Ask a Physicist

What amplitude, frequency and wavelength gravity waves are we expecting to find?
Submitted by Kostas from Australia

Each detector can find them only in a certain range of frequencies.

LIGO and the other ground-based detectors are sensitive to frequencies from a few tens of Hertz (cycles per second) to a few thousand Hertz. These are the frequencies the human ear is sensitive to, but the amplitudes are much smaller. Your ear can pick up sound waves from air pressure changes as small as one part per ten billion. The strongest signals LIGO might see are less than a part in ten billion of THAT - maybe a thousand times less, though we hope not. These signals have wavelengths from ten miles to about the diameter of the Earth.

LISA, which will share the Earth's orbit around the Sun, will find waves at frequencies from a tenth of a Hertz down to a ten-thousandth of a Hertz. This means wavelengths up to ten times the diameter of the Earth's orbit. LISA's biggest signals will be from galaxies colliding, with amplitudes thousands of times those of the LIGO signals. That's still a few billionths of the fractional pressure changes your ear can detect.

The next frequency window is around a few billionths of a Hertz, or periods of years. Any signals here would have wavelengths about the distance from the Sun to the next star. We'll look for these with a "pulsar timing array." Pulsars are spinning stars which act as very good clocks and are scattered around our galaxy. "Array" means we'll look at lots of them for years (we're just starting). If a gravitational wave at these frequencies passes between us and the pulsars, it makes a pattern of changes in their apparent spin rates as it distorts spacetime between us and the pulsars in different directions. Such waves would be coming from the early universe, which we don't know much about, so we can't predict the amplitudes well.

Last but not least are frequencies around a billionth of a billionth of a Hertz. These waves have waved only a few periods over the entire age of the universe and have wavelengths about the size of the universe. They'll show up as patterns in the cosmic microwave background (the fading glow from the Big Bang from the edge of the universe). We haven't seen the pattern yet, so the waves must be pretty small. But they're coming from right after the Big Bang when the universe was microscopic, so if we find them with a future satellite we'll learn something drastically new.