|La Muse Verte Absinthe: A birthday gift to myself|
But what is time? – I ask this question in a physical sense, not philosophically or metaphysically.
There are certain realities of time. While it is relative to space and energy, it moves at an absolute pace in any given frame of reference. Whether I'm here on Earth or outside an event horizon or traveling near the speed of light, time will always pass at the rate of one second, every second. In other words, when I look at my wristwatch, time will run perfectly normal, regardless of my motion.
We speak of time as being a fourth dimension of space, i.e. spacetime, but it's not "real" like the three dimensions of space. Rather, time is an emergent property. While we observe, in our personal experiences, cause and effect such as a glass breaking, we don't see that same, one way flow, at the subatomic level.
This is similar to ocean tides on Earth. There is no physical "tide property" of water. Tides only emerge when we collect a lot of it and introduce other forces such as gravity from the sun and moon along with the Earth's rotation, i.e. it's an emergent property.
What is Time?
Time is literally nothing more than the passage of events – it's neither energy nor physical. Think of it as hole flow in an electrical circuit. We measure time by the ticking of a watch's second hand. Imagine the universe's clock as the motion of atoms, such as an electron orbiting an atom's nucleus. The motion of matter marks time similar to a computer's clock ticking off cycles for the CPU. Different computer clocks move at different rates, and time also moves at different rates depending on how much energy is around it. In your frame of reference, a watch's second hand will always tick away one second, every second, no matter if you're at rest or moving close to the speed of light.
In the presence of huge amounts of energy, time literally slows down, from the perspective of an outside observer. This is simply due to the atoms, in the presence of large amounts of energy, struggling to move through this energy molasses. But, since all the atoms in this frame of reference experience the same slowing, everything looks normal. Imagine if all the clocks and watches (and atoms) in your home slowed down because they had dirt or grit in the "gears" – everything would continue to seem normal even though things are moving slower. But, to an outside observer, your passage of time would be different.
So, is time travel is possible? Absolutely not. You can slow down how fast time passes, but that's about it; you can't speed it up. The concept of traveling through time to a different period makes as much sense as traveling through tides (or love, etc). While we know what causes the flow of the time or tides, it is, as I mentioned earlier, an emergent property.
For time travel to be possible, every particle in the universe would have to move in the opposite direction (backwards). Doing this would move the entire universe back to an earlier state that it was in. However, the atoms in your body would still need to move forward. So, while the universe is moving backwards, your being is moving forward and that would allow you to travel backwards in time. Then, at some point, to return to the present you left, you'd need to slow down all the particles in the universe and/or speed up the particle motion in your body to catch up.
Changing the state of the entire universe simply isn't possible. There is only one state of the universe, The Now.