Causality, relativity and quantum gravity: a thread

Relativity is just the study of a fancy type of rotation.
Suppose we take two guys, standing at an angle, and ask them to choose x and y axes.

They do so like in the picture. Their axes will be rotated with respect to each other.
Relativity is rotation in space-time. In this case, instead of standing at an angle, the two guys are moving at different speeds.

Their clocks and rulers won't agree with each other. That is because their space and time are rotated with respect to each other, just like the different x and y axes in the last example.

This is a special kind of rotation, called 'hyperbolic rotation', which is why the picture looks different.

Time disappears in Quantum Gravity?

A thread for everyone 1/n Take two people who are carrying two different maps. One map is rotated with respect to the other.

They may disagree on what 'go straight' means, but they will agree on all essential things. Like if you ask them about the distance between A and B, they will agree.

Proof that black holes have zero mass, by Abhas Mitra.

Looking at Eq 20 and 21: My brother in Christoffel, that’s…not how it works. Mitra’s response to a paper which pointed out the problem

I am going to write about three instances of remarkable progress in the foundations of physics that happened within the last 20 years. 🧵

https://twitter.com/skdh/status/1667395312397213696

1. Double Copy: What if I told you gravity = (strong force)^2 ?

Gravity and strong force are two disparate forces that have nothing to do with each other.

But in 2008, inspired by results in string theory, a group of physicists found a remarkable result that relates the two

Step two, approach a senior big shot whose name can attract funds. Step three, invite subject experts from different areas to speak at your conference who coincidentally turn out to be all male

Kids these days don't fully appreciate the advantages they have over earlier generations.

If they were students of physics in the 1870s looking to learn electromagnetism, they would have to contend with a set of 20 equations.

It was character-building stuff as you can see But nowadays, they learn just 4 equations.

They have it too easy

A thread on BMS symmetries. 🧵

We start with an example. Take a complicated distribution of charges. It will produce a complicated electric field.

But if I could get far enough away from all the charges, the entire distribution would look like a point charge to me. The electric field then falls off as the inverse square of distance from the charges. It approaches zero.

In this case we have a symmetry emerges -- the electric field is the same in every direction.

This is the broad idea of asymptotic symmetry.

The physics version of chicken-egg problem is fields vs particles.

Is the universe made of fields or particles? 🧵 To understand the concept of fields, we go back to its origin -- magnets.

A magnet will exert a pull on all magnets around it.

One way to understand this pull is that a magnet creates a magnetic field around it, and other magnets feel and respond to this field.

Black holes don't exist? 🧵

In this thread, we will look at a paper() that claims black holes don't exist.

The author is Abhas Mitra, who has received some media limelight in India in the past for such claims,
like in here:


1/6arxiv.org/abs/0904.4754
ndtv.com/world-news/nas… First, I am going to distill the math of the paper into its essence and write it out in a single tweet.

This will make the mistake in the paper obvious.

What follows in the next tweet is the math of the analysis made in the paper.

See if you can spot the mistake.

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The holographic principle of quantum gravity blows the mind of anyone who first encounters it.

It is also like a can of juice.

A thread🧵 There are two characters in the story, best known by their acronyms AdS and CFT.

AdS or Anti-de Sitter space is a hypothetical universe.

Unlike our expanding universe, AdS is fenced in by a boundary surface.

Which makes it look like an infinite can of juice.

A couple of days ago, a number of science magazines reported that '(some) physicists have created a wormhole in the lab.'

To which other physicists responded with an outraged 'No, they did not!'

What's going on? Let me give you the tl/dr.

1/n What is a wormhole?

Imagine you go to work everyday via a major road with lots of traffic.

Then one day you discover a little-known back-alley between your home and office, and now you can reach office much quicker.

Wormholes are the back-alleys in space-time.

2/n

During my PhD days, I was often asked in family gatherings what is this 'quantum gravity' thing that I work on.

I came up with an analogy which worked really well with my mostly-IT relatives!

This is what I used to say.

1/n Your computer is running a program.

You give some input and get some output. Suppose you are now told to guess the program.

That is what scientists do.

They try to guess the programs nature is running, from our observations of how the output changes with the input.

2/n

I know a prof who napped in every seminar and always muttered "yes, yes, that's correct." as soon as he woke up

https://twitter.com/caridoesphysics/status/1597320338786103297

One time, in a seminar on a topic not at all in his field, he woke up in the middle of a heated argument about some intricate point and muttered his usual 'yes, yes, that's correct,' and got glared at by one of the debaters, who thought he was supporting his opponent.

A thread🧵about Einstein's first salvo against quantum theory.

The Solvay conference of 1927 is the venue of the famous group photograph of physics hall-of-famers.

It was here that Einstein made his first public argument for the incompleteness of quantum mechanics

1/11 Quantum mechanics doesn't deal in certainties. If you measure the position of an electron, QM won't tell you where you will find it.

Instead it will give you a 'wave function', which specifies probabilities of finding the electron at different points.

2/11

One of the approaches to quantum gravity is causal set theory (CST).

I will share the basic idea of this interesting approach (with the caveat that I am an outsider to this field).🧵

1/8 Let's start with something simple. We know that nothing travels faster than light.

This means that in the light-cone below, the event A can only cause events like B that lie inside the light-cone, but not C.

A and B have a 'causal relation', while A and C don't.

2/8

Einstein's special relativity revolutionized our understanding of space and time. The main mathematical result -- Lorentz-Fotzgerald transformations -- was known before Einstein.

The first to hypothesize them was the almost forgotten Irish physicist George Fitzgerald.

1/9 Maxwell's theory of electromagnetism had a strange result -- it specified a speed of light while being silent on the frame in which light would have that speed.

It was believed that the speed of light was with respect to the rest frame of the hypothesized medium aether.
2/9

Every mathematician and physicist is aware of Green's theorem, which has numerous applications.

TIL that Green's theorem was self-published by George Green(1793-1841), who was entirely without formal mathematical education at that point.

1/5 Son of a miller-turned-baker, Green spent most of childhood and youth working in his father's bakery and later windmill.

He had little formal education growing up.

When he was thirty, he subscribed to the local library, where he likely taught himself mathematics.

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I asked people who work on string theory/ holography about that one wow moment that sold them on the subject. Here are some of the responses 🧵 Everyone who has encountered this knows that counting the number of black hole microstates and reproducing the Bekenstein-Hawking entropy is nothing short of magic

https://twitter.com/dvijmankad/status/1571273152881168384

In 1974, Alice Sheldon writing under the name James Tiptree Jr published a story called "The Girl Who Was Plugged In" that is very interesting to read in the light of modern aspects of celebrity.

1/4 The story is set in a future where ads are banned. To get around this, companies create celebrities who covertly advertise products by using them in public.

'Create' is meant literally. The celebrities are lab-grown, with beautiful bodies and non-functioning brains.

2/4

Time in quantum mechanics

When it comes to measuring time, Asher Peres showed in 1980, quantum physics "carries the seeds of its own demise."

A 🧵

1/8 Time and energy have a similar relation to position and momentum.

Homogeneity of space implies conservation of momentum. Homogeneity of time implies conservation of energy.

As [X,P]=1, why not define a time operator for which [T,H]=1?

2/8

Quantum mechanics is secretly classical mechanics.

Only, it is classical mechanics in infinite dimensions 🧵

1/8 In classical mechanics, if we know the position and momentum of an object at a given point of time, we can predict its future. The space of position and momentum is called the phase space.

Phase space is the arena of classical mechanics.

2/8