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A Time Travel Dialogue

John W. Carroll

4. Thursday

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1It is 7:00 am. Dr. Rufus and Willie are sitting at the computer console with cups of coffee, talking. Tad enters.

2Tad: Wow, I thought that I was getting an early start!

3Carlene [looking up]: Oh, Tad, I didn’t hear you come in; we’re having an absolutely stimulating conversation.

4Tad [yawning]: I think I need to have some stimulating caffeine before I can have any stimulating conversation.

5Willie: Sorry, Tad.

6Tad [seeing empty coffee pot]: Hey, don’t you know that the first one in is supposed to make the coffee?

7Willie: I do, and I did.

8Tad looks from the empty coffee pot to Willie, who holds up his cup and tips it upside down to make the point.

9Willie: It was good to the last drop.

10Tad: Uh oh, what does that much caffeine do to a philosopher?

11Willie: Hey, if the Department of Energy wants this work done, they’ll have to lend me some of their energy. You can make the next pot; I think I saw some decaf in the cabinet.

12Tad [mocking horror]: Decaf?!

13Carlene: Settle down, Tad. Willie’s just teasing.

14Tad [recuperating]: So, is that the program that turns off the trigger when two particles appear in the chamber?

15Carlene: No, it isn’t. Willie’s been trying to sell me on some new ideas for variations on the experiment.

16Willie: After lunch yesterday, while you two attended the rest of the conference, I wrote the program that deactivates the trigger when two particles are present. I also wrote a program that is, in a way, the opposite; instead of turning the trigger off when the second psi-lepton appears, this program doesn’t turn the trigger on until the second one appears. It’s loaded right now.

17Tad: Wait, tell me whether I have this straight. The trigger is initially off, but if two particles appear in the chamber, the program will turn the trigger on?

18Willie: Now you are the master of summary.

19Tad: Sure, Willie. But why are we bothering to run this program at all? I think it’s obvious what will happen: regardless of whether the time-travel hypothesis is correct, if the trigger is turned off, then it’ll look exactly like the trial without the trigger that we ran on Monday, Trial 16; the psi-lepton will live its life and decay normally.

20Carlene: Maybe, maybe not. We thought we knew what our data would look like the first time we isolated the psi-lepton, but we were certainly wrong then. In order to approach this problem scientifically, we need hard data, not speculation. Our knowledge of the psi-lepton’s behavior is based almost entirely on our theory, and this trigger throws another unpredictable factor into the equation. I really don’t think we can assume anything at this point.

21Tad: What else could happen?

22Willie [pressing a few keys]: Well, let’s find out. (See Figure 4.1)

  • 1 To see an animation of any of the Thursday illustrations online visit (...)

Fig. 4.1 A Causal Loop?1

23Carlene [smiling]: And that’s why we should never assume that we know what the outcome of an experiment will be.

24Tad grabs a printout of Trial 19.

25Tad: How is this possible? These results look just like the other trials in which the trigger was turned on.

26Carlene: So, the so-called second particle, the time-traveling psilepton, turned on the trigger that caused the time travel.

27Tad: That shouldn’t have happened even if the time-travel hypothesis is true. When the program was started, there was no trigger, and no trigger means no particle. Therefore, unless this just happened to be the one in eight bazillion times that another particle spontaneously appeared in the chamber, the experiment should have gone just like Trial 16.

28Willie: Well, the trigger remaining off would have been consistent with our hypothesis; however, I believe that the actual results are consistent as well.

29Tad: How can you say that? The second particle is clearly present in these results! According to your theory, the second particle shouldn’t appear unless the trigger causes it, but the trigger was turned off in this program, so there was nothing to cause the second particle.

30Willie: Not so fast, consider this: the anomalous second particle appeared, causing the program to activate the trigger; the trigger then caused the psi-lepton to change its temporal direction, thus traveling to the earlier time. The graph would look just like it does on the printout you’re holding.

31Tad: But there was nothing to cause the time travel.

32Willie: Yes, there was; the trigger caused it.

33Tad: But the trigger was turned off.

34Willie: The trigger was initially turned off, but the presence of the time-reversed psi-lepton in the chamber turned it on.

35Tad: But the particle shouldn’t have been there!

36Willie: But it was.

37Tad [containing frustration]: But there was nothing to cause it to be there.

38Willie: The trigger caused it.

39Carlene [interrupting]: Guys, you’re going around in circles.

40Willie: What a fortunate phrase! Yes, we’re talking about the possibility of a causal loop here, wherein each event is among its own causes.

41Tad: A causal loop? Oh please. What could have caused the loop itself? There’s no reason that we should have a particle – trigger loop rather than no loop at all.

42Willie: There seem to be a lot of possible causes; for example, by starting the accelerator, we could have caused the loop.

