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Can two people who are travelling in opposite directions through time communicate?

Paul Nahin (Time Machines. 1993) draws on the work of Murray MacBeath to demonstrate how a scenario in which two people, travelling through time in opposite directions, might be able to communicate. His two characters, Jim and Midge are separated by a window.

“Now, at t0 Jim brings a computer to the window. He programs it to wait for four days, until t4, and then to display the following message on the screen: ‘This message is from Jim, who experiences time in the sense opposite to yours. Please study the following questions and display your answers on a computer screen three days from now.’

Diagram .1 (Nahin’s Figure 9)

earlier............Jim (normal time)................later
t0...........t1............t2.............t3..........t4

T4...........T3............T2.............T1..........T0
LATER..............MIDGE (reversed time)............EARLIER


Jim’s message ends with a list of questions. Since this will appear at t4, Midge sees it at what we will now call T0. As requested, she brings her computer to the window, enters the answers to Jim’s questions, and programs the machine to display them (after a three-day delay) on its screen. Thus, at T3, which is Jim’s t1, Jim sees Midge’s computer screen light up with: ‘Hi, Jim. This is Midge. The answers to your questions are at the end of this message. Now, I’ve got some questions for you. Please display the answers two days from now.’ Midge’s message ends with Jim’s answers and her list of questions. Jim sees Midge’s message at t1, enters the answers to her questions and sets the machine to answer after a two-day delay. At t3, which is Midge’s T1 – and by now you see how the process goes. It’s cumbersome, but it does work.”

I have found a few reasons why I think this would not work, but would be interested to know the thoughts of others with more scientific knowledge than myself.

He's wrongly assuming computers are somehow fundamentally different from humans in how they travel through time and make decisions.

What if the computer changes its questions at t2? Jim's already seen the answers to the modified questions before the changes have been made. He's seeing into his future.

What if Jim turns off his computer at t3? He's already got the answers but the questions will never be asked.

I think Nahin really didn't think this through. Unless he has some other requirements that you didn't tell us.

It is quite some time since I read Nahin's book, but if I remember correctly he was just giving this as an example of how MacBeath had countered some other philosopher's argument that people travelling in opposite directions through time would not be able to communicate.

I don't think MacBeath was claiming that this would work in every event, or was proof against sabotage.

Personally, I think the use of the computers is a spot of mis-direction.

I tried using days/dates instead of time symbols to see if that would improve things.

This is another extract from my notes:

……I can think of no good reason to assume that Midge’s perception of the order of days, or dates, would be different from Jim’s. The simplest scenario would be to assume that t0 and T0 were both Monday 10th, and that Jim and Midge experience the days and dates as running in the same direction. Diagram 2 shows how this would look. I have retained the use of the lower case for “normal” time and capitals for “reversed” time, in our frame of reference.

Diagram 2(After Nahin’s Figure 9)

Earlier.................Jim (normal time)..........Later

m.10th......t.11th......w.12th........th.13th.......f.14th

F.14th......Th.13th.....W.12th........T.11th.........M.10th
LATER.............MIDGE (reversed time)........EARLIER


We now have to ask whether Jim and Midge could communicate if the days they were experiencing were in fact different. If Jim’s message was displayed on Friday 14th, would Midge be able to see it on Monday 10th? Would Wednesday 12th actually be the only day on which they could have any chance of communicating? And, if so, what would be the length of time for which they were able to communicate?

Got it! Diag. 2 really played me up. There must be an easier way.

I don't think there's any easier way. This kind of puzzle always leads to paradoxes. When you have paradoxes you can't come to any conclusions.

Can Midge see Jim's house the whole time or does it just pop up for an instant on Wed 12th as you suggested? So she has no time to actually communicate because it's only instantaneous. If it pops up for an instant, what happens to the stuff that was already in its place in Midge's world? What about the air around Jim's house that's suddenly superimposed onto Midge and her house and everything?

If they can see each other the whole time, then they'll see broken teacups reassembling themselves. And they'll see that each other's calender doesn't match the position of the sun in the sky - except on that magic crossover time.

It wasn't reaching a conclusion that caused the trouble; I didn't even hope to do that. The trouble was that Diag 2 persisted in re-arranging itself when I tried to post it, I had to edit it 4 times.

As far as the air around Jim's house, and that sort of thing, is concerned, I should have included the fact that Nahin stipulated that the only thing that could pass through the window was light.

My original notes contain a further diagram in which the days/dates for both Jim & Midge run in the same direction. This solves nothing, it just changes the problems a little.

Excuse my misunderstanding :P


That doesn't really solve the problem. Everything in Midge's world would be going at the same time as her, so all of it would get dumped on top of Jim's world when they meet. Otherwise she'd suddenly find the rest of her world disconnected from her house and replaced by Jim's world. In that case, what happens to the glass of the window when half its atoms are replaced by new ones, in slightly different locations?

But I think Narin's version has them always able to see each other, so they don't just pop up for an instant. So that bypasses the whole problem.


I thought of doing that too. I think it would solve the problem of the sun being in the wrong position in the sky. But then it creates a new problem of causality going backwards for Midge, which would perhaps lead to her not even knowing she was moving backwards and everything would reduce to just normal real life without time travel.

Seems as though we reached much the some point.

Elsewhere, I've met considerable disagreement as to whether or not time travel would be involved if both were travelling normally through time in their own reference frames.

Some authors of science fiction have explored the possibilities of reversed living, with all its revolting implications with respect to alimentary and eliminatory functions, but although this might work in fiction, I can see no hope for it in real life.

Haha