On Sep 12, 5:14Â am, Tom Roberts
sbcglobal.net> wrote:> funkensteinwrote:
>> Positrons (anti-electrons) are hitting you right now from cosmic ray
>> showers.
>
> Yes.
>
>> In high energy cosmic rays, the ratio of observed
>> antiprotons to protons approaches unity.
>
> Not for the PRIMARY particles, only for the secondaries from
> very-high-energy primary particles. The primaries are predominantly
> matter, not antimatter; mostly protons, with a few percent of
> light-element nuclei. Sorry, I have no reference for this, but it is
> general knowledge among physicists.
>
Thanks for your reply Tom.
Well, approaching unity might be an exaggeration, sorry :) Thanks for
the reference dlzc. It looks like the antiproton/proton ratio is
measured up to 10^10 eV or so, where it is about 10^-3, and rises the
whole way. Models suggest it will turn down again at higher energies,
but measurements would sure be nice (AMS!). In some ways going to
higher energies represents a sample of a larger region of the galaxy.
One CR researcher told me they could not tell from air shower arrays
(ultra-high energy CR measurements) if the primary particle was
antimatter or koinomatter. A pity.
>> However, there is no
>> observation of macroscopic anti-matter, nothing bigger than a single
>> anti-hydrogen atom as far as I know.
>
> Yes. Indeed, those are QUITE rare, and only a handful have been produced
> so far at accelerators. AFAIK no observation of anti-hydrogen has been
> made from cosmic rays. Antiprotons and anti-electrons are common, but it
> is non-trivial to get them to form an anti-hydrogen atom.
>
Neutrals are really hard to detect and characterize, and cosmic rays
are basically all ions. Detecting a neutral anti-hydrogen atom in
space seems like a tough task even if there were some floating
around.
>> Hannes Alven speculated the nearest star could be made of anti-matter,
>> and we would have no way to know. Â The strongest evidence against this
>> is that not a single anti-helium cosmic ray has ever been detected,
>> but even the lower limits there aren't that low on a cosmic scale.
>
> Actually, the strongest evidence is that there are no 511 keV gammas
> coming from e+e- annihilation in the vicinity of the nearest star, or
> from anywhere in the observable universe. We know the inter-galactic
> medium permeates the cosmos, and since no e+e- annihilation is observed,
> everywhere within the observable universe it must be matter with
> essentially no antimatter.
>
I disagree. The galaxy is a awash with gamma background. Also, the
amount of 511keV gammas we would expect if e.g. alpha cen were
antimatter is small. Even assuming all their stellar wind was being
annihilated in the interstellar medium and all to gammas, the gamma
flux would still be too small to see here.
Assume a sun-like stellar wind with 10^31 particles/second. Assume
isotropy, and that all the energy goes to gamma rays, none to heat,
other particles, or other electromagnetic channels (unlikely but an
upper limit on gamma flux). Take alpha cen as 4 light years away, so
the sphere with alpha cen at the center and us on the edge has a
surface area of 6*10^33 meters. This would give us an expected flux
of 511keV gamma photons from alpha cen of ~0.002 ph/m^2/s. That's
2*10^-7 ph/cm^2/s, well under galactic background.
>> They (antimatter) have mass and respond to gravity, but their active
>> gravitational potential (ability to attract other matter) has never
>> been measured.
>
> Experimentally, the gravitational response of antimatter to gravity has
> never been measured. There are several proposals to do that, and I am
> involved with one at Fermilab. This is a difficult experiment....
>
Wow, that sounds very interesting! Good luck! Am I wrong then that
their response to the Earth's gravitational field has been observed?
Could positrons be unaffected or even repelled by our mass? Sorry
about gravity being so weak, makes your job a lot harder :)
Cheers -
