[HN Gopher] An antimatter experiment shows surprises near absolu...
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An antimatter experiment shows surprises near absolute zero
Author : nsoonhui
Score : 114 points
Date : 2022-03-17 11:20 UTC (11 hours ago)
(HTM) web link (www.quantamagazine.org)
(TXT) w3m dump (www.quantamagazine.org)
| mikewarot wrote:
| It took reading it a few times to figure out that you can slam
| antiprotons into liquid helium and end up with a shelf stable
| helium/antimatter hybrid atoms.
|
| That is the part that amazes me... I had no idea that was even
| possible. I wonder what other atoms this would work with?
| waterhouse wrote:
| > shelf stable helium/antimatter hybrid atoms
|
| Well. Depends on the scale you consider to be shelf stable:
| _The antiproton can thus orbit the nucleus for tens of
| microseconds, before finally falling to its surface and
| annihilating._ I think that _is_ a relatively long time in
| terms of particle-physics experiments, but it 's not something
| you could stockpile.
|
| https://en.wikipedia.org/wiki/Antiprotonic_helium
| gameswithgo wrote:
| Doesn't it only last a few microseconds?
| mmmBacon wrote:
| I skimmed the article looking for something about the
| spectroscopy of antiprotonic helium. I'd assume that the
| spectroscopy of such an atom is very different from normal helium
| but the article says nothing about how the spectroscopy is
| different or if the spectroscopy observed is what we expect
| (thinking of anti Hydrogen here). The article does point to
| Helium made from a pion though where the atoms spectroscopy was
| observed.
| Pet_Ant wrote:
| Will any anti-matter/matter annhilate or does it have to be the
| complementary type? Like can a proton and positron annihilate?
|
| PS- hate the article title, pure vague clickbait
| Xcelerate wrote:
| I've always thought "annihilate" was a poorly chosen word to
| describe what happens here. It makes it sound like stuff is
| disappearing, and if you consider "stuff" to be only those
| particles that have rest mass, then fine, the aggregate value
| of the rest mass property of the system becomes zero, but most
| of the "substance" of everyday objects isn't due to rest mass
| anyway. Also, the notion that rest mass "turns into energy" is
| similarly silly, as energy is just a scalar value that is
| conserved over time due to the time translational invariance of
| the laws of physics. It would be like saying I converted an
| onion into the number 7; it doesn't make any sense. The energy
| of the system is the same after the collision as it was before
| the collision.
|
| A better way to describe matter/anti-matter annihilation is
| that different types of particles interact in all sorts of
| different ways, and a collection of particles can turn into
| other types (and number) of particles during these interactions
| as long as the complete system obeys certain conservation laws.
| Matter/anti-matter collision is no more special in this sense
| than any of the other types of interactions in the Standard
| Model.
| chasil wrote:
| A star that is undergoing a "pair-instability" supernova
| would express rather dramatic opinions on exactly what is
| meant by "annihilation" as it is destroyed by positrons.
|
| https://en.wikipedia.org/wiki/Pair-instability_supernova
| raattgift wrote:
| Annihilating positrons don't really consume an appreciable
| amount of the star's ordinary matter. Their presence mostly
| serves to allow a big star to compress into a much smaller
| space than otherwise, leading to a more powerful
| thermonuclear detonation.
|
| One way to think about the gamma -> lepton-pair is that the
| parent (the gamma photon) must have _at least_ the same
| momentum-energy as it 's lepton-pair children. That is, the
| positron and electron each have less momentum than the
| gamma, and since each half of the pair can go in a
| different random direction, a gamma whose entire momentum
| may be outwards might turn into a positron that, for
| example, heads inwards and an electron that heads outwards,
| each carrying about half the gamma's original outward
| momentum. The outward momentum flux is what does the work
| of keeping the star from imploding gravitationally, and the
| balance is usually fairly fine; when a substantial amount
| of that momentum flux is turned in the wrong direction, the
| implosion is inevitable. There is nothing particularly
| special about the positrons (indeed, muons and antimuons,
| and other cascades of particles likely are produced too)
| other than that they and their partner electrons represent
| a redirection of mostly-outward momentum.
