctorum of HN to expose themselves to the plebian
| knowledge and interests of others to be a waste of time. In
| other words, with people who wear their ignorance as a
| badge of elitism.
|
| Not unrelated, HN has a few actual subject matter experts
| who are often aghast at how _wrong_ HN tends to be about
| any field or subject not directly related to programming,
| and how often their confidence is in inverse proportion to
| their knowledge.
|
| You can safely assume that while there may be actual,
| credible particle physicists on HN, their voices are likely
| to be drowned out by armchair expositors of bullshit
| ranting about dark matter being a hoax perpetrated by
| cultural Marxists or somesuch. Do the work yourself. Read
| the article yourself. Enrich your own mind and satisfy your
| own intellectual curiosity, don't expect HN to do it for
| you, or to be better than the mainstream in terms of
| general expertise, or you'll be misled and disappointed
| more often than not.
| jodrellblank wrote:
| "In Comments: Be kind. [...] Please don't sneer,
| including at the rest of the community. -
| https://news.ycombinator.com/newsguidelines.html"
|
| HN and Reddit are filled with people sneering at how dumb
| everyone else on HN and Reddit are. It makes for tedious
| reading, and a cringeworthy "I'm superior because I see
| how dumb everyone else is" status grab. If something is
| wrong, correct it or downvote it or ignore it; don't
| conjour up some imaginary mind-reading fantasy about how
| great the author must think they are and then sneerily
| tear down the strawman.
| krapp wrote:
| What status do you think I'm trying to grab? The only
| thing that's going to happen here is that I get
| aggressively downvoted and the problems I'm complaining
| about will continue unabated, because mentioning them
| makes for "tedious reading."
| echelon wrote:
| You're projecting here. I only stepped in to defend
| someone asking for an explanation from the HN community.
|
| I'm not putting aside skepticism and intellectual rigour
| when I read the comments here.
|
| I also think you're wrong about the members of this
| community. There are a tremendously wide assortment of
| backgrounds represented here.
|
| I come to Hacker News mostly for, not in spite of, the
| comments.
|
| I think telling people to not ask questions is dangerous.
| The person I originally responded to was not doing that
| exactly, but it felt borderline enough to remind them
| that the community here is filled with experts and
| teachers.
|
| It's also worth pointing out that we shouldn't silence
| curiosity. Attacking what you perceive as a lazy mind can
| only harm the desire to learn.
| krapp wrote:
| I'm not objecting to asking questions, I'm objecting to
| preferring the HN commentariat over at least engaging
| with the posted content. Asking questions _after_ reading
| the article is fine - that 's part of creating fruitful
| discussion.
|
| But HN does have a problem in that _no one_ believes
| reading the article is worth the effort, and _everyone_
| prefers to just read the comments. The end result of that
| is a negative feedback loop of insularity and
| incuriosity, more people only engaging with the subject
| (or what they believe the subject is based on their
| reading of the title) on a superficial level rather than
| real intellectual discussion.
| AnimalMuppet wrote:
| That article didn't clear up a whole lot for me personally.
| Maybe some others are like me. I also read the actual CERN
| article, which this article copied almost entirely.
|
| But after reading twice, what seems to be going on is that a
| particle has a slightly different mass from its antiparticle.
| (That is the most unbelievable sentence I have written so far
| this year! It's just... wow. I didn't think that was even
| theoretically possible.)
| dukwon wrote:
| > a particle has a slightly different mass from its
| antiparticle
|
| This is not the case, and that would violate CPT symmetry.
|
| There are two states with different masses (the mass
| eigenstates) but they are not antiparticles of eachother.
| They are linear superpositions of the flavour eigenstates:
|
| |D1> = p|D0> + q|D0>
|
| |D2> = p|D0> - q|D0>
|
| where p and q are complex coefficients.
|
| The difference in mass between D1 and D2 is directly
| proportional to the oscillation frequency.
|
| If p/q [?] 1, you have CP violation in this mixing. The
| amount of CP violation in this measurement was found to be
| consistent with zero.
| treeman79 wrote:
| Spin two toy tops in opposite directions. When they bump into
| each other they both stop. All fun stops.
