# 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.......................T 0
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.

Well the question is "can you travel through time?" and once you get the answer to be yes, it comes down to how you are doing so when it comes to your communication question.

Regardless, surely the easiest way to communicate would simply be to go to a point in the other persons past and leave a note - perhaps with an 'open on' date.

Given we can't currently travel through time, there's no real answer to your question. Any hypothesis on the matter is valid unless there's a gaping flaw in it.

jarednjames, I take your point about time travel, but, surely there is no actual time travel involved here. Jim is experiencing time as we all seem to experience it. Presumably, Midge is doing the same, in her F of R. Both are living their lives "normally". I suppose the question of any possible communication is philosophical rather than scientific, but scientists do seem to become involved in such considerations, at least, if P S books are to be believed.

You said they are travelling through time in opposite directions. In which case there is time travel involved.

As far as I'm aware time only travels in one direction - it can appear to slow down and speed up based upon frame of reference but never reverse direction.

No frame of reference is going to result in something travelling backwards through time.

As far as I'm aware time only travels in one direction

Do you subscribe to the view that time moves, rather than that it static, and we move through it?

If time is static, it doesn't move in any direction.

Do you subscribe to the view that time moves, rather than that it static, and we move through it?

If time is static, it doesn't move in any direction.

All 4 dimensions are static until we move through them. However, unlike the first 3 we are always moving through time and it is always in one direction. (Strictly speaking we're always moving in the first 3 as well because we can't be truly stationary - but that's another issue.)

Regardless, it doesn't matter what reference point you take. You can make your reference point so that the first three dimensions are static but you can't put time as static. Your reference frame may alter the speed at which time passes, but it doesn't stop (at least as far as I'm aware) and it never reverses direction.

All 4 dimensions are static......you can't put time as static.

Is it me, or is there a contradiction there?

Is it me, or is there a contradiction there?

Nope, not if you take the whole post and not specific sections:

"All 4 dimensions are static until we move through them."

"However, unlike the first 3 we are always moving through time and it is always in one direction."

And as we are always moving through time:

"you can't put time as static."

I'm not getting into some philosophical debate about time. The fact of the matter is there is no reference point you can take where time appears to be running backwards - hence for your initial scenario to occur you need some form of time travel.

OK. I can live with that.

Marvellous, now so far as communication goes, unless you have a signal that you can send to a specific time position for communication, the only way I see to communicate is to interact with a person before / during the point you need them to get the message.

Your system appears to be leaving a message - but in a relatively complex manner.

If time is reversed on each side of the window, identifying a point before/after you need them to get the message can become complicated.

I tried substituting days/dates for the time intervals in Diag. 1, but that seemed to raise more questions than it answered.

Just a point - I am not sure of its implications - In a Feynman diagram, an antiparticle is the equivalent of a particle moving backward in time. The positron can be viewed as an electron moving backward in time (or vice versa).

Containment

The person in the past could send a message to the person in the future by leaving a note.
The person in the future could send a message to the person in the past by refusing to read it.

So the best way for these two people to communicate is to... not.

Those who do not remember history are doomed to repeat it???

Anyhow I think you should read some stuff on causality, and logic/reason.

Rap, moving backward in time presents no problem in a Feynman diagram, but if that is what they are doing in reality, how are electron/positron collisions achieved in particle accelerators?

The most "sensible" stuff on causality I have read, as well as every logical instinct I possess, lead me to the belief that cause must precede effect. This is probably why I try to understand why intelligent people seem to be able to hold the opposite view.

The most "sensible" stuff on causality I have read, as well as every logical instinct I possess, lead me to the belief that cause must precede effect. This is probably why I try to understand why intelligent people seem to be able to hold the opposite view.

What people consider sensible isn't necessarily logically sound or correct. For example, people don't want nuclear plants built near their houses - due to irrational fears based on unsound logic and poor knowledge of the subject area.

Again, depending on the idea of time travel used determines the ability for cause to come after effect.

You refer to "intelligent people" holding the opposite view - perhaps that tells you there's something wrong with your understanding? Perhaps you're "sensible stuff" is only what you choose to be sensible and not what actually is.

The way you word that last part annoys me to some extent, because it appears you are saying these people are wrong and yet your only basis for this is your own possibly flawed understanding. I'm not sure what you know about the subject and how much you understand of the surrounding subjects but do you consider yourself below, above or equal to the people working in those fields?

jarednjames said:
You refer to "intelligent people" holding the opposite view - perhaps that tells you there's something wrong with your understanding?

That was precisely my point. I am a non-scientist, trying to understand and learn. One of the best ways I know to achieve this is to question why other people hold the views they do.

I certainly don't consider myself equal to or above those working in the field. I am more like a hitch-hiker, asking the driver why he takes a particular route, in the hope of learning something.

Rap, moving backward in time presents no problem in a Feynman diagram, but if that is what they are doing in reality, how are electron/positron collisions achieved in particle accelerators?

In the Feynman diagram, what happens in your time depends on how your time coordinate "goes through" the diagram. The physics is not dependent on who is viewing the process, or how their time coordinate goes thru the diagram. Take a piece of paper, draw a dark line going up and to the right, then stop and without lifting your pencil, draw a dark line going down and to the right. Then, at the vertex where they meet, draw a dotted line going up.

