Then he applies this more than infinite power to travel at two hundred and four thousand miles per second for _ten_ seconds. He has then penetrated the past. How far?"

Again I hesitated.

"I'll tell you. _One second!_" He glared at me. "Now all you have to do is to design such a machine, and then van Manderpootz will admit the possibility of traveling into the future--for a limited number of seconds. As for the past, I have just explained that all the energy in the universe is insufficient for that."

"But," I stammered, "you just said that you--"

"I did _not_ say anything about traveling into either future or past, which I have just demonstrated to you to be impossible--a practical impossibility in the one case and an absolute one in the other."

"Then how _do_ you travel in time?"

"Not even van Manderpootz can perform the impossible," said the professor, now faintly jovial. He tapped a thick pad of typewriter paper on the table beside him. "See, Dick, this is the world, the universe." He swept a finger down it. "It is long in time, and"--sweeping his hand across it--"it is broad in space, but"--now jabbing his finger against its center--"it is very thin in the fourth dimension. Van Manderpootz takes always the shortest, the most logical course. I do not travel along time, into past or future. No. Me, I travel across time, sideways!"

I gulped. "Sideways into time! What's there?"

"What would naturally be there?" he snorted. "Ahead is the future; behind is the past. Those are real, the worlds of past and future. What worlds are neither past nor future, but contemporary and yet--extemporal--existing, as it were, in time parallel to our time?"

I shook my head.

"Idiot!" he snapped. "The conditional worlds, of course! The worlds of 'if.' Ahead are the worlds to be; behind are the worlds that were; to either side are the worlds that might have been--the worlds of 'if!'"

"Eh?" I was puzzled. "Do you mean that you can see what will happen if I do such and such?"

"No!" he snorted. "My machine does not reveal the past nor predict the future. It will show, as I told you, the conditional worlds. You might express it, by 'if I had done such and such, so and so would have happened.' The worlds of the subjunctive mode."

"Now how the devil does it do that?"

"Simple, for van Manderpootz! I use polarized light, polarized not in the horizontal or vertical planes, but in the direction of the fourth dimension--an easy matter. One uses Iceland spar under colossal pressures, that is all. And since the worlds are very thin in the direction of the fourth dimension, the thickness of a single light wave, though it be but millionths of an inch, is sufficient. A considerable improvement over time-traveling in past or future, with its impossible velocities and ridiculous distances!"

"But--are those--worlds of 'if'--real?"

"Real? What is real? They are real, perhaps, in the sense that two is a real number as opposed to √-2, which is imaginary. They are the worlds that would have been _if_-- Do you see?"