# Basic time dilation question do I really understand it?

Hello Forum,

if one observer A is at rest, its clock measures proper time. Observer A could look at the clock on a spacecraft moving at a large fraction of the speed of light and see that the clock on the spacecraft ticks more slowly, leading to time dilation...
To look at that clock in the spacecraft, the clock is placed by one of the windows of the spacecraft....
The acting of seeing, looking at that clock on the spacecraft implies that light is going from the spacecraft to observer A.....

So if observer A and observer B (on the spacecraft) looked at each other and at each other clocks through one of the spacecraft windows, they would see each clock tick at a different time...Each observer would see the other observer clock tick more slowly....reciprocity...

IS this correct? Am I getting the concept right?

thanks
fisico30

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Janus
Staff Emeritus
Gold Member
Hello Forum,

if one observer A is at rest, its clock measures proper time. Observer A could look at the clock on a spacecraft moving at a large fraction of the speed of light and see that the clock on the spacecraft ticks more slowly, leading to time dilation...
To look at that clock in the spacecraft, the clock is placed by one of the windows of the spacecraft....
The acting of seeing, looking at that clock on the spacecraft implies that light is going from the spacecraft to observer A.....
When considering time dilation we are not concerned with the light traveling between spacecraft and observer and any observational effects caused by this are factored out.

Ok, but how does observer A, on Earth, find out that the clock on the spacecraft is ticking slower then?

Ok, but how does observer A, on Earth, find out that the clock on the spacecraft is ticking slower then?
By looking at the rocket clock and factoring out the classical Doppler shift and light travel times. What is left is the time dilation.

Alternatively, observer A places multiple clocks all over the place that are stationary and synchronised with his own local clock. He notes the time on the spacecraft clock each time it momentarily alongside one of the stationary synchronised clocks and calculates the time dilation of the spacecraft clock. This way light travel times and classical Doppler shift are not a factor, because we only compare clocks that are alongside each other.

Chestermiller
Mentor
By looking at the rocket clock and factoring out the classical Doppler shift and light travel times. What is left is the time dilation.

Alternatively, observer A places multiple clocks all over the place that are stationary and synchronised with his own local clock. He notes the time on the spacecraft clock each time it momentarily alongside one of the stationary synchronised clocks and calculates the time dilation of the spacecraft clock. This way light travel times and classical Doppler shift are not a factor, because we only compare clocks that are alongside each other.
A variation of yuiop's experimental methodology is for A to station other observers within his frame of reference along the trajectory that B follows, and for each of them to write down the time on their clocks (which are synchronized with A's clock) when B passes them, together with the time they observe on B's clock. They can then get together afterwards and compare notes. They can plot a graph of the time on B's clock as a function of the time displayed on their own clocks when he passes each of them. From this graph, they will find that the slope of the line is less than 1.

When the observer that is stationary with planet Earth looks at the clock on the moving spacecraft (he can factor the classical Doppler shift and light travel times), does he really, visually, see the clock arms moving slower?
It may not be a feasible situation, but I wonder if the observer on Earth really sees that clock ticking slower.
I know it is not, but it would seem that the spacecraft observer and earth observer "see" different things, as if it was an optical illusion or a vision impairment...

if one observer A is at rest, its clock measures proper time.
The clock observer A carries is always her proper time....."rest" or motion makes no difference.

So if observer A and observer B (on the spacecraft) looked at each other Each observer would see the other observer clock tick more slowly....reciprocity...IS this correct?
yes. Each observer sees the other clock tick more slowly...and sees the other as length contracted along the line of relative motion.

[QUOTE....I know it is not, but it would seem that the spacecraft observer and earth observer "see" different things, as if it was an optical illusion or a vision impairment... ][/QUOTE]

The hard thing to comprehend is that it IS a physical phenomenon!! It is also SEEMS an 'optical illusion' that the speed of light is always c locally; that everybody measures that as a constant.....but it,too, is REAL. In other words, no matter how fast you go, light still whizzes past you at 'c'.

A key to remember is that everybody's local clock always ticks at the same rate, their proper time, and that everybody measures lightspeed at 'c' locally [right where they are]. It is when distant comparisons are made that things 'seem' strange. So from everyday appearances, you think space and time are fixed constants, but surprise, they are not: it is lightspeed that is the 'constant'.

It took an "Einstein' to figure all this out!!!

ghwellsjr
Gold Member
When the observer that is stationary with planet Earth looks at the clock on the moving spacecraft (he can factor the classical Doppler shift and light travel times), does he really, visually, see the clock arms moving slower?
It may not be a feasible situation, but I wonder if the observer on Earth really sees that clock ticking slower.
I know it is not, but it would seem that the spacecraft observer and earth observer "see" different things, as if it was an optical illusion or a vision impairment...
You haven't specified how the spacecraft is moving relative to the Earth. If it is far away and traveling towards Earth then each observer will visually see the other ones clock ticking faster than their own. Factoring out the classical Doppler shift and light travel times is just an arbitrary calculation and does not change what they actually see.

[...]
Many of your questions were discussed in the thread "simple question regarding the twin paradox", which has by now migrated down to page 2 (on my browser, at least). I think you'll find that thread helpful.

When the observer that is stationary with planet Earth looks at the clock on the moving spacecraft (he can factor the classical Doppler shift and light travel times), does he really, visually, see the clock arms moving slower?
It may not be a feasible situation, but I wonder if the observer on Earth really sees that clock ticking slower.
I know it is not, but it would seem that the spacecraft observer and earth observer "see" different things, as if it was an optical illusion or a vision impairment...
To appreciate that its a real physical effect I like to think of a real process like a chemical reaction or an exploding bomb.
you might use the laws of physics (quantum mechanics) to calculate that it takes a second for a bomb to explode. the same laws of physics according to relativity should tell you that when you put the same bomb in a spaceship you will see it exploding for an hour (factoring out motion of the light itself)

Chestermiller
Mentor
If a flash of light goes off on the spaceship once every second, the person on earth will measure the interval between flashes as being greater than one second.

ghwellsjr
Gold Member
If a flash of light goes off on the spaceship once every second, the person on earth will measure the interval between flashes as being greater than one second.
Not if, as I said in post #8, the spacecraft is moving towards earth. If you are talking about calculating time dilation, then it depends on the arbitrary Frame of Reference that is used.

If a flash of light goes off on the spaceship once every second, the person on earth will measure the interval between flashes as being greater than one second.
The word "measure" is ambiguous. To be clear and avoid confusion, you should say "If a flash of light is emitted from a spaceship once every second in terms of the standard inertial coordinates in which the spaceship is at rest, then the time interval between flash emissions is greater in terms of the standard inertial coordinates in which the Earth is at rest (provided the spaceship is in motion in terms of those coordinates)."

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Chestermiller
Mentor
Not if, as I said in post #8, the spacecraft is moving towards earth. If you are talking about calculating time dilation, then it depends on the arbitrary Frame of Reference that is used.
ghwellsjr is correct in his comment #12 regarding my reply #11. If there is a team of observers at rest within the earth's rest frame who are deployed along the trajectory of the space ship, then when each flash goes off on the space ship, there will be an observer from the earth's rest frame present at each flash. The time interval that these two observers measure on their synchronized clocks will always be greater than the time interval measured on the space ship. This is the basic setup for time dilation. If a single observer on earth measures the time interval between flashes, he will find that the interval will be greater than the time interval measured on the space ship if the space ship is moving away, and will be less than the time interval measured on the space ship if the space ship is moving toward the earth. This is the basic setup for the relativistic Doppler effect.

Chet