cwjohn, I think your post, and third paragraph in particular, perhaps is the best direct answer to the OP's question. And Bert, there was no need for the blast of abuse at the original question either IMHO. It's an entirely good question
, especially if you have not done much physics. It's also a much more difficult question than it initially appears, as shown by the range of answers!
Yes, there
is indeed a lot of information out there that you can learn yourself, but on the Internet you might not know when you're being told porkies, if you don't already know better. There are pages that will really convincingly try and tell you that the Sun is made of iron, that climate change is a global conspiracy (mind you some Aussie politicians fall for that one
), that the world is going to end in seven months time, or that aliens regularly visit Earth! It's best to answer a straight question honestly, then if the questioner turns out to be a troll, they will surely show themselves to be such. And it's a teaching moment for many others lurking on the thread!