|
| Another way of thinking about it is that a hot gas of
| gammas becomes a cooler gas of gammas+leptons, and "hot gas
| rises" is what keeps the star inflated. Cool down the
| middle, and the star contracts, because there is less hot
| gas rising. Initially, the sudden central cooling may re-
| heat the star's middle (compressive heating as outer layers
| sink inwards), which produces further central cooling
| (pair-production), which produces yet more heating, and so
| forth, with the swings from heating to cooling becoming
| more extreme over time. In some stars undergoing this
| process, the heating wins, and a thermonuclear detonation
| completely obliterates the star, scattering its remains to
| infinity. In other stars other factors help "lift" the
| outer envelope (e.g. the star's rotation, or the presence
| of many atomic nuclei heavier than helium) and cooling
| wins, with a super-dense phase (likely a black hole)
| holding on to some of the detonating material.
|
| More technically, the lepton-pair has more degrees of
| freedom than the pair's parent gamma, and as one adds more
| degrees of freedom to a system, one increases the system's
| _heat capacity_. This is why one calls the process of pair
| production "cooling": effective temperature goes down when
| specific heat capacity goes up. For a nice rabbit hole,
| visit negative heat capacity in stars: https://en.wikipedia
| .org/wiki/Heat_capacity#Negative_heat_ca... with a general
| attitude of "momentum can condense, or become frozen into
| structures with large numbers of internal degrees of
| freedom (DoFs); in fusing stars, that's ever-heavier atomic
| nuclei. Black holes have _enormous_ numbers of such DoFs
| inside the horizon, with number of DoFs increasing with
| black hole mass, which in turn is why more massive black
| holes are _colder_ than small black holes ".
| czzr wrote:
| Has to be the complementary type. In your example, they will
| repel each other (since both are positively charged).
| willis936 wrote:
| What about a neutron and positron? They could collide with no
| electrostatic repulsion but have quark-antiquark pairs.
| jfengel wrote:
| The electrostatic repulsion isn't really an issue. If you
| shoot an electron hard enough, it will overcome the
| electrostatic repulsion.
|
| The bigger issue here is that a positron is a lepton, and a
| neutron has a 0 lepton count. So they can't react directly
| while conserving lepton count.
|
| However, a neutron can decay into a proton, an electron,
| and a anti-neutrino. The neutrino is a lepton. So there is
| a path where the neutron and positron react to form a
| proton, anti-neutrino, and two gamma rays.
|
| I don't know if that actually occurs. There are even more
| factors to be considered. But it illustrates how you have
| to go about conserving all of the things, including
| obscurer factors like leptonicity.
| db48x wrote:
| No, a positron is a lepton but a neutron is not. Since
| lepton number must be conserved, these two will not
| annihilate with each other. Similarly, one has a positive
| charge and the other neutral, so electric charge would not
| be conserved either.
|
| Are you thinking of a neutron plus an antiproton? Those can
| annihilate with each other. They are not direct
| antiparticles of each other, but the neutron is really an
| up quark and two down quarks, while the antiproton is
| really two anti-up quarks and an anti-down quark. There are
| several different ways that they could annihilate with each
| other.
|
| Worse, all hadrons (like neutrons and protons) contain a
| confined gluon field which you can think of as containing
| additional quark-antiquark pairs. There's a chance that the
| gluon fields of the two particles will interact as if a
| quark-antiquark pair from one had annihilated with a quark-
| antiquark pair from the other, further complicating the
| results. This can cause the creation of other exotic
| particles as the quarks combine and recombine.
|
| You can read more about it on Wikipedia, of course:
| https://en.wikipedia.org/wiki/Annihilation
| ellis-bell wrote:
| lepton number does not need to be conserved. it is an
| approximate symmetry of nature. if lepton number were
| conserved, neutrinos could not oscillate.
