- Hello, my name is Ilya Mandel.
I'm a professor of theoretical astrophysics here
at the Monash School of Physics and Astronomy.
And I'm going to try to use this lycra universe,
to explain how gravity works.
Now, you might be wondering what this thing is doing here.
I'll explain in just a second,
but first let me remind you how gravity works
according to ideas that Newton developed 400 years ago.
So, in Newton's notion of gravity,
there are two massive objects,
they exert a force on each other, they attract each other,
that force is proportional to their masses
and inversely proportional to the square of the distance
between them.
But there are some problems with this.
One of the problems is that gravity in Newton's theory,
propagates instantaneously,
whereas Einstein realized early in the 20th century,
that that doesn't make any sense because information cannot
travel faster than the speed of light.
There are a few other problems with this idea.
And after pondering on this for more than a decade,
Einstein came up with something that he called,
the general theory of relativity.
Which is really Einstein's theory of gravity.
And it says something like the following.
It says that, gravity is actually a curvature of space
and time.
Now, I don't quite know how to show curvature
in four dimensions,
so I'm going to use this lycra universe
to show it in two dimensions.
Now, the first idea is that if we have a flat space time,
look, here's a flat surface.
Then any object is just going to move along,
with a constant speed, in the same direction
it was initially moving in.
So here is my little marble, I roll it along
and you can see it's following a fairly straight path
along this flat surface.
But now suppose that it's a big, massive object.
What this massive object does is it's gravity curves
the space and time around it.
You see it here as a funnel, in this lycra universe.
And now, if I take my little object
and I again, try to move it on a straight line
by giving it some initial speed,
you will see that what it actually does is it curves around.
It zooms around this big, massive object, right?
Let's try it one more time.
You can think of this as a kind of planet
that's orbiting around the sun, for example,
at the center of the solar system.
Now, that's all well and good and okay in general,
if there was no friction, this orbit would continue forever.
But you can imagine a different situation,
where now my second object is also fairly heavy.
Now, here of course this simple illustration
it's just going to have a little bit more friction.
But in reality, what's going to happen
is a phenomenon that Einstein predicted
called gravitational waves.
These are ripples in the fabric of space time,
that you can see here.
You can see that there is some shaking
of my lycra universe, the shaking of space and time.
And of course you can see that this object
spirals in much faster now than the little pebble did,
because it's losing extra energy that propagates out
as the ripples in space and time.
And these ripples, in fact, were predicted by Einstein
in 1916,
but were not observed until exactly 100 years later,
when the Laser Interferometer
Gravitational-Wave Observatory,
a big international collaboration
that Monash is a major player in,
first detected gravitational waves.
And this is one of the very exciting topics of research here
in the School of Physics and Astronomy at Monash,
where we have a big group that plays a major part
in the LIGO collaboration,
trying to detect gravitational waves,
from the mergers of two black holes
or two neutron stars, very dense, very compact objects,
and trying to understand what they mean
as far as the evolution of massive stars
and the tests of general relativity
under the most extreme scales imaginable.
Thank you.