Tess and Harry on the Dock
“Why can’t we have a television here?” nine-year-old Tess had asked her father one night. “You said you watched the moon landing from the lake on a television.”
“That’s true, but it was my parents’ decision to bring a TV that summer, and it was the only year we ever did. And the reception was terrible. Besides, it’s not like we sat around watching game shows all summer. We watched the moon landing and that was it. It was a special occasion.”
“There are other special occasions,” said Tess sulkily.
“We have a radio.”
“It’s not the same. There are no cartoons on the radio.”
“Radio waves are the most beautiful things in the world, Tess. They can send us music and entertainment, and they can teach us about the stars and the origin of the universe.”
“And that’s what you do at your work, right? Listen to the radio?”
“Not exactly. I teach at the university, you know that. But I also work for SETI analyzing radio waves.”
“Listening for aliens, right?”
“Something like that,” said Harry.
“If you ever hear a message, what will you say back?”
“Think about this, Tess. We believe that the earth is near the edge of the Milky Way Galaxy. That galaxy is something on the order of fifty thousand light-years across.”
“And a light-year is really far, right?”
“If we sent a radio wave to a star one light-year away, it would take a year to get there.”
“So really far.”
“Really far. Now, let’s suppose that there is an intelligent, technological civilization on a planet just halfway across our home galaxy.
When you consider the size of the entire universe, that’s practically like a next-door neighbor.
Suppose that civilization sends a radio signal that reaches Earth and suppose we discover a way to reply.
Never mind about different languages or modes of communication.
Now, if that signal left this nearby planet today, we would receive it twenty-five thousand years from now and if we responded immediately, they would receive our response fifty thousand years after they sent the original signal.
Fifty thousand years ago, our species was making crude tools out of stone and maybe starting to think about painting on the walls of caves. ”
“So, if they sent a signal and it took fifty thousand years to get a reply,” said Tess, “they might not even be there anymore.”
“And even if they were, they might have no record of the original signal. Or their means of communication might have changed so much they couldn’t understand our reply.
Our species has a hard time planning things a few years in advance—imagine making plans to be carried out two thousand generations from now. ”
“You couldn’t,” said Tess.
“When we listen at SETI—when we listen for one of those signals, we’re not just listening for signals from far away, we’re listening for messages from long ago. We’re listening to the past.”
“You’re time traveling,” said Tess excitedly.
“In a way,” said Harry with a smile, “we are.”