Thursday, 23 April 2015

Time - as we know it, and as we don't.

I asked this question to a few of my friends - "How much is one second?". The question wasn't from a spiritual or a psychological perspective, and I also wasn't expecting the answers in that way. It is a very straightforward question that I want you to answer to yourself before you read any further. A purely mathematical or scientific answer.

Technically, the International System of Units (SI) defines one second of time as the duration of 9,192,631,770 cycles of microwave light absorbed or emitted by the hyperfine transition of Caesium-133 atoms in their ground state (Just for explanatory purpose, don't bother about the details). What it does not mention is that the definition is only valid on Earth. First things first - The human experience of time is not an effective measure of time. To have a better understanding of what time really is, we need to consider a few other factors that affect time. In an earlier post titled 'The depths of my mind', I had discussed about spacetime and how time is different from the way we understand it to be. We know three dimensions - The X,Y and the Z axes which we call 3D in short. All objects move relative to these three dimensions. Along with these three dimensions, when we take time into consideration (which we call the 4th dimension) together, these four dimensions are known as Spacetime.

The foremost factor that affects time on Earth is Gravity. Our measure of time or our perception of its measurement is relative to the amount of gravity being exerted on us. It is said that the understanding of Gravity is the solution to many problems. In fact, the problem in approaching many problems is the lack of sufficient knowledge about gravity. We know that gravity is a fundamental and a naturally occurring force by the nature of which all objects attract one another. We were first taught about gravity as an attracting force back in our 6th grade and we used the concepts of gravity till we completed school, or even college for some. But from my very limited knowledge that I have gathered from my interest in reading about metaphysics, I understand that Gravity isn't an attractive force as taught to us. What's more, it cannot be classified as a force by itself. At the very basic level of quantum physics, it would be safe to say that all objects distort (bend or have an effect on) space and time. It essentially means that any object that has a mass has an effect on space and time in and around it. And the bigger the object (by mass), the more is the distortion. The gravity and time that we experience is therefore a distortion of time and space due to the Earth's mass. Let's take a very simple example:

Assume that spacetime is simply a stretchable membrane, much like a trampoline. We place a heavy ball (the Sun) exactly at the center of the trampoline so that it stretches the trampoline downwards. Next, we take a lighter ball (the Earth and other planets) and place it at the edge of the trampoline. We would see that the ball gets drawn to the center of the trampoline and falls towards the larger (heavier) ball at the center. Consider another case - while placing the ball at the edge, if the ball had considerable velocity that would match or perfectly negate the force by which it is drawn towards the center, it would simply keep revolving around the bigger ball. This would explain why Earth rotates around the Sun and doesn't fall towards the Sun due to its gravity. Again, this gravity is technically the distortion of spacetime by the Sun. But why and how is this distortion caused? That's something I would be curious to know myself. The best accepted theory we have on gravity is general relativity, which doesn't in itself explain why mass bends spacetime, just that it happens. We are moving right now through both space and time. Einstein said that these were not two different things but really one thing, called spacetime.

Now, that we have a (good?) understanding of gravity, let's link it to time and see how gravity has an impact on time. As discussed about Caesium atoms earlier, you may know that they are used as timekeepers in atomic clocks. There are atoms of other elements that are used in atomic clocks too, and they are the most accurate clocks that measure time (The most accurate atomic clock loses only one second in 15 billion years, so we are talking about that much level of precision). A direct relationship could be established by saying that time passes at the current rate we know because of gravity. The gravitational pull isn't constant, and it is different at different altitudes on Earth. Therefore, on Earth itself, two clocks at different altitudes would show different times, but the difference would be extremely small as the difference in gravity itself would be very small. Establishing a more precise relationship, time passes slower where gravity is higher. Time on the top of a mountain would pass faster than time near the core of the Earth. But considering this very short distance, the effect would be negligent.

Take an out of the Earth example - clocks on the International Space Station and on the satellites run at a slower pace than the ones on Earth. This shows that gravity bends time in its own way. This phenomenon of time being affected by gravity is known as Gravitational Time Dilation. Take a bigger example of astronauts traveling at a very fast speed to some place and then getting back to Earth. When the astronauts return back to the Earth, they find that they haven't relatively aged as much as the people on Earth have (the basis on which the Interstellar movie was made). Time has passed slower for them, while it has passed faster for those on Earth. Although everyone experiences the effect of time dilation, nobody notices a difference within their own frame of reference (the place they exist).

To simplify, velocity and gravity each slow down time as they increase. Velocity has increased for the astronauts, slowing down their time, whereas gravity has decreased, speeding up time (the astronauts are experiencing less gravity than on Earth). Nevertheless, the ISS astronaut crew ultimately end up with 'slower' time because the two opposing effects are not equally strong. The velocity time dilation (explained above) is making a bigger difference, and slowing down time. The (time-speeding up) effects of low-gravity would not cancel out these (time-slowing down) effects of velocity unless the ISS orbited much farther from Earth. Now, if an object were to move faster, at the speed of light, theoretically, there would be no lapse of time at all - which means that if an astronaut were to travel in a spaceship at the speed of light, he would never become older. Let's say that you are on a chair and spin really fast. It would mean that parts of you are aging at different rates. If you could measure accurately enough, your head and your feet are not the same age!

Now that we have discussed so much about spacetime, the next time you go to the beach to watch the tides, do remind yourselves that the cause of those tidal forces is due to the varying strength in the spacetime distortion with position in space. Not quite simple to think of it that way, is it? I thought life was easier when I simply believed that it was due to the Moon's gravitational pull :)

If you have some more information or explanation on any of this, I'd be glad if you could share it in the comments section below. Do share this article if you found it to be informative :)

Edit: I found this answer on Quora that deals with bending space and time. Quite thoughtful.