|
| quarks, the particles composing protons and neutrons,
| have fractional charge; these would be the particles that
| would interact with an electron or positron. the charges
| wouldn't work out (charge is conserved (as far as we
| known...)) so there wouldn't be a fundamental
| electromagnetic interaction between a single quark and a
| e+/e- (i.e., an annihilation). But there are fundamental
| _weak_ interactions between quarks and e+ /e-; these
| processes are known as inverse beta decay and are used
| for pet scans.
| db48x wrote:
| As a rule of thumb it's good enough for most particle
| interactions.
| lanstin wrote:
| To quote Wikipedia: "Lepton flavor is only approximately
| conserved, and is notably not conserved in neutrino
| oscillation.[6] However, total lepton number is still
| conserved in the Standard Model."
|
| The beta decay gives rise to e.g. a positron and a
| neutrino (or an electron and an anti-neutrino).
| floxy wrote:
| What is going on in neutron stars? The layman's answer is
| that the electrons get squeezed into the protons due to
| the extreme gravity, leaving only neutrons. But I suppose
| (?) there needs to be a anti-neutrino or similar that
| comes along to "complete" the reaction?
|
| Edit: I guess it is inverse beta decay:
|
| https://en.wikipedia.org/wiki/Inverse_beta_decay#Electron
| _in...
| fsh wrote:
| Only hadrons (neutrons, protons, mesons,...) consist of
| quarks. Positrons are leptons which are elementary
| particles.
| aardvark179 wrote:
| For particles to annihilate everything has to be conserved. A
| proton and a positron cannot annihilate because they both have
| positive electric charge, but you also need a bunch of other
| properties to cancel out as well.
| breezeTrowel wrote:
| How is the article title clickbait? They expected the spectral
| lines of hybrid atoms to widen the same as regular atoms when
| immersed in a dense fluid. Instead the spectral lines narrowed.
| This was so surprising they thought they made a mistake and
| spent years checking over their work only to conclude that,
| yes, the finding was in fact real.
| whimsicalism wrote:
| It's not clickbait. And obviously a title about a complex
| physics topic is going to be vague.
|
| It's just classic HN bikeshedding, as usual.
| gus_massa wrote:
| > _PS- hate the article title, pure vague clickbait_
|
| I agree. From the article:
|
| > _Yet physicists have found nothing amiss, no conclusive sign
| that antimatter particles -- which are just the oppositely
| charged twins of familiar particles -- obey different rules._
|
| So no surprises. Also, the " _near absolute zero_ " is true,
| but it's just to make the experiment easier, it's not important
| from the theoretical point of view. Anyway, it's an interesting
| experiment in spite of the press article title.
|
| > _Will any anti-matter /matter anhilate or does it have to be
| the complementary type? Like can a proton and positron
| annihilate?_
|
| There are a lot of conservation rules, for example
|
| * proton + positron --> gamma rays
|
| is impossible because the electric charge is conserved
|
| * proton(+1) + positron (+1) --> gamma rays (0)
|
| so +2 charge before and 0 after the annihilation, so it's
| imposible.
|
| There are other number that are conserved, like the number of
| quarks (a quark is +1 and an anti-quark is -1) so
|
| * proton(+3) + positron (0) --> gamma rays (0)
|
| And the number of leptons, particles like electron are +1, and
| particles like positron are -1
|
| * proton(0) + positron (-1) --> gamma rays (0)
|
| And there are other examples of numbers that must be conserved.
| (And there are numbers that are almost conserved and they can
| change but usually the decay/annihilation is "slower".)
|
| It's interesting how this work with changes that are common,
| for example
|
| * neutron(0) --> proton(+1) + electron(-1) + anti-neutrino(0)
| [charge]
|
| * neutron(+3) --> proton(+3) + electron(0) + anti-neutrino(0)
| [quarks]
|
| * neutron(0) --> proton(0) + electron(1) + anti-neutrino(-1)
| [leptons]
|
| or another example
|
| * electron(-1) + positron(+1) --> two gamma rays(0) [charge]
|
| * electron(0) + positron(0) --> two gamma rays(0) [quarks]
|
| * electron(+1) + positron(-1) --> two gamma rays(0) [leptons]
|
| Note that as said before there are more numbers that are
| conserved and more complicated rules. If one of these 3 rules
| are broken the transformation is impossible (AFAWK). If these 3
| rules are ok, then you must check all the other rules.
