Research and discussion topics

30 May 2008 16:28:00 BST

A rippling yarn

There is a famous detective story in which the hero, Sherlock Holmes, is able to solve a mystery because a dog doesn't bark in the night. Science is like that sometimes. We can learn almost as much from things that don't happen as from something that does.

So when a team of scientists searching for gravitational waves given off by the Crab Nebula found none at all, they weren't too disappointed.

Something from nothing

Ripples in space-time caused by a catastrophe like the collapse of a star have never yet been detected. But scientists have been searching, with more and more sensitive instruments, for forty years. So there will be great excitement when these gravitational waves are finally found. It will open a whole new window on the universe. It will also be one more test passed by Einstein's general theory of relativity.

But the Crab that isn't waving – like the dog that didn't bark – can still tell science detectives a great deal.

"It means for one thing that the neutron star at the heart of the Crab Nebula is an almost perfect sphere, with hardly any irregularities," says the University of Glasgow's Graham Woan. He is co-leader of an international group of scientists that have been looking for traces of gravitational waves in the data from LIGO – the Laser Interferometer Gravitational-Wave Observatory .

Inside the Crab

The Crab Nebula is the remains of a huge star that blew itself to bits almost seven and a half thousand years ago. How do we know? Well according to Chinese documents written at the time, in the summer of 1054 a star appeared in the sky so bright it could be seen in daylight.

This "guest star" slowly faded. But it could still be seen at night for almost two years after it first appeared. There was no explanation at the time, but scientists now know that the guest star was a supernova. This is the explosion at the end of a large star's life, when it has used up all the fuel it needs to shine brightly. The star collapses, then the outer layers bounce back out again from the small, hard centre and splatter themselves all over the neighbourhood.

What remains right at the centre of where the star once was is a small, very dense ball of matter. #a5A teaspoonful of this matter weighs as much as all the ships on Earth. This is a neutron star.

Crab with a pulse

Compared to normal stars, a neutron star is tiny. If a giant hand placed the Crab neutron star gently in the middle of Glasgow, its outer limits wouldn't go further than the rough circle formed by Milngavie, Kirkintilloch, East Kilbride, Newton Mearns, Paisley and Clydebank.¹ That's a pretty small star by any standards.

But it packs a terrific punch.

Neutron stars spin – just as the Earth, planets, sun and stars all spin. But while the Earth spins on its axis once a day, the Crab neutron star spins 30 times a second.

How do we know? Because like many neutron stars it sends out radio waves that sweep across the sky like lighthouse beams and can be detected here on Earth as radio pulses. Thirty pulses a second in the case of the Crab. Neutron stars that give off pulses have a special name. They are called pulsars.

Spin down

Pulsars spin at phenomenal speeds – the fastest so far found spins 716 times a second. But they don't spin forever. From the time a pulsar is born, spinning rapidly in a supernova, it starts to slow down.

A spinning object slows down by giving off energy. The more energy it gives off, the more it slows down. Tops stop spinning ² as their energy is changed into heat through friction. The Crab pulsar, as we'll see, is losing energy in different ways from a top. But the effect is the same - it slows down.

From the rate a spinning object slows down we can work out how much energy it's giving off (if we know its mass and shape). Scientists have done the sums for the Crab pulsar. They've observed that each second it slows down by just over a third of a thousand millionth of a revolution per second (3.7×10⁻¹⁰ revs per second to be exact).

Braking energy

This sounds pretty puny. It amounts to just a hundredth of a second in a year. But it's an unusually high rate of slowing down for a pulsar. So the Crab is "radiating energy at a prodigious rate," says Woan.

How much energy? Well the sums show that to spin down by just a hundredth of a second a year the Crab pulsar must be sending out 4.4×10³¹ joules of energy every second.

This is way beyond any amount of energy we're used to here on Earth. It is a thousand million million times as much energy as the whole world uses in electricity in a year.

Spin-down limit

Now if all this energy were coming off the Crab pulsar as gravitational waves, the scientists would be able to see it in their LIGO measurements.

But they know before they measure anything that it can't all be coming off as gravitational waves. There's an awful lot going on in the Crab Nebula and most of it is powered by energy from the pulsar. This energy is coming off the pulsar as electromagnetic radiation or fast-moving particles.

The best estimate until now of how much energy these two mechanisms carry away from the pulsar was at least 60% of the total spin-down energy (the 4.4×10³¹ joules per second) of the pulsar. That still meant that gravitational waves might be carrying away 40% of this total energy.

But the new results from the LIGO team show that they aren't. Nowhere near it. The new measurements and analysis are so good that if gravitational waves were carrying away more than 4% of the spin-down energy, the LIGO team would be able to see them in their data. And they can't. Not a trace.

Lot of balls

What the team has achieved with this new research is to set an upper limit on the amount of energy the Crab pulsar can be giving off as gravitational waves. The small size of this limit tells us something very interesting - remember the dog that didn't bark?

