Suppose you are in space. As far as your position coordinates are concerned, you are free to move in all the possible directions. 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, 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 the “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.” Over the years, several researchers worked in this regime. Hence, today, we have a list of seven Arrows of Time that will be explained briefly in this article. 

The thermodynamic arrow

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 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, any isolated or closed system’s net entropy will always increase. This asymmetry can be used empirically to distinguish between the past and the future.

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. 

Read all the articles of our Basics of Astrophysics series here

The cosmological arrow (expansion of the universe)

After developing the thermodynamic arrow of time, Eddington turned towards the cosmological arrow, which 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 crucial arrows. It is even responsible for driving some of the other arrows. Without expansion, a static universe would settle into thermal equilibrium. An absence of change indicates that there is no increase in entropy to show Eddington’s thermodynamic arrow. 

Arrow of Time: How Does Science Explain The Direction In Which The Time Moves? 2
The cosmological arrow of time (Image: Smithsonian magazine)

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.

The radiative arrow

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 seen in nature so far. Hence, some researchers also regard this asymmetry in the flow of radiation as one of the probable causes for time asymmetry.

Ripples in water as explained by Radiative arrow of time (Image : Science abc)
Ripples in water as explained by Radiative arrow of time (Image : Science abc)

When the radiations radiate away from their source, it suggests an increase in entropy, while the reverse process, i.e., convergence, suggests increased order, which means decreased entropy. Keeping this behavior into consideration, the radiative arrow may also be linked to the thermodynamic arrow. 

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The quantum arrow

Whenever we talk about quantum mechanics, Schrödinger’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? To understand this, let’s take an example.

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.

Arrow of Time: How Does Science Explain The Direction In Which The Time Moves? 3
Collapse of wavefunction (Image : pinterest)

The particle physics (weak nuclear force) arrow

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. Specific subatomic interactions involving the weak nuclear force violate both parity and charge conjugation conservation, but only very rarely. An example of this behavior is the kaon decay. According to the CPT theorem, this means that they should also be time irreversible, and so establish an arrow of time.

Arrow of Time: How Does Science Explain The Direction In Which The Time Moves? 4
Kaon decay

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. You can read more about parity transformation and its violation in weak decay in this article.

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The causal arrow:

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 will you 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

The psychological or perceptual arrow of time can most likely be considered 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 the 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, if the direction of the time were to reverse, the theoretical statements that describe them would remain valid. Yet, at the macroscopic level, it often appears that this is not the case. 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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[…] and general relativity, the ‘cosmological constant,’ the redshift of spectral lines, ‘time’s arrow,’ unified field theory, quantum mechanics and much […]


[…] Arrow Of Time: How Does Science Explain The Direction In Which The Time Moves? […]

Abhinandan Mehra

Cheer Up to the team of The Secrets of Universe I have done a course of thermodynamics and from there I came to know about the direction of time that is exercised by entropy as it make ua manifest in which direction time actually moves also the expansion of universe tells us about the direction of timw too

Umakant Mani
Umakant Mani

Amazing I couldn’t imagine that such a bewildering topic time could be categorised in just seven arrows of time.

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