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A Time Travel Dialogue

John W. Carroll

1. Monday

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It is 2:00 pm. Dr. Rufus and Tad welcome Willie to the control room and begin to explain why they have called on his computer expertise.

1Carlene: Tad, would you mind showing William to the control console? Do you go by ‘Bill’?

2Willie: ‘Willie’, actually. It looks like you’re running a pretty powerful system. You must have two dozen new HP-UXs in here.

3Tad: Twenty, actually, with about eighty Motorola VMEs for input – output control. Coffee?

4Willie: Thanks, but it’s a little late in the day for caffeine.

5Tad [pouring another cup]: Suit yourself.

6Willie [looking around]: Where does the funding for all this equipment come from?

7Carlene: The Department of Energy, primarily, but NASA, the NSF, and a few research universities are interested in what we’re doing. We just might be on to something.

8Willie: Something big?

9Tad: Yes, well, no… something very, very tiny. We think we’ve found a new fundamental particle.

10Willie: Wow, really? What is it?

11Carlene: We’re trying to isolate the elusive psi-lepton. We’ve found something, but we’re not sure whether we’ve found it or some unanticipated cousin. The particle we’re observing was behaving exactly according to our theory, that is, until recently.

12Willie: My particle physics is pretty rusty, but I think I remember reading something about the psi-lepton. It seemed like pretty speculative stuff, even for the fringes of particle physics.

13Carlene: Most of my associates have been quite skeptical, which is why we’re taking such care in trying to understand our particle’s anomalous behavior.

14Willie: What’s anomalous about it?

15Carlene: I’ll let Tad fill you in. I have some results to look over and some more calculations to do. Tad, would you mind telling Willie about our dilemma, and then accessing the accelerator program files so he can get to work?

Dr. Rufus returns to her office, leaving Willie with Tad at the terminal near the control console.

16Tad: Okay, so here’s the story. The process we’re running involves the ultra-relativistic collision of a uranium isotope and a heavy helium ion. We’ve set up B fields — uh, magnetic fields — to filter out every particle predicted to escape the collision except, so we expect, the psi-lepton. We have software in place to analyze the data from the detection devices, which track position and energy. The results were exactly what we were hoping for; it seemed undeniable that we’d observed a psi-lepton. Unfortunately, we haven’t been able to replicate our results since early last week. Since then, although we can create a psi-lepton — again, assuming that’s what it is — it vanishes long before our theory predicts.

17Willie: So, your particle shows a faster decay rate than it should?

18Tad: It’s not that simple; it doesn’t appear to be decay at all. We observe a second particle that appears out of nowhere, apparently annihilating the psi-lepton.

19Willie: Out of nowhere?

20Tad [pointing to the monitor]: Here’s the weird data. (See Figure 1.1)

  • 1 To see an animation of any of the Monday illustrations online visit www. openbookpublishers. com/is (...)

Fig. 1.1 The Unanticipated Results1

21Willie: Tell me what I’m looking at exactly.

22Tad: It’s a standard position-versus-time graph that also indicates energy levels. The dotted line-segment emerging from the origin marks the position of the psi-lepton within the chamber at different times. The dotted line that emerges near the top of the graph at t = 3 marks the position of the unexpected second particle. The horizontal dashed line marks the total energy within the chamber at the various times. The increase in energy at t = 3 results from the presence of the second particle. Everything goes just as expected from t = 0 until t = 3, but at t = 3 we find the second particle in the chamber that appears to collide with the psi-lepton at t = 5, after which there’s no sign of either one. Trial 15 is our most recent trial.

23Willie: But you weren’t getting this result before the middle of last week, right?

24Tad: That’s right. Before then, we’d run eleven trials, and for each of those trials the psi-lepton was all by itself in the chamber and decayed at t = 7, just as our theory predicts. Then we got four straight trials of who-knows-what.

25Willie: And that’s why I’m here? To tell you whether there’s a bug in the accelerator’s program that’s giving you these strange results?

26Tad: You’ve got it. Here are the accelerator program files. By the way, are you really a philosopher?

27Willie: Well, I have a PhD in philosophy. I loved doing metaphysics, epistemology, and the like, but a full time job was hard to come by.

28Tad: I guess you can’t really expect DOE funding for that kind of stuff. I’ll let you get to work.

Willie begins to study the programs that run the accelerator. After a few hours, he finishes and approaches Dr. Rufus in her office.

29Willie [knocking]: May I?

30Carlene: Please, come in. Did you find anything?

31Willie: I did, actually; there was a glitch. Since I was able to bypass the problem using an alternate code, my guess is that some kind of hardware problem cropped up last week. The new code is functionally equivalent, but the simulations run much more smoothly. As far as I can tell, the accelerator should work fine.

32Carlene: Excellent, Willie. Now we can see whether we really have the psi-lepton. Won’t you stay for our first trial with your code?

33Willie: Thanks, I will.

Dr. Rufus and Willie leave the office and walk back to the control console.

34Carlene: Tad, Willie thinks we’re ready to go. Would you mind readying the accelerator?

Tad sits down at the console, entering the necessary commands.

