What did Einstein mean when he said the word "giddy" from the phrase about quantum physics?

He was delighted with how beautifully it all worked together.

The actual quote is “anyone who is not shocked by quantum theory has not understood it,” and it was said by Niels Bohr. What you are asking about is a variant or rephrasing by someone who either didn’t remember the original or didn’t think it was interesting enough in its original form. In fact, it’s a variant of a variant since the quote is more often rephrased as “if you think you can talk about quantum theory without feeling dizzy, you haven’t understood the first thing about it.”

Einstein said that quantum entanglement was “spooky action at a distance” which has a similar meaning. He didn’t really believe it possible though it has since been proven to be a fact.

@flutherother What Einstein called “spooky action at a distance” was a particular interpretation of quantum entanglement according to which entangled particles could affect each other instantaneously at a distance (which violates the special theory of relativity). The current understanding of quantum entanglement does not say any such thing.

@SavoirFaire As I understand it entangled particles do influence each other at a distance but in a way that doesn’t allow the transfer of information and doesn’t violate the Special Theory of Relativity. Wasn’t that what John Bell’s experiments showed?

@flutherother Yes. But faster-than-light transfer of information or energy is what Einstein was worried about. That’s what made it “spooky.” What Bell showed is that the sort of action at a distance that quantum entanglement allows is non-spooky (at least, as far as Einstein was concerned). It’s a nuance that a lot of science writing misses due to Einstein’s explanations being easy to misinterpret.

Trying to explain Einstein’s work in a way that was less prone to misinterpretation was, in fact, another thing that Bell tried to do over the course of his career.