What
follows is adapted from David Harrison's introduction to antimatter,
the best one on the web. In 1930, Paul Dirac was musing
about why energy in quantum mechanics is always a positive
number. He could think of no reason why this should be so.
Applying a common physicists' assumption, that what is not
forbidden is required, he concluded that there must be negative-energy
objects like electrons. The next question, then, is why don't
we observe these negative energy electrons? Dirac concluded
that it must be because there is an infinitely dense, totally
homogeneous sea of these electrons everywhere in the universe.
And since it is homogeneous it is unobservable.
Inayat Khan tells a Hindu story that has some relevance to
this unobservable sea. A fish went to the Queen fish and asked:
"I have always heard about the sea, but what is the sea?
Where is it?" The Queen fish replied: "You live,
move, and have your being in the sea. The sea is within you
and without you, and you are made of sea and you will end
in sea. The sea surrounds you as your own being."
Now imagine that some light energy collides with a negative
energy electron in Dirac's sea, knocking it into a positive
energy state. This electron is observable. The hole it leaves
in the sea is also observable, since it differs from the homogeneous
negative energy sea around it. In fact, the hole will appear
to have the mass of an electron. Further, its charge will
appear to have the magnitude of the charge of the electron
but will be positive. In Dirac's theory the hole is the positron.
When Dirac got this far, the positron was unknown. He tried
to interpret the hole as being a proton, but the mathematics
of this theory didn't quite work out. When Carl Anderson discovered
the positron, Dirac immediately realised his theory described
it perfectly. This theory of the positron is still routinely
used. What appears to be the annihilation of the positron
when it collides with some other electron, then, is just the
electron falling into the hole, giving up its energy as light.
In 1949 Richard Feynman devised another theory of antimatter.
An electron travels along, interacts with some light energy
and starts travelling backwards in time. An electron travelling
backwards in time is what we call a positron. The positron
then interacts with some other light energy and starts travelling
forwards in time again. Note that throughout, there is only
one electron. Note also that in Feynman's theory, there is
no pair production or annihilation. Instead the electron is
just interacting with electromagnetic radiation, ie light.
Thus the whole process is just another aspect of the fact
that accelerating electric charges radiate electric and magnetic
fields; here the radiation process is sufficiently violent
to reverse the direction of the electron's travel in time.
Nambu commented on Feynman's theory in 1950:
The time itself loses sense as the indicator of the development
of phenomena; there are particles which flow down as well
as up the stream of time; the eventual creation and annihilation
of pairs that may occur now and then is no creation or annihilation,
but only a change of direction of moving particles, from
past to future, or from future to past.
See "Day
and Night" by MC Escher. Note that both Dirac's and
Feynman's theory of antimatter are represented. This was not
Escher's intention, of course. Instead, in this and much of
his work he was focusing on figure-ground studies.
Thousands of years ago Chuang Tsu wrote a commentary that
seems apropos for both Escher's drawing and these two theories
of antimatter:
It comes out from no source, it goes back in through no
aperture. It has reality yet no place where it resides;
it has duration yet no beginning or end. Something emerges,
though through no aperture - this refers to the fact that
it has reality. It has reality yet there is no place where
it resides - this refers to the dimension of space. It has
duration but no beginning or end - this refers to the dimension
of time. There is life, there is death, there is a coming
out, there is a going back in - yet in the coming out and
going back its form is never seen.
In the very early days of the universe, matter was created
through pair production of electrons-positrons and protons-antiprotons.
Does this mean that one-half of the matter in the universe
is antimatter? David Harrison's answer is a good one:
Almost all experts in this field know the answer to the
question. The problem is that some of the experts think
the answer is "Yes" and others think the answer
is "No."
A hydrogen atom is a positively charged proton with a negatively
charged electron in orbit around it. We can similarly have
an anti-hydrogen atom with a negatively charged antiproton
with a positively charged positron in orbit around it. This
anti-hydrogen atom has been experimentally produced. Similarly,
we can have anti-helium, anti-molecules, etc. It's possible
that there are antimatter galaxies, with anti-solar systems
and maybe anti-people. In Dirac's theory we might say that
the anti-people are holes in an infinite sea of negative energy
people. They, of course, would say that we are holes in an
infinite sea of negative energy anti-people.
Wheeler once called Feynman in the middle of the night, saying
"There is only one electron in the universe!" If
we assume the universe is closed, then in Feynman's theory
of antimatter, all the worldlines of the electrons and positrons
connect so throughout there is only the one electron. This
'explains' the mysterious fact that all electrons are totally
identical. Hmm.
I've read in many places that photons are their own antiparticles.
You can't really time-reverse a photon, because the concept
of time really has no meaning for a photon. Relativity tells
us that light always travels through space at the speed of,
well, light, so it consequently has no movement through time.
Time-reversing a photon is like changing the direction of
motion of a motionless object. Interestingly, though, time-reversing
a photon reverses its spin, its handedness. Mirror-imaging
something, a process known in mathematics as eversion, is
the same as turning it inside-out. You
can also think of eversion as a one hundred and eighty degree
turn in hyperspace.
So is antimatter charge-reversed, inside-out matter? Physicists,
have I got this right? If so: Cool.
© ethan hein 2007 | back
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