The only two “arrows of time” we have don’t match | by Ethan Siegel | Starts With A Bang! | Jul, 2024

By examining this strobe image of a bouncing ball, you cannot tell for certain whether the ball is moving towards the right and losing energy with each bounce, or whether it’s moving towards the left and getting an energetic kick with each bounce. The laws of physics are symmetric under time-reversal transformations, and yet we only ever perceive time’s arrow as running in one particular (forward) direction. The reason why is not yet known. (Credit: MichaelMaggs; Edit by Richard Bartz/Wikimedia Commons)

Our thermodynamic arrow of time explains why the entropy of any isolated system always increases. But it can’t explain what we perceive.

Most of us, in our day to day lives, experience time as something that’s fixed: always ticking by, in the forward direction, at an easily measurable rate that all observers can agree on. But when two observers compare what they each experience, for themselves, as one second, they don’t always find themselves agreeing with one another. This was only explained in the early 1900s, with the arrival of Einstein’s theory of relativity: the surprise that time itself, long considered fundamental and universal, is actually relative. Different observers, so long as they move through space at different speeds or in different directions, will experience the flow of time differently from one another. Whether two events occur simultaneously or one-before-the-other depends entirely on the observer’s point of view.

And yet, despite how ambiguous time is, there are some facts about it that all observers can agree on. Perhaps the most fundamental of these facts — and yet, perhaps the most puzzling among them as well — is that everyone, in their own inertial reference frame, always sees time moving forward at the same rate: one…

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