Who is Stephen Hawking from Extremely Loud and Incredibly Close and what is their importance?

The real-life eminent astrophysicist, long confined to a wheelchair with amyotrophic lateral sclerosis, Hawking appears throughout the novel. Oskar imitates his computer-synthesized voice, reads his A Brief History of Time, which is far beyond his reading level, and repeatedly writes him in Cambridge, England, asking to be his protégé. Hawking sends form letters regretting he cannot keep up with the volume of his mail, but finally after a health crisis responds personally, inviting him to visit and see how boring it is to be an astrophysicist. Hawking wishes he were a poet.