|
| This experiment says when they bump into each other ones
| spinning a certain direction always have a little spin to it
| left over. A tiny bit of fun still exists.
|
| That little bit of fun is all physical matter in the universe.
| adrian_b wrote:
| The title is confusing, because the whole particle does not
| switch between matter and antimatter, only its internal
| components switch.
|
| The hadrons, i.e. the particles made of components bound
| together by the so-called strong nuclear forces are partitioned
| into 3 groups: particles made of 3 quarks, particles made of 3
| anti-quarks and particles made of 1 quark together with 1 anti-
| quark.
|
| Particles made from more quarks than in these 3 groups are
| possible in principle, but they would disintegrate extremely
| quickly, i.e. in times many orders of magnitude less than 1
| nanosecond.
|
| The reasons why only those 3 kinds of hadrons are normally
| encountered is that exactly like the electric forces attempt to
| neutralize the electric charge and prevent the negative charges
| to separate from the positive charges, the strong nuclear
| forces also try to neutralize a similar quantity, with the
| difference that this quantity is 2-dimensional, unlike the
| electric charge, which is 1-dimensional.
|
| The condition of neutralizing that 2-dimensional quantity can
| be satisfied by the 3 kinds of quark combinations listed above.
|
| The lightest hadron made of 3 quarks is the proton and the
| lightest hadron made of 3 anti-quarks is the anti-proton.
|
| The proton and the similar hadrons can be considered as matter
| and the anti-proton and similar hadrons can be considered as
| anti-matter.
|
| None of these hadrons made of either 3 quarks or 3 anti-quarks
| can switch between matter and anti-matter and this remains true
| for their compounds, like nuclei or anti-nuclei.
|
| On the other hand, the 3rd kind of hadrons, which are usually
| named mesons, and which are made from 1 quark and 1 anti-quark,
| are in fact neither matter nor anti-matter.
|
| The article linked here refers to a particle of this kind,
| where the total number of quarks is +1 -1 = 0.
|
| So in mesons it is possible that the pair of 1 quark + 1 anti-
| quark will transform in another pair of 1 quark + 1 anti-quark,
| but where the quarks are different types of quarks than before
| the transformation. This transformation is possible because the
| total number of quarks is 0, both before and after the
| transformation, so any such transformation is possible as long
| as the new types of quarks are such so that the electrical
| charge remains the same.
|
| The compositions of mesons can oscillate between different
| pairs quark + anti-quark, but the heavier mesons decay
| extremely fast so it is exceedingly difficult to observe such
| oscillations.
|
| The linked article reports the experimental observation of such
| an oscillation, which was known as possible since decades ago,
| but it was not seen in experiments due to technical
| difficulties.
|
| So in these transformations some quarks and anti-quarks appear
| and disappear, but the total quantity of matter and anti-matter
| is the same before and after, which is why the title is
| misleading.
| Y_Y wrote:
| No. Five-year-olds can't do particle physics. It may as well
| imply nothing at all.
| burundi_coffee wrote:
| Relevant xkcd: https://xkcd.com/1364/
|
| Some things are indeed too complicated to be explained in
| simple terms and analogies.
| BobbyJo wrote:
| Then just don't respond homie.
| steve76 wrote:
| Water drips out of a faucet, and makes a sink wet. If you are
| nature, and have no internal mind, and can interact on extreme
| limits such as exists/non-exist, you can drip dryness from the
| sink, up through the faucet, through the pipes and into the
| water tower. Nature always takes the easiest route. If that
| route includes changing "drip wet down" or "drip dry up" it
| will take it.
|
| Electricity works with positive and negative. At very high
| temperatures in very precise regions, a third pole leads to
| better calculations. Better calculations led to better
| applications, such as plasma, measuring and manipulating the
| Sun, quantum biology where the molecules of genes are abandoned
| for a much higher resolution in the subatomic particle
| interactions, or giving your nervous system a therapeutic
| sunburn with a positron emitter.
