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Thread: Matter universe - observational evidence for lack of anti-matter?

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    Matter universe - observational evidence for lack of anti-matter?

    Except for CP violation, matter and anti-matter seem to be opposites (and wrt gravity, of course - to gravity matter and anti-matter seem the same).

    Yet there seems little in the way of anti-matter locally, and certainly no massive, condensed objects made of anti-matter.

    Why this apparent imbalance is an interesting question, in particle physics, and cosmology.

    But what about the observational basis of our conclusion that we live in a matter universe? What are the observations from which we make this conclusion?

    That's what this thread is about - why are we so confident there are no anti-matter planets in our solar system? why no anti-matter stars in the Milky Way? why no anti-matter galaxies out to z ~ 6?

    (Note: that there is anti-matter, in particle form, has been known since the positron was discovered ... in cosmic rays. INTEGRAL mapped the distribution of positrons in the ISM in the Milky Way. And so on.)

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    Quote Originally Posted by Nereid
    Except for CP violation, matter and anti-matter seem to be
    opposites (and wrt gravity, of course - to gravity matter
    and anti-matter seem the same).
    Can you provide any support for the idea that antimatter
    reacts to gravity in the same way as ordinary matter?
    General relativity theory appears to imply that it must,
    but I'm not so sure that it actually does, and I'm not
    aware of any observational evidence for the behavior of
    antimatter in response to gravity. At the moment I'm
    mainly interested in observational evidence.

    -- Jeff, in Minneapolis

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    It's an interesting question, but I think the problem is, there is no antimatter energy source that can curve spacetime in a measureable way, nor is there ever likely to be one, so this question will never be answerable. All we can say is that the simplest understanding of antimatter says that it should produce the same gravity that matter does, and this is a part of the simplest theory of gravity, so we will never have cause to introduce a more complicated theory to describe the unknown result of an impossible experiment.

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    Quote Originally Posted by Jeff Root View Post
    Can you provide any support for the idea that antimatter
    reacts to gravity in the same way as ordinary matter?
    General relativity theory appears to imply that it must,
    but I'm not so sure that it actually does, and I'm not
    aware of any observational evidence for the behavior of
    antimatter in response to gravity. At the moment I'm
    mainly interested in observational evidence.

    -- Jeff, in Minneapolis
    One example: what is the path of a positron, in a mass spectrometer, a cloud chamber, a spark chamber, a bubble chamber, ...?

    Ditto an anti-proton?

    What is the wavelength of the electron-positron annihilation line?

    (etc)

    This establishes the value of the mass of these (anti-)particles, at least in an absolute sense.

    The part that I (dimly) remember, re me+'s sign, is storage ring design - there are HEP facilities which store positrons (and anti-protons?), for weeks (months?). Those designs assume me+ is positive; the storage rings work, ergo ...

    There is at least one CERN experiment will, if it proceeds as planned, provide direct evidence of the sign ...

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    Unless you assume an ultraviolet cutoff at some scale, the energy contained in the vacuum runs to infinity. The last time I checked, this is substantially larger than the energy contained in any real (non-virtual) field. Is this trivial in your cosmology?

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    Quote Originally Posted by Nereid
    Quote Originally Posted by Jeff Root
    Can you provide any support for the idea that antimatter
    reacts to gravity in the same way as ordinary matter?
    One example: what is the path of a positron, in a mass spectrometer,
    a cloud chamber, a spark chamber, a bubble chamber, ...?

    Ditto an anti-proton?

    What is the wavelength of the electron-positron annihilation line?

    (etc)

    This establishes the value of the mass of these (anti-)particles,
    at least in an absolute sense.

    The part that I (dimly) remember, re me+'s sign, is storage ring
    design - there are HEP facilities which store positrons (and
    anti-protons?), for weeks (months?). Those designs assume me+ is
    positive; the storage rings work, ergo ...
    The inertial masses of antiprotons and antielectrons have long
    been known. The gravitational masses have not been measured
    at all.

    Quote Originally Posted by Nereid
    There is at least one CERN experiment will, if it proceeds as
    planned, provide direct evidence of the sign ...
    I had long hoped that the ATHENA apparatus would be connected
    to an experiment chamber which would measure the response of
    anti-hydrogen atoms to gravity, but apparently ATHENA was not
    able to produce anti-hydrogen atoms which were cool enough.
    The article says they only lasted for microseconds.

    Now we'll have to wait for ALPHA to do what I expected from
    ATHENA.

