Okay, one day you find yourself contacted by a weeping angel (from Doctor Who). It explains, in writing, that wants to try working alongside humans to better benefit both species... a symbiotic relationship if you will.

A weeping angel freezes whenever they are observed (they appear to turn to stone about a milisecond before anything has the chance to observe them) but can move incredibly quickly otherwise, so naturally this makes working alongside humans a difficult thing to pull off.

Also, they claim to feed on the "potential energy" of sapient lives. Basically, they find a moderately healthy human who has the potential to life many happy years, then they send them back into the past where they live out their remaining years. All the choices and actions they would have otherwise done with their lives are then collapsed to feed the angel. In the grand scheme of things, they are like predators that eat people with the side effect that their 'victim' get to live out the rest of their lives in the past.

In regards to their ability to send people into the past, the weeping angel explains that their natural method of sending things into the past is "almost completely" paradox free. If they send someone back into the past, that person simply cannot alter the timeline in any really dangerous way. They can use all the future knowledge they want to try and invent things before their time, or they try to save random people from disasters they know of, but events will conspire to ensure that history takes it's previously established course. People can however use their time in the past to set things up for the point after they get sent back.

There have been cases of paradoxes and "time poisoning" but the angel insists those were due to "over farming" or generally being careless. This angel has no intention of messing up reality, it just wants a steady (and preferably ethical) method of getting nourishment.

That said, the angel is perfectly willing to offer its services to you so long as it can be arranged to send at least one human 30 or more years into the past per month. Or a larger number of people back 1 year or more. At minimum, it can send people back in time two weeks, but anything less than that uses up more energy than it gets back. It can also just teleport people. Teleporting uses up energy, but less than sending people back in time less than two weeks.

Weeping angels cannot send you into the future, put you in stasis, or otherwise prolong your life or give you immortality. They can however give you a decent approximation of how long you have to live. Alternately, it can attempt to infuse you with "the essence of an angel" which could turn you into an angel yourself. The weeping angel makes no guarantee whatsoever that such a transformation would be pleasant, or that the resulting angel would truly be you instead of just its own offspring that consumed your body and memories. It never really saw the appeal for such activities.

If you can help it get a steady supply of humans willing to get sent into the past, it will gladly provide its help and maybe get some more weeping angels that might be interested in assisting with such an operation. However, trying to keep it locked up or forcibly observing it for long periods of time (which are essentially the same thing as far as it's concerned) will earn its ire.

How would you best exploit this situation to your advantage?

Interesting.

If an angel sends a human back in time less than the duration of that human's lifetime, can the human interact with their past self? And conversely, can the pre-sent-back human observe the past activities of the sent-back human?

Stable time loops aside, can the human that will be sent back take advantage of known future events as long as they don't seriously affect the timeline? For instance, can they win the lottery, or inform someone about the outcome of an experiment before they perform it?

How far back can a human be sent? Can they bring objects with them?

2546543
Considering how many people would love to return to this or that decade, charging them for the privilege would be a pretty lucrative little side gig. I could collect a finder's fee (in addition to a personal shopper's fee from the angel's rightfully larger share, since s/he can't exactly spend it in all the regular ways), although seeking those people out sounds really tedious and I wouldn't want to spend the bulk of my time doing it. Maybe I'd just make a big wad of cash after a few years and then live on the interest.

The spirit of the premise seems to be that you can't change the past or profit unduly from any temporal shenanigans, but I'd definitely have to spend some time conversing with the angel to feel out just what the mechanics of the situation were and if there were any loopholes.
I suppose I'd ask people beforehand to set aside accounts or objects that would appreciate in value (maybe that could be their form of payment for the service), which I could then retrieve and hawk for profit (or just keep around if they're neat).

If the angel can't send me into the future, though, after giving me a ballpark figure of my lifespan, s/he really doesn't have anything I want in regards to special abilities (maybe the angel is really cool to hang out with, though, who knows).

