Time, as we experience it, flows only in one direction – forward. We cannot easily reverse the ‘arrow of time’ as it is called. An example is that it is easy to squeeze a toothpaste container and bring out the paste, but well-nigh impossible to push it back without making a mess. It is a fascinating exercise to see how physicists view this concept.
Physics and time
The laws of elementary particle physics remain the same when time is reversed. That is, take the questions which govern gravitational, electromagnetic and strong-nuclear forces and replace “t” by “–t” and the equations are invariant. Does this mean that time reversal is indeed a possibility? We do not see it in practice, hence there must be something defining the arrow of time. This is the second law of thermodynamics, which says that a quantity known as the entropy of the system will either remain a constant or increase with time. The entropy is directly related to the disorder in a system. The more the entropy the greater the disorder. So, we can break an egg and go from an ordered state into a disordered state, but the reverse – broken bits of egg joining to form a whole – does not happen. Thus, the direction of increasing entropy determines the arrow of time. This is a popular way of defining the arrow of time.
Energetics and time
Professor Mahendra K. Verma from the Physics Department of IIT Kanpur has come up with a different way of defining the arrow of time, which is described in a paper published in The European Physical Journal B.
The concept is readily illustrated taking the example of milk being stirred into coffee decoction in a cup. First the milk swirls in large blobs, then it dissolves into smaller and smaller blobs until it gets dispersed in the decoction. Therefore, there is a transfer of milk from large blobs to smaller blobs and then to still smaller blobs. In the same way, energy gets transferred from a large scale to the small scale, there by defining a direction for the arrow of time.
Such alternative definitions of arrow of time are needed, for example, in cosmology to explain cosmological models like the oscillating universe. According to this model, the universe, which we know to be expanding, will reach a maximum size and then start contracting once again due to gravity. In such a contracting phase, entropy may actually decrease. If this happens, it will mean the arrow of time defined using entropy will reverse, and that sounds physically impossible.
Oscillating universe
“The second law of thermodynamics encounters difficulties in explaining cosmological arrow of time for oscillating universe. However, energy transfers can predict the arrow of time for the collapsing universe,” says Prof. Verma.
In a gravitating system, such as the collapsing universe, cluster or star formation is somewhat similar to the formation of cyclones or hurricanes. “[In contrast to the example of coffee] here the energy flows from small scales to large scales. For such systems, the clustering or structure formation is in the forward direction of time,” he adds in explanation.
