# Confused about time slowing down

Ok, before I go any further, let me ask this: Why do the clocks on Earth and Planet X HAVE to be out of sync? What if I have someone on a planet that is equidistant from both Earth and Planet X send simultaneous signals to both planets to set their clocks together? The signal would take the same length of time to reach Earth as it would to reach Planet X, so that both clocks would be set to the same starting time.
Yes, but if the clocks on earth and the other planet at rest in the same frame are synchronised to an observer (planet/transmitter) also at rest in that frame, then they cannot be synchronized as far as an observer moving relative to them is concerned. This is absolutely basic to SR. Even if you could arrange to start them at the same time according to some observer moving relative to them, they would not remain synchronized for that observer.

Matheinste.

Yes, but if the clocks on earth and the other planet at rest in the same frame are synchronised to an observer (planet/transmitter) also at rest in that frame, then they cannot be synchronized as far as an observer moving relative to them is concerned. This is absolutely basic to SR. Even if you could arrange to start them at the same time according to some observer moving relative to them, they would not remain synchronized for that observer.

Matheinste.
All that my scenario requires is that when the traveler "passes" a point on Planet X, his AGE at that point can be documented. If so, then the calculations can be made back on Earth (I don't care WHEN they're made) to determine whether the traveler aged slower than the stay-at-home DURING his constant velocity trip.

All that my scenario requires is that when the traveler "passes" a point on Planet X, his AGE at that point can be documented. If so, then the calculations can be made back on Earth (I don't care WHEN they're made) to determine whether the traveler aged slower than the stay-at-home DURING his constant velocity trip.
If the earth (for our present purposes) and the traveller are each at rest in inertial frames of reference which are moving relative to each, then being at rest or moving is meaningless.

Matheinste.

Janus
Staff Emeritus
Gold Member
Ok, before I go any further, let me ask this: Why do the clocks on Earth and Planet X HAVE to be out of sync? What if I have someone on a planet that is equidistant from both Earth and Planet X send simultaneous signals to both planets to set their clocks together? The signal would take the same length of time to reach Earth as it would to reach Planet X, so that both clocks would be set to the same starting time.
Again, this is only true for someone at rest with respect to Earth and Planet X.

This will not be true as far as the Traveler is concerned. Remember, the speed of light is a constant relative to any inertial frame. Thus according for the traveler, the two signals travel at c relative to him (not Earth and Planet X). Thus Earth is fleeing away from its signal and Planet X is rushing towards its signal. Planet X gets its signal first, and its clock starts running from the Agreed upon starting value first. By the time that Earth has gotten its signal, quite a bit of time has already past on Planet X.

Janus
Staff Emeritus
Gold Member
What happens if the ship stops at planet X, no longer is earth fleeing away at .866c, so a signal sent will take 10 years by the travellers clock and 10 years by the earth clock, arriving only 12.88 years after departure?
Then the ship has undergone an acceleration and changed inertial reference frames. Form the Traveler's perspective, the Earth clock jumps forward to read the same as Planet X's clock.

Again, this is only true for someone at rest with respect to Earth and Planet X.

This will not be true as far as the Traveler is concerned. Remember, the speed of light is a constant relative to any inertial frame. Thus according for the traveler, the two signals travel at c relative to him (not Earth and Planet X). Thus Earth is fleeing away from its signal and Planet X is rushing towards its signal. Planet X gets its signal first, and its clock starts running from the Agreed upon starting value first. By the time that Earth has gotten its signal, quite a bit of time has already past on Planet X.
That doesn't make sense to me at all. Let's forget a traveler for the moment. I can visualize a clock on Earth and a clock on Planet X being set in sync simply by having simultaneous signals sent from a THIRD planet that is equidistant from both Earth and Planet X. Forgetting travelers, forgetting relativity, forgetting everything, I can do that! Surely, you will allow that scenario.

That doesn't make sense to me at all. Let's forget a traveler for the moment. I can visualize a clock on Earth and a clock on Planet X being set in sync simply by having simultaneous signals sent from a THIRD planet that is equidistant from both Earth and Planet X. Forgetting travelers, forgetting relativity, forgetting everything, I can do that! Surely, you will allow that scenario.
If you forget relativity then your question is no longer meaningful! You can add an infinite array of intermediate planets, and your "difference" will become smaller and smaller at each step, but it's never eliminated. What you would require is that light not be the upper 'speed limit' at work, and have a means to send signals faster than light. That in essence, removes the entire need for and concept of, Relativity.

Janus
Staff Emeritus
Gold Member
That doesn't make sense to me at all. Let's forget a traveler for the moment. I can visualize a clock on Earth and a clock on Planet X being set in sync simply by having simultaneous signals sent from a THIRD planet that is equidistant from both Earth and Planet X. Forgetting travelers, forgetting relativity, forgetting everything, I can do that! Surely, you will allow that scenario.
I never said that you couldn't. The problem is that not everyone will agree that the Earth clock and Planet X clock are in sync.

Much of Relativity doesn't seem to make sense when compared to everyday experience.

We expect it to be the same time in Atlanta and New York regardless of whether we are standing along the road between them or driving along the road between them. Relativity says otherwise. It says that if you are driving from one to the other, it will not be the same time in the two cities. We just don't notice it because at the speed of a car the difference is too small to measure.

Relativity requires that we rethink how we consider time and space and toss out some of the notions about it that we got by observing the slow moving world around us.

I really think the link I gave to The Elegant Universe's relativity section is a pretty simple and straightforward explanation. *shrug*