What Is the Twin Paradox? A Simple Guide to Einstein’s Theory of Relativity

Did you know that NASA conducted a study of the effects of spaceflight on twins. This was done to test Einstein's famous twin paradox experiment which is a result of relativity. Identical twins Scott Kelly and Mark Kelly were chosen and while Mark stayed on Earth, Scott spent a year aboard the international space station. You can read the results here.

What exactly is twin paradox? Let us understand Einstein's original idea first.

Time machine

Surely the idea of time machine takes our mind on a wild imaginary ride when we attempt to understand it. Time travel is a kind of travel not in space (which we know has three dimensions, length, width and height) but it is the travel in the fourth time dimension. Is it possible to travel in the past or in the future?

In non-relativistic or classical physics, the concept of time is that of absolute time, which is independent of any observer and is same throughout the universe. Same time flow on Jupiter as on Earth. This was thought of first by English scientist Sir Isaac Newton back in the day... who proposed that time progressed at consistent pace for everyone everywhere and is essentially imperceptible and mathematical in nature.

But in Einstein's relativity, time is not absolute. Meaning that time is perceivable and is not the same everywhere and for everyone. And we now know that the rates of time actually run differently depending on relative motion for different observers; time effectively passes at different paces so it might not be the same flow of time on Jupiter.

So, making a time machine might be possible if we can control the flow of time.

Twin paradox

There are two types of time travel: to the future and to the past. We already are moving into the future all the time at the tick of the seconds hand, but we're doing so at a regular rate. Could we make it so that this pace of time going forwards is increased, such that we go into the unseen future?

There is a way. Sending elementary particles on round trips in a particle accelerators at 99% of light speed is routine. The result is that the inner clock of such a travelling particle, say electron, runs much slower than that of a particle of the same species that remains at rest. Time slows for the fast moving particle.

Can this result from subatomic particles extend to larger human bodies? Einstein thought so... in his famous experiment "twin paradox" while he was working on the theory of relativity. In this, a hypothetical astronaut returns from a near-light speed voyage in space only to find his stay-at-home twin many years older than him, because travelling at high speeds has allowed the astronaut to experience only, let's say, one year of time, while ten years have gone by on the Earth.

The real paradox happens from the fact that there is "no favourable reference frame" in relativity. Why can’t the twin in the spaceship define himself as being at rest, let's say? And everyone on Earth is moving in that frame, oppositely. The Earth moves away at high speeds before returning to the still spaceship.

And if that is the frame, couldn’t the travelling twin apply time-dilation to everyone who stay on the Earth? By that argument, shouldn’t it be the humans of earth that remain younger once the twins are reunited? We all must eventually agree though that only one of the twins' perspective has to be the correct one. Which one is it then? So this is the actual "twin" or "dual" paradox of time dilation as put forth by Einstein in the 20th century.

Mind bending.

From general relativity, we can say that time passes more slowly for objects in strong gravitational fields than for the objects which stay far from such fields. There are all kinds of space and time distortions near black holes, where the gravity can become very intense. Thus if one of the twins is orbiting around a black hole and the other's orbiting around the earth the question of the paradox, "which twin is older" is answerable.

Back to the past

We have all watched "Back to the Future" and wondered how messed up it could get if we too did actually move backwards in time? And many scientists say the very premise of pushing a button and going back to yesterday violates the law of causality. However there are also some who think otherwise. Professor Michio Kaku has said, "Time is a river. It speeds up, meanders, and slows down. It can also have whirlpools and even fork into two rivers."

That last bit, "fork into two rivers," is important because then moving backwards in time could become at least thinkable. Because as soon as we push the button we go back into an alternate world or reality. We do not cause harm to our previous reality as in the case of "grandfather paradox". The idea was first proposed by British physicist David Deutsch who used the terminology of multiple universes to solve the grandfather paradox. Deutschian time travel involves the time traveler emerging in a different universe other than his own but very similar to his own.

