How were pulsars discovered?

Daniel recounts the story of how pulsars were discovered and what they tell us about the death of stars.

Learn more about your ad-choices at https://www.iheartpodcastnetwork.com

See omnystudio.com/listener for privacy information.

Episode Transcript

Speaker 1 (00:08):
What is the moment of scientific discovery actually? Like I mean,
in the movies, it always seems so crisp. Scientists find
something in her data or an experiment suddenly dramatically works.
We go from ignorance to knowledge in a moment, from
failure to success. That kind of drama works for the

(00:29):
movie screen, But how does it happen in real life?
Is it a slow and steady march rather than a
sudden leap, or are there actually real moments of insight
where all of a sudden light penetrates the darkness and
the scientists learned something new about the universe that no
human has ever known before. Hi, I'm Daniel. I'm a

(01:07):
particle physicist and I've been doing particle physics experiments for
decades but never discovered a new particle. And Welcome to
the podcast. Daniel and Jorge explain the Universe, in which
we examine everything about the universe, from its origins to
its ends, from its biggest things to its smallest things,

(01:28):
from all of its mysteries and all of our discoveries.
Our goal in this podcast is to open our minds
to all of the craziest, biggest, deepest, most important questions
the one that frame the context of being human, the
ones that tell us what it means to be in
this universe and how this universe works. We tackle all

(01:48):
of those questions and we go right to the forefront
of scientific knowledge. We take you right to the edge
where scientists are currently working, and we explain all of
it to you in a way that we hope it
makes and maybe even occasionally makes you laugh. My co
host Orgy him the Creative PhD Comics, can't be here today,
so I'm gonna share with you one of my favorite

(02:08):
stories of scientific discovery. And I mentioned earlier on that
I have never discovered a new particle. That's not a true.
My career and particle physics spans from the mid nineteen
nineties till today, and in the mid nineteen nineties was
the discovery of the top cork or and I did
a really fun episode about that whole, amazing, hilarious, dramatic story.

(02:31):
But I just would have joined the field right when
that had already happened, So I wasn't around when the
top cork was discovered. I didn't get to participate in
that moment of discovery. I was, however, part of the
team that discovered the Higgs boson. But you have to understand,
this was a really big group of people, thousands of
thousands of people who all contributed little bits here and there,

(02:52):
and there wasn't really a dramatic moment when we said,
ah ha, the Higgs is there. It's solely emerged out
of the a a sort of the way a treasure
chest might be revealed in the sand of a beach,
as a tide pulls out inch by inch, showing you
more and more of it. That was sort of the
way the Higgs boson discovery went. We saw a little peak,
we thought it might be it. It got bigger and

(03:13):
bigger and bigger, and there was never really a moment
of than the official announcement when we could say, now
we have discovered the Higgs. But that sort of was
a bureaucratic choice and artificial choice. There was no single
aha moment. And part of that is because we knew
what we were looking for. We suspect that the Higgs
was there, we knew how to find it, we knew

(03:33):
how to look for it, we knew what to expect,
and so when we saw it, it was just sort
of this slow creeping realization that we had found what
we had been hunting. But that doesn't mean it's always
like that. There are moments of discovery in science. Usually
they happen when we're more surprised, when we see something
we didn't expect, when you go looking for one thing

(03:54):
and you find something else. Moments, for example, like the
discovery of the cosmic microwave back round that we talked
about a few episodes ago. Today, we're going to tell
the story of one of those moments when discovery came quickly,
when someone went looking for one thing and found something else,
something alarming and astonishing, a moment of insight about the universe. Actually,

(04:16):
we're gonna tell a story of two of those moments,
because this discovery has multiple parts, and for one of
those parts, we happen to have real historical audio of
those scientists realizing their discovery in real time as it happens,
so you'll get to hear what it actually sounds like

(04:36):
when scientists are astonished when they make a real life discovery.
So that's super fun, and for me, it's always really
interesting to try to understand what it was like to
make that discovery. You know, it's easy in hindsight to say,
all these things exist, here's how you look for them
that when did it bad? A boom but being done.
But you have to go back to what it was

(04:58):
like before we knew it was there, to put yourself
back in that mental position of ignorance, not knowing whether
something is out there, not understanding whether you live in
the universe where it's real or where it's just an idea,
not knowing which direction human knowledge and science will take.
Science is so easy in hindsight and so difficult in foresight.

(05:19):
When you stand in the forefront of human ignorance, you
don't know necessarily which way to go. So it's really
valuable to revisit these moments when we took a step forward,
when we went from ignorance to knowledge, and understand what
was required, how it happened, and the bravery it took
to make that claim to say I have found something new.

