After a century since Albert Einstein came up with General Theory of Relativity, in February 2016, we witnessed the experimental discovery of gravitational waves, proving Einstein’s prediction. It would be an understatement to call it a groundbreaking discovery. It’s much more than that and it has taken us closer to understanding the universe on a whole new spectrum. Let’s take look back at how far we’ve come and how an abstract and incredible theory from 1915 had given us a beautiful vision of the universe.
General theory of relativity
Einstein’s theory of General relativity states that the motion of the planet and everything else in the universe is a consequence of how their mass warps the four-dimensional space-time around it. In other words, the theory states that massive objects curve the space-time around them, resulting in what we call as gravity.
Objects in motion around such massive objects would follow the curvature rather than a straight line in a four-dimensional space-time domain. Einstein thus disproved the Newtonian notion of gravity where it was considered as a mutual attraction between massive bodies. For decades that followed, physicists confirmed and verified all the consequences of GR experimentally. They are well known to us as the tests of general relativity.
Predictions and tests of general relativity
Throughout the time between 1915 and 2016, all of GR’s predictions have been experimentally verified. Beginning with the explanation of Mercury’s orbital precession, every other predicted notions of the time have been proved true with repetitive experiments.
When a massive object like our Sun, curves space-time around it, everything around it follows the same path, including photons. GR proposed that if we were to observe the position of stars or galaxies right around the edges of the massive bodies like the Sun or a galaxy cluster, then those stars should appear displaced, and sometimes duplicated.
The total solar eclipse of 1919 helped astronomer Arthur Eddington to observe the effect gravitational lensing for the first time, just four years after Einstein proposed General relativity. The total occultation of the Sun made it possible to observe the stars right along the edges of it, which appeared to be deflected due to the lensing effect. It was verified from two different locations and subsequently by other astronomers during the eclipses that followed.
Gravitational time dilation
Another consequence of the space-time curvature is that an object in a stronger gravitational field will experience time slower than the one in a weaker gravitational field. This is due to the fact that near strong gravitational field time for any events slows down due to the geometry of space-time and its effects on events that determine time. This has been proved by experiments like the Pound–Rebka experiment, Gravity Probe A, and Hafele–Keating experiment. As a matter of fact, our current GPS satellites account for time dilation effects as they orbit in a weaker gravitational field compared to the receivers on the surface of the Earth.
As photons are affected by gravity too, the significant curvature of space-time affects light by changing their wavelength to account for the time dilation. Visible light that’s travelling outward a stronger gravitational field would appear shifted towards the red end of the spectrum as their wavelength appears stretched. If the light is moving towards a stronger field, the shift would be towards the blue end. This experimentally verified by the Pound–Rebka experiment and the Gravity Probe A as well.
While most of the estimates of GR had been proved experimentally, the existence of gravitational waves remained a mystery for a long time. It was the final piece of the puzzle. According to GR, any gravitational interaction, like two massive objects colliding or orbiting one another should produce a disturbance or ripple in the space-time. Until 2015, no such waves were ever observed.
The Laser Interferometer Gravitational-Wave Observatory (LIGO), on 2016, announced that they have successfully observed gravitational waves for the first time. With a giant pair of Michelson interferometers of arms 4 km long, fitted with powerful lasers, we were able to detect the stretching and squeezing of the space-time due to the gravitational waves for the first time in the history of mankind. For this observation, Kip Thorne, Rainer Weiss, and Barry Barish were awarded the Nobel Prize in Physics in 2017.
If you are curious, this is how the gravitational waves sound like.
The discovery of the century – why the excitement?
What advanced LIGO detected are the effects of two black holes that merged about a billion light-years away. This not only proves general relativity but also opens an entirely new and awesome way to explore the universe. Astronomy just got massively interesting!
So far, we’ve been only able to look at the universe through the electromagnetic spectrum. We have telescopes that show us how astronomical bodies look, what they’re made of, how they behave — all with the help of observing the electromagnetic spectrum from gamma rays to radio waves. But now, we can detect gravitational waves!
To put this in an interesting perspective, our quest for gravitational waves and its discovery has expanded our search by giving us something new to look at. In the days to come, we will be able to detect black holes and any massive merger events that would happen billions of light years away.
For what the universe has to offer, we shall keep exploring, as one race.