| macksd wrote:
| >> the Big Bang should have produced matter and antimatter in
| equal amounts, and over time that all would have collided and
| annihilated, leaving the cosmos a very empty place. Obviously
| that didn't happen
|
| Why do we expect the Big Bang would have created little more
| matter than we observe? How do we know that almost all matter
| wasn't annihilated, and what's left really isn't just a rounding
| error after ~50% annihilated ~50%?
| dokem wrote:
| Wouldn't this just produce energy in the form of photons which
| are neutral in terms of being matter or antimatter which could
| then turn back into matter? I don't see how anything has to be
| lost in this situation. Other than a large amount of entropy
| being produced.
| AnimalMuppet wrote:
| Tangentially related, but veering off-topic very quickly:
|
| We define "matter" to be what there is in greater abundance in
| the local neighborhood. Thus we define protons and neutrons (or
| up and down quarks) as matter, along with electrons.
|
| But what if that's wrong? What if electrons are anti-particles?
| That would go some distance toward "matter and antimatter in
| equal amounts" (though not, I think, all the way there).
|
| Is there any physical reason that, if we identify the up quark,
| say, as matter, then the electron also has to be matter rather
| than antimatter? (I think there _is_ reason that if the up
| quark is matter, the down quark has to also be matter, but I
| could be wrong on that one, too.) Can anyone with better
| knowledge than mine shed some light here?
| Filligree wrote:
| Arguments about matter-antimatter balance are actually per
| quantum field.
|
| Electrons can't, in any sense, be "antimatter". That's
| because there antimatter as a concept only applies to a
| particle's relationship to its antiparticle -- it's not a
| feature of a single type of particle, only of _two_ types of
| particle. Which must be variant excitations of the same
| quantum field.
|
| That's almost irrelevant, however. The big bang should have
| created roughly equal amounts of matter and antimatter _for
| each quantum field_ ; an excess of protons _cannot_ be
| balanced by deficit of positrons, as I believe you were
| suggesting.
|
| Protons and positrons do not annihilate, nor would protons
| and electrons for that matter. Only protons and anti-protons,
| or electrons and positrons, have that reaction.
|
| (Though, aside: Protons are composite particles. None of the
| composites are electrons, though, so this still applies.)
| ben_w wrote:
| This reminds me of a question that popped into my mind
| earlier this year. Forewarning: I'm not a professional
| physicist.
|
| If the Big Bang didn't create an equal quantity of matter
| and antimatter, and given that antimatter is still being
| produced by various cosmological sources and at a lower
| rate for proton-antiproton pairs than for electron-positron
| pairs, would that imply that the universe has a non-zero
| net electric charge which is still changing over time?
|
| And if so, what would a universe with a significant non-
| zero electric charge look like?
|
| And if not, because Noether, could the combination of e.g.
| antiprotons and positrons into anti-neutrons turn out to be
| stable and one possible candidate for dark matter?
|
| (Full blogpost:
| https://kitsunesoftware.wordpress.com/2021/02/09/baryon-
| asym...)
| adrian_b wrote:
| As far as we know, during Big Bang, after the matter had
| cooled enough for heavier particles to decay but before
| cooling enough to allow the formation of nuclei and
| atoms, there were equal numbers of protons, neutrons and
| net electrons (i.e. electrons - positrons), so the
| electric charge of the universe was zero.
|
| All the events that happened after that, e.g. the decay
| of a part of the neutrons into protons and electrons, the
| annihilation of positrons with electrons and various
| generations and annihilations of particle/anti-particle
| pairs, have not changed the total electric charge, so it
| has remained zero.
|
| The electric forces are extremely strong and they ensure
| that matter is on average electrically neutral, so the
| only long-distance forces are magnetic and gravitational.
|
| Any region with electric charge would generate strong
| electric fields with various obvious effects.
| eigenket wrote:
| Two reasons:
|
| 1. Because there isn't even close to the amount of radiation
| around we would see from such incredible amounts of
| anihlillation
|
| 2. Matter/antimatter annihilation is symmetric - it removes the
| same amount of matter and antimatter from the equation so
| however much stuff we started with we expect to still see the
| same amount of matter and antimatter and the antimatter doesn't
| seem to be around
|
| Its worth emphasising that there is some matter/antimatter
| asymmetry in the standard model, they don't behave as exact
| "mirror opposites" particularly in the context of some kaon
| physics, but there isn't even close to enough asymmetry to
| explain the amount of matter and the lack of antimatter we
| observe.
