Why do we get the impression that time travels in a straight line from yesterday to today?

Question

Why do we get the impression that time travels in a straight line from yesterday to today?

Answer 1

"We" don't have that impression.  My impression, for example, is that time is a scalar and, therefore, has no direction.  However, it is monotonically increasing, like a pile of sand at the bottom of an hour glass.  That pile has no direction, but it is getting bigger, it is increasing.  And so also does time.

Time is the only asymmetric dimension of our four dimensions.    Because time is asymmetric, we cannot go back, we cannot decrease the size of the sand pile.  This is due to something called entropy, which in the net, like time, is also monotonically increasing throughout the universe.  The June 2008 "Scientific American" has an excellent article on why time cannot go backwards...in our known universe.

Scientists have often puzzled on why time is the only asymmetric dimension.  After all, we can go up or down, forward or back, left or right along the spatial dimensions, why not forward and back along time?  Well, perhaps time can go backward, but not in our universe.

The multiverse concept is that we are but one of many, perhaps infinite, number of universes.  And in some of those other universes, time may travel backwards relative to our time line.  We will never know because we cannot cross the boundaries between the universes, but the backwards time in other universes would make time symmetric in the wider, multiverse picture.

Answer 2

It might be for the same reason we think there are three detentions.  That is, how we perceive it.

Answer 3

We don't get that impression, it's simply an artificial construct, at least in one sense of your question.

The real answer to your question is that the second law of thermodynamics tells us that entropy never decreases, and since the Universe is constantly expanding and increasing in randomness, it would be impossible for us to spontaneously go back from today to yesterday because that would require a decrease in entropy (things becoming more ordered). We know this because if the Big Bang was a singularity, that was the most possibly ordered and non-random arrangement of atoms in the Universe, and as time goes "forward" the Universe expands and this is a more random arrangement of atoms.

A classic example is what happens if you drop a coffee mug off of a table. It will shatter when it hits the ground, but you never see broken coffee mug shards on the floor spontaneously get up and return together to fix themselves on top of the table. This is because the coffee cup went from a more ordered state to a less ordered state. Doing it in reverse would be a decrease in entropy and is impossible according to the second law of thermodynamics.

So this is why it appears as though time only flows in one direction.