[Science Stories] Guess who first translated Einstein's General Theory of Relativity into English!

The first ever English translation of Einstein and Minkowski’s papers on Theory of General Relativity, written in German was published by Calcutta University in 1920.

The translators were the famous Indian physicists Meghnad Saha and S.N. Bose and a historical introduction was provided by P.C. Mahalanobis

Those were the golden days of Presidency College. The great JC Bose and PC Ray were the teachers. SN Bose, Meghnad Saha, JC Ghosh were all their students, that too of same batch (+/- 1 year).

Saha was 27 years and SN Bose was 26 years old when they translated Einstein's General Theory of Relativity into English.

Further, it is said that SN Bose learnt German just for the purpose of translation.

Einstein later sort of paid his debt by translating Bose's famous foundational paper which came to now be called as "Bose-Einstein statistics" to German and got it published. Here's Bose's letter to Einstein



"I have ventured to send you the accompanying article for your perusal and opinion. I am anxious to know what you think of it. You will see that I have tried to deduce the coefficient <number> in Planck's Law independent of classical electrodynamics, only assuming that the ultimate elementary region in the phase-space has the content h3. 
I do not know sufficient German to translate the paper. If you think the paper worth publication I shall be grateful if you arrange for its publication in Zeitschrift fr Physik. Though a complete stranger to you, I do not feel any hesitation in making such a request. Because we are all your pupils though profiting only by your teachings through your writings.  
I do not know whether you still remember that somebody from Calcutta asked your permission to translate your papers on Relativity in English. You acceded to the request. The book has since been published. I was the one who translated your paper on Generalised Relativity." [Credits]

Einstein agreed with him and translated Bose's paper titled "Planck's Law and Hypothesis of Light Quanta".

Story credits: Albert Einstein: His Annus Mirabilis 1905

No! We are NOT made up of Higgs Bosons

Higgs Boson is one of the most talked-about scientific discoveries. As with any popular idea, it got misinterpreted, misunderstood, manipulated, and even abused! 

A part of the reason is the analogies physicists used to simplify complex mathematical equations into a lucid story. As with any analogy, it breaks under pressure because they are not precise and everything about the analogy is not literally correct. 

People who use the Higgs Boson phenomenon to dress-up their woo-woo theories in scientific terms is the other and primary cause for abuse of its concept.

For instance, the famous abuse of the Higgs Boson is to take its nickname "God particle" literally and say that "Physics now has proven the existence of the God". No! It doesn't prove god. God particle is the short form of Higgs Boson's nickname "goddamn particle". It was called "goddamn particle" out of frustration because at the time it was coined, scientists were working hard to find it for nearly 40 years without success. Journalists turned goddamn particle (original name of a book by Nobel Laureate Lederman) into god particle (name of the published version of the book).

The other abuse is to say that Higgs Field is all-pervading and hence it is the "God". Other variations of this abuse include - there is a mention of an all-pervading entity that's the cause for the reality in old scriptures and holy books. Since Higgs Field is all-pervading and it gives mass to objects, which is necessary for their existence, the discovery of Higgs Field validates holy books.

Just in case if you are wondering if these interpretations are true, the one word answer is "No". The one sentence answer is "It is a pure concocted nonsense." These guys don't even understand the concept of Higgs Mechanism properly. They are just playing with words. It is a popular trick used by sophisticated English speaking con-men to dress up their woo-woo in scientific terms, so that it appeals to an average educated person, fooling such people to think that the woo-woo of the con-men is scientific.

In this post, we deal with a different misinterpretation of Higgs Boson, different from the above two. It is as follows:

Higgs Boson is popularly described as "giving mass" to the object. This makes people form a mental picture where every object is partly made up of "Higgs Bosons", that are the cause for the mass of the objects. An extension of this reasoning is that, if you remove the Higgs Boson from an object, its mass should decrease.

This is a genuine and legitimate question that's bound to arise given the popular description of Higgs Boson as a particle giving mass to objects. It's not the typical woo-woo kind although some woo-woo guys have caught onto this interpretation and are giving a scientific dressing to their woo-woo.