Time travel will remain only conceptual and debatable except if we are able to develop enough advanced technology for it to become achievable. Until then we will use our earth bound telescopes as time machines. Because when you look into one you'd actually be looking into the past stages of the universe.. meaning that the star you observe today might not even exist in the first place. Turns out that if aliens knew exactly where to point their telescopes they could see dinosaurs at least in principle.

What Physics Tells Us About a Hypothetical Nuclear War Between India and Pakistan

India and Pakistan were recently involved in fresh tensions resulting in military actions by both countries. World leaders called for maintaining peace as any further escalation could potentially lead to a nuclear war in south Asia.

What impacts would a hypothetical nuclear war between India and Pakistan have on the world? Let us explore the physics behind the potential consequences of nuclear war in South Asia.

1. Blast destruction: Nuclear explosions release immense energy (E=mc²), causing blast waves and thermal radiation that kill millions instantly. The explosion can release energy as much as ~10²³ joules, causing a shockwave, flattening structures.

2. Underground or surface detonations release energy as mechanical waves, causing artificial localized earthquakes (seismic waves).

3. Thermal radiation from the fireball (~several million °C) ignites flammable materials, leading to large-scale fires. Shockwaves (pressure >10 psi) destroy buildings and critical infrastructure.

4. Radioactive isotopes (e.g., Cs-137, Sr-90) from fission reactions decay, emitting gamma rays and contaminating air, water, and soil. Post nuclear war, it would be impossible for any species to survive in the place.

5. High-altitude detonations produce gamma rays that ionize the atmosphere, generating electromagnetic pulses (EMPs) that disable electronics.

6. Soot from the explosions absorbs sunlight, reducing surface temperatures by 5-10°C for years due to aerosol scattering. This would lead to an unbearable nuclear winter.

7. Nitrogen oxides from high-temperature explosions catalyze ozone breakdown, increasing UV radiation exposure. This would not only impact the south Asia region but also the rest of the world for decades to come.

8. Reduced sunlight and temperature from nuclear winter disrupt photosynthesis, leading to crop failures.

9. Radiation sickness follows. Ionizing radiation (alpha, beta, gamma) disrupts cellular structures and DNA, causing acute and chronic health effects.

10. The nuclear war would speed up global disarmament efforts. Countries would volunteer to destruct their nuclear arsenal because people around the world could clearly see how dangerous nuclear weapons are.

credit: Geralt on pixabay

Why Was Albert Einstein Not Religious?

Science without religion is lame and religion without science is blind. This popular quote of Albert Einstein has been repeatedly used, particularly in science versus religion debates. But from this statement alone can one say that Einstein was arguing for religion? A large number of believers definitely think so, referring to this adage and thus claiming the greatest scientist of the 20th century as one of their own.

However, Einstein had also famously written: "The idea of a personal God is a childlike one," in a letter to a friend dated 28 September 1949.

Einstein even went on to say, "You may call me an agnostic but I do not share the crusading spirit of the professional atheist whose fervor is mostly due to a painful act of liberation from the fetters of religious indoctrination received in youth."

From this saying alone, we can conclude that Einstein was neither a religious man in the usual sense nor was he a staunch atheist. Einstein was agnostic in belief. If you think about it, agnosticism really is the essence of science, whether ancient or modern.

Being an agnostic simply means that a man shall not say he knows or believes that which he has "no scientific grounds" for professing to know or believe.

Does God play dice? (Einstein's most famous quote)

Einstein was expected to make many statements on the origin of life, the universe and existence of God. Some of the views resonated with religious groups, but that does not make Einstein a believer. Albert Einstein was in fact one of the most famous agnostics in America, others being Edwin Hubble, Carl Sagan, John Bardeen, etc. and yet Einstein's name and his quotes are selectively chosen as merely "tools" by debaters to silence an opposition.

What had Einstein meant really, when he said: Science without religion is lame and religion without science is blind?

