Think of it this way...

Even IF I (as a twin of yours) leave here near lightspeed and come back and you are now much older than me. But there we are NOW at the same time. We are still both there in the same reference frame. I just aged slower than you due to my speed. At the point we meet, we experience time exactly the same.

CAUTION! I got demerits from the Physics and Math forum for posting a link to Miles Mathis' papers. He is not peer reviewed and his site is his publishing site. I will post the link with the warning. Here: A Revaluation of Time

Miles' views intrique until they go beyond my education. Here are the first three paragraphs. His concise expansions on this thought are at the site with many more papers on his insight to physics and even an explanation for the out of position Pioneer and Voyager spacecraft.

So without further ado, Miles Mathis.

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The most merciful thing in the world ... is the inability of the human mind to correlate all its contents... The sciences, each straining in its own direction, have hitherto harmed us little; but someday the piecing together of dissociated knowledge will open up such terrifying vistas of reality... That we shall either go mad from the revelation or flee from the deadly light into the peace and safety of a new dark age.� - H. P. Lovecraft

You hear that kind of description a lot, but I think there are actually two minor inaccuracies in it that nevertheless can be a barrier to understanding relativity. The first is that one "aged slower" than the other-- they aged at the same rate, but they simply "match up" their ages with each other differently. That "matching up" process is very bizarre, and is different for the two, and is only required to be the same matching up at the beginning and the end when they are together. It was also said that they "experience time exactly the same" at the point they meet, but in fact they always experience time exactly the same, time is time. It's not time's fault that there is a different amount of it that has elapsed between matching points. An analogy might be that you might choose to match up the beginning credits of two movies, and the ending credits of those same movies, as they are "corresponding moments in the movie". However, that neither implies that the movies must be the same duration, nor that one "happened faster" than the other.

Just going by what Carl Sagan told me in Cosmos. "It's not an illusion. Atoms really do decay more slowly the closer you get to the speed of light."

ETA: This is why they use accelerators when doing particle experiments. It actually slows the decay to a point that they can observe it before it vanishes.

Yes, I would say that Carl is promoting a misconception there, though what he is saying is not wrong depending on what is meant by the words. But taking their most natural interpretation, his words seem to cling to an idea that relativity tells us we should let go of: that there is an absolute time, such that if two people experience time differently from that absolute, one must be "sped up" or the other "slowed down"-- instead of just, they experienced a different amount of time, end of story.

Let's look at the experiment he has in mind: an unstable particle in its rest frame is likely to decay in some amount of time (sometimes quite short), but if it is moving relativistically in our laboratory, we will clock it as lasting much longer than that. This is the observable that we must agree on, the question is, what implicit assumptions do we make when we say it "decayed more slowly"? That's like saying it decayed at a slower rate, but a rate has to be referenced to some time interval. Sagan is using the laboratory clock as his reference, and there's nothing wrong with that as long as it is clear-- but one should not state it as though it were somehow absolutely true that the particle is decaying slowly. That is entirely on us, we made that true, by choosing the laboratory clock as our reference. We are choosing to "match up" time intervals on different clocks, between when the particle decays on its "own clock", and when it decays on the laboratory clock, and we must not implicitly assume that the same amount of time should have transpired on the two, such that if it doesn't, one process must have "run slow". Because if we take that mindset, we are surprised to find that in another reference frame, that same process may not be happening slowly, or may even be happening more quickly (in general relativity).

Given all this complexity, the gold standard is to use the clock moving with the process to gauge how long that process takes, and that clock is never too slow or too fast to get the answer we depend on. That's the rock of relativity, everthing else is the shifting sands of converting from that unique clock to all the other arbitrary clocks.

Yeah. I get what you are saying.

If I stood still and my friend ran circles around me near the speed of light, the same amount of time would pass for both of us, but I would age quicker.

Very very difficult to wrap minds around. But real.

I stil do believe that time is static. And how we experience it is reletive. We move "through" time...just like moving through space. The space doesn't move, nor does the time. We just go from here to there. The difference with time (as opposed to space) is that we can only move in one direction.

