All the different kinds of waves we currently know about – radio, microwave, visible light, X-rays and so forth – are caused by one and only one process: electrons in atoms giving varying amounts of energy to particles of light called photons. These photons are merely vehicles that carry the energy FROM atoms undergoing various electrical and magnetic reactions TO our Walkman, our popcorn, our digital camera, our X-ray film. Thus the electromagnetic energy transfer process is carried out THROUGH space (FROM an antenna TO a radio), but it does not physically ALTER the space between the origin and destination points.
Gravity waves are fundamentally different. To begin with, they are not caused by atoms gaining and losing electromagnetic energy. Gravity waves are caused instead by the mere motion of atoms in space, just like waves in a pond are caused by the mere motion of a pebble thrown into the water. Just like a ripple in water, gravity waves cause a ripple in space.
A ripple in space: such a thing is hard to imagine, to say the least. For now, imagine it like this. Say you had a bathroom sponge sitting on a set of bathroom scales. The sponge represents Our Universe, so it’s made not of plastic foam, but made instead of quantum spacetime foam (QSF). (How does QSF differ from polyurethane, you ask? Check these SciScoop pages in the middle of the 22nd Century and we’ll get back to you). This sponge even has some flecks of mold in it; that’s extraterrestrial life – including us. Somehow a lit match gets dropped on this sponge, it catches fire, and it burns with a bright flame. You can see electromagnetic energy in the form of visible light from the flames. In a very crude way, this is the story of the Big Bang. When we take pictures of the Cosmic Microwave Background with NASA satellites like COBE and WMAP, we’re sort of taking electromagnetic snapshot pictures of the burning QSF sponge in which we live.
But while our QSF sponge is electromagnetically burning its way into a cold ash, the bathroom scale on which it sits has not registered any change at all! Let’s change that right now by dropping a steel ball bearing on the top of our sponge. BAM! The impact squishes our sponge momentarily and the bathroom scales jerk for a moment as they register a spike of weight. A camera can’t capture the internal squish of the sponge – that’s not its job – but the bathroom scales DOES. And that pulse of squishiness in the foam is for our purposes a gravity wave.
Nobody has yet discovered a gravity wave, but we’re trying. Our latest attempt is with some new billion-dollar bathroom scales we’ve built in Louisiana and Washington called the Laser Interferometer Gravitational Wave Observatory, which so far hasn’t seen a thing – but then, it just recently got turned on this year and is still undergoing fine-tuning.
Sinking that kind of money into bathroom scales just isn’t done unless there is a belief that some pretty heavy ball bearings are getting dropped on the spacetime foam in which we live. And indeed they are, in the form of double stars. Most solar systems have two suns revolving around each other – ours would be that way if the planet Jupiter were a little bigger and had ignited into the star it could have been. In their old age, a few double star systems become special super-dense stars called neutron stars that spiral in to collide with each other eventually. This spiraling dance of binary neutron stars in itself is believed to create gravity waves that are possibly too weak to be measured directly with LIGO – but just thinking about them has been good for a Nobel Prize anyway. When the two neutron stars finally collide, tho, they create a black hole. THAT collision is one heck of a big steel ball bearing dropped smack dab in the middle of the spacetime foam in which we live – one that LIGO is designed to “see.”
So how long do we have to wait before LIGO registers one of these star crashes? At last, dear reader, we’re at the “current news” paragraph of this story. Last week it was announced the wait may not be as long as we’d thought. The discovery of a new dual neutron star system was announced, and this one is scheduled for impact in a mere 85 million years!!! While LIGO won’t be tuning in on THAT impending gravity wave, the study published in Nature suggests that binary neutron stars may be much more common than we had thought in the universe as a whole – and that means the gravity wave from one could be rippling through our solar system on its way to LIGO measurement sooner than we think.
Newton had it easy in Quicksilver and in real life when he teased out the secrets of electromagnetic waves. He only needed a glass prism, and he had a steady source of sunlight every day. Today’s scientists need LIGO and have to wait for an as-yet undertermined amount of time for a gravity wave from outer space to slither through the Louisiana swamps. But hey, you’ve got to start somewhere in the trek for godlike mastery of an entirely new wave spectrum.