(05:39):
I now know something about the universe that no human
ever knew before. And so today we're gonna be telling
one of my favorite stories of discovery, one about a
really weird kind of star, a very fast, very dense,
very bizarre kind of star that we've talked about on
the podcast before and so today's episode we'll be answering
the question how we're pulsars discovered? And so, as usual,

(06:08):
before we dig into the topic and tell you the
story today, I wanted to know how much people already
knew about this sort of famous story. So I went
out and solicited volunteers from the internet to tell us
what they knew about various questions and science of this
being one of them. So thank you to all of
those who participated and give us their speculation without the
opportunity to look into any reference material whatsoever on the

(06:32):
honor system. Of course, if you'd like to participate and
hear your voice on the podcast in the future, please
don't be shy. I promise you it's fun. Send me
an email to questions at Daniel and Jorge dot com.
But in the meantime, think to yourself, do you know
the story of how pulsars were discovered? Here's what people
had to say. I am a d percent sure that

(06:56):
pulsels were discovered when they stuck a stepiscope pots to
the whole space telescope. I'm guessing pulsars were discovered by
scientists who observed these stars that were kind of flashing,
so dimming and brightening in these regular pulses. Hence the

(07:17):
name pulsar. I realized I just described what a pulsar is,
not how they were discovered, so sorry about that. UM.
For what a pulsar is, I would say it was
discovered as a rapidly blinking source of light in the sky. Um.
They were discovered by I think she was a graduate

(07:42):
student ince in the sixties. Something they were They discovered
through listening to some radio signals, and first they thought
there was extracted see in life, because they called that

(08:03):
little Green Men l g M. But I always confused
pulsars and quasars. I'm going to guess that someone saw
repetition of light in some part of the sky over
and over and that led to an investigation that found

(08:23):
the pulsars. Pulsars were discovered by a woman, and I
believe it was in the nineties seventies, but I'm not
sure how or why or where even There was a
woman astronomer radio astronomer whose name, unfortunately I cannot remember,
was doing some sort of sky survey when she noticed

(08:45):
a set of pulses that were incredibly regularly spaced. She
actually annotated them as l g M for Little Green Men.
That one time they thought it might have been discovery
of aliens, but later they discovered that it was actually
a rotating uh neutron star and the magnetic field was

(09:11):
exciting the gas molecules molecules around it and giving off
radio energy. Alright, so congratulations to our excellently informed listeners. Together,
they really do have most of the story there. There's
a lot of really insightful stuff and a lot of
bits of the story are there in pieces here and there.
So let's dig into it and to really understand how

(09:32):
pulsars were discovered, we have to understand, of course, first
what a pulsar is, how we came to the idea
of it existing in the universe, and that will help
us understand how it was seen and how we knew
what we were seeing, all right, So first of all,
what is a pulsar. A pulsar is a very very
compact object. Neutron stars and white dwarfs are more famous

(09:55):
as the sort of like densest things in the universe,
and a pulsar is a version of these. It's most
commonly considered to be a version of a neutron star,
but it can also be a white dwarf, but both
of them are essentially are the end points of stars.
Stars have these incredible life cycles where you start out
as a big molecular cloud, huge blob of gas and

(10:18):
dust that somehow shocked to collapse into a hot and
dense object a star, which burns for billions and billions
of years in this incredible, incredible balance between gravity that's
pulling it together, trying to turn it into a black
hole or something very very dense, and fusion which is
erupting and sending radiation out to prevent the collapse of

(10:39):
that star. And it always amazes me that these things
go on for so long, these two cosmic forces so different,
both so powerful, can be so balanced for so many
billions of years. Well, at some point the star gives
up because it's burned most of its fuel and its
core has become very very heavy, and it's filled with
things that it can no longer fuse. When the war

(11:00):
of the star is filled with iron, for example, I
using iron doesn't generate heat, it actually costs energy, so
it cools the star. So now the star no longer
has that power from fusion to resist gravity, and it collapses.
There's some intermediate stages in there will skip over, such
as it becoming a red giant, but depending on the
size of the star, this collapse generally triggers a supernova.

(11:21):
So you have this collapse where the materials racing inwards,
which then causes that back reaction outwards, a massive explosion
where a huge chunk of the stuff that used to
be the star is now spread out into a new nebula,
like a big sprawling cloud of gas and dust. At
the core of it, however, is a very dense, very
hot remnant, and that remnant can either be a white

(11:44):
dwarf or a neutron star or a black hole, depending
on the mass of the original star. So smaller stars
end up as white dwarfs, which are basically just like
huge hot chunks of metal that are resisting collapsing because
their fermions and they don't like to overlap too much,
or if they are larger, they become neutron stars, where

(12:06):
gravity now pushes them together and forces all of the
protons and the electrons together into forming new neutrons, and
you have this really weird material that's sort of like
the nucleus of an atom, but the size of a mountain.
So it's incredibly dense, incredibly weird stuff, something we even
still today do not understand in detail. And then, of course,
if the star is more massive, it would become a

(12:28):
black hole. So the gravity totally wins and nothing prevents
the collapse and it becomes a black hole. But it's
the first two categories that are more interested in. And
let's focus on the neutron star category because that's the
majority of pulsars. So you have this very dense object, right,
and the object is a huge chunk of the material
that used to be a star, not all of it.
Some of the material is lost in the supernova and

(12:49):
some of it remains in this cloud that surrounds the
neutron star. But this neutron star is a very very
dense object and very very small because gravity is really
pulled it together. And with that means is that it's
spinning really fast. Why is it spinning fast, Well, the
star itself was spinning because everything in the universe is spinning.
And the reason is simple is because angular momentum is conserved.