| fb13 wrote:
| Thanks for the information. Does dark matter ever enter the
| discussion in terms of "there isn't even close to enough
| asymmetry to explain the amount of matter and the lack of
| antimatter we observe."
| eigenket wrote:
| I'm not really an expert on that so I can't comment
| definitively, I think the answer is a tentative "yes" but I
| don't think theres really enough evidence to favour any
| proposals there.
| ephimetheus wrote:
| We don't know a ton about dark matter. Basically (and I'm
| simplifying a bit) you need CP violation (and more than is
| explained by the standard model) to account for the
| asymmetry. If your dark matter model of choice can generate
| CP asymmetry, then it could enter the discussion.
| adrian_b wrote:
| There is no reason to believe that matter and antimatter was
| produced in equal quantities in the Big Bang.
|
| 2 particles, one of which is the anti-particle of the other
| form a system where the sum of all quantities that are
| conserved can be 0, so a transformation generating or
| destroying both particles is possible.
|
| Nevertheless there are also other systems of particles where
| the sum of all conserved quantities can be 0 so there should
| exist transformations that generate all of them
| simultaneously.
|
| Besides the systems of 2 particles, where generation and
| annihilation is well known, there are systems of 4 particles
| with zero sum of all conserved quantities (e.g. a quark, an
| anti-quark, an electron and an anti-neutrino) and also
| systems of 8 particles e.g. the 3 kinds of u quarks + the 3
| kinds of d-quarks + electron + anti-neutrino.
|
| The systems of 4 particles can actually be generated and
| destroyed in weak interactions, similarly like the the
| systems of 2 particles can be generated and destroyed in
| electromagnetic interactions.
|
| For the systems of 8 particles, there is currently no theory
| about a mechanism of generation or annihilation, but my bet
| is that this is how matter was created during Big Bang (those
| 8 particles aggregate into 1 proton, 1 neutron, 1 electron
| and 1 neutrino).
|
| For now we have a theory of Big Bang not from the starting
| point but from a little later when there already was normal
| matter as we know it.
|
| There is no theory yet for the origin of Big Bang, but there
| is absolutely no reason to believe that at the beginning
| there was an electromagnetic interaction generating
| simultaneously equal numbers of particles and anti-particles.
| For that to be true, the matter and anti-matter should have
| been preceded by electromagnetic waves i.e. photons, of
| equivalent energy with the mass of the generated matter and
| anti-matter, which would then annihilate restoring the
| previous photons. Such a theory goes nowhere, so it does not
| match reality. Whatever started Big Bang, it was not an
| electromagnetic field generating pairs of particles and anti-
| particles.
| canadianfella wrote:
| > There is no theory yet for the origin of Big Bang
|
| There are many.
| adrian_b wrote:
| None of them can explain anything besides what is already
| explained without them.
|
| The Big Bang theory that is useful starts with matter at
| very high temperature and density, but otherwise not
| different from the matter that we know. That happens at
| some uncertain time interval after the beginning of Big
| Bang.
|
| We may try to extrapolate towards time 0 and increasing
| temperatures and densities, but that reaches soon values
| of temperature and pressure high enough that we do not
| really know how matter behaves in those conditions and it
| does not matter anyway because it does not influence what
| happens later.
|
| For the later evolution, it is enough to start the
| modeling of Big Bang from the moment when the temperature
| became low enough, e.g. of some tens of MeV, i.e. when
| matter was too hot for nuclei and atoms to exist, but
| cold enough so that it consisted of a plasma composed of
| protons, neutrons, electrons, positrons, photons and
| neutrinos, with only negligible quantities of heavier
| particles.
|
| For the time 0 there is no theory that can predict
| anything quantitative.
| eigenket wrote:
| Theres a lot of stuff here that I'm not going to pretend to
| be expert enough to talk about but some is definitely
| wrong.
