Thread: time dilation -confusion

according to relativity if a person takes a rocket and say travels near the speed of light moving towards a particular direction,then time slows down for him and the rocket.
But can't i say that the the rest of the universe say the earth and all other stuff moves at the above speed in the opposite direction with respect to the rocket and hence time should slow down for us and not for the rocket ( or basically from our new perspective time speeds up in the rocket).

is it that since mass of the universe >> the mass of the rocket that the later does not hold true.
But even then doesn't this imply a correction to the current equation for time dilation by considering a ratio of mass of the rocket to that of the remaining part of the universe.
Basically if hypothetically the rocket had half the mass of the universe , the time flow in the rocket and the earth should still be the same with respect to each other.

But can't i say that the the rest of the universe say the earth and all other stuff moves at the above speed in the opposite direction with respect to the rocket

Yes, you can say that. The two frames of reference are symmetric, i.e. they are interchangeable.

hence time should slow down for us and not for the rocket ( or basically from our new perspective time speeds up in the rocket).

No. From the point of view of the earth, the clock inside the rocket is dilated. From the point of view of the rocket, the clocks back on earth are also dilated. The two frames are symmetric, and thus experience the exact same laws of physics.

But even then doesn't this imply a correction to the current equation for time dilation by considering a ratio of mass of the rocket to that of the remaining part of the universe.

Relative time dilation depends only on the state of relative motion, not on masses.

Basically if hypothetically the rocket had half the mass of the universe , the time flow in the rocket and the earth should still be the same with respect to each other.

If the rocket had half the mass of the universe, it would collapse into a black hole

No. From the point of view of the earth, the clock inside the rocket is dilated. From the point of view of the rocket, the clocks back on earth are also dilated. The two frames are symmetric, and thus experience the exact same laws of physics.

you are contradicting the very theory you support,if the rocket and the rest of the universe ( ie two objects under consideration) show time dilation independent of their mass and obviously have the same speed with respect to each other then as you said Relative time dilation depends only on the state of relative motion, not on masses.
this implies time dilation occurs equally in the two bodies equally. Thus an atomic clock in the rocket and one in the earth should have the same timing (which contradicts time dilation).
time dilation theory obviously fails if you don't consider mass wrt that of the remainder of the universe.

If the rocket had half the mass of the universe, it would collapse into a black hole

This is just a shady way to get around the original question

Originally Posted by vinoo

you are contradicting the very theory you support

No I am not. Everything I said is exactly in accordance with the model of relativity.

if the rocket and the rest of the universe

The "rest of the universe" is not a valid frame of reference; it is better to choose a fixed point such as a distant star or galaxy.

you said Relative time dilation depends only on the state of relative motion, not on masses.

Yes, that is what I said. Take note of the term "relative time dilation".

this implies time dilation occurs equally in the two bodies equally

No, it doesn't imply that at all. You see, there are two very distinct types of time dilation - one is relative time dilation, which is a coordinate effect, and the result of relative motion only ( hence does not depend on masses ). The other type is gravitational time dilation, which is a result of the presence of energy-momentum ( mostly mass-energy, but other forms of energy can play a role, too ). The total time dilation an observer sees is the sum total of these two distinct contributions. Note also that this will depend where exactly the observer and the observed are located.

Thus an atomic clock in the rocket and one in the earth should have the same timing (which contradicts time dilation).

Total time dilation in this scenario depends on both the observer ( relative motion, and location inside the gravitational field ), and masses. If the rocket is moving slowly and is far away, then the clock at the earth's surface will measure less proper time due to gravitational time dilation. It is very important here also to distinguish between proper time and coordinate time.

time dilation theory obviously fails if you don't consider mass wrt that of the remainder of the universe.

Rest mass ( or more generally the energy-momentum tensor ) is a covariant object, so it is the same for all observers. You don't need the rest of the universe to calculate gravitational time dilation; you only need to know the components of the energy-momentum tensor in a particular frame. If that is a rest frame, and the object does not possess charge or angular momentum or any kind of stresses, then quite simply the total mass will suffice.

This is just a shady way to get around the original question

Not at all. Despite the smiley I was being entirely serious - if you concentrate half the mass of the universe into a region the size of a rocket, you will end up with a gravitational collapse and a supermassive black hole. However, time dilation follows the same laws even in that scenario, so it is really quite trivial.

