Wait… listen… do you feel it? The inexorable passing of time: each consecutive moment lost to the past as we travel this one-way street from each fleeting moment to the next. Independent, inescapable, and ever-present, as you try to get a firm grasp on the elusive idea of time, it crumbles in your hands.
What is time? In everyday life, it is thought of as an absolute concept, independent of any observer and universal; all processes unfold over the same duration for everyone, everywhere. However, in 1915, Albert Einstein published his General Theory of Relativity, in which he showed that space and time are not independent but interwoven in a single fabric, space-time. Like the axes of a graph, space and time are perpendicular dimensions, creating the 4D structure of the universe containing all matter and energy over all time.
Moreover, objects with mass, from stars to people, curve space-time, analogous to water displaced by objects placed on it. The water flowing near the object takes curved paths and moves faster and inward towards the object—time being like the flow of water. Therefore, the closer you are to a massive object, like the Earth, the more space-time is warped, distorting time so that the duration of events passes at different rates depending on your position. Those at a higher altitude experience a faster passage of time than those at lower altitudes. Although small, this difference is even measurable with precise atomic clocks.
This is taken to an extreme in a black hole. Space-time is warped so much that space becomes time-like, forcing you to travel in one direction—inwards. You only remember events that happened farther from the centre than you are. Additionally, time becomes space-like, as light can travel to you from events in the past and future, so you can literally see them!
So, time's duration is not ubiquitous or independent and varies depending on your location. Yet, there is still a present when things happen "now." No matter where I am, we can agree that if an event happens “now,” right? Unfortunately, not. Again, Einstein deconstructs any intuitive notion of time in his Special Theory of Relativity. Einstein described how time depends not only on your proximity to massive bodies but also on your velocity. Imagine a river with a constant rate of flow: the faster you go relative to the river, the slower the waters seem to flow. This is similar to the flow of time.
Let’s imagine you were to travel from Earth at 99% the speed of light for 7 years before turning around and returning. Although you would experience a 14-year journey, for a "stationary" observer on Earth, 100 years would have passed, with your return being 100 years after you left! Moreover, if on Earth, 14 years after you left, I asked, “What are you doing ‘now’?” what would the answer be?
Our first inclination might be that what you are doing "now" is what you are doing 14 years after you left. However, to you, 14 years after you left is 100 years for me—so "now" is in 86 years? Instead, what if “now” is 14 years after you left from my perspective, when you are 14 light-years away in space? However, if I could see directly to that position 14 light-years away, the light would have taken time to travel, so I would be seeing that position 14 years ago, before you even left.
So in this case, to see me “now” at that point 14 light years away, you woud need to look in 14 years, 28 years after I left, at which point in my perspective, I am 28 light-years away after having travelled for 4 years. It appears asking what is "now" is a fruitless question. From our perspective, “now” is just what happens in the bubble of the universe around us, but if we expand our view, nothing is truly present.
So, there is no “now,” but time still flows as we travel this one-way street from the past to the future. Yet the physics of the universe doesn’t depend on the flow of time, so why do we feel this invasive illusion? In most cases, the physical laws describing the universe do not distinguish between past and future unless heat is involved. The movement of heat from hot to cold highlights the arrow of time.
Imagine a still glass of water. Although seemingly calm, on a microscopic level, water molecules are constantly agitated and exciting surrounding molecules—heat is just a measure of their average energy. The transfer of heat is like shuffling a deck of cards; shuffling disorders them, and similarly, heat also increases the disorder of a system. This is what gives our perception of time: the movement from order to disorder.
Imagine our deck of cards is ordered perfectly—half red and half black. This is a specific, ordered configuration, which becomes disordered when shuffled. But it is specific only with respect to colors; equally, you can specify by suit or odd numbers. However, every configuration possible can be specific, as each can be characterized in a unique way by the cards’ positions. Therefore, certain configurations are specific only when we restrict our view to certain aspects (like color). This is analogous to disorder; states becoming more disordered are only apparent from a blurred perspective. The movement of heat, and from lower disorder in the past, is only apparent when looking with this incomplete view. If you could view every detail of the microscopic state of the universe, the seeming flow of time disappears, as the "past" and "future" could be determined by the present state.
In conclusion, time has lost any familiar aspects, but it is still part of our human experience. Like the distinction between up and down, it is an illusion to the extent that it has no meaning in outer space. I will leave you with a quote from Einstein on the death of his closest friend: "Now he has departed from this strange world a little ahead of me. That signifies nothing. For those of us who believe in physics, the distinction between past, present, and future is only a stubbornly persistent illusion."
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