|
| I think the main point that is missing is that the standard
| model is very close to symmetric, so if you have some
| process that generates systems of 4 or 8 or however many
| particles from a neutral boson then exactly the same
| process should occur producing all the anti-particles of
| the above e.g. if you think you make
|
| 1 proton, 1 neutron, 1 electron and 1 neutrino
|
| in such a way that all the numbers balance then you should
| also see a process that makes
|
| 1 anti-proton, 1 anti-neutron, 1 anti-electron and 1 anti-
| neutrino
|
| Its also worth pointing out that pair production is not
| specifically an electromagnetic thing. For example the Z0
| boson pair produces fermion+anti-fermion pairs in a very
| similar way to the photon and W bosons decay to
| lepton+anti-neutrino pairs in a similar way (although W
| bosons are charged so its a little different). The Higgs
| boson also decays into quark-anti-quark,lepton-anti-lepton
| or even boson-anti-boson pairs.
| adrian_b wrote:
| You are right that that the sum of the conserved
| quantities can be zero also for the 8 quarks & leptons
| composing 1 anti-proton, 1 anti-neutron, 1 anti-electron
| and 1 neutrino.
|
| I did not claim that this is how matter was generated in
| the Big Bang, especially because for now there exists no
| theory for such a process.
|
| My point is that this is a strange coincidence as it
| would provide matter in the right proportions. Unlike for
| electromagnetic interactions, there might be some
| asymmetry making the process generating the particles
| more probable than the process generating the
| antiparticles. Because the 2 processes are decoupled,
| they might have happened at different rates, which is not
| possible with electromagnetic generation and
| annihilation.
|
| Of course, the correct explanation for Big Bang might be
| completely different, only the fact that there was no
| simultaneous generation of matter and anti-matter is
| certain.
|
| The neutral Z0 boson behaves like the photons, so I did
| not mention it.
|
| The charged W bosons cannot generate particle/anti-
| particle pairs because that would violate the
| conservation laws. They generate simultaneously 4
| elementary particles: quark, anti-quark, lepton and anti-
| lepton.
|
| The quark and the anti-quark remain bound as one meson.
|
| The reverse event is also possible, but highly
| improbable, because it is unlikely for a meson, a charged
| lepton and a neutrino to collide simultaneously, in order
| to annihilate into W bosons. What happens frequently is
| the equivalent event when 2 of the 3 collide and
| transform into a W boson together with the antiparticle
| of the third.
|
| These type of events involving W bosons are what I was
| referring to as the generation/annihilation of systems of
| 4 elementary particles through weak interactions.
| eigenket wrote:
| The pair production processes like
|
| W^+ -> positron + electron neutrino
|
| W^- -> muon + muon antineutrino
|
| are allowed as are processes like
|
| W^+ -> up quark + anti-down quark
| adrian_b wrote:
| The facts are more complex than that.
|
| Unlike the stable photon, the W bosons have a negligible
| lifetime.
|
| So in fact those processes listed by you that result in 2
| particles are 4-particle processes.
|
| Two particles collide, either a quark and an anti-quark
| giving other 2 particles, a lepton + an anti-lepton (in
| your exemples positron + electron neutrino or muon + muon
| antineutrino) or a lepton collides with an anti-lepton
| giving a quark and an anti-quark (in your exemple up
| quark + anti-down quark).
|
| So there are always 4 particles, either quark, anti-
| quark, lepton and anti-lepton, or there may be both at
| input and at output a lepton and an anti-lepton, which
| includes the case of scattering through weak
| interactions, or there maybe both at input and at output
| a quark and an anti-quark, which includes the case when a
| meson decays into another meson through a weak
| interaction.
|
| All the interactions involving W bosons are 4-particle
| interactions, where the W boson is an intermediate
| particle that exists only during a negligible time.
|
| The 4 particles may be all at the input, all at the
| output, 3 at the input and 1 at the output, 1 at the
| input and 3 at the output or the most frequent case, 2 at
| the input and 2 at the output.
|
| Like all the interactions between elementary particles
| all these cases are equivalent and all the variants are
| interchangeable by moving a particle from the input to
| its anti-particle at the output or vice-versa.
|
| In most cases you need to pay attention only to the 4
| particles participating in the weak interaction and not
| to the intermediate W boson.
