Causal Noose

"Bob!" Dirk positively leaps out of his chair. He storms around his desk and shakes Bob vigorously by the hand before Bob is even two steps into the office. "Good to see you."

"It's only been half a week," says Bob, mildly startled.

"I know, I know. But stuff has been happening. Wheels have been turning. Have a seat."

Bob Pritchard is small, dwarfed somewhat by his own reputation. Some years ago, Bob had a single, inconceivably brilliant idea about the internet, money, and intellectual property rights. From his idea, Bob made a computer program. From his program, Bob made some money. From his money, Bob made a business and from his business, Bob went on to make a much, much larger amount of money and a much, much bigger business.

Since he was a teenager, Bob has always had casual side projects. Now, he gives them staff, and funding, and a manager: Dirk.

Dirk has the firmest handshake you've ever felt. Dirk is expansive, both in size and in character. Dirk fills rooms. Dirk came with a string of recommendations as long as Bob's arm. He Gets, and has Gotten, Things Done. "Which wheels in particular have been turning?" asks Bob. Dirk manages a diverse collection of wheels for him right now.

"Let me lead you into this, as it were, by asking you a question. What would you say is the major problem, or risk, of time travel?"

"Changing history?"

"Good answer. But what would it even mean to 'change' history? 'Change' is when something moves from one state to another over time. How can time itself 'change'? How can the year nineteen sixty-nine or whatever 'change' when there's no 'time' for it to change over because it's still nineteen sixty-nine? Don't try to answer that. I've got guys I pay to think about this stuff harder than you could ever imagine. Anyway, the answer is that you can't 'change' history. That's not the risk. History's fixed. Cigar?"

Bob holds up a hand. "Dirk, I threw a lot of money at this--"

"I know you did, Bob, I know you did." Dirk lights his cigar, causing Bob to shuffle away a bit because he doesn't like the smell of smoke. "You threw a ton of money at us and told us to go away and puzzle out a time machine and that's what we've done. All I'm working my way up to saying is, it's not going to be like you think. It's not a Delorean. You can't punch in a date from the past and go there. Stuff can't be sent back in time, because, like you say - that could be abused to alter history. What we can do instead is pull stuff from the future to the present. We have set up a machine which, when we press the button, looks forward in time and randomly selects an object and wrenches it back to the current day. It's guaranteed not to change history because, at the moment we push the button, history hasn't happened yet. No risk at all."

"And this is what you've done?"

"Dynamite tie, by the way, Bob."

"Uh, thanks." The tie's lemon yellow, silk.

"Yeah. So, I have to admit, we never told you when we finished up the theory a month or so ago. Since then, we've just been keeping our heads down, building the thing as a physical piece of equipment, getting it working perfectly. You said you wanted results, but I got the feeling that you meant you wanted something to rap your knuckles against, if you know what I mean? Not figures. Product. So that's what we've got. Now. Today.

"Walk with me."

Dirk leads Bob to the elevator. They go down three floors, which is enough to put them underground. The doors open on some deserted and fairly echoey underground corridors. They were painted beige last time Bob saw them; now they're dark red. "Did some interior decorating?" remarks Bob.

"Well, you know how it is, Bob. Image. If we're going to bring people to see what we've done, we have to have something worth seeing, you know? You know what I mean. You know what image is. We've got a little bit of a tour worked out. Let me see if I can find my notes." Dirk holds his cigar between his teeth and unfolds a piece of paper pulled from his pocket.

They stop at an archway leading into a wide-ish, tall, rectangular room. It has two curtained alcoves on one wall, two more on the other, and at the far end is a large, armoured door which Bob knows leads to the main laboratory. In the middle of the room is a scale model of the first iteration of the time machine - a six-foot-wide elevated octagonal platform circled with circuitry and readouts. "Tour as in museum tour," Bob observes. "You've got a little bit of a museum worked out."

"Yeah, it's good, huh?" Dirk goes to the first alcove on the left and pulls a nearby cord, causing a large curtain to draw aside and reveal a glass box on a pedestal, lit by a single spotlight from above. Inside the display case, with a small printed note in front of it, is a shiny metal sphere.

Dirk consults his notes. "See, time travel occurs on the subatomic scale all the time. Suppose you have a particle and an antiparticle appear out of nowhere. Then suppose the antiparticle collides with a second regular particle and they both annihilate, leaving just the first particle behind to do its thing. So it looks like there were three particles in this system, right? Two particles and one antiparticle. But if you see the antiparticle as just a regular particle travelling backwards in time, right, then there is only one particle, zig-zagging through time. See?"