| breezeTrowel wrote:
| > > _PS- hate the article title, pure vague clickbait_
|
| > _I agree. From the article:_
|
| > > _Yet physicists have found nothing amiss, no conclusive
| sign that antimatter particles -- which are just the
| oppositely charged twins of familiar particles -- obey
| different rules._
|
| > _So no surprises. Also, the "near absolute zero" is true,
| but it's just to make the experiment easier, it's not
| important from the theoretical point of view. Anyway, it's an
| interesting experiment in spite of the press article title._
|
| I'm sorry but did you actually read the article? The first
| quote is from the very first paragraph talking about
| physicists in general. And the cooling to near absolute zero
| was not "just to make the experiment easier." And it
| definitely is important.
|
| The surprise, in case you haven't read the article, is that
| one expects spectral lines of atoms in an ultra dense fluid
| (i.e. supercooled helium) to be smudged due to the
| interactions with surrounding atoms. But, when performing
| laser spectroscopy on hybrid atoms (regular nucleus but with
| antiprotons taking the place of electrons) the spectral lines
| narrowed instead of widened. See, for example, this
| paragraph:
|
| > _Where the spectral lines of most atoms would have gone
| completely haywire in the increasingly dense fluid, widening
| perhaps a million times, the Frankenstein atoms did the
| opposite. As the researchers lowered the helium bath to icier
| temperatures, the spectral smudge narrowed. And below about
| 2.2 kelvins, where helium becomes a frictionless
| "superfluid," they saw a line nearly as sharp as the tightest
| they had seen in helium gas. Despite presumably taking a
| battering from the dense surroundings, the hybrid matter-
| antimatter atoms were acting in improbable unison._
|
| This was a big surprise. That's why the next two lines are:
|
| > _Unsure what to make of the experiment, Soter and Hori sat
| on the result while they mulled over what could have gone
| wrong._
|
| > _"We continued to argue for many years," Hori said. "It was
| not so easy for me to understand why this was the case."_
|
| So, no, the article title is not clickbait and, yes, this is
| "important from the theoretical point of view".
| gus_massa wrote:
| <handwaving>The sharper espectral lines doesn't look too
| surprising, because in the superfluid liquid the Helium
| atoms "want" to be in the state creates the Bose
| condensate, so they don't "want" to colide too much with
| the disolved impurities. So the impurities are not
| disturbed too much and you get a nice
| spectrum.</handwaving>
|
| You may not like my handwavig, and in that case I agree
| that my handwaving is not a proof. Perhaps this has more
| info: https://en.wikipedia.org/wiki/Bose%E2%80%93Einstein_c
| ondensa...
|
| Anyway, other teams already used superfluid helium to make
| spectroscopy of molecules (without antimater). The second
| result in my Google search is https://www.tandfonline.com/d
| oi/full/10.1080/23746149.2018.1...
|
| > _For more than two decades, encapsulation in superfluid
| helium nanodroplets has served as a reliable technique for
| probing the structure and dynamics of molecules and
| clusters at a low temperature of [?]0.37 K. Due to weak
| interactions between molecules and the host liquid helium,
| good spectral resolution can usually be achieved, making
| helium droplets an ideal matrix for spectroscopy in a wide
| spectral range from infrared to ultraviolet._
| cupofpython wrote:
| Side question. I understand the positive and negative
| charges, but when we say "+1" or "-1" what are the units? I
| never understood that part. an electron is -1 and a proton is
| +1, but what makes them equal? Why / how do all these things
| of varying sizes and properties have the same magnitude for
| charges?