(13:13):
You know how momentum is conserved. If you push on something,
it stays in motion until something else pushes on it,
or if you don't push on something, it stays still
until something does push on it. That's conservation of momentum.
Those are Newton's laws. Well, there are similar laws for
angular momentum. That is that something spinning tends to keep spinning,
and to make something spin, you've gotta give it a push.

(13:35):
So if you leave something alone, it will keep spinning
the way it's always been spinning. Right, that's conservation of
angular momentum. And so the original gas cloud that formed
that star had some spin to it, and that spin
can't go away. It needs to stick around. And as
the gas cloud gets smaller and smaller and turns into
a star, the star spins faster. Now that might sound

(13:58):
like it violates conservation of because the momentum because it's
spinning faster. Right, Well, the velocity of the star's spin
is not what's conserved. It's the angular momentum, which is
the product of the velocity and their radius. So things
that are larger spin slower with the same angular momentum
as things that are smaller than spin faster. You know

(14:18):
this because if you're a figure skater and you pull
your arms in, you spin faster. You have the same
angular momentum. You're not pushing against anything to spin faster,
but you spin faster because your radius is smaller. So
to have the same angular momentum, you've got to go faster.
That's why the star spins faster than the original gas cloud.
And that's why the super compact, dense, little neutron star

(14:41):
that has a huge chunk of the star's mass but
is much much smaller. We're talking about something only kilometers
in size, you know, maybe ten fifteen kilometers, has to
be spinning really really fast to have the same angular
momentum as most of the original star. So that's why
these things are spinning so fast because they are so
small as they are so dense. In addition, some of

(15:02):
these things are highly magnetic. There's a magnetic field of
these stars, just like every star and most planets have
a magnetic field, and that's because the motion of charged
particles inside it. A neutron star is mostly neutrons, but
there are protons and there are electrons, and they are
moving around sometimes on the surface, and the flux of
the particles on the inside can create these magnetic fields.

(15:23):
So you have this object that's spinning really really fast,
and it has a magnetic field. In addition, it's generating
a huge amount of radiation. The magnetic field of the
thing is rotating, which generates an electric field which accelerates
the protons and the electrons on the surface of the
neutron star, and that creates a bunch of radiation because

(15:43):
when you accelerate particles, they radiate photons. So you have
this magnetic fields on this neutron star that's rotating and
is generating an electric field which pushes the electrons and
protons on the surface of the star, creating a lot
of radiation. And that radiation doesn't go in every direction
because there's a strong magnetic field. That radiation tends to
go along the magnetic north and the magnetic south because

(16:06):
magnetic fields are really good at bending the path of
charged particles. The reason that we don't get a lot
of radiation from space is because we have a magnetic
field here on Earth, and when particles come from space,
they are bent around those magnetic field lines. The magnetic
field lines are sort of like the lines on a
basketball right they run from north to south, and if

(16:27):
a particle comes from space, it gets bent by those
magnetic fields and goes out in another direction where sometimes
they loop around those magnetic field lines all the way
up to the north or the south pole, and then
they can slip in between the magnetic field lines. And
that's for example, why we have the Northern lights and
the Southern lights, because magnetic fields guide charged particles in

(16:49):
the same way. If you generate radiation on the surface
of the planet, it's also bound by those magnetic fields,
and so in this case, the magnetic fields are even
much more powerful, and essentially all of the radiation from
the neutron star gets guided towards the north or the
south pole of the magnetic field. So you get these
beams of radiation shooting off of this crazy neutron star.

