The rush of investors and entrepreneurs to start companies and businesses based on future successes would be interesting. Would Bezos and Zuckerberg have any claim on starting Amazon and Facebook or is anybody free to attempt to be the first of it’s kind? And how about intellectual property? 20 years worth of songs everyone knows but don’t exist yet. Is it whoever can record and publish them first gets ownership?
Yes, thanks. 
This. It’s a whole new ballgame for everybody.
(Except those who were conceived after August 14, 2000. They’re SOL. After all, we’re talking about the perfect case of that chaos-theory phrase, sensitive dependence on initial conditions. A couple conceived a child on August 15, 2000? This time he comes a few seconds earlier or later, a different sperm cell wins the race to the egg, so a different child gets born. If your children are under 19 years and 3 months of age here in 2020, you’d never see them again; you’d just have memories of them.)
That would be one HUGE amount of computational power. Have we even ‘solved’ chess yet? Last time I looked, even ‘simulating’ all the futures of 32 pieces on a 64-square board where only one piece can act at a time, and each piece can only act in a very limited and specific number of ways, was beyond our capabilities.
Compare with 7.5 billion ‘pieces’ who can all act at any time in a virtually infinite number of different ways. And that’s just the people, since after all you’ve also got everything from animals to weather systems.
I think the OP is equivalent to this statement of the hypothetical: suppose the physical world around us, including our physical bodies, suddenly assumes all the forms it had 20 years ago?
See, calling this “20 years in the past” assumes that the physical state of things defines future and past. But it’s truer to say that our mental state, especially if it’s collectively shared, defines the future and past. So it’s more accurate to say that physical objects all become 20 years newer, with things that had disappeared reappearing, and things less than 20 years old disappearing, and me regaining a bunch of teeth…
Yeah, do you think she would be sitting around thinking “I can’t believe I need to wait another 12 years to stab that dude!”
That may be true, but we’d be surrounded by markers that say it’s August 2000, from wall calendars and daily newspapers to the positions of the planets and moons in their orbits.
And we’d not just be 20 years younger, we’d be where we were 20 years ago. If you were living in Cleveland in 2000, and now you’re living in New Orleans, you’re back in Cleveland. You’ll be living in the house you were living in in 2000, and if you were in a marriage that’s since broken up, you’ll be sharing that house with your ex. And while there’d undoubtedly be a lot of people changing jobs soon, right now if people want to keep drawing paychecks, they’d have to go back to their 2000 jobs. Their 2020 jobs might not even exist yet.
So it wouldn’t be a rejuvenated 2020; it would be August 2000, Mark 2.
This was one of the first things that occurred to me.
It seems to me that there’s a few interesting possibilities here. On the one hand, it might lead to a vastly more diverse and fragmented online economy. Instead of eBay and Amazon and Facebook and Google and Uber and a few other massively dominant platforms, you might end up with three or five to ten or twenty competing and successful companies in each area of internet commerce.
On the other hand, even if you know what’s coming, getting it off the ground still requires a considerable amount of capital input, and the determinant of how this new online economy proceeds might depend, as much as anything else, on where the people with investment capital are willing to put their money, and how willing they are to compete with one another for a (possibly smaller) share of the pie.
Also, the people who had already been working for companies like Google in the first iteration of the timeline would have the knowledge and experience to get their product up and running much more quickly the second time around. Technology that only emerged slowly would suddenly appear almost overnight, or at least as quickly as companies could get the schematics laid out and the manufacturing processes up and running.
I’m sure this would work differently in different industries. For example, computer processor advances rely on particular types of advances in chip construction and microarchitecture, and I don’t know enough about exactly how this works to guess at how quickly our 2020 knowledge could be adapted to the reality of the year 2000. If you handed the folks at Intel, in 2000, all the specs and knowledge required to make a 2020 chip, how long would it take them to actually get one up and running? A month? A year? Five years? Would the iPhone 11 of 2020 become the iPhone 1 of 2001, or the iPhone 1 of 2005? And even if you knew exactly how to do it, it would still take a long time and a bunch of work to build out infrastructure like high speed internet. Or would we skip the cable and fibre altogether, and move straight to 5G wireless for everyone, including home internet users?
