LIVE MINT | Of LIGO, gravitational waves and a revolution in physics (Long Interview)
Karan Jani, a member of the team working at LIGO, talks about black holes, gravity and why good days are in store for scientists in India.
So, these gravitational waves. Are they as big a deal as we are led to believe?
They are way bigger deal than what is understood by many among the public so far. It is a transition of mankind’s knowledge about the universe. A step change. An inflection point.
Wow. That is a pretty serious claim to make. How is it transformational? Does it change the way we think about the universe? Or does it confirm suspicions? How would you explain it? (We will get into what they really are in a bit.)
Whatever we know about our universe so far is mainly due to a nice introductory physics concept—’light’, or more technically, electromagnetic waves (which gives X-ray, radio, rainbow lights, etc.). But now we have a new ‘sense’, beyond the five, to comprehend the universe… we are now probing the universe with the most fundamental ingredient of its composition—gravity.
Does this make our existing understanding of universe false? No, but what it does is add much more nuance to the current picture.
I find the notion of gravity being a fundamental ingredient of the universe interesting, but also very difficult to appreciate. Gravity, if I remember my high school physics, is a quality of the universe. But not something that made it what is. But now you are telling me it is a fundamental ingredient. This is like saying that Sachin is not just good at cover drives, but that cover drives are like 25% of Sachin’s substance… In other words, Karan, what is gravity?
Imagine all we had to learn about human body was the aura coming out of it, and the fact you can see wounds with your eyes… and then the electron microscope happened and now you can look at the cell, go even further deeper and look at the genes, all because of electron being the ‘fundamental ingredient’. There is a similar relation between gravity and the universe, and through gravitational waves we get fundamental information about the universe.
From a cricket fan analogy, gravitational waves are to mankind’s quest to universe what Sachin is to Team India. Fundamental!
So, these waves are almost like a new wavelength of light that we can now see? In a manner of speaking.
Think this way—light travels through vacuum (or more technically, the space-time fabric, light can bend in vacuum due to gravity), but instead now the whole stage on which light travels about is also moving… these disturbances or ripples in space-time is essentially what we call gravitational waves. It is the fundamental bouncing and jumping of our universe, that we all are part of, but we now can “hear” it.
Fascinating. So, essentially, we are seeing an entirely new aspect of our universe. Is it fair to say that this is a little bit like how the microscope revolutionized biology?
Absolutely! It’s a transition from measuring heartbeats to analysing genes!
That makes sense. That explains the excitement. Which brings us to the fundamental question: what are gravitational waves? Where do they come from? And why did it take so long to detect them? Please answer these one at a time in the English language!
In terms of an analogy, gravitational waves are equivalent to the radiation we see everywhere (light, X-rays, etc). The reason your eyes can see the laptop screen or read the newspaper is because the tiny, extremely light electrons that form the pixels and inks are accelerating nearly at the speed of light. That causes radiation in visible light. This light propagates and reaches your eye.
Now, imagine that there are two black holes accelerating on your laptop screen. Would you see them? No, because they don’t emit light (they will suck in everything around you, including all the light, and not to mention you yourself). But in the process, they also emit radiation, and that radiation is gravitational waves.
Just like ‘every’ form of matter makes light (radiation), every form of matter also makes gravitational waves.
OK. So they are all around us all the time?
Yes even while we play carrom we create gravitational waves. The question is, which kind of gravitational waves can we detect?
Imagine you were Aryabhatta, with all the understanding of how moon moves around earth, etc. And then one day somebody comes and asks Aryabhatta, how cold is the moon? He wouldn’t have the answer, because he cannot detect the infrared radiation coming from the moon which tells you about the temperature.
Ask this same question in the 1900s, by which time we have discovered infrared radiation, and whoa, we now know the temperature of the moon.
So, gravitational waves, like light, exist all around us. But which kind we can detect (i.e, what wavelength) depends on the technological advancements of the age.
I see. And then, of course, we get better and better at processing light radiation information and now we know what the temperature is on the surface of stars million and millions of miles away, right?
Exactly. So right now, on earth, we can look for gravitational waves in a frequency band of 10-1,000Hz. That limit is set by how big my gravitational wave detector is.
So, then, what in the universe can create space-time ripples with that frequency and which is loud enough to be seen beyond the noise activity of earth (and also lasers)? The only sources that come to mind are astrophysical in nature.
Why is it hard to detect them?
Gravity is the oldest of all siblings among the “fundamental forces”. But it is like Faizal from Gangs of Wasseypur. Very weak.
When you pick up an apple off the ground, the gravity of the entire earth is pulling the apple down, but your fingers can still challenge the whole earth and pick up the apple. That is how weak gravity is!
So the gravitational waves barely affect anything around when it passes by. Which makes them very very difficult to detect them.
Ah. So, you need really, really big events to create these waves?
