Wednesday, November 4, 2009

What exactly is time?

I came across today the following paper reviewed on the arXiv Blog and also on arXiv: Can the Arrow of Time be understood from Quantum Cosmology?

The question itself used as the title is flawed. There is no such thing as the "Arrow of Time" in Nature and after reading the paper suspect this will lead to another Not Even Wrong theory. This is a common misunderstanding in Physics so what exactly is time? It's about time we have a look. There's a wiki article however I prefer the explanation on p40 of the first volume of Motion Mountain:
"Time is what we read from a clock."
That's it! Your first intuition is correct ie you check your clock to find out what time it is. Note that clocks can be the Moon, the Sun, sundials, atomic clocks, Harrison's H1 clock etc.

Schiller goes on:

"Time is a concept introduced specially to describe the flow of events around us; it does not itself flow, it describes flow. Time does not advance. Time is neither linear nor cyclic. The idea that time flows is as hindering to understanding nature as is the idea that mirrors Page 71 exchange right and left. The misleading use of the expression ‘flow of time’, propagated first by some flawed Ref. 36 Greek thinkers and then again by Newton, continues. Aristotle (384/3–322 bce), careful to think logically, pointed out its misconception, and many did so after him. Nevertheless, expressions such as ‘time reversal’, the ‘irreversibility of time’, and the much-abused ‘time’s arrow’ are still common. Just read a popular science magazine chosen at random.

The fact is: time cannot be reversed, only motion can, or more precisely, only velocities of objects; time has no arrow, only motion has; it is not the flow of time that humans are unable to stop, but the motion of all the objects in nature. Incredibly, there are even books written by respected physicists that study different types of ‘time’s arrows’ and compare them with each other. Predictably, no tangible or new result is extracted. Time does not flow."

There you have it, if you were thinking of becoming a time traveller with exotic machinery don't waste your time. However time machines are available for purchase if you want one ;-), they only allow you to see back in time though and we are constrained to seeing the observable Universe.
Because light from the Sun for eg takes just over 8 minutes to reach us here on Earth, astronomical objects you see in the sky are essentially as they were when the light left them and because of the vastness of space and the speed of light, it takes time for the light from these objects to reach us. For eg light from our neighbour the Andromeda Galaxy takes 2.5 million years to reach us so if you look at it with a telescope (or with binoculars), you are seeing the galaxy as it was 2.5 million years ago. The bigger the telescope, the further you can see back:

At time t = 0, current Physics cannot explain what happened as the Physics cube mentioned in the previous post isn't complete. And before that? Roger Penrose gave a good talk in 2007 at Darling Harbour titled: What happened before the Big Bang?

Photos: Claudia I coming into Blackwattle Bay the other day delivering more concrete supplies.


  1. Provocative and entertaining post, Paul. Love the new blog and will link to it now from Centauri Dreams.

    Paul G.