o time-travel was impossible.

On a higher technical level, there is just one law of nature which seems
infallibly true, since its latest modification to allow for nuclear
energy. It is the law of the conservation of mass and energy. The total
of energy and matter taken together in the universe as a whole, cannot
change. Matter can be converted to energy and doubtless energy to
matter, but the total is fixed for all time and for each instant of
time. So, if a ship could move from one time-period to another, it would
lessen the total of matter and energy in the time-period it left, and
increase the total when--where--where-when it arrived. And this would
mean that the law of the conservation of mass and energy was wrong. But
it wasn't. It was right.

Soames tried to reconcile what he had to accept with what he knew. He
failed. He provisionally conceded that the children's civilization did
something which in his frame of reference was impossible. They had other
frames of reference than his. He tried to find their frame of reference
in something simpler than time-travel. He picked one impossible
accomplishment and tried to duplicate it, then to approach it, then to
parallel it. He scribbled and diagrammed and scowled and sweated. He had
no real hope, of course. But presently he swore abruptly and stared at
what he had drawn.

* * * * *

He'd begun a second set of diagrams when the two physicists of the
investigation-team came back. There was a short man and a thin one. They
looked dazed.

"They are children," said the thin man in a very thin voice, "and they
are human children, and their science makes us ridiculous. They are
centuries ahead of us. I could not understand any device they had. I
could not imagine how any of them worked."

"It is impossible to talk at a distance," said Soames.

"What do you mean?" asked the thin man, still numb from what he'd seen.

"Sound diminishes as the square of the distance," Soames explained. "You
can't make a sound--unless you use a cannon--that can be heard ten miles
away. It's impossible to talk at a distance."

"I feel crazy too," said the short man, "but there are telephones."

"It's not talking at a distance. You talk to a microphone at a few
inches. Someone listens to a receiver held against his ear. You don't
talk to the man, but the microphone. He doesn't listen to you, but a
receiver. The effect is the same as talking at a distance, so