# Motion vs Rest: How Does Relativistic Time Differ?

• ag048744
In summary: It is interesting to think about what would happen if two observers were in different inertial frames of reference.
ag048744
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?

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.

## 1. What is the difference between motion and rest when it comes to relativistic time?

Relativistic time refers to the concept of how time is experienced differently by objects moving at different speeds. In the context of motion vs rest, the key difference is that when an object is at rest, time passes at a constant rate (as we experience it in our everyday lives). However, when an object is in motion, time appears to slow down for that object relative to an observer at rest. This is known as time dilation and is a fundamental aspect of Einstein's theory of relativity.

## 2. How does the speed of an object affect its experience of time?

According to the theory of relativity, the faster an object moves, the slower time appears to pass for that object. This means that for an object traveling at extremely high speeds (close to the speed of light), time would appear to almost stand still. This effect is known as time dilation and has been confirmed through numerous experiments and observations.

## 3. Can an object experience time differently depending on its location?

Yes, the theory of relativity also states that time can be affected by an object's location in a gravitational field. This means that time would appear to pass slower for an object in a stronger gravitational field (such as on the surface of a planet) compared to an object in a weaker gravitational field (such as in space). This phenomenon is known as gravitational time dilation.

## 4. Is motion or rest more relative when it comes to time dilation?

Both motion and rest are equally relative when it comes to time dilation. This means that an object in motion would experience time dilation relative to an observer at rest, but from the perspective of the moving object, it is at rest while the observer is in motion. This is due to the principle of relativity, which states that all motion is relative and there is no absolute frame of reference.

## 5. How does the concept of time dilation impact our understanding of time and space?

The concept of time dilation has significantly impacted our understanding of time and space. It has challenged our traditional understanding of time as a constant and absolute quantity and has shown that it is relative to an observer's frame of reference. It has also led to the development of complex theories and ideas about the nature of space and time, such as the theory of relativity and the concept of spacetime. Additionally, it has practical applications in fields such as space exploration and satellite navigation.

Replies
12
Views
1K
Replies
16
Views
1K
Replies
14
Views
1K
Replies
84
Views
5K
Replies
20
Views
1K
Replies
25
Views
3K
Replies
34
Views
2K
Replies
4
Views
1K
Replies
31
Views
4K
Replies
14
Views
1K