This gets to to crux of the issue the author (and others in contemporary physics, Hawking included for that matter) was raising. We can conjecture, via theoretical physics, a whole slew of possible models of the universe that combine general relativity and quantum mechanics. But, there is a lack of experimental evidence (or interpretation of existing results) to resolve the ongoing "debates". And there hasn't been any real progress, in that respect, in the past 80 years.
We need to test the edge cases of both theories as best we know how. For example, tunnel to the center of the earth (or another planet?) and verify our existing models in the gravity well, maybe repeat the double-slit experiment remotely while we're there. I'd say a task much more difficult, engineering-wise, then flying a human being to another planet and back.