Arrow of Time

in Space

The Flow of Time and Human Perspective

Repost of a Quora answer found here.


In a physics sense time is widely accepted as being a real thing.

Sir Arthur Eddington coined the term the ‘Arrow of Time’ referring to the linear nature of our experience of times passage. That it moves in one direction from the ‘past’ to the ‘future’. Whereas space has no directional orientation.

Eddington makes three points regarding the arrow of time;

  1. It is vividly recognized by consciousness.
  2. It’s equally insisted on by our reasoning faculty, which tells us that a reversal of the arrow would render the external world nonsensical.
  3. It makes no appearance in physical science except in the study of organisation of a number of individuals. Here the arrow indicates the direction of progressive increase of the random element.

The Physics of Time

Eddingtons third point is the most interesting as it hints at the underlying physics. Newtons second law is that the universe tends towards more entropy. Or in other words the arrow of time points towards increasing entropy.

However the laws of physics are ‘time reversible’ meaning that if the arrow of time was reversed they’d still work just fine. Below is the best image I could find of the ‘Arrow of Time’.

I think of the cone as being the amount of entropy present and the more complex structures outputs of that. You can easily imagine the arrow being reversed and it flowing back into the ‘Big Crunch’.

Why Does Time Flow Towards More Entropy?

The best explanation for that I’ve found is from Carrolls “From Eternity to Here”.

Basically he argue that because the universe was created in a low entropy state orders above where we are now, and so it flows towards a high entropy state.

He goes into some theories of why the universe started low entropy that are beyond me. So if you’re interested in that it’s a well written book for tackling an imposing physics subject.

Spacetime then is the ‘dimension’ of time and space woven into the fabric of the universe. It’s a smooth fabric that’s distorted by the presence of energy.

That’s why moving near the speed of light changes the movers experience of time and space. The fabric of spacetime has converged around them because of the massive quantities of energy involved. Shown below you can see how the structure of the mover changes relative to a stationary observer as the mover approaches the speed of light.

The Contraction of Matter as it Approaches Light-Speed

Below is an example of the length contraction of matter moving towards the speed of light. A 100 foot long time-space ship traveling at 60% the speed of light would contract by 20% and would become 80 feet in length. That same ship moving at 0.87c would contract by 50%.

This shows that time and space are both dynamic or not absolutes. Relative to the ‘speed’ or energy output in motion of the body moving the spacetime fabric warps around them. This doesn’t prove the existence of time absolutely but it does show that theres more than just the movement of matter at play here.

A Human Perspective of Time

Above is the physics perspective. Just to give some context lets briefly explore the human perspective of time and space.

  1. Humans don’t see particles or the fabric of spacetime. We feel the wind, see the stars distantly, hear each other laugh. We experience ‘space’ as sensory inputs like the vibration of particles for sound which our sensory organs translate into what we consider voices or the brush of someones hand and so on.
  2. Time we experience as the passing of these events. By arranging events in a linear fashion we construct a narrative of them, imbuing them with meaning and symbolism.

The Narrative Self

  1. There’s an interesting theory known as the ‘narrative self’. It holds that what defines our identity is the story that we construct for ourselves out of the events we’ve experienced.
  2. That’s sort of what ‘time’ is. It’s the sense of continuity we feel in our lives. It’s the feeling of remembering things that have happened, anticipating new things happening and then mentally organizing those into a ‘timeline’.
  3. As intelligent people continue studying and unravelling these physics issues, they’ll make new discoveries and uncover further understanding of these physics issues. Meanwhile we as people will continue experiencing time and space in the same linear, sensory manner.
  4. So in that sense it does ‘exist’ because we feel it does. It’s hard to convey that easily with words. That’s an example of ‘tacit’ knowledge and there’s still debates about whether its actually knowledge or not and whether that makes it real.

I hope that helped give you an overview of time and what constitutes it being ‘real’. I’m more confused now than when I started. I guess it’s time to direct my arrow of time towards the state of me having a drink (hehe).

Sources

Does Time Really Exist? A Physicist’s Perspective

LIGO | MIT

Albert Einstein and the Fabric of Time

The 6 Types Of Knowledge: From A Priori To Procedural

-Andrew


Written by Andrew Walls

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