35Tad: All set, Professor. Shall I start it?

36Carlene: Please. (See Figure 1.2)

Fig. 1.2 The Anticipated Results

37Tad: Willie, you’ve done it! Professor, the data indicate we had a perfectly stable psi-lepton in the chamber, just as our so-called speculative theory predicts. It’s the same result we were getting before last week.

38Carlene [containing excitement]: Thank you, Tad, but don’t be hasty. Right now I’m curious about what was happening before Willie bypassed that glitch. If the particle really is a psilepton, how can we account for the strange phenomena we were observing? Willie, do you know how the glitch was affecting the accelerator?

39Willie: No, not really. Like I told you before, I suspect that there was some kind of hardware problem; something took place in the chamber when the program was run with the original code. Without pulling apart all of this beautiful equipment, that’s about all I can tell you. If it’ll help, though, I could probably figure out when the pesky event took place by restoring the original code.

40Carlene: It’s not much, but every bit of information could be useful. Do you mind?

41Willie: Not at all. Reinstating the original code should take only a second. Then I’ll attach a diagnostic log that will show when the glitch kicks in.

Willie sits back down at the terminal and types for a few minutes.

42Carlene: Is that it?

43Willie: That’s it.

44Carlene: Tad, would you mind running the creation process again?

45Tad: One psi-lepton, coming right up! (See Figure 1.3)

Fig. 1.3 Backwards Causation?

46Carlene: The anomalous particle is back. Once again, it originates at t = 3 nanoseconds as it did in trials 13, 14, and 15.

47Willie [remaining fixated on his terminal]: I know this is going to sound strange, but the event seems to take place at about t = 5. Everything’s normal until then.

48Tad: That is strange. If Willie’s right, the glitch didn’t cause the second particle to appear. Its origin is still a mystery.

49Carlene [tapping her finger on her nose]: Hmm.

50Tad: What are you thinking, Professor?

51Carlene: First, since we seem to be in control of this event, perhaps we shouldn’t refer to it as a glitch. Second, in contrast to what you said, Tad, if the event doesn’t take place, then the anomalous particle doesn’t appear. When it takes place, however, the particle does appear. Maybe we should refer to the event as a trigger — it does, after all, now appear to be the cause of the second particle.

52Willie: Backwards causation? You’re suggesting that the trigger at t = 5 causes the origin of the second particle at t = 3?

53Tad: What?! You’re not serious, Professor.

54Carlene: It’s the closest thing to an explanation for the appearance of the second particle we’ve come up with so far. Why couldn’t it be a case of backwards causation?

55Tad: Because that’s wacky! The future can’t cause the present or the past. Backwards causation is just the kind of confusing thing Hollywood takes up, not serious researchers.

Dr. Rufus raises her eyebrows in mock offense.

56Willie: You know, Tad, what Hollywood seems to care about more than anything is entertaining and making money, thus the glut of paradoxical time-travel films. But serious researchers actually study this kind of thing; philosophers think about backwards causation and time travel quite a bit. In any case, backwards causation is the only real issue with our experiment. Carlene, if I’ve got you right, you believe that the trigger causes the anomalous particle to appear before the trigger is, well, triggered; then the new particle moves off and collides with the psi-lepton. No time travel, only backwards causation.

57Carlene: It would be imprudent to believe that or anything else at this point in our investigation, but I’m considering the idea.

58Tad: Time travel or no time travel, it just doesn’t make sense. The problems that come up in the time-travel movies aren’t due to their time-travel aspect so much, but from the presence of backwards causation. Take Back to the Future, for instance: the time controls of the DeLorean are set in 1985, Marty ends up back in 1955, and young Biff chases Marty until Biff gets covered in manure as a result. The 1985 event caused the 1955 events. Okay so far, I guess, but say Marty time-travels to the past and permanently breaks up his parents’ meeting, which he does only temporarily in the movie. If he had done that, he wouldn’t exist any longer, but if he didn’t exist, then he would never have been able to go back in time to break up his parents’ meeting in the first place. It doesn’t take long for things to get ridiculous.

59Willie: I’ll grant you that many time-travel films are hard to make sense of, but that’s because the writers want to entertain. The logical absurdities slip in because the writers are more worried about dramatic effect and humor. But the problems in these films don’t even begin to undermine the actual possibility of either time travel or backwards causation. Besides, there are other films that are coherent: Terminator, for one.

60Carlene: I’m afraid I’m not familiar with that film.

61Willie: So, our society of the future is destroyed by cyborgs that were created by the company, Skynet. A devastating war between the machines and the humans takes place. During the war, the technology for a time-travel device is discovered. Events unfold, and one man and one terminator cyborg are sent backwards in time. The terminator is programmed to kill Sarah Connor, the mother of the leader of the human rebels, which will in effect prevent her from giving birth to her son.

62Carlene: That doesn’t sound like a logical time-travel plot. How can the terminator succeed in killing the mother of a leader who later exists before she gives birth to him? Killing the mother would change the future; her son would never have been born and would never have fought the cyborgs, which the film evidently tells us he did. There appears to be a dilemma.

