Ripples in spacetime
In Einstein’s general theory of relativity, the gravitational force is not best pictured as a pull reaching through empty space. Instead, mass and energy shape spacetime, and the shape of spacetime guides how things move. A rough image is a stretched rubber sheet with a heavy ball on it: the sheet curves, and nearby marbles roll toward the ball. The image is imperfect, since real spacetime has four dimensions and no outside, but it conveys the key idea.
If curvature can change, the change can spread. Einstein showed in 1916 that his equations allow ripples in spacetime that travel at the speed of light. When massive objects accelerate in the right way, such as two stars orbiting each other, they should emit gravitational waves. As a wave passes, it stretches space in one direction and squeezes it in the perpendicular direction, then reverses, repeating as the wave goes by.
Why they are so hard to detect
The effect is extremely small. Ordinary events on Earth produce undetectably weak waves. Strong ones come from violent cosmic events involving very dense, massive objects, such as two black holes spiraling together or two neutron stars merging. Even then, by the time the wave reaches Earth from a distant source, the change in length it causes in an arm several kilometers long is far smaller than the width of a proton. Einstein himself doubted that they could ever be measured.
Early indirect evidence
Before direct detection, there was strong indirect evidence. In the 1970s, astronomers studied a pair of neutron stars orbiting one another, one of which is a pulsar emitting regular radio pulses. Over years, the orbit shrank at the rate predicted if the system were losing energy by emitting gravitational waves. That agreement convinced most physicists the waves were real, even though no detector had caught one.
How detectors work
The detectors that finally succeeded are laser interferometers. The basic design looks like a giant letter L. A laser beam is split into two, and each part travels down one of two long perpendicular arms, several kilometers long in the largest facilities, bounces off a mirror at the end, and returns. The beams are then recombined.
When no wave is passing, the two beams are arranged so that they cancel out at the output. If a gravitational wave stretches one arm and squeezes the other by a minuscule amount, the travel times differ slightly, the cancellation is no longer perfect, and a small amount of light appears. By measuring this, scientists can infer the wave’s strength and pattern over time.
Making this work required extreme engineering:
- Isolation from vibration. Mirrors are suspended so they are shielded from ground motion, traffic and even distant ocean waves.
- Vacuum. The laser beams travel through very high vacuum so air does not disturb them.
- Multiple detectors. Using several detectors in different places helps rule out local noise, because a real signal should appear in all of them with small timing differences. Those differences also help estimate where in the sky the source is.
- Careful data analysis. The signals are buried in noise, and scientists compare data against calculated waveforms to pick them out.
The first detections
Gravitational waves were directly detected for the first time in 2015, from the merger of two black holes, and the result was announced in 2016. The signal showed the characteristic “chirp” of a rising frequency and amplitude as the objects spiraled in and merged. Since then, more events have been detected, including mergers of black holes and a few involving neutron stars. In 2017, a neutron star merger was seen in both gravitational waves and light across the electromagnetic spectrum, a landmark event for what is now called multi-messenger astronomy.
What they can tell us
Gravitational waves offer a new way of observing, complementary to telescopes that collect light. Light can be absorbed or scattered, while gravitational waves pass almost unhindered. Possible uses include:
- Studying populations of black holes, including ones too dark to see by other means.
- Testing general relativity in strong-gravity conditions, where any deviations from predictions would be notable. So far, results have been consistent with the theory.
- Providing an independent way to measure distances, which could help with disagreements over the rate of cosmic expansion.
- Investigating the behavior of extremely dense matter in neutron star mergers.
Researchers also plan or operate other approaches, including detectors in space and methods that use timing of pulsars, which aim at longer wavelengths than ground-based interferometers can sense.
What to keep in mind
Gravitational waves are not a source of energy or a communication method, and they do not travel faster than light. Their importance is scientific: they let us study events that would otherwise be invisible and test our best theory of gravity. As detectors improve, the number of observed events is expected to grow, though exactly what will be learned remains to be seen.