Which is the viewpoint of the survey guy from Earth in the last few chapters of Farmer.
Before they find the walker, their boss is explaining how bad it will be and that’s why he’s staying- he’s been growing his beard out for the task.
If you bring the clocks back together, then at least one of them must have changed frames of reference. If you don’t bring them back together, then there is no meaningful way of saying which one is fast or slow.
I still don’t get this (I’m not arguing, I’m totally lost 
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I don’t know how to figure out the real numbers, but let’s just pretend the numbers I use are real (I know they’re way too high)
There are 5 atomic clocks across the globe: California, NY, Italy, Moscow and Japan. There’s one more in a plane in California. They’re all set to 6:00 am Greenwich time. The plane takes off at near lightspeed and zooms around the earth for (say) 11 hours. The pilot looks at the clock on the plane it says that it’s (this number is way too high, but let’s pretend) 4:00 pm Greenwich time. When the pilot calls the other 5 sites, they all say their clocks say it’s 5:00 pm.
First, is that right (not the math, obviously the difference would be a fraction of a second, not an hour, but the concept)?
If so, What frame of reference changed? And, I guess, what does “frame of reference” mean here?
(I love physics, but I don’t understand a lot of it. 
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And I guess I understand why, when you bring the clocks back together, they’re in the same reference frame and therefore you can tell which one moved and which one didn’t.
But couldn’t Tau Ceti and Sol be in (roughly) the same reference frame? You don’t have to travel back to Earth to compare clocks, you can compare clocks by sending radio messages that will take 8 years to complete, but since we know the distance from Tau Ceti to Sol we could easily calculate the difference in clocks and therefore which twin is older and which is younger.
Or is matching velocities with Tau Ceti what puts the two twins in the same reference frame and therefore lets them compare clocks?
In my opinion, Heinlein probably thought there were two important bits in that interchange, neither of which directly dealt with the speed of light.
The first was
This was in the protagonist’s voice, and Heinlein might have regarded it as a “teaching moment” in what is, after all, a YA novel.
The second is
which I think is classic Heinlein world-building–just a couple of words that imply a big difference.
Finally, while it is possible to read this passage as the engineer really knows the answer to the question and doesn’t feel that this is the time or place to get into it, the last couple of lines make that a pretty tortured reading. Like I said, I doubt if Heinlein regarded it as very important to his story either way.
For all his concern for realism and drive to tutor his readers, Heinlein largely predated the Forward/Clancy/ultra-hard sf era of trying to get every nut, bolt, control and theory precisely correct. The story mattered more - as it does to me, which is why I don’t read the dogged technovore school of fiction.
Graphite would be my choice for lubing a slipstick. If they’d stayed popular a bit longer, we’d have them made from teflon.
Technically, you can only compare clocks when they are in the same location, because the concept of “simultaneity” only works in relativity for “events” that occur at the same location in space and time. If you have two clocks (‘a’ and ‘b’) that are light years apart, there may very well be reference frames in which a is ahead of b, frames in which b is ahead of a, and frames in which a and b are in synch. You could choose the frame in which Earth and Tau Ceti are motionless as the “official” frame for comparing the clocks (as you suggest above), but even that doesn’t help for the time period when the twins are still moving. This side discussion should probably spawn off a detailed discussion of relativity as it applies to “Time for the Stars” (which is a book I really enjoyed, by the way) and I hope if such a thread pops up, that Chronos jumps in to provide clarity.
"Slipstick"s explanation here is great, but if we shut up when we had nothing useful to contribute the Board would collapse, so…You can add to the thought experiment that the traveling twin carries his own clock, which measures his time passing…and also carries another clock, which is programmed to calculate “current” time on Earth based on the local clock and his course and acceleration. The Earth twin can also have a computer-driven clock that shows the “current” time for the traveling twin, assuming that the traveling twin sticks to his original itinerary. Both calculated times will show the twin’s clock running slow, and the two pairs of clocks–the calculated Earth time and the real Earth time, the calculated traveling time and the real traveling time–will agree when the traveler returns to Earth (assuming the traveler sticks to his itinerary).
The problem is that there is *no way *to compare the clocks while the traveling twin is still en-route. If you have more than just simple grammar-school calculus I could explain it to you (or somebody else could, at least) but briefly, if there was any way to communicate instantly between different frames of reference, there would be a way to send a message backwards through time, which would threaten to wreck the comforting linkage of cause to effect. The Eschaton frowns on that kind of thing, so don’t do it.
You’re saying telepathy implies time travel?
Yes; faster than light travel would also allow travel backwards in time. Faster than light communication would allow communication backwards in time.
Telepathy that traveled *at the speed of light *or less would not violate relativity, of course.
Or another way to put it is that time travel will allow you to travel faster than light, since you can travel to Tau Ceti at 99% the speed of light for 8 years, then set your Wayback machine for 8 years in the past, and hey, you arrive at Tau Ceti in only a week!
Or to put it a third way, time travel/communication and faster than light travel/communication are instances of the same sort of thing, and both would break causality if they are possible.
Not sure I follow. Instantaneous telepathy saves you the travel time required by a radio message or light beam.
But it’s a fixed amount of displacement, calculated on the speed differential. That’s not time travel, and it’s not exactly “communicating backwards in time”.
So I must be missing something- what is it??
Communicating instantly with someone 8 light years away is the same as communicating with someone 8 years in the past.
No, it just redefines “now” outside of the way we understand relativity. Einstein might be wrong, you know. It’s happened to better people. 
Ok good. But you only get that one exact displacement, right? You don’t get to “choose where you re-enter the time axis”?
Well, it’s not quite that simple. But if you can communicate “instantaneously” in your reference frame with someone 8 light years away, then there is some frame of reference where you’re communicating with someone up to 8 years in the past. And then there’s no reason that they can’t, in an appropriate reference frame, pass the message on to someone even further in the past, and so on.
“Is some frame of reference” doesn’t apply- there’s only one other telepath. You can choose one or the other but not both and not an arbitrary frame.
Assuming telepathy is irrelevant to spacetime and instantaneous I mean. As the author asserts in the novel. Right?
One twin is eight years in the past of the other, by any einsteinian communcation method?
Heinlein’s telepathy method involved close relatives. There was an oddity where age made a factor, and one could become “in tune” with a new relative. But not just anybody.
If one could tune in to anybody, or multiple partners, then in theory one could stack telepathic links, run a telepath relay.
The telepaths only being attuned to specific individuals doesn’t matter, since telepathy isn’t the only form of communication available. The first telepath tells his twin something through the link, then that twin tells another, unrelated telepath using ordinary speech, and then that other telepath tells his twin through his link. Presto, relay.