# What do we mean by “particles” when talking about identical particles in QM?

A molecule of O$_2$ and a molecule of my DNA are not identical. Therefore, some sort of restriction must be placed on the word "particle", but what sort of restrictions and why them, not others?

Perhaps the particles must be "of the same thing" e.g. two electron or two hydrogen atoms. Then again, two strands of DNA are not identical but still count as "same thing".

• This post may be very helpful to you. If the math or quantum mechanics in that post are beyond what you can understand, let me know and I can write a more introductory type version of the ideas there. – DanielSank Jul 5 '17 at 20:04
• Two particles are indistinguishable if they are excitations of the same field. – gautampk Jul 5 '17 at 20:09

Two identical particles are indistinguishable. Therefore, whether or not two free electrons (without specifying the spin state $m_s$) are considered to be identical particles depends on the environment:
• However, if a B field exists and one of the electrons has $m_s = 1/2$ and one $-1/2$, then the two electrons are distinguishable. Hence, they are not identical.