We always wish we could turn back time to correct a mistake or relive old memories. However, currently, this is entirely impossible. So why can’t time be reversed like winding back a clock, or is there a possibility that we just haven’t discovered yet?
Why Can’t We Reverse Time?
In a study published in the journal Physical Review Letters, a group of physicists re-examined the definition of the “Arrow of Time”—a concept describing time’s continuous journey forward—and offered a different perspective on how time manifests on universal scales.
For a long time, time has been described by a “past hypothesis,” which assumes that any given system begins in a state of low entropy and subsequently
Even if time doesn’t flow, we can still assign it a dimension, called a time arrow. This is an abstract concept that simply means we can define an order of events. A time arrow points from the past toward the future, from events that occurred earlier to those that occur later. It is a dimension in time in which everything happens. The crucial distinction here is between a flow of time and a dimension of time.
Imagine looking at individual frames in a film reel. We can easily define a time dimension pointing in a specific direction along the reel based on which frames come before and which come after. We do this despite the fact that we are looking at static photographs of events with no movement within the frames. Each frame is a frozen snapshot in time.
If this concept is applied on a universal scale, it is hypothesized that the Big Bang created the universe in a state of low entropy—that is, a state of minimal thermodynamic force. Over endless time, as the universe expanded and gradually cooled, the entropy of this large-scale system increased. Therefore, according to the hypothesis, time is fundamentally linked to the level of entropy, or disorder, in our universe.
However, immediately after the Big Bang, some observational evidence indicated that the environment was extremely hot with a state of extreme chaos among fundamental particles. As the universe matured and cooled, gravity began to have a greater influence, making the universe more orderly and complex. From cooling gas clouds, stars formed, and planets evolved from gravitational collapse. Ultimately, elements combined into organic matter, evolving to bring about life and humans, thereby forming concepts of space and time. Thus, on a universal scale, “chaos” significantly decreased, rather than increasing as the “past hypothesis” assumed.
Physicist Flavio Mercati of the Perimeter Institute for Theoretical Physics in Ontario, Canada, a co-investigator in the study, argues that the issue is how entropy is measured. Since entropy is a physical quantity with different dimensional representations (like energy and temperature), an external reference frame is needed for measurement. “This can be done for subsystems of the universe when the rest of the universe is set as their reference. But the entire universe—by definition—has no corresponding exterior to determine these things,” Mercati wrote in an email to Discovery News.
Furthermore, physical equations do not even provide a direction in time. Time can run backward, and the laws of physics remain unchanged. One might argue that this is just a coincidence for physicists. If the direction in which time points is missing from physical equations, they cannot tell us the whole story. The inability to derive a dimension for time from mathematical equations does not mean there is no time dimension in the real world.
Mercati provided a somewhat complex example, noting that even in the real world, at the atomic level, most processes are time-reversible. If, in a subatomic process, two particles, A and B, converge and collide, they typically bounce off each other and separate. If you watch a film of such a process and then watch it in reverse, you cannot tell which way the process occurred. Time reversal still obeys the laws of physics.
To be more specific, Mercati used an example of two new particles, C and D, being created and flying away from each other. Again, you cannot be sure of the true order of events if you watch a film of this process because the laws of physics state that the reverse process is also possible. Particles C and D could collide to create particles A and B. Therefore, you cannot assign a time arrow indicating the sequence in which the process occurred.
This contrasts sharply with everyday events, where we have no trouble determining the direction of time. For example, you never see smoke above a chimney gather toward the chimney and be swallowed back inside. Similarly, you cannot “un-stir” sugar from a cup of coffee once it has dissolved, and you never see a pile of ashes in embers “un-burn” to become a log again.
So what distinguishes these events from subatomic events? How is it that most phenomena we see around us never occur in reverse? Surely everything is ultimately made up of atoms, and at that level, everything is reversible. So at what stage in the progression from atoms to smoke rising above a chimney, to a cup of coffee and logs, does a process become irreversible?
The Second Law of Thermodynamics
This is all due to an important law in physics called the Second Law of Thermodynamics. The field of thermodynamics studies heat and its relationship to other forms of energy. Astronomer Arthur Eddington once asserted that the Second Law of Thermodynamics holds supreme status among all laws of nature. There are three other laws of thermodynamics related to how heat and energy can convert into each other, but none is as crucial as the second law.
