I should clarify I mean the bucket contains water, and is itself in the universe. If this is not understood then I will be confusing.
Newton’s Bucket is one of the innumerable great physics problems involving buckets. By innumerable I mean two. The other one is the problem of how many buckets you can place over the head of a person sleeping in physics class before they startle awake. But this problem is of only historical interest anymore. Now that they have cell phones students don’t sleep in class, or anywhere else. This has helped them reach the level of maturity where we’re all tired and anxious. That surely helps.
So this is the other great bucket-based physics problem. And the fun thing is that you can work on it without having a bucket of your own. You don’t even need your own Newton, which makes things easier on all of us. First, imagine if you had a bucket. If you really do have a bucket, your imagination is either quite vivid or it’s not good at all. I don’t have a preferred interpretation. You can fight it out with your friends. Make sure to bring up the time they volunteered at the improv night and froze up on stage. That will surely help.
But take your bucket, imaginary or not, and put some water in. You can get water by imagining that you’ve burned some amines and filtered out the carbon dioxide and nitrous oxides. Or you can use the tap if you aren’t any fun. Imagine spinning the bucket, though. As you twirl it around, your arm gets very tired and, if you don’t stop, it falls off. So we maybe imagine hanging the bucket from a rope or a chain that can twirl around without being your arm.
The question is: while you’re spinning the bucket, how do you know the bucket and water are spinning? How do you know they’re not actually staying still while the entire universe spins around them, like the way it works when you’re riding a carousel? For us this is easy to answer. As the bucket spins, it spills water onto your socks. This makes you growl and set the bucket down. You go to change into oh, it turns out those were your last clean socks. Great. Now the day is spilled. You mean spoiled, but it’s spoiled by being spilled, so maybe they’re the same word after all and just look different and mean different things. Anyway your setting the bucket down settles the matter. If the whole universe were spinning instead, it would have to be that your socks spilled into the water. And you know that didn’t happen, unless you stepped into the bucket.
That’s the practical matter. Now imagine this, though. What if there were nothing in the universe except the bucket and the water and the chain it hung from? Then how would you know it was the bucket and water spinning instead? And this is the question that makes cosmologists say “whoooooah” to each other. Meanwhile the other people in the physics department? The ones working on, like, the dispersal of shock waves in rarefied fluids near a phase transition? They’ll say things like, “Yeah, they’re not with us” and “that person with the physics degree and the post as physics professor with an office in the physics department is … uh … we’re going to say in Evo Psych.” The evolutionary psychologists, happy to have someone talking about them in a non-derogatory way, agree.
But this does nothing to answer the question, which we have to do for some reason. So if there was almost nothing in the universe. There’s no distant galaxies. No planets. No things that wiggle. No pillows with flower prints. No battery-powered plastic candles. No nothing except this bucket and this water, how would you know if it were spinning? And the answer is that you don’t know, because you don’t exist. Not unless you’re either the bucket or the water. And we can be sure you’re not the bucket, not with your sense of dignity.
Can we be sure you’re not the water? This is a harder question to answer. To say for sure that you’re water would require pressing yourself into a sponge, pulling back out, and seeing if you can’t. But as the question set out, there are no sponges in the universe. So therefore there’s no way to do this experiment and thus tell.
And so we see the importance of Newton’s Bucket. Thanks to it we understand how fortunate we are to live in a world with sponges, as we would otherwise have no knowledge of which things were spinning. This surely helps.