Dirk shows Bob the diagram.

"Now, these really-really-really small reversals happen all the time, all over the universe, because of - quote - 'quantum energy fluctuations in the vacuum'. But, you'd never notice. Ordinarily, those vacuum fluctuations are very tiny, which makes your BIG spontaneous causality violations, like actual person-sized stuff, impossible. Or, at least, astronomically unlikely. Now, here's the clever bit. Our machine overcomes this by leveraging an amount of energy to make large vacuum fluctuations occur. This encourages macroscopic time travel to occur spontaneously. Does that make sense?"

"You know your stuff," says Bob.

"Are you kidding? I love this stuff," says Dirk. "So anyway, the first time we pushed the button on the machine, this is what appeared. A steel ball, most probably a ball bearing of some kind, pulled back in time from some time in the future. We don't know where or when, as yet, seeing as the ball is completely lacking in identifying marks. But we do know for a fact that, at some point very shortly after the experiment, a steel ball identical to this one, somewhere in the world, vanished without trace. Between those two times, there were two identical steel balls in the world. Plus, I guess, the 'signal' connecting them together. When something appears in our time machine, it spits out a virtual signal. The signal goes forward in time and seeks its counterpart in the future and annihilates it when they meet each other. Or, to look at it another way, the signal is the ball travelling backwards in time."

"Interesting. On an abstract level, anyway," says Bob. "The first ever hard causality violation, here in front of us. But... like you were saying, Dirk..."

"Image. I got ya. Not sexy enough. Fully understood, Bob."

"I guess what I'm waiting for is for you to show me what's behind the other three curtains."

"It does get better," says Dirk, who moves over to the next alcove and pulls the next cord, revealing another glass case with a human skull in it.

Bob jumps.

"It's a human skull," says Bob.

"Damn, I meant to show you this third, not second. Yes, it's a fossilized human skull," says Dirk, grinning broadly.


"Think about it," says Dirk, waving his cigar a little. Bob isn't listening, he's transfixed by the skull's hollow-eyed glare. It seems to be looking right at him. "It's a human skull and it's a million years old. There simply is no such thing. Homo sapiens only evolved a few tens of thousands of years ago. The only place something like this could possibly come from is the future. This is absolute proof that the system works. We can forward this to the Nobel Prize gang for radioisotopic dating and peer review. You win that, that's about two million dollars in grant money. It's not much, but it's a start, am I right? Hah hah hah."

"Hah," says Bob. "Are there many of these in the future? Hah. Human skulls, I mean."

"Oh, let's be honest, probably billions upon billions of them. That's a great suit you've got on today, Bob, is it tailored?"

"Uh, thanks..." The suit's dark blue, pinstripe, Armani.

Dirk crosses the room and violently yanks open the penultimate curtain. The third display case contains a small green piece of circuit board with hexagonal circuits and chips. One central chip is much larger than the rest. "This is a microprocessor. Instead of transistors, it seems to have a hexagonal layout of biological cells. Or something along those lines, at any rate. We haven't succeeded in activating it yet and the serial numbers are meaningless. So we're not sure where or when it comes from. But it's not current-generation processor technology. It's at least ten years ahead of its time."

"Much better," remarks Bob. "Do you think we can reverse-engineer this?"



"Suppose we did it. Suppose we ended up manufacturing this original chip in bulk and then one of those chips disappeared and came here and turned into our original inspiration all over again. Then that means that what we've effectively done here is created an entire new processor technology from thin air. Instinct should tell you that that's impossible. There are numbers to prove it, too."

"So what do we gain from this?"

"Somewhere out there, sometime in the future, the original chip is going to be manufactured by someone else. What we can do is look out for that person. Look for somebody researching something similar and provide them with funding or investment. They may have the chip designed by the time we find them, but we can be certain there will be millions of these chips in the future one day, which means we can be certain of a successful investment."

Bob brightens. "I like that. Going back in time and investing in IBM--"

"--was in your original brief, that's right, Bob, I remember. It's the same thing, except we don't have to engage in any risky time travel."

"That's good."

"I said to them that's what you'd say, Bob. I said you'd say that, and there you have it. Anyway, we can cover this later." Dirk moves to the final alcove. "One last one before we get to the main event. By which I mean the, uh, lab. Now, before I open this last curtain, please remember that what you're about to see is, like all things in this museum, a fixed, immutable, inescapable and necessary part of future history." Dirk pulls the last cord.

