Re: What is negative energy, is there something called cahsmere effect ?

Hello!

In classical physics, it is certainly possible for a particle to have negative potential energy; in fact, the absolute scale for potential energy is not classically measurable. One can arbitrarily pick the point where the potnetial energy is zero, as only differences in potential energy can be measured, and if an attractive force is involved (such as gravity) it is customary to pick the point where the potential energy is zero to be at infinity, so that the potential energy at all finite distances is negative. The total energy is then negative as well as long as the particle is in a bound state (does not have enough kinetic energy to escape the attractive force). This is also true of electrons in an atom (for example).

Another situation arises in particle physics, when you include the rest energy - then the square of the total energy is equal to the square of the rest mass (times the fourth power of the speed of light) plus the square of the momentum (times the square of the speed of light):

E^2 = p^2 c^2 + m^2 c^4

Clearly, the square of the energy cannot be negative, but when you take the square root, both signs of the energy are possible. These negative energy states have been interpreted as antimatter particles (first by Dirac), and antimatter has certainly been seen!

Here is a nice introduction to negative energy states:
http://www.duke.edu/~gek4/phys/page1.html

Note that it is true that, in general relativity, one expects the vacuum energy density to be equal to the cosmological constant, which until recently was thought to be zero; also, the rest masses of all known particles are either zero or positive. So one would not expect the *rest* energy of a particle to be negative, and would expect the energy density of the vacuum to be zero. (Even if we are not dealing with general relativity, we can of course always *define* the energy density of the vacuum to be zero).

The Casimir effect is a very small attractive force between parallel conducting plates, and is an example of a situation where the energy density of a vacuum (the space between the plates) has a lower energy density than the surrounding space; the closer the plates are, the lower the energy is, resulting in the attractive force. So in this situation, it is possible to have an energy density less (but *very slightly* less) than the energy density of the vacuum; this could be thought of as "negative energy".

Here is a page about the Casimir effect, from the Physics FAQ:
http://www.weburbia.com/physics/casimir.html

Here is an article describing the measurement of the Casimir force:
http://focus.aps.org/v2/st28.html