    -- Jeff, in Minneapolis

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    Quote Originally Posted by Ken G
    It's an interesting question, but I think the problem is, there
    is no antimatter energy source that can curve spacetime in a
    measureable way, nor is there ever likely to be one,
    Not anywhere nearby, no. But there's a reason I'm posting in
    an astronomy forum.

    Quote Originally Posted by Ken G
    so this question will never be answerable.
    Huh? We just need to watch some neutral anti-hydrogen atoms
    and see what they do in response to Earth's gravity!

    Quote Originally Posted by Ken G
    All we can say is that the simplest understanding of antimatter
    says that it should produce the same gravity that matter does,
    and this is a part of the simplest theory of gravity, so we will
    never have cause to introduce a more complicated theory to
    describe the unknown result of an impossible experiment.
    Nothing impossible about observing the response of neutral
    anti-hydrogen atoms to Earth's gravity. Just difficult enough
    that it hasn't been done yet. Various experiments have been
    promising to do it Real Soon Now for at least 20 years.

    I'm optimistic that it won't be too much longer.

    -- Jeff, in Minneapolis

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    Quote Originally Posted by turbo-1
    Unless you assume an ultraviolet cutoff at some scale, the energy
    contained in the vacuum runs to infinity. The last time I checked,
    this is substantially larger than the energy contained in any real
    (non-virtual) field. Is this trivial in your cosmology?
    Was that addressed to me? It is beyond my level of understanding.
    But I agree that infinity is substantially larger than most
    everything that is real.

    Edit to add: Was it to me, but posted in the wrong thread?

    -- Jeff, in Minneapolis
    Last edited by Jeff Root; 2006-Aug-18 at 05:41 AM.

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    Quote Originally Posted by Jeff Root View Post
    Huh? We just need to watch some neutral anti-hydrogen atoms
    and see what they do in response to Earth's gravity!
    Yes, I now see that your question was will antimatter react to gravity similarly to regular matter, but we already know that-- light is antimatter (light is its own antiparticle, anyway), and it reacts normally to gravity. That was the big test that gravity is indeed an effect on spacetime, adding antihydrogen would be a nice but relatively minor extension. I thought you were asking if antimatter generated gravity in the same way as matter, that is what we will likely never have any way of knowing. Perhaps precision cosmology will be able to establish at least that light produces gravity in the normal way, using the early universe (maybe it has already, in which case we already have pretty much have the answer, though it's always nice to be sure by extending from the antimatter properties of light to more obvious forms of antimatter like antiprotons).

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    Quote Originally Posted by Nereid View Post

    But what about the observational basis of our conclusion that we live in a matter universe? What are the observations from which we make this conclusion?

    That's what this thread is about - why are we so confident there are no anti-matter planets in our solar system? why no anti-matter stars in the Milky Way? why no anti-matter galaxies out to z ~ 6?
    Of course matter dominates the present universe through some process of asymmetry, the Big Bang model predicts that equal amounts of matter and antimatter should have been created with the initial explosion. Experiments continue to search for this apparent asymmetry. But is it not true, that for anti-matter galaxies or for anti-matter stars in the Milky Way to exist, or even for anti-matter planets to exist in the Solar System locale because of the fact that this would create a matter-antimatter boundary with the medium of space, thus producing gamma rays, which we have been unable to detect. Is this not the observational data we portray to conclude that we live in a matter universe.
    (2000), Sci.News 158, 86.
    (1997), Science 278, 226

    CJ
    Last edited by ToSeek; 2006-Aug-24 at 02:30 PM. Reason: Fixed quote tags

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    Quote Originally Posted by Ken G View Post
    light is antimatter (light is its own antiparticle, anyway), and it reacts normally to gravity.
    This is one of those quirks of nature that I find fascinating. Is this fact a clue of some kind? All matter(+or-, fission, fusion, and annihlation) when converted to energy produces the same thing, an ordinary photon.

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    Quote Originally Posted by Ken G
    Yes, I now see that your question was will antimatter react to
    gravity similarly to regular matter, but we already know that--
    light is antimatter (light is its own antiparticle, anyway), and
    it reacts normally to gravity. That was the big test that gravity
    is indeed an effect on spacetime,
    Yes. You got right to the big question for which I came to
    BAUT in the first place, but hadn't asked, yet: Is the photon
    really it's own antiparticle? If so, then clearly all photons
    behave the same way in all gravity fields. If not, then
    antiphotons may behave differently from ordinary photons.

