I have a time machine and I want to do good with it. I want to save lives.

But I'm worried any change that I could make could have larger and larger ripple effects that change history drastically causing even worse atrocities.

This hasn't been done before now so it is unknown if a change that effects my past or my ancestors could erase me or my chance to use this marvellous machine. I need to limit the large scale global ripples or the butterfly effect of my trip.

My ancestors have lived in an isolated part of the world for the last 100 years but I dare not go back farther than 100 years, for fear of messing with my own past.

I could try to stop terrible wars or kill powerful terrible men but then I know nothing about how the wars and people that come after them will change. I also bear the risk that one of those I save will be a great genius or leader and change history but there is no way around that so I have to accept it.

I can send myself back in a one way trip to any place on the surface of the earth in the last 100 years with up to 200 lbs of equipment available in the world today.

Where and when should I go what should I do, to save the most lives without sending large changes rippling through world history?

  • $\begingroup$ How well does the time machine move groups of people? $\endgroup$ – AndyD273 Dec 8 '15 at 7:33
  • $\begingroup$ @AndyD273 The time machine only moves me one person and is a one way trip. I can act for years after arriving but there is no way back. $\endgroup$ – sdrawkcabdear Dec 8 '15 at 7:38
  • $\begingroup$ @sdrawkcabdear One way to explain the rules in your comment is to say that time is not looping, but rather you are pushing a new branch of history -- the function of the time machine is to pinch one thread of time forward, the index point of the beginning of one single light cone of potentiality is yanked forward like a sweater yarn pulled on a nail and pushed back in the wrong place. This could be Superman-scienced away plausibly enough that its discovery by the character in the story could be shocked once he realizes that he's not saving lives, he's seeing different echoes of them. $\endgroup$ – zxq9 Dec 8 '15 at 9:22
  • $\begingroup$ You can't go back in time to save lives and not drastically alter history. (at least it is very very very unlikely) This is potentially idea generation but is in the same vein as many of our questions to I am voting to leave it open. $\endgroup$ – James Dec 8 '15 at 16:18
  • $\begingroup$ What is your limit on the butterfly effect? People disagree on what it takes to send large changes rippling through history. Some stories require you to kill someone in power to have large ripples. Others believe the mere act of breathing is sufficient. $\endgroup$ – Cort Ammon Dec 8 '15 at 17:58

Where and when should I go what should I do, to save the most lives without sending large changes rippling through world history?

You should go behind your time machine, and pull the power plug out of the wall.

Predicting the effects of causality with time travel is a wicked problem, or a problem with "incomplete, contradictory, and changing requirements that are often difficult to recognize."

But why does that mean you shouldn't go back in time? After all, there's a chance you might save some lives!

Let's look at the two extremes:

Saving maximal lives: 1.2 billion saved

How many lives could you save? According to the World Health Organization, here are the leading causes of death, worldwide:

WHO death causes

Source: WHO

An estimated 56 million people died worldwide in 2012, most of them by natural causes (e.g., diseases are natural causes. Bullets are not.).

So assuming you do not have a cure for old age or heart disease in your kit yet, let's very optimistically say, maximum, you could have saved ~24 million people in 2015. I have no idea how you'd do that, but there it is.

Now, the population of the world has increased dramatically in the past 100 years, from about 1.5 billion to 7.3 billion. I'll assume the death count scales with population (a bit dubious, but more in-depth analysis would be prohibitively difficult). That means you could save about 1/300 of the population, maximum.

Your best bet might be to bring a printed record of every accident, murder, terrorist attack (with documentation on the attackers), etc. So, best case, assuming everyone listens, and doesn't cause other accidents in the process of avoiding the original ones, you could save 1/300 of the population of every year.

That would add up to about 1.2 billion. (See note 1)

Worst-case scenario: negative 7.3 billion

The Cold War was a very precarious time in world history. There were some well-known close calls, and possibly more that have yet to be declassified and published. However, if your far-reaching life-saving efforts were to cause any "ripples" that affected any of these events (or instigated others), and the US and Soviets unleashed their arsenals, the explosions, fallout, and potential nuclear winter could end life as we know it.

