# Time in Custom and Concept

1. Oct 25, 2009

### oldman

Time in Custom and Concept

There are tensions between the customary way one thinks of time, and concepts of time in
modern physics. Evolution has conditioned us (and probably many of our fellow creatures) to
accept that time is a mysterious and unceasing progression through our consciousness of instants that separate the past from the future. We call the most important of these instants ‘now’ and accept for practical purposes that the world is what we see around us ‘now’ --- I’ll call it the natural ‘now’. This wysiwyg approximation has served sighted creatures like ourselves well throughout evolutionary history and is now a prejudice deeply ingrained in the perspective with which we humans have come to analyse our physical environment. We are also slow-moving (compared with light), tribal and chatter a lot. These attributes may have engendered in us our widely-held belief that natural now is the same for everyone, and that time flows evenly everywhere.

But we have long been aware that this perspective is ambiguous when it comes to the quantitative assessment of time. Indeed it has probably been appreciated (since Rømer’s 1676 analysis of the motion of Jupiter’s moons) that because light travels with a finite speed your natural ‘now’ must in fact differ slightly from mine. The changing world we see around us does not look quite the same to both of us. And modern physics prefers the natural ‘now’. For a hundred years in physics ‘now’ has been defined as the instant where future and past light cones meet along an observer’s world line, as drawn on Minkowski diagrams in Einstein’s relativity. This physics ‘now’ is observer- dependent --- it is personal. For instance for antipodal persons at rest on the Earth’s surface physics ‘now’ differs by about 70 milliseconds. Nevertheless for practical purposes we accept that Greenwich Mean Time (GMT) can be used to export a standardized ‘now’ around the world, so that antipodal persons can standardize their ‘now’, in accord with the universally held belief that everyone’s natural now in the same. Here the tension between the custom of taking into account light-travel times (as in GMT) and our strongly held concept of a universal natural ‘now’ is resolved by compromise --- and the fact that it is not important enough to worry most folk.

In physics, time has to do with more than one universally believed-in instant, more than one
‘now’. It has also to do with duration --- how time intervals are measured and compared in
unfamiliar remote places and extreme situations. At a fundamental level physics --- or rather its theory of relativity --- uses the concept of the Local Inertial Frame, which to me seems to be a most ingenious and wise compromise between the concept of a personal natural ‘now’ (that of an observer at the coordinate origin) and standard GMT-like protocols for synchronising clocks and defining coordinates at points not at the origin --- elsewhere and elsewhen.

With this compromise in place it turns out that both simultaneity and duration, as measured by relatively moving observers, each equipped with their own LIF’s, depend, firstly, on their relative motions (as in the well-validated theory of Special Relativity) and second: on the distribution of mass and energy (as in the elegant theory of General Relativity). It also turns out that the compromise concept of the LIF neatly resolves tensions (between the customary way one thinks of time and concepts of time in modern physics) but only on the mesoscopic scales of time and distance we move about in, and the tranquil backwater we inhabit. Here, and on these scales, the tensions are for most practical purposes weak enough to ignore.

But when it comes to sorting out the information generated from observations of large scale
happenings in violent and remote times and places, as in cosmology, the tensions become very evident. Fortunately the tensions are matters that concern the imagination, rather than our daily lives. We needn’t worry too much about whether time flows universally or whether we live in a block universe where past, present and future may coexist forever. Unless we are physicists. Do philosophers worry about such tensions?

2. Oct 25, 2009

### vectorcube

Re: Time in Custom and Concept

Let me see if i...

Suppose we defind a light cone for the earth.

Suppose the sun exploded "now"( called this event A)

It would not have any effect on us, because event A is outside the earths light cone.

The sun  s effect on us would only come when the sun  s light cone intersect with the earth s light cone 8 minutes latter.

Great! So what is the problem?

When the sun explode, it occur ( event A), and we feels it s effect 8 minutes from event A. Why can  t i define NOW to be the point for the point in space-time for all observers in their trajactory from their past light to cone to their future light cone?

3. Oct 25, 2009

### oldman

Re: Time in Custom and Concept

You can define 'now' anyway you like. For example in SR or GR the way you define 'now' depends on how you slice up spacetime. In cosmology it is conventional (no more) to define 'now' for all observers as the an instant when you measure the same temperature for the microwave background. Here on Earth it is conventional to define now and measure time with GMT (or UT). Light cones are just a convention, too! An'd they're personal. Different folk have different light cones. But this is not what most non-physicists think.

4. Oct 25, 2009

### vectorcube

Re: Time in Custom and Concept

This is vague. Why can  t i define time t=0 as big bang, and use that as the universal "clock" for the universe?

we can slice up time in the following way: Let [0, d/c] be the time for the observable universe. where d is the radius of the observable universe, and c speed of light. we can set our clock to this universal time All of us define NOW to be d/c.

5. Oct 25, 2009

### magpies

Re: Time in Custom and Concept

I believe the reason you cant do that is because according to physics ppl there was something before the big bang and we dont know what. Or you could just be sain and say yeah sure.

6. Oct 25, 2009

### vectorcube

Re: Time in Custom and Concept

Problem i see is that we don  t have information in the form of light or anything that would tell us about what happen before the bb, and thus, we cannot set our clocks to something unknown.