gino_segre's picture
Professor of Physics & Astronomy, University of Pennsylvania; Author, The Pope of Physics: Enrico Fermi and the Birth of the Atomic Age
Gravitational Radiation

Humans as well as most other mammals detect, from birth onward, electromagnetic radiation in the form of visible light. But it was not until the 1887 experiments by Heinrich Hertz that scientists accepted that what they were seeing was nothing but a frequency band of the electromagnetic radiation generated by accelerating electric charges. These experiments confirmed the prediction James Clerk Maxwell had made less than three decades earlier.

The realization opened the door to studying electromagnetic radiation of all frequencies, from radio waves to X-rays and beyond. It also stimulated scientists to ask if an entirely different form of radiation might ever possibly be observed. Accelerating masses rather than electric charges would be its source and it would be called gravitational radiation.

Maxwell’s equations had shown what form electromagnetic radiation would take. Albert Einstein’s 1916 general theory of relativity offered a prediction for gravitational radiation. But since the gravitational force is so much weaker than the electromagnetic one, there was doubt that a direct observation of this radiation would ever be possible.

But the seemingly impossible has taken place! In an experiment of almost unimaginable difficulty, the LIGO (Laser Interferometer Gravitational-Wave Observatory) reported that they detected on September 15th, 2015, at 9:50:45 GMT (Greenwich Meridian Time), a signal that corresponded to two black holes. They were 1.3 billion light-years away, one with a little more than thirty solar masses and the other a little less, spiraling into one another to form a larger black hole. Theory predicted that the equivalent of three solar masses had been emitted as gravitational waves in the last second of the two stars’ death-spiral.

The general theory of relativity tells us that gravity is a curvature in space caused by the presence of masses and energy. The signal for gravitational radiation or alternatively gravitational waves is therefore a deformation in the space of a detector which the waves are passing through. The LIGO detectors, one near Livingston, Louisiana and the other near Richland, Washington consist of L-shaped structures containing a vacuum tube in which a laser beam travels back and forth from one arm to the other.

In distorting space, the gravitational waves from the spiraling black holes altered the relative length of the detector’s two arms by approximately a billionth of a billionth of a meter, an incredibly small distance but sufficient to change the interference pattern of the laser light recombining at the detector’s nexus.

The need for two detectors was obvious. Despite every effort to eliminate backgrounds such a miniscule effect would be hard to take seriously unless it had been observed simultaneously in two widely separated locations. This was the case; the detection at Livingston and Richland was separated by only seven milliseconds.

Four hundred years of observational astronomy, from Galileo’s telescope to the Hubble Space Telescope, has enriched our view of the universe immeasurably. The study of gravitational radiation offers the potential for us to take the next step. The universe only becomes transparent to electromagnetic radiation when the universe has cooled sufficiently for atoms to form, some 380,000 years after the Big Bang. Gravitational radiation does not suffer any such limitations. Some day we may even use it as a tool to observe the inflationary expansion the universe is presumed to have undergone immediately after the Big Bang.

We are at the dawn of a new era in astronomy. If all goes well and the importance of studying gravitational radiation is appreciated, twenty years from now we will anxiously be waiting reports from LISA (Laser Interferometer Space Antenna), interferometers separated by five million kilometers.