the great uh cosmic ray mystery appears
to have been solved this is the question
of where do these high-energy cosmic
rays that are bombarding the earth where
do they come from
basically we've known about for about
100 years well exactly 101 years to be
precise there was a guy called hess who
was floating around in a in a hot air
balloon and he noticed there was some
radiation hitting him he detected this
radiation and it's basically very very
high energy protons they're just coming
from somewhere and they're hitting the
earth and they're
we really didn't know where they came
from super super high energy
far far in excess of any kind of proton
that's flying around the lhc for example
so for example at the lhc protons have
um around well when it's at full whack
which it will be once it starts up again
uh what we call seven tv of energy per
proton
which is a lot tv stands for air terror
electron volts that's 10 to the 12
electron volts and basically the ones
that the high energy cosmic rays that
were coming from outer space the highest
ones have been about 300 million
tv so an awful lot more to get a picture
of how much energy is installed in those
highest energy cosmic rays if i were to
like whack a tennis ball like your brady
a hundred miles an hour the kind of
speeds that you know the top players
andy murrays of this world are serving
at
that's the kind of energy we're talking
about but it's packed into a single
proton but actually when it comes to
smacking the earth most of them would
just pass straight through because the
earth isn't dense enough in most cosmic
rays come through the atmosphere come
past us go through the earth go through
the core of the earth yeah come out the
other side and keep going through space
yeah most of them will pass straight
through that's correct yeah yeah it's
amazing yeah one of the questions is so
why do we not know where these things
are coming from you know they're
supposed to be very sophisticated we've
known about these things for 100 years
now why did we not know where they come
from the problem is
is these guys are charged okay they're
protons they're charged so charged
particles get moved around in magnetic
fields and there's a lot of magnetic
fields out there in the galaxy and
beyond so let's say right i have some
crazy magnetic field going on in this
room and there's a proton sitting
outside the room and it you know it
comes through the door it gets moved
around the magnetic field twists it all
the way around and it goes smack
straight into your face brady which
director did it come from
well i'm going to think it came from
that way absolutely you think it came
from that way but didn't it was hiding
outside the room before you even started
this uh this video so we throw up our
hands in the air and say we're never
gonna know no of course we don't do that
we find other ways
to uh to work out where it came from
there was an idea that
really dates back to a guy called fermi
and interestingly enough the satellite
which the which has made the
observations which have led to the
discovery that was announced today is
also called the fermi satellite so it's
a it's a nice coincidence but anyway
these ideas go back to filming what he
suggested was that when a star dies in a
supernova when a star reaches the end of
its life it runs out of fuel in certain
situations it can go supernova
and one of the things that happens with
a supernova is that uh you send out a
shock wave
okay this very very powerful shock wave
it basically happens because the star
tries to collapse and then it gets the
collapse is sort of jolted by the
neutrons in the star not wanting to be
put too close together you can get this
sharp
sort of abrupt halt in the collapse and
that causes the shock wave we will hit
the ceiling in a second
that's what a supernova is essentially
anyways you have this shock wave so you
have a situation where you have a proton
which is
moving around in the magnetic field
surrounding this dying star
okay so it's moving around the magnetic
field every so often it passes the shock
front
when it passes that shock front it gets
a kick from the shark
okay it gets a kick from the shark and
it gets uh accelerated a bit
and then it moves around a bit more
going in all higgledy pld directions
because of this magnetic field
eventually it passes the shock front
again and it picks up more gets more of
a kick more kinetic energy each time it
picks up about one percent of its
kinetic energy it
increases it by about one percent so
it's getting more and more and more and
more energetic eventually gets so
energetic that it escapes this is the
idea that this might be the source of um
of
of hydrogen cosmic rays
so
we start looking at things objects like
that what might we see so we're looking
at let's look at a supernova and ask
what might we see and in particular they
looked at two particular supernova they
got typical astronomers names which i'm
not going to one of them was called the
jellyfish
nebula they so that's quite at least we
can remember that they're both of the
order of five thousand nine thousand
light years away so the faraway objects
these guys have gone supernova so this
should be if this effect is correct high
energy cosmic rays coming from there but
we're not going to see the protons
coming from there because they've moved
around so much but what we can see
are photons
because photons don't move around in a
magnetic field unlike cosmic rays gamma
rays travel to a straight from their
sources in particular if you've got a
proton one of these really high energy
protons what it'll do is if there's
interstellar sort of dust around there's
you know some interstellar gas cloud or
something around this this remnants of
this supernova
when the high energy proton
hits another proton that's of lower
energy but sitting in this cloud
then basically what will happen is it
can produce what's called a pion
if it's neutral ion it's just it's a
combination of quarks
okay so it can produce this pion and
then that pion can decay into two
photons and they'll be high energy
photons and those are the things that we
see and those are the things that would
come direct to us from this supernova
now the important point is is that these
photons that come from this process
have a minimum energy
and the reason they have minimum energy
is because they were produced by the
pion itself right so they're at least
picking up the energy which is
essentially the rest mass of the pion
which is about 135
uh gev giga electron volts
if this effect is correct you would
expect to see the uh the sort of drop in
the number of photons around 150 or 130
gev you would because basically you're
not producing them from these pound
decays anymore and so that's what they
look for
and that's what they saw
this is not the end of the story brady
it's so we say we may so maybe okay we
think this is where it looks like this
is where these high energy guys are
coming from
it's at the end but there is another
mystery and that's the there's something
called the gzk cutoff and this basically
says
that there should be a maximum energy
that any high-energy cosmic ray could
have
right
now it's about 50 million
tv
now remember i said there were ones
about 300 million tv so it's a little
it's not hugely more so one could say
it's a bit blurry but it's like six
times more well yeah but six times in
chronological terms and so bad is it so
it's a little bit more but okay so why
is there a maximum amount that we expect
right the reason is you take one of
these very high energy protons
and if it's going at sufficiently high
injuries it will start to interact
with the cosmic microwave background
radiation itself right
and you can get a process whereby again
pions the proton interacts with the
cosmic microwave background photons
you produce a pion and so you lose your
energy into these pions
okay and then they decay into other
photons but but the point is the protons
themselves would lose energy into these
planets so you basically get reach a
maximum where you say right if the
protons got too much energy it will just
start losing them into pions and end of
story you stop you shouldn't see
anything above that scale
but we do see some
is it really a mystery one you know
i think i think the jury's still out on
that but it's certainly an interesting
question