What is time?

6 Nov 2011, 7:07PM

Time, everyone knows what it is, how long it takes to cook an egg, its the numbers on your watch, or it allows us to know when the News is on T.V. (6 o'clock), but few people can actually provide a definition for it. This largely leads to a lot of misconceptions.

A case in point from my last New Scientist (and I paraphrase):
"Given two identical clocks, we place one on Earth and one in a strong gravitational field (e.g. near a black hole). Time will flow differently at each point (slower in the stronger gravitational field) and the clocks will therefore differ to reflect this. Now if we can connect these two points in space-time, e.g. via worm-hole, then we can travel backwards or forwards in time."

It then goes on to discuss how the energy required to keep open the worm hole will require the entire output of a number of stars for a year. It makes me cringe. There's a number of fallacies here but the real flaw in the argument is making assumptions and then continuing to build on them with more assumptions.

Lets address the major issues, first worm-holes are theoretical and so may not exist at all. Second just because two identical clocks run at different speeds it doesn't mean that they run in the past and future. Think of a mechanical clock that is cooled until its movement (and therefore time keeping) slows. If this clock slows down it doesn't mean its suddenly in the past.

I am a firm believer that if you criticise something then you must provide a better alternative. If you can't then you shouldn't be criticising as what you are criticising is better than you can provide. Having said that lets examine what time is and provide a clear definition of it.

First, how do we measure time? We have...

- Years (time taken for Earth to complete one orbit around the sun)
- Months (time taken for the moon to orbit around the Earth)
- Days (time taken for the Earth to complete one full rotation)
- Hours (A day broken up into 24 units)
- Minutes (An hour broken up into 60 units)
- Seconds (A minute broken up into 60 units)

The first thing we notice is that these are all based around our location and our conventions. They are pretty much meaningless to another alien civilisation in another solar system, not to mention that the year, month and day are not constants, e.g. they are changing over time.

We must note here that the second has been redefined in terms of radiation from the caesium atom. This provides a definition independent of the solar system but as we will see is still dependent on something and will vary for different locations in the universe.

So how do we measure time?

We initially used sun dials, the passage of the sun across the sky providing estimates to the time of day that were good for all intensive purposes back then. We then developed mechanical clocks, which through cogs and gears kept track of the time. These would be set and then would slowly deviate depending on the craftsmanship of the watch and would need resetting. There were also pendulums (grandfather clocks). We then developed digital clocks, using electricity and crystals to keep track of time. These would still slowly deviate from the initial time.

Today we have atomic clocks, measuring the the decay of atoms which on average gives a very accurate time. There are a number of variations on this and our time keeping is now so accurate that it would have lost 1 second since the big bang, 13 billion years ago.

The accuracy stated is misleading though. We know through theory (and experiment) that an atomic clock travelling at high speed will slow down compared to one at lower speed. This creates a difficulty whereby a clock that looses only 1 second since the big bang can (theoretically) loose 1 second in a year by travelling at the right speed. Our accuracy is not really what we have stated!

The first thing to note is that all of these clocks (we'll ignore the atomic clocks and the like for now) consist of interactions between atoms (or mechanical parts). So we are really measuring time by the interacts of the constitute atoms. The atomic clock we can picture as having moving parts within the nucleus (we know neutrons and protons are not fundamental and hence they can decay) and as such are similar, just on a much smaller scale (which we know much less about).

So we have the pieces of the puzzle, why do clocks keep time differently depending on the speed (or gravitational field) they are at/in?

The answer would seem to me to be thus:

- All matter is limited by the speed of light.
- Matter within a clock, any clock of any devising, will consist of interacting sub parts. These sub parts are subject to the speed limit of light.
- Therefore if they are travelling at the speed of the light in a given direction (lets say along the x-axis for example), then THEY CANNOT TRAVEL IN ANY OTHER DIRECTION IN RELATION TO EACH OTHER. E.g. they have come to a complete stand still in relation to each other.

Maybe this seems obvious, if they are travelling at the speed of light already, they cannot move in any other direction (other than to slow down) otherwise they would be travelling faster than the speed of light.

So what does this look like to us at the macro level? All atoms have stopped moving, it looks like time has stopped! Of course it has not. Time continues and the atoms keep moving, simply they have stopped moving in relation to each other due to the speed limit of light. We can also summise that from our resting state (that of being on Earth which is not really resting) to that of travelling at the speed of light, we will have some formula or interpolation of movement from normal every day interaction between atoms (or time flow) to no movement between atoms (relative to each other).

E.g. perceived time will go from normal time flow (on Earth) to stopped as we increase the speed of the atoms in the direction of travel.

So what am I saying here?

We measure time as the interaction of atoms, relative to each other. Our own aging, the passing of the day, the time taken to cook an egg, these are all interactions of masses of atoms, bouncing off each other and so forth.

Given there is a limit to the speed of light (and all matter), (and we have lots of experimental proof that this is the case), then...

The atoms (or constituent parts) will therefore be subject to, and effected by, the speed of light limit. It will have little effect at low speeds relative to the speed of light, but increasing effect as we approach the speed of light.

Therefore the interactions between matter (atoms and smaller parts) will slow down as they approach the speed of light and their overall speed is taken up mainly in the direction they are travelling in. Since our definition of time keeping is via the interaction of atoms (or smaller parts) then we can say that (our definition of) time slows down as we approach the speed of light.

This is one outcome of Einstein's Theory of Relativity (time dilation) and has been proven to exist experimentally. But we can see that our conception of it is possibly wrong. Rather than say that space and time are inextricably linked into space time and that time slows down in large gravitational fields and at high speeds we might be better off saying that our ability to keep time (and therefore our definition of time) is tied to the speed of the constituent parts in a given direction (note not random direction, as in extremely high temperature, this is a discussion for another day).

It would seem that there is no more place for space-time than say space-wind (the value of wind speed at any given point in 3D space, confined to Earths atmosphere for the sake of my example, perhaps a poor example but trying to highlight the tying of an arbitrary value to space). It never made sense to me how a spaceship travelling close to the speed of light could warp space-time around it. In fact this idea is very strange, what is the boundary of this warping? Is it exactly the boundary of the ship, flying through space? Does space-time flow continuously from the normal time outside the ship to the slowed down time inside the ship or is their a sharp boundary?

It is better to picture this as a group of atoms (the spaceship) travelling very fast, hence the movement of the atoms (e.g. temperature is average kinetic energy or movement, blood cells flowing in your blood vessels are movement, processes of digestion require movement, everything requires movement!) will be increasingly taken up with the movement in the direction of travel, until they are travelling at the speed of light and no movement (e.g. Brownian motion) can exist in any other direction other than to slow down.

Hence we see that our definition of time is the problem. It is dependent on movement between atoms which ca be effected by certain phenomena, e.g. speed and temperature also effects time (e.g. cooling slows aging in the body). So our definition is lacking, but can we provide a better one?

It is perhaps worth pointing out here that by removing the concept of space-time we are not in any way (yet) invalidating Einsteins theory of relativity, it is simply (so far at least) an equal theory with a different premise, just like dismissing space-wind does not mean that wind speed does not exist at different locations on Earth or that they have no effect on things.