# Synchronised clocks

## Main Question or Discussion Point

if 2 clocks are synchronised in a stationary frame of refernece and then that frame of ref is accelerated up to a constant velocity will the clocks still be synchronised ?

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tiny-tim
Homework Helper
Welcome to PF!

if 2 clocks are synchronised in a stationary frame of refernece and then that frame of ref is accelerated up to a constant velocity will the clocks still be synchronised ?
If both of them remain in the same frame, then the answer should be 'yes'. I'm no expert on Relativity, though. It's probably best to wait for a real scientist to weigh in with some answers for you.
Hi rab99! Welcome to PF!

Yes, if they're attached to a spaceship, say, and the spaceship accelerates, and then stops accelerating, the clocks should still be synchronised.

A lot depends on the method of acceleration method used. Born rigid acceleration (keeping the seperation between the the nose and the tail of the rocket at the appropriate length contracted length) and bell acceleration (keeping the seperation of the nose and tail constant in the non accelerating frame) will result in the clock going out of syncronisation. See https://www.physicsforums.com/showthread.php?t=216113

A lot depends on the method of acceleration method used. Born rigid acceleration (keeping the seperation between the the nose and the tail of the rocket at the appropriate length contracted length) and bell acceleration (keeping the seperation of the nose and tail constant in the non accelerating frame) will result in the clock going out of syncronisation. See https://www.physicsforums.com/showthread.php?t=216113
Nice work.

I didn't realize that it could go out of sync under born-rigid acceleration.

It is a little unclear from your explanation but I interpret it that both Bell and Born cause the clocks to become unsynchronised.

From examining the thread that you referred me to if:
all the appropriate parameters are measured, by the appropriate measuring device, in the appropriate frame, Then the clocks can be adjusted (say by moving the hands on the clock) to allow for the amount they became unsychronised, during the acceleration, to make them synchronised again yeh ?

Of course at a constant velocity contraction is constant and time is constant

I guess the question I want answered is , is it possible to have two clocks perfectly synchronised in a moving frame of reference yes or no? I suspect the answer is yes

I need to clarify my last post a little

From examining the thread that you referred me to if:
all the appropriate parameters are measured, by the appropriate measuring device, in the appropriate frame, Then the clocks, ONCE THEY ARE AT A CONSTANT VELOCITY, can be adjusted (say by moving the hands on the clock) to allow for the amount they became unsychronised, during the acceleration, to make them synchronised again yeh ?

AND

I guess the question I want answered is , is it possible to have two clocks perfectly synchronised in a CONSTANT VELOCITY moving frame of reference yes or no? I suspect the answer is yes

JesseM
I need to clarify my last post a little

From examining the thread that you referred me to if:
all the appropriate parameters are measured, by the appropriate measuring device, in the appropriate frame, Then the clocks, ONCE THEY ARE AT A CONSTANT VELOCITY, can be adjusted (say by moving the hands on the clock) to allow for the amount they became unsychronised, during the acceleration, to make them synchronised again yeh ?

AND

I guess the question I want answered is , is it possible to have two clocks perfectly synchronised in a CONSTANT VELOCITY moving frame of reference yes or no? I suspect the answer is yes
Well, do you understand about the relativity of simultaneity in relativity which says that different frames define what it means for clocks to be "synchronized" differently? In relativity, if I'm moving inertially and I have two clocks at rest relative to me, I synchronize them in my frame using the "Einstein synchronization convention" which is based on assuming that light moves at the same speed in all directions in my own frame--so, for example, I can set off a flash at the midpoint of the two clocks, and then as long as they both read the same time at the moment the light from the flash hits them, I define them to be "synchronized" in my frame. But it's not hard to see that if each observer synchronizes their own clocks this way, they will say that the clocks of other observers are out-of-sync. For example, say I'm on a rocket which is moving forward in your frame, and I synchronize clocks at the front and back of the rocket by setting off a flash at the midpoint and setting the two clocks to read the same time when the light reaches them. In your frame the clock at the front of the rocket is moving away from the point in space where the flash was set off, while the clock at the back of the rocket is moving towards that point, so naturally if you assume the light moves at the same speed in both directions in your frame, that means the light will actually hit the clock at the back before it hits the clock at the front, so if both clocks read the same time when the light hits them that must the two clocks are out-of-sync in your frame.

