Thread by @AstroKPJ

Starting a recurring thread on BLACK HOLES. From history to our modern understanding and, in between, any bizarre facts about it.

1. The idea of "dark stars" - from where light cannot escape - became obvious since we figured in the late 1600s that the speed of light is indeed finite. Here is a quote by philosopher-scientist John Michell from 1784.

(Source: Wikipedia)

2. After Einstein published his breakthrough paper on Special Theory of Relativity in 1905, it was known: (i) nothing travels faster than the speed of light, (ii) speed of light is constant for all observers. Therefore, a "dark star" is 'black' for all observers of the Universe.

3. Black Holes are essentially ONE WAY streets in our Universe. Once any form of matter or massless particles (like light) enter its boundary, no known force in the Universe can bring them back.

Worst one cannot even recover information about this disappearance!

4. Until Einstein published his most ambitious & celebrated work in 1915 - the General Theory of Relativity - it wasn’t clear how light interacts with gravity. Therefore, the idea of “Dark Star trapping light” couldn’t be developed further.

5. In 1915, Einstein concluded lecture series at Prussian Academy of Sciences in Berlin by deriving a set of equations- now known as Einstein Equations. Unaware they would go on to later describe phenomena like black holes & expansion of Universe!

Text from my PhD thesis 👇

6. Physicist John Wheeler had a poetic way to explain Einstein Equations:

“Mass tells space-time how to curve, and space-time tells mass how to move”

He is also often credited for coining the term “Black Hole”

7. Wheeler produced a record number of PhD students that shaped the modern understanding of gravity. (The most famous being Richard Feynman)

His PhD student from 1960s, Kip Thorne, co-founded LIGO that made the first ever direct observation of black holes in 2015.

8. Months after Einstein published General Theory of Relativity, a weatherman (& astronomer) in the German Army during World War 1, Karl Schwarzschild, published a paper.

He derived first analytical solution to Einstein Equations. It’s now known as Schwarzschild Black Hole👇🏻

9. While the intention of Karl Schwarzschild in 1916 was simply to explain a simple star, there was something bizarre in his solution to Einstein Equations. The spacetime "blew up" to infinity at a particular point.

As we now know, this point is the SINGULARITY of the black hole

10. The singularity of a black hole is a point of infinite density. All the laws of physics as we know break down here. But can such singularity exist in the real Universe? Or is it simply that our knowledge has not advanced enough to understand it?

The latter imo is the answer.

11. The Schwarzschild solution describes a black hole that has no spin & no charge. But it is incredibly unlikely that a #BlackHole born in our cosmos has no spin! On the other hand, black holes would almost always be electrically neutral <for you sir, no charge- joke>

12. A black hole is, generally, a corpse of a big star. They are in a true sense the ghosts in our cosmos - you can't see it, but it can trap you to eternal hell.

13. Black Holes (BHs) have three families based on their size (mass):

(i) Stellar BHs: formed only from stars (~10x bigger than Sun)
(ii) Intermediate BHs: 1000x bigger than Sun
(iii) Galactic BHs: at the center of galaxies (million-billion of Sun)

Pic from my phd thesis 👇

14: Black holes are the only known object in our Universe whose size (mass) can scale over 10 orders of magnitude, yet their fundamental properties and behavior remain EXACTLY same.

15. “The BLACK HOLE teaches us that space can be crumpled like a piece of paper into an infinitesimal dot, that time can be extinguished like a blown-out flame, and that the laws of physics that we regard as ‘sacred,’ as immutable, are anything but.”

~ John Wheeler

16. 63 years ago on this day, Albert Einstein passed away. His work on General Theory of Relativity, which provides the mathematical framework for black holes, remains unchallenged till date.

Below is an iconic pic of his office taken hours after he died (Image: Ralph Morse)

17. Just like how moon 🌙 revolves around Earth, and 🌏 takes 1 year to orbit the Sun, our ☀️ revolves around the monstrous black hole which is residing at the center of our Galaxy.