It tells us that the neutron star is very smooth and regular. Why? Well irregular spinning objects give off far more gravitational waves than smooth ones. Imagine a rugby ball spinning on the surface of a swimming-pool. Waves sweep out from the long ends of the ball.

Now picture a brand new football spinning on the water. You get far fewer waves. If you use an old, scuffed football you get more waves again, but not as many as from the rugby ball.

Notice too that the new football spins longer than the rugby ball because it's giving off less energy in waves.

Not waving at all?

So the Crab pulsar makes billiard balls look like furry dice.

But is that really what these new results are telling us? There is at least one other possibility.

Gravitational waves have never yet been detected. Maybe they don't exist.

This is possible but very unlikely, says Glasgow University's Jim Hough. For one thing it would mean there was something seriously wrong with general relativity. But Einstein's theory of space, time and gravitation has passed every test thrown at it in almost a hundred years.

Secondly binary pulsar

So when will the first gravitational wave be detected and from what kind of event? Quote Martin.

1. Don't try this at home. The neutron star's enormous gravity would crush the whole Earth flat.

2. Try saying that five times fast.

Audio Interview with Jocelyn Bell

Little Green Men by Jocelyn Bell.

Little Green Men by Cambridge University.

Discussion

stardust human bodies supernovae

Gravitatonal waves on your PC: http://einsteinathome.org/index.html

3 take a look at the latest job vacancies if you're interested.

http://wwwcascina.virgo.infn.it/CollOrganization/Press-Release-Feb-2007.htm

Question for Woan, Hough et al

Personal and human, scientists as well as science

why are there so many names on the paper?
explain how we can tell the smoothness of the pulsar and how incredible that is.

Beating the Spin-Down Limit on Gravitational Wave Emission from the Crab Pulsar
The Astrophysical Journal Letters. Volume 683, Issue 1, Page L45–L49, Aug 2008
analyze the Crab Pulsar

http://arxiv.org/abs/0805.4758
At the heart of the Crab Nebula is one of the strangest kinds of object in the universe.

"The Crab neutron star is relatively young and therefore expected to be less symmetrical than most, which means it could generate more gravitational waves," says Graham Woan.

Professor Keith Mason, Chief Executive of the Science and Technology Facilities Council, which funds UK involvement in gravitational waves said:

"This is an exciting result which adds to LIGO's continuing success. The project has allowed us to study the Crab Pulsar in a new and unique way and has provided us with some fascinating information. This is also the first time we've been able to estimate the role played by gravitational waves in the slowing down of a neutron star and is an important stepping stone on the way to actually detecting gravitational waves."

Matt Pitkin, also of Glasgow University, who worked on the data analysis said:

"LIGO measures changes in the relative separation of masses, suspended several kilometres apart, caused by the passage of gravitational waves through the instrument. We looked for regular variations in this separation synchronised with the rotation of the neutron star in the Crab nebula.”

Research is carried out by the LIGO Scientific Collaboration, a group of 600 scientists at universities around the United States and in 11 foreign countries.

The LIGO Scientific Collaboration interferometer network includes the LIGO interferometers (including the 2 km and 4 km detectors in Hanford, Washington, and a 4 km instrument in Livingston, Louisiana) and the GEO600 interferometer, located in Hannover, Germany, and designed and operated by scientists from the Max Planck Institute for Gravitational Physics and partners in the United Kingdom funded by the Science and Technology Facilities Council (STFC).

The next major milestone for LIGO is the Advanced LIGO Project, slated for operation in 2014. Advanced LIGO, which will utilize the infrastructure of the LIGO observatories, will be 10 times more sensitive. Advanced LIGO will incorporate advanced designs and technologies that have been developed by the LIGO Scientific Collaboration. It is supported by the NSF, with additional contributions from the U.K. STFC and the German Max Planck Gessellschaft.

The increased sensitivity will be important because it will allow scientists to detect cataclysmic events such as black-hole and neutron-star collisions at ten-times-greater distances and to search for much smaller "hills" on the Crab Pulsar.

Further information:

Dr Graham Woan, Department of Physics and Astronomy,
University of Glasgow, Tel: 0141 330 5897

Hints and tips for teachers

Relevant Curriculum for Excellence outcomes:

Science

Level 2

I can use simple models to communicate my understanding of size, scale and relative motion in our solar system. I can observe or research a feature of space that I find fascinating and describe this to others.

I can demonstrate and describe energy transfers in everyday situations and devices to develop my understanding that energy cannot be created or destroyed.

By carrying out investigations into friction I can explain how it affects movement, and can use my understanding of friction to design or improve a product.

Level 3

I have carried out activities to investigate the effect of gravity on objects and can describe its effects to others. I can predict what might happen in different situations on Earth and in space.

Level 4

From my research into a current space development, I can use information and data to show how new technology is used to make observations and measurements that increase our knowledge of the Earth and our Universe.

I can apply my knowledge of electromagnetic waves and their detection to explain how images and information from space help us understand the structure of the Universe and how it has changed over time.

I have explored the relationship between mass and volume, and as part of a team I can use my knowledge to solve a challenging problem related to density.

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