>> All right, good evening and welcome
to a guest presentation at Space Place.
Tonight, I'm very pleased to introduce
Professor Ellen Zweibel,
who is in the UW physics and astronomy departments.
She's a plasma physicist, among other things,
and won a major award last year, the Maxwell Prize
of the American Physical Society.
So I have imposed on her to come talk to us tonight,
and her title is "The Fastest Particles in the Universe."
So, Ellen, thanks for coming.
[applause]
>> Thanks, Jim.
I'm not new to speaking, but I am new to television.
So I apologize in advance for any awkwardness.
The story of the fastest particles in the universe,
or cosmic rays, is really a story of curiosity
and where it goes, but, where it goes, as I said,
but before I launch into the main part,
I just want to acknowledge my wonderful colleagues
here at the UW.
I couldn't work without them.
So everybody from undergrads, [inaudible] and Cory Cotter,
to the most senior faculty, yes, and in between,
our graduate students and postdocs.
And they really do so much to make life a joy.
So the story really begins back in the 18th century
when Coulomb discovered that air is partially ionized.
So I've indicated that schematically here
with some little pluses and minuses,
and everything else is neutral atoms and molecules.
And we expect hot gases to be ionized,
but we don't expect air to be ionized.
So part of the art to science, and there really is a lot of art
in science, is knowing when something is interesting
and really worth thinking about
or whether it's just a detail that you can discard.
Just kind of an extraneous detail.
And, in this case, they made the right decision,
that this was really something worth thinking about.
In 1896, radioactivity was discovered,
and so this led to the hypothesis that the ionization
of the air is caused by high energy particles emitted
by radioactive elements in the Earth.
So that was the hypothesis, but then, in 1911-1912,
an Austrian physicist, Victor Hess,
went up in a balloon to test this idea.
So if the ionization is produced by particles coming from within
the Earth's crust, it should drop off with height.
But what he showed instead is that it increased with height.
And then he thought, well, maybe these are particles
coming from the sun.
So, fortunately there was a solar eclipse.
Sometimes you have to wait a long time,
although there will be one this summer,
and he went up during a solar eclipse,
and he still found that the ionization
dropped off with height.
And so he concluded that whatever was doing the ionizing,
it came from the sky; it didn't come from the ground.
Now, another sort of universal thing in science is that
when you think of something, someone else has thought of it long before.
And, in fact, this idea had been tried previously
with the Eiffel Tower.
But the Eiffel Tower is not tall enough to show this effect.
You really have to go up several kilometers.
And Victor Hess got the Nobel Prize in physics
in 1936 for this work.
So the name that he gave this ionizing radiation is one of
those sort of descriptive names that you give something when you
don't really know what it is,
like adding -itis to a body part. [laughter]
So, okay, cosmic ray, and so the question is:
is this high energy light or is it charged particles?
And to answer this question, studies beginning in the 1920s
showed that cosmic rays arrive preferentially from the west.
And so the idea is something like this.
Here's the Earth.
And here is its magnetic field,
like the magnetic field of a bar magnet.
And if we kind of zoom in close up to these field lines,
charged particles are gyrating around them
in kind of helical orbits.
And the size of the orbit depends on the energy of the
particle, and the direction of circulation depends on whether
the particle is positively or negatively charged.
Now, I put this all on one slide,
but it did not come easily.
There was a 10-year effort on the part of Arthur Holly Compton
really to go all over the world and to work at sea level,
to work on mountaintops.
He rounded Cape Horn something like five times.
He climbed Mount Evans in Colorado.
Two of his research assistants were killed on Mount McKinley.
And he-- But he established that there was a geographic
variation in the arrival of cosmic rays that correlated
with the geomagnetic field.
Now, you're going to see the name Millikan again.
So, the New York Times, one of their front-page headlines,
December 31, 1932, "Millikan retorts hotly to Compton
"in cosmic ray clash."
So these were two of the greatest physicists of the era,
and they disagreed over whether cosmic rays were charged
particles or photons.
Now, I want to take a little detour and emphasize that
when I talk about the energy of a particle,
I mean Einstein's definition of energy.
And, according to Einstein,
well, according to Newton or just us,
if you apply a force on a particle,
you increase its speed.
But what Einstein showed is that
the speed will not increase indefinitely.
It'll level off at the speed of light.
You cannot get this particle to go faster
than the speed of light.
