"How do you know?" Marek said.
"I'll show you," Gordon said. And he reached for a manila file that said "ITC/CTC Technology."
He took out a blank piece of paper, and began drawing. "Very simple experiment, it's been done for two hundred years. Set up two walls, one in front of the other. The first wall has a single vertical slit in it."
He showed them the drawing.
"Now you shine a light at the slit. On the wall behind, you'll see - "
"A white line," Marek said. "From the light coming through the slit."
"Correct. It would look something like this." Gordon pulled out a photo on a card.
Gordon continued to sketch. "Now, instead of one slit, you have a wall with two vertical slits in it. Shine a light on it, and on the wall behind, you see - "
"Two vertical lines," Marek said.
"No. You'll see a series of light and dark bars." He showed them:
"And," Gordon continued, "if you shine your light through four slits, you get half as many bars as before. Because every other bar goes black."
Chapter 5
Marek frowned. "More slits mean fewer bars? Why?"
"The usual explanation is what I've drawn - the light passing through the slits acts like two waves that overlap. In some places they add to each other, and in other places they cancel each other out. And that makes a pattern of alternating light and dark on the wall. We say the waves interfere with each other, and that this is an interference pattern."
Chris Hughes said, "So? What's wrong with all that?"
"What's wrong," Gordon said, "is that I just gave you a nineteenth-century explanation. It was perfectly acceptable when everybody believed that light was a wave. But since Einstein, we know that light consists of particles called photons. How do you explain a bunch of photons making this pattern?"
There was silence. They were shaking their heads.
David Stern spoke for the first time. "Particles aren't as simple as the way you have described them. Particles have some wavelike properties, depending on the situation. Particles can interfere with one another. In this case, the photons in the beam of light are interfering with one another to produce the same pattern."
"That does seem logical," Gordon said. "After all, a beam of light is zillions and zillions of little photons. It's not hard to imagine that they would interact with one another in some fashion, and produce the interference pattern."
They were all nodding. Yes, not hard to imagine.
"But is it really true?" Gordon said. "Is that what's going on? One way to find out is to eliminate any interaction among the photons. Let's just deal with one photon at a time. This has been done experimentally. You make a beam of light so weak that only one photon comes out at a time. And you can put very sensitive detectors behind the slits - so sensitive, they can register a single photon hitting them. Okay?"
They nodded, more slowly this time.
"Now, there can't be any interference from other photons, because we are dealing with a single photon only. So: the photons come through, one at a time. The detectors record where the photons land. And after a few hours, we get a result, something like this."
"What we see," Gordon said, "is that the individual photons land only in certain places, and never others. They behave exactly the same as they do in a regular beam of light. But they are coming in one at a time. There are no other photons to interfere with them. Yet something is interfering with them, because they are making the usual interference pattern. So: What is interfering with a single photon?"
Silence.
"Mr. Stern?"
Stern shook his head. "If you calculate the probabilities - "
"Let's not escape into mathematics. Let's stay with reality. After all, this experiment has been performed - with real photons, striking real detectors. And something real interferes with them. The question is, What is it?"
"It has to be other photons," Stern said.
"Yes," Gordon said, "but where are they? We have detectors, and we don't detect any other photons. So where are the interfering photons?"
Stern sighed. "Okay," he said. He threw up his hands.
Chris said, "What do you mean, Okay? Okay what?"
Gordon nodded to Stern. "Tell them."
"What he is saying is that single-photon interference proves that reality is much greater than just what we see in our universe. The interference is happening, but we can't see any cause for it in our universe. Therefore, the interfering photons must be in other universes. And that proves that the other universes exist."
"Correct," Gordon said. "And they sometimes interact with our own universe."
"I'm sorry," Marek said. "Would you do that again? Why is some other universe interfering with our universe?"
"It's the nature of the multiverse," Gordon said. "Remember, within the multiverse, the universes are constantly splitting, which means that many other universes are very similar to ours. And it is the similar ones that interact. Each time we make a beam of light in our universe, beams of light are simultaneously made in many similar universes, and the photons from those other universes interfere with the photons in our universe and produce the pattern that we see."
"And you are telling us this is true?"
"Absolutely true. The experiment has been done many times."
Marek frowned. Kate stared at the table. Chris scratched his head.
Finally David Stern said, "Not all the universes are similar to ours?"