Leonard wants to travel back in time and prevent himself from buying a time machine. Sheldon points out the futility of his plan.
Sheldon: Leonard, it’s two in the morning.
Sheldon: So it’s my turn. Why did you set it for the day before yesterday?
Leonard: Because I want to go back and keep myself from getting a time machine.
Sheldon: You can’t. If you were to prevent yourself from buying it in the past, you would not have it available in the present to travel back and stop yourself from buying it, ergo you would still have it. This is a classic rookie time travel mistake.
Leonard: Can I go back and prevent you from explaining that to me?
Sheldon: Same paradox. If you were to travel back in time and, say, knock me unconscious, you would not then have the conversation that irritated you, motivating you to go back and knock me unconscious.
Leonard: What if I knocked you unconscious right now?
Sheldon: It won’t change the past.
futility = the fact of having no effect or of achieving nothing
ergo = therefore
rookie = a person who is new to an activity
to knock someone unconscious = to hit someone so that they become unconscious; You could also say to knock someone out.
unconscious = in the state of not being awake and not aware of things around you
irritated = annoyed or angered
(definitions taken from the Cambridge Dictionary)
Another funny scene from Big Bang Theory:
Martin hat es sich zum Ziel gesetzt, Blogbeiträge, Videos und Onlinekurse zum Thema ‚Englisch lernen‘ zu verfassen, die abwechslungsreich, effektiv und lehrreich sind. Vor allem solltest du dadurch Englisch schneller und mit mehr Spaß lernen können als bisher.