But you're putting energy into it,
and so instead its effective mass will increase.
And the energy can rise without limit.
And to kind of illustrate what this means,
so let's say we take one proton, which it turns out that's what
most cosmic rays are, and if that proton has an Einsteinian
effective mass of 10 protons,
it's going at .995 of the speed of light,
and if it has an effective mass of 100 protons, .99995 c.,
and this is the last one I'm going to read,
1,000 protons, .99999 c.
The highest energy cosmic rays are going this close
to the speed of light.
There are, I think, 21 nines here.
But they're still going less than the speed of light.
That's the law.
So how does this compare
to the most powerful particle accelerators on Earth?
So right now that would be the Large Hadron Collider at CERN,
where the Higgs boson was discovered.
And the Large Hadron Collider can accelerator protons up to
about 6500 effective proton masses,
and that corresponds to a speed of about so.
So not as many nines, not as high an effective mass
as the highest energy cosmic rays.
And that's why a lot of particle physics was really done
first on cosmic rays.
So here's a list of all the exotic particles that were first
discovered in the cosmic rays, including that neutrinos,
the ghost particles, have a mass.
But to kind of bring this down to Earth,
it turns out that the highest energy cosmic ray ever detected
has an energy about one-quarter of the fastest recorded serve.
So here's Serena Williams killing the ball.
So the tennis ball is more energetic than that cosmic ray,
but that tennis ball has 3.6 times 10 to 25 zeros,
and the cosmic ray is going 4.3 million times faster.
So, morally, maybe it's sort of a tie.
So to get back to what this means for how we detect cosmic
rays on Earth, the Earth has these radiation belts,
which are due to the magnetic field,
and low energy particles with small magnetic orbits are
trapped in the radiation belts but high energy particles with
large magnetic orbits, like cosmic rays,
can just come in and they're kind of deflected
and they arrive preferentially from the west.
So that explains what Clay and then Compton
and so many others later saw.
Now, the highest energy cosmic rays,
the sort of tennis ball equivalent cosmic rays,
and I'm sometimes going to abbreviate ultrahigh energy
cosmic ray or ultrahigh energy cosmic ray as UHECR.
So astronomers love acronyms, and instead of saying this,
sometimes I'll say this.
And so this is what our galaxy probably looks like.
Of course, we're deep inside it, so we can't verify that.
But the orbit of a cosmic ray, ultrahigh energy cosmic ray in
the magnetic field of the galaxy looks something like this.
Whereas the cosmic rays that we detect mostly in Earth have an
orbit about the size of the Earth/sun distance.
So how come and are these UHECR?
Well, they're not common but they're not terribly rare.
About 40,000 strike our country every year.
And if you estimate how much area our population takes up,
that would translate to about one person
every two-and-a-half years.
Now, this is actually a smaller number than the number of people
who are killed every year by being hit by baseballs.
That's a couple of people every year.
But you've never heard of anyone being killed by a single cosmic
ray, and that's because these high energy cosmic rays break up
in the atmosphere and they create these showers of high
energy particles called extensive air showers.
There's another way that we can think about it.
So these highest energy cosmic rays are very rare.
The lower energy ones, still moving at nearly the speed of
light, are more common, and that adds up to about one in a
billion particles in interstellar space.
But the energy in that one billionth is equal to the energy
in the rest of the gas.
And if that happened in this room,
if we could make that happen in this room,
the temperature would drop by one half.
So from whatever it is to 35 degrees or something like this.
And one billionth of the molecules would
become absolutely lethal.
So we don't expect this to happen in rooms on Earth,
but somehow it happens in interstellar space,
and part of our job is to explain that.
So let's get back to the origin of cosmic rays.
So here's Millikan again, and here's another headline in the
New York Times: "Cosmic Rays are the Only Thing Immortal."
This is back in 1932.
"Neither stars nor worlds, sunlight or heaven
"can science admit to be eternal.
"Only one thing known to science can be called immortal:
"the cosmic rays investigated, amongst others,
"by the famous California physicist Dr. RA Millikan.
"These rays may even be relics of days before there existed any
"universe as we know it now."
But there was another idea that came along a couple of years
later, and this was due to two other Caltech astronomers,
Walter Baade and Fritz Zwicky, and they proposed that cosmic
rays originated in supernovae.
So here's an image of a young supernova remnant.
This is the Crab Nebula, seen actually many forms
of light are put together.