2546665
Yeah, I was gonna say, lots of crotchety young people would pay to be sent back some distance in time.

Thoughts:

1. What is its definition of a person, and how far back can it send people. Can it send a non-sentient embryo back 10 billion years and be satisfied for... around 28 million years?

2. If you have a scientist looping back 2 weeks for a while, could they get any interesting research done, from our perspective, instantly?

3. If you were to make a promise to yourself that, in 50 years time, you would send someone back with all of the scientific and technological advances from the next 50 years, what would happen?

4. Weeping angels can send back the clothes people are wearing as well as the people, what is their limit for non-people matter? Could they send an entire spacecraft with someone? A planet?

5. If 1 and 4 turn out in the right way, we could send someone back to the early universe to observe things. Would there be any way to get the information they find back?

6. How many people can it send back in a given amount of time? Could we send everyone in the world back if there was some incoming disaster? Although I suppose we could just turn a lot of people into angels if this situation came up.

7. If you send a 100kg person back 30 years, the universe has 100 more kilograms in it for 30 years, and 100kg equals... about 9*10^18 joules, can that be taken advantage of, or does the fact that after 30 years the same amount of matter effectively disappears render this pointless?

8. What if you send someone back during a significant event where they would have to be noticed, like Stephen Hawking's time traveler party that no one showed up to?

9. What are the limits on its teleportation ability? Would it make colonizing other planets and stars nearly trivial?

10. Could its abilities be used to defeat entropy? Continuously teleporting water up a river with hydroelectric dams along it, for example.

That's all I got for now, great question!

2548613
5 is relatively trivial (depending on 4); all you need is an information storage medium that can stand the test of time, and it'll catch up to you all on its own. If your prospective reporter is particularly clever they might even be able to have light itself do the job- if they take a device with them capable of transmitting a message on loop for, say, a thousand years, and have it start transmitting 14 billion years ago from a point 14 billion light-years away from Earth, then receiving is just a matter of pointing a satellite dish in the right direction...
I wouldn't want to be the poor sucker who gets to live out the rest of their days at the start of the universe though.

2554428

I think you mean the rest of their femtoseconds.

2548613

1. What is its definition of a person, and how far back can it send people. Can it send a non-sentient embryo back 10 billion years and be satisfied for... around 28 million years?

Lets say that weeping angels only get energy by "destroying destiny" (or some similar magical sort of rule). Or more specifically by forcing people to live their lives in the past (where all decisions have already been made) as opposed to the future (which supposedly hasn't been written yet).

So, the angels get energy for every year that the victim lives in the past (but the act of sending them into the past could use up energy as well). So, if someone has the potential to live a good forty years longer and the angels send him forty years into the past, and he survives those forty years... or lets be fair, suppose he survives thirty six years due to not having access to the advanced medicines and such that he would have gotten if stuck around and saw the advances the future brings.

If he spends thirty six years in the past, the angels get energy for those thirty six years. Since the act of sending people back also uses energy, angels would tend to attack people with the potential to live long lives and only send them far enough back to ensure that their victim died of old age around the time they got sent into the past (which fits in with the angels actions in the episodes).

So... as for the embryos. I'd say that it would only work if that embryo actually did grow up into a human being who lived at least some of those years in the past. I suppose, in the future people could genetically modify humans to be immortal or not suffer from age., then they create an embryo with the potential to live forever (or at least a long time) and then send that embryo back to be implanted into a surrogate mother in the past, and that child could grow up healthy and strong and possibly live to the time it was originally sent back.

But just sending inert lumps of flesh probably isn't going to do anything. Sending a frozen embryo back to say... Antarctica where it somehow stays frozen but alive for centuries might work. But I'd say it would give out significantly less potential energy than sending a sentient or active person. You might make a case if that embryo was legit going to be grown into a human but was effectivly 'killed' by getting zapped into the past.

2. If you have a scientist looping back 2 weeks for a while, could they get any interesting research done, from our perspective, instantly?

Probably. Time travel gets weird in Doctor Who but in the episode Blink, a time loop arose where the Doctor was able to send a message to the future to save himself in the past, by using information brought about by him saving himself.

