Time as known flows only in one direction, i.e., forward, The question is whether it is possible to reverse the arrow of time. There are some thought provoking views on the matter.
View of Physicists
Physicists are of the view that the laws of elementary particle physics remain the same when time is reversed. For example, take the questions which govern gravitational, electromagnetic, and strong-nuclear forces and replace ‘t’ by ‘–t’ but the equations do not vary. Can we say that time reversal is possible? Practically, it does not seem to be, however, there must be something defining the arrow of time.
The second law of thermodynamics says that a quantity, known as the entropy of the system, will either remain a constant or increase with time. The entropy is directly linked with the disorder in a system. The more the entropy, the greater the disorder. Therefore, we can break an egg and go from an ordered state into a disordered state, but the reversing it does not happen, i.e., joining back the broken bits to form a whole. In this way, the direction of increasing entropy determines the arrow of time. And it is a popular way of how the arrow of time is defined.
Time and Energetics
Besides, there is another definition of arrow of time given by Professor Mahendra K. Verma, Department of Physics, IIT Kanpur. He illustrates the concept by 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. Thus, energy gets transferred from a large scale to the small one, thereby defining a direction for the arrow of time. This concept was described in a paper, published in The European Physical Journal B.
This and other such alternative definitions of arrow of time are necessarily required in cosmology to explain cosmological models like the oscillating universe. As per this model, the universe, which is considered 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. In that condition, the arrow of time defined using entropy will reverse. However, it does not sound physically possible.
Oscillating Universe
According to Prof. Verma, the second law of thermodynamics encounters difficulties in explaining cosmological arrow of time for the oscillating universe, whereas energy transfers can predict the arrow of time for the collapsing universe.
He explains that in a gravitating system, like the collapsing universe, cluster or star formation is somewhat similar to the formation of cyclones or hurricanes. However, unlike 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.
Some Interesting Facts about Time Travel
Definition of Time
Physicist Albert Einstein said that time is an illusion and is relative, i.e., it varies for different observers depending on the speed through space. He views time as the ‘fourth dimension.’ Space is regarded as a three-dimensional arena, which provides a traveller with coordinates like length, width, and height, which show location. Time provides another coordinate, called, direction, which conventionally moves only forwards.
Einstein’s Theories of Relativity
As per the theory of special relativity, time slows down or speeds up depending on how fast you move relative to something else. Approaching the speed of light, a person inside a spaceship would age much slower than his twin at home. Also, gravity can bend time.
When anything that has mass sits on a piece of fabric, it causes a dimple or a bending of space-time, which causes objects to move on a curved path and that curvature of space is what we know as gravity.
Both these theories—general and special relativity—have been proven with GPS satellite technology that has very accurate timepieces on board. The effects of gravity and the satellites’ increased speed above the Earth, relative to observers on the ground, make the unadjusted clocks gain 38 microseconds a day.
This effect, called time dilation, means astronauts are time travellers, as they return to Earth very, very slightly younger than their identical twins that remain on the planet.
Through the Wormhole
As per NASA, general relativity also provides scenarios that could allow travellers to go back in time. However, the equations might be difficult to physically achieve. One possibility could be to go faster than light, which travels at 1,86,282 miles per second (2,99,792 km per second) in a vacuum despite Einstein’s equations showing that an object would have both infinite mass and a length of 0 at the speed of light.
To some physicists, this appears to be physically impossible, while some others have extended his equations and said it might be done. As per NASA, a linked possibility would be to create ‘wormholes’ between points in space-time, which would collapse very quickly and would only be suitable for very small particles. Moreover, scientists haven’t actually observed these wormholes yet. The technology needed to create a wormhole is also not available so far.
Other Time Travel Theories
Apart from Einstein, some groups have proposed alternate solutions to jump back and forth in time.
As per astronomer Frank Tipler, there is a mechanism, called, Tipler Cylinder, where one would take matter that is 10 times the sun’s mass, then roll it into very long but very dense cylinder. As per the Anderson Institute, a spaceship nearby could get itself on a ‘closed, time-like curve’ following a very precise spiral around this cylinder. However, there are certain limitations with this method, including the fact that the cylinder needs to be infinitely longer to work for this.
Moving a ship rapidly around a black hole, or artificially creating that condition with a huge and rotating structure may be another possibility. As per physicist Stephen Hawking, they would go around and around, experiencing just half the time of everyone far away from the black hole. They would be travelling through time. However, the crew would need to travel around with the speed of light. Another limitation is that if one used a machine, it might fall apart before being able to rotate that quickly, says physicist Amos Iron, the Technion-Israel Institute of Technology in Haifa, Israel.
Cosmic Strings
Yet another theory for potential time travellers includes something, called cosmic strings. They are narrow tubes of energy stretched across the entire length of the ever-expanding universe. Left over from the early cosmos, they are predicted to contain huge amounts of mass, and therefore, could warp the space-time around them.
These strings are either infinite or they are in loops, with no ends. The approach of two such strings, parallel to each other, would bend space-time so vigorously and in such a particular configuration that might make time travel possible theoretically.