Someone on Quora asked the following question

My answer is below

The real picture of Atomic Structure

School level

Atom has electrons, protons and neutrons
  1. Nucleus has protons and neutrons
  2. Electrons revolve around nucleus
College level (Quantum physics)

Electrons are not particles. They are described as waves with a probability distribution. We have a mathematical function for each electron which tells us the probability of finding an electron at a given point in space. Electron has higher probability of being found around - what we earlier thought to be the traditional orbit of revolution. However, there is a non-zero probability of finding an electron at other points too. It’s just that such probability decreases as we move radially away from the “traditional orbit”.

When we plot the probability distribution around the nucleus, we get a picture with dark pixels around the traditional orbit and the density of pixels reduces as we move away. It looks like a “cloud”.

So, unlike the traditional picture of orbits, as taught in school, with particle electrons revolving in that path, the quantum mechanical picture of electron is that of an “electron cloud” spread across the space. (Image source)

Coming to Protons and Neutrons, we thought of them in school as indivisible particles. But it turns out that they aren’t indivisible. Protons and Neutrons are made up of quarks.

Quantum Physics for Dummies

On Quora, someone had asked me to explain Quantum Physics in simplest terms. Here is my answer

The essence of Quantum Physics (QP) can be easily understood if we compare it with Classical Physics (CP), the notion of our everyday experience.

Classical Physics essentially deals with the motion of macro objects. Following are the key aspects of CP.

1. Variables to describe motion: We need two variables to describe the motion of an object - position and momentum.

2. The equation of motion: Newton's famous F = ma describes the change in momentum with time. A classical physicist would say - give me the position and momentum of an object, I will tell you the future trajectory of the object (assuming no other force) using the F = ma equation.

In other words, "position" and "momentum" are the key variables required to describe the motion of an object completely.

3. Deterministic: If we say that an object is at a point A, it is at point A. We can measure and verify this observation.

4. Particle nature: In CP, we are used to clearly distinguish particle and wave. I am a particle, a ball is a particle. Water Ripples is a wave.

The above principles are different in QP. The following notions are necessary to appreciate the basic essence of Quantum Physics (QP) and to be able to distinguish from Classical Physics (CP), the physics of everyday objects.


1. Wave-Particle duality: In CP, we perceive objects as “particles”. In QP, they are said to have both wave and particle nature. Wave description of light explains the interference patterns, while the particle description explains the photoelectric effect.



2. Wavefunction: In CP, we need two parameters to describe the motion of an object - momentum and position. Give me these two, I can describe the particle’s motion completely. These two parameters are said to have “complete information” about the object.


The QP equivalent of momentum and position is “wave function”. The wave function is supposed to have all information needed to describe the object.


3. Schrodinger’s equation: Schrodinger's equation is the CP equivalent of F = ma. In CP, given momentum and position, you can its trajectory, using Newton’s force equation (F=ma). We discussed that “wave function” in QP is “momentum-position” equivalent of CP. Similarly, in QP, given a wavefunction, Schrodinger’s equation describes the “evolution” of “wave function”, the CP equivalent of the trajectory of the object.


4. Uncertainty: You might have heard of Heisenberg’s uncertainty principle. It says that you can’t measure the position and momentum of an object simultaneously below certain error.


This has nothing to do with the precision of your measuring instrument or dimension of the object. It’s just a mathematical consequence. You can’t measure position and momentum simultaneously. That’s it. Such measurement will always have a certain error.

This relation is not just for momentum-position pair, it also applies to other pairs like energy-time etc.

Contrast this with CP, where we could say that a particle is at point A with speed X. We can't make such statements in QP. We have to say a particle is within this position range within this speed.

5. Probability: In CP, we could say that a particle is at A deterministically. In QP, we don't say that it is at A. We say that the probability of finding that particle at point A is p (p < 1). Different points in space have different probabilities of finding the particle. Such probability can be calculated from the wave function of the particle.

If we calculate the probabilities of finding an object at each point in space, you get the probability distribution of wave function. It’s more probable to find the object at certain places than others.


6. Superposition: The most important one. In CP, a body is in state A or state B. An electron is in "up spin" state or "down spin state".


It's different in QP.  If A and B are valid states of a particle then aA + bB are valid states as well (there is a mathematical restriction on the values a and b, can take which we won't go into now).


For instance, it means that if the “up-spin” and “down-spin” of the electron are the valid states, then the electron can also be in a “combination of up and down spins” with different weights to up and down spin. This is counter-intuitive but true.


