Spinning Through Space-Time
What does an old-fashioned amusement park ride have to do with space and time? Plenty! Climb aboard the Rotor for a mind-blowing adventure with three of the world’s greatest scientists – Ike, Ernie, and Al – to find out for yourself.
The Rotor is a ride you either love, or would love to forget. Just last year you had an unfortunate incident involving this attraction and a plate of cheese fries. But this time will be different – you hope. Inside the Rotor, a round floor is edged by a roughly-padded wall. As you, Ike, Ernie, and Al press your backs against that wall, the whole contraption starts to spin. You feel yourself pinned against the padding and, faster than you can say “lose my cookies,” the floor falls out from under you. Instead of dropping along with it, though, you find yourself stuck, like a fly on paper, to the wall behind. Faster and faster the Rotor spins, pressing you deeper into the padding. Will the madness never end?
“I don’t get this,” you say above the whine of the motor, “Why aren’t we falling?”
“It is friction,” shouts out Al, a kind-looking man with wild hair and a woolen sweater.
“Yes,” agrees Ernie, his voice traveling through his rough beard, not surprisingly, at Mach one (the speed of sound), “You provide the outward force, the wall responds with an inward force, and the roughness of the wall provides an upward force to keep you in place.”
“But,” replies Ike, a disagreeable sort wearing, of all things, a powder-white wig, “the deeper question is this: why do we feel ourselves pressed to the wall at all?”
Al and Ernie both groan. Either they’ve heard this before, or they’re feeling their lunch of bratwurst and sauerkraut. As your own stomach churns, Ike expositates on the nature of rotation. “Objects in motion tend to stay in motion in a straight line,” he recites, “Only a force, such as that supplied by the wall of our spinning cylinder, can change straight-line motion into curved motion. To do this, the wall must push on us, otherwise we’d continue in straight line motion, traveling past the cylinder until we finally splatter on the midway beyond,” he declares with a slightly evil grin.
“But suppose,” Ike goes on, “that not just our cylinder, but the ground below, and in fact the entire Universe, were spinning along with us. I submit that we would still feel the force of the wall. We spin, not relative to any outside object, but relative to Absolute Space itself!”
“No,” Ernie breaks in, “this is not correct. We cannot measure absolute space, so we cannot claim it exists. The Rotor must spin in relation to real objects. If the ground below were to spin with the Rotor, then the Rotor’s motion could be compared to more distant objects, such as the distant stars.”
“Are you suggesting,” Ike replies with a sneer, “that in a totally empty Universe, a Universe containing only us and this infernal spinning cylinder, we would not be pressed against said wall, but rather would float alongside as if it were motionless?”
“Precisely,” says Ernie. “Consider: in an empty Universe, how could one judge that the Rotor is spinning? There are no outside landmarks: no coney stand, no Matterhorn, no Dunk the Clown and Win a Prize. Not even the distant stars could serve as markers, for in this scenario they no longer exist. In a totally empty Universe, there is no difference between a still Rotor and one that is spinning madly. In such a Universe, we would not be pressed to the wall, but instead float alongside it, exactly as if we and the Rotor were motionless.”
“That is ludicrous,” spits out Ike, “How could the faraway stars affect events here and now?”
“And I,” replies Ernie, “might ask how one can base a scientific theory on something, such as your Absolute Space, that by definition cannot be measured?”
“Ah,” says Al, speaking finally, as a young child at last finding his voice, “but you forget, Ernie, that the Rotor exists not just in space, but in time, as well. When we combine our ideas of time and space, we see that the Rotor is spinning with respect to something: it spins with respect to its own past self!” The kindly eyes blaze with passion as Al reaches a crescendo, and you are certain he must be right. He goes on.
“We move through time always, in a straight line from then to now. When we move through space, we affect this motion through time. However, as long as our space motion does not change, in either speed or direction, we feel no force, because our path, not in space, not in time, but in space-time, remains a straight line.
Now, if we speed up, slow down, or change direction in space, we feel a pressing against ourselves. This is because we have changed our path through space-time from a straight line to one that is curved.
Al continues. “Imagine, as Ernie suggests, that we and the Rotor are alone in the Universe. Suppose it moves, without spinning, in a straight line at a single speed through space. No experiment we can do will reveal this motion to us. Why? Because in space-time this motion is no different from not moving at all. Both trace straight line paths. But if the Rotor spins, its motion through space-time is distinguishable, even in an otherwise empty Universe. The path traced by the spinning Rotor, and by those riding within, is not a straight path, but a curve. It is for this reason that, even in an otherwise empty Universe, we still feel the pressing of the Rotor walls against us. We move, not relative to the imaginary entity called Absolute Space, but relative to our own normally straight-line paths through space-time!”
The motor’s whine diminishes, and the Rotor slows. You and the three scientists slide down the wall until your feet gently touch the floor. Your head spinning in more ways than one, you forget all about your churning stomach and race your three new friends to the bumper cars.