So there's about one supernova every 50 years in the Milky Way,
but we see them much less frequently than that because
most of them are obscured by interstellar dust.
And at the time they made their suggestion,
Baade and Zwicky had absolutely
no real physical basis for saying so.
It was really just a speculation based on intuition.
Supernovae have a lot of energy.
Cosmic rays have a lot of energy.
Maybe somehow, they're connected.
But Millikan's hypothesis was tested in 1948
by Phyllis Freier, who was then a graduate student,
one of the real pioneer women in science.
And she showed that cosmic rays contained many different kinds
of atoms: hydrogen, helium, carbon, nitrogen, oxygen, iron,
but the only elements that were created in the Big Bang,
so by 1948 people already knew about the Big Bang,
are hydrogen, helium, and a little bit of lithium.
So that was pretty definitive proof that cosmic rays have not
been around since the beginning of the universe.
They've been created much more recently.
And so beginning in the late '40s, 1950s, 1960s,
there was really kind of this golden age
of cosmic ray discoveries.
And I'm showing here two different tracks of high energy
particles made in plastic, and here's another one,
and here are just a few of the pioneers
and they contributed in so many different ways.
Some to the technology, some to the theory,
some of them by climbing mountains.
It was a huge human enterprise.
Some of the other tools were high altitude balloons.
The first research job I was offered in college was with
a group that launched these balloons,
not from Manitoba but from Hudson Bay,
and I tried to picture myself driving around
on the frozen tundra in a giant truck.
And then I took a job with a radio astronomer.
It worked out, worked out very well.
A couple of decades ago, we developed, really,
a new window on cosmic rays with the ability
to detect cosmic gamma rays.
And so it turns out--
So this is a gamma ray view of a supernova remnant,
and it turns out that supernovae remnants
are just lit up in gamma rays.
Here's the Large Magellanic Cloud,
the small dwarf galaxy
that you can only see from the southern hemisphere,
and here's a region where a lot of stars are forming
and supernovae are going off
and it's all lit up in gamma rays too.
And what this means is that 70-80 years after Baade and
Zwicky made their intuitive suggestion, we've shown, really,
that they're at least partly right.
Cosmic rays do have something to do with supernovae remnants,
the origin of cosmic rays.
So another way in which we can detect cosmic rays
relies on a sort of sonic boom phenomenon in cosmic ray,
in light that we call Cherenkov radiation.
So here's an airplane breaking the sound barrier,
a very sleek-looking airplane.
And what Cherenkov radiation is about
is that the speed of light in water is slower
than the speed of light in the vacuum.
And so if a cosmic ray goes tearing through a tank of water,
it's not going faster than light in a vacuum but it's going
faster than light in water.
And so the radiation that it emits can't spread out in all
directions from it the way it can for a normal particle.
It gets concentrated in this cone.
And this is called Cherenkov Radiation,
and it's similar to a sonic boom.
And so the UW is a partner in a high energy cosmic ray detector
called HAWC is a Cherenkov telescope.
It's being built in partnership
with many institutions, primarily Mexico.
So it's near Puebla.
And everything you see here is a giant water tank.
And when cosmic rays break up in the atmosphere above HAWC,
their particles come sort of streaming down
and they set off Cherenkov Radiation in all these tanks.
And by looking at the timing when the Cherenkov Radiation is
seen in the tanks, you can estimate how much energy the
particle had and where it came from.
And I'll be showing you some results from that
a little bit later on.
There's also theory. That's what I do.
Again, many, many players.
This is a former graduate student of mine, Tova Yoast-Hull.
And I just love this picture because look at this blackboard
and she looks so exhausted.
But she did get her PhD,
and she's now a postdoc at the Canadian Institute
for Theoretical Astrophysics in Canada.
And this is the kind of thing we calculate.
So I've mentioned these unique facilities.
This is the largest HAWC-like telescope, Cherenkov telescope.
This is called the Pierre Auger Observatory.
It's 1660 water tanks spread out
over this large area of western Argentina.
And this is an artist's conception of particles
from an air shower as a cosmic ray breaks up,
setting off these different water tanks
across 3,000 square kilometers.
So this is the largest cosmic ray detector ever built.
But, in addition to these unique facilities,
there are some not unique facilities.
You can turn your cell phone into a cosmic ray detector.
So my colleague in the physics department,
Justin Vandenbroucke, developed this project called DECO,
Distributed Electronic Cosmic Ray Observatory.
