"We accept the reality of the world with which we are presented"
Christof, The Truman Show.
Time exists. Or does it? You have probably never questioned whether you believe in time or not, but there are many out there that argue that time is simply an illusion. The quote above powerfully portrays the idea of direct realism theory, meaning that we accept what our senses are delivered. For us, sensory normality is time moving forwards and we frankly go along with this, with no surface-level reason not to.
The idea of time’s arrow, or asymmetry of time, was developed in 1927 by the British astrophysicist Arthur Eddington, and is still to this day one of the most complexing unsolved general physics questions.
This proposition of questioning time is similar to questioning life’s purpose, directly correlated with the manner in which humans react with mere acceptance. In physics, the constantly dumbfounding question of why life exists is such a foundational one, inspiring answers ranging from pure luck to overlapping multiverses hosing every conceivable reality, while billions of people neglect these fundamentally shocking questions and peacefully and unassumingly settle for the notion of never receiving answers.
According to theoretical physicist Carlo Rovelli, our naïve perception of time’s flow does not correspond with physical reality at all. Some have concluded that time is simply a human construct, backed up by Albert Einstein’s theory of relativity, stating that space and time are part of a four-dimensional structure where everything that has ever happened has its own coordinates in space time.
Nonetheless, it is understandable that we believe in time, since it agrees with our observations and plays an incredibly significant role in the physical theories of the universe and the social structure of our world. Moreover, time is arguably an invention and not a discovery, creating structure in our lives, helping us to develop skills, and allowing us to reach goals.
Back onto the arrow of time, one of science’s largest mysteries. Within the realm of statistical mechanics, Ludwig Boltzmann and many other greats of physics tackled the origins of thermodynamic entropy increase. So, every moment that passes us transports us from the past, to the present, and into the future, whilst never standing still or reversing, hence being a constant arrow.
However, if we examine the laws of physics, all the way from Newton’s to Galileo’s, all are time-symmetric, resembling that they do not have an inclination to which way time flows.
Many believe that there might be a connection between the arrow of time and a quantity called entropy. While most people normally equate entropy with disorder, rather, entropy is a measure of how much thermal energy could be turned into useful mechanical work. The second law of thermodynamics states that the entropy of a closed system can only increase over time and never go down, meaning that over time, the entropy of the entire universe must be increasing.
Consequently, time is not really the progression of events, but instead it is the manner in which the contents of the universe continuously are becoming increasingly disordered and chaotic. Using a matter of logic; there are more disordered arrangements of particles than ordered, and so as things change, they tend to fall into disarray.
To help you envision, here are some examples where entropy explains the arrow of time for a number of phenomena: coffee and milk mixing but not unmixing, ice melting into a warm drink but not spontaneously arising along with a warm beverage from a cool drink, egg never resolving back into an uncooked separated albumen and yolk. In all of these instances, an initially lower-entropy state moves into a higher-entropy state as time has moved forwards.
Except, entropy is not completely irreversible, given the caveat that we are only referring to the entropy of a closed system. A way to reverse this reaction is to introduce an external entity, hence disrupting time. With our current measurements of entropy in the universe, we only know of one decrease in the entirety of cosmic history; the end of cosmic inflation and its transition to the Big Bang.
We are left wondering why entropy was so low in the past and why the universe was so unnaturally organised. Well, the circumstance of gravity being spread evenly is rare and therefore low-entropy, but it being clumped is common and therefore high-entropy. The energy of the universe was spread evenly after the big bang, and therefore, with gravity, it was low entropy.
One last query you might have. Does this mean that time travel could be possible? The short answer, yes.
The more complex answer is that through time dilation and coining the phrase ‘spacetime’, Einstein re-envisioned space itself, fusing the three dimensions of space and one dimension of time into a single term. Instead of treating space as a flat and rigid place that holds all the objects in the universe, he thought of it as curved and malleable, able to form gravitational dips around masses that pull other objects in. Einstein inferred that by taking advantage of the intense gravitational acceleration caused by black holes or by travelling closet to the speed of light, time travel to the future may be possible. But with our current technology, jumping a few microseconds into the future is all we can manage.
Unfortunately, time dilation cannot be used to return to the past, but in spite of this, the past should not forever be inaccessible, due to the proposed idea of an Einstein-Rosen bridge. This type of wormhole is a theoretical area of spacetime that connects two distant points in space, if it were ever stable enough.
In terms of time’s arrow, entropy can be reversed only in the sense that it can be redistributed as we travel into the future. In my opinion, travelling into the past is meaningless since the past exists only as memory, and is synonymous with the entropy of the current moment. We cannot change the past, only update our memory of how we regard the past.
In conclusion, we understand the arrow of time from a thermodynamic perspective, which obviously proves to be incredibly valuable knowledge. Nevertheless, if you are curious as to why there is an immutable past, tomorrow will arrive in a day, or how the present is constantly relocating, thermodynamics will not give you the answer. Actually, nobody will.