Actually, that is the usual perspective you hear, as in Sagan's remark. I think it is more conceptually useful to say it the opposite way-- different amounts of time would pass for both of you, but you would both age at the same rate. You would be like two movies of different length from start to finish, in which all the action happens at normal speed.
And I don't see anything wrong with that picture, it's just that we move through our own time, not someone else's. They may interpret us as moving through their time, but that is kind of an egocentric perspective.

OK.. Like 1 movie that is 2 hours long playing at SP and 1 movie that is 2 hours long playing at EP. Although the tapes are traveling at different speeds, they both end at the same time?

I get it I get it!

(i think)

If one person was going near the speed of light, and the other person was as still as possible.... and the universe ended suddenly.... BOTH would be gone at the same instant. No one gained anything.

It just "felt" like more time to the person traveling near lightspeed.

The problem is that you are still in effect embedding the playing of the tapes into your sense of time. You have to leave your sense of time behind, it is arbitrary when thinking about the tapes. The tapes are of different lengths of time, yet begin and end at the same moment. In between, they simply don't exist in the same time, but one can match up their times into pairs of simultaneous "nows" in any number of different ways, depending on what reference frame you choose to consider the situation. It just has to work out that the beginning and ending get matched up together in all reference frames, if the tapes begin and end with the same events.

There is no such thing as "the person traveling near lightspeed", there is only relative motion between the two people. Everything else is just in how they match up each other's time into pairs of simultaneous "nows", and they won't form the same matchings-- each will think the others' "nows" are lagging their own, as long as they keep separating without turning around and meeting up again (at which point they are forced into the same matching-up at beginning and end, but not at every point in between).

You are much better at explaining it than I am. But I do understand it.

The start and endpoint are the same, but the experience in between is different for the two. Longer or shorter.

An analogy if I may.

I go to bed at 10pm and my wife sits in a chair awake. Time for her lags, and I sleep until 8am. When I wake up, it almost seems instantaneous, but for her, it seemed like forever.

(I know it isn't the same thing.. just stating the way it is relative)

Yes, now you've got it. That analogy has a lot going for it. It may even be true that her body ages more than yours does-- people say animals don't age when they hibernate (I don't know how they would know that).

You may have uncovered some evolutionary truth there!

In the early years we had to sleep at night because we couldn't see in the dark. We lived longer. Then we got light (fire & eventually lightbulbs) and lived shorter and shorter. But then we got real medicine and lived longer again.

Maybe the amount we sleep (or not) DOES have to do with time-decay!

That could be true-- I really don't know anything about what makes the body decay. There does seem to be a "burning your candle at both ends" effect.

Ok.. I'm beginning to understand how everything perceives things differently ( every observer, creating a different story, as it were) and we only have to match the "nows" (and take into account the effect of motion/direction etc.? that alter the "nows" between the two observers), when they meet in the same time reference frame?...

Is it time that is causing this effect or is it the motion, curvature of space-time, direction of motion etc.?

Yes I think you are thinking along the right lines here, but note they can do the matching without actually meeting up, it's just that the meeting up is what provides an unambiguous test that they did the matching in a valid way--- it's clear enough they must agree on what is real if they are at the same place and time. If they are separated, they have to use intermediaries (typically light), and that's what leads to the discrepancies in how they interpret what is happening (even after correcting for time of flight effects).

It isn't curvature until you get into gravity, but it's an interesting question to ask "what is causing this effect". One answer is that it emerges whenever you require that two observers in a vacuum can only use light to detect a single relative velocity, not the two velocities of the two observers. That coupled with each observer having a normal concept of time, and the finite speed of light, all conspire to require the strange time behavior to "cover the tracks" that might allow you to infer two separate velocities (as you could with sound, for example).

Two observers seperated by any distance are both in each others past.

If I explain the 'Illusion' that revolves around 'simultaneity' here, I will be banned, so...

I will ask the one question that every physics major has been taught to "Ignore"...and use 'tactics' to get around having to answer it straight forwardly...

SO, Ken G, Grant, Tim, Publius...etc...here it is...

Take off in your spaceship, "At the Speed Of Light", from Earth and travel to Alpha Centauri, ~4 light years from Earth.

How is it possible that you traveled 4 light years into my past virtually instantaneously???

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RussT
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Everything is, as it should be, otherwise, it wouldn't be!