1. Feb 27, 2013

### peterspencers

There is a spaceship moving close to c on a journey to a planet. The observer on the planet, sees through his telescope, that the clocks on the ship appear to be running slower than on the planet. The observer on the ship, sees the clocks on the planet appear to be running faster than on the ship. Is this correct?

2. Feb 27, 2013

### bapowell

No, to the observer on the spaceship, the clocks on the planet are moving slow. The rule is "moving clocks run slow" and since there is no absolute sense of motion, both clocks appear to run slow to the observers they are moving relative to.

3. Feb 27, 2013

### ghwellsjr

No, the observer on the planet, sees through his telescope, that the clocks on the ship appear to be running faster, not slower, than on the planet. In fact, the observer on the ship will see the clocks on the planet appear to be running faster than on the ship by exactly that same amount. It's a symmetrical relationship. What they actually see through telescopes is called Relativistic Doppler.

Your question is not about Time Dilation which is what bapowell gave the answer for. Time Dilation cannot be observed through a telescope or by any other means. It is the ratio of the Proper Time (the actual time) on a clock to the Coordinate Time of an arbitrarily defined Inertial Reference Frame (IRF). Each different IRF in which you choose to describe or analyze your scenario can assign a different speed to each moving clock which assigns a different Time Dilation to each clock. So the Time Dilation of a clock is related to the IRF you select and not to what the observers can see.

4. Feb 27, 2013

### peterspencers

So both observers 'see' each others clocks as running faster than their own? when the ship arrives at the planet and de-celerates to the same IRF as the planet, the two observers then compare clocks. Assuming both clocks were in sync at the beginning of the crafts journey, do they find that less time has past on the ships clock?

5. Feb 27, 2013

### nitsuj

yuppers, do you feel the assumption about the clocks being in sync is important? If so how?

6. Feb 27, 2013

### ghwellsjr

Yes.

Last edited: Feb 27, 2013
7. Feb 27, 2013

### Staff: Mentor

The deceleration greatly changes the physical situation. You may want to search here and Google for "twin paradox", and especially work through the explanations at http://math.ucr.edu/home/baez/physics/Relativity/SR/TwinParadox/twin_intro.html

As you read the various threads, be alert for the many people (myself included, I'm afraid) who carelessly say that the observer "sees" something, when they mean that the observer sees something and then calculates a correction based on the light travel time between whatever the observer is "seeing" and the arrival of the light at the observer's eyes or measuring instruments. GHwellsjr was using the word precisely and in the first sense in his post earlier in this thread.

8. Feb 27, 2013

### peterspencers

ok, so im on the planet and I have just witnessed a clock on a spaceship, on a journey towards me, running faster than my own, so that say, 5 minutes had passed on my clock, whereas I can now see that 10 minutes has passed on the ships clock. When the ship is decelerating (say very close in the atmosphere) do I see its clocks become slower than my own, and does the ship stay decelerating for long enough such that by the time it is in my IRF, more time has passed on my clock than the ships?

9. Feb 27, 2013

### Staff: Mentor

To answer questions of this sort, it helps to imagine that the clock is a light that flashes once a second (as measured by someone who is at rest relative to the clock, which is the only way that most of us interact with clocks most of the time).

Now the question "how fast is the clock running, as I see it?" is answered by considering the frequency with which the flashes reach your eyes: If it's more than once a second the clock is "running fast" and if it's less than once a second the clock is "running slow".

10. Feb 27, 2013

### Janus

Staff Emeritus
What each visually sees goes like this:

Upon leaving his origin and achieving a near c speed with respect to the planet, the ship observer will see the planet clock run fast up until the time he reaches and comes to rest with respect to the planet.

For the planet observer, however, there is a delay between the ship leaving and his seeing the ship leave. For example if the planet is 1 light year away from the ship's point of origin, the planet observer will not see the ship leave until one year after it left. During which time, the ship will have crossed most of the distance to the planet. Once he sees the ship start its journey he will see its clock run fast. Very shortly after, the ship arrives.
He only sees the ship clock run faster for that short period between his first seeing the ship leave and the ship's arrival.

What ends up happening is that the ship observer sees the planet clock run fast for a longer period than the planet clock sees the ship clock run fast.

11. Feb 27, 2013

### nitsuj

The direction of the ships velocity is still towards you, so the frequency that you receive the "signals" is still greater than once per second (or whatever).

wow im a slow thinker Nug & Janus beat me to it. And with more detail lol

12. Feb 27, 2013

### peterspencers

Awesome :) and thankyou all for helping me. Im always blown away by how helpfull you guys are on this forum, for someone studdying this stuff in their spare time without the benefit of a university, its greatly appreciated.

13. Feb 27, 2013

### Staff: Mentor

If the ship decelerates until it is at rest relative to you (that phrase "it is in my IRF" is something you'll see a lot, but it is dangerously sloppy), then while it is at rest relative to you, you will be seeing one flash per second so will say it's running at the same rate as your clock.

The total amount of time that has passed between two events for you is the number of flashes of your clock that you've counted between the two events. The total amount of time that has passed for the shipboard guy is the total number of flashes of his clock that he has counted between two events.

In your original scenario (ship approaching from infinity) it's easy to decide when to stop counting (ship has decelerated to be at rest relative to you) but not at all clear when either of you should start counting. That's why the twin paradox is usually set up as a round trip: they start at the same place at the same time, and they end in the same place at the same time, and they each count the flashes of their clock between the two points.

14. Feb 27, 2013

### ghwellsjr

The ship isn't approaching from infinity. He said in post #4 that "both clocks were in sync at the beginning of the crafts journey."

15. Feb 27, 2013

### Staff: Mentor

Ah - right. He didn't say how that synchronization was achieved, nor specify that common starting point and I may have read too much into that omission.

As long as that omission is there, the "when to start counting flashes" question has to be pointed out.