The intellectual property thing is interesting too, because for things like trademarks and copyright, you need your intellectual property to be not just in your head (or other people’s heads), but committed to a physical medium, often accompanied by some sort of application and/or registration. We would all know that Taylor Swift’s songs are Taylor Swift’s songs, but would she be able to assert a legal right to them if someone else wrote them down first and registered them with the Copyright Office?
Speaking of which, the government would probably need to allocate a massive budget increase for hiring more workers at the US Copyright Office and the Patent and Trademark Office. 
Also, half the people doing this would be kids. You’d have ten- and fifteen-year-olds designing chips and writing code. Would society try to prevent that somehow? After all, the law would also deprive these people of a chance to vote in their own interests, so maybe we can just force them all to go back to school and do their chores. 
I think that there are going to be quite a number of people very upset about waking up and finding that their children no longer had ever existed. (Not sure if I got the tense right, that’s the worst part of temporal paradoxes.)
Would we build the super collider again in order to reprove the Higgs particle exists? Would we perform any of the experiments that we already know the answers to?
Yeah, this would really be a big deal, because while you could always have more kids, you could never have the same kids. It would be as if the kids from the first timeline died. Anyone under 20 years old gone, never to return. My nephew would be alive as a two-year-old, but his younger sister would never exist.
Speaking of which, does the OP’s retention of memory model respect the limitations of age? That is, would my two-year-old nephew be able to understand and appreciate the significance of his 22 years’ worth of memories, or would those memories be too much for a two-year-old brain to comprehend?
Foreknowledge of natural disasters and personal medical conditions would save millions of lives. The dikes around New Orleans could be beefed up. The coasts on the Indian Ocean could be evacuated by Christmas, 2004. The Smiths could change the batteries in their smoke detectors. Bob could get that “mole” removed…
Overall I endorse @Riemann’s various comments.
The 2nd go-around of 2000 will, within days, diverge so wildly from the first go-around of 2000 that much of the macro foreknowledge will be useless. Not that that will stop people from trying to act as if they are the only one with the clue. Which means real quickly the OP’s very intriguing set-up just turns into another “humanity goes nuts” dystopia.
Treating only a short time interval around a major event might work much better as a story.
Such as immediately pre- and post- 9/11 or Katrina or Pearl Harbor or the fall of the Berlin wall. Have everybody jump back from 2 weeks after to 2 months before. Enough time before to influence the immediate outcome, but not so much that the larger world situation on the Big Day can have diverged too far.
I also like the variation with a slowly spreading population of rewinders and the slowly dawning awareness that I’m not the only one with this new magic.
A whole world realizing what they actually would be capable of doing, saying and tolerating because they would have memory of what they did, said and tolerated when push came to shove? I think that much light would burn out a lot of souls.
Well, yes. I didn’t intend to imply we could do it next week. More like - at the Type III civilization stage.
Not sure it’s even theoretically possible. To be complete, the simulations would have to include the simulations themselves, since the simulations would be part of our world. They’d really need to be happening in some meta-universe inaccessible to us.
Reminds me of this story
So, not next week, but a week from next Monday.
Overall the 20 year reset seems pretty awesome to me. I’d be just going into my senior year of high school so even if we kept “kids” as kids I would only have one year of weirdness but I’d have maximum flexibility to adjust my future. I agree that with everyone else going back I wouldn’t have any practical foreknowledge though if we could rewind to the beginning on the summer I’d appreciate one last summer of freedom.
One question I have, will the dead be brought back? My high school football coach died a couple of years ago so does he come back with 3 years of after life memories or does he only have the memories up until his death or does he not come back at all since he isn’t here to be part of the reset? Coming back from the dead would have to seriously mess with people. In my coach’s case he drank himself to death 15 years after the stillborn death of his only child. The kid died right before the reset would occurr (spring of 2000). There doesn’t seem like a good way for him to recover from dying and successfully coach us again.
Too late to edit. What about those kids that died in a car accident a year after graduation. They’ve been dead for 18 years. Do they get brought back? On one hand they get a second chance on the other they are the only people in their “age” group to not have adult memories on one hand if it just 18 years of nothing then they probably drop back into high school really easily if they have 18 years of afterlife or something then they would be really messed up.
It’d probably take longer than that…
But I’d read it, too. I’m a sucker for amnesia or time travel, or ‘protagonist gradually realizes…’ books.
Or, I’m thinking this’d work as a multi-season tv series.
It’s been done, on a more limited scale: Seven Days (TV series) - Wikipedia