"If no one comes from the future to stop you from doing it, then how bad can it be?"
This slogan has been doing the rounds on social media. It is in a photograph of the title page to a spiral bound document. The document is attributed to US Robots and Mechanical Men Inc., a fictitious company in Isaac Asimov's Robot stories.
In this post we shall assume that people from the future could come back to our time and make adjustments on the basis of what they could see would be the consequences of actions taken now. They might do so in order to make things better for some people (perhaps us or themselves) at some time later than now. And they might do so indirectly, by leaving information that was relevant to predictions somewhere where we would find it. Even if they intervened directly, for example by changing people's brains just before they made certain decisions or by introducing fatigue into machines that people were using so that the machines did not carry out the actions that the people had chosen, the affected people would not be aware of the fact of the interventions. They might in retrospect be puzzled that they had taken decisions which were out of character, or that previously reliable machines had failed at critical moments, but that would be all.
What consequences might there be for some of our decision-making, and what would be the scope of and limits on the actions of the time-travellers?
In order to keep track, we shall give numbers to years.
Year 1 is now.
Year 600 is the year in which the time-travellers who visit us live. We shall assume that after visiting us they would go back there. It would be where all their friends lived and where the lifestyle was one to which they were accustomed.
Year 900 is a time when everyone alive in year 600 and in the next few generations after them will have died.
We shall speak of people living in, for example, year 600, to mean people who would be confined to that year and decades either side of it if there were no time travel. That is, the people who live in year 600 are the people for whom that year and decades around it are their home period of time.
The benefit for consequentialists
One of the problems with selecting actions in year 1 on the basis of their consequences is that it is impractical to work out more than a few of the consequences, or to work out the long-term consequences. It is possible that an apparently harmless action would in fact have disastrous consequences years into the future, although it is also arguable that little blame should attach to a specific action in year 1 because many other actions not yet taken would also influence what happened years into the future. Consequentialists would love to be able to foresee all significant consequences of actions, but they cannot do so.
Once one reached, say, year 50, it would at least theoretically be possible to see what the long-term consequences up to that point of an action in year 1 had been. It would not be easy, because many other things would have happened in the intervening years. And to do the job in a way that would be of practical help to consequentialists in year 1 who had magically gained access to information that was in fact only available in year 50, one would have to work out how the world would have turned out following all of the alternative actions or inaction that might plausibly have produced a better outcome. That might remain impossible, or it might require the running of many simulations on the lines envisaged by the simulation argument that Nick Bostrom puts forward.
This is the message of the slogan. If actions in year 1 would lead to disastrous consequences, people living in year 600 would come back to year 1 and either prompt us to different actions or make other changes so as to break the causal chains that would have led to disaster. Then consequentialists could do the best they could in year 1, and be confident that their truly massive mistakes would be nullified. But would people living in year 600 bother? Could they coherently intervene? And what might people living in year 900 do?
Would people living in year 600 bother?
We might expect people living in year 600 to intervene if it would make life better for themselves. And it might be that year 1 would be the easiest point at which to intervene, before the consequences of actions in year 1 had spread too widely and deeply. On the other hand, it might be hard to foresee the effects in year 600 of adjustments to year 1. Perhaps the best option would be a broad-brush adjustment to year 1 followed by finer adjustments to years 100, 200, 300, 400, 500, 550 and 575, making adjustments to each year as it would be modified by adjustments to earlier years. (Later we shall note a reservation about making adjustments to years close to year 600.)
Would people living in year 600 care about people earlier than themselves? They might foresee bad consequences of year-1 actions for people living in year 2, or year 50, or year 300. Those people would include the ancestors of people living in year 600. There is a large philosophical debate about what we owe to our near and distant descendants. Here we ask whether, given the opportunity, people should benefit their ancestors.
There is a natural feeling that people probably would do so, assuming no significant cost to themselves. We care about the living elderly. We may not give much thought to the states of mind of the dead, given that there are no such states of mind and there is no current way to affect the states of mind of the dead when they were alive. But if we could do something that would have benefited them back in their lifetimes, doing so would be a natural extension of our current habit of caring for the living.
Having said that, if there were a significant cost to people living in year 600 of benefiting their ancestors, one can envisage them saying that if they did not act the ancestors would never be aware of the lost opportunity to be helped. But then, if intervention in the past became routine, people living further forward (say in year 800) might complain that they seemed to have get things wrong, say in year 799, making year 800 a bad year, and that they had not been helped by people living in year 1200. There might come to be an expectation that future generations, able to estimate the consequences of actions, should help. That expectation might be seen by those future generations as creating an obligation on them to help.