Actually, he was making a reference to a large part of human history in which science and religion were intertwined or interdependent. He put it like this, indicating that the interdependency still existed in the modern society.

This does not suggest in any way that Einstein was a deeply religious person and nor does it provide any surface to anyone to interpret it in such a way. If truth be told, Einstein had strongly asserted in one of his statements - "The word god is for me nothing more than the expression and product of human weaknesses."

So if Einstein wasn't even religious in the most ordinary sense, why his name is often dragged in trivial debates? Because it is assumed by a large number of people that in science "Einstein" is the authority. But they are wrong, because in actuality there is no authority in science. Feynman said: You can be the most amazing minds, if your ideas do not agree with experiment it is wrong. No matter who you are.

This is precisely how science progresses, by challenging, by having no authority, by questions and doubts; whereas religion has not progressed for hundreds and thousands of years.

Einstein's views were simply, that nature is not nurtured. That nature itself is nurturing. This is the ultimate essence of Spinozism a philosophical system which was largely advocated by Einstein. Spinoza belief is the unbounded admiration for the structure of the world, the universe, so far as our science can reveal it.

Just a year before his death, Einstein had replied to a fan in a letter, "It was of course a lie what you read about my religious convictions, a lie which is being systematically repeated. I do not believe in a personal God and I have never denied this but have expressed it clearly."

How Ohm's Law Was Discovered?

Georg Simon Ohm was a German mathematician and physicist who discovered notably one of the most important laws in physics and engineering. Ohm's law is on every high school student's lips like A,B,C and that is what makes it so great - its simplicity and fundamental nature, qualities which make Ohm's law easily memorable.

V = IR

where,

V = voltage

I = current

R = resistance

Pretty neat, isn't it? But what does Ohm's law imply and how did Georg Ohm discover his now famous law?

Background of Ohm's law

Georg Ohm (1789 - 1854) was born to Johann Ohm, not formally educated and a locksmith by profession, who wanted his son to receive excellent education. Ohm's mother died when he was only 10.

In 1806, Ohm accepted a position as a mathematics teacher in a school in Switzerland. By the early 1820s, Ohm had also started teaching physics to high school students. Teaching high school science was a decent job, but Georg didn’t want to just teach - he wanted to make science.

Ohm was fascinated by electricity, which at the time was still something of an unknown magical force. The electrical battery had just been invented 20 years ago and there was a lot of scope of new discoveries in the field.

Discovering Ohm's law

Ohm didn’t have university funding or state-of-the-art equipment. He just wanted to do experiments at home for fun. Ohm only used a simple battery, copper wires and a home made galvanometer (a scale used to detect and measure small electric currents).

One day while testing different wires, Ohm connected a longer wire to his circuit, and the current dropped drastically. Initially Ohm thought that something had gone wrong with his archaic battery. But then he switched back to a much shorter wire and saw the current rise again.

At that time, it was not understood how or why electricity flowed through the wires... Ohm was puzzled at this strange phenomenon. Could it be that the length of the wire itself was resisting the flow of electricity?

Ohm began to systematically test wires of different lengths, materials, and thicknesses and jotted down the results of his experimentation.

Pattern of Ohm's law

Ohm noticed the following pattern:

1. The longer the wire, the less current was read by galvanometer.  

2. The thicker the wire, the more current.  

3. Different materials changed the flow too.

Ohm came to conclusion that each material offered different resistance to the flow of current. This quality is inherent to the material itself. Furthermore, length of the material also influenced the amount of current. 

Ohm was the first to describe electricity in mathematical terms, showing that: 

... the graph between current (I) and voltage (V) of battery was a straight line. When you divide voltage by current, you get a new physical quality - resistance - of the material used. Ohm's law can be visualized mentally as voltage pushing the current further down the wire, but the wire's resistance in turn preventing the flow of current.