Originally Posted by vinoo

No. From the point of view of the earth, the clock inside the rocket is dilated. From the point of view of the rocket, the clocks back on earth are also dilated. The two frames are symmetric, and thus experience the exact same laws of physics.

you are contradicting the very theory you support,if the rocket and the rest of the universe ( ie two objects under consideration) show time dilation independent of their mass and obviously have the same speed with respect to each other then as you said Relative time dilation depends only on the state of relative motion, not on masses.
this implies time dilation occurs equally in the two bodies equally. Thus an atomic clock in the rocket and one in the earth should have the same timing (which contradicts time dilation).
time dilation theory obviously fails if you don't consider mass wrt that of the remainder of the universe.

You are falling into the trap a lot of people do when they encounter the concept of time dilation; You are trying to treat it as an isolated effect. This is not the case. there are two other aspects of Relativity that have to be accounted for: Length contraction and the Relativity of Simultaneity.

Here's how they come into play:

From the point of view of the Earth, your ship, as it travels to its destination undergoes time dilation and length contraction. If we assume that the clocks at the destination are in sync with the Earth clock, then, as far as the Earth or destination is concerned, you can just take the difference between clock readings at departure and arrival to get the time of the trip in this frame. The ship, being time dilated, will show less time. Thus if the time dilation factor is 1/2, and the trip takes 2 years by the Earth's clock, 1 year will pass on your clock.

Now from the point of view of your ship. For one thing, it will the Earth and destination that undergoes length contraction, and this includes the distance between them. The length contraction factor is the same as the time dilation factor. So for instance, if the time dilation factor is 1/2, you will measure the distance between Earth and the destination as being halved. Thus, for you, the trip takes less time by your clock because the distance of the trip is shorter.

Secondly, the clocks on Earth will be time dilated as compared to your own clock. Again using the same factor as above, they will run 1/2 as fast as yours. If you measured 1 year for the duration of the trip, 6 months will pass for the Earth and destination Clocks.

Thirdly, there will be the effect of the Relativity of Simultaneity. This can be the hardest concept to wrap your head around. Basically what it means is that if the Earth and destination clocks are in sync in their own frame, they will not be so for your ship. The clock at the destination will be 1 1/2 years ahead of the Earth clock. In other words, if it is Jan 1, 2014 at both the Earth and the destination according to the Earth, then according to you, when it is Jan 1, 2014 on Earth, it will already be ~Jul 1, 2015 at the destination.

So for you, you leave Earth when its clock reads a certain time, and you arrive at the destination when its time reads 1 1/2 +1/2 = 2 years later, while having aged 1 year. This is exactly the same end result as seen by the Earth, but it was arrived at by a different means.

Actually, if you consider the frame of reference in which observer A is travelling at , and observer B is travelling at , and consider which time intervals are actually being compared when A observes B to be time dilated and B observes A to be equally time dilated, you'll see that there is no contradiction at all. [A spacetime diagram would help, but I don't have one to display].

What elementry structures does time dialation affect the most.

It affects all processes.

Twin paradox - Wikipedia, the free encyclopedia

All bullshit. Time progresses at it's own rate, observable by an individual in one frame of reference, but excluded from the individual in the next frame over. Thus, each "observer" is unaware of the "rate" at which the other's progresses.

What do you think of that? jocular

This evokes in me thoughts of:
Einstein on the illusion of simultaneity...
The ability to see something limited by the time light takes to reach the eye (ignoring brain processing)...
The empirical evidence of time dilation....
The fact that the actual ability to perceive each other's clocks is irrelevant, because the effect happens regardless...
If the effect were severe enough, one could see the rate of change increasing significantly outside of one's own local reference frame in distant space... (imagine yourself standing in an area of extreme gravity, such that you experience one second for every year experienced on earth, while the solar system was close enough for you to see it... and for the sake of not muddling up this thought experiment, this hypothetical extreme gravity is not affecting what you are looking at because you are using a super telescope to view the solar system, and are extremely far away, and you have the super power of being able to withstand extreme gravity)...

Your super telescope also magically adjusts for blueshifting of light, and the distortion caused by the light's path being affected by the extreme gravity. To make the observation of the solar system meaningful to you, rather than say, a bright dot.