Physicists say past, present, and future all exist at the same time
Time is something we experience day in and day out. It is so intrinsic to our life that most people do not give too much thought about it. Where does it come from? Why has it always moved forward? Is it the same everywhere in the Universe?
How do you feel if you were told that time is not real, it is an illusion, and our intuitive perception of time flow is just a perception ― past, present, and future all exist now?
How is something not real when it feels so real?
Is time fundamental or emergent?
Time always seems to be there, flying like an arrow. We can tell the difference between past, present, and future — World War Three would occur after WW Two and WW One; we can measure its length — one hour is longer than the duration of “right away”; we can determine its order — raw eggs before broken eggs before scramble eggs.
However, all these features — the arrow of time, the duration of time, and the order of time, as well as time itself — are not realities, at least not physical entities, according to a bunch of really, really, ridiculously smart physicists.
To better understand this, think about the directions “up” and “down” — seemingly intrinsic features of our nature, are convenient concepts we invented to correspond to our proximity to the Earth’s surface. Now imagine an astronaut floating in outer space, and there would be no up and down, “all directions in space are created equal”, says Sean Carroll, a cosmologist at the California Institute of Technology in his book “The Big Picture”.
We invented the time, a useful tool, to help us construct a sense of unfolding order of events, just as we invented maps to help us establish the relationship of different geographical locations. Events do not need time to unfold, just as those locations do not need maps to exist.
Why does time always go forward, not backward?
We notice time has a direction and things unfolding in time – milk added into a cup of coffee would never be spontaneously unadded and scrambled eggs would never automatically become raw eggs again.
Yet the laws of physics say the above processes are time-reversible, i.e. any process can be preceded both forward and backward.
If time’s arrow can go both ways as most physicists claim, then why do we perceive it as always going forward? There, scientists have explanations. And it has something to do with the Big Bang, the fancy concepts of entropy and decoherence, and our brain.
Entropy
Entropy is often interpreted as a measure of molecule disorder or the amount of randomness in a closed system. Entropy always increases, a fact enshrined in the second law of thermodynamics. You can cool things down using a refrigerator or an air conditioner, but only at the expense of generating even more heat, therefore increasing entropy, around the outlet.
Entropy increases because the Big Bang, the known origin of our universe, happened to create an exceptionally low-entropy universe 13.8 billion years ago, for unknown reasons.
The earlier low-entropy universe consists of fundamental particles — protons, neutrons, electrons, and so on. The interactions of those particles lead inevitably to an increase in randomness. Though there is nothing in the underlying laws of the universe to prevent the decrease of randomness from happening, the probability of those particles acting in concert to reduce the amount of randomness is extremely low. For “there are far more states with high entropy than states with low entropy”, according to Carroll. An analogy of it is that a deck of playing cards, being shuffled randomly, would render far more disordered combinations mixing black and red cards than the ordered arrangements where all the black cards are on the top (or bottom) and all the red ones on the bottom (or top).
Decoherence
Entropy is defined as a measurement of molecule disorder at a macro-level in a closed system. At a micro-level, between two or more large physical systems, such as milk being poured into hot coffee in a cup, there are lots and lots of particles involved. When perfectly isolated, a system is considered to be coherent under the laws of quantum physics and possesses quantum properties such as the wave function. However, when a system interacts with the environment, the coherent state is interrupted and the system becomes entangled with the environment. This entanglement increases entropy. Increased entropy means time’s arrow points unambiguously in one direction.
The human brain’s experience of time’s arrow
If time’s arrow points one way in physics, then questions arise — how does our brain align the physical arrow of time with the psychological one? Are these two processes intimately connected and if so, how?
Imagine you were to watch a video of two molecules colliding and bouncing for 10 minutes. It is hard to tell if it was being shown forward or backward. However, if you were to watch a slow-motion of milk spilling, it is easy to determine which direction the video is being run.
Our brain’s neural network operates by organizing the sequence of events we experience in an orderly manner and interpreting the ongoing flow of experience as time passes. Such passage is encoded in our brain as a series of static time frames. Each frame relates to a piece of experience. Just like watching a movie, all you see is a sequence of still images being run successively, which gives you an illusion of continuous motion. Our brain, through our eyes, makes a succession of snapshots seem linear. This is our brain’s way of making sense of time and time’s forward flow.
Is there a distinction between past, present, and future?
Since the basic laws of physics do not specify a direction of time — permitting any process to proceed both forward and backward — and time’s arrow only exists in our mind, is there a distinction between past, present, and future? Or like time’s arrow, they are merely products of our mental construct corresponding to the entropic change of events. If so, why do the past and the future seem so different to us? Why can we remember the past, but not the future?
Again, some clever physicists proposed the theory of a “block universe”.
The “block universe” is a static four-dimensional block — three space dimensions plus time — in which time does not flow and is an illusion. The “block universe” is timeless. This being the case, events associated with the past, present, and future are all equally real and still there in spacetime. There is no special moment called “now”, and nothing is “happening” and nothing is “becoming” at the galaxy scale.
A better way to understand it is to image the “block universe” as “a succession of pictures, a succession of snapshots, changing continuously one into another”, says Julian Barbour, a British physicist. We are living inside those static snapshots, just like characters (not actors) inside a film, being watched by audiences at home who can choose freely to run forward or backward to see the storyline of those characters.
Since we are trapped inside the “snapshot” universe, the past to us is what is stored in our memories, and the future is our expectations or predictions. Though the nature of time gives us no reason to believe that the future cannot be experienced as “memory”, the network of neurons in our brain, from which memory arises, prohibits us from storing the “memory” of the future.
Our brain is a biological structure functioning on chemical reactions, which also follow the laws of quantum mechanics and thermodynamics. As previously stated, the past is where the entropy is lower, and the future is where the entropy is higher. Higher entropy indicates the occurrence of the entanglement between observers (i.e. us) and measured state. For the future experience to be stored in our present memory, the entropy has to be decreased. This leads to disentanglement between our memory, which stores what we have observed, and future events that we may have lived out. Such disentanglement leads to the erasing of memory about the future experience. We might experience the future, but we are not able to remember it. Thus we can remember the past, but not the future, by accumulating memories through entangling them with observed events to push the entropy higher over time.
Conclusion
Time is not real in the sense that it is not an intrinsic property of the physical world. The Big Bang gives us an illusion of the flow of time, just like living on the Earth’s surface gives us an illusion of up and down.
You can argue time is useful and thereby subjectively real, as most people can go by with their daily lives without needing to know the fundamentals of the universe, quantum mechanics, entropy, and all that. Will the realization — that time is not real —upend some people’s worldview? I don’t know.
Key points:
- time is not real and time is an illusion under the laws of physics
- time is emergent
- time is symmetric
- past, present, and future all exist now
Definition
Entropy: a thermodynamic quantity representing the unavailability of a system’s thermal energy for conversion into mechanical work, often interpreted as the degree of disorder or randomness in the system. – Oxford Language
Block universe: according to the block universe theory, the universe is a giant block of all the things that ever happen at any time and at any place. On this view, the past, present, and future all exist — and are equally real. – ABC Science
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References and links
Carroll S. (2016). The Big Picture: On the Origins of Life, Meaning, and the Universe Itself. Dutton.
Rovelli C. (2018). The Order of Time. Riverhead Books.