[Harri (OP)]

In general, a clock runs slower on the surface of the earth than it does higher and further away from the earth. Is it accurate also to say that 'time' runs slower nearer the surface of the earth and faster further away? Doesn't this depend on an agreed definition of 'time' ?

If I said time is the measured rate of 'change' then does change happen much slower away from the force of gravity throughout the whole universe?

[Colin2B]

For a person next to a clock on the surface of the earth time appears to run at the rate we all experience. This is also true of someone far away from the influence of earth’s gravitational field, time passes normally.
However, if the person on earth views the clock next to the person in deep space they would say it is running faster, whereas the person in deep space would say that the clock on earth is running slower. This is true for all clocks* including our body clock and aging processes, also chemical reactions and nuclear decay.

* note some clocks eg those with pendulums don’t work without a gravitational field so can’t be used for this.

[evan_au]

In developing his theories, Einstein ran many thought experiments on relativity and time dilation.

After developing his theories, he used thought experiments to illustrate them.

Illustrations about time often referred to a clock which is next to an observer (ie stationary, in the observer's frame of reference, and at the same level in a gravitational field). This observer is then able to observe clocks that are moving relative to himself, or clocks that are at different levels in a gravitational field (or both).

Today, with accurate atomic clocks, we can do these experiments and measure the results.
- Every time you use GPS to find your destination or track your position on a map, this uses Einstein's equations to calculate the right rate of the atomic clocks in orbit (which is different from those same atomic clocks, sitting on the ground before they were launched).
- Some of the most accurate atomic clocks can detect a difference in elevation of 1 foot (30cm), due to the different time dilation.
- If we could put an atomic clock outside the Solar System (away from the Sun's gravitational well), the time difference would be more extreme
- If we could put an atomic clock outside our galaxy, the time difference would be even more extreme
- We have no way of traveling outside the galaxy (or communicating with a space probe outside the galaxy) with today's technology. But physicists expect that Einstein's predictions would hold to a high degree of accuracy.
- My observation is that it is a brave physicist who bets against Einstein (except when it comes close to a black hole - Einstein knew that he didn't have a solution there)

See: https://en.wikipedia.org/wiki/Einstein_synchronisation

[Bill S]

A “timely” thought experiment

“In a galaxy far away”, there exists a star identical to our sun, with an identical “solar system”.
Earth is planet A with physicist A (PA) and clock A (CA).
Earth’s twin is planet B with physicist B (PB) and clock B (CB).
Each physicist can see his own clock and the other’s with no time delay for information transfer.

Three assumptions:

1.  PA and PB each sees time passing at 1 sec per sec on his own clock.

2.  Each physicist sees the rate of time on the other’s clock as unaffected by gravity.

3.  Any difference between the perception of the rate of time between CA and CB, observed by either physicist, will be due to the degree of relative motion between the two galaxies; or relative motion resulting from differential motion within the individual galaxies.

[Halc]

A “timely” thought experiment

“In a galaxy far away”, there exists a star identical to our sun, with an identical “solar system”.
Earth is planet A with physicist A (PA) and clock A (CA).
Earth’s twin is planet B with physicist B (PB) and clock B (CB).
Each physicist can see his own clock and the other’s with no time delay for information transfer.

This last one is unrealistic unless both clocks are stationary relative to each other, in which case both clocks will run at the same pace in any frame.

If they're moving apart as distant things tend to do, then each clock will run slower in the frame of the other, but they will appear to run even slower than that due to Doppler effect.  That Doppler effect is why is it unrealistic to assume no delay in looking at each other's clocks.

2.  Each physicist sees the rate of time on the other’s clock as unaffected by gravity.

No.  If there was another clock in the same frame as the distant clock but not as deep in a gravity-well such as these planet-bound physicists, the clocks on any planet would appear to run slower.  Hence they all appear to be affected by gravity, but equally since both CA and CB are in similar gravity wells.

3.  Any difference between the perception of the rate of time between CA and CB, observed by either physicist, will be due to the degree of relative motion between the two galaxies; or relative motion resulting from differential motion within the individual galaxies.

Yes.

[Bill S]

Thanks, Halc.  I put that post together in a hurry without due thought for the possibilities for misinterpretation.

Each physicist can see his own clock and the other’s with no time delay for information transfer.

This was badly expressed and invited your response.  I was thinking of two clocks, in different galaxies, experiencing an identical gravitational situation, and thinking that, by reason of gravity alone, the clocks would be seen to tick at the same rate, were it possible for their rate to be compared without any influence other than gravity.  (I’m not sure that’s any better).   

.  Hence they all appear to be affected by gravity, but equally since both CA and CB are in similar gravity. wells.

Rather than “unaffected by gravity”; I should have said “affected equally by gravity”.  Essentially, that’s what I was trying to get at.