Your cell phone camera, so you have to leave you cell phone on,
and it can detect the particles produced by air showers.
And if you download two apps, the data will be automatically
uploaded and sent for analysis, and you can become part of a
high energy cosmic ray detection project.
All you need is a smartphone, which I ironically do not have.
[laughter]
Okay, so now let's talk about where the highest
energy cosmic rays come from.
And this is going to be-- We're not going to--
We may not get to the answer,
but we'll go through kind of a process of elimination.
So here's a view again of what our galaxy probably looks like.
So this is, of course, not our galaxy,
but it's the nearest galaxy similar to ours,
the great nebula in Andromeda.
So a spiral galaxy.
This is how we see the galaxy from where we are,
namely inside it.
So the galactic plane, so most of the matter,
the visible matter in the galaxy, is in a flat disc.
And so the galactic plane,
which we know as the Milky Way at times that we can see it,
is the brightest structure in the sky.
Now, why isn't the Milky Way bright?
It's not extremely bright because most of the starlight
is absorbed by interstellar dust.
And that is what gives the kind of patchy,
lumpy patches of black.
And when we see kind of irregularities in the plane,
they turn out mostly to be associated
with nearby structures.
So this is something called the North Polar Spur.
It's related, actually, to supernovae remnants.
So this is how we see our galaxy.
So, okay, let's hypothesize that ultrahigh energy cosmic rays
come from within the Milky Way.
So there aren't very many of them.
You can actually plot the direction of each one,
determined in the way that I explained using many,
many water tanks and Cherenkov Radiation.
So this is their distribution on the sky.
These red dots.
And these blue objects are some sort of famous galaxies
that we'll talk about.
And so, if ultrahigh energy cosmic rays come
from within the Milky Way, you might think, well,
then they should have a distribution similar
to the light that we see from the Milky Way.
It should all be concentrated in this plane, but it's not.
So that is maybe a clue that cosmic rays,
that these ultrahigh energy cosmic rays don't come from
within the Milky Way.
But there's a possible way out, and that is suppose if they
actually come from very close to the sun.
So now you have to put up with my cartooning.
This is kind of a little piece of the disc of the galaxy in
which the sun is embedded, and the galactic center is this way.
And this kind of purple scrolling is just all the matter
in the disc of the galaxy.
And these dots here represent the source of cosmic rays,
and they are of an unknown nature.
We don't know what they are.
But if they're close enough, if they're all concentrated close
to the sun, kind of like bees in a nest,
then we would see them coming from pretty much
the same direction everywhere.
So that would work, but we've never identified what these
nearby objects could be that are capable of accelerating
particles to such high energies.
So let's put this idea on the back burner.
So another possibility is that UHECR come from galaxies
with supermassive black holes.
And so here's an image, and this is a composite,
not an artist's conception but a composite of emission of light
at many different wavelengths.
So here is the galaxy Centaurus A,
and at its center is a supermassive black hole
probably about a hundred million solar masses.
And activity in the vicinity of the black hole has launched
these enormous jets of hot plasma
that are thousands of light years,
hundreds of thousands of light years in size.
So now this is a sort of Baade- and Zwicky-type argument.
Energetic thing, energetic thing,
could they somehow be related?
Well, we don't see any concentration of ultrahigh
energy cosmic rays coming from objects like Centaurus A.
so we can't establish any correlation.
That's a suggestion maybe that's not where they come from.
Can we say anything about where they come from?
Well, it turns out that we can.
So in the 1960s,
three physicists simultaneously, roughly,
Greisen, Kuzmin, Zatsepin,
thought about what would happen if
cosmic rays collided with light waves.
And the light waves would most probably be the cosmic microwave
background radiation left over from the Big Bang.
And so a very high energy.
So, you know, we don't really think of colliding
with a light wave as doing much damage.
But that's because we're not a cosmic ray.
So just as when a jet plane hits a chicken,
it can do a tremendous amount of damage,
when an ultrahigh energy cosmic ray
hits a very feeble little microwave photon,
it can break up.
And they estimated that given the amount of radiation in the
universe, the highest energy cosmic rays must be produced
within 150 million light years.
Now, that may not sound like a very small range to you,
but it's a tiny, tiny fraction of the volume
of the known universe.
So we can at least say this much:
that the ultrahigh energy cosmic rays
come from within 150 million light years.