Behind the curtain, hung from the wall, lit by a single spotlight like the rest, is an upside-down crucifix with an upside-down man nailed to it. He is wearing a tattered pair of trousers and his shirt has been ripped open so someone can carve a deep pentacle across his chest, with arms of the five-pointed star extending across the man's shoulders, arms and face. Blood has poured out of the cuts and stigmata and completely drenched the man's shoulders and face with a red slick before clotting up, making his facial features difficult to make out, though he obviously died in horrific pain.

The man's skinny.

"This came through our machine about a week ago," murmurs Dirk, exhaling a puff of cigar smoke and laying a hand on Bob's shivering shoulder with just a little weight, just enough to stop him from instinctively bolting in terror. "He was still twitching on arrival."

Skin crawling, Bob squints a little closer at the man's tattered clothes. He notes the pinstripe pattern on the trousers. His eyes widen when he spots a bloodied fragment of yellow cloth around the neck.

"We'd have shown you this earlier, but we had to wait, you understand," explains Dirk. He glances behind him and nods to the two scientists who have been waiting quietly out in the corridor for the last few minutes. They advance. "Causality. We had to wait until the day you turned up for work wearing the right clothes."

The white-coated newcomers seize Bob's arms from behind, forcibly. They lift him off his feet and begin dragging him towards the lab door, which is yawning open.

"There's a signal, Bob," says Dirk, as Bob begins screaming. "And we have to help it find its way to you."

It's too dark to see in the lab, and much too hot. There's a heavy crucifix on the floor. Bob catches glimpses of books and ropes and tools and white coats. Dirk's voice and the smell of his cigar follow him into the gloom. "I have to say, we didn't like the idea to begin with, but after a while we kinda warmed to it. This is the way we know history turns out, see? And history has to turn out the way it turns out."

Dirk throws the cigar stub away and closes and bolts the door behind them, leaving Bob in darkness, eyes unable to adjust.

"I mean, we're scientists, Bob. You understand that, right? We're not bad people."


Discussion (65)

2009-01-11 15:44:29 by Matt:

I like the story, but surely they're breaking the same principle as they would be by reverse-engineering the computer chip. Crucified Bob being pulled out of the future causes Bob to be crucified... same basic thing as the chip being pulled out of the future and causing the development of the chip, which he said can't happen.

2009-01-11 16:13:12 by qntm:

Unless the scientists were already very likely to perform the sacrifice anyway, i.e. they were already murderous Satanists plotting to kill Bob. DUN DUN DAAH

2009-01-11 17:50:59 by Josh:

Well, that wasn't creepy at all. And I did notice a double standard with the chip and Bob. Either they made up that thing about reverse-engineering the chip being a bad idea, or they want to test what would happen if they did form a causal loop, as a sort of test. Still, I don't think they needed to do something so horrific. If they chose not to, then somebody else would have done it later. At least their hands would be clean.

2009-01-11 19:05:29 by qntm:

Did... did you read my previous comment, Josh?

2009-01-11 19:14:17 by Ian:

As Sam said, unless they were already planning on sacrificing Bob. Those damned dirty Satanists.

2009-01-11 19:46:30 by Thrack:

At Ian, Planet of the Satanists. This is an interesting way to murder someone. You can get rid of the evidence too (sorta).

2009-01-11 20:32:10 by Felix:

What if Bob freaked out and dug his nails into his arm, say deep enough to leave a scar? That would make the body sent back a different person from him, since the body Bob saw on the cross didn't have such a scar. Of course, since history can't be changed, he would be crucified anyways, just the body sent back would have a scar on his arm, and even then, Dirk probably wouldn't have noticed. Gosh this is making me think. I love the twist ending, by the way.

2009-01-12 04:38:17 by Randall:

While I see what you're trying to do with the "messages from the future can't be used to cause those messages to arrive from the future" bit, I'm unconvinced that there's a significant difference between "reverse-engineering the chip to invent it" and "funding it from the ground floor, knowing whom to fund from time travel." What if that chip would never have been successful save for the massive funding boost provided? And the version they brought back wouldn't have been invented if the initial one they fund wasn't successful? Even without actually doing any reverse-engineering or inventing, they still created the causal loop. I mean, I don't actually have any problems with causal loops, but any use of future knowledge to take actions one would not otherwise have taken leads to either causal loops or making the observed future not happen.

2009-01-12 06:34:29 by William:

Unless, Randall, the special computer chip also got funding from other sources.

