I was not going to mention Einstein in my posts anymore. But, the man stays relevant even after a century. Note to the young physicist: if you want to be remembered in the next century solve the mystery of gravitation. Newton and Einstein came up with theories explaining how it works but we still do not know why it is weaker than the other forces of nature and why it cannot be shielded. Gravitation is fundamentally different from other forces and this needs to be explained. Young physicist: while at it you can explain how gravitation works at the Planck scale too. That would be nice!
On Feb 11, 2016, 100 years after Einstein’s prediction, the LIGO collaboration announced that they detected gravitational waves coming from the merger of two black holes that happened in a far away galaxy more than 1 billion years ago. Yes, you read it right. The violent event occurred about 1.3 billion years ago. The detection on Earth occurred on September 14, 2015 at 09:50:45 UTC. LIGO consists of 2 observatories. Both observatories (LIGO Hanford, WA, and LIGO Livingston, LA) detected the same signal. Since then the LIGO collaboration (more than 1000 scientists) have been checking and re-checking every detail and analyzing data. Their scientific paper was published on Feb 11, 2016 as well. Here’s the link to the scientific paper:
This detection event has been named GW150914 which stands for “gravitational wave of September 14, 2015“.
GW150914 proves few things:
- black holes certainly exist
- binary black holes exist and they can collide
- there are black holes more massive than 25 solar mass (the 2 black holes that produced the GW150914 wave were each heavier than 25 solar mass)
- collision of black holes is extremely violent – so violent that the shaking of the fabric of space-time is felt in a radius as large as few billion light years
- Waveforms of the GW150914 event at both the LIGO Hanford, WA, and LIGO Livingston, LA conform to the waveform predicted by Einstein’s General Relativity theory
- A new era of astronomy is beginning. The LIGO observatories in USA will soon join forces with similar observatories around the world. The sensitivity of these gravitational wave observatories will improve 10-fold.
- There will be a space based gravitational wave observatory in the future: eLISA
GW150914 proves that space-time is a physical entity.
At the risk of confusing you, I would like to remind you about the discussions centered around space-time being emergent. See this article for example. Also remember the discussions of nonlocality in the context of Quantum Mechanics. After the detection of GW150914 we can be sure that space-time is not just a mathematical construct. Space-time is a physical entity. Space-time may be emergent but physical nevertheless.
Direct detection was not possible until this year
Until this year the direct detection of gravitational waves was not possible. The detection became possible only after the state-of-the-art technological improvements were made in the LIGO instrumentation in both locations. The Advanced LIGO instrumentation is so sophisticated that they deal even with quantum noise. I sometimes think of the Advanced LIGO as the best noise cancelation headphones ever invented. The noise cancelation is so good that it allows us to hear the sound (gravitational waves) of the heavens.
YOU MUST WATCH THIS NYT VIDEO
The video is so good that after watching this 5 min video my post becomes redundant. If you read the rest of my post I thank you.
Best expository articles after the announcement
Gravitational Waves are Discovered at Long Last by Natalie Wolchover
Gravitational Waves Detected, Confirming Einstein’s Theory by Dennis Overbye
Einstein’s gravitational waves were found at last by Davide Castelvecchi and Alexendra Witze
There was indirect evidence for gravitational waves
There was indirect evidence for gravitational waves from the measurements of binary neutron stars. Russell A. Hulse and Joseph H. Taylor Jr. received the 1993 Nobel Prize in Physics for the discovery of a new type of pulsar: “the orbiting period of the pulsar around its companion gradually diminishes with time – extremely little, but in exactly the way the general theory of relativity predicts, as a result of the emission of gravitational waves.” 
What is a gravitational wave?
Gravitational waves (GW) are distortions in the space-time fabric. The GW distortion travels like a wave. When it passes through an object it will cause very small temporary distortions in the shape of that object. The Advanced LIGO experiment can detect shape distortions thousand times smaller than the size of a proton. This amazing feat is the result of very advanced technology.
What creates a gravitational wave?
Accelerated motion of mass-energy (any concentration of energy in space) creates ripples in the space-time fabric. These ripples are known as gravitational waves. Accelerated motion means speeding up, slowing down or rotation of mass-energy. Any accelerated motion of mass-energy will cause ripples in space-time but for ordinary objects the ripples will be extremely weak to measure. On the other hand, if a black hole accelerates the ripples will be stronger. When 2 massive black holes merge the gravitational wave will be very strong. In fact, the GW150914 event at its source was so powerful that we can hear it even after 1.3 billion years.
Gravitational waves propagate at the universal speed limit
No information (waveform, pulse, ripple, disturbance, particle) can move faster than the speed of light in vacuum. Now, we can express this universal speed limit by saying that no information can move faster than a gravitational waveform.
Future plans for Advanced LIGO
Other information found on the Advanced LIGO web site