| jdmichal wrote:
| https://en.wikipedia.org/wiki/Elementary_charge
|
| And, more specifically:
|
| https://en.wikipedia.org/wiki/Elementary_charge#Charges_les
| s...
|
| Basically, quarks seem to have charges that are 1/3 _e_.
| However, they can only be stable in groups of three, and
| those groups must have an integer value. So the only
| possibilities are -1, 0, and 1.
|
| As to _why_ the above is true, it just seems to be a rule.
| We don 't have any good reasoning for why that rule exists.
| ellis-bell wrote:
| mesons are composed of two quarks and can have neutral or
| non-neutral charge. baryons are composed of three quarks
| and can have neutral or non-neutral charge. both mesons
| and baryons are classified as _hadrons_
|
| charge, baryon number, isospin and strangeness are all
| related by the Gell-Mann-Nishijima formula[1].
|
| the overarching principle underlying this is that in
| quantum chromodynamics, the theory of the strong
| interaction, quarks are representations of an underlying
| SU(3) gauge group containing color charge (a quantum
| number) and electric charge quantized as -1/3e and +2/3e.
| QCD is a gauge theory (like all other theories of nature)
| and the gauge boson, the gluon, also pops out of
| demanding the theory be invariant under local SU(3) gauge
| transformations.
|
| that said, no one has any clue why all of our physical
| theories are gauge theories :-)
|
| https://en.wikipedia.org/wiki/Gell-
| Mann%E2%80%93Nishijima_fo...
| cupofpython wrote:
| very interesting! I am rather unfamiliar with quantum
| chromodynamics but I am going to enjoy the gauge theory
| rabbit hole
| hnuser123456 wrote:
| Would a theory of the history of the universe be a non-
| gauge theory since there was fast initial inflation, a
| relative slowdown, and now accelerating expansion again?
| Nothing seems conserved here. It seems symmetries were
| broken as the forces split apart, anti-particles were
| annihilated, etc
| gpvos wrote:
| I am now visualizing this as a series of assert() statements
| that abort particle action when a constraint is not met.
| Sharlin wrote:
| I would expect the conservation rules to be statically
| encoded in the type system.
| WJW wrote:
| It should be statically typechecked of course, but
| especially in the quantum physics code we couldn't encode
| all the invariants correctly and runtime exceptions can
| still occur in rare occasions. A fix has been proposed
| and will hopefully be merged in time for the next
| universe release window.
| wadkar wrote:
| I don't understand how an anti-proton can orbit a nucleus without
| annihilating into pure energy with a proton from the nucleus.
|
| What am I missing here?
|
| Honestly speaking I am really amazed by these researchers ability
| to separate and then "shove" an anti-proton in a Helium gas.
| Iwan-Zotow wrote:
| > I don't understand how an anti-proton can orbit a nucleus
| without annihilating into pure energy with a proton from the
| nucleus.
|
| it means there is bound meta-stable state of combined
| proton+nucleus interacting system.
| mynameismon wrote:
| Related: https://news.ycombinator.com/item?id=30700633
| icodestuff wrote:
| Very cool. I had no idea antiprotons -- well, an antiproton and
| an electron -- could orbit a nucleus. Does it fill up the usual
| quantum state of the first electron, or does it have its own set
| of quantum states to fill? Can they get a second antiproton to
| take the place of the second electron? I imagine that should be
| at least somewhat stable as the electrons of the enclosing
| container should repel the antiprotons just fine.
| AnimalMuppet wrote:
| > Does it fill up the usual quantum state of the first
| electron, or does it have its own set of quantum states to
| fill?
|
| Not a physicist. But if I understand correctly, it fits the
| same equations the electron would, except you have to use the
| proton mass instead of the electron mass, which means it's much
| closer in. The remaining electron (I think) doesn't have Pauli
| exclusion with the anti-proton, so it looks like the electron
| in a hydrogen atom. (Since it's much further out, it sees the
| effective charge of +1 for the "nucleus" of two protons and an
| anti-proton.)
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