|
| The existence of the W boson matters only when you need
| to compute various things because this intermediate
| particle transforms diagram nodes with 4 edges that
| cannot be computed into diagram nodes with 3 edges that
| can be computed with the methods of quantum
| electrodynamics.
| scotty79 wrote:
| Can radiation be absorbed by matter and and turned into
| kinetic energy? Because matter in the universe has plenty of
| that when super massive blackholes and things that fly around
| them run away from each other.
| eigenket wrote:
| Sure, but the amount of kinetic energy (in sensible frames
| of reference) isn't really significant compared to the
| amount of energy locked up as mass energy of stuff.
|
| To close approximation the kinetic energy of something is
| 1/2 * mv^2 and the mass energy is m c^2 so to have a
| meaningful contribution to the total energy thespeed has to
| be close to the speed of light. We generally don't observe
| big things (stars, black holes, galaxies) moving anywhere
| near to that fast and their energy is basically entirely
| due to their mass rather than their speed.
| ganzuul wrote:
| Wouldn't that stuff have taken leave right at the
| beginning?
| eigenket wrote:
| Things can't really "leave", well they can, but the best
| guess we have is that there is no particularly special
| place in the universe, everywhere is roughly the same. So
| if even if all the really speedy stuff from here "left"
| we'd see the speedy stuff arriving from other places.
| ganzuul wrote:
| I thought 'no special place' was meant to say physics was
| the same everywhere; not the environment.
|
| If you have a sparse gas with a random distribution of
| velocities and you let it sit for a while, you will find
| that the outliers on the upper end have have moved
| further away from cluster center. They have boiled off.
| eigenket wrote:
| The point is that as far as we can tell there isn't a
| "center" of the universe.
| ganzuul wrote:
| Because it is an open set?
| 8note wrote:
| On average though, you'll find similar outliers in
| whatever direction you look.
| scotty79 wrote:
| Really? Distatnt galaxies move away from each other at
| insane speeds. That seems like a plenty of kinetic energy
| to me...
|
| What if kinetic energy is the only thing responsible for
| galaxies moving away from each other? Or at least the
| bulk of it? Maybe the part that we interpret as a period
| of super fast inflation right after the big bang? Maybe
| this rapid inflation is just a way of interpreting matter
| having huge amount of kinetic energy relative to each
| other right from the beginning?
| eigenket wrote:
| That would mean that we are (as in our galaxy is) in a
| very, very special position because we see all distant
| galaxies moving away from each other with velocities
| proportional their distance from us.
|
| This is exactly what we would expect if the universe is
| expanding isotropically but under the "galaxies actually
| moving away from each other" hypothesis this is only
| possible if Earth is exactly at the center of a sort of
| "explosion of galaxies".
|
| A second issue with this theory is that we see objects
| with redshifts that would mean that if their apparent
| motion is actually real motion then they would be moving
| away from us faster than the speed of light. As far as we
| know this is completely impossible for actual motion but
| is exactly what we would expect from expansion.
| scotty79 wrote:
| Not really special. Try simulating this:
|
| Start with bunch of objects at coordinates 0,0,0. Give
| them random velocities from 0 to c. Then just let them
| move according to Newtons law.
|
| Now focus on single random object and narrow down your
| field of view so you won't see the edge. Look at other
| objects. They will seem to be moving away directly from
| the point you focused on with velocities proportional to
| distance from it.
|
| I made such simulation and made calculations to ensure
| that the velocities of other points face directly away
| from the point I'm observing. And they really do.
|
| Even in completely flat Newtonian universe there could
| have been a sort of Big Bang with epicenter and it could
| be as simple as "give matter random speeds" and we living
| on a one speck of matter would have no way to figure out
| that there is a center or where is it.
|
| When I asked about this on physics stack exchane I got a
| shurg that, yeah cosomology is basically that but with
| Einstein not Newton.
|
| All that talk about spacetime inflating is just a result
| of matter 'dragging' the spacetime along as it moves.
|
| The faster than light galaxies far away are not a problem
| because the speed of their movement that we measure is
| sum of their kinetic movement (which could be almost at
| light speed) and expansion of the space time between us
| and them as the spacetime is 'dragged' by them with GR.