Yes. Very loud events! Much louder than what we usually see in the universe.
Ah, yes, yes. So, now, I want you to tell me about two things. First, what was this blockbuster event that gave rise to the waves you just detected, and how big and sensitive is the machine needed to detected it?
How a gravitational wave detector works is a quite nuanced. The gravitational waves, when they reach the detector, change the space-time around the detector too.
That means gravitational waves change every measuring device, every piece of the detector.
But what it does not change, thankfully, is the speed of light bouncing between the mirrors. We rely on this light to detect change in detector’s size, which is then investigated as an effect of gravitational waves passing by.
But then how much does the size of the detector change? If you have a 4km length of detector, the change in that detector because of gravitational waves is 1000th the size of proton. The fact we humans could detect an effect so small is as impressive as the fact that black holes exist!
The LIGO detector is the most sensitive instrument ever made by mankind.
And that is what really took time. It took us nearly 20 years of experimental effort to make it, and in the process, we made sophisticated data analysis techniques that can look for a signal buried deep in the noise.
I am fascinated by how much time and homework it takes before we get these outcomes. I was recently reading Jon Butterfield’s book on the Large Hadron Collider. Decades of work and preparation leading up to that one moment… Was it like that for LIGO too?
Oh yes. I will argue that it was even longer! The theory came 100 years ago. The notion that we can even detect a signal from the theory (i.e., the experimental concept) came some 40 years ago. Pretty much since then physicists have been writing proposals to government agencies, refining the experimental concepts and at the same time working strongly on modelling possible gravitational waves from our theories of astrophysics.
So, what was the astrophysical event we just detected at LIGO?
This astrophysical event would have brought relief to Einstein’s soul. It came from two concepts his theory “exactly” predicted: black holes and gravitational waves. We didn’t need any physics beyond Einstein’s general relativity to analyse the astrophysical origins of this signal.
What exactly happened was that two black holes, each about 30 times the mass of our sun, collided. The collision formed a bigger black hole, slightly less than the total mass of the two black holes, and during the collision, gravitational waves were released.
The final black hole has the surface area very close to the state of Rajasthan. Imagine 60 times size of our sun condensed into a state of India! In this black-hole desert of Rajasthan, each grain of sand will weigh more than the total weight of all Indians combined!
But the key is how powerful the gravitational radiation was during the collision. The waves carried 50 times more energy at that instance than all the stars in the universe! Or, for a national security lover, it released 10^27 times more energy than all the nuclear weapons in the world combined!
Insane. I am presuming this is energy from the mass left over after the creation of the new, slightly smaller black hole?
Yes. Literally, E = m c^2. And here m is about three times the mass of our sun.
I know that formula. So, when and where did this take place? I am assuming very, very far away.
The black holes collided 1.3 billion light years away. And that we have strong proof that gravitational waves travel at speed of light—meaning this collision happened 130 crore years ago. This leads to an existential moment—1.3 billion years ago, there was no major life form on earth. Just single-celled life. To be alive at an epoch when conscious species on planet earth can actually comprehend such signals gives a strong faith in evolution!
So, waves from that collision have been washing over us for millions of years? Without us knowing?
The waves travelled 1.3 billion years, reached earth on 14 September 2015 (just days after LIGO resumed taking data) and the wave stayed for half a second on earth and left.
How did you guys know it came from that collision?
Within that half a second, both LIGO detectors (one in Washington, another in Louisiana) saw the gravitational wave. The wave had a signature look of what we call “a chirpy signal”. In the black hole collision simulations I do at Georgia Tech, we can see such chirpy signal very evidently. So, we were quite sure since the day we detected it that if the signal was truly astrophysical, it had to be two black holes colliding.
Wow. Which brings us nicely to the topic of yourself. Before we go back to the waves, tell us a bit about yourself. How did you end up becoming a gravitational waves guy?
Ha! That depends on how much you want me to indulge in early-life anecdotes.
I did my schooling in Baroda, Gujarat, and after finishing 12th, had an existential crisis. Didn’t give JEE, thought I was too dumb for medical, had no interest in arts, lacked the entrepreneurial skills to join commerce, and was way too unfit to be part of Baroda cricket culture.
I remember an acquaintance at the time passed her worn-out copy of Stephen Hawking’s Brief History of Time, and though I didn’t understand most of it, the desire to learn about black holes became a quest.
That was back in 2006. Three years after that, I was in Stephen Hawking’s institute working on black holes.
I joined Pennsylvania State University for my bachelors in astrophysics and physics. That place was a temple of gravity research in the US. In my first year, I went to the director of the Institute of Gravity and Cosmos, Abhay Ashtekar, the father of quantum gravity, with embarrassingly silly questions on relativity. But his response made me a fanboy!
I started working on a gravitational waves experiment called LISA since my very first year of undergrad. The second year I interned at the Albert Einstein Institute in Germany and learned a lot about LIGO and gravitational-wave astrophysics. And now for nine years, I have been doing the same.