7. Observation: If an electron can be in a combination of “up and down spin” states, what happens if you observe an electron, to measure its spin? Do we get to measure the detect the intermediate spin states of the electron?


Surprisingly, you will always find either an “upstate” or a “downstate”. You won’t find the other intermediate states, which are the combination of up and down. This is called Observer Effect. 

There are many interpretations of why and how this happens. The classical Copenhagen interpretation says that before measurement, the electron was in a superposition of states but on making the observation, the wave function “collapses” into one of the classical states. This is the idea behind the famous Schrodinger’s cat.

There are other interpretations like Everett’s Many-Worlds interpretation, which I think is too much for beginners. Essentially, the point is that we know the math of how it works but we don’t know the underlying mechanism yet.


8. Quantization: In CP, we are accustomed to the idea of continuous energy. In QP, energy comes only as a multiple of a minimum possible number. This is the reason for the name “Quantum” in QP. Quantum here signifies a “chunk”. It’s given this name because the fundamental characteristics of objects always come in multiples of minimum chunks. It is not continuous. For instance, energy is always a multiple of a minimum chunk. The charge is a multiple of minimum chunk etc. The intermediate values are not possible.


All of this might sound crazy, counter-intuitive, weird and even nonsensical. But hold on, we don’t get to decide what’s non-sensical. Universe has no obligation to make sense to us. All of this is true and has been experimentally verified. So, we are the ones who are to update our notions!


Consequences:

To appreciate the consequences of quantum phenomenon, let’s see some examples.

1. Electron cloud: We learnt in school that electrons revolve around the nucleus in circular orbits. We pictured electrons as if they revolve like planets, in definite paths.

But QP says that electrons have a wave function which gives it a probability of existing at several points. If we plot the probability of finding an electron in each space, we find that the highest probability of finding is around the classical circular orbit. But there’s some non-zero probability of finding it outside it too! We call this probability distribution as an “electron cloud”.

2. You cannot reach absolute zero temperature: We have learnt in school that there’s absolute zero temperature (-273 C). QP makes it impossible to reach absolute zero.

It’s because the temperature is essentially vibration of individual molecules. Absolute zero means that all the vibration stops and everything is at rest. But from the uncertainty principle of QP, we know that we can’t precisely measure the position of molecules. It means that they cannot be at absolute rest. There will be a little bit of motion. When there’s motion, there’s temperature!

You can go close to absolute zero, but cannot reach it.

3. Quantum Fluctuations: As discussed above, just like there is no “absolute position of rest” due to uncertainty, which results in fluctuation of “position” of molecules, a similar phenomenon happens in case of energy too.

As per CP, you look at empty space and can say that there’s zero energy. But QP doesn’t allow for “absolute zero” energy, there is always a fluctuation of energy. This is called quantum fluctuation.

4. Hawking radiation: From the famous E = mc^2 equation, we know that energy and mass are equivalent. If you have enough energy, you can get mass.

From QP, we know that there is always a fluctuation of energy. This quantum fluctuation sometimes results in the creation of “particle-antiparticle” pairs, which is often called as “particles popping in and out of existence”. These pairs annihilate themselves quickly.

Hawking applied this concept to blackhole. We know that there’s a boundary around the black hole, beyond which nothing can escape. Hawking argued that if such quantum fluctuation resulting in particle-antiparticle creation happens at the boundary of a black hole, there is a chance that one particle falls inside and other falls outside. Since the particle that fell inside can’t come out, the particle that fell outside will travel outwards, resulting in what we call “Hawking radiation”.

This has led to famous black hole information paradox.

5. Quantum leaking in semiconductors: If the energy of an object is X, CP says that it can’t overcome an energy barrier which is more than X. But QP talks of probabilities. It gives a non-zero probability of objects passing through the high energy barrier. We call it quantum tunnelling.

Quantum tunnelling is accounted for while making semiconductors because it results in “quantum leakage”.

Using gravitational law to verify proclaimed effects of moon’s gravitation on humans’ mental health during full moon day

Lunar eclipse is an eventful day on earth because of various beliefs attached with it. Two popular beliefs are that food gets poisoned and the mental health of humans is affected, with pronounced effects in people with previous history of mental health issues. Though this is seen specifically on lunar eclipse, people also claim that similar effects on mental health front also happen on full moon day.

