8.1 Classical Physics

What is Classical Physics? It covers the development of fundamental physical laws starting from the time of Newton to the twentieth century. The most significant contributions being Newton’s and Einstein’s laws.

They all have a central theme - The world we observe has matter changing dynamically in the arena of space and time. This matter is made of particles. These particles interact via forces. If we know the position and momentum of every particle, of the world at one point of time, we can predict both its future and its past.

This could be a lot to swallow. Let me break it down.

The world we observe has matter. It is easy to assume matter is just solid or liquid substances. It also includes gaseous substances. The chair you are sitting on is made of matter. The water you drink has matter. The air you breathe has matter in forms of nitrogen, oxygen and carbon dioxide.

The matter changes dynamically in the arena of space and time. This is fairly straightforward.

This matter can be broken down further into smaller particles at different levels, each having their own properties.

These particles interact via forces - Gravitational and electromagnetic forces. An apple falling from the tree is because of Gravitational force. Electric motors and Magnets work because of electromagnetic forces.

Imagine that the whole universe you can perceive is just a cube you cannot get out of or move around in. All you can see are two balls in front of you which are bouncing about at the same speed and they do not hit each other and you cannot touch either of them. The idea of classical physics is that if you know the position, the speed and direction of the two balls at one moment, you can predict the future of the balls and by that, you know the future of the entire universe you live in. You just have to apply the laws of physics to know where the balls will be after 100 years.

We are far from predicting the future of the universe this way. But this is the basic idea behind the laws of Classical Physics. How far did we reach when it comes to our ability to predict with the Laws of Classical Physics? That is what we will see in this chapter.

Newton’s Laws

We all have heard of Newton’s three laws of motion, especially the third law - Every action has an equal and opposite reaction. The third law has been used in silly dialogues in movies and books as much as it has been used in science. There is also Newton’s law of Gravitation which explains the effect of Gravitational forces between two objects. You know, the apple falling from the tree story.

You put them all together, and you have a basic set of laws which allow you to predict the trajectory of objects in our day to day life. You can calculate how hard and how high you have to throw a ball, for it to land in the basket ball net. But we don’t have the time to do these calculations in a real basketball game. And even if we did, we may not be able to actually throw the ball accurately based on our calculations.

Taken further, you can calculate how hard you have to throw an object so that it never falls back on the earth. This is the calculation they use in the rockets to determine how to send it out of the Earth’s atmosphere.

Maxwell’s equations on Electromagnetism

James Clarke Maxwell came up with an equation which describes how electromagnetic forces behave. What happens to magnets when they are brought near each other? How does electricity flow through conductors? We used to think that electricity and magnetism are fundamentally different phenomena. But they were both unified under one equation.

This equation is one of the most important equations used in our day to day life in all the electrical appliances around us.

Einstein’s Laws of Relativity

With Newton’s Laws and Maxwell’s equations, we had a good toolkit that was capable of explaining most things we observe around us in our day-to-day life. Many thought that physics was very close to being finished. But there was one lingering issue which took years to figure out - The mystery about the speed of light.

Before Einstein came up with his Laws of Relativity, we made some fundamental assumptions about space and time. We treated them as different and absolute entities. If I measure the length in space as 20m, you will also measure the same. If I measure the time in my stopwatch as 20 seconds, as long as you start your stopwatch at exactly the same time, you will also record 20 seconds.

Forget Newton’s laws. These are also basic concepts we live by. If Einstein’s Laws of Relativity weren’t a thing, I would have added them as brute statements in the layman’s system. But it turned out, they were wrong assumptions.

The story goes like this. If you measure the speed of an oncoming car while you are standing on a road, then you measure the speed of the same car while you are also in a car driving towards that car, the second measurement of the speed will be higher than the first measurement. The speed of any object is usually measured in relative terms. In our day to day life, we don’t normally talk about what the speed is relative to. We just say that the train was going at a speed of 80kmph. Others assume that it was measured from the perspective of someone standing next to the railway track.