That is a great story. It is enjoyable work?
Oh yes! How often can you tell your co-passenger in a flight that your job profile is to probe the formation and existence of our universe?
But more than that, what keeps me going is not being conscious at any point that I am working. There is no longer a boundary between my work schedule and my regular life, which I hope translates to that I am never bored, and perpetually occupied. That is all I can demand as a finite conscious entity in an otherwise infinite universe.
So, where were you when the signal came through?
From 14 September (Monday), the first official data collection of upgraded LIGO detectors was supposed to start after a five-year break. And from the trial data-collection earlier that month, I had anticipated it was going to be a crazy six months. So, the weekend before, I went to Key West, Florida, to be at the beach, without any Internet access. I came back to Atlanta at 2am on Monday. At around 5am, I woke to emails from the LIGO scientists at the Albert Einstein Institute in Germany about an unusually interesting signal in two LIGO detectors.
The rest of the US was obviously sleeping back then, but by the time we reached our research centres in the morning, we knew this was going to be something historic.
Was it amazing?
I’ll say the answer is very subjective. The post-doc in my lab has been part of LIGO team for over a decade and he was quite sceptic. I was naive and hyper, trying to estimate which type of black hole (mass and spins) could have generated it, and their possible distance from earth.
So, what happens next?
We have seen the waves. LIGO works. The celebrations have ended. And the science has started!
Now, we know gravitational waves exist, black holes exist. And we have some estimates on how many such black hole collisions LIGO can see. So, we gear up our analysis to make sure we are sensitive to as many black hole collisions as we can. For example, I look for black holes in LIGO that are extremely massive (hundreds of times the mass of our sun).
A lot of interest is also in detecting collisions between a highly dense object (called a neutron star) and black hole, as they emit both gravitational waves and light that conventional telescopes can see.
Super. Do you think we will get lots more positives now?
If we do see heavier black holes than what we saw, we will have to ask a more difficult question. How can nature produce such big black holes? So, beyond this black hole hunting game is another goal—trying to paint a more consistent picture of the universe.
At the same time, a lot of effort goes into further testing of Einstein’s theory.
And LIGO will help in all this?
We know general relativity agrees remarkably well with what we saw in LIGO (which is a statement limited by how clean the signal is), but will all such black holes would match exactly with Einstein’s general relativity?
And more importantly, how much can you change in Einstein’s theory and still get consistency with the same signal?
Hmm… So, there are a lot of questions to answer. Did you just find a special case of Einstein’s general theory or a more general thing?
Those are the questions which will shape the next generation of theories of fundamental physics. I won’t say we found a special case of Einstein’s general relativity, but what we know is that it is the simplest theory of space-time we know so far. And we know that it is not a perfect theory because it doesn’t tell us anything about quantum physics. Hence, there has to be a more fundamental theory of nature.
Fascinating. And India is doing a lot here too, right?
Oh yes! Achhe din for Einsteins in India!
The third LIGO detector is going to be built and operated in India. The LIGO-India project got the Union cabinet’s approval right after the discovery.
The final location of this mammoth 16 sq. km detector will be either the Udaipur-Chittorgarh area of Rajasthan or the Marathwada area of Maharashtra.
We can say that we not only Make in India now, we also Discover in India!
Awesome. Will you be involved in this?
Yes, I am part of the Indian Initiative for Gravitational Wave Observatory (IndIGO), which leads the efforts for the LIGO India project. The project will be managed and operated, in collaboration with the LIGO Labs (MIT, Caltech), by IUCAA (Pune), IPR (Gandhinagar) and RRCAT (Indore).
In fact, in the detection paper by LIGO, there were over 30 Indian researchers, and an acknowledgement to our funding agencies like the HRD ministry and the department of science and technology. What not many people in India (at least in the media) realize is that Indian physicist C.V. Vishweswara was the first to predict gravitational waves from the final black hole in 1968 (the one about the size of Rajasthan).
Also, many active researchers from the International Center for Theoretical Science (Bengaluru) and the Inter-University Center for Astronomy & Astrophysics (Pune) are building a strong gravitational-wave physics community in India.
Great to hear. So, tell me this. What do you think we will know about the universe by the end of our lifetimes?
We have two major unanswered question about the universe—the existence of dark matter and the existence of dark energy. The latter I am not sure will be solved in our lifetime. But with gravitational waves, we will have a better picture of our universe, and the ability to resolve fundamental features about the growth of the universe that we may not have noticed so far. This in turn will let us test the next generation of fundamental theories (like string theory) that predict existence of dark matter-energy in our universe.
The most interesting thing in my lifetime I hope to see is a more widespread acknowledgement among the public about our understanding of the universe. We know the age of the universe till the first decimal point! And yet, we have people that want to believe in zodiac signs and Vastushastra.