What to add in MS thesis presentation I am about to defend my MS thesis in Physics. How much detail of the methods should I add in the presentation? In my thesis, I have compared the results of my method to an other method's results. Should I give a brief introduction of that method too?

One of the members of the defense committee is from the other field of research. Should I make my presentation very detailed so that he can understand it? But it will make the presentation very long. Probably, more than 40 minutes long.
Your advisor can give you the best advice. You might also attend another student's presentation to get an idea of what is typical at your institution. 

But probably you should cover the ground lightly but be ready to supply more detail as requested. Focus more on your own contribution, but don't neglect the basis on which your work rests. Probably a 70/30 or even 80/20 split seems about right. 

If you haven't given enough detail, I assume you will be asked questions, so be prepared for that with a more complete explanation.