Time:- Symmetry and the Arrow
This article is a part of a series of articles on time. This is part 2/5. If you haven’t read the first one, read it here!
In the last article, we saw that time is a frozen river. It does not move, Spacetime moves. But, another question arises from the loaf-of-bread approach we used in the last article, does time have a sort of symmetry? What if two observers, at two same times, cut the loaf at the same angle? And if time does not have a symmetrical pattern, is it really frozen, or the “arrow” of time is non existent? We will try to answer these questions in this article.
Time’s unusual ways
Time, for a lot of people is fairly simple, but not for physicists. Many physicists argue that time, in all it’s glory is symmetrical. What really is astonishing is that it can be experimentally verified. Here is an example.
Consider you are watching a movie. In the movie, a box is being pushed around a place, with some amount of force. And the same exact clip is played at the end of the movie, but just in reverse. So, in the first part, the box is being pushed from the left to the right. And in the second part, from right to left. If you were asked, which clip is the original one, what would you say? You may say that the first is original while the second is played on rewind. But there is also a probability that the second clip is the original one, and the first was played on rewind.
Classical physics, and the laws and equations it governs, treat the future and the past on an equal footing. If you take a tennis match, and watch Nadal return to Djokovic, and Djokovic return to Nadal, it is possible Djokovic returned to Nadal first, and then Nadal returning to Djokovic. Unless you know any other information, such as who served, or have concreate proof of some eyewitness, you will not know what the original series of events was. The laws laid down by classical physics do the same in such cloudy situations. It says that if you can reverse the velocities and position in time of a travelling object, you can, in fact treat past and future on equal footing(velocity is a vector quantity, with both magnitude and direction. If velocities are reversed, direction is also reversed.) Unless someone provides you with true information does it help to clear the situation. This can commonly happen in small systems, as seen in situations when conservation of linear momentum is seen. Classical physics have a lot of weird things, which not only give rise to things which manhandle with our intuitions, but also bamboozle us. But why don’t we see symmetry in our everyday lives, where a book that has been printed, was printed in the past, spontaneously un-printed itself, and printed to get what it is now? Perhaps the answer to this is question is that this is not a very good question, according to physics. You see there are a lot of elements in this system, the book, the printer, the person who operates the printer, and not to mention, about 585 x 10²² atoms that are present in every cubic centimeter. All these constituents in this sort of system, give a lot to work with, since all of them have forces acting on the book. This can result in the book gaining so many different unique velocities and positions, (each small nudge from an atom for example, can move the book about 0.1 plank lengths, or about 1. x 10-³⁴ centimeters or 1.6 x 10-³⁶ meters.)each of them so minute and indifferent than the other, that reversing so many velocities and positions over time becomes a long, energy consuming, and nearly impossible task. That is why we don’t see time symmetry in our daily lives.
Does the arrow really exist?
Reading the previous section, you maybe asking,- does the arrow of time really exist? Well, the answer to this is both yes and no. In systems with small number of objects(such as the tennis match or the box movie), there seems to be no distinction between the past, present and the future. On the other hand, the systems with larger objects(such as the one the book was in), have a clear distinction between the past. present and the future. So, the question now becomes,- Why is there a arrow in some situations, but not in others? Well, the answer lies in other concepts, which require some knowledge of mathematics, but are easy to grasp(and will be discussed in the next article), but for now, what really is interesting, is that it all depends on you vantage point, and on the system. Go back to the universal loaf we talked about in the previous article(don’t go, I have the picture here), there, we see the evolution of the universe, with galaxies, planets, black holes, quasars and even more things which we are yet to discover.
This clearly shows the arrow of time. But, if you look right on the left corner, at the exact moments before the big bang, you won’t see the arrow, there was singularity then there before, and there was singularity moments after, but the arrow started some more moments after the big bang. (all this, remember is from a imaginary vantage point) Similarly, people inside a stationary car in a working hours wont feel the arrow , since everyone is working, no one is coming out of their offices, there is no movement, while in rush hour, when everyone is rushing home, one can see cars moving, pedestrians walking, etc. Remember, classical physics, is dependent on motion for it’s existence, all the laws, laws of motion, gravitation and magnetism, of classical physics, follow some kind of motion.(motion of objects in gravitational or magnetic field for example)Thus, the arrow is nothing but something made up by ourselves, and it’s existence depends on our point of view, and the system we are observing.
The other part which explains the arrow, more precisely, will be discussed is the next article, along with answers to why spilt milk doesn’t un-spill or why splattered eggs don’t un-splatter. Coming, up is the answers to all these questions, and a little more insight into the arrow and symmetry:- Time- Entropy and the Arrow.