The Nobel Prize

It was a quite autumn day, and Jack was enjoying an ice cream as he waited in his detective agency. The door opened to another neighbourhood boy, “Hello, ” he started, “My name is Jonathan Davis.”
Jack recognised him. Jonathan Davis was in the first team for Rugby and Football but he held his own in knowledge as well.
“I have a chance to buy two outstanding pieces of memorabilia: Nobel Prizes.”
He explained further, Jonathan was a collector of anything to do with physics, and Ernie told him he had two Physics Nobel Prizes in his possession.
As Jack once again walked up the road to the gang’s den, he noticed them playing cards outside. As Ernie saw Jack, he said: “Distance makes the heart go fonder. Now please go away.”
Jack asked: “Can I see the Nobel Prizes?”
Ernie proudly showed his prizes.
“The first one,” he boasted, “belonged to Stephen Hawking for winning the prize in 1996.” He pointed to a old medal. “The second one, ” pointing to the second prize, “was given to Einstein for the theory of Relativity in 1921.” His gang cheered at these Nobel Prizes.
“It looks pretty real to me,” said Jonathan. “It’s only £500,”
“You’re not paying for that,” said Jack “£500? It’s a fake!”
How did Jack know?