Supporters of the claim that moon affects human mental health on lunar eclipse or full moon day, claim that, such an effect is due to the gravitation force of moon on human head, that creates micro tides in our head, in the way moon creates tides on earth. This post cross checks this claims using Newton’s formula for gravitational force between two bodies. The structure of the post is as follows
  1. Background: Simple explanation for cause of tides on earth due to Moon
  2. Tidal gravitational force of Moon on human head
  3. Tidal gravitational force of a 100 kg terrestrial object on human head
  4. Conclusion: Comparing tidal gravity of moon and 100 kg object on human’s head.

Excerpt from “Discovery of India” on “Scientific Temper”

Following is the excerpt from Nehru’s “Discovery of India” on “Scientific Temper”. It’s from a chapter titled “Religion, Philosophy, and Science”

“India must break with much of her past and not allow it to dominate the present. Our lives are encumbered with the dead wood of this past; all that is dead and has served its purpose has to go. But that does not mean a break with, or a forgetting of, the vital and life-giving in that past. We can never forget the ideals that have moved our race, the dreams of the Indian people through the ages, the wisdom of the ancients, the buoyant energy and love of life and nature of our forefathers, their spirit of curiosity and mental adventure, the daring of their thought, their splendid achievements in literature, art and culture, their love of truth and beauty and freedom, the basic values that they set up, their understanding of life’s mysterious ways, their toleration of other ways than theirs, their capacity to absorb other peoples and their cultural accomplishments, to synthesize them and develop a varied and mixed culture; nor can we forget the myriad experiences which have built up our ancient race and lie embedded in our sub-conscious minds. 

We will never forget them or cease to take pride in that noble heritage of ours. If India forgets them she will no longer remain India and much that has made her our joy and pride will cease to be. It is not this that we have to break with, but all the dust and dirt of ages that have covered her up and hidden her inner beauty and significance, the excrescences and abortions that have twisted and petrified her spirit, set it in rigid frames, and stunted her growth. We have to cut away these excrescences and remember afresh the core of that ancient wisdom and adapt it to our present circumstances. We have to get out of traditional ways of thought and living which, for all the good they may have done in a past age, and there was much good in them, have ceased to have significance to-day. 

Reason as an instrument to determine morality — II

In my earlier post, I had made three arguments
  1. Morality has multiple resources.
  2. Morality can exist independent of religion.
  3. Reason can be an instrument to determine morality and we often use it, even if we may not be aware of it.
Sean Carroll adds a nuance to it, saying that empathy should be a component of such “reasoning process”.

I agree with Prof Carroll’s argument. Empathy is necessary part of reasoning. Reason and empathy are not opposed to each other.

Reason as an instrument to determine morality — I

It is often said that “reason” is useful only in the arena of science and that it is not useful to determine morality and the norms of human interaction. The corollary of this argument is that, we should hence turn to religion for moral questions, and that only religion can be the prime source of morality. Some people in fact wonder — how can one get sense of morality without religion. These notions aren’t necessarily true.

Morality has multiple resources. People learn qualities of empathy, compassion, and how to be moral even from reading fictional books like Harry Potter.

Morality can exist independent of religion. Around 70% of the population in countries like Netherlands declare themselves as not having a religion. If morality does not exist without religion, such countries should have been in chaos by now, which we know is not the case.

Reason” can be an instrument to determine morality and we often use it, even if we may not be aware of it. The recognition of universal human rights and the provision of fundamental rights in our constitution is a good example. The reasoning behind recognition and provisioning of these rights was not based on a religious pretext. It is based on the “reasoning” that all humans are equal.

LGBTQ rights is another contemporary example where we used “reason” to determine morality and norms of social relationships. The acceptance of the sexuality of LGBTQ rights was not based on religious reasons or writings in holy books. It was based on the “reasoning” that every human has a right to choose and live their preferred sexual life. Religion had no role in the narrative that helped make LGBTQ acceptable in our society. If any thing, it only contributed negatively.

The increasing acceptance of love marriages is another example where reason was used an instrument to determine its morality.

In summary, religion may have a part in determining one’s morals but it isn’t a monopoly. More importantly, it is completely possible to have morality independent of religion, and reason can be an instrument to determine morality. In fact, we often use secular reasoning to determine morality, it’s just that we may not be consciously aware of it.