It takes our ☀️ about 230 million (23 crore) years to finish just one such orbit!!

18. The black hole at the center of our Galaxy is 4 million times heavier than our Sun. All the 100 billion stars in our galaxy orbit around this Master.

19. There is an ongoing effort to take a picture of the central Black Hole of our Galaxy. Yes, you head that right, an actual picture.

This likely is the only #BlackHole for which humans could take a 'picture' of EVENT HORIZON!

For updates on this: eventhorizontelescope.org

20. We have a solid measurement of the 'size' of this central Black Hole in our galaxy, thanks to the recent measurements of stars orbiting VERY CLOSE to it.

They are so close, that they "feel" the Einsteinian effect of our Master Black Hole.

21. As much as we'll like to feel Master Black Hole of our Milky Way Galaxy is special, it's clearly not. Almost all galaxies have a monstrous black hole at their center.

S5 0014+81 hosts #blackhole of about 40 BILLION times heavier than Sun (10000x bigger than Milky Way's BH)!

22. While we had ample of evidence of black-hole-like objects in the Universe for some decades (coz what else can be so massive & not emitting light?), it wasn’t clear if they are THE Black Holes as predicted by Einstein’s Theory of Relativity.

Of course, until 2015 (Hint: LIGO)

23. Three dimensional gravitational waves from the collision of black holes. A supercomputer simulation of Einstein’s general relativistic equations.

The white eggs 🥚 are the event horizon of the black holes.

24. There is a hypothetical population of black holes. We don’t really know if the universe can host them, but with LIGO we are searching for it across the cosmos. upi.com/Goldilocks-bla…

25. Did a guest lecture today in General Theory of Relativity class on black hole horizons. It’s only when you explain to someone that it strikes (again!) that there is something so fundamentally flawed about the ‘global’ behavior of black holes in Einstein’s theory.

26. Almost two years ago from today, signals from a black hole arrived at the two LIGO detectors in the US.

This black hole was 49 times heavier than our Sun but only as wide as New Delhi to Chandigarh.

27. Throwback - 9 years ago - a talk on the “ringing” of black holes at the @Perimeter Institute for Theoretical Physics.

This phenomenon was first proposed by the ‘Black Hole Man of India’, Prof. C. V. Vishveshwara

28. When a #blackhole is born, it is still “shaking & crying”. But quickly becomes quiet, and remains so till eternity.

That cry right after it’s birth (called Ringdown) releases a gravitational wave signal.

It is to inform the rest of the universe about a new singularity.

29. Black holes, as found in Einstein’s theory, is quite a bizarre object to really exist in the Universe.

Many have proposed other exotic scenarios, such as stars made up completely of bosons.

30. For any observer in the Universe to know if a cosmic object is indeed a BLACK HOLE (and not some other exotic star), all one has to do is hear a sound like ringing of a bell.

Next few tweets about a maverick Indian theoretical physicist that figured this deep connection.

31. C. V. Vishveshwara left us this day 2 years ago. The FUNDAMENTAL work that he published about BLACK HOLES in 1960s was finally proved right in 2016 with the detection of gravitational waves by LIGO.

A long journey journey with a climax that cannot be more stellar!

32. An obituary on Vishu (as we used to fondly call him) and his contribution to black hole physics. Co-authored this with his PhD advisor Prof. Charles Misner for @PhysicsToday.

33. As is the case with most of us Indian scientists, the recognition & respect from the country & the academic community of the land came much late for Vishu. It still pains that he is not a known name to students.

Share more about his work: @SwarajyaMag
swarajyamag.com/science/we-wou…

34. Want to take the picture of a black hole?

Your best shot is to point the camera (radio telescopes) at the Sagittarius ♐️ constellation.

35. At Sagittarius A* resides the biggest black hole of our Milky Way galaxy.

It is 4 million (40 lakh) times heavier than our Sun ☀️