To understand this question, I need to get us to think about each of the concepts that come up. The first is time. According to the RAE dictionary meaning most relevant to this context, time is “the physical quantity which makes it possible to order the succession of events, to establish a past, a present and a future, and whose unit in the international system is the second”. If nothing had evolved, we would not be able to imagine the passage of time. The least we need to talk about the passage of time is a clock to measure it, and the ticking of a clock in an otherwise empty universe would already imply some change: a needle moving. Therefore it makes no sense to speak of time without change, we have a flow of time.
Also, time not only flows, but always moves forward. This is known as the arrow of time. We know how to recognize when we are replaying the frames of a series backwards or forwards, because a glass that breaks into a thousand pieces does not spontaneously reassemble. This is something we believe to be true. We remember the past and dream of the future. This arrow of time is known as the psychological arrow of time.
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On the other hand, general relativity is the theory that allows us to study the evolution of the universe. This theory is formulated in the understanding that time cannot be understood entirely independently of space, but forms the space-time in which all events that have taken place and will take place can be found. Time would be almost like another dimension of this block of four dimensions, albeit something different from the spatial ones. But how do we know in which direction time is moving in this block of space-time?
Time cannot be understood as completely independent of space, but as constituting the space-time in which all events that have taken place and will take place are found.
The universe is expanding. The galaxies are getting further and further away from us. This happens because the space we fill that separates them expands. The universe has been expanding since its earliest infancy, before the galaxies (or the matter that formed them) were closer together and the temperature of the universe was higher. Cosmic expansion is a constant change that can be used to define the direction of time, and indeed the cosmological arrow of time is the one that takes into account that we can define the direction of time as that in which the universe is expanding. In a way, according to this arrow of time, the answer to your question is yes, time advances because the universe is expanding. If we think of a universe that didn’t evolve, time wouldn’t progress. But if the universe started contracting, which ours doesn’t seem to be, the cosmological arrow of time would reverse. Therefore, taking this train of thought to its ultimate implications, the universe would always expand by choosing this arrow of time.
This is not the only arrow of time that we can consider from physics. The thermodynamic arrow of time is probably the best known and most closely matches our intuition. Most physical laws are symmetrical under temporal inversions, that is, once formulated, they seem to hold equally if time is considered to flow into the future, or conversely, if time is assumed to flow into the past. However, when we speak of macroscopic physical systems, this is no longer the case, especially when they are subject to irreversible processes.
The thermodynamic arrow of time is based on the observation that the increase in randomness of an isolated system is not spontaneously reversed, i.e. it is irreversible.
The thermodynamic arrow of time is based on the observation that the increase in randomness of an isolated system is not spontaneously reversed, i.e. it is irreversible. This increase in randomness is related to an increase in a system’s entropy and provides us with a way of determining the direction in which time is progressing. Thus, the second law of thermodynamics, which states that the entropy of an isolated system never decreases, breaks the time-invariance of the physical laws for macroscopic systems that are subject to irreversible processes.
Of course, we should expect that time will not flow in one direction or the other, depending on which branch of physics we are looking at. In order for the thermodynamic arrow of time to be compatible with the cosmological arrow of time, the universe must have been in a state of very low entropy to begin with, from which it evolved through expansion. There is no consensus as to why the initial state of the universe had such low entropy, dare I say there isn’t even one as to whether we need to look for a reason.
On the other hand, it might appear that these two arrows of time would be incompatible in models of the universe that began to contract at some point. However, it has been argued that this would not necessarily be the case if the final state of the universe after contraction was also very low entropy and the thermodynamic arrow also reversed when contraction started. Fortunately, the fact that our universe is expanding rapidly makes it unlikely that it will enter a phase of contraction in the future. In this way we would avoid a reversal of the arrows of time that would cause us to lose memories instead of accumulating them.
Prado Martin Moruno She is a PhD physicist, researcher and contract professor at the Institute of Theoretical Physics at the Complutense University of Madrid.
Question sent by José Díaz.
Coordination and writing: Victoria Bull
We Respond is a weekly scientific consultation hosted by the Dr. Antoni Esteve Foundation and the L’Oréal-Unesco program “For Women in Science” and answers readers’ questions about science and technology. They are scientists and technologists, members of AMIT (Association of Women Researchers and Technologists) who answer these questions. Send your questions to [email protected] or on Twitter #weanswer.
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