The Second Law of Thermodynamics states that everything wears down, cools, separates, ages, and decays. It explains why sugar dissolves in coffee but never the reverse. It also states that an ice cube in a glass will melt because heat always transfers from the warmer water to the colder ice cube and never the other way around.
Complexity and the Arrow of Time
Complexity is a dimensionless quantity that, in its most basic form, describes how complex a system is. Therefore, if you look at our universe, complexity is directly linked to time; as time passes, the universe becomes increasingly structured, i.e., more ordered. Mercati explained: “The question we sought to answer in our research is: what established these systems in such a very low thermodynamic state at the beginning? Our answer is: gravity and its tendency to create order and structure from chaos.”
From the Big Bang, the universe began in the lowest state of complexity (likened to a “hot soup” of chaotic particles and energy). Subsequently, as the universe cooled to a state where gravity began to take hold, gas clustered together, stars formed, and galaxies evolved. The universe became more complex, and gravity was the driving force behind this increase in complexity.
As the universe reaches maturity, the subsystems become sufficiently independent for other forces to establish conditions for the time arrow to appear in low-entropy systems. In these subsystems, like daily life on Earth, thermodynamics can dominate, creating a “thermodynamic time arrow.”
On a universal scale, our perception of time is driven by the continuous and irreversible growth of complexity, but within these subsystems, thermodynamics dominates. “The universe is a structure with increasing complexity,” Mercati said. “The universe is made up of large galaxies separated by vast distances. In the very distant past, they clustered closer together. Our hypothesis is: our perception of time is the result of a law of the irreversible growth of complexity.”
The next step in the research will be to seek observational evidence, which Mercati and the team are currently doing. “…we don’t know if there is any support, but we know the type of experiment that can test our idea. That is cosmological observations.” He did not reveal which cosmological observations will be studied but said they would publish information in an upcoming interesting study.
Additionally, Professor of Physics Jim Al-Khalili of the University of Surrey—author of the highly acclaimed book Black Holes, Wormholes and Time Machines—stated that the so-called “flow of time” is merely an illusion, and the laws of physics say nothing about the flow of time. They tell us how things like atoms, pulleys, levers, clocks, rockets, and stars behave when subjected to different forces at specific moments in time, and if we know the state of a system at a certain moment, the laws of physics give us the rules to calculate its potential state at some future time.
However, nowhere is there any sign of time passing. The concept of time flowing, or motion in a certain way, is completely absent in physics. We find that, like space, time simply exists; that is all. Furthermore, Albert Einstein also held the view that time is an illusion, and he even expressed this when trying to comfort the widowed wife of a close friend, telling her she should ease her mind with the realization that the present moment is no more special than any other moment in the past or the future; all moments exist together.
So we can see how confusing time is when viewed as a separate concept. Jim Al-Khalili even joked that if you try to understand how time works, you should be ready to familiarize yourself with Einstein’s theory of special relativity, in which he bound time with space into four-dimensional spacetime. Professor Jim Al-Khalili concluded that reversing time is like fooling human perception on a quantum physical level, although it may still be possible if performed on a quantum scale.
Indeed, in February 2015, Professor Kater Murch of Washington University and his colleagues conducted a quantum experiment. They placed a circuit board in a microwave and then shot photon particles of light into it, where the particle’s quantum field interacted with the circuit board. After the first photons passed through, the computer’s analysis results were “hidden,” and the experts jointly predicted the results of the next shot. According to Murch, this is like predicting future events, and the highest probability of being correct is only 50%.
In contrast, Murch’s team argued that if they knew the future state and development of each particle, the ability to correctly predict the outcome of the photon’s flight in the experiment reached 90%. According to him, atomic particles are not shaped until humans measure them. This means that in the past, the particles were undefined, but when humans conducted research (in the future), they took on full shape, weight, or speed. In simple terms, the action in the future changed what happened in the past.
If this theory is correct, then according to scientists, time and space are symmetrical. In other words, we only perceive the rapid passing of time today, but in reality, time is a two-way “arrow” and can be completely reversed.