2009-01-12 07:20:50 by Randall:

Sure, but if you follow that line of reasoning, you're going to eventually run into chaos theory. "If I only make minor changes to my actions, ones which won't affect the future, then it won't matter; the same objects will come back." Yes, the same objects will *usually* come back, but not always, and it's not (easily) predictable which actions will affect the future too much. And even if you have some magic computer which can ensure that your actions aren't severe enough to cause causal loops or change the future, it still means that the risk is there, that the laws of physics aren't stopping those events. If the only thing preventing you from screwing up history is your future-calculating computer, you might as well have a full-on Time Corp who go into the past to fix what once went wrong.

2009-01-12 09:04:36 by qntm:

The premise of most time travel stories is that a causal loop forms. At the end of the story, it turns out that whatever the time traveller did was destined to happen all along and the loop is complete. However, macroscopic causal loops of this kind are astronomically unlikely to form in reality. The premise of "Causal Noose" is that you CAN have macroscopic causal loops, provided you inject a lot of energy at the beginning (i.e. earliest point in time) of the loop, forcing it to form. The amount of energy is roughly equivalent to the amount of "new information" that the loop contains, i.e. the amount of information which goes all the way around the loop and has no origin point. It *is* a loop that is formed, though. There is no paradox, because if there was a paradox, no loop would be formed. If it was possible to avert Bob's death, then his corpse would not have appeared in the machine. Hence the term "Causal Noose".

2009-01-12 09:36:01 by Randall:

OK, so causal loops are possible, but the energy/information/whatever needs to "come from outside the loop," in that something needs to happen of sufficient magnitude that wasn't caused by the loop that leads to the desired future? Such that if one were to imagine the universe in which the object was not brought back from the future, the same sort of future would have arisen, maybe a little later, but still close enough to produce the same object? Sounds to me like a self-stabilizing causal loop: there needs to have been a "first go-round" where the future arose on its own, and the events we see are the result of many iterations until the past causes the same future it observes. This still runs the risk of such loops blowing up instead of stabilizing, but having the device pick some random object in the future (rather than searching for something specific) provides a hand-wavey out for that.

2009-01-12 11:03:11 by qntm:

I don't understand what you're saying, but no, there is no self-stabilising or iteration going on. This model of time travel does have causal loops, i.e. things which appear to have no origin point, but *the loop itself* does have a cause, which completely precedes it.

2009-01-12 12:39:29 by CJ:

Although it's a bit of a cop-out way to think about it, Dirk WAS planning to murder Bob all the way through the conversation. He could have just been "talking out of his ass so the sacrifice doesn't escape", to use technical language. Of course, what one ought to do when presented with a twitching corpse from the future is to immediately encase it in concrete, dump it in the sea, and begin research into animatronic dummies which can be made to look like whomsoever one desires. You build one (only one is needed! The signal _must_ find its source) and wait for it to disappear. Have to admit, tis more time consuming than murdering your employer, but in the long run probably more lucrative.

2009-01-12 16:18:34 by Deek:

Wow. That was truly chilling. What I seem to understand, from the comments, is as follows. Dirk killing Bob is not a causal loop, since he planned to do so anyway. However, Dirk waiting for Bob to show up in the right clothes is a causal loop. I think that the fact that Dirk already planned to kill Bob, regardless of any sci-fi, reduces the impact of "we're not bad people" somewhat.

2009-01-12 22:49:46 by John:

Really interesting. It actually raises a point that I've been thinking about while watching Heroes. In Heroes, a character called Isaac Mendez paint pictures of future events. Now, some of these future events are of people dying. And some of these people look at their own picture. The picture with them dead in it. The pictures that always come true. My first reaction would be to look at my clothing and NEVER WEAR THAT OUTFIT AGAIN. Unless it showed me being too hedonistic and dying in a wave of pleasure that overwhelmed my senses. That'd be aces.

2009-01-13 01:32:02 by Thrack:

John, what if you'r naked? Then you would have to revise your plan and never be in those surroundings. Though that could be difficult depending on what the surroundings are, for instance maybe you're out next to the road on a foggy day/night standing beside a broken down car. That's bound to happen sooner or later. Of course a simple solution is to stay inside the car for dear life. Like hiding from an invisible killer who can pass though walls and see everything but is picky about killing people. About causal loops, some people have said that you can cause a causal loop if you put enough energy into it, but does this mean that causal loops have a beginning but no end? If that is true then there must also be causal loops with an end but no beginning. I'm not sure how that would work though.

2009-01-13 05:22:28 by Tyler:

Man this is confusing >.< although rather liked the story.