    Can you (or anyone) explain what evidence exists that the
    photon is its own antiparticle?

    Quote Originally Posted by Ken G
    I thought you were asking if antimatter generated gravity in the
    same way as matter, that is what we will likely never have any
    way of knowing.
    If the photon is not its own antiparticle, then antiphotons may
    be deflected differently by the gravity of ordinary matter from
    the way ordinary photons are deflected. Ordinary photons may be
    deflected differently by the gravity of antimatter from the way
    they are deflected by the the gravity of ordinary matter.

    If large regions of antimatter exist in the Universe, then the
    difference would be visible as a unique gravitational lensing,
    producing faint, straight radial lines as opposed to the bright
    circumferential arcs that have been seen so far.

    -- Jeff, in Minneapolis

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    Quote Originally Posted by jlhredshift
    All matter(+or-, fission, fusion, and annihlation) when
    converted to energy produces the same thing, an ordinary photon.
    I would agree if it were the case that a particle and its
    antiparticle annihilating created a single photon. But that
    is not what happens. Two photons are created. How do we
    know that one of those two is not an antiphoton?

    -- Jeff, in Minneapolis

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    Quote Originally Posted by Jeff Root View Post
    Two photons are created. How do we
    know that one of those two is not an antiphoton?
    I'm not a particle physicist, but I think one argument would be that the two photons created by that annihilation behave in an indistinguishable way. One has to be matter and the other antimatter to obey the desired conservation laws (abandoning those laws would need a good reason), yet they behave just the same in all ways we can test, so we'd have to invent new physics to explain how a photon could not be its own antiparticle. But perhaps the bottom line is, we don't know it, but there is as yet no justification to doubt it. And if antiphotons behaved differently under gravity, then why would we not observe two half-bright images of stars as they are gravitationally lensed by the Sun?

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    Quote Originally Posted by Jeff Root View Post
    I would agree if it were the case that a particle and its
    antiparticle annihilating created a single photon. But that
    is not what happens. Two photons are created. How do we
    know that one of those two is not an antiphoton?

    -- Jeff, in Minneapolis
    I did not know that. Or, I should say that after looking at Feynman diagrams it didn't dawn on me that there were always two photons coming out. And, your'e last question is my question too.

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    Quote Originally Posted by jlhredshift View Post
    This is one of those quirks of nature that I find fascinating. Is this fact a clue of some kind?
    I think the theoretical idea must be that a particle with no rest mass does not have enough internal degrees of freedom to distinguish a matter mode from and antimatter mode. It is its own antiparticle by virtue of not having an avenue by which to be different from its antiparticle. I suspect it would violate relativity if the two photons were distinguishable yet had no medium and no rest mass.

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    Quote Originally Posted by Ken G View Post
    I think the theoretical idea must be that a particle with no rest mass does not have enough internal degrees of freedom to distinguish a matter mode from and antimatter mode. It is its own antiparticle by virtue of not having an avenue by which to be different from its antiparticle. I suspect it would violate relativity if the two photons were distinguishable yet had no medium and no rest mass.
    So, then a neutrino has more degrees of freedom?

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    The jury is still out on whether any neutrinos are massless, but your point is well taken-- the very fact that theorists consider it a possibility that the electron neutrino is massless (and it is likely that trinitree will comment that other possibilities exist for all neutrinos, but I'm giving the mainstream view here), suggests that my understanding is incomplete. Or maybe a massless neutrino would be its own antiparticle as well, that's a good question.

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    Well, I am a particle physicist, and I can tell you that there are many particles besides the photon that are their own anti-particle. The neutral pion comes to mind, but others also exist. A quick tutorial. Mesons such as pions consist of a quark and an anti-quark. If the meson in question consists two quarks of the same flavor you get a particle that is it's own anti-particle. So, neutral pions are a mix of up-upbar and down-down bar. Phi's are s-sbar. J/Psi is c-cbar (as is my own namesake, the eta c).

    Why, you may ask don't these quarks internally annihilate and destroy the particle? They do. That's the primary decay mode for most of these particles (and for the J/psi the only one).

