Space and time are individually absolute quantities but are relative to each other and are combinedly known as space-time. This fact establishes that these are intrigued quantities which need to have similar physical properties or the way in which they in nature should be same. But this is not so. Time is asymmetrical while space is symmetrical.
Space is SYMMETRICAL
If you take a top view of the Solar System and show it to Newton, he could not be able to tell the motion of the planets. He would even agree to the fact that if we decided to set the Solar System in the clockwise direction, its counter-clockwise motion is also possible. Thus Newton’s laws are not asymmetrical with respect to time. Anything that can , the opposite can happen.
The fundamental laws of quantum mechanics say the same thing. You cannot take the picture of atom and tell that the atom is moving forward or backwards. The laws are completely symmetric.
But this fact of space being symmetric and existing on both sides of the graph is not shown by time. Time is a one-way street. It is always uni-directional. You can clearly establish a picture of space as past and a picture of space as future even though it’s highly unlikely of space to look like it in the future. This uni-directional characteristic of space is explained by the Second Law of Thermodynamics. The Second Law of Thermodynamics states that the entropy always increases. Entropy of a system is defined as the amount of chaos and confusion in the system.
Boltzmann’s contribution to explain this is regarded to be the greatest. He said that “Entropy almost always increases”. To understand, the second law of thermodynamics is not exactly a law; like conservation of energy or F=ma. It’s not always true. It’s almost always true. It’s a statistical law, a probabilistic law. Doesn’t mean you can see something crazy happen backwards, it just means most of the times you don’t.
Example. Consider two atoms in a closed room. Both are on the left side of the room. If you perform an experiment on locating these two molecules after some time, there is a one in eight probability that you will find the molecules in the exact same place. That is, if you observe the molecules at eight different times, only once you will find them at their original positions. Since our usual observation field can never have just two molecules so to find something happen in the reverse direction would take long long time.
Boltzmann had a very modern idea about how to think about this. He was interested in why the universe proceeded in one direction one model of universe that he made was a universe in a box. It’s like a room without any doors or windows and full of whatever it was full of.
Let us take a completely empty universe. In a completely empty universe nothing happens. It doesn’t go forwards; it doesn’t go backwards. It just stays there so it’s completely symmetric. Let us start the universe in a very unusual situation. One that is very special. Here is a special situation. A large number of molecules (total number of protons in the visible universe) start all in the corner of the box. What happens next:-
- They might squeeze themselves tighter into the corner of the box. Doesn’t sound right. Although they can do that.
- But much more likely, most of the times if you find molecules like this, you would find them streaming away from that point.
The molecules will swarm away and in the process of swarming away, during the time they were flowing away from the corner, in the process of flow some turbulence might create little swirls or tornadoes and these swirls might have structures in them.
Ultimately if you wait long enough, everything evaporates, everything turns into evaporation products (photons, elementary particles). Nothing is left but particles in thermal equilibrium. Particles in thermal equilibrium mean particles uniformly filling the box, randomly moving around. As per a physicist it’s heat death or entropy death.
This isn’t completely right. If you wait long enough (more than 13.7 billion years), just by accident, by the laws of large numbers just by statistical probabilities, the molecules will reassemble themselves into the corner of the room and the whole thing will happen over again. These are called recurrences.