Sunday morning "Mass" Equivalent.




A discussion about light.



Q & A: How much mass-equivalent light in the universe?
Has anyone calculated the amount of light existing in the Universe? I say amount in terms of matter if light qualifies as matter. If no one can or has done it then that may be the missing force or matter above and beyond dark matter. Light energy is traveling through the Universe at all times from all sources so how much energy is out there. Dumb question, but what do I know. Thanks Wade
- Wade Hampton (age 62)
Phoenix, AZ USAA:This is a profound question and has been studied by many eminent cosmologists. First let's get some basics straight.
First, one should call it mass-equivalent and not simply mass because light has no rest mass i.e. the mass, usually denoted by mo , when a moving object is brought to rest. Since light always travels at the speed of light, it cannot be brought to rest. The mass-equivalent is the energy of a photon or collection of photons divided by the square of the velocity of light, c .
Second, we should not consider the total amount of light or matter since the universe being infinite in extent the total amount is infinite. So what you do instead is to consider the density of light or matter i.e. the amount per cubic meter. That is a well defined quantity.

cont..

[link to van.physics.illinois.edu]