(17:12):
Right like it's not crazy enough, it's already super hot,
super dense, super small, spinning, super fast, really magnetized, and
now on top of that, it's shining these two crazy
flashlights out into the universe, one from its magnetic north
pole and the other from its magnetic south pole. And
these beams don't come for free. They are very bright,

(17:33):
they cost a lot of energy, and this energy comes
from the spinning of the neutron star. Because that's what's
generating this electric field, the rotation of the magnetic field,
and eventually it's going to slow it down. Like these pulsars,
they generate these beams and they last for maybe ten
or a hundred million years, but they don't last for
their whole lifetime. At some point the beams turned off

(17:55):
because the neutron star has slowed down, it's not generating
that radiation anymore. What that means is that for most
of a lifetime, the pulsar is actually quiet. They don't
emit these beams, and so something like nine percent of
the pulsars out there aren't actually emitting any radiation anymore.
They are quiet. The universe is filled with dead pulsars,

(18:16):
pulsars that have gone quiet. So we've explained what a
pulsar is and how it emits these beams. But why
do we call it a pulsar. Are these beams themselves
like pulsing? Do they turn on and off? The beams
don't turn on and off. I mean, they last for
millions of years and they eventually fade, but they don't
like flicker on and off. The reason we call it

(18:36):
a pulsar is because we only see those beams as
they pass by the Earth, because the beam is shooting
up and down along the magnetic field lines. But that's
not necessarily the same as the axis that the pulsar
is spinning around. So if it were, if the magnetic
north and the magnetic south were the same as the
north and south of the actual stars, so it was

(18:59):
spinning around on the north pole, then it would always
be shooting the beam north and the beam south. However,
if instead the magnetic field is tilted so that it's
like spinning along one axis, but it's beams are shooting
off a little bit skewed, then when it spins around,
the direction of that beam changes right. It's like if
you're holding a flashlight and you point it straight up

(19:21):
and then spin, the direction of the flashlight doesn't change.
But if you hold a flashlight straight out and then
spin right, then what happens. Then Your flashlight's gonna sweep
around three sixty degrees every time you rotate. And what
does somebody see if they're standing in front of you
watching you spin, they see a flash. They see a
pulse of light only when the flashlight is pointed in

(19:43):
your direction. So it's this difference between the direction of
the pulsars magnetic field and it's actual spin axis, which
makes it a pulse. Are right, that's what makes it
appear to pulse. They don't actually pulse. They're sending bright
streams of light contain usually out into the universe until
they fade. But we see them pulsing because that beam

(20:05):
sweeps across Earth and that's what we see. So that's
what a pulsar is. An introduction to these weird things
in the universe. Next, we're gonna talk about why we
suspect that they might exist and how they were actually found.
But first let's take a quick break. All right, we're

(20:33):
back and we're talking about the incredible story of the
discovery of pulsars. And we reminded ourselves that pulsars are
a tiny, very hot, very dense, very quickly spinning stars
the left over heart of a supernova. They're shooting a
beam of light out into the universe, and they are
also spinning, and so that beam of light passes over
the Earth and looks like pulsations. It looks like pulses

(20:56):
from something out there in the universe. And before we
discovered these things, we had a suspicion that they existed.
People have been thinking about the life cycle of stars,
and in nineteen thirty four people suggested that when you
had a supernova, it might not all blow out into
the universe, that you might get this small, dense core

(21:17):
left over, and if so, it would have this really
weird state of matter. These neutrons would form, that would
be in a really dense state, this thing that's sort
of like nuclear matter, the things in the heart of atoms,
but now on the sides of like a mountain, something
kilometers wide. Imagine that the nucleus of an atom, but
kilometers wide. So this was a novelty, but nobody had

(21:38):
ever seen one before. We didn't know if neutron stars
existed in nineteen thirty four, and they would be difficult
to detect because these things don't have fusion anymore. They
don't glow the same way that very bright stars do.
So to see neutron stars seemed like a puzzle. But
then decades later people said, well, you know, they might
have very strong magnetic fields, and if so, they might

(21:59):
be rotating, and if so, then they might be pulsing.
And so this idea sort of came into existence in
the sixties idea the pulsars as weird, spinning, magnetized, beaming
light neutron stars might be out there. But you have
to remember that there are lots of crazy ideas for

(22:19):
what might be out there. The astronomy literature is filled
with people speculating maybe these things exist, maybe boson stars exist,
maybe these other things exist. Now, with a hindsight of history,
we can go back and trace the development of this
one thread of an idea that turned out to describe
something in the actual universe. But don't forget it was
buried at the time in a forest of other crazy,

(22:41):
wrong ideas about what might be out there in the universe.
You know, pulsars exist, and if you took a time
machine back to the sixties, you might say, I know
these things exist, and I know how to find them.
It's not actually that hard. But without the hindsight of
that history, of course, it's hard to pick the wheat
from the chaff. So let's get to the story of
how they were actually discovered. They were found by a
graduate student at the University of Cambridge, a woman named

(23:04):
Joscelynn Bell, and she was not looking for pulsars. In fact,
she wasn't looking for stars at all. She was trying
to study quasars. Quasars are at the heart of really
large galaxies. They are the accretion disks around black holes,
the stuff that has not yet fallen into the black
hole but is swirling around. And because of the tidal

(23:25):
forces and the incredible gravity, these things get really hot
and they radio a lot of light. And we had
seen these things, and we knew that they were very,
very far away, because these quasars have existed for a
long time. They were formed in the very early universe,
like a billion years after the Big Bang, but they're
still super duper bright. And for a long time they
were big mystery because people thought, well, what could it

(23:47):
be that is so incredibly bright and so far away,
so at its source, it's got to be like mind
bogglingly bright. What could that even be? People thought for
a long time, this was a mistake. It's not really
a thing. We must be misunderstanding how these things work.
And Jocelyn Bell was trying to understand these quasars. She
was trying to understand how these quasars twinkle, how they scintillate.

