Suppose you are in space, you can move there in all the possible directions as you want to, as far as your position coordinates are concerned. But, can you do the same for your time coordinate as well? As you already know, the answer is “No”! As far as we have experienced the macroscopic world till now, we can only move forward in time. And this uni-directedness of time is what is termed as the “Arrow of Time “.
The idea of ” Arrow of time ” was first developed in 1927 by the British astrophysicist Arthur Eddington, and is still an unsolved question of general physics. Eddington first came up with his “Thermodynamic arrow of time” and over the years, several researchers worked in this regime and hence today, we have a list of seven Arrows of Time that will be explained briefly in this article.
Thermodynamic arrow of time:
Given by Arthur Eddington, the thermodynamic arrow of time is derived from the second law of thermodynamics. According to the second law of thermodynamics, entropy tends to increase with time, or at least stays constant but never decreases. In simple words, Entropy can be thought of as a measure of randomness or disorder in a system. Thus, the second law implies that time is asymmetrical with respect to the amount of order in an isolated system. As a system advances through time, it becomes more statistically disordered. So, as one goes forward in time, the net entropy, of any isolated or closed system will always increase. This asymmetry can be used empirically to distinguish between the future and the past.
Although some violations of the second law of thermodynamics have been found to exist at the microscopic level, none of the observed violations are significant to reverse the thermodynamic arrow of time. This arrow of time is the most fundamental arrow of time known to exist and seems to be related to all other arrows of time in one way or the other.
The Cosmological Arrow (expansion of the universe):
After developing the thermodynamic arrow of time, Eddington turned towards the cosmological arrow. The cosmological arrow of time points in the direction in which the universe is expanding. There exist a good number of reasons to regard this as one of the most fundamental arrows. It is even responsible for driving some of the other arrows. Without expansion, a static universe would settle into thermal equilibrium, thus there would be no changes. No changes indicate that there would be no entropy increase to show Eddington’s thermodynamic arrow.
The thermodynamic arrow of time and the second law of thermodynamics are thought to be a consequence of the initial conditions in the early universe. So, they ultimately result from the cosmological set-up. This indicates that without the cosmological arrow, all the other arrows could not be realized.
Radiative arrow of time:
All kinds of waves, whether light, radio waves, sound waves, or even water waves are always radiative. They appear to expand outwards from their point of origin. Although the theoretical equations do allow for the opposite to happen, that hasn’t been ever seen in nature so far. So some researchers also regard this asymmetry in the flow of radiation as one of the probable reasons for the asymmetry of time.
When the radiations radiate away from their source, it suggests an increase in entropy while the reverse process ie convergence suggests increased order which means decreased entropy. So, keeping this behavior into consideration, It is possible that the radiative arrow may also be linked to the thermodynamic arrow.
- The concept of Schrodinger’s cat in quantum mechanics
- What is entropy, exactly?
- The experiment that introduced the concept of spin in quantum mechanics
Quantum arrow of time:
Whenever we talk about quantum mechanics, Shrödinger’s equation and wavefunctions are always among the first words to pop up in our minds. The whole conventional Copenhagen interpretation of quantum mechanics is based on Schrödinger’s equation and the collapse of wave functions, and this is what appears to be a time-asymmetric phenomenon. But how is this time irreversible? In order to understand this, let’s take an example!
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The wavefunction of a particle gives various probabilities of finding a particle at different positions. The wave function only collapses when the particle is actually observed. At that point, the particle can finally be said to be in one particular position, and all the information from the wave function is then lost and it cannot be recreated In any way. In this respect, the process is time-irreversible, and hence a possible arrow of time is created.
Particle physics (weak nuclear force) arrow of time:
Of all the four fundamental forces in physics, the weak nuclear force is the only one that sometimes fails to manifest a complete time symmetry. Certain subatomic interactions involving the weak nuclear force violate the conservation of both parity and charge conjugation, but only very rarely. An example of this behavior is the kaon decay. Now, According to the CPT theorem, this means that they should also be time irreversible, and so establish an arrow of time.
But, the combination of parity and charge conjugation being broken together is so rare that it means that this arrow only “barely” points in one direction, thus setting it apart from the other arrows whose direction is much more obvious. So, to a limited extent, there can exist a weak force arrow of time. This is the only arrow of time that appears to be completely unrelated to the thermodynamic arrow.
- The mirror of the universe and the Ozma problem
- The standard model of physics
- The concept of Feynman diagrams in physics
Causal Arrow of Time:
Can you feel full without actually eating something? No! Right? A cause always precedes its effect! This means that a causal event has to occur before we get to experience its effects. So, you will eat first, and only then you will feel full. Thus, by causing something to happen, we, to some extent are controlling our future, whereas whatever we might do, we cannot change or control the past. Although it is not directly related to physics, causality appears to be intimately bound up with time’s arrow.
Psychological/Perpetual arrow of time:
The psychological or perceptual arrow of time can most likely be considered as a variant of the casual arrow. We all possess a perception that runs from the known past to the unknown future. We anticipate the unknown and automatically move forward towards it. We plan and often execute actions that intend to affect the course of events in the future. But, none of them can affect what has already occurred in past.
According to Stephen Hawking, when we remember past things, they form a relatively small set compared to the potentially infinite number of possible disordered future sets. Thus, even the psychological arrow of time is ultimately dependent on the thermodynamic arrow.
The concept of the arrow of time is still one of the most mysterious problems in physics. Although, most of the physical processes at the microscopic level are believed to be either entirely or mostly time-symmetric, which means if the direction of the time were to reverse, the theoretical statements that describe them would remain true. Yet, at the macroscopic level, it often appears that this is not the case, and thus there is an obvious direction of time, as indicated by the above mentioned “Arrows of Time”, as far as the current advances are concerned.
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