In 1955, the world’s most famous physicist, Albert Einstein, wrote in a letter “For we convinced physicists the distinction between past, present, and future is only an illusion, however persistent.” He was conveying how the future and the past were virtually indistinguishable in almost all physical laws and equations at that time, from Newtonian mechanics to quantum theory. Most physical processes appear to have a symmetrical quality, looking as if they would work perfectly well if the universe were played in reverse. This suggests no concrete reason for why the direction of time should necessarily be moving forward towards the future.
The geometry of space-time brought about by Einstein’s theories of special and general relativity has led some people to speculate about the potential for time-travel. However, this idea is distinctly at odds with the results that Einstein developed: only travelling faster than the speed of light could enable time travel. Furthermore, the linear concept of time is engrained into the concept of the Second Law of Thermodynamics, one of the most crucial laws of physics. Simplistically, the First Law states that the amount of energy in the universe is constant and that energy cannot be created or destroyed but can be transformed from one state to another. The Second Law determines the direction of energy transfer and leads to the concept of ever increasing entropy, or disorder, of a system. Appropriately, the word entropy in Greek means ‘transformation’.
The Second Law is unique in that it is not derived from theory (such as Quantum Theory or the Theory of Relativity), but is empirical. No entirely satisfactory theoretical proof exists for the Second Law, and only when a Unified Field Theory encompassing the broadest stretch of Physics is developed will it be proven. And yet it has been established as one of the most fundamental laws in physics, with British physicist Sir Alfred Brian Pippard writing that “no evidence has ever been presented that the Second Law breaks down under any circumstances.” In the past, many have claimed that the Second Law is paradoxical: the Loschmidt paradox, Gibbs paradox and Poincare recurrence theorem have all challenged the basis of the Second law, but each paradox has been resolved on further investigation, each being founded upon faulty reasoning.
Entropy is the key concept which disallows the possibility of time travel. Energy is in an inevitable, continuous and irreversible process of becoming increasingly randomized. We can see this entropic process in everyday life: ice tends to melt because molecules of H2O are more ordered in ice crystals than in water form, which is a more disordered state. Similarly, water spontaneously evaporates, as the structure of a gas is more randomized than that of a liquid. Considering the example of salt crystals (with an ordered, crystalline structure) being dissolved into a more disordered state in a beaker of water, entropy dictates that the solution of salt cannot revert back to crystals unless external energy is introduced to the system to evaporate the water. This would result in an overall increase in entropy in the universe, as although the salt would return to its less entropic, more ordered state, the poor efficiency of the process would ensure entropy increased elsewhere.
Hence, because all natural processes result in an unavoidable increase in entropy, we cannot ‘reverse’ this effect. Entropy will always increase, and this ensures a particular direction of time: towards the universe in a more disordered state. At the origins of existence, the Big Bang, the finite amount of energy in the universe was established. Initially, energy was extremely concentrated and ordered (low entropy), but as the universe has expanded, the energy become increasingly spread out or disordered. This boundary condition results in natural laws which ensure that entropy always increases. The implications are significant, as reiterated in a profound statement by Lincoln Barnett:
“The universe is thus progressing toward an ultimate ‘heat death’ or, as it is technically defined, a condition of ‘maximum entropy’. And there is no way of avoiding this destiny. For the fateful principle known as the Second Law of Thermodynamics, which stands today as the principal pillar of classical physics left intact by the march of science, proclaims that the fundamental processes of nature are irreversible. Nature moves only one way.” [Lincoln Barnett, The Universe and Dr. Einstein (1957)]