Transcriber: Tijana Mihajlović
Thank you.
Hi, hello.
My name's Arnau and I'm a nuclear physicist.
That doesn't generally go very well at the parties.
(Laughter)
But today I'm not here to talk about the atomic nucleus,
the tiny object at the center of the atom
that defines the elements that we find around us;
today I'm here to talk about neutron stars,
and why these are objects
that redefine our ordinary understanding of astronomy and astrophysics.
Now, before I move into neutron stars, though,
I also need to talk about an extraordinary event
that occurred just two weeks ago.
If you were paying attention to the news two weeks ago,
you would have seen something like this:
the Laser Interferometer Gravitational Observatory, LIGO,
announced the first direct detection of gravitational waves.
This was an event that looks like the video on the right-hand side in there.
Two black holes around 30 times as heavy as the Sun each
collided and merged,
creating an even larger black hole in just a fraction of a second.
So that movie was a slowed down version.
The power output of this event - to put it into context -
was larger than the power output of all the stars in the Universe.
It was really extraordinary.
Now, there are several reasons why this event is important.
There's a technological aspect to it.
There's the understanding of astrophysics that it brings forward;
we didn't know before that two of these black holes could collide,
and we didn't know how often they would do it.
But really, why this event matters
is because it opens a new window
into the way that we look at the Universe.
Let me give you an example of how the Universe looks
when you look at it instead of with your own eyes
with different kinds of eyes.
What you can see here is the Crab Nebula.
The Crab Nebula in the top-right corner,
that's how you would see it with the telescope,
with the binoculars, if you are lucky enough;
not with your own eyes.
The Crab Nebula sits in a place in the sky
where Chinese astronomers in 1054 detected the presence of a new star.
And we think that was a supernova explosion,
the cataclysmic event at the end of the lifetime of a star,
leaving behind, inside and at the middle, a tiny tiny neutron star.
When you look at the Crab Nebula with your own eyes,
you see a beautiful, colorful nebula.
When you look at the Crab Nebula, though, with other eyes -
for instance, looking at the most energetic events, like X-rays -
you see things like the picture at the center down here,
which shows a tiny object with flows of matter
and incoming, inspiraling accretion disks.
So, it's a completely different picture.
You see something extraordinarily different.
The same way if you look at it in ultraviolet light,
in infrared, or in the radio spectrum.
We don't know yet what the Crab Nebula looks like in gravitational waves,
but when we do, I'm sure we're going to find something extraordinary.
If you want an analogy to this, imagine that suddenly, one day,
you lose one of your five senses, let's say hearing.
You would stop listening to music, hearing people speak,
you would miss a lot out of the Universe that surrounds you.
And if, by chance, suddenly you got hearing back,
let's say while you're walking in a station,
and you could suddenly hear the crowds, the noise of the trains,
the announcements, the radio, the music,
you would feel incredibly excited.
That's how excited the physicist feels when the new window is opened
to look at the Universe
because we are going to see completely new dimension
of the events in astrophysics and in the Universe.
Now, since we are talking about hearing,
it turns out that we can also listen to the Crab Nebula.
You just point a radio telescope to the Crab Nebula,
plug it to your usual radio and listen to it.
Let's see how that sounds.
(Noise)
It's a terrible sound.
(Laughter)
We expected music, but we got this instead.
(Laughter)
If you paid attention there,
you could hear a repeating, high-pitched frequency sound,
more or less like a car motor.
This was about 500 Hertz signal,
and the reason that you heard that
is that the neutron star at the center of the Crab Nebula is actually rotating.
It’s a bit like a lighthouse.
Neutron stars have the largest magnetic fields in the Universe,
and the particles trapped in those magnetic fields emit light
along the north and the south pole of the star,
in a very narrow beam.
If the neutron star is rotating,
and that beam happens to sweep across the Earth,
you would hear a beep.
And if that happens at the frequency of 500 Hertz,
you would hear the sound that you've just heard
of the Crab Nebula.
So, it's really extraordinary.
We can see these massive magnetic fields
and these sounds coming from objects in the sky.
Now, why are neutron stars so extraordinary?
Let's look at their properties.
In terms of mass, neutron stars are very massive.