63Willie: Well, you’re right that there’s a logical problem, but it’s only with the plan. It seems the cyborgs didn’t think it through. But what do you expect from cyborgs? The way things actually turn out, the terminator they sent wasn’t successful; he didn’t kill Sarah Connor. There’s no suggestion that something both did and didn’t happen. Sarah lives to give birth to her son, who later leads the humans against the cyborgs. It seems to me that as long as there’s no hint that at one and the same time something both did and didn’t happen, then the plot could be consistent. Back to the Future is a fun film, but it’s hard to make sense of it if time is one-dimensional.

64Tad: One-dimensional time? As opposed to what?

65Carlene [interrupting]: I’m going to have to cut this off. I want to get back to what’s going on with the psi-lepton. Have a close look at this printout. Consider the possibility that at t = 3 nanoseconds the second particle decays rather than emerges. Maybe t=3 nanoseconds is its instant of termination rather than its instant of creation.

66Tad: Professor, just think about what you’re suggesting. The particle can’t decay then because it keeps going! It exists after t = 3, until t = 5. How can you say it’s decaying at t=3?

67Carlene: Hold on, Tad. You shouldn’t be so quick to ignore your own thoughts. Didn’t you bring up time travel just a few minutes ago?

68Tad: You can thank Willie for that.

69Willie: Carlene, are you suggesting that the psi-lepton is time-traveling?

70Carlene: I think it’s a candidate explanation for what we’re observing. Wouldn’t you agree, Tad?

71Tad: A time-traveling psi-lepton? I really don’t know whether that’s worth our time.

72Carlene: Time travel could explain a lot of the data we’ve collected. Willie said that the trigger occurs at nearly the exact moment the two particles supposedly collide. What if there was only one particle? The trigger might cause the psi-lepton to reverse its temporal direction rather than directly and over a gap of time cause the birth of the second particle at t=3 nanoseconds. At t=5 nanoseconds, perhaps the psi-lepton ceases traveling from present to future, and begins traveling from present to past. What we’ve been thinking of as two particles could be just one particle, a psi-lepton traveling forward in time and the same psi-lepton traveling backward in time. Its lifespan would end when it decays at 3 nanoseconds, 7 angstroms from the origin.

73Willie: In what way is that a better explanation than the hypothesis that merely posits backwards causation?

74Carlene [holding a new printout]: The graph only shows the data for the position and energy levels of the psi-lepton, but you can see in these supplementary data tables that in this trial the mass – energy, momentum, charge, spin — you name it — all have the predicted values. The data also indicate that there was no disturbance in the chamber’s magnetic field anywhere near the supposed collision. If there had been a collision, then there should be some recorded disturbance in the chamber’s magnetic field at t=5 nanoseconds.

75Willie: I see; if the trigger caused the particle to turn around in time, to the effect that there was no collision, then we would have an explanation for the lack of magnetic field disturbance and for the origin of the second trace. We also don’t need to posit any mysterious action at a distance between the trigger’s occurrence at t=5 and a second particle at t=3. Wow, we might have just witnessed a case of actual backwards time travel!

76Tad: Come off it, Willie! That doesn’t mean anything. Terminator notwithstanding, time travel is a fantasy. It would generate all these crazy, impossible situations. I could go back in time and shoot my grandfather, but then if I had shot my grandfather, I wouldn’t exist because my grandfather would never have fathered my father, and my father would never have fathered me. But then you’d have my grandfather lying dead in the street back in 1930, shot dead by a killer who never existed. Impossible!

77Carlene: Tad, how do you propose to explain the fact that the trigger seems to cause the anomalous particle to appear as well as there being no magnetic field disturbance after the supposed collision?

78Tad: I don’t know yet. Perhaps it’s just a wild coincidence. Besides, you know those B fields can be pretty dicey sometimes.

79Carlene: Not just one wild coincidence, Tad, lots of them. We’ve run this experiment four times in the last few days and every time — excluding the time we ran it with the alternate code today — the so-called second particle has appeared.

80Willie: Yeah, if we were always content to dismiss anomalies as coincidences, then I don’t see how anyone could make progress in science. At least the time-travel hypothesis appears to explain away the coincidences.

81Tad: But this is an extraordinary hypothesis, one that has its home in science fiction more than it does in actual science, and you’re asking us to consider it on the taxpayer’s dime to boot. Maybe a big dose of levelheadedness is in order.

82Carlene [sighing]: Look, guys, this talk really won’t get us as far as more experiments will. We need to run more tests tomorrow. Do you mind returning tomorrow, Willie? We may need you to remove and restore the original code a few more times.

83Willie: Sure, I don’t have any pressing projects, and I think the Department of Energy can afford me for another day.


1 To see an animation of any of the Monday illustrations online visit www. openbookpublishers. com/isbn/9781783740376#resources or scan the QR code.

Table des illustrations

Légende Fig. 1.1 The Unanticipated Results1
Fichier image/jpeg, 68k
Légende Fig. 1.2 The Anticipated Results
Fichier image/jpeg, 120k
Légende Fig. 1.3 Backwards Causation?
Fichier image/jpeg, 64k


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