Can a physicist say why there is something rather than nothing?
And so what if they can't?
If you are Heidegger please do be very sophisticated (i.e. contemporary)
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It depends on what you mean by "nothing". Modern physics has shown that empty space has energy ("dark energy") and other properties suggesting that it is "something", even though by any ordinary standards an empty void would be considered "nothing". Physics may indeed be able to explain why, given empty space as we know it, there is something of substance in that space. However, there are the further questions of why space has those particular features, and why there is space at all (and whether this last question even makes sense to ask). These questions are beyond science at least as it exists today.
I don't think philosophy has much chance at addressing this either, except to investigate the meaningfulness of the questions themselves.
I don't think anyone at all can explain why there is something rather than nothing. For this reason philosophers such as Quine (see his interview on YouTube, for example) thought this to be a nonsensical question.
Think about it, for any possible answer to such a question, for example, there is something because of X, you can still consistently ask, "And why is there X?"
Several recent books address this topic at least partly from the standpoint of contemporary physics. Jim Holt's "Why Does the World Exist?" and Lawrence Krauss "A Universe from Nothing."
The answers, such as "quantum fluctuation" or whatnot, are not terribly convincing. It is clearly an area where physicists leave the confines of physics proper to engage in metaphysics, often without much knowledge of the philosophical tradition here. The concept of "ex nihilo" simply cannot fit within any rules of "physics."
Yet such metaphysical speculation, by definition beyond experimental confirmation, is perhaps necessary for the groping advancement of physics. Causality, space, and time are being redefined under new extensions of mathematics and instrumentation.
Not sure what your last sentence means. Bit of a ramble. Anyway, it was Leibniz not Heidegger who first posed the question in this way, and it seems to me best resolved by Kant's limits on knowledge, which act as the "enabling constraints" of free speculation. It is generally understood that physicists who address such questions are "off duty."
No, because physicists are in the business of explaining the behavior of the universe in terms of underlying laws. But what law underlies the most fundamental law? The question is nonsensical.
This is a very old question and there are books tackling it. Some of them are written by physicists. Those ones will disappoint you as they take the existence of something for granted, because they tend to equate "nothing" with "vacuum". If you ask a physicist why there is something instead of nothing they will tend to interpret that as "how can matter arise from a vacuum through quantum mechanical effects?"
Physics, as a discipline, seems to eschew the question as I think you intend it, leaving it for philosophers, many of whom (all?) have gnashed their teeth at it at some point.
Why Does the World Exist?: An Existential Detective Story by Jim Holt is an interesting book on this subject. He asks various physicists and philosophers this question. None of the answers are particularly satisfying, but that is to be expected given the question.
What is the significance of physicists being unable to answer this question?
Practically, it probably doesn't make much difference. However, philosophically, it is a bit of an Achilles heel for all scientific disciplines as scientists have to simply assume the existence of a universe. That assumption then underlies all scientific results, and no one likes assumptions.
By the way, a related question is whether your premise is even correct: does something exist? And is there a fundamental difference between something and nothing? Is nothing the lack of something or is something the lack of nothing? As you can see, the very definition of "nothing" is problematic.
Can a physicist explain why […]
Nope. Not its job. Physics isn’t about why, it’s about how.
Physics concerns itself with how the universe (whatever that is) behaves. It does so through observation, via the help of models that it tries to disprove (but, hey, until the model is disproved, it is a pretty good model and we can use it to predict stuff).
The reasons behind why the models work? Not in its scope.