(24:11):
You know that when you look at a star in
the sky, you see a twinkling, and that's mostly because
the stuff between you and the star is interfering with
the star light. That's why planets, for example, don't twinkle,
but stars do because the light from the star has
to go really really far. So quasars kind of twinkle
as well. They do this thing called scintillation, and it's

(24:33):
due to fluctuations in the densities of particles in the
solar wind. So the way we see quasars is not
by looking usually at visible light, but by looking at
radio waves. These things come from really really far away
and with their best seen in the radio spectrum. And
in the radio spectrum, an obstacle is the solar wind.
Remember that the Sun doesn't just shoot out photons. It

(24:54):
also shoots out a bunch of charge particles, protons and
electrons and other crazy stuff, and this is what we
call the solar wind. And when a radio photon enters
our solar system from somewhere really really far away, it
hits this barrage of radiation coming from the Sun and
interact with it. It's radio signal made of light and

(25:14):
electromagnetic radiation. Essentially, photons comes from these quasars billions of
years away, they sometimes get deflected or interfered with by
these particles in the solar wind, and so that's what
makes these quasars scintillate. So she wanted to study this
because she wanted to understand quasars. People at that time
didn't know that black holes were real, so they didn't

(25:35):
know what was powering these quasars. What could possibly be
generating so much radiation from so far away. So she
built a radio telescope. And a radio telescope is just
a bunch of antennas. But the thing about radio waves
is that their wavelength is very, very long. They could
be meters or hundreds of meters. So to capture a
radio photon, you need a big antenna, you need something large.

(25:59):
So she builds some thing which was four and a
half acres, Like this thing is big. She spent two years,
and for her, doing astronomy meant every day pounding fence
posts into the ground and stringing wire among them. Imagine
one of those old fashioned TV antennas. It was like
a grid of metal that could capture a signal. That's
essentially what she built. And she strung hundred and twenty

(26:22):
miles of wire over two years to build her radio
telescope to capture the signal from these quasars to look
at them scintillating. She wanted to see the fluctuations in
these signals. And that's really key because what she did
is get these radio signals and look at them and
develop her own personal sense for what this data should
look like. She was looking for characteristic wiggles changes in

(26:45):
this data as they studied the pulsar. And this is
back in the day before they had computers and before
people could just like you know, dump the data onto
the screen and analyze it bump bump bump. Her data
came out directly onto a printer like her radio telescope
captured this turned it into an electrical signal which was
directly sent to a printer which dumped it onto paper.

(27:07):
So her output from her telescope was stored on a
hundred feet per day of printer paper, which is like
came out steadily and she would stand there and look
at it. She would get to know it. She was
like a natural neural network where she learned if I'm
looking over here that I'm going to see this thing
which we're pointing at the sun, and I'm gonna see
this kind of radio waves. And this isn't the kind

(27:28):
of thing that she could easily point right. This thing
is just something you build in the ground. But the
earth turns, and as the Earth turns, this thing is
essentially pointed in a new direction. She herself is like
sweeping her instrument across the sky, examining different parts of
the universe. And you can get some directional information from
a radio antenna based on like when the signal arrives,

(27:50):
does it arrive first on the eastern part of the
antenna or first on the western part of the antenna.
But it's not great at telling where something is coming
from exactly. So she came really good analyzing these signals,
and then one day, November nineteen sixty seven, she saw
the signal that she did not understand, something she had
never seen before. What she saw were pulses separated by

(28:13):
one and the third seconds. So it was like whoop,
poop poop, and she would get these pulses of radio waves,
and the regularity of it, the exact distance between the
pulses is what made it seem really weird. And at
first she thought, oh, this must be a signal from
something here. On Earth, because, of course, there are lots

(28:34):
of sources of radio waves here on Earth. Almost everything
we do with our electronics generates radio noise. Every time
you turn on your television, certainly every time you use
your cell phone, and of course there are radio transmitters
all over the planet. And so first she had to
rule out various sources of human interference, like other radio astronomers,

(28:55):
people sending pulses off the Moon to measure the distance
to the Moon, television signals, beeps from orbiting satellites, even
like you know, possible effects from large corrugated metal buildings
near the telescopes. She went through this whole list, and
you gotta do that when you see something weird in
your data, you gotta first look for the boring explanation like, oh, well,

