98 Not-Out

It’s now almost a century since Albert Einstein published his general theory of relativity and we’re still trying to find a flaw in it. It remains our best theory of space, time and gravity. But, as ever in science, we’re constantly striving to test it in new ways in the hope that some experiment or observation will disagree with the theoretical prediction. We hope this because it’s certain that general relativity isn’t the full picture.

Recent discoveries just might help reveal what we’re missing.

There has been a lot of reporting on the LHC at CERN; their hunts for the/a Higgs boson and supersymmetry (among others). These particle physicists are concerned with the world of sub-atomic particles, which we call the quantum world. Down at these tiny scales counter-intuitive quantum theory prevails, explaining experimental results very nicely. Their work has given us the standard model, a magnificent achievement (other, more exotic models are available).

It may surprise you then to hear that quantum physics cannot describe gravity. You know, that force we all know and love. Yeah… the standard model ignores it completely. Gravity is just far, far too weak to affect the high energy particle physics experiments that have shaped our view of the quantum world. And so we’re faced with perhaps the most puzzling problem in modern physics; how to marry our understanding of gravity with our understanding of quantum physics and develop a theory of quantum gravity. If both GR and the gravity-less quantum theory are reinforced by ever more experiments, we’re rather unlikely to do this any time soon. We’d like nature to give us a hint, please.

Extreme views

The bright star in the centre of this image is Sirius A, the brightest star in the night sky known colloquially as the ‘dog star’ due to its location in the constellation of Canis Major, one of Orion’s hunting dogs. The small pinprick to the lower left is Sirius B, the nearest known white dwarf to Earth. These two orbit each other in a binary, and were born together as a pair. The larger star of the two ran out of fuel first and became a white dwarf upon its death, while the smaller is still burning brightly. Credit: NASA, ESA, H. Bond (STScI), and M. Barstow (University of Leicester).

If we have any hope of spotting a deviation from GR, we probably need to look at the extremes of the theory; gravity at its weakest and strongest limits. We can’t hope to create extremely strong gravity in the lab, so we need to look outwards into space for things like black holes, neutron stars and white dwarfs.

White dwarfs are the hot, dense leftover cores of ex-stars. Average to small-sized ones like the Sun, in fact the vast majority of all stars. Give it 4-5 billion years and the Sun too will leave behind a white dwarf once the outer layers have been cast off as a nebula, completely annihilating whatever is left of the Earth, which is a cheerful thought.

These stellar remnants have a significant proportion of the mass of the star but are only about the size of the Earth, and they glow white-hot purely from the heat of their formation. After trillions of years they are expected to cool down to form so-called ‘black dwarfs’, but the Universe isn’t old enough for this to have happened to a single one yet.

When larger stars die they do so in explosions called supernovae. If their cores are above 1.44 times the mass of the Sun, the forces which hold up a white dwarf against its own weight are no longer sufficient, and the core will instead collapse to form a much smaller but heavier object. This collapse happens in a fraction of a second and triggers the supernova, leaving a neutron star (or a black hole in the cases of the very largest stars).

It’s safe to say that neutron stars are among the coolest things in the Universe (not temperature wise; they’re actually extremely hot). They can be anything from 1.4 – ~3.0 times the mass of the Sun, yet only the size of a city.

The first neutron star seen in visible light by the Hubble Space Telescope. Most are seen either in X-rays or as radio pulsars.

We know very little about exactly what goes on inside them, since the pressures and densities are so absurdly great we simply cannot recreate them here on Earth (for comparison, if all 7 billion of us here on Earth were squished into a cube only 1.4 cm a side we’d be about as dense).

However, at their surfaces the physics is still so extreme that we can use them to probe strong field gravity. For example, a neutron star with a mass double that of the Sun will have a radius of only about 11km (7 miles). This means the gravitational field at its surface is more than 200 billion times stronger than at the surface of the Earth! If an object were to fall from shoulder height above the surface (1.5m) it would hit the surface in only a millionth of a second while travelling at 2,565 km per second (that’s a swift 5.7 million miles per hour). Due to this ridiculously strong gravity, a number of possible challengers to Einstein’s GR predict behaviour near the surface of large neutron stars that differs from the standard GR prediction. If there were a way of observing something moving very close to such an object, we might stand a chance of learning something very interesting and take a step toward solving the problem of gravity.

Oh look, what’s this over here…

In the last week or so a set of results were published, observations of a binary system consisting of a neutron star and a white dwarf. The neutron star happens to be the largest yet observed at, wait for it, 2 solar masses (that’s handy!) and the two binary companions orbit tightly in only 2.46 hours, ie. they are close to one another.

Artist’s impression of the neutron star – white dwarf system. The neutron star is the smaller of the two size-wise, but is more massive. As it spins the jets sweep out an arc, and the Earth intersects one of them, causing the radio pulses which make it a pulsar. The rippled blue mesh represents the gravitational waves rippling through spacetime. Credit: ESO/L. Calçada

GR predicts that such systems should gradually shrink, the two bodies moving closer together in faster and faster orbits as they radiate away energy as gravitational waves. Some competing theories predict different evolution of such a system since they have different descriptions of how gravitational waves might be emitted. Fortunately, this neutron star happens to be a pulsar, one which is seen to pulse regularly in radio frequencies as it spins about its axis 25 times per second, like a cosmic lighthouse. So by looking at these radio pulses, the astronomers involved could use them like a ticking clock to measure the evolution of the system accurately. Perhaps this system will point us to an updated theory of gravity!

And the result so far:

Our radio observations were so precise that we have already been able to measure a change in the orbital period of 8 millionths of a second per year, exactly what Einstein’s theory predicts.

– Paulo Freire (co-author), Max-Planck-Institut für Radioastronomie.





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