Motion vs Rest: How Does Relativistic Time Differ?

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Discussion Overview

The discussion revolves around the relativistic effects of time as experienced by observers in different inertial reference frames, specifically comparing a moving observer in a plane to a stationary observer on the ground. The conversation explores the implications of these perspectives on time measurement and the effects of acceleration and gravity.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Some participants propose that the observer in motion will perceive time on the ground as slowed down due to their relative velocity, while the ground observer will see the clock in the plane as slowed down.
  • Others argue that the interaction between the two observers upon reuniting raises questions about their respective measurements of time and relativistic effects.
  • A later reply introduces the complexity of Earth's rotation, suggesting that a plane flying west at the same speed as Earth's rotation would be effectively at rest, complicating the time measurement comparison.
  • One participant references the Hafele-Keating experiment, which involved atomic clocks on aircraft, to illustrate real-world measurements of time differences due to relativistic effects.
  • Another participant suggests simplifying the scenario by assuming an infinitely flat ground and no gravitational effects, questioning what the observers would conclude about their measurements.
  • Further discussion includes a hypothetical scenario of spaceships in space, where one travels in a circular path and returns, leading to a conclusion that the traveling clock would show less elapsed time.
  • There is a query about the effects of minimal non-inertial maneuvers on clock differences, with some asserting that significant speed or distance is necessary to observe measurable differences.
  • One participant reflects on the implications of non-inertial acceleration on symmetry in spacetime diagrams, drawing a parallel to the twin paradox.

Areas of Agreement / Disagreement

Participants express multiple competing views regarding the effects of motion and acceleration on time measurement, and the discussion remains unresolved with no consensus on the implications of these relativistic effects.

Contextual Notes

Limitations include assumptions about the nature of the ground and the plane, the effects of gravity and acceleration, and the specific conditions under which measurements are taken. The discussion also highlights the complexity introduced by non-inertial frames.

ag048744
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There are two separate clocks, each set in a plane and on the ground. Assuming inertial reference frames, how can this be?
Perspective of the observer in motion:
The observer in motion on the plane will have recorded some time duration. Since the the world outside the plane is moving at a velocity relative to the plane, time on the ground would have slowed down.

Perspective of the observer on the ground:
The observer at rest would have seen the clock inside the plane slowing down since it moving relative to the ground.

Observer vs observer
When the two observers do meet again, what will they have to say about each other's observations? Would they have measured the same relativistic effects? Would the pilot say that the ground observer's clock had slowed down, or would the ground observer say that the pilot's clock had slowed down?
 
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ag048744 said:
There are two separate clocks, each set in a plane and on the ground. Assuming inertial reference frames, how can this be?
Perspective of the observer in motion:
The observer in motion on the plane will have recorded some time duration. Since the the world outside the plane is moving at a velocity relative to the plane, time on the ground would have slowed down.

Perspective of the observer on the ground:
The observer at rest would have seen the clock inside the plane slowing down since it moving relative to the ground.

Observer vs observer
When the two observers do meet again, what will they have to say about each other's observations? Would they have measured the same relativistic effects? Would the pilot say that the ground observer's clock had slowed down, or would the ground observer say that the pilot's clock had slowed down?

It's a good question, but thinking about planes makes it difficult, because there's also the rotation of the Earth to take into account. Imagine a plane flying West as fast as the Earth was spinning, then that plane would effectively be at rest, while the clock on the ground would be orbiting the centre of the Earth.

There is a famous experiment that put atomic clocks on commercial aircraft and measured exactly what happened to the clocks relative to the clocks on the ground:

https://en.wikipedia.org/wiki/Hafele–Keating_experiment
 
Oh I see. Let's just assume that the ground is infinitely flat and the plane and the ground are inertial frames of reference. no mass, no gravity and just a centripetal acceleration for when the plane starts to return to the ground observer. What would the observers have to say, when they compare measurements?
 
ag048744 said:
Oh I see. Let's just assume that the ground is infinitely flat and the plane and the ground are inertial frames of reference. no mass, no gravity and just a centripetal acceleration for when the plane starts to return to the ground observer. What would the observers have to say, when they compare measurements?

Take it a step further: space ships in space far from Earth. One sets off on a circular trip and returns to the starting point. The other remains "at rest". The one doing the circular trip will find its clock shows less elapsed time upon its return.
 
What if the spacecraft turned around as minimally as possible, to the point that non inertial effects are negligible?
 
ag048744 said:
What if the spacecraft turned around as minimally as possible, to the point that non inertial effects are negligible?

There is no mimimum. If the spacecraft did a short, slow circular manoeuvre, the difference in the clocks would be negligible. In order to see any measurable difference you would need significant speed and/or time/distance travelled.

In the Hafele-Keating it was less than 300 nanoseconds difference over about 50 hours commercial flying.
 
Hmm I guess there is no way to say as nature does not allow it. The space time diagrams would have lost symmetry while undergoing a noninertial acceleration. It is a similar situation with the twin paradox.
Thank you for your assistance, PeroK. This curious thought has always come up when I study relativity.
 

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