Einstein’s swag research paper

On the occasion of Einstein’s birth anniversary (March 14th), sharing a witty incident in Einstein’s life.

Check the abstract of a paper written by Einstein, submitted to the famous journal “Science”
Some time ago, RW Mandl paid me a visit and asked me to publish the results of a little calculation, which I had made at his request. This note complies with his wish.
Research papers are usually written in a professional manner, sometimes with jargon. Einstein, on the other hand, wrote it in a casual language, demonstrating swag.

This paper is on “gravitational lensing”, a widely used technique these days for astronomical observations. Einstein’s General Theory of Relativity says that masses bend spacetime around them. When light passes through the bent spacetime, they appear amplified, thereby the masses acting as a lens.

Gravitational lensing is now used for several purposes. For instance, by measuring the lensing effect on the light by an object in the interstellar space, one can measure the mass of that object acting as a lens. Such technique is useful to study dark matter, especially in its estimation.
A “little calculation” that Einstein had made is of significant utility today!

The physics of the acts that claim to create objects in the mouth

Creating objects in the mouth is a common technique used by some to demonstrate that they have supernatural powers.

Such acts are debunked umpteen number of times, showing that these are nothing but carefully orchestrated magic. Such people keep the object in a hand-kerchief and on the pretext of wiping the mouth, take the object into them mouth. It is later taken out and demonstrated as if the person has created it in the mouth.

Even if we aren’t aware of the exact techniques, we can also examine such claims using our knowledge of physics. We all know the famous Einstein’s equation E = mc² where c is the speed of light (3 x 10⁸ m/sec). It says that energy and mass are equivalent. In other words, for a given mass, we can calculate the energy required to create that mass.

Let’s assume that the mass of the object created in the mouth of the performer is 1 gm. Usually, it is much more than 1 gm, but let’s consider 1 gm for sake of demonstration.

If an object of 1 gm mass is created in the mouth of the person, then the energy required as per E = mc² formula is:
= (1 x 10^-3) (9x 10¹⁶) J
= 9 x 10¹³ J
= 90 TJ

Creating 1 gm mass requires 90 TJ of energy.

To give a sense of scale, the energy released in Hiroshima and Nagasaki nuclear explosion is around 63 TJ and 85 TJ.

In other words, the person creating an object of 1 gm in the mouth has created more energy in the mouth than the energy that was created in Hiroshima and Nagasaki nuclear explosion. It’s obviously ridiculous.

Even high school children are familiar with the famous the E = mc² equation but it’s just a matter of application. The lesson is that knowing and applying simple physics can help cross check such claims.

Hawking Radiation — How does radiation come from black holes if nothing can come out of it?

Hawking Radiation is Stephen Hawking’s most cited paper. Hawking radiation is the radiation from black holes, which will lead the black holes to eventually evaporate!

But didn’t we all hear that nothing comes out of black hole? If so, how is radiation coming from it? The short answer is that — Hawking Radiation is not coming from inside the black hole. It is coming from the surface just outside the black hole.

The physics of Hawking radiation goes like this.
  1. As per quantum physics, space everywhere is filled with fluctuating fields. Even the space just outside the black hole’s event horizon (surface beyond which there is no return) has tiny fluctuating fields.
  2. The quantum fluctuations can lead to creation of particles from the quantum fields (more on it below). It’s happening all around us but such particles exist only for a very short period of time and get annihilated.
  3. When such particles are created just at the surface of black holes, it may happen that one particle falls into the blackhole, while the other falls outside the black hole. The one that falls into the black hole doesn’t return, while the one that falls outside the blackhole is emitted as “Hawking radiation”
  4. In case of Hawking radiation, there is a net creation of particles (that don’t annihilate). The energy for creation of such particles must come from somewhere. It comes from the blackholes itself (technical version of this phenomenon in FAQs below). It means that as the Hawking radiation is emitted, blackholes lose a part of their energy. If you wait long enough, black holes lose all their energy and evaporate!
The Hawking radiation and subsequent evaporation of blackholes is a significant breakthrough in physics. Two direct consequences of the formulation of Hawking radiation are:
  1. Negligible threat of blackhole catastrophe in CERN’s Large Hadron Collider (LHC): There is a theoretical possibility of creation of tiny black holes in LHC. But we don’t need to worry about them because they evaporate in a short time, due to Hawking radiation

  2. Holographic principle: Holographic principle says that our 3D world may just be a projection of the information on the 2D surface at the outer edge of the cosmos, just like holograms where we see a 3D object on a 2D surface. The physics of holographic principle will be a topic of another post but just note that Hawking’s radiation led to the formulation of holographic principle. Holographic principle was also part of the famous Big Bang Theory TV series.

