Physics

Is the universe infinite? And if it isn't, what is outside it? Are there lots of universes, or is it all just fractals? And what about other dimensions? Is it possible that time and laws of physics work differently in other universes? Helen from Worcester, age 12

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Hello Helen from Worcester! Thank you for your excellent questions!

Let's start with whether the universe is infinite. The answer is: We don't know! But suppse it is NOT infinite. Then what is outside of it? Perhaps the answer is that there is no such thing as "outside" of it. The universe consists of all of space and time. An "outside" would be a location -- in space -- that is outside of the universe -- and so outside of all space. That is a contradiction, right? (It would be in space and outside of space at the same, um, time.)

Suppose that the universe is not infinite. Suppose you head off in your spaceship in a particular direction, and just keep going straight. Since the universe is not infinite, will you eventually get to a wall that you cannot penetrate: the edge of the universe? No! Instead, you might just find yourself approaching the place from which you started, but from the opposite direction. Space might be curved so that there is only a finite amount of it, but there is no edge to it. (Suppose you were a flat creature living on the surface of a very large balloon. If you headed off in one direction on the balloon's surface, you would eventually arrive back where you began, even though you never reversed the direction of your travel at any point in your trip.)

Finally, you asked whether the laws of physics might work differently in other universes. I don't know whether there are any other universes besides the one in which we live. Some cosmologists think that there are. In fact, some cosmologists and philosophers think that every possible universe exists. (That would nicely explain why our particular universe exists: because every possible universe exists. On the other hand, it would leave us with the question: Why does every possible universe exist?) If every possible universe exists, then of course, the laws of physics in some of those universes are different from the laws in our universe.

On the other hand, perhaps there is only our universe. In that case, the fundamental laws of physics would seem to be arbitrary, brute facts: they could have been different, and there is no reason why they aren't diffferent. They just aren't.

Some philosophers and physicists have speculated that the fundamental laws of physics (whatever they really are) could not have been just a *little bit* different from the way they actually are. If you tried to make them just a little bit different, then the differences would have various consequences, and those consequences would have consequences, and ultimately, you would find yourself dividing by zero, or with probabilities that are greater than 1 or less than 0, or something logically impossible like that. If that's right, then the fundamental laws of physics could not have been a little bit different. But they still could have been a lot different. They just aren't.

Hope that gives you something to think about! Thanks so much for your questions.

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