Okay, let's look around the universe
and say where are there objects
with high energy strong magnetic fields,
strong electric fields.
And so one thing that comes to mind are pulsars.
So here's the same image of the Crab Nebula
that we looked at a few minutes ago.
And here is zeroing in on the center of the Crab Nebula is
this X-ray radiation that's produced by the Crab pulsar.
And so an artist's conception
of a pulsar would be a neutron star.
So a neutron star is an incredibly compact ball
of mostly neutrons.
So up to three times the mass of the sun
but concentrated in a sphere maybe 10 kilometers in radius.
So really the size of Madison, that's a neutron star.
And they have super strong gravity and super strong
magnetic fields, and they spin on their axis hundreds
or even up to a thousand times a minute.
So they're among the most extreme objects
that we know in nature.
And there's a class of pulsars that are called magnetars.
And their magnetic fields are even larger.
Their magnetic fields are so strong,
so this is again an artist's conception,
they're strong enough to crack the crust of this neutron star.
And so a theory developed by Professor Chris Thompson
at the University of Toronto is that these magnetars,
the rotation of these magnetars with their strong magnetic
fields, is what produces the ultrahigh energy cosmic rays.
And here is a high school senior,
Meghan Gialluca from Hollis Brookline High School
in New Hampshire, and she just missed by this much being the
Young Astronomer of the Year.
She was beaten out by two guys who worked in radio astronomy.
But she made a very nice study of glitches and anti-glitches.
So a glitch is a little change in the rotation rate
of these magnetars.
So, again, citizen science.
Anyone can contribute.
So the magnetar model has some advantages.
There are super strong magnetic and electric fields
that accelerate particles to high energies.
So they're formed rarely in any individual galaxy,
but there are enough galaxies, probably doesn't surprise
you, within 150 million light years
to supply the UHECR that we see.
And the galaxies don't have to be special in any way.
They don't have to have an active nucleus,
a nucleus with a supermassive black hole
that's actively accreting.
Any galaxy in which stars are forming
can make these magnetars.
So this, except for how the particles actually acquire
their energy, this model really fits what we know.
So we can go a little further.
So here's another international project
in which Wisconsin has an important role.
In fact, we're leading it.
This is the IceCube Observatory at the South Pole.
And these are, this is again an artist's conception
of deep holes that are drilled in the Antarctic ice
that are lit up when high-energy particles come through.
So the IceCube Observatory has seen a lot.
It's detected a lot of cosmic rays,
and it's detected high energy neutrinos.
And this is what the sky looks like in high energy neutrinos.
So purple is a neutrino.
There are so few neutrons that they can be given individual
numbers, although they haven't been given names.
Are the sources of these neutrinos
the same as the sources of these UHECR?
Well, they share the same list of "nots."
They're not concentrated to the galactic plane.
They're not preferentially coming from active galaxies.
And for now, that's all that we can say.
So the study of cosmic rays began with the observation
that air was ionized.
So if we kind of bring it all back home and say, okay,
there are these high energy particles that are impinging on
the atmosphere and breaking up into this cascade
of other energetic particles.
Do they do anything?
Do they do anything noticeable to the atmosphere of the Earth?
And there's pretty good evidence that they can trigger
lightning strikes by kind of pre-ionizing the air.
So the same ionization of the air that Culham discovered back
in 1785 is sort of providing a conduit for lightning strikes.
And here's a dramatic lightning--
This is the best picture I found.
This is actually a little town in Belgium.
But lightning is the same all over the Earth.
And so a fair question would be:
Do cosmic rays affect the chemistry of the atmosphere?
Do they affect the climate?
And this has been thoroughly studied because cosmic rays
are always there, and the answer seems to be
a little bit but not very much.
So they are primarily a cosmic phenomenon, and that's, I think,
where they're going to stay.
So, what's the summary?
So cosmic rays are an extreme form of matter.
The highest energy particles that we've ever detected.
Much higher energy than we can make
in any accelerator on Earth.
And there's no single person who made a decisive contribution
to the study of cosmic rays.
It's just been this vast web of human effort.
And it's been both pure science and applied science.
The creation of new plastics.
The creation of counters to measure
to faint Cherenkov Radiation.
Contributions from all sides that have allowed us to
understand them as well as we do.
But there's so much more work to be done.
So, that is the end.
This is how you can find me.
And thank you.
[applause]