Reaction to Ohm's law

The scientific community of Germany thought that Ohm’s work was too simple and too mathematical a result to have any great physical influence. One reviewer said it was "a fantasy, not physics." The rejection hurt Ohm so much that he resigned from his teaching post. Imagine discovering a law of nature… only to be told you're imagining things.

It wasn’t until a decade later, as electrical science progressed, that Ohm was recognized for his innovation. Scientists like Kirchhoff and Maxwell helped establish Ohm's law as undeniably right. In 1840s, Ohm had earned himself a position at the University of Munich.

Ohm's law today

Today, Ohm’s Law is taught in every physics classroom around the world. It’s one of the first scientific formulas students learn in physics and electronics. Ohm's fun with wires ultimately led to the creation of a formula which is taught to every high school physics student.

How Physics Affects Technology and Vice Versa

The knowledge of physics has resulted in a wide range of technological applications. Steam engine, for example, the first great industrial invention, arose from the discipline of thermodynamics, at the very start of eighteenth century. Within just a few decades, steam engines were being used in all sorts of applications including factories, mines, locomotives, and boats. Of course, the progress rate of human civilization increased manifold in a short time.

Then, in the nineteenth century, Faraday discovered the law of induction which became the basis for the invention of transformer. Further discoveries by Tesla and current war between Edison and Westinghouse led to the commercialization of electric power. Which created a ground-breaking change in the way we lived life as people. Radio came along soon when Hertz found the way to transmit and receive radio waves; the magic of long distance communication happened.

In first half of the twentieth century, two great inventions took the world by storm. Firstly, there was this enormous, uncontrolled atomic energy, employed as weapons of mass destruction in the second world war. Which would later be controlled in the nuclear reactors to harness clean electrical energy for the 21st century society. The second big invention of the early twentieth century was the television; all the world inside a box; first in greyscale then in color.

In next half of the twentieth century, humankind exceeded all their expectations. Landing on the moon, flying past the planet Saturn and invention of the internet. Towards the end of the twentieth century, personal computers became a reality. Productivity of man reached an all time high. Furthermore, the continually developing semiconductor physics led to manufacturing of even smaller computers and then ultimately smartphones.

From a mechanical age to electrical; from electrical to space age; and from space to a digital age; how far have we come; and marching onwards still! The main point is, that there is a fundamental connection between physics and technology. Without knowledge of physics, there could be no technological advances in the society. Let's see with the following table a list of some important technologies and the principles of physics they are based on.

Steam engine	Laws of thermodynamics
Nuclear reactor	Controlled atomic fission
Radio and TV	Generation, transmission and reception of electromagnetic waves
Laser	Stimulated emission of radiation
Computer	Digital logic
Rocket propulsion	Newton’s laws of motion
Radar and sonar	Reflection of sound
Ultra high magnetic field	Superconductivity
Electric generator	Faraday’s law of induction
Hydroelectric power	Conversion of energy
Airplane	Fluid dynamics
Particle accelerators	Lorentz force
Optical fibers	Total internal reflection
Electron microscope	Wave nature of matter
Photocell	Photoelectric effect
Fusion reactor	Magnetic confinement of plasma
Telescope	Reflection and refraction of light
Transistor	Semiconductor physics

 

Now a question arises: can technology give rise to new physics? Yes but rarely is it so. For example, measurement of time, an otherwise ancient technology, was perfected only in the era of Huygens and Galileo. This new form of technology was an accurate pendulum clock, which led Galileo to understand the physics of velocity and acceleration.

Another example of technology giving rise to new physics is the particle accelerator such as the large hadron collider. Inside a particle accelerator, there are a thousand possibilities; discoveries of strange particles. It is not surprising that whenever a new particle is found, new physics is unraveled.

Thus, the link is kind of two-way. However, physics leading to new advances in technology is far more likely than vice versa. Whatever is the case, the ultimate benefit, should be going back to the common people. It is the duty of those in power to make it so and to not misuse physics or technology to fulfill their eccentricities.