Yes

Sigh of relief! 

[set fair]

For the purposes of the OP's question we can say clocks measure rates of change. A quartz crystal on earth would appear to vibrate more slowly if an astronaut could observe it from the space station. And the astronaut would see a quartz clock on earth running slower than a quartz clock on the space station.

[Bill S]

Hi, Set fair; it's always good to look back to the OP when thread drift has moved the discussion away. Mea culpa

If I said time is the measured rate of 'change' then does change happen much slower away from the force of gravity throughout the whole universe?

My “layman” thought is that time is, as you suggest, a measure of rate of change. It becomes convenient to talk of changes in the rate of time’s flow, or of the rate of our passage through time, and to treat time as though it were an entity with existence that was independent of the things being measured and those doing the measuring; which may not be the case.  It certainly streamlines scientific thought and discussion. 

Thinking along those lines suggests that change does happen at different rates in different gravitational situations; however, reference frames and observer dependence must not be overlooked.

[Colin2B]

It becomes convenient to talk of changes in the rate of time’s flow, or of the rate of our passage through time, and to treat time as though it were an entity with existence that was independent of the things being measured and those doing the measuring; which may not be the case.  It certainly streamlines scientific thought and discussion. 

The problem with using ‘rate of change’ or ‘rate of time’s flow’, is that it implies measurement against time, or that there is another deeper time flow. That’s why the clock ticks are usually used; you count the clock ticks in your frame using your time standard.

[Bill S]

The problem with using ‘rate of change’ or ‘rate of time’s flow’, is that it implies measurement against time, or that there is another deeper time flow. That’s why the clock ticks are usually used; you count the clock ticks in your frame using your time standard.

Rate of change is measured using time; why does this involve a deeper time flow?
Wouldn’t a deeper time be necessary only if time is considered as a separate entity?

[alancalverd]

Time is what separates sequential events. If we build an atomic clock, we know that the separation between sequential events is absolutely fixed by the nature of the internal fields of the atoms.

Now we place the clocks in different gravitational potentials. Each appears to run at a different rate from the point of view of the other. The difference is calculable and experimentally verifiable.

Any further discussion rapidly becomes philosophical or metaphysical and therefore pointless intellectual vanity, by definiton.

[Colin2B]

Rate of change is measured using time;

Exactly, but what time?
Best to keep things simple as @alancalverd suggests. That's why counting the clock ticks in a different frame from your own timed against your own std clock avoids the philosophical discussion on whether 'rate of time' exists or whether the 'rate of passage through time' has meaning.. Relative to you their clock is either faster or slower, whether those clocks are atomic, chemical or biological.

[Bill S]

Alan, as usual, a neat explanation, with which I have, essentially, no problem, but as a “certified and practicing” nit-picker, I have to ask about “Time is what separates sequential events.” If that is “pointless intellectual vanity”, so be it.  Julian Barbour would probably disagree.

If time separates sequential events, does this mean that something exists between events that is not, in itself an event, and that that something is time?  (Smolin)
Alternatively, does it mean that our perception of time allows us to distinguish as sequential events something that is essentially continuous? (Barbour)

[Colin2B]

If time separates sequential events, does this mean that something exists between events that is not, in itself an event, and that that something is time?  (Smolin)
Alternatively, does it mean that our perception of time allows us to distinguish as sequential events something that is essentially continuous? (Barbour)

Sequential = following in a logical order or sequence

Let’s say you are born, then you die. Those are sequential events. Hopefully they are separated by as much time as possible.
Read the above quotes in the light of those 2 events.

[Harri (OP)]

And talking of death! If I die and no longer observe time passing and experience change no longer, does time still exist?

[alancalverd]

The bit between birth and death is called life. You can count  the clock ticks if you like, but generally the earth goes round the sun between 50 and 100 times. History suggests that this sequential and repetitive event is not affected by the death of anyone.

[Bill S]

And talking of death! If I die and no longer observe time passing and experience change no longer, does time still exist?

Yes, otherwise your body would not decompose.

[Colin2B]

You can count  the clock ticks if you like,

There is probably a meditation method somewhere that suggests this

And talking of death! If I die and no longer observe time passing and experience change no longer, does time still exist?

Well, you certainly won’t experience it (other religious theories are available), but there is no reason to believe that it doesn’t.
To suggest otherwise implies you are the only person in existence and everything is in your imagination.

[Harri (OP)]

I am the only person in existence and everything is in my imagination!? I'm not sure why, but I find that quite interesting.

[yor_on]

Depends on definitions Ham.

Time dilation is a measure done from your local clock. That clock also contain your life expectancy. Relative other 'frames of reference' aka 'the universe' you might to seem aging 'slower' or 'faster', and actually you can do both of them 'simultaneously', just by changing reference frame, but relative your own wrist watch, you won't.