    As to the observational evidence for a matter universe, John Baez gives a brief description on his physics FAQ. Pretty much the same reasoning I gave in the ATM thread. Baez acknowledges that the theoretical explanations need some work. The observational evidence is basically that we don't see areas of the universe that behave as we would expect them to if certain regions consisted of anti-matter (no annihilation at boundries, etc.).
    "I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind." - William Thompson, 1st Baron Lord Kelvin

    "If it was so, it might be, and if it were so, it would be, but as it isn't, it ain't. That's logic!" - Tweedledee

    This isn't right. This isn't even wrong. - Wolfgang Pauli

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    Quote Originally Posted by Eta C View Post
    Well, I am a particle physicist, and I can tell you that there are many particles besides the photon that are their own anti-particle. The neutral pion comes to mind, but others also exist. A quick tutorial. Mesons such as pions consist of a quark and an anti-quark. If the meson in question consists two quarks of the same flavor you get a particle that is it's own anti-particle. So, neutral pions are a mix of up-upbar and down-down bar. Phi's are s-sbar. J/Psi is c-cbar (as is my own namesake, the eta c).

    Why, you may ask don't these quarks internally annihilate and destroy the particle? They do. That's the primary decay mode for most of these particles (and for the J/psi the only one).
    That answers what was going to be my next question, thank you.

    What about force carriers, matter antimatter, distinguishable?

    And, I like the idea of 1/137.000000000......

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    Quote Originally Posted by Ken G
    I'm not a particle physicist, but I think one argument would be
    that the two photons created by that annihilation behave in an
    indistinguishable way. One has to be matter and the other
    antimatter to obey the desired conservation laws (abandoning
    those laws would need a good reason), yet they behave just the
    same in all ways we can test, so we'd have to invent new physics
    to explain how a photon could not be its own antiparticle. But
    perhaps the bottom line is, we don't know it, but there is as yet
    no justification to doubt it. And if antiphotons behaved
    differently under gravity, then why would we not observe two
    half-bright images of stars as they are gravitationally lensed by
    the Sun?
    I'm suggesting that ordinary matter produces ordinary photons,
    and antimatter produces antiphotons. In which case we would
    not see antiphotons very often.

    I'm also suggesting that ordinary matter and antimatter have
    been separated from each other on the scale of galaxy clusters
    or superclusters. (If a cluster is part of a supercluster,
    it would be the same kind of matter as the other clusters in
    that supercluster.)

    If an antiphoton is not identical to an ordinary photon, does
    it necessarily have to behave differently in some way that is
    noticeable in ordinary experiments? Couldn't there be a
    difference which hasn't been noticed because it wouldn't
    show up in any experiment done so far?

    -- Jeff, in Minneapolis

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    Quote Originally Posted by Ken G View Post
    The jury is still out on whether any neutrinos are massless, but your point is well taken-- the very fact that theorists consider it a possibility that the electron neutrino is massless (and it is likely that trinitree will comment that other possibilities exist for all neutrinos, but I'm giving the mainstream view here), suggests that my understanding is incomplete. Or maybe a massless neutrino would be its own antiparticle as well, that's a good question.
    No. Nu-bars are different from Nu. They have to be in order to conserve lepton number. When an atom undergoes beta decay the emitted electron is accompanied by an anti-neutrino. We know this to be true as when one gets a beam of neutrinos, their interactions only produce positrons, just as anti-neutrino interactions only produce electrons. If the neutrino were its own anti-particle we'd see a mix.

    This being said, neutrino mixing implies some violation of lepton number. But that's more between electrons, mu's and taus. That is, we see electron neutrino beams producing mu+. I need to look into the lit more to see if mu- are also observed. I tend to think not. I'm fairly sure that the particle/anti-particle lepton number is still conserved.
    "I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind." - William Thompson, 1st Baron Lord Kelvin

    "If it was so, it might be, and if it were so, it would be, but as it isn't, it ain't. That's logic!" - Tweedledee

    This isn't right. This isn't even wrong. - Wolfgang Pauli

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    Quote Originally Posted by Eta C View Post
    No. Nu-bars are different from Nu. They have to be in order to conserve lepton number. When an atom undergoes beta decay the emitted electron is accompanied by an anti-neutrino. We know this to be true as when one gets a beam of neutrinos, their interactions only produce positrons, just as anti-neutrino interactions only produce electrons. If the neutrino were its own anti-particle we'd see a mix.

    This being said, neutrino mixing implies some violation of lepton number. But that's more between electrons, mu's and taus. That is, we see electron neutrino beams producing mu+. I need to look into the lit more to see if mu- are also observed. I tend to think not. I'm fairly sure that the particle/anti-particle lepton number is still conserved.
    Holy cow, let me get this straight:
    neutrinos produce anti-electrons
    anti-neutrinos produce electrons

    Could I conclude that at the level of ten to the minus something that we live in a sea of anti-particles?

