I Is time really moving backwards?

[darth boozer]

"The closest I can make is to observe that Monday morning comes at different times for frames in motion with respect to the Earth."

Monday morning comes between 11 and 13 hours (depending on time of year) earlier in New Zealand than in UK, both of which are not in motion with respect to the Earth or each other (ignoring plate techtonics).

 

[Ibix]

Monday morning comes between 11 and 13 hours (depending on time of year) earlier in New Zealand than in UK, both of which are not in motion with respect to the Earth or each other (ignoring plate techtonics).

A more precise statement of the point I was making is that frames in relative motion have different notions of simultaneity. That means that what one frame calls "now" another frame will call partly in the past, partly in the future, and partly now. This is not the same concept as different locations that share a notion of simultaneity having different names for the same time, which is what your example is.

 

[jbriggs444]

A more precise statement of the point I was making is that frames in relative motion have different notions of simultaneity. That means that what one frame calls "now" another frame will call partly in the past, partly in the future, and partly now. This is not the same concept as different locations that share a notion of simultaneity having different names for the same time, which is what your example is.

To make this a bit more concrete, suppose that I have a Zoom conference at 8:00 am EST and my colleague in London joins the meeting at 1:00 pm GMT.

Per my time coordinate system (EST) we both join the conference simultaneously at 8:00 am EST.
Per my colleague's time coordinate system (GMT) we both join the conference simultaneously at 1:00 pm GMT.

From the perspective of time zones, we both agree that we both joined simultaneously [to within 20 milliseconds or so anyway]. Relativity of simultaneity is not a problem.

 

[etotheipi]

From the perspective of time zones, we both agree that we both joined simultaneously [to within 20 milliseconds or so anyway].

I'm not sure I understand this remark as written; certainly the spatially separated clocks will appear desynchronised to an observer with relative speed $v$ to the Earth by $\delta t = Lv/c^2$, and if $v \sim c$ then this is $\delta t \sim L/c \sim 10^{-2} \, \mathrm{s}$ between the UK-clock and the USA-clock. Is that what you mean by 'from the perspective of time zones'?

 

[jbriggs444]

I'm not sure I understand this remark as written; certainly the clocks will appear desynchronised to an observer with relative speed $v$ to the Earth by $\delta t = Lv/c^2$, and if $v \sim c$ then this is $\delta t \sim L/c \sim 10^{-2} \, \mathrm{s}$ between the UK-clock and the USA-clock. Is that what you mean by 'from the perspective of time zones'?

The two joins to the Zoom conference are off by only 20 milliseconds at most, no matter what frame one chooses. Not the 5 hours that the time zone offset calls for.

At a relative velocity of only 1000 miles per hour or so and nearly at right angles to the displacement, the actual discrepancy between a Washington, DC frame and a London England frame's assessment of the delta T between the joins would be far less than even the 20 milliseconds.

[Just checked. I'm seeing 79.8 ms RTT to London right now from Nashua, New Hampshire. Not bad. It's within 50% of light speed]

 

[PeterDonis]

A more precise statement of the point I was making is that frames in relative motion have different notions of simultaneity.

Even more precisely, inertial frames in relative motion have different notions of Einstein clock synchronized simultaneity.

The qualifiers are important, as will be apparent in a moment.

This is not the same concept as different locations that share a notion of simultaneity

But the notion of simultaneity that is shared by people at different locations on the Earth, that let's them all call into a Zoom call at the same time even though they are in widely different time zones, is not the same notion of simultaneity that inertial frames have. It is not based on Einstein clock synchronization. It can't be, because it is impossible to use Einstein clock synchronization to establish a global notion of simultaneity for a rotating family of observers. (In more technical language, it is impossible to use Einstein clock synchronization to establish a global notion of simultaneity for a family of observers whose worldlines form a congruence with nonzero vorticity. Observers at rest on the rotating Earth are such a family of observers.)

certainly the spatially separated clocks will appear desynchronised to an observer who is using the Einstein clock synchronization notion of simultaneity for the inertial frame in which he is at rest

See the qualifier in the quote above. Clocks at rest on the rotating Earth and using the common time standards that we all use for our clocks will appear desynchronized in any inertial frame that is moving relative to the center of the Earth, if we use that frame's notion of simultaneity. The clocks we use on Earth are simply synchronized using a different notion of simultaneity (roughly, the notion of simultaneity of the inertial frame in which the center of the Earth is at rest, with the clock rate adjusted to the rate on the geoid).