Waves come to shore only a few nanoseconds before

That incident sent waves of excitement (yes) in scientific circles. In part, it was sheer engineering and astronomical ingenuity that detected that wave (see my article Waves come ashore, https://bit.ly/43telsJ). This was accomplished by the two Laser Interferometer Gravitational-Wave Observatories (LIGO) stationed in the US. (India will also build its own LIGO.)

But the excitement was also because these waves proved one of Albert Einstein’s predictions. More than a hundred years ago, he theorized that cosmic events create ripples in the fabric of space-time, and that we should be able to detect those ripples. And when we finally found him, it spoke once again about the amazing mind of this man.

Astronomers concluded that the wave they first detected was triggered by the merger of two black holes. The mass of each was about 25–30 times that of the Sun, and they were about 1.8 billion light-years away. It might make you shudder a little, as it did me, because you totally understand what happened to those black holes. I mean, start by thinking for a moment about dropping a large rock into a calm lake. You will see a tremendous splash, and the waves will go far in every direction, possibly even reaching the far end of the lake. But within minutes, you’d expect the lake to be quiet again. However, with black holes… if we know that after 1.8 billion years there are still signals of that merger that can be detected 1.8 billion light-years away – well, take a moment to imagine how much Must have been a cataclysmic event.

In the years since then, scientists have used LIGO to detect several dozen more such waves. But the gravitational waves detected today are qualitatively different. What’s more, they were found to be using a completely different mechanism.

Here are the fundamental objects called pulsars. When a massive star – up to 25 times as massive as our Sun – reaches the end of its life and collapses in on itself, the resulting small, dense object is known as a neutron star. Some neutron stars rotate, emitting beams of radiation—the behavior can be best understood by thinking of the rotating beam of a lighthouse. These rotating neutron stars are known as pulsars (‘pulsating radio sources’).

If the pulsar’s beam was pointed toward us on Earth, we would “see” it as a star that varies in brightness. That is, it pulsates. A remarkable thing about such an object is that it rotates at a constant speed, so that the vibrations are as regular as clockwork. What is even more remarkable about such an object is that it can spin at an almost unimaginably high speed, so the pulse frequency is also unimaginably high. We know of pulsars that have only milliseconds between pulses. To give this some perspective, remember that it takes one day for our Earth to rotate once. Imagine that the planet completes one rotation not in a day, but in, say, ten milliseconds. Pulsars can spin so fast.

The clockwork (and better) periods of pulsars make them indispensable astronomical tools – cosmic clocks, if you will. In particular, they helped astronomers detect gravitational waves.

Five different groups of astronomers around the world – including one in India – took on the responsibility of monitoring a set of pulsars. These sets are called Pulsar Timing Arrays (PTAs).

What were these teams looking for in their Pulsar sets? Try this thought experiment. Your friend Eva sends you a letter every day by courier. Each day’s letter takes exactly 24 hours to reach you. One week, Sunday’s letter reaches you on Monday. On Monday, on Tuesday. But on Tuesday, someone drives Eva to your house, where she writes her letter for the day, hands it to you, and returns home: it all happens on that Tuesday. Rest of the week will pass normally. So, Eva has written a letter for every day of the week, as usual, but you have received them like this: Monday, Tuesday, Tuesday, Thursday, Friday, etc. That is, even if Eva sticks to her once-a-day habit of writing letters, her Tuesday visit means that the frequency of you receiving her letters has changed.

Equally, if astronomers find variations in pulsar frequencies, this would suggest a disruption: a gravitational wave that distorts space-time and thus moves the pulsar slightly. So, yes – this kind of variation is exactly what the five teams were trying to find out in their PTA. For example, the Indian Project (InPTA) used the Giant Metrowave Radio Telescope in Maharashtra to observe its PTAs.

And – drum roll please! – they got exactly such a variety. In fact, all five PTA teams did this. In total, they analyzed pulsar data collected over 25 years. InPTA said in a press release, “It revealed that there are distinctive irregularities in the measured rate of ticking of these cosmic clocks … This is consistent with an effect produced by gravitational waves.”

The press release continued: “As radio signals travel through space and time, the presence of gravitational waves impacts [sic] Their path is in a specific way: some pulses will arrive a little late (less than a millionth of a second), some a little earlier.

There is still verification work to be done. But pay attention to what happened here. In our thought experiment above, Tuesday’s letter from Eva arrived a day earlier than expected. Of course, it’s easy to find out. But here we have pulses from pulsars that arrive less than one millionth of a second earlier (or later) than expected.

In those tiny fractions of a second, strong evidence of gravitational waves generated by some massive cosmic event. In fact, PTA projects have found that these waves are coming from everywhere around us. One report put it this way: “There is constant pushing and shoving in random directions.” This is equivalent to the haphazard flow of water on the surface of a pond due to rain.”

Unimaginably distant, unimaginably long-ago events all around us. Their mark, in the form of almost unimaginably small pieces of time, is upon us today. There’s just something about astronomy.

Dilip D’Souza, a one-time computer scientist, now lives in Mumbai and writes for his dinner. His Twitter handle is @DeathEndsFun.

Updated: 06 July 2023, 10:39 PM IST