Could people living in year 600 coherently intervene?
The risk of incoherence
A common issue in discussions of backward time travel is that of coherence. If someone living in year 600 comes back and changes something in year 1, it seems likely that things in year 600 would have to change too because the intervening course of history would have been different. Moreover, all the history between years 1 and 600 would need modification.
There are extreme concerns over coherence, for example that someone coming back from year 600 might not have existed under the re-written history, which would at the least seem to make it difficult for them to return to year 600. Then there are apparently more modest concerns that things would have had to be a bit different in various years.
Concerns of both sorts would however be of comparable severity in one sense. It is a fundamental principle that the world has, from any particular point of view, one history. At least, this is so subject to the isolation of some regions of spacetime from others on account of the finite speed of light and the expansion of space.
Call the history up to year 600 absent any intervention History-F (for first). An alternative, History-S (for second), would have to displace History-F from the point of intervention onward. Then what would have happened to History-F?
One answer involves the idea of multiple histories. History-F continues to exist and play out, but at the point at which a visitor from year 600 to year 1 intervenes, History-S starts to run in parallel. Anybody after that point of intervention is either in History-F and can only look back over that history, or in History-S and can only look back over that history. The histories over which they can look back are the same at all times before the point of intervention.
Whatever merits this approach may have in the interpretation of quantum mechanics, it does seem a bit much at the level of human life. It could be correct, but it would raise further questions about which was the real history, where there would be space for all the alternatives, and so on. The counter-intuitive nature of an approach does not show that it is mistaken. We must put up with a great deal that is counter-intuitive in physics. But at the human level, it at least intuitively (and therefore perhaps circularly) seems that intuition should carry more weight, if not as a guide to what is correct then at least as an indicator of the manifest incorrectness of some ideas.
Such concerns would arise even if a split into History-F and History-S was followed by a merger well before year 600, so that the picture was like that of a main railway track and a track that branched off to one side, ran in parallel to the main track, and then rejoined it. There would be some points at which there would be two different histories running in parallel. Indeed, concerns over coherence would be more acute. Without a subsequent merger one might at least say that there was no instant of time at which two histories were running, because each history would have its own time. But with a merger there would be much more pressure to accommodate a common timescale with two histories running at the same time, simply because one could count backward in time from a post-merger point. Moreover, there would be a question of which history should be considered to have preceded the merger. It would be a condition of merger that only one set of records and memories existed. So only one history would be written, and there would be no awareness of the branching and the merger. Nonetheless one could consider the unverifiable possibility of those events and the period of parallel histories having happened. This would challenge the intuitive idea that however obscure the causal pathways, the present was caused by a single past.
(We may note effects on motivation if there were such a merger, so that year 600 and years around it would be the same with or without the intervention. On the one hand the merger would make people living in year 600 more willing to intervene in the past, because it would make no difference to their lives. On the other hand they might say that there would be no benefit to them from intervening, so they might not bother. If the point of merger were before year 400, there would not even be any benefit to anyone whom anyone alive in year 600 had ever met.)
Now let us put all such concerns, with or without a merger, to one side. Just suppose that multiple histories would be created and that someone comes back from year 600 in History-F. They intervene and make a change to the benefit of people in some year later than year 1 (it need not be people living in year 600). That would make some people in History-S better off than they would have been had their history been the same as History-F. But it would leave unchanged the lives of everyone in History-F, including all of the intervener's friends back in the year 600 as it was when they set out on their journey (and as it would be on their return if there was a merger).
We could then ask what the benefit of the intervention would be. If it was to save people from some disaster, there would be a new set of people who did not experience the disaster but the old people, in History-F, would still experience it. So there would be extra people who would have a better life. If it would overall be a good life, we could see the point of the intervention. But then, would people living in year 600 bother to create better histories for people who would otherwise not have existed at all? There are connections both with the question we posed above as to whether one should care about people who no longer exist, and with the view that every positive life is worth creating which leads to Parfit's repugnant conclusion. (Intervention in the case we consider might however not be as repugnant as the breeding of as many people with even marginally positive lives as possible, because in History-S, and even in History-F, lives might be far better than marginally positive.)
One alternative which would remove many of the concerns we have raised would be to have the intervention wipe out History-F, so that it no longer occurred. This would however not be a panacea. It would go against our intuition that once a course of events has unrolled in time, the events cannot be undone. And interveners would have to leave a year 600 from History-F before returning to a year 600 from History-S. A question of the identity of the leavers with the returners would arise, although one might borrow from work on possible worlds and in particular David Lewis's counterpart theory. Alternatively one might claim that History-S was the only history there ever was, it being fixed from the start that the interveners would do their work. One might compare that idea with the conjecture of superdeterminism in quantum mechanics.