43Tad: Wait, no, that can’t be right. As we’ve already seen, lots of things could have happened after we started the accelerator; we got different results in the past. Starting the accelerator wasn’t enough to guarantee that the loop occurred — assuming there’s a loop at all — so how could it have been the cause?

44Carlene: Well, if we hadn’t started the accelerator, then the causal loop wouldn’t have occurred. Remember yesterday when we said that the Big Bang caused everything that’s happened since? It’s the same kind of thing; if the Big Bang hadn’t occurred, none of the events since would have. But it’s not clear that the Big Bang guaranteed any of the particular events that followed any more than turning on the accelerator guaranteed the causal loop.

45Tad [staring blankly]: Okay, Professor, I see what you’re saying, but I’m still not sure how some event could be the cause of another if it’s possible for the first to occur without the second. Here’s what I was getting at. Once the experiment started, there must have been some conditions that led to the occurrence of the loop rather than the ordinary life and decay of a single psi-lepton.

46Willie: Our actual results were just one way that this could have gone; nothing about our set-up entailed that we would get the results that we did, with the trigger, rather than without the trigger.

47Tad: Then how do we explain the results we got?

48Willie: For starters, not all events are completely determined by initial conditions and the laws of nature alone; for example, quantum mechanics, on one standard interpretation, is an indeterministic theory. Given the laws of nature and the state of the universe at a time, there’s only a certain probability that some possible state of the universe will follow, and that probability isn’t 100 percent. If this is true — and if all events require an explanation — then we need some way of explaining undetermined events.

49Tad: Which is?

50Carlene: We think about explanation in terms of causes all the time, Tad, often like this: some event causes another if and only if the effect-event is less likely to occur when the cause-event doesn’t occur. During this trial we used Willie’s new program, which starts with the trigger off but turns it on whenever two particles are detected in the chamber. If we had simply left the trigger off, then the causal loop probably — almost certainly — wouldn’t have occurred, so it’s reasonable to think that using this program — in addition to starting the accelerator, the Big Bang, and so on — caused the loop to occur. That seems like a pretty good explanation.

51Tad: But we started with the trigger off; that’s exactly how we’ve started several other trials. There was no difference in the initial conditions that led to the occurrence of the loop.

52Willie: How can you say that the initial conditions were the same? Dr. Rufus just mentioned how the computer was running a different program and how that affected the possible outcomes.

53Tad: The trigger was turned off initially, so I don’t see how that makes any difference. In terms of probability, the chance of a second psi-lepton spontaneously appearing in the chamber is on the order of — well, the probability is essentially zero. But that’s the only way another psi-lepton could have found its way into the chamber, and that’s the only way the program could have turned the trigger on.

54Carlene: Willie, even though you make a plausible case, I have to admit to finding the causal-loop idea dubious.

55Willie: Okay, maybe we should consider a different take on our results. You both seem to be assuming that every phenomenon in our universe admits of an explanation, is caused, and so on.

56Carlene: This is the best attitude to take in science.

57Willie: Methodologically that may be correct, but theoretically that may be a little presumptuous.

58Dr. Rufus raises her eyebrows in genuine offense.

59Carlene: Tread lightly.

60Willie: Look, maybe it’s a causal loop, which itself may be inexplicable. The Big Bang might be inexplicable, too, as well as why the universe is lawful in the manner that it is. Any event that quantum mechanics tells us was extremely improbable may also be inexplicable; citing low probabilities hardly explains why the improbable event occurred. Our causal loop — if that’s what it is — is, I think, a good candidate for being an inexplicable sequence of events. Would it matter if it were inexplicable? Would you really suggest that we ignore the results in front of us because of a philosophical position on explanation?

61Tad: Do you even know what ‘tread lightly’ means?

62Carlene: It’s okay, Tad; Willie has a point. We do have a result that we need to contend with, and the best way is to get more results.

63Willie: I think that part of the problem is that we’re used to thinking about causes and effects in a linear fashion. Normally causes are independent variables, and effects are dependent variables; the causal chain doesn’t usually link up to itself.

64The three sit in silence for a bit.

65Carlene: So, in our supposed causal loop, every event is both its own cause and its own effect.

66Willie: Yeah, I don’t think any event in our loop really fits our usual assumptions about cause – effect relationships; I don’t think we should try to identify the cause of our loop in terms of the ordinary linear relationships we’re used to dealing with.

67Tad: Maybe I’m too linear in my thinking. Then again, maybe these loops don’t make any sense.

68Carlene: Right now I think we ought to try our first idea, the program that turns the trigger off when a second particle is detected. I believe this trial will really be the one to give clear evidence for or against the time-travel hypothesis. We have all the time in the world for data analysis and philosophical speculation.

69Willie: I’m uploading the first program right now.

70Tad: I still don’t think causal loops make any sense. They make all sorts of ridiculous situations possible.

71Willie: Such as?

72Tad: Well, I once saw this movie called Somewhere in Time, but I don’t remember much about it.

