Newton’s laws were widely accepted at first. But soon, a discrepancy was discovered. Although his laws correctly explain why apples fall from trees and why the Earth orbits the Sun, they also claim to work backwards. This is obviously not true.
According to the laws of Physics, we can reverse any event in our lives at any time.
So then, when you can break an egg, why can’t you un-break it? Or un-burn matches? Or un-sprain an ankle? Or even un-tear paper?
Milk spills but why doesn’t it un-spill? We always grow older but why not younger? Why does time run forward but not backwards?
These sound like simple questions but to answer them, we need to dig deep into the atomic realm and go back to the birth of the universe.
"You might mix up east and west, but you would not mix up yesterday and tomorrow," says Sean Carroll, a physicist at the California Institute of Technology in Pasadena. "The fundamental laws of physics don't distinguish between past and future." (1)
Scientists’ attempts to explain these commonplace observations spurred the development of thermodynamics and statistical mechanics and led to profound insights about the properties of matter, heat, and time’s arrow. Central to this is the thermodynamic concept of entropy. 150 years ago (in 1865), Clausius famously said, "The energy of the universe remains constant. The entropy of the universe tends to a maximum." (2)
This problem was tackled by Austrian physicist Ludwig Boltzmann. He found that entropy measured the number of ways atoms, and the energy they carry, can be arranged. When entropy increases, it's because the atoms are getting more jumbled up (3). Boltzmann’s take on entropy explains why it always increases, which in turn suggests why time always moves forward. As a result, the universe as a whole always moves from a state of low entropy to a state of high entropy. It is for the same reason that one cannot un-break an egg, un-sprain an ankle, or un-tear a piece of paper.
However, there were some flaws in this reasoning as well. Boltzmann said that entropy increases as you go into the future, because of the probabilities that govern the behavior of small objects like atoms. But those small objects are themselves obeying the fundamental laws of physics, which don't draw a distinction between the past and the future. Boltzmann suggested a past hypothesis to solve the problem, which stated that “at some point in the distant past, the universe was in a low-entropy state.”(3)
If that is true, then the future and the past look very different, because the past has much lower entropy than the future. So eggs break, but they don't un-break.
Even though this proved Boltzmann’s reasoning, another question arose: How do we know that the past hypothesis is true?
The Big Bang Theory was a stepping stone toward answering this question. It appeared that even gravity affected entropy. Then came the Theory of Quantum Mechanics and the General Theory of Relativity. Unfortunately, none of them could successfully explain “The Theory of Everything,” an all-encompassing theory of how the universe works.
After much analysis, most physicists have concluded String Theory to be the most promising one. As per this theory, “all particles are actually tiny vibrating strings and each type of vibration corresponds to a different particle.” (4)
In order to explain the “arrow of time” (Lebowitz, 2008) (5), we need to explain the low-entropy state of the universe at some point of time during its birth. That will actually be the theory of everything—maybe String Theory or something else.
At this point of time, the Large Hadron Collider (LHC), “a particle accelerator that accelerates protons or ions to nearly the speed of light, seems to be our biggest hope.” (6)
The scientists at CERN hope to use the experiments conducted using the LHC to catch a glimpse of new and unexpected fundamental particles as well as further insight into the nine dimensions of reality as stipulated in String Theory.
References:
1) The Greening of Pharmaceutical Engineering, Theories and Solutions. (n.d.). Retrieved June 30, 2020, from https://books.google.co.in/books?id=9EWPDAAAQBAJ
2) Clausius R (1865) On several convenient forms of the fundamental equations of the mechanical theory of heat. Ann Phys 125:353.
3) Becker, Adam. “Earth - Why Does Time Always Run Forwards and Never Backwards?” BBC, BBC, 9 Mar. 2015, http://www.bbc.com/earth/story/20150309-why-does-time-only-run-forwards.
4) What is string theory?: Explore: Physics.org. (n.d.). Retrieved June 30, 2020, from http://webcache.googleusercontent.com/search?q=cache%3AeaqSqpar2akJ%3Awww.physics.org%2Farticle-questions.asp%3Fid
5) Lebowitz, Joel L. “Time's Arrow and Boltzmann's Entropy.” Scholarpedia, http://www.scholarpedia.org/article/Time's_arrow_and_Boltzmann's_entropy.
6) “Facts and Figures about the LHC.” CERN, https://home.cern/resources/faqs/facts-and-figures-about-lhc.