2009-01-13 07:43:15 by DonJaime:

Since the laws of physics are symmetrical, Sam has given us a practical way of creating causal loops/realising time travel: rather than starting with the unknown causes of something just appearing in the present, all we have to do is work out the effects of something just disappearing in the future, which are much easier to calculate. Then we make them happen backwards now, and the result is the creation of the causal loop from the other end, and time travel into the future.

2009-01-13 12:50:53 by frymaster:

Not _quite_ as disturbing as Gorge, but enough to give me nightmares none the less :( Well played, sir

2009-01-13 16:04:34 by John:

If I was naked I would get a really big tattoo.

2009-01-13 18:43:16 by AaronMcDaid:

The victim of any bad news from the future couldn't have been Dirk or any of his engineers. It it was, they would have taken whatever drastic steps necessary to stop it, thereby breaking the causal loop (and thereby making the loop impossible 'in the first place'). So the only bad-news-causal-loop possible is a loop that affected somebody ignorant of the experiment. So the question now is - why did it have to be Bob and not just some random person from anywhere else in the world? It's hard to imagine the bad news causing the random person to fall victim. However Bob was always destined to be taken on a tour of his experiments, i.e. brought towards the machine into near the point in the causal loop which is at the machine at the time it pulls from the future. In conclusion, the first ignorant person to visit the experiment was always going to be Bob. Whether it was a time machine, or just a batmobile, the plan was always that Dirk works on it and then Bob is shown the final result. Therefore, Bob had to be the victim of the bad news. I think the microchip is relevant in an interesting way here. Dirk doesn't want to mess with the development of the chip, but he does want to make the money from it. And perhaps Dirk believes that Bob is the person most likely to get in his way of making the money from the chip. (I had to be careful writing the last sentence - I almost wrote 'perhaps Bob was supposed to develop the chip himself'). Aaron (

2009-01-13 21:01:13 by AaronMcDaid:

I have some further theories to build around my last comment. The 'universe' had to find some way to stop them using the time machine too much. If they did use it too much, they'd be pulling many things from the future causing many complicated interfering causal loops. Much simpler for the first causal loop to be such that it discourages further time travel. Now that the 'universe' wanted to discourage further use, the question is how to do so. A bribe, in the form of the microchip, is the obvious answer. Dirk can now make lots of money and would want to limit further use in case in jeopardized his money making. In particular, if the general public could get lots of free data from the future then everybody would have a great money making scheme (like really efficient transport, for example, or simply stock market moves) and Dirk would find himself no richer than anyone else. Dirk will want to dismantle and bury the machine to be safe. But will Dirk and his colleagues be able to keep it all secret? Here, the universe steps in to cause them to conspire in a murder. This murder will bind them together. As explained in my last post, Bob was always destined to be the victim. In conclusion, the universe will have settled on the minimal causal loop. Only one person is killed, and a tightly-knit group of the knowledgeable are incentivized to keep the loop on course. I wonder if there's any significance in the face that Bob came through alive, barely, through the machine? This would have tempted Dirk to let him die in order to keep the money for himself. Once they've done this little murder (letting a severely beaten man die) it was easy to then start the beating. We shouldn't have to rely on Dirk being evil before the loop, but instead just make it easy for a successful (i.e. cutthroat (metaphorically cutthroat!)) manager to take that little step of letting a commercial rival die. And we don't need to worry about who developed the chip first. The causal loop is satisfied by Dirk placing the chip in the right place in the right time, even if it's just five minutes after they abduct Bob.

2009-01-14 17:08:50 by strangexperson:

They are, in fact, bad people. After time-traveler Bob arrived, they could have called for medical help and revived him.

2009-01-14 20:32:43 by CJ:

AaronMcDaid: I disagree that it could not have been Dirk or an engineer. The universe would simply have been arranged such that their attempts to sabotage the machine were ineffective. Your other thoughts, although interesting and consistent, I think are an addition to the above story rather than something already contained therein.

2009-01-15 09:08:08 by AaronMcDaid:

Perhaps they found it easier to kill Bob, as opposed to a fellow engineer or Dirk, because Bob was somebody few of them worked with regularly? I'm just trying to find the least complicated causal loop that might emerge and would be consistent with the facts we've been given. Many other theories are certainly possible. strangexperson, Once Bob came through on a crucifix, he had to be crucified. The only question is the prior motivations of Bob. If Bob was already contemplating murder, which was of course possible, then the universe would just leverage that to its own advantage. I'm just trying to find a causal loop that relies on what little we are given - namely that Dirk is a successful manager of an ambitious secret project who may not have actually comtemplated murder before. Those types of people who 'Get Things Done' may tend to have murderous tendencies and just need to be given a little push.