They're very massive for our standards,
but they are more or less as massive as a normal star,
so a neutron star weighs more or less the same as the Sun;
that's to say, about 500,000 Earths, so it's a pretty heavy object,
but it's in the right scale, what we would expect from a normal star.
Now, what makes neutron stars extraordinary objects
is that they are tiny; they are very small.
The radius of a neutron star is just around 10 kilometers.
In comparison, the radius of the Earth is 6,000,
so 600 times larger.
The radius of the Sun is almost a million.
So it's a much larger object.
Neutron stars are tiny,
and they have tons, large amounts of mass inside of them.
Just to give you an idea of how tiny that object is,
this is my commute every day from London to Guildford.
You could fit two neutron stars in there.
It would be a bad idea because the density is so large
and the gravitational field is so large
that you would probably be crunched immediately,
but still, you could put one of those in there.
(Laughter)
If you make the ratio of the mass over the volume of that star,
the density that comes out is, again, a very large number.
A teaspoon of that material would weigh about a billion tons.
It's very difficult to picture what that means
because even the more dense materials on Earth, like gold,
are just 10 to 13th orders of -
10 to 13th times less than that,
so one followed by 13 zeros less dense than that.
There's only one other place in the Universe
where you actually find these extremely large densities.
And that's the interior of the atomic nucleus,
the systems that I study every day,
where you have neutrons and protons interacting in a very tiny space.
Again, putting a lot of mass in a very small place
gives you a very large density.
So, in principle, if I know how neutrons and protons behave inside of a nucleus,
I can also figure out how neutrons behave surrounded by a crowd of neutrons
and how they, for instance, make a neutron star.
That's why I can make the connection
between one of the smallest objects of the Universe, the nucleus,
and an extraordinary astrophysical object like a neutron star.
Now, on 11th February, as a physicist,
as a lecturer in physics, I was extremely excited.
I was thrilled.
This opened a new window to the Universe.
What we will see with gravitational waves will be extraordinary for sure.
As a nuclear physicist, though,
on 11th February, I was slightly disappointed.
The reason I was disappointed is that, for all we know in terms of astrophysics -
how our galaxy works, how stars live their lives -
we expected that the first event that we would see
would be with the same likelihood either a black hole,
black hole binary, like we observed,
or a neutron star, neutron star binary.
Obviously, as a nuclear physicist,
I would have preferred to see two neutron stars colliding
rather than two black holes.
(Laughter)
Now, that's not only because I like neutrons,
but that's also because we would have learned a lot
about the properties of these objects from this collision;
we could have figured out their masses with better precision,
possibly their radii with better precision, too.
But there's another reason why neutron star collisions or mergers matter.
And I think it's better if we see a movie of one of these events to explain it.
So, there you have two neutron stars colliding.
They rotate around each other,
and as you can see, they shed a lot of mass around.
The center is a very dense object
that eventually will evolve into a black hole,
but all the material around the inspiral
is being ejected into the astronomical medium.
Now, this is very hot; the color coding here is temperature.
So, this is hot material and these are two neutron stars,
so this is neutron-rich material.
And as nuclear physicists,
we know that to explain the existence
and presence of the elements that are heavier than iron -
think about gold, platinum, mercury, lead -
all those elements are not produced in normal stars;
they have to be produced in cataclysmic events
with large amounts of energy and large amounts of neutrons.
And we believe that the neutron stars mergers
could be one of those places.
So, if we could get the gravitational wave signal
coming from one of those mergers,
and if we could also observe it, say, with light,
we would learn so much,
not only about the gravitational interaction between these two objects,
but also about nuclear physics and the processes that create the elements
that make us and make the Earth.
That is a prospect that really excites me,
and that's why I was so disappointed on February 11th.
Well, to close down I want to show you a picture.
This is what a pulsar looks like.
These are the radio pulses coming from a specific object.
And perhaps you don't recognize that image,
but if I actually invert it,
those of you who love vinyls might actually see
that this is the cover of Joy Division’s famous album "Unknown Pleasures."
It's a beautiful example of what happens when you mix science with art.
But what I want to take out of it is a message of "Unknown Pleasures."
Working at the forefront of research within nuclear physics,
in gravitational waves, in astrophysics, in any field, actually,
provides an immense amount of joy.
You always find new ideas,
extraordinary events
that really give an immense amount
of unknown intellectual pleasures.
Thank you.
(Applause)