The concept of spacetime tells us that both space and time are the same.
One typical example is that the age of the universe is the edge of the universe. As it takes time for light to travel from one place to another, what we see at present, strictly speaking, should be what happened in the past.
The sun we see right now is the sun that existed around eight minutes ago, as it took time for the sunlight to reach our eyes. The moon we see right now is also the moon in the past, around a few seconds ago.
Similarly, the coffee cup in front of me right now, strictly speaking, existed in the past. We can never see the things that are in the exact moment of our very act of observing. And, the extreme case is that the age of the universe is the edge of the universe. The farther we reach, the older we return. Therefore, the remotest space we observe should be the ultimate beginning.
It takes “time” for “light” to travel in “space” from one point to another. That is the fundamental basis for spacetime.
We usually sense velocity as the combination of time and space. When one object moves from one place to another, the time spent can indicate the speed. These relations can be the primary basis for us to capture spacetime in Newtonian physics.
In this classical physics, both time and space are supposed to be the static entity to formulate the universe. That is also our experiential intuition to see and feel the universe in our consciousness. And it works in the physical reality as well to some extent. Both time and space are static, and yet time passes irreversibly. We can go back to the place we left. But we can’t go back to the past. In this sense, time seems different from space. Is that so?
The special and general theories of relativity provided us the paradigm-shifted attributes about time and space.
- Both time and space are not static but dynamic, which causes gravity.
- On the other hand, the speed (space/time) of light is invariant even in this dynamism.
While the combination of time and space gives us a sense of velocity, its acceleration causes gravity, which bends space, even modifies time. As we know, space nearby the sun deforms itself due to intense gravity, and the object moves with the speed of light delays time in it. And these takes place even in the small scales.
Thus, both time and space is variant, which relates to the gravity controlling the dynamism of the entire universe. But then, there is one static limit in this universal dynamism. That is to say, the speed of light is invariant.
For example, when the sun we see right now is the sun that existed around eight minutes ago. On the other hand, can we also see the sun that exists at the very exact moment when we do the act of observing? That is not possible. For, we can never exceed the speed of light. We can’t see the light which is not yet arrived.
What if we launch a missile from the spaceship that moves with the speed of light? Can this missile exceed the speed of light? It can’t. The speed of light seems absolute among all the spacetime dynamics according to the theories of relativity. Even in quantum physics, light places the special position. We call it the superposition that light is a particle at the same time a wave. We can never determine which should be the true face.
Why does light seem so exceptional? As mentioned above, “light” traveling “space” requires “time” itself. Light is the basis for integrating both time and space into spacetime, acting as a universal physical constant. Moreover, the quantum particles also are in the same condition as the subsets of light, including a hypothetical particle such as graviton.
Thus, the speed of light (c) serves as a universal physical constant of proportionality, linking the formerly disparate concepts of mass (m) and energy (E). We all know the famous equation:
- E = mc2
Perhaps we can pursue how it works. But then, the answer for why it is and does so seems agnostic, similar to the question of:
- Why there is something rather than nothing, why there is anything at all.
Without light, there is no spacetime. Without spacetime, there is no gravity. Gravity is the essential connectivity for the dynamism of the whole universe. Light is the ultimate source of the universe. Without light, there is no universe. Without the universe, there is no light.
And God said, Let there be light: and there was light.Genesis 1:3
Because of light, we can see the irreversibility is the attribute of time, even that of spacetime as the whole. We can’t go back to the past. We can’t go to the future. In the same way, we can’t stay in more than two places at the same time.
It seems we can go back to the place we left. That is not true. Even if we go back to the place we left, it is no longer the place we existed before. It seems we can move around from one place to another. Can we consider this move as something similar to time-travel? I don’t think so.
The effort of time-traveling should be the same attempt to stay in several places at the same time. That is not possible. We can stay only in one place at a time and can exist only one moment at a time. In this regard, time is nothing special as compared to space. Spacetime alone is.
Both time-traveling and staying in several places simultaneously can be the same (impossible) capability. If ever it is possible, we call it ubiquity. Light is ubiquitous, and only beyond spacetime could there be such ubiquity. Only if both time and space were invariant, then we could see the true light beyond the beginning and end, the Alpha and Omega.
I am Alpha and Omega, the beginning and the ending, saith the Lord, which is, and which was, and which is to come, the Almighty.Revelation 1:8
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