Well,Take a galaxy for example. Find out the orbital velocities of all the stars revolving a common center of the galaxy. Well,we can calculate mass with the help of this.But what was the other way they used to calculate the mass which lead to disagreement between them and introduced the concept of dark matter?
Hi ash64449, The Compton Wavelength represents rest mass while the reduced Compton wavelength represents relative mass. If you compare relative mass with rest mass you have a discrepancy that is a percentage of the ratio 2*Pi:1 or 6.3:1 depending on how representative your sample is. http://en.wikipedia.org/wiki/Compton_wavelength
The other way (simplified enough for me to understand) is to look at the galaxy and see how much light-emitting matter there is.
what is the relationship between them? How can we know the total mass by looking how much light emiting matter is there?
pete,i think answer to this was given by first poster... Compton wavelength.. Which is planck constant divided by rest mass times speed of light.