# Relavance of speed to time

1. Dec 19, 2009

### Cranebale

What is the speed, of time?

2. Dec 19, 2009

### dmtr

Definitely relevant. The speed of light have very much to do with the 'speed' of the time flow.

3. Dec 19, 2009

### dmtr

Welcome to PhysicsForums, Cranebale!

You may want to try reformulating the question, or formulating it more precisely. You also may want to consider the idea that you can not measure the time directly. When you measure the time duration, in fact what you are measuring is some difference between positions, not the time (you look at the clock, you see the position of the hands).

I'm also not sure if the QM forum is the right one for that question. General Physics is likely to be more appropriate.

4. Feb 9, 2010

### lucien86

I was looking for an answer on a similar question - put precisely what is the propagation speed of time through space, specifically in general relativity.
In the past I've had conflicting information -

My original suggestion was that if Relativity restricts everything to the speed of light, shouldn't that include time itself. I was told (on PF) that this wasn't correct - that time is actually instantaneous which allows a simple solution to general non-locality and solves problems like space time coherence.
But now looking again for more detail (at WP and PF) I'm not finding any really coherent answers that totally agree with this and would like a definitive answer.

The problem of course is that if space time has no non-local component then space time has no FTL coherency and the restriction on things like FTL travel becomes rather weak, or am I wrong about this as well?

I'm not promoting any new theories here I'm just asking what is the true situation?

5. Feb 9, 2010

### Rasalhague

If by speed you mean the magnitude of the rate of change in position with respect to time, how are you defining the "position of time"?

6. Feb 9, 2010

### lucien86

Start with a point in time and space, Relativity says that the speed of light defines the maximum velocity of propagation of information in or out of that point. Does this define the velocity of time either locally or universally (ie across the whole universe) ?

The big problem is coherency, the point of simultaneity shows that different observers can experience time at different rates, but do they all experience 'now' at the same time? Logically of course they do, but I don't trust logic here, non-locality certainly explains things clearly but it hardly goes with the spirit of Relativity.

7. Feb 10, 2010

### Kracatoan

Technically, the 'speed' of time is 1 second per second everywhere. Time only changes with other factors if you compare it to somewhere else, otherwise it appears to be constant. For example, if you were travelling close to the speed of light, time for you would appear to be travelling at the normal 1 second per second from your perspective, but from someone outside your system, your time would be going much slower.

8. Feb 10, 2010

### Rasalhague

Not sure what you're getting at here.

There is no natural way of defining a universal, coordinate-independent "now" in relativity. In special relativity, take any pair of coordinate systems moving at different velocities to each other; they will not agree on which events are simultaneous unless the events happen at the same place as each other. Each coordinate system has its own three dimensional "hyperplanes" of simultaneity (hyperplanes of events with the same time coordinate).

The idea is logically consistent in relativity; what's untrustworthy in this territory is not the logic, but our naive intuition.

9. Feb 10, 2010

### lucien86

Thanks Rasalhague, that kind of answers my question. Maybe I can rephrase it this way. Is the time 'now' simultaneous, specifically in the ftl region? The answer seems to be no or rather that it is impossible to answer.

The contrary explanation I had been given was to do with the dimensional stability of general space which is probably an instantaneous variable. Making some aspect of time instantaneous is one way of allowing a simple connection between General Relativity and the universe on larger scales where the speed of light approximates to zero.

This is really a very complex question and I suspect that only a true master of the subject will be able to answer it definitively if anyone can. None of my own materials supply the answer either, maybe I should start hunting for an advanced book or two.

Last edited: Feb 11, 2010