[Physics] Clocks in relativity

[Thomas Goodey]

On 14 Nov 2016 at 12:00, physics-request at tuks.nl wrote:

> Einstein says that "moving clocks run slow". 

He doesn't say that. He says that, from the point of view 
(i.e. in the reference frame) of observer A, a clock moving 
with respect to observer A runs slow.

> Just using the
> rules and logic of SRT, please tell us which clock is
> running slow, and why that one. 

Each one of two mutually moving clocks is running slow, in 
the reference frame of the other.

> Until that question is
> answered, I stick by my assertion that the time dilation is
> illusory, and quite simply caused by perspective.

Depends what you mean by "illusory". This behavior is a 
brute fact.

> GRT says that gravity increase causes the slowing
> of clocks which use the speed of light as their basic time
> keeping mechanism. 

As the best clocks do.

> Use a pendulum clock, and you will find
> that time speeds up (more precisely the clock 
> speeds up) as
> gravity increases.

Well, obviously. But, in the context of a changing 
gravitational field (changing acceleration), a pendulum 
clock is a very poor clock. This remark is so obvious as to 
be a stupid one.

> Use a stellar clock, ie a star passing
> the meridian (or any other reference point) every 24 hours
> apx. , and you will find that that clock is unaffected by
> gravity. 

Actually it is the meridian that passes the fixed star.

And you are wrong. If, for example, you are on the surface 
of the Earth (thus in a gravitational field), and then the 
Moon gently coalesces with the Earth (thus increasing the 
gravitational field that you are in), you will find that 
the rate of this "meridional clock" changes.

Thomas Goodey
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