Text Version

Gravity: Making Waves

Video transcript
The video is 7 minutes and 38 seconds long.
Produced by the American Museum of Natural History, November 2004.

Video begins here.

Visual: Black and white photo of Albert Einstein.

Speaker: Ray Weiss, Professor Emeritus, Massachusetts Institute of Technology

Einstein's view of gravitation was very different than Newton.

Visual: Historical drawing of Isaac Newton.

Newton was thinking of forces between objects. And he didn't quite imagine how the forces would project from one object to another. That was always a mystery.

Visual: Ray Weiss in laboratory

On the other hand, Einstein replaced that spookiness with probably something that's just as spooky. And that was this notion of gravitational waves. 

Visual: Title: Gravity: Making Waves

Montage: Livingston, Louisiana, A long concrete tube-like structure extends for kilometers along a canal in cleared wilderness.  A van drives up a road through a scientific facility, past a large storage tank reading “LIGO”.

Speaker: Gabriela Gonzalez, Assistant Professor, Louisiana State University

LIGO stands for “Laser Interferometer Gravitational-Wave Observatory”.

Visual: Gabriela Gonzalez sitting in front of large, laser-firing apparatus.

The "W" didn't fit in there, so we just hyphened it. 

Visual: Scientists monitor schematics on computer screens.

LIGO's goal is to detect gravitational waves.

Visual: Gabriela Gonzalez in LIGO facility.

Gravitational waves were predicted by Einstein's theory, but they have never been directly detected. We have very strong indirect evidence, and we all believe his theory.

Visual: Scientists in control room, monitoring computer screens.

But we never saw these waves directly, so that’s why we want to measure them. 

Visual: Mike Zucker in office

Speaker: Mike Zucker, Director, LIGO Livingston Observatory

Newton's description of gravity was basically that it was a force that was proportional to the masses of bodies and inversely proportional to the square of their separation. 

Visual: A 3-D blue wireframe grid in space.  An orange orb moves through the grid, warping the lines toward it.

Einstein's concept was to look at the bodies’ influence on the space they're embedded in.

Visual: A smaller green orb approaches the orange orb, and its path is altered.

And so in Einstein's picture, a massive body makes a depression in the space-time around it so that other bodies following their natural paths tend to be attracted to this depression. 

Visual: Mike Zucker in office.

The two ideas sort of come up with the same answer when we're describing, say, the orbit of the Earth around the Sun, or the Moon around the Earth, or the falling of an apple. But they differ if you think about what is the effect of a very rapid change.

Visual: Computer simulations of stellar collisions and mergers.

For example, if two stars collide or a star explodes somehow, the change in the mass distribution, the presence or absence of a star where there was one before, has to somehow be communicated throughout the whole universe.

Visual: Mike Zucker in office.

And in Newton's picture, there's a problem with that because there isn't any way for that information to take some finite amount of time. Somehow the whole universe must know about everything instantaneously. It's called the problem of action at a distance.

Visual: A 2D blue wireframe grid.  A stellar event occurs in the center. The blue wireframe ripples outward, making waves.

So going back to Einstein's picture, if you think of it in terms of massive bodies affecting the space-time, you can immediately think of a way to communicate information about those bodies from place to place, through ripples in space-time.

Visual: Ray Weiss in lab, stretching and collapsing a mesh wine bottle protector.

The waves can be represented by this object I found on a wine bottle. And it's a mesh that you can see. And the waves cause transverse to the direction in which they're moving. They're moving forward, and transverse to that the space gets tugged like this, and collapses like that. Tugged like this. And if you look carefully at this, and I'll do this a few times, you'll notice that the little squares in this, how they're exercising a motion where along one direction, it's obvious which direction--I mean, the direction I'm pulling in--space is getting expanded. But transverse to that, up and down, space is getting contracted. And that's the key to the whole thing. 

Visual: An aerial photograph of the LIGO facility, a giant L-shape stretching kilometers long.  A blue wireframe grid is superimposed, the gridlines matching up with the arms of the facility.

Visual: Enormous concrete tubes in cleared wilderness, stretching to the horizon.

Speaker: Joe Giame, Chief Scientist, LIGO Livingston Laboratory.