(29:15):
maybe I'm just measuring what happens when somebody turns on
the microwave in the break room or something like that.
You don't go straight to I've discovered something new in
the universe. So she very carefully went through all these
different explanations and eventually even borrowed somebody else's radio telescope
to confirm her observations. She wanted to make sure it
wasn't just like some weird blip in her telescope, so

(29:36):
she knew it wasn't just her telescope. She ruled out
all sources of human earth bound interference, and she saw
that it tracted with a particular location in the sky.
And that's a great clue that tells you that it's
not from Earth, because if it's from Earth, then it
doesn't matter which direction the Earth is pointed. If it's
not from Earth, then you will only see it when

(29:57):
the Earth is pointed in a certain direction, only when
the mess it itself sweeped across your radio telescope. So
where did their minds go? The strange regularity of it,
the fact that it came like every one and a
third seconds, made them think not of some new after
physical object, because nature is not often that precise, right.

(30:17):
Nature is messy. When you go out into the world.
You don't see like rocks that are exactly square. You
don't see like ten rocks exactly the same size. You
don't see the sort of regular patterns. I mean, sometimes
you do in crystals and other places, but nature is
more often messy than precise and regular. So their media

(30:37):
thought was like, wow, maybe this is alien intelligence, you know,
she says, quote, we did not really believe that we
had picked up signals from another civilization, but obviously the
idea had crossed our minds, and we had no proof
that it was an entirely natural radio emission. It is
an interesting problem if one thinks one may have detected

(30:58):
life elsewhere in the universe, how does one announced the
results responsibly? So they really didn't know what they had.
They were wondering, is this something weird and new? Are
these aliens? Or is this some natural source of radio
emission that's weirdly regular. So in their internal notes they
called this thing l g M, one for Little Green Men.

(31:18):
And so here you can see the process of discovery
in motion, like there existed in the literature, the speculation
that these things might be out there, that spinning neutron
stars might generate pulses, and here they are discovering pulses
in the radio spectrum, essentially exactly what was predicted. But
they couldn't put it together because, as we mentioned before,

(31:40):
there are lots of predictions out there in the literature,
only in hindsights you know exactly who to listen to.
It's like picking one of Nostradamis's predictions. Right, most of
them are nonsense, and if you look back through all
of them, you can always find one that seems to
make sense. So what they did was they kept looking,
and pretty soon they found in other pulsars somewhere else

(32:01):
in the sky. And I told him it's probably not aliens,
because their signals coming from two very different, very distant
locations in the universe, so probably it's a natural source.
And then by Christmas of nineteen sixty seven, right just
like weeks after the first discovery, they had found four
of these things, so four pulsars, and early the next

(32:24):
year they publicized their results and they wrote a nice
paper and this was a huge discovery, and then everybody
with the radio telescope started looking at these things, like, wow,
oh my gosh, these things are out there. The incredible
thing is that once you know to look for them,
they're not that hard to find. Pulsars are pretty bright.
Radio telescopes were kind of new. Optical astronomy was dominant

(32:46):
at the time, but there were a lot of radio
telescopes out there, and by the end of nineteen dozens
of these things had been found, and it was another scientist,
a guy named Thomas Gold, that put the story together,
who said, Ah, these pulsars are the rotating neutron stars
that we've been thinking about. What these folks have seen

(33:06):
out there in the universe is exactly what we thought
might happen in some circumstances at the end of a supernova.
So that was a really incredible moment to say, like, Wow,
these things, these crazy, weird little blobs that we've predicted
might be there as like the tombstone on the end
of a supernova, actually are out there and they do

(33:26):
this weird thing that lets us find them. I think
the discovery that really put a pin in it was
the discovery of a pulsar at the heart of the
crab Nebula. Crab Nebula is a huge cloud of gas
and dust. It's the remnant of an old supernova star
that blew up and spread most of its stuff out
there in the universe. So then when we looked with
the radio and we saw that at the heart of

(33:48):
crab Nebula was a pulsar, we thought, that's what this is,
and that completes the story that tells us that at
the heart of many nebula there may be these neutron stars.
Not all of them become all stars, but pulsars tell
us that the neutron stars are there, that this supernova
remnant has this hard little nub at the core of it.

(34:08):
But remember that we're using radio waves so far to
find these pulsars, and radio waves are not very good
at telling the direction of a signal. It's not like
an optical telescope, where the photons of very short frequencies nanometers,
and you can capture them with a telescope point in
one specific direction and you can tell exactly where on
the lens it hit. These things are captured by very

(34:29):
large antenna and it's hard to tell what direction they're
coming from. So while we say we saw a pulsar
in the direction of the crab nebula, it's not like
we could really pin down its location exactly. So there's
a second part of this discovery story, a part that
was caught on audio tape that I want to share
with you. But first let's take another break, all right.