So, if done properly I'm sure the time loop could be created to get stuff done pretty quickly relative to the rest of world. If done poorly it could potentially result in paradoxes that do weird stuff like make the universe reset itself to fix the problem.

I'd say that using this for simple things like just fitting "a years worth of work into a single month" without sharing too much info between loops would be totally safe and fit in with the whole "angels destroy destiny" sort of thing. If the scientist was doing things "the natural way" then he could get his years worth of research done in a year, experienced that year in a "natural human life" and maybe made friends or seen history go by or whatever. By reliving those same few weeks to do research, he's sacrificing whole weeks of life and experience to "merely" get his research done faster.

If he does a year of research in a month, that means he will chronologically die 11 months earlier. The angel feeds on those 11 months and he get his research.

3. If you were to make a promise to yourself that, in 50 years time, you would send someone back with all of the scientific and technological advances from the next 50 years, what would happen?

No idea, future hasn't been written yet. Though I'd guess it depends on how likely it would be for the right people in the future to send the information back considering the steps you made to ensure it happens. Depending on how much the universe likes the idea, you could either get nothing coming back, you get the info you want, or you get a message reading "Do not mess with time!" from your future self. I guess it also depends on how likely that information would "pollute" the timestream.

Like say... if you get research saying that in 50 years they almost perfect brain uploading, then you use that info to actually perfect brain uploading, and then the guys who were going to send you that info originally send the new info on how they perfected brain uploading... then you've got a paradox and crap happens.

So, it could work if you're careful.

4. Weeping angels can send back the clothes people are wearing as well as the people, what is their limit for non-people matter? Could they send an entire spacecraft with someone? A planet?

No idea. I think in the episode Time of the Angels, an angel sent a ship off course (though I'm 99% sure it was just through space and not through time). I'd say that normally one angel can only send back one person with however much that person could comfortably carry on themselves. For spaceships or planets, you'd need to get more angels (who in turn would need food to sustain themselves and you'd need to keep an eye on them since they are still technically predators).

5. If 1 and 4 turn out in the right way, we could send someone back to the early universe to observe things. Would there be any way to get the information they find back?

The angels themselves are said to be older than the universe (according to the Doctor), and the image of an angel supposedly becomes an angel itself. So, depending on how the heck you plan to survive the beginning of the universe or acquire meaningful data (lets assume sending people back there doesn't result in huge butterfly of doom problems) you could probably send an angel, an image of one, or a human turning into an angel back to ensure the data gets taken an delivered.

However, I'm assuming that sending stuff into the past still takes energy and that sending stuff farther back than the estimated survival of the target would reduce the energy gained from the process, and going too much into the past would require some kind of extra supply of "potential energy" to power the process.

I'm guessing that sending someone to the beginning of the universe is one of those things where you'd need a huge ritualistic orgy of human sacrifice to pull off... I'm talking millions of angels gorging on the lives of billions of humans on a whole planet at the near center of the universe... generations of humans living in the same few years as their children, grand children, and their current selves watching their families past present and future die of old age to fuel a ritual of massive proportions. This isn't something your everyday Rationalist is going to pull off on a weekend by themselves, and it isn't going to be without cost.

Probably better to just invent a time machine or something.

6. How many people can it send back in a given amount of time? Could we send everyone in the world back if there was some incoming disaster? Although I suppose we could just turn a lot of people into angels if this situation came up.

Dunno. From the episodes, it seems like angels can be satiated once they have enough energy so if an angel does keep sending lots of people back then they won't be getting energy for every one (and might even start running into health problems even if they can keep sending people back). They can send people back "in the blink of an eye" so I'd say if you put a single angel in a dark room and herded people in a huge line it could probably zap them all with about the same speed as if you had them running into a straight up portal. So long as nobody looks at it or the angel doesn't run into any problems of "overfeeding" or whatever.