Intuitive explanation of “slowing of time” due to motion

One of the consequences of Einstein’s special theory of relativity (speed of light is constant in all reference frames) is that your clock ticks slower when you move. If you move at the speed of light, time stops!
This is a counter intuitive concept. In my earlier post, I had discussed an intuitive way to understand this. Let’s denote the speed of light by c.

c = distance/time = a measure of space/a measure of time

Einstein’s special theory of relativity says that c should be constant in all reference frames. It means that space (numerator) and time (denominator) adjust, which leads to change in time due to motion.
I find another exposition of time dilation by Brian Greene pictorially appealing.

Let’s suppose that you move along the North with speed v, and let’s say that you cover a distance of D, in unit time. The red line in the picture below denotes this.
Now, you change your direction of motion, while traveling at the same speed. The new direction of motion is depicted by the green line.

Note that in the same unit time, you now cover less distance along vertical direction (d < D), even though you travel at the same speed. It’s because, some of your motion along the North is converted to your motion along the East.

Similar analogy can be used to depict the time dilation.

When you are at rest, your coordinates along space dimension don’t change but since the time is moving forward, your coordinates along the time dimension change. Motion of a stationary body (in space) along time dimension is depicted by the red line.
Now, the body starts moving along space dimension too. The new motion will be along the green line.

Note that the moving body covers less along time dimension (t < T), because some of its motion along time dimension is now converted to motion along space dimension.

It means that measurement of time is slower in moving bodies. Time dilates with motion.
I found this as a good intuitive explanation. Hope you liked it.

This principle is used in GPS to determine the precise coordinates, as explained in the earlier post.

From Special Relativity to General Relativity

Einstein proposed his Special Theory of Relativity (SR from now on) in 1905 where he gave new set of equations for mass, time and velocity for objects moving with constant speed.
The concept of spacetime is an important aspect of SR. It says that space and time are related. Two interpretations of spacetime are useful to remember.
  1. Each one of us carry our own coordinates and our own time. There is no absolute space or absolute time.
  2. My time is a mixture of your spacetime, your spacetime is a mixture of my space and vice-versa.
Brief summary of General Relativity (GR): I will give a brief idea of GR, before going to narrate the story of its evolution from SR.

GR in brief says that bodies with mass warp the spacetime around them. The gravitational force on a second body is nothing but the motion of second body in the warped spacetime of the the first body.
It’s commonly explained with the example of a dense ball placed on a trampoline sheet. The ball creates a dent on the sheet. A small ball in the surrounding falls into the dent. Gravity acts in a similar fashion.

Just a clarification — In case of trampoline, the gravity from beneath the sheet makes small balls fall into the dent. In case of spacetime, you don’t need a force from beneath. Warped spacetime means that the first body changes the shape of the spacetime in which the second body moves. So, the second body has to move along the curved spacetime, which appears to be gravity.

In other words, the gravity of first body acts on the second body by shaping the trajectory of the second body. It is similar to shaping the curves of a railway track. By changing the shape of tracks, you change the trajectory in which the train moves. This is equivalent to applying force on the body to change its trajectory.

One might argue, let someone else change the shape of rail tracks. It can change trajectory only once the train starts moving. How can changing trajectory make the train move in first place?

Note the difference between space (x,y,z) and spacetime (x,y,z,t) we discussed in previous post. We are accustomed to thinking in space framework. So, we think that we are at rest. In spacetime framework (x,y,z,t) you are always moving because time is moving. Therefore, you have no option but to start moving along the trajectory and no option but to travel along the warped trajectories, caused by gravity.