2009-01-15 10:04:43 by Val:

I don't quite understand why it is necessary to feed the causal loop? What if they decided not to kill him? Why is this paradox different from the grandfather-paradox?

2009-01-15 19:35:30 by AaronMcDaid:

Val, The conventional grandfather paradox is based on the idea that the operators of the time machine would be able to choose the time and location to travel to and have freedom of action there and then. But in this story, the operators do not have that choice; instead, they are simply given something random from the future. Therefore, the universe will ensure to give us something that it 'knows' we will return in the future. Consider the ball bearing example. The only way to stop that happening might be to gather up all the nearby ball bearings and quickly melt them down. But that would take some time, and if we were going to do that then the machine would plan to remove the ball bearing before we've collected them all. So if there is a way to sabotage the device and change history, it's going to have to involve dealing with whatever we are given with the machine. We aren't given much details of how the machine works. Do the operators just twiddle a dial called 'energy' and hope for something interesting? Could the machine be put into orbit such that it would have to bring back either a pocket of vacuum (or a piece of itself?), allowing us to sabotage it by flooding the device with some gas; "A pocket of nitrogen came back, so we're going to flood it with oxygen instead. Let's see what the universe thinks of that!".

2009-01-15 21:20:00 by qntm:

There's no grandfather paradox in this model of time travel. If there was any way that Dirk could change his mind or something, the time machine simply wouldn't have worked.

2009-01-16 18:00:28 by strangexperson:

Aaron, Mutilated future-Bob wasn't dead yet. "He was still twitching on arrival." It wouldn't have broken causality to have a Bob emerge from the loop scarred but alive, and just leave the satanic ritual culminating in a flash of virtual-particle interactions to be performed by some anonymous third party.

2009-01-16 22:49:47 by Jacob:

My explanation for Dirk's "we're not bad people" comment: he was Lying. Interestingly enough, I didn't trust Dirk throughout the story specifically because of the way he kept commenting on Bob's clothes. Also, when I first read the bit about the human skull, I thought it was a skull from a live person as opposed to a fossil. For which I assume there are no safeguards against. Yaaaay

2009-01-18 13:54:41 by Val:

So it seems this time machine needs to have infinite computational power, and the ability to somehow ignore the Heisenberg uncertainty. To know what kind of signal to emit, it should be able to predict everything down to quantum level. (so it's not possible to fool it by creating something similar to the produced object.. it needs to be exactly the same) Either that, or the universe is fully predictable, we have no free will, everything is just a movie we cannot alter.

2009-01-18 13:57:47 by Val:

On second thought, the two situations presented above are the same.

2009-01-18 21:58:35 by Daniel:

This is no less changing history than sending something back in time. When the ball bearing is sent back in time, the result is the same, no matter when the time machine that causes it is activated. I doubt there'd be billions of fossilized human skulls in the future. Does anyone know the probability of a given skull fossilizing? Does anyone notice the sheer unlikelihood of getting those items, even if those were just the most interesting ones out of years of running the machine? Why doesn't it just get random rocks, over and over again? If your going to suggest that it was just much more likely to get those four, and that's it, then for it to be coincidentally avoiding paradoxes for as long as that machine exists (actually having paradoxes would be impossible, so that doesn't count), that still doesn't explain the first three, as the last one alone should have done that. If it wouldn't, it could have brought back an exploding bomb from a nuclear test. It could have just brought back a random rock, but in such a way that the Brownian motion would have caused a hurricane over the lab, or any other freak accident. Why did they kill Bob? They knew he was going to die, but not that they'd kill him. They could have at least pointed everything out to him, and told him that he can either have them kill him now, in which case he could be sedated, or he could wait for the fate later, in which case he wouldn't. So long as he wears those clothes a lot, he could put it off for a while. I don't know much about quantum physics, but when that particle-antiparticle annihilation happens more than a planck time after the formation, doesn't it require that they replaces themselves with something equally energetic? If so, wouldn't they be causing nuclear-sized explosions sometime in the future, and need that amount of energy in the present to run the machine? Wouldn't using that machine be stealing from someone in the future?

2009-01-18 22:54:20 by qntm:

It's true, they *should* get random rocks (or random chunks of vacuum) each time they ran the experiment. But the complexity of the causal loop each experiment creates is proportional to the amount of energy they put in. A ball bearing is a very small and boring causal loop, but the later results (circuit board, twitching corpse) are much more complex because more energy was used. Also, there would easily be billions of fossilised human skulls in the future if Dirk and his evil cohorts took over the world and slaughtered everybody on Earth. This, at least, is what Dirk thinks is likely, in the story.

