1. Cosmic Acceleration... I was raised knowing the universe is expanding, and cannot imagine the shock it must have been to the old-timers who had to change their view of the world from a static one to an ever-growing one. Now another shocker is smoldering, the universe is expanding faster than a California fire fueled by Santa Anna winds, and nobody knows why. Hmm.
2. Dark Matter... Imagine an intelligent race born and raised on an island with sand and palm trees, and ask them to describe the rest of the earth. Since we have seen only a small percentage of all of the kinds of matter in the universe, that is exactly what we are trying to do.
3. Quantum Gravity... This is the hardest problem there is. What else can I say, except that no other problem has attracted so many great physicists and has remained unsolved.
4. Origin of the Universe... I think that one of the greatest accomplishments of physics is that we can even think about this issue. One well exercised strategy to deal with difficult issues like this is: Stop asking about it. But I would like to ask about it anyway.
5. Strings... As far as we can tell, an electron is just a point. It is there, can be measured, but it has no size (that is what a point is). Try to imagine this (spend a few minutes--no cheating). Everything known about particle physics is based on point particles. Now try to imagine that a particle is really a string.
6. Renormalization... What is infinity minus infinity? Mathematicians will frown on such a question, but in physics it is very often something small, but not zero.
7. The Higgs Particle... The Nobel Laureate Leon Ledermen wrote a book, “The God Particle.” Since he was referring to the Higgs particle, I hope this tells you how important it is, even though it has never been observed and never measured.
8. Monopoles... Everybody knows a magnet has a north pole and a south pole. We call it a dipole. If you chop it in half we all know the extraordinary result, you get two dipoles. So how do you get a single magnetic pole, a magnetic monopole?
9. Pioneer 10... Here is a pesky problem. Past Pluto and zooming toward distant stars, Pioneer 10 is experiencing a mysterious force. Some unknown power is trying to pull it back to our sun, but nobody knows what this is.
10. GZK paradox... Theory tells us that the energy of cosmic rays has an upper limit. Today we are measuring the energy of cosmic rays that far exceed this limit. Once again, the impossible is possible. Something is rotten in the state of Denmark. Read more