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    Well, no. Given that neutrinos generally don't interact at all. Also, any positrons produced would almost immediately annihilate and produce photons.

    When physicists talk about this they usuall do so in term of the C or charge conjugation operator. Basically the C operation involves flipping the sign of all quantum numbers (mostly electric charge, but others such as lepton number as well). If a particle's quantum nubmers are unchanged by C, then it's its own anti-particle. This includes photons, the aforementioned pi-zero, and others. Obviously, all particles in this class have zero charge, but not all zero charge particles fall into this category (e.g. neutrinos)

    To get to the questions about the force carriers. Most fall into this class (photons, etc). One exception may be the W+/-. The oppositely charged W's may be particle and anti-particle (I'll do some checking). They certainly decay into charge conjugate modes (W+ to a positron and neutrino, W- to an electron and a nu-bar). The Z0 is definitely its own anti-particle.

    I'd recommend that people do their own reading on this. There are any number of non-technical books on particle physics out there (Those by Close are good). As with many scientific topics, speculation without understanding can lead to confusion. Heck, I understand this and I'm being careful with my speculations.
    Last edited by Eta C; 2006-Aug-18 at 03:33 PM. Reason: spelling & clarification
    "I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind." - William Thompson, 1st Baron Lord Kelvin

    "If it was so, it might be, and if it were so, it would be, but as it isn't, it ain't. That's logic!" - Tweedledee

    This isn't right. This isn't even wrong. - Wolfgang Pauli

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    And the graviton?

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    I think that would be it's own antiparticle, if it exists.

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    Getting back to my question: sea of anti particles

    Muons make it to the surface and decay.

    Anti-neutrinos whiz through us from all directions.

    At the quantum level particle anti-particle pairs pop into and out of existence.

    Realativisistic He (cosmic rays) hit from all directions.

    The numbers are tiny but don't we live with these occurences all the time?

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    Quote Originally Posted by Jeff Root View Post
    [snip]

    I'm also suggesting that ordinary matter and antimatter have
    been separated from each other on the scale of galaxy clusters
    or superclusters. (If a cluster is part of a supercluster,
    it would be the same kind of matter as the other clusters in
    that supercluster.)

    [snip]
    Answering this properly would take quite a few posts, so the following is just a few highlights:

    * there are observations of some clusters interacting; clearly those cannot be of different kinds (otherwise the annihilation fireworks would be obvious)

    * while the inter-cluster medium is extremely tenuous, it is not a vacuum; matter-antimatter boundaries would produce fireworks (albeit faint fireworks)

    * DRAGNs have lobes that extend a long way (several Mpc?); if the extremity of any such (matter) lobe 'hit' an anti-matter region (or vice versa), there'd be fireworks (none observed)

    * while the source of UHECRs is still uncertain, it seems that at least some come from AGN jets (which we see as BL Lac objects and blazars). These UHECRs are matter (some uncertainty), not antimatter, so the lack of antimatter UHECRs suggests that there are no anti-matter AGNs 'nearby' (to ~100 Mpc, say)

    * the universe was denser in the past, with a much higher level of interaction between 'neighbouring' clumps of mass than we see today. So matter-antimatter boundary regions should have been much more intense, as photon sources, then than today. There are no footprints of such regions, in deep surveys, whether in the x-ray, UV, optical, NIR, FIR, microwave, or radio wavebands.

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    Quote Originally Posted by Eta C
    Well, I am a particle physicist, and I can tell you that there are
    many particles besides the photon that are their own anti-particle.
    The neutral pion comes to mind, but others also exist.
    The photon, the neutral pion, and the (ahem) eta-meson. And as
    you say in a later post, the Z-zero. Is that the list?

    -- Jeff, in Minneapolis

    I think I didn't read the whole post before replying, but I'll let it
    stand as is.
    Last edited by Jeff Root; 2006-Aug-19 at 01:24 AM. Reason: I happened to read what I had already replied to.

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    Quote Originally Posted by Jeff Root View Post
    Was that addressed to me? It is beyond my level of understanding.
    But I agree that infinity is substantially larger than most
    everything that is real.

    Edit to add: Was it to me, but posted in the wrong thread?

    -- Jeff, in Minneapolis
    It was meant for the OP. If we include virtual particles in our inventory, the apparent dominance of matter in our universe is a trivial excess. The reason that the excess exists is the big question.

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