2009-01-19 09:04:53 by Val:

Maybe the time machine has a built-in improbability drive.

2009-01-21 00:10:29 by Mick:

I just feel bad that Bob absolutely, irrivocably 'had' to die like that. I honestly can't think of a worst way to go, but then I'm not much of a sadist. I suppose if they told him that his mother never loved him just before he vanished... It would be terrible to live in a future where these machines existed. You would live in a state of constant fear, aways expecting your cat, briefcase, or hell, you leg to vanish. Dangerous tech indeed.

2009-01-29 00:29:45 by Scott:

I am more concerned about where they got the energy rather that what happens on the other end. By my calculations depending on how much bob weighs, the amount of energy they would need would be equivalent to a 1-2 GIGAton nuke.

2009-02-09 01:27:42 by Mick:

Well, if they're harnessing time travel technology, than it's safe to assume that they have high tech sources of energy. Either that, or their machine is increadibly efficient and effective.

2009-06-17 18:49:30 by Boter:

Aaron, your theory of the universe itself minimizing the impact is interesting because it draws parallels from Sam's "Fine Structure", where the universe also takes an active role in minimizing damage (by changing the Script).

2009-06-18 17:12:59 by Tony:

(forgive me if my language is less an elegant, it's my first time posting) My issue is with the idea of the future. Dirk explains that you cannot go back in time to change history but can pull objects to the "present" from the "future." Doesn't this assume an absolute present future? If that ball bearing was an integral part of a machine that failed and people died, wouldn't that change their past assuming that there is some future that exists for them without the time machine? The skull presumably came from approximately 2 million years in the future, but Bob's body came from only a few minutes in the future. Taking objects from a "nearer" future would change events in the past of the "further" future, thus violating Dirk's initial premise, wouldn't it?

2009-07-02 21:45:12 by Phil:

Great story, and interesting comments all around. I know it's been a while, but if anyone's still reading these I've got a question: Assuming Dirk wasn't lying the entire time, how would the particle/anti-particle thing affect such large objects? For example here, if the "signal" that sends Bob back is an enormous wave of anti-particles, does each anti-particle then grab something from the present and go on its merry way into the future/past? Perhaps the energy needed to pull him back from the future simultaneously gathers enough particles needed for the complete annihilation of the anti-particles? Ps. John, you're right, that would be aces. And I'd love to join you.

2009-11-17 15:57:05 by Thor:

In retrospect the revelation that the scientists are satanists would explain why they repainted the walls of the corridors dark red, the colour of blood.

2009-12-01 17:52:49 by luda:

Oh well, when i saw the crucified man i immediately thought that the whole event took place in distant past and they just pulled Jesus out of our timeline hence his disappearance. That ending is good too.

2009-12-15 01:44:03 by Xartavion:

Incredible stories, incredible site. I sincerely hope "Fine Structure" comes to a head before I run out of reading, otherwise I'll slip into the DT's at work mid-week one day.

2009-12-15 01:44:37 by Xartavion:

Delete the above comment - I put it in the wrong place :(

2010-01-09 12:20:26 by Ryan:

"Also, when I first read the bit about the human skull, I thought it was a skull from a live person as opposed to a fossil. For which I assume there are no safeguards against. Yaaaay" Wouldn't that be a surprise... One second you're doing fine; the next, your head collapses into mush and you die. But hey, maybe you'll get lucky and have your skull replaced with a rubber piggy instead!

2010-01-29 21:36:00 by Boter:

strangexperson:"After time-traveler Bob arrived, they could have called for medical help and revived him." Ooh. Good point. Bob-A comes through crucified, crew-B help him and heal him up, Bob-B takes a tour, sees Bob-A (who is still in a bad way, it hasn't been that long), then crew-B beats up Bob-B, crucify him, and let him go back in time for crew-C to deal with. Bob gets to keep living (though he'd probably fire all their asses... hard to tell; he might have realized how they took the hard way to save him and keep them around).

2010-02-04 03:00:17 by Belinda:

@CJ: maybe the reason it was Bob (and not Dirk or an engineer) is because Bob was so little, and Dirk was huge. And the energy needed to power the machine, as Scott points out, is so huge... so the easiest person to 'pull' from the future would be the smallest.

2010-07-23 00:07:26 by Sean:

Actually, Bob could have meant that metaphorically. And still, a human is a human, right?