73Carlene: What do you remember?

74Tad: I remember that there’s a young man visited by an old lady in the 1970s, and she gives him a watch before leaving and saying only, “Come back to me”. Many years later, the young man sees an old picture in a hotel of a beautiful actress that fascinates him. He does some research and finds a picture of that actress as an old lady, and he realizes that it’s the same one who’d given him the watch. After talking to some weirdo who’d written a book about how to time-travel through self-hypnosis, the young man goes back in time to 1920. Once there, he finds the young actress, and they fall in love. Before returning to the 1970s, the young man gives the actress the watch that she will give to him when she’s older. And so the history of the watch forms a complete loop, which makes no sense.

75Willie: That’s a pretty decent recall, I’d say. But what’s wrong with the watch? It sounds to me like it had a completely consistent causal history: the man gave it to the actress in 1920; she carried it with her until the 1970s at which point she gave it to him; then he returned it to 1920 and gave it back to her.

76Tad: What’s wrong?! Nobody ever made the watch!

77Willie: So?

78Tad: So, that’s impossible! Watches don’t just appear out of thin air, Willie!

79Willie: But it didn’t; the watch’s first appearance in 1920 was supposed to be the result of the time travel from the future.

80Tad: And, as I’ve said all along, that’s the underlying problem, the backwards causation. There’s another problem, too. In order for the story to be consistent, the watch would have to be exactly the same when the actress first received it and when the man took it back in time to give it to her, right?

81Willie: Definitely.

82Tad: If that’s true, though, then the watch wouldn’t be able to age at all. The actress carried it around for fifty years or so; even if you assume the watch didn’t rust or get scratched or whatever, its entropy would still have increased over that time.

83Willie: Then the outside world must have expended energy to return the watch to its initial state. All that goes to show is that the longer the time between the arrival and the departure of the watch, and the larger it is, the more energy that’s required to return it to the prior state. This could happen when the watch time-travels. Oh, the program is ready to go.

84Tad: I still say there’s no need to run this program.

85Willie [looking amused]: Are you scared that you’ll be proven wrong if we run this trial?

86Tad: No, because there’s no doubt in my mind that I’ll be proven right; I’m just trying to save some money for the DOE. If we can turn off the trigger after the appearance of the second particle that was caused by the trigger — which we surely can — then we would obviously end up with a contradiction. That means that backwards causation is impossible, just as I’ve said all along.

87Carlene: Are you as confident about your predictions about this trial as you were about the last one?

88Willie: Hey, we might all be surprised by the results. Tad, you ready?

89Tad: Always.

90The three gather intently. (See Figure 4.2)

Fig. 4.2 The Really Weird Results

91Tad [studying the monitor]: Well, I’m glad that we ran this trial, but as I expected it looks as though the time-travel hypothesis is shot. If the second particle really was the original particle traveling backwards in time as a result of the trigger, then it shouldn’t have appeared. Now we have the anomalous particle appearing without the trigger. It lives its short life and decays just in time for the psi-lepton to rush past it. It looks like we’re back to the drawing board.

92Carlene: Well, that’s assuming the program ran as intended. Willie?

93Willie: I’m almost positive that it worked. Based on the previous trial, the computer obviously can turn the trigger on or off in the interval of only a couple nanoseconds. Let me check. Yeah, the diagnostic log shows that the trigger was turned off after the appearance of the second particle.

94Willie thinks to himself while Tad and Dr. Rufus quietly discuss the results.

95Carlene: What do you think, Willie?

96Willie: I have to admit that the possibility that the trigger caused the psi-lepton to travel backwards in time is now looking doubtful; there was no trigger-event, but there’s still the path of what we thought was the time-traveling psi-lepton. Everything we’ve seen before suggests that this anomalous particle shouldn’t have appeared, but it did, this time with a seemingly completely normal psi-lepton also in the chamber.

97Carlene: If the trigger wasn’t causing the second particle, then why didn’t it appear in Trial 18, or any of our earlier trials, when the trigger was never in place? Why now? For that matter, when there were the two particles, why did they both disappear? It never really looked like a collision or annihilation. But look here at the data tables: now we are seeing a slight disturbance in the magnetic field at both t=5 and t=7 nanoseconds, but there’s no collision since the normal particle continues along its expected trajectory. We have the second particle, no collision, and the normal particle living its full life.

98Willie: Do you think that time travel might still be involved?

99Carlene: Well, I have to agree with Tad that the time-travel hypothesis isn’t looking so good at the moment. This was only one trial, though; there’s more work to do. Who knows? Let’s wrap this up for now. I’ll see you fellows tomorrow.


1 To see an animation of any of the Thursday illustrations online visit or scan the QR code.

Table des illustrations

Légende Fig. 4.1 A Causal Loop?1
Fichier image/jpeg, 65k
Légende Fig. 4.2 The Really Weird Results
Fichier image/jpeg, 69k


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