Vikas recently wrote a post about Unsolved problems in Physics. The post covered the development of a theory of Quantum Gravity and the question about the beggining of the Universe. However, the mother of all physics problem is the problem of vacuum or zero-point energy. Lets get some perspective first. When a result differs from experimental value by an order of 10 it is acceptable, by 2 orders(i.e. 100) its a disgrace and by 3 orders is just pathetic. But what if i told you that the best physicists in the world using all possible physical theories, trying to solve the same problem for over a century have got the result wrong by 120 orders of magnitude ! Its so wrong. that its funny.
So What is vacuum energy ? Isnt the vacuum supposed to be just empty space ? Then where does the question of energy come from ? Like all problems in physics if the question appears inexplicable the answer is simple- Quantum Mechanics. According to Quantum Mechanics Everything That can happen does happen. Vacuum is not just empty space- it is teeming with particles flipping in and out of existence. Vacuum is not a state of zero energy but rather minimal energy. Energy gravitates. A positive vacuum energy would tend to impart a repulsive force on a universal scale. This would tend to counteract the effect of gravitational attraction thus affecting the expansion of the Universe and directly its ultimate fate.
If the vacuum does have energy then there are severe limits to its magnitude. Too big and it would to distortions in the path of bodies travelling in free space. But there are no reasons on why the vacuum energy should be small. According to the current model of physics this energy should be huge if not infinite. So here’s the crux of the problem – Let 10 to the power 116 Joules/ c.c be called one unit. Then each kind of particle whether fermion or boson gives an energy of roughly 1 unit. The total energy must add upto some incredibly small energy density in units. In fact a vacuum energy density bigger than .000000000000000000000000000000000000000000000000000000000000 0000000000000000000000000000000………..1 would conflict with astronomical data. For a bunch of numbers, none of them particularly small, to cancel one another to such an amazing precision would be a numerical coincidence so incredibly absurd that there must be some other answer.
Such a numerical coincidence is highly unlikely. Most physicists have infact been looking for a deep and mysterious reason as to why the vacumm energy should be exactly zero. Finding that reason has been regarded as the highest, most difficult and deepest problem in all of modern physics. Every attempt to do so has failed.
PS: There is a lot more on this problem. Perhaps i will write some more. and my fridge is working again.