This is simplified - The dark line is the same particle which can become either an electron or a positron, and the dotted line is a photon. If you view it from bottom to top, it looks like an electron and a positron colliding and annihilating to produce a photon. If you view it from top to bottom, it looks like a photon decaying to produce an electron and a positron. If you look at it from left to right it looks like an electron absorbing a photon, and that same electron going off in a different direction. From right to left, it looks like a positron absorbing a photon, and that same positron going off in a different direction. Feynman's point was that the physics of all four processes are the same, just viewed from different directions in spacetime.

Thanks Rap, you make Feynman diagrams seem quite understandable.

I still have trouble getting my head round the mechanics of electron/positron collisions, if positrons are travelling backward in time.

Thanks Rap, you make Feynman diagrams seem quite understandable.

I still have trouble getting my head round the mechanics of electron/positron collisions, if positrons are travelling backward in time.

Well, if a particle can reverse its direction in time, during the time before that reversal there must be two versions of that particle, and after the reversal, none. Still, energy and momentum have to be conserved, and that's where the photon comes in. The energy and momentum of the electron and the positron is equal to the energy and momentum of the photon that is left after their collision.

Also looking at the diagram, you can see that there is no difference between saying that a positron is an electron moving backward in time or that an electron is a positron moving backward in time. Electrons and positrons are the same type of particle moving in different directions in time. Its the direction that YOU are moving in time that determines whether YOU experience an electron or a positron.

Regarding this thread, the point is that maybe a person travelling backward in time is just a person composed of antimatter. Feynman diagrams don't deal with how you experience them, because the equations of microscopic physics are just as good running forward in time as they are backward in time. But then there is the second law of thermodynamics, which only runs "forward". In other words, if you had a movie of something happening, you would see no physical laws being violated if you ran it backwards except the second law of thermodynamics. There is something more to the idea of time than is expressed in microscopic laws like the Feynman diagram. I don't know how this applies to a person composed of antimatter, but I do know that antimatter does not disobey the second law. In other words, I think a person composed of antimatter would "experience" time in the same way as we do, not backwards. The second law is what prevents a person from traveling backward in time. But, then, the second law is just a statistical law, it tells you what will very probably happen, not what will certainly happen.

vanesch
Staff Emeritus
Gold Member

This is not really "general physics", because we are talking here about *subjective experience*, which is all that distinguishes a "forward-in-time-moving", ah, person from a "backward-in-time-moving" ah, person.

From a behavioral viewpoint, if you see a person's body, and that person's body obeys the laws of nature as we know them, then you will be able to explain that behavior by "forward-moving" laws. For instance, the brain of that body will function according to "forward-moving" laws of nature, including memorisation of past events and everything.
So on Monday, you will see that body in a certain state, and you can talk to it, and that body will respond you as the laws of nature (the normal ones, forward in time) will have it respond. On Tuesday, it will respond to you differently, in agreement with the laws of nature still.

Whether some subjective experience experiences that body on Tuesday, "and then" on Monday, is entirely a matter of philosophy, but won't alter, in any way, the physical reactions of the body.

In other words, as seen behaviorally, a body associated with a "subjective experience back in time" will not appear, will not behave any differently than a body associated with "a normal subjective experience moving forward in time", because both bodies are governed by the same laws of nature.

Maybe I'm a strange person, who experienced "first" rising up out of his coffin, becoming younger and younger, end ending up by crawling into the womb of my mother in the end, but "to the outside" this will appear as a normal life.

So before talking about a PERSON experiencing back-in-time life, maybe you should tell us what you think something like a stone does, travelling back in time. And then a mixture of milk and water travelling back in time. That's easier to start with. I think you'll find out that the direction of time is simply given by the second law of thermodynamics.

i have a question?
is it possible to travel back in time in any way?we can travel forward in time according to time dilation...

is it possible to travel back in time in any way?

We don't know. There are theories which say you may be able to but nothing is certain.
we can travel forward in time according to time dilation...

We can travel forward in time... because we travel forward in time. Time dilation has nothing to do with us travelling forward in time, it is merely the difference in perceived time for two different reference points.

ok....thanks......!

vanesch said:
So before talking about a PERSON experiencing back-in-time life, maybe you should tell us what you think something like a stone does, travelling back in time. And then a mixture of milk and water travelling back in time. That's easier to start with. I think you'll find out that the direction of time is simply given by the second law of thermodynamics.

I assume that by "a mixture of milk and water travelling back in time", you are looking at the mixing process. I feel sure that an observer travelling with the mixture would see it being mixed. However, for an observer travelling in the opposite direction through time it seems logical to imagine that she would see it un-mixing. When considering just a brief period of time this may seem to present few serious problems, but I have grave difficulty if I try to extend the period of time over which an observation is made.

In fact, the more I think about it, the more I am able to convince myself that no actual observation would be possible in a reversed time scenario.

epenguin
Homework Helper
Gold Member

?uoy era woh sey