(35:01):
So we are in the late sixties and the field
of astronomy was very excited because people had been discovering pulsars.
But these pulsars had been seen in the radio frequency,
which means they were hard to pin down exactly where
they were, and people were wondering, are their pulsars out
there where? The beams of light that they are shooting
are visible light, not just a radio noise, but like

(35:22):
actual visible beams that our eyes and our telescopes could see. Well,
most pulsars, we think are brightest in the radio or
in the X ray. But the idea was that there
might be some optical pulsars. So there are a couple
of theorists named John Cook and Mike Disney, and these
were not experienced astronomers, but they were curious about whether
or not you could see one of these pulsars in

(35:43):
the optical So they decided, hey, let's give this thing
a shot. Let's sign up for some telescope time pointed
at one of these pulsars and see if we can
see any flashes. So these guys not experimentalists, right, they
didn't really know how to use a telescope. This their
first time using like real serious astronomical scientific equipment, and
they went down to kid Peak near Tucson, and they

(36:03):
signed up for a couple of days of observing time,
and what they had going for them was that they
were going to point this thing at the crab nebula,
and they already knew the frequency of the pulsar, so
they knew like what frequency of light flashes to look for.
So what they did is they pointed this telescope at
the crab nebula and then they looked at the light
that came in. But remember that this again was before

(36:24):
like dedicated computers where you could rapidly inflexibly analyze your data.
But they needed was some sort of like dedicated electronics
that could turn their flashes of light into blips that
they could study. So there was a guy there who
was really good electronics, and he happened to have exactly
what they needed. So they could plug their telescope into
this thing and it would analyze the frequency, like the

(36:44):
time between blips and make a little plot from them
on a very small screen. So it's sort of like
a dedicated computer exactly to do this. They happen to
stumble across this guy who had exactly this equipment to
do what they needed. So they went out there for
their first day. They were very ided, thinking, Wow, maybe
we're going to discover something, and they turned it on
and they saw nothing. And but they didn't know at

(37:05):
the time was that they had made a mistake in
their calculations and they had like tweaked the knobs on
this thing wrong, so they shouldn't have seen anything because
they were looking at the wrong sort of frequency spectrum.
The next two nights that they had were both cloudy,
and so they lost all of their observing time and
they never would have seen this thing that hadn't been
for somebody else's bad luck. The person with the telescope

(37:26):
next after them, his wife got sick, so he decided
he was going to stay home and take care of her,
and he gave them his telescope time, so they got
an extra bonus of a couple of days of observing
time that they didn't expect to get. And the clouds
cleared and they had a beautiful night, and they set
their thing correctly. And they also had a tape recorder
running which recorded their conversation as well as the data

(37:50):
coming from the telescope. So this little box not only
makes a lit depiction on their screen that shows from
the frequency, it also made a little tick for every blip,
so you'll hear those ticks. On this tape, you'll also
hear them reacting in real time to the discovery they're making. Hey, ah,

(38:18):
I was supposed heping ca. So you hear them saying
that it's bang in the middle of the period. Remember
that they knew what to look for. They knew the
period of this pulsar. They knew their frequency of which
it should flash, so they were looking for a repeated
pattern of flashes with just the right period. They had

(38:41):
zoomed in on exactly what they were hoping to see,
but of course they never knew whether the universe would
show it to them or whether it wouldn't. Hear the
rest of their recording really looks something mhm too, Oh yeah,

(39:12):
it look so you can hear literally the excitement in
their voice. One of them is astonished, look at that
bleeding pulse, and the other one is like, I can't
believe this is happening right now, it's getting bigger and bigger.
You can see them discovering it. You can hear in
their voices that they're realizing that they've caught it, that

(39:32):
they've seen this pulsar flickering invisible light, that they've pointed
this telescope at this weird, far away object and they've
caught it doing its thing. So that's a super fun
little follow up discovery. They published that paper, and this
must have been a really fun moment for these guys,
because again, this is the first time they ever went
to a telescope. This is the first time they ever

(39:52):
like looked out into the universe. Most of their science
was done with pencil and paper and just sort of
thinking about what might be out there. And so I'm
glad they got to go out there and actually experienced
this moment of discovery. And it also really helped us
understand what these pulsars were because with the optical telescope
with a visible light, you could really pin down exactly
where this thing was, and we knew then that really

(40:14):
was at the heart of the crab nebula and it
really was a pulsar. So very exciting discovery and very
quickly appreciated, of course by the scientific community. And in
nineteen seventy four, just a few years later, Jocelyn Bell's
advisor is the first astronomer to ever win the Nobel
Prize in physics. That's right, her advisor won the Nobel

(40:36):
Prize now, of course he was involved, right, You know,
a graduate student never works alone. He gave lots of guidance,
lots of ideas, probably provided the funding. But it's clear
that she's the one who made the discovery. She built
that thing, She was out there day to day, she
saw it in the data. And there's a lot of
discussion these days about why she was left out of it.