So, the short answer to your question is, yes, whenever two clocks are moving at constant velocity it is always possible to synchronize them in their own rest frame using the Einstein synchronization convention, but this will result in the clocks being out-of-sync in the frame of other observers moving at constant velocity who see the clocks moving relative to themselves.

So assuming I have an observer within the same frame as the clocks and I have an observer external to that frame (or in a different frame) it is entirely possible thru measurement and adjustment to synchronise the clocks so the observer within the frame will say they are synchronised

and

thru measurement and further adjustment to synchronise the clocks so the observer external to the frame will say they are synchronised

Of course the adjustment for the external observer will be different than the adjustment required for the internal observer

and

of course it is impossible to adjust the clocks so they are simutaneously synchronised for both the internal and external observer if the internal and external observer are in different frames ie frames travelling at a different velocity or direction relative to to each other .... is this correct ?

JesseM
So assuming I have an observer within the same frame as the clocks and I have an observer external to that frame (or in a different frame) it is entirely possible thru measurement and adjustment to synchronise the clocks so the observer within the frame will say they are synchronised

and

thru measurement and further adjustment to synchronise the clocks so the observer external to the frame will say they are synchronised

Of course the adjustment for the external observer will be different than the adjustment required for the internal observer

and

of course it is impossible to adjust the clocks so they are simutaneously synchronised for both the internal and external observer if the internal and external observer are in different frames ie frames travelling at a different velocity or direction relative to to each other .... is this correct ?
A "frame" is just a coordinate system, it isn't localized to a particular region of space, so I don't see what you mean when you distinguish "external to the frame" and "in a different frame". But other than that I'd say your correct, it's possible to adjust the clocks so they're synchronized in the rest frame of an observer who sees the clocks as being at rest relative to himself, or it's possible to adjust them so they're synchronized in the rest frame of an observer a different observer who sees the clocks in motion relative to herself, but not both (except in the special case where the observer who sees the clocks in motion sees them moving exactly perpendicular to the axis between the two clocks).

I always think of a frame as being a space ship. One person is standing on the ground observing the space ship moving and there is a person in the space ship. If the clocks are in the space ship then the pseron in the space ship is at rest relative to the clocks the person on the ground is not at rest relative. I guess the inside the space ship is a regoin of space that is a moving frame as opposed to the air just outside the skin of the space ship or the person on the ground?

The main thing is it is possible thru measurement and adjustment to make two clocks synchronised wrt any observer, regardless of the frame that they happen to be in

JesseM
I always think of a frame as being a space ship. One person is standing on the ground observing the space ship moving and there is a person in the space ship. If the clocks are in the space ship then the pseron in the space ship is at rest relative to the clocks the person on the ground is not at rest relative. I guess the inside the space ship is a regoin of space that is a moving frame as opposed to the air just outside the skin of the space ship or the person on the ground?
No, you're thinking of "frame" incorrectly. A frame is a coordinate system that fills all of space and time, which may be infinite. When physicists talk about the ship's frame, all that means is a coordinate system in which the ship is at rest--i.e. its coordinate position does not change with the passage of coordinate time in that system. A frame is not localized to any particular region of space.
rab99 said:
The main thing is it is possible thru measurement and adjustment to make two clocks synchronised wrt any observer, regardless of the frame that they happen to be in
Clocks are not "in" one frame or another, since every frame covers every region of space, including whatever region the clock is occupying. But yes, any pair of clocks can be synchronized in any frame.

If there is one coordinate system for the moving space ship and there is separate coordinate system for the person standing stationary on the ground is there a corordinate system common to both?

surely there must be as space is continuous

Are coordinate systems just a mathematical construct used to make it easier to visualise reality ?

tiny-tim
Homework Helper
… why are there no mathematicians at the North pole … ?

Are coordinate systems just a mathematical construct used to make it easier to visualise reality ?
Yes!

Don't get confused between mathematics and reality!

Mathematicians can define anything they like, to make calculation easier!

For example, there's a mathematical singularity at the North pole (of the Earth) … but physicists know there's no singularity there!

JesseM