# I can't understand time dilation in absence of absolute motion.

• Soumya_M
In summary, according to the theory of relativity, there is no absolute standard of rest. When two observers, Alice and Bob, are floating in space at speeds comparable to that of light, their time will be dilated in both frames. Both frames will have time dilation with respect to each other. When Alice and Bob compare their clocks, they will see the other person's clock running slower. This is because time dilation is relative, meaning that it depends on the observer's frame of reference.
Soumya_M
According to theory of relativity there is no absolute standard of rest. Now, if Alice and Bob are two observers floating in space, so that Alice thinks that she is at rest and Bob is moving away from her at a speed comparable to that of light and Bob thinks that he is at rest and Alice is moving away from him at a speed comparable to that of light, whose time will be dilated?

Soumya_M said:
whose time will be dilated?
In which frame?

Soumya_M said:
whose time will be dilated?
The other person's.

DaleSpam said:
In which frame?

In both the frames. Could it be that both frames have time dilated with respect to each other? How does time dilation make sense that way?

Doc Al said:
The other person's.

What will happen if Alice and Bob compare their clocks? Alice's clock will be faster than Bob's or the other way? If both show the same time - where is the dilation?

Soumya_M said:
What will happen if Alice and Bob compare their clocks? Alice's clock will be faster than Bob's or the other way? If both show the same time - where is the dilation?
The only way for them to directly compare their clocks is for one or both of them to turn around and join the other. That changes everything. (Look up the 'twin paradox'.)

But the important point is that between inertial frames the relativistic effects (time dilation, length contraction) are completely symmetric. To ask which clock is really dilated is to miss the point.

You cannot have "absolute time" if you don't have "absolute speed". If you have relative speed then you must have relative time.

That is, Alice, observing Bob's clock as he goes whizzing past her, sees Bob's clock as running slower then hers. Bob, observing Alice's clock as she goes whizzing past him, sees her clock as running slower than his. That is what "time dilation" means.

Soumya_M said:
Could it be that both frames have time dilated with respect to each other?
Yes.

Soumya_M said:
How does time dilation make sense that way?
Because it is relative, as HallsOfIvy mentioned.

## What is time dilation?

Time dilation is a phenomenon in which time appears to pass at a different rate for an object or observer in motion compared to one at rest. This is due to the effects of relative motion and is a key concept in the theory of relativity.

## How does time dilation occur?

Time dilation occurs due to the fact that the speed of light is constant in all frames of reference. This means that as an object or observer moves faster, time appears to slow down for them in relation to a stationary observer. This effect becomes more significant as the speed of the object approaches the speed of light.

## What is the role of absolute motion in time dilation?

Absolute motion does not play a role in time dilation. This is because there is no such thing as absolute motion in the theory of relativity. Motion is always relative to a particular frame of reference, and it is the relative motion between two frames that causes time dilation.

## How is time dilation measured?

There are several ways to measure time dilation, including using atomic clocks or observing the decay rates of particles moving at different speeds. These methods have been used to confirm the predictions of the theory of relativity and have shown that time dilation does indeed occur.

## Why is understanding time dilation important?

Understanding time dilation is important because it is a fundamental concept in the theory of relativity and has been proven to be true in numerous experiments. It also has practical applications, such as in GPS systems, where the effects of time dilation must be taken into account for accurate measurements.

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