You’ve probably heard that what happens in Las Vegas stays there. Obviously, it’s not true, but what if it was? The answer is of course “everyone dies.” The surprising thing is exactly what causes of death are competing to kill everyone.
What the saying is referring to is not that nothing can leave the city, but that no information about events on the inside can leave. This is, naturally, even worse. The first side effect is that if you enter the city, you stay in the city. Otherwise your knowledge about what you saw would leave, and that isn’t allowed. Even if you could magically forget everything, or keep from collecting any knowledge while there, the fact that you left and what kind of condition you were in when you did would still give information about the inside.
That means the entire city is full of people who can’t leave even if they want to, and anyone who enters is stuck. It wouldn’t take long for the outside to find out that they have lost all contact, and soon imports stop. People who were already panicking about not being able to leave suddenly find out that they have to worry about competing for finite resources. It becomes postapocalyptic in there, and the city descends into a lawless wasteland. More so than it is already, anyway. Nobody can escape, because then someone outside Las Vegas would know what was happening, and they can’t call for help for the same reason. The city descends into violent anarchy in a matter of weeks at best. Some people, the lucky ones or the prepared ones, might be able to survive until the city’s water runs out. Or they could, if they lasted that long.
The postapocalyptic Lord of the Flies thing isn’t even the biggest problem. Since nobody on the outside is allowed to know what happens in the city, they must not be able to find out by simply looking. That means, no light can leave the city. It’ll look like a perfectly black dome surrounding it, where nobody can see in. People or things can go through perfectly easily, but if they do then nobody on the outside ever hears from them again. From the inside, the wall is completely transparent but impossible to break through. Of course, the people on the outside don’t know that.
The rule is that what happens in Vegas stays in Vegas, not that what happens outside Vegas stays out. This means that light is going in but not coming out. This is unstable. It’ll make the inside of the city heat up until it’s an uninhabitable desert. Again, more so than it is already. And then it will continue heating it up indefinitely. Very quickly it will become unsurvivable.
The city of Las Vegas takes up 135.8 square miles, or 3.7 billion square feet. At noon on a bright day, about 100 Joules per square foot reach the Earth’s surface from the Sun every second. Ignoring all other sources of heat and counting only the Sun, there’d be 370 billion Watts of power entering that black dome and none leaving. That will stack up quickly. (Incidentally, whatever wish-granting genie decided to make sure that everything that happens in Vegas stays in Vegas must be at least a .55 on the Kardashev scale, minimum. Probably more.)
Assume “the city of Las Vegas” extends 60,000 feet vertically. There has to be some arbitrary boundary, and this (the highest extent of controlled airspace) is as good as any. There are then over two hundred trillion (2.2*10^14) cubic feet of air in the city. It takes about 37 Joules of energy to heat up a cubic foot of air by one degree Celsius (or Kelvin), so it would take about eight quadrillion Joules to heat the entire city by one degree. Since we have 370 billion Joules coming in every second, it’d take about 22,000 seconds to do that. That’s about one degree every six hours. Everyone dies of heat stroke in less than two weeks.
Or they would if they lasted that long. How else can information get transmitted, other than light? If you said sound, you’re completely correct. If you said large boulders launched from trebuchets in pre-arranged coded patterns, then you are also correct, as well as being unbelievably cool, but right now we’re looking at sound. Whatever is blocking all the information has to block sound, or else the news could just be shouted across the border. Unfortunately, this gets even more destructive than the thing with the light.
Whatever is happening keeps everything in and nothing out. “Everything” must include “air molecules,” otherwise sound can travel. The good news is that there is no danger of running out of air and suffocating. The bad news is that this is because of the instantaneous planetwide Armageddon. Did I forget to mention the instantaneous planetwide Armageddon?
Air molecules can get in, but not out. So every molecule that enters the dome stays there. Normally, the air around Las Vegas has about the same pressure as air anywhere else, and so things stay pretty stable. But when this one-way wall appears, the air in the city stops pushing back and an entire atmosphere’s worth of air blows into the city. It would be surprisingly similar to what happens if some idiot opens a wormhole to outer space: All the air flies through because there’s no pressure resisting. Or in this case, all the air flies in, and then stays there. It’s like Maxwell’s Demon turned up to eleven and destroying life as we know it.
From the perspective of anyone on the rest of the Earth, the atmosphere suddenly starts going away. For people anywhere close to Nevada, they notice ridiculously strong winds blowing toward Las Vegas. It’s a perfectly black dome sucking everything toward it and not letting anything out. It looks a lot more like a black hole than a black hole does. (Literally. Picture a black hole. Black holes are just weird and look nothing like that whatsoever, but if you tried to picture one then your mental image probably looks a lot like what is happening here.)
In very little time, most of Earth’s atmosphere is packed into a measly few trillion cubic feet. I’m sure you were all wondering, how much time? Would you believe there is actually research on the subject? How about on a different subject that kind of applies here? The air moves in at the speed of sound, and the boundary between the moving air and the stationary air moves away at the same speed. It takes ten hours for the air on the other side of the world to notice that all the air around it has started moving and it should probably do the same. Then it takes another ten hours for the last of the atmosphere to go in and never come out. After that, it’s over.
If anyone on the inside survived the incredible winds and flying debris (nobody did), they are crushed by the air pressure hundreds of thousands of times normal. The entire city is at pressures great enough to synthesize diamonds. If it were still at room temperature, it would be high enough pressure for oxygen to be a solid. Meanwhile, everyone on the outside dies of exposure to vacuum. (It’s not actually a total vacuum. But what remains of the atmosphere is far too thin to matter.) Nobody ever finds out that everything happening in Vegas staying there was the cause of the whole thing.
This is why you always have to be careful when messing with the laws of the universe. Adding in a new law is one of the more dangerous things you can do. One wrong move can go horribly right.