2010-09-05 01:17:11 by Charlie:

Hmmm... Dirk (big guy) says that by taking objects from the future is not changing history because it hasn't happened yet. He's wrong. When you go back in time you have a good chance of changing history. But when you pull objects from the future, they could still change history (today)because they go back in time, just like you would [change history] if you went back in time. In this story, they DID change history. Bob got killed because of pulling objects from the future. Dirk is also hypocritical because of this statement because the only reason they would want to time travel is TO change history (make more money, for example). Actually, I can kind of think why that might not be true, but point made. Regardless, I'm sure he's thought of that and figured it's still a great story anyway, which it is. Sam, I love your free thought so much. Don't stop writing these awesome stories.

2011-03-10 02:18:06 by Sysice:

Gonna say, this is a lot creepier than any of the stories on the node you linked to on The Vending Machine Murders. You should write horror more.

2012-01-31 21:57:19 by Omegatron:

I just thought of a difference between reverse engineering the chip and just funding it which could explain why the former was impossible and the latter wasn't. If they reverse engineered the chip then knowledge (the chip design) will be created out of no where but if they fund it but don't design it themselves then someone will come up with the design and nothing will be created from nothing. Does this make sense?

2013-05-05 23:22:18 by Dirk:

To be fair, Bob did have it coming.

2014-01-11 16:45:19 by VMLM:

You know what would've been interesting? If at the end, Bob's body didn't disappear as it should.

2014-06-20 21:08:06 by holomanga:

I think I've worked it out. Since they found Bob crucified with a pentagram carved into his chest, they knew that would happen; there's no way that couldn't happen. However, what they didn't know is how it would happen. So, rather than leaving it to chance (e.g. satanists break into the lab and destroy lots of expensive equipment), they simply killed Bob under controlled conditions, to reduce the risk of anything bad happening.

2014-10-21 21:41:00 by K.:

@holomanga: That last clause really needs an "else" in it.

2014-11-26 06:45:52 by 27chaos:

There is no meaningful difference between sending a chip back in time then reverse engineering it and sending a person back in time then murdering him in a certain particular way. Recreating the chip and recreating the murder are fundamentally similar processes. Imagine if, rather than carving a pentagram into his chest, they instead carved the design plans for the chip. Do you see now why the distinction doesn't make sense? Information is information, no matter how big or small.

2016-02-17 09:13:45 by Ezo:

So, here's what Bob should do: after he's closed off in the 'lab', he should bite off his little finger. I presume that's possible. Then, if body pulled out of the future had all fingers intact, that would create paradox. That would mean they pulled out from the future something which is not there. So that would never happen. So they wouldn't pull upside-down cross with Bob on it from the future. Problem solved.

2017-10-16 02:09:45 by Irrevenant:

They're going to be hella embarrassed when Bob doesn't vanish and they realise they actually have a cloned Bob from 2171 behind the curtain rather than the current one...

2020-06-13 08:50:58 by eyqs:

Chilling story. Took me a re-read to realize that the significance of "It's only been half a week" was because Bob wasn't wearing the right clothes the first meeting. Coincidentally, Bob Pritchard happens to be the name of one of my composition professors, and although the real Bob is quite tall and well-built, I'm sure that for his sake he'd be quite relieved if you changed the name! (Obviously, you don't have to.)

2021-10-10 12:53:10 by Zimby:

Well, there goes their funding

2021-12-07 18:18:42 by Amalthea:

It took me a reading of the story to realize it said “Causal” and not “Casual”

2022-08-31 23:47:27 by Anamorphic:

It is absolutely confirmable that the scientists either wanted to kill Bob, or were terribly naive about the nature of time travel. In any version of the universe where a causal loop exists, a corresponding version without it is also a valid solution to the laws of physics because you can blindly extrapolate the laws of physics from a Cauchy surface outside the loop; in particular, even if you grant that blind extrapolation of a Cauchy surface inevitably leads to the machine being turned on and pulling Bob back, there's no reason (besides murderous intent) that it has to pull him back dead. So if you see a causal loop happening that you don't like, you can deliberately force a temporal paradox by interfering in it or refusing to complete it—the fact that you would, makes it an inconsistent solution to the laws of physics that will not ever actually occur, and instead one of the consistent solutions (one must always exist, by the above principle) will happen in its place. (Essentially, when people talk about a temporal paradox destroying reality—that's actually a good thing if you don't like your reality and other self-consistent ones exist that are better!) Anyone knowing this and killing Bob anyway is cementing the causal loop on purpose.

2023-10-04 05:48:39 by Tux1:

I feel like that line of logic just proves my hypothesis that this is some sort of cult.

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