“Ladies and gentlemen, we have detected the gravitational waves! We did it!”
The whole world is fascinated by this exciting announcement. For the first time, scientists have observed a temporary gravitational signal generated by two black holes crashing together, confirming a prediction from Albert Einstein 100 years ago.
Experiments of detecting gravitational waves have been conducted since the 1960s. However, as the waves are incredibly weak, even Einstein was skeptical that they would ever be detected.
So how did scientists finally detect gravitational waves?
The answers lie in the Einstein’ s theory of general relativity and the most precise detector ever built, Advanced LIGO!
- What is gravitational waves?
In Einstein’s theory, massive objects in the universe distort both space and time, generating a warped space-time.
When the most energetic event happens, such as merging black holes, it creates gravitational waves spreading as ripples in a pond. These waves transport energy through the universe, squeezing and stretching the space-time continuum.
- How were gravitational waves detected?
On September 14, 2015, Advanced LIGO (Laser Interferometer Gravitational Wave Observatory) finally observed this extremely tiny signal. The size of which is about 1/1000 diametre of a proton.
Inside the LIGO, a single laser beam is split into two and sent down to two precisely equal arms. The two beams bounce off the mirrors and recombine at where they split.
If there are any gravitational waves passing by, they distort the space and alter the length of the arms. This variation makes the two beams return to the splitter at different times, transmitting the strain of the waves proportional to optical signals.
To increase the LIGO’s sensitivity to detect gravitational waves, special mirrors are modified to maximize the conversion of strain to optical signal.
Ultrahigh vacuum and extremely low pressure are also required to isolate external noise.
Following the detection of the signal, scientists conducted a series of tests to confirm the validity of it. These tests, such as measuring LIGO’s response to magnetic and radio-waves, proved that the signal was from gravitational waves instead of other environmental or instrumental disturbance.
- How do we know the waves were produced by black holes merging?
The observed signal matches the prediction of Einstein’s equation for two black holes merging.
By analyzing features of the signal, scientists found two black holes, 30 times the mass of the sun, producing gravitational waves 1.3 billion years away. They spanned around each other at half the speed of light, losing energy, eventually collapsing into a massive one.
Just for a double check, the mass and the spin of the final black hole were estimated by computer model. It turned out that the data was consistent with Einstein’s general relativity.
You may keep wondering what is the big deal of this discovery.
Now, we can not only confirm the existence of paired black holes system, but also witness other big events happening in the universe by analyzing the gravitational waves washing through our plant.