Changes at a safe distance
Another answer is offered by the possibility of changes made in year 1 by visitors from year 600 making no difference that would bother humanity before year 601. It is not that no change would be made anywhere. Information and its transmission have energy costs. There is no free information, and no free causation. But if the difference made, which would technically create a split in the universe, were parked somewhere out of sight, for example on a planet that orbited a distant star, the idea of a split history would not be so disturbing. Nobody would have to contemplate the existence of alternative human beings who had split off from them and who would have noticeably different lives, even if there were such splits in theory. The effects of the distant change would then work their way back to Earth at the appropriate time.
The challenge of possible incoherence would not go away, but at least it would not be so directly disturbing to our idea of how human lives play out in time. The effects of the change made by interveners who came back from year 600 would not enter into the human realm until year 601 or later, so there would be no split in human history.
The motive of interveners who came back from year 600 would also be clear. They would be making life better for their own society, including themselves once they returned to year 600. Indeed, the approach suggested here would not allow the interveners to arrange any benefit to people between years 1 and 600, on pain of reintroducing incoherence that would be visible in the human realm.
On the other hand, if that were all that would be done, it would not be clear that there was much point in intervening in year 1. Why not just use, in year 600, the technology then available to change how things would be in year 601 or later years? It is plausible that if people living in year 600 had the technology for time travel and to effect changes far away from Earth, they would have the technology to effect local changes at reasonably short notice. And that would probably be safer, both because time travel might not work as intended and because the causal chain from some distant planet over a period of 600 years would be highly uncertain.
What might people living in year 900 do?
People living in year 900 would have more information than people living in year 600. They would know how the world turned out in years 601 to 899, and they might attribute some of the things that happened in that timespan to interventions made in years before 600 by people who lived in year 600. They might also have more information relevant to the choice of interventions that would influence years before 600, because they might have developed better methods of simulation of alternative realities.
It would therefore not be surprising if people living in year 900 intervened to over-write the work of people who lived in year 600, in addition to making changes in years from 601 to 899.
The layering of interventions on interventions in the period from year 1 to year 599 would not generate new problems of coherence, although the problems already identified would still arise, and in the slightly stronger form that there would be a need to handle three or more alternative histories instead of just two.
There would be a new concern for people who lived in year 600. They could have no confidence that their interventions would be final. But there would also be new reassurance for them. If their interventions were not in fact ideal, and if people who lived in year 900 cared enough, those people would intervene to make corrections.
The process could of course go on, with subsequent generations making further interventions. There would however be two possible restrictions, both of which might also apply to people who lived in year 600 who made the first set of interventions.
The first possible restriction is this. People at any time T might be reluctant to make changes directly within the lifetimes of people who were still alive at time T or who had been personally known to people still alive at time T. The thought here is that there might be a reluctance to disrupt the lives of people one knew or had known, in whatever sense of disruption was appropriate given how the problems of coherence and multiple histories were handled. And direct disruption at points within the lifetimes of those people might be considered to be significantly more offensive than disruptions at earlier points in time which would likewise change the lives of the people in question. Any such gradation of offence would probably be based on a feeling that people at the point of intervention would feel a sudden jolt. That would be likely to be a mistaken feeling, but unless the effects of intervention in the past became much better understood by year 600 than they are now, it would be hard to displace the feeling.
The second possible restriction is this. People might eventually lose interest in the welfare of people in the distant past, and would also think it safer to adjust their own welfare by making interventions in the more recent past so as to keep the causal chains shorter and their simulations less prone to serious error. So people living in year 600 might want to intervene as far back as year 1, but people living in year 900 might not go back further than year 300, people living in year 1200 might not go back further than year 600, and so on. This would limit the number of layers of intervention in any particular year that would build up.
The Prime Directive
In the science fiction stories of Star Trek, there is the Prime Directive. There does not seem to be a canonical text, but the gist is that Starfleet personnel shall not interfere in the normal development of any alien society, for example by introducing technology the society has not yet discovered for itself. Even intervention in order to save a starship or its crew is not allowed.
It is possible that people living in year 600, while they had overcome the technical challenges of time travel, would decide that the philosophical challenges were too great. Then they might adopt their version of the Prime Directive and leave us alone, even though we could hardly be called an alien society.