I guess you'd have to call that bumper sticker "appeal to authority." At least that's the English term I've heard. I don't know if there's a Latin equivalent. The reader is supposed to be persuaded that the statement is more likely to be true because Einstein said it, than if some unknown person said it.

In science, who said anything is supposed to be irrelevant. You don't prove that relativity is true by demonstrating that Einstein was a genius. You prove it by examining the evidence, and the logic of the way the evidence is interpreted. For those of us who don't know how to interpret the evidence, we have to trust somebody. I think relativity is true because my physics professor said so, and I don't think he's the kind of person who would lie or could be easily deceived.

The Nazis fell into the "ad hominem" trap by assuming that relativity couldn't be true because Einstein was a Jew. Good thing, too, because it prevented them from developing the atom bomb. (Or so I've heard.)

In science, experiments are supposed to be repeatable by anyone who has the right equipment. Repeating the experiment and getting the same results confirms the theory.

In law, the kinds of events that are discussed in trials are often not repeatable. You can't rerun a car accident to see who was at fault. So you have to rely on witnesses (or not, if the witnesses aren't credible). The character of the witnesses certainly is relevant.

In politics, practically any statement you hear is about the aggregate behavior of thousands, perhaps millions of people. What does it mean to say a nation is preparing for war? Certainly, within any large population, some people are preparing for war while others are preparing for peace. Even within a government, different agencies may be pursuing different ends. I'd be very surprised if this weren't true. Politicians make their living by oversimplifying the case.