(40:56):
It's because she was a student. While there are lots
of other cases when a student participated in discovery and
was included in the Nobel Prize. Discovery. Wholes and Taylor,
for example, was a graduate student advisor pair that discovered
binary pulsars just a couple of decades later, and they
were both given the Nobel Prize even though one of
them was a graduate student. Of course, there's the question

(41:19):
of whether or not it was sexism. In the history
of the Nobel Prizes, very few women have been given
the prize and many have been qualified, so it seems
like an obvious case of injustice. Burnell herself is very
gracious about it. She recently was given the Breakthrough Prize
and Fundamental Physics, which comes with millions of dollars, which
she then donated to advancing the cause of having more
women in physics. But of course she didn't know that.

(41:41):
The journalists didn't ask her science questions. They tended to
ask her questions about like how many boyfriends she had.
But this kicked off a whole really exciting era of astronomy,
because every time you discover something new out there in
the universe, it gives you another handle, it gives you
a way to learn things. It reveals new things about
the universe that you didn't know before. And just a
few years after that, we discovered things like millisecond pulsars.

(42:05):
These are things that's been around so fast that we
see a pulse from them, not every second, but every
mill a second. So these stars are spending a thousand
times faster than the original pulsar spun right every one
point six seconds. This incredible, enormous dense object spins around.
These things are moving really really fast, spinning like tens

(42:27):
of thousands of times per minute. The fastest pulsar we've
ever seen, we talked about on our episode about the
fastest spinning things in the universe is sixteen kilometers in
radius and the surface of it is moving at a
quarter of the speed of light. That's how fastest thing spenning.
I won't tell you the name because it's a ridiculous
series of letters and numbers, but it's spinning at seven

(42:49):
hundred and sixteen hurts. That means every second, this entire
mountain sized blob of nuclear matter spins seven hundred times
around and it's eighteen thousand years from Earth in the
constellation Sagittarius and is sending us pulses very very regularly.
The other amazing thing about these pulsars is that they
are precisely timed. It's not just like roughly seven sixteen hurts,

(43:13):
it's like exactly and every second it's the same. These
things do not change. It's astounding when you see something
in nature that is so regular. These things have the
regularity the consistency that rivals that of atomic clocks. You
can use them as a probe of the rest of
the universe because they send out these very very regular pulses.
For example, a pulsar was actually the first way that

(43:35):
we had evidence of a planet around another star. Because
when a pulsar has a planet around it, that planet
is tugging on it gravitationally as it orbits, and it
means the pulsar moves towards us sometimes and away from
us other times, and this velocity changes the frequency of
the pulse are by a very small amount. Because the
pulsars are so precise and so accurate, we can detect that.

(43:57):
And if it's a regular shift in the free and
see the pulsar, you can deduce the presence of a
planet around the pulsar. How do you have a planet
around a pulsar? It's crazy, right, because a pulsar comes
from when the Sun was destroyed, So probably some chunk
of that nebula has now reformed, some planet which is
orbiting the pulsar, or some planet happened to amazingly survive

(44:20):
of the supernova explosion that created the pulsar. And you
can also use them to navigate around the galaxy. Because
every pulsar is different, each one has like its own
unique fingerprint. You can tell which one you are listening to,
and you can also tell where you are in its cycle.
Is that pointing towards me or away from me? And
if you look at multiple of these things, you can

(44:41):
tell like how many cycles you are away from multiple pulsars.
Lets you triangulate exactly where you are in the galaxy.
But a whole fund podcast episode about navigating deep space
using pulsars, and people have crazy plans for how to
use pulsars. For example, they want to use them as
gravitational wave detector. Remember that we have seen ripples in

(45:02):
the fabric of space by seeing how these gravitational waves
stretch and shrink the distances here on Earth. Well, there
might be really massive ones that we can measure their
stretching and shrinking the entire galaxy, and those would affect
the pulses from these pulsars. And so a bunch of
really precise clocks sending us dings from all around the

(45:23):
galaxy can be used to detect gravitational waves. So there's
a bright future for the signs of pulsars, as well
as a fascinating story that tells us exactly how they
were discovered. So thanks for coming along with me on
this ride of historical exploration to understand how we actually
make these breakthroughs, how people actually win Nobel prizes or
are sometimes cut out of it by their advisor, but

(45:45):
how scientific knowledge is very slowly, very painstakingly, but very
excitingly accumulated. Thanks for joining us tune in next time.
Thanks for listening, and remember that Daniel and Jorge Explain
the Universe is a production of I Heart Radio. For

(46:07):
more podcast for my heart Radio, visit the I heart
Radio app, Apple Podcasts, or wherever you listen to your
favorite shows. Yeah.