Let’s say you’re in a nondescript room with a box that can move things backwards in time. Every hour, on the hour, it moves whatever is in it to fifteen minutes ago, but offset by a few meters so that in ends up in a different box (the boxes are labeled). You have a thousand dollars.
(Edit: I came across this post while searching Google today, and I feel the need to clarify that statement about stable time loops; it’s very possible to make money if you have one, because you can look up stocks or lotto numbers or use what’s referred to as Time Loop Logic. However, in the scenario presented, it is indeed impossible.)
The second circumvention method is to send information back in time. Since information is practically free (for small amounts, you only need a pencil and paper, and you get to keep the pencil when you’re done), it’s something that you would do without any incentive – unlike the example wherein you waste $1000, it has some chance of actually happening. The real problem is finding some bit of information that’s worth more 15 minutes ago than it is at the time the T-box activates.
And here’s where we have to take the thought experiment outside the idyllic realm of nondescript rooms and boxes. The first thought that comes to mind is lottery tickets. Fifteen minutes isn’t a long enough time, because a play on any of the numbered lottos rolls over to the next week more than fifteen minutes prior to the drawing (generally one to two hours before the drawing, depending on what state you live in). However, a similar effect can be achieved with the use of scratch-off games. Your course of events would look something like this:
1) Wait by the R-box to see if any tickets come back. If they do, go to step 2b.
2a) Run to the nearest gas station and buy some scratch-off tickets (if you’re too far away from one, you can coordinate this by phone with another person).
3a) Record all the numbers on the cards.
4a) Have the employee run them through to see if you’ve won (this is faster than actually using a penny and checking for yourself).
5a) Make a note of the winning numbers.
6a) Put the sheet of paper with your records in the T-box.
2b) Take the sheet of results to the nearest gas station.
3b) Only buy the winning tickets (most clerks, if it’s not busy, will let you buy scratch cards out of sequence, especially if you act really superstitious)
Of course this plan requires some initial investment money, because you have to actually play the cards to know what they contain. However, you’ve given yourself a huge edge, because for each time you use this system there’s a 50% chance that you’ll be able to skip the whole “losing” part of gambling. However, most of us would be willing to spend $3 on a making hundreds of dollars.
But we can actually increase those odds considerably. At the point you hit 4b, you should have two things: a sheet that tells you which tickets to buy and some amount of cash. One of those things is now worthless to you. Here is our replacement step that we’ll do instead of screaming “Profit!”:
4b) Put the sheet of paper with your records in the T-box.
What happens now is that instead of two universes, one in which you gamble and one in which you win without gambling, there are a nearly infinite number of universes. In one of those, you gamble, while in the other infinitude of universes you win without gambling. This means that for any time you try this system, it’s a freak occurrence for you to actually have to gamble, because all the versions of you on the b-track outnumber the single version of you on the a-track.
(Note that step 4b should be moved down to 5b, and the new step 4b should be something like “If the piece of paper is getting unreadable, write all the numbers down on a new sheet of paper and put that one in the T-box”. You’ll note that if those steps were followed as is, the piece of paper would still be aging with every cycle. A million cycles of 15 minutes is about 30 years. By copying the information onto a new sheet of paper, you can ensure that information is the only thing traveling. Of course, then you get into the problem of transcription errors, but whether that would actually happen depends on what you believe about the universe as far as chaos theory and quantum mechanics goes – whether the physical laws mean that things are predetermined, and how quickly events diverge.)
So using a single player, with a highly restrictive time machine, the best method of exploit is to send information back to yourself, especially if there’s minimal cost associated with that information. Time to expand the scope. Let’s say someone else finds this room, works out the principles of how this mysterious box works, and turns it into just another piece of consumer electronics, which can translate anything of any size to another time and place.
Lotteries would immediately stop, as would most other games of chance, because the house edge would absolutely evaporate. The stock market would most likely stop serving any useful function, because information from the future would (in almost all realities) be flooding in. A large number of information services would fold, because it’s only necessary to pay for an application once, at which point you simply send the source code back to before you paid for someone to create it. Crime would be stopped before it happened, because the date and time of nearly every murder, theft, arson, etc. would be known beforehand. There would also be a mass exodus to the past, where information about the future is even more valuable than in the present, and the further back you go the easier it is to fake an identity.
Of course, this assumes a reality which has experienced more instances of time travel than the initial reality. Arranged left to right, we could order these universes in order of causality, with the leftmost universe having been the root cause of time travel and having never actually experienced anything coming from the future, and the rightmost universe being one in which nothing has ever actually traveled to the past because every attempt has been interrupted by people from the future.
The impossibility of time travel has often been disputed with the simple fact that if it were possible, we would be knee deep in people from the future. However, in this model, we can clearly see that this doesn’t prove anything; it merely means that we find ourselves in a universe further to the left.