However, there is the issue of where all those people would go. My interpretation of "Angels take Manhattan" is that the angels are sensitive to paradoxes and that causing too much trouble in the timeline can poison them (which is why they don't go around screwing everyone over with their at-will time travel powers). If an evacuation becomes necessary, it would be best to have a plan on where/when to send everyone or the angels might refuse or they get poisoned (which retroactively kills them and then all the refugees get sent back into the present regardless).

7. If you send a 100kg person back 30 years, the universe has 100 more kilograms in it for 30 years, and 100kg equals... about 9*10^18 joules, can that be taken advantage of, or does the fact that after 30 years the same amount of matter effectively disappears render this pointless?

Dunno, I assume the change of mass doesn't drastically change anything in the universe. Or if it does, the fact that timetravel exists and all the rebooting and stuff is a bigger issue.

Though that could be related to how they feed... since sending people into the past means that the universe now has X larger amount of mass in it. Or their "potential energy" is more information related or something.

8. What if you send someone back during a significant event where they would have to be noticed, like Stephen Hawking's time traveler party that no one showed up to?

I'd assume doing that would poison the timeline and risk killing the angel (or at weaken them enough that they are retroactively prevented from sending the person to that point).

9. What are the limits on its teleportation ability? Would it make colonizing other planets and stars nearly trivial?

No idea, I think in the show the longest teleportation was sending someone from say... England to Scotland or some other country. But that was one angel sending one woman to a place it knew she wouldn't be heading back from before dying of old age.

However, the weeping angels can apparently travel between planets and I can't imagine them having their own ships or being able to reliably hijack them whenever they want. So, I'm going to assume that angels can teleport things to other planets, but they still need 'fuel' in the form of the years they force people to live in the past. I'm guessing they would need to feed on multiple people to get the energy needed to send one or more people to another planet.

10. Could its abilities be used to defeat entropy? Continuously teleporting water up a river with hydroelectric dams along it, for example.

Dunno, I doubt it. They have displayed a number of "spooky" abilities like sealing off rooms to trap someone, draining power from spaceships, dimming lights, and the like. But my headcanon is that they don't so much defeat entropy as they have a way of hitting it in unexpected ways.

At best, they might be able to do the seemingly impossible and get energy from nothing... but the method they use to do that results in people getting sent into the past where they can't see what the future brings. The angel might be able to teleport water but doing so would drain energy. They might be able to power some kind of macabre Human Wheel where they send people up to the top of a giant wheel that takes two weeks for the wheel to lower them to the ground (I'm arbitrarily dubbing two weeks to be the point where the energy to send a person back is equal to the energy gained from the lost potential. Essentially, they can send people back two weeks with no loss or gain of potential energy).

Or more practically, you could just ask the angel to generate electricity and power it through a series of wires for easy use (considering what they can do, it wouldn't surprise me). But in order to do that you'd have to supply it with an equivalent supply of people to harvest potential energy from.

Anyway, good questions!

2562372
A guy from a couple years after they perfect brain uploading could get you the date they're going to get it right, or the names of the researchers involved. Then it's just a matter of enforcing that.

I think the fundamental issue with a lot of these plans is the implication that they would TAKE time energy, rather than produce it. If the energy gotten by sending someone back in time is the difference between how 'productive' they would be in the past and how 'productive' they are otherwise, minus some base cost ('digestion' so to speak), then we would first need to supply the angel with a fair bit of time energy before pulling tricks like generating lots of information from nothing, since in those cases the energy gotten would be negative. So the first task would be to figure out how to get energy. Here is an example which seems like it would work:
Precommit the following: If I catch a serial killer, I will either have them sent back in time by the angel into a prepared jail cell where they will live out the rest of their life unproductively, or give them access to grenades, machine guns, whatever helps them and let them go.
Catch a serial killer (for this example. Hopefully safer similar setups could be developed). Send the serial killer back in time. The angel now gets energy based on the difference in 'productivity' between the serial killer spending its remaining life in jail doing nothing, vs the serial killer being let go with above mentioned weapons and such. This should be a massive difference. Once the angel has enough energy, start using some of its energy (based on some agreed on split) to pull fun things like ontological paradoxes and such.