Sure, there might be a few survivors (hopefully your "isolated" kinsfolk have a good fallout shelter and plenty of supplies), but it's an unfortunately plausible worst case.

But what's likely?

As mentioned, causality is extremely hard to predict.

To quote a strangely apt observation from WOPR (WarGames) on the Cold War, which seems to apply to this question: "a strange game. The only winning move is not to play."

In other words, I don't know, but I hope this answer helps to put some bounds on the possibilities.


  1. I arrived at the 1.2 billion figure by coming up with a rough plot of world population growth, dividing $y$ by 300, and calculating the area under that curve between 1915 and 2015.
  • $\begingroup$ "hopefully your isolated kinsfolk have a good fallout shelter and plenty of supplies" If they don't, then the protagonist is never born, the time travel never happens the way the protagonist did it, and the die-off never happens that way. $\endgroup$ – user Dec 8 '15 at 15:08

If all you are concerned about is changes in the present, you can avoid butterfly effects by not going back very far. 1-2 days tops.

You see on the news terrorists blew up a plane last night, so you go back to yesterday morning and take recorded footage of the news reports to the FBI.

When they finally let you talk to someone and they don't believe you, make them a deal that when it happens they'll give you evidence to give past him.

Wait the day for the event to either happen or be stopped, make contact, then take what he gives you to past him.

Repeat this 1 day loop until the terrorists are foiled and arrested before hurting people.

Wait till the next disaster and take it to your contact, which will be easier this time since you already earned his trust.

You'll probably only be able to save a few people at a time, and it's mostly letting the authorities do the work, but that's ok, since people's lives will be saved.

How to turn it into a dystopia story:
So our hero has been going back to stop major incidents for a while, and the law enforcement community is looking really good, having foiled several major attacks around the globe.
The hero's contact calls him up one day, says "hey, you've helped us out a lot with stopping these major events, but you could do more, if you want. Last week across the country there were 200 murders, 57 rapes, and a bunch of other crimes we didn't do anything about. How'd you like to save those people too? I'll just give you a flash drive, you go back a week, deliver it, and we'll put you up in a nice hotel with room service."
Sounds really good, huh?
Now we have pre-crime, along with a host of other ways that the system could be abused.

  • 1
    $\begingroup$ I feel like this would make a good crime show: a time traveler that travels back in time by a few days and informs an agent of an upcoming crime/disaster that will happen soon, and then they have to work together to stop it from happening. $\endgroup$ – Mike.C.Ford Dec 8 '15 at 9:23
  • $\begingroup$ Like the show "7 Days"? $\endgroup$ – JDługosz Dec 8 '15 at 13:32
  • $\begingroup$ @JDługosz I don't know, never heard of that one. I picture it being really iterative. So he hears about another attack, takes back whatever information he can get and delivers it to the people that can do something. The attack still happens because of unknown factors, so he collects the new information and takes it back, over and over until they get it right and the bad guys are caught, saving lives. Unless you have all the intelligence the first time, multiple trips are probably unavoidable. $\endgroup$ – AndyD273 Dec 8 '15 at 13:58
  • $\begingroup$ Like how DIY problems can be classified as "will need 3 trips to the hardware store" even if you plan ahead as best you can. $\endgroup$ – JDługosz Dec 8 '15 at 14:00
  • $\begingroup$ Note that the OP said it's a one-way, one-time trip to the past, without any ability to jump back and forth. $\endgroup$ – user Dec 8 '15 at 15:09

I think you could only save a maximum of one or two people.

Saving large numbers of people would increase the potential for changing the world too much. Changing the course of the Titanic for example would put roughly an extra 1500 people into the timestream to cause chaos.

On the negative front there wouldn't be any need to track icebergs and more shipping could have been affected. Especially given the international rules around lifeboats following that disaster.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.