But in scientific terms, it is necessary to mention what it is measured relative to. You could claim that you are stationary because you are sitting somewhere and reading this. But you are on the planet earth and earth itself is moving around the sun.The Solar system itself is going around the Galaxy and the Galaxy is also in motion. One common example that is used to explain relativity of motion is when you are in a train that is stationary on a platform and there is another train that is next to you. When one of the trains moves, it is sometimes hard to say which one is moving just by your visual perception. If your train accelerates, then you will feel it and that will give you a hint that your train is moving. And lack of that feeling gives you a hint that your train is really stationary. But otherwise, it is hard to say which one is moving.

Going back to your story, Maxwell’s equation and Michelson Morley’s experiment showed that the speed of light is a certain huge value(299,792 km per second) usually represented as C. But what was it relative to? We just understood how speed is always measured relative to something else. It turned out that the speed of light remains constant no matter what your state of motion is when you are measuring it. You can stand on the top of a rocket and measure the speed of light coming from the sun. It will still be the same.

Einstein took the idea and calculated what happens to our understanding of space, time, and other physical laws when we assume that the speed of light is constant irrespective of the state of motion of the observer. What resulted was his special theory of relativity. The famous E=MC² was an outcome of this theory.

Then, Einstein used what is called the equivalence principle to develop the general theory of relativity. If you are in a lift that is on a free fall from a really tall building, the things in the lift float, similar to how things float in a spaceship that is not affected by any meaningful gravity. Similarly, the feeling you get while resting on the earth is the same as the one you get when you are on a lift that is going up. Einstein saw the equivalence between the effect of gravity and an accelerating body and came up with the general theory of relativity.

The special theory of relativity is mathematically very accessible while the general theory of relativity gives you impostor syndrome. But you don’t have to understand the mathematics of the theories to be stuck in awe of the implications of the theories, which are:

Space and Time aren’t separate like we assumed. They are a single intertwined entity named spacetime.

Space and Time aren’t absolute like we assumed. Imagine I am in a space station outside the solar system. You took a space ship from the station and travelled very close to the speed of light. The length of objects you measure is smaller from the length of the same object I measure. When you are travelling, the time elapsed in your clock will look slower than the time elapsed in my clock if I could observe you. And the time observed in my clock will look slower than your clock if you could observe me. When you do a round trip and come back to the station, the time elapsed in your clock will be slower than mine.

Objects with mass bend spacetime. As a result, if you go near a heavy planet or if you go near a blackhole, the time in your clocks will be slower than the time on mine. This is also how gravity really works. Because huge masses bend spacetime, it changes your path in spacetime in a way that you are pulled towards the huge mass.

There is a limit on how fast you can travel. It is the speed of light.

Mass and energy are inter-convertible. This is an interesting outcome of the theory of relativity, popularly known by the mass-energy equation E=mC^2.

The fact that time runs differently for different people is probably one of the most profound philosophical realisations that is heavily used in science fiction. Imagine this. If there is a black hole near us. You can go there and come back every hundred years on earth, which is probably just a year for you and see how human civilization evolves over such a long period of time.

Philosophical implications of classical physics

Classical physics is a vast subject and I only touched the surface of it with too many simplifications that I am not particularly proud of. But to convey my thoughts about philosophy, what we have seen now is enough. There are two major philosophical implications from classical physics.

Determinism - If the classical physics assumption that if we know the position and momentum of all particles in the universe, we can predict the future state is true, does it mean that the universe’s fate is pre-written? But what about free-will? We feel like we are in control of our actions. If it is all pre-determined, does it mean that free-will is an illusion?

Nature of space and time - This one is on relativity. The idea that spacetime isn’t absolute and can be bent breaks down or at least asks some difficult questions to many philosophies. For example, many religions rely on a God watching the world and affecting things in the world based on people’s actions. If time goes differently in different parts of the Universe and for different people, how does time flow for God? It’s not just Religion. Many philosophers before relativity wrote their philosophies on the fundamental assumption that space and time are absolute. Turns out they are not.

8.1 Classical Physics

Newton’s Laws

Maxwell’s equations on Electromagnetism

Einstein’s Laws of Relativity

Philosophical implications of classical physics

Further Reading