The following post is structured as follows
  1. Why did Einstein feel the need to come up with GR?
  2. The two important principles for deriving GR — equivalence of acceleration and gravity, and; link between space and time.
  3. Happiest thought of my life: Deriving equivalence between acceleration and gravity.
  4. Proving that space is warped.
  5. Proving that time is warped.
  6. Understanding gravity using an example.
  7. Einstein’s equations for GR
  8. Experimental evidence for Einstein’s GR.
  9. Applications of GR
  10. Conclusion

Origin of the “Special Theory of Relativity” and the concept of “spacetime”

Einstein’s Special Theory of Relativity (Special Relativity) and General Theory of Relativity (General Relativity) are most known concepts of physics. Even a non-Physics person is likely to know the equation E = mc².

This post discusses the origin of the word “Relativity” and uses it to understand the concept of “spacetime”.

Definitions

Laws of motion are those that describe the laws of motion (movement) of bodies. For instance, Newton’s equation F = ma is a law of motion because it describes the motion of the body.

A reference frame is a frame from which we observe a body. If we observe a falling apple, the ground is the reference frame here (assuming that we are standing on the ground while observing the apple) and apple is the body we are observing. A reference frame is called an inertial reference frame if it is at rest or moving with constant speed.

Galileo

Galileo first said that laws of motion are independent of the inertial reference frame. It means that the same equation of laws of motion are applicable to a person observing the object by staying still and another person observing the object while moving with constant speed.
Laws of motion are the same in all inertial frames.
This is called Galilean Relativity (1632).

Newton

Decades later, Newton (1643–1727) gave the formula for the law of motion: F = ma. This equation satisfies the Galilean Relativity.

[For those comfortable with mathematics, you can do this transformation and check for yourself.

In a reference at rest, Newton's equation of motion of a body is d²x/dt².

If a body is in an inertial reference frame moving with a constant velocity k,  then its x coordinate becomes x+kt. So, we replace "x" with "x+kt" in Newton's law of motion. Doing that we get d² (x + kt)/dt².  We notice that this is equal to d² (x)/dt² which is the equation of motion for a body at rest.

We call this replacement of "x" with "x+kt" as a transformation. Newton's laws of motion do not change with this transformation. In technical words, it is invariant under this transformation.]

But Newton had a deeper question — “relative to what?”. You are at rest with respect to your neighbour. But you both are moving with respect to a person in the solar system (because earth is moving). For a person outside the solar system, the whole solar system is moving and so on.

Newton said that there is something called “absolute space”, a static one, relative to which we can measure the motion of bodies. [Read “Newton’s bucket” to understand how Newton demonstrated it.]

Story of the winners of 2017 Nobel Prize in Physics



2017 Nobel Prize in Physics was awarded to Rainer Weiss, Barry C Barish and Kip S Thorne for their work on LIGO (Light Interferometer Gravitational Wave Observatory) that detected the gravitational waves.


A lot has been said about the gravitational waves. Here, I give a brief idea of gravitational waves but I focus on the story of the heroic efforts of Rainer Weiss, Barry C Barish and Kip S Thorne. Newyorker has a detailed piece but I believe it didn’t cover some aspects. I would be focusing on them.

First, I give a brief idea of theory of gravitational waves. Next, I give a brief history of efforts to detect gravitational waves. This is followed by the story of the story of this year’s Nobel winners — the three challenges they faced and how they dealt with the challenges. The post ends with the next steps for LIGO.

Theory of Gravitational Waves

Newton gave us the formula for gravitational force telling us about the quantity of force (F = GMm/r²) but didn’t say about the mechanism behind the force. Regarding the mechanism of gravity, in his book Principia, Newton just said “I will leave it to the consideration of the readers”.

In his General Theory of Relativity, Einstein explained how gravity arises. Einstein said that “masses warp the space-time” that causes other bodies to fall towards it. We can understand this using the famous trampoline sheet analogy.



If you stand on a trampoline sheet (rubber sheet), a big dent is formed. Einstein said that “mass” has a similar effect on space. It causes big a dent. Other particles around it fall into this, which we call gravity. This is a simplistic version. We are accustomed to thinking of space as something static but note that Einstein said that it’s not. Space can be bent!

This is for a static object. Now imagine that the object rotates. A rotating creates ripples in space and sends out “gravitational waves”. This is similar to ripples formed on the surface of water.

Why are gravitational waves special?: Because they can reach places in the universe where light can’t reach. So, we can learn a lot about universe!