LIGO is laid out in an L shape, and each side of the L is four kilometers long. Four kilometers was chosen because the American taxpayers are willing to pay for it.

Visual: LIGO person driving in a van.  The van drives on a road along the length of one of the arms.

And it's far enough to get an effect that we think we can see. 

Speaker: Joe Giame

Visual: An aerial photograph of LIGO, with a blue wireframe grid superimposed.  The grid stretches and contracts along the length of the LIGO arms.  Red markers at each end of the LIGO arms move with the contraction and expansion.

The reason it's an L shape is because gravitational waves cause objects to get closer in one axis and farther in a perpendicular axis. And then vice versa.

Visual: A van speeds along one of the LIGO arms.

So we fill the two arms with light from a laser in the corner station.

Visual: Joe Giame in van, driving.  The concrete tubes of LIGO speed past outside.

And the light spends about one one-hundredth of a second in this arm and one one-hundredth of a second in the other arm, and then the light comes back to the corner station.

Visual: An aerial view of the LIGO facility, with blue gridlines overlaid on it.  Yellow orbs, representing the travel of the lasers, move back and forth along the LIGO arms.

And the light that spent time in this arm comes back a little bit before or a little bit after the light that spent time in the other arm. And that's how we would detect the effect.  

Visual: A van pulls up to the LIGO facility.  Gabriel Gonzalez in the LIGO facility.

Speaker: Gabriela Gonzalez

A lot of people try to think about how to produce gravitational waves so you could measure them, and then you have an experiment like you do with radio. You produce the waves, you detect them, and then you prove that they exist. But with gravitational waves, it turns out that you need a lot of mass and very high accelerations in order to make any distance change at all, by even a minuscule amount that you can measure.

Visual: Crowded city sidewalk.  Cars speed past on a city highway.  Computer simulations of stellar collisions.  Colored orbs meld into each other and swell.  Gabriel Gonzalez in LIGO facility.

So everything produces gravitational waves, but it’s only big things out there, it’s big black holes, big neutron stars that are accelerating and colliding and exploding, and it’s that kind of event that produce gravitational waves of such magnitude that we can measure them.

Visual: Mike Zucker in office

The prediction is that gravitational waves are always occurring.

Visual: Computer animation of stellar, gravity wave producing event. 

The reason why we don't notice them is that they're exceedingly weak. They're very hard to imagine how weak they are. But for our detector, for LIGO, we're looking for displacements of the test masses at the two ends that will be something like a thousandth the diameter of a proton over a scale of two and a half miles on a side. So we're looking for substantially smaller displacements than the kind of things that people normally experience and measure.

Visual: LIGO technicians at computer stations. 

Speaker: Gabriela Gonzalez

When we begin detecting gravitational waves it’s going to be in the newspaper everywhere because it is going to be the first time we see gravitational waves.

Visual: Gabriela Gonzalez in LIGO facility.

But once we begin seeing them routinely and we can classify them—

Visual: Black hole in center of galaxy NGC4261

these come from black holes, these come from neutron stars,

Visual: Nebula around Crab pulsar.  Supernova remnant SNR 054069-3

these come from supernovas—

Visual: Sombrero galaxy

then we can learn a lot about these sources. 

Visual: Mike Zucker in office.   Computer simulation of stellar collision.

Speaker: Mike Zucker

I think one of the really big motivations for looking for gravitational waves, in addition to all the wonderful forms of energy that we observe our universe in, is precisely this fact that they are entirely responsive to the distribution of matter.

Visual: Computer simulation of stellar collision.  Mike Zucker in office. 

And so, if we can detect them, we'll see what's going on in the cores of the objects, the most dense and the most active parts, and even though it's quite challenging, and it hasn't been done before, the promise is that we'll reveal effectively a different universe than we've seen before.

Visual: Ray Weiss sitting in laboratory.

Speaker: Ray Weiss.

And that’s what makes it so exciting for most of the people in this business. It isn’t just one number you’re looking for. Or this experiment isn’t over when we make one discovery.

Visual: The sun and Earth in space.

It has a lovely distant wonderful future with many, many new things that will come.

 

Video ends here.