| But you can equally well interpret the math and data as
| galaxies at rest and all the speed coming from spacetime
| expansion and for I have no idea what reasons people
| actually prefer to do that.
| serf wrote:
| >Really? Distatnt galaxies move away from each other at
| insane speeds. That seems like a plenty of kinetic energy
| to me...
|
| US 708 is going _fast_ and it 's still only 0.4 percent
| the speed of light.
|
| if you're talking about relative speeds between
| astronomical objects being very high due to cosmic
| expansion, i'm not well enough versed in the physics to
| know why or explain how, but the energy locked up in that
| movement doesn't seem as practical to talk about with
| regards to creating _work_.
|
| we know how to harness kinetic and thermal energy, as far
| as I know we're not yet able to surf the cosmic
| expansion, except inadvertently.
| scotty79 wrote:
| What if only the part of the speed of ftl remote galaxies
| is due to expansion and part (lower than c but
| arbitratily close to c for distant galaxies) is due to
| kinetic energy?
|
| Their sum might easily be faster than light and they
| still might have insane kinetic energy.
| laurowyn wrote:
| I'm not reading that as the matter/antimatter didn't annihilate
| each other after the big bang. Rather, that matter and
| antimatter were not produced in equal amounts - if they were
| produced in equal quantities, why do our observations show such
| a large amount of matter and not equal amounts of antimatter?
| production and annihilation are balanced processes, so why the
| heavy sway to one side?
|
| Th obvious suggestion is that if there was equal amounts of
| antimatter near us, we would likely be annihilated already. But
| that doesn't explain why so little can be found remotely
| either.
|
| The potential for matter and antimatter to alternate is an
| interesting proposal that could explain our little island of
| stability in the universe.
|
| Or it could be something else entirely. I'm a software
| engineer, not a physicist.
| philipov wrote:
| My baseless speculation of choice is that if antimatter is
| basically regular matter moving backwards in time, all the
| antimatter we expect to see from the big bang went flying out
| into negative time, into an anti-universe temporally disjoint
| from our own.
| scotty79 wrote:
| That would mean we should see the events where anti-particle
| flies, absorbs (or emits, or does none of that) some energy
| and turns into normal particle. Unless the moment of big bang
| was totally unique time when such things happened.
| ben_w wrote:
| Isn't that a valid description of both pair-creation and
| pair-annihilation events?
|
| (Serious question; I'm _not_ a professional physicist)
| scotty79 wrote:
| No. Pair creation is two photons disappearing and turning
| into particle and antiparicle. And annihilation is
| turning particle and antiparticle into two photons.
|
| When particle and photon meet the result is particle and
| photon just moving differently. You probably might think
| of this as old particle and photon disappearing and new
| paricle and new photon appearing.
| philipov wrote:
| Shouldn't you be able to rotate the Feynmann diagram in
| all directions and get a valid interaction, though?
| ben_w wrote:
| Ah, I misread your original post. Sorry!
| whatshisface wrote:
| Antimatter isn't matter flying backwards in time, it just has
| some symmetries that make it appear so in some cases.
| scotty79 wrote:
| How can you tell? I thought you cand pretty much swap
| matter with anti-matter and reverse time and all the
| physics seems the same.
| ben_w wrote:
| Almost but not quite:
|
| https://en.wikipedia.org/wiki/CPT_symmetry
|
| https://en.wikipedia.org/wiki/CP_violation
| scotty79 wrote:
| So you can't run an experiment and tell apart matter in
| normal time and from anti-matter in mirror coordinates
| with time reversed?
|
| Because while CP symmetry is violated CPT symmetry always
| holds as far as we know?
| ben_w wrote:
| I believe that's the case, yes.
| scotty79 wrote:
| So you might think of anti-matter being matter but with
| all the spacetime coordinates (including time) reversed,
| right?
|
| Traveling in opposite directions and back in time?
| philipov wrote:
| Could you please explain why it is wrong to think of the
| appearance caused by those symmetries as real?
| filmor wrote:
| I think that the experimentally found CP-violations show
| that anti-matter doesn't necessarily behave _exactly_ the
| same as time-reversed matter.
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