Einstein’s Thought Experiments 1

Rumor has it that some philosophers have come out against “thought experiments,” calling them “intuition pumps” and suggesting that these useless situation scenarios prove nothing, and only serve to intensify the beliefs people already have. It would probably be disingenuous to mention the particular philosopher’s name in this context, since I have no references for it, but apparently the subject of this criticism is one thought experiment in particular: The Chinese Room, by John Searle, an explanation of which can be found on Wikipedia.

Actually, the use of thought experiments goes back much farther, and has better-known examples in the science of physics, because this method of study was a fondness of Albert Einstein’s. The most famous thought experiment I know is Einstein’s description of the

Principle of Equivalence.

Simply stated, the Principle of Equivalence is the idea that the force of Gravity is the same as the force you feel on your back when a car accelerates, or that throws you sideways when the car makes a turn. Einstein gave this description of the idea: Imagine you’re standing in a room in a spaceship, with no windows. You can’t look out, and you can’t see what kind of situation or environment the room is in. The rocket engines fire, and you begin to accelerate. Now the question you have to answer is, Is the force you feel the force of gravity or the force of the rocket’s thrust?

Einstein’s answer to the question was that there is no experiment you can make that will differentiate the two causes of the force you feel. In other words, they feel exactly the same. And, indeed, when you get in an elevator and the elevator starts to go up, you feel your weight increase, and the pressure of your feet on the floor increases, just as the elevator starts to move. This feeling dissipates when the elevator achieves constant motion. This fact is so well known, now, that the force of acceleration the pilot feels when a fighter jet pulls out of a steep dive is called “g-force,” where 1 g is one gravity, 32 ft/sec/sec, the force we feel due to the earth’s gravity.

Einstein thought, if you can’t discriminate the force of gravity and the force of acceleration experimentally, then there really is no difference. The two concepts are just different ways of describing the same thing. In other words, gravity is not an external force reaching out to you like rays propagating from the earth’s core, it’s actually the inertia of your own body being deflected from its normal path, just like the car turning induces a centrifugal force and throws you to the side.

Very good, but this raised another question. How are we being deflected from a path of motion when we’re sitting quietly on a chair in our room? By working out the answer to this question, Einstein developed concepts of space, curvature of space due to the locality of mass, and an entire geometry of the space-time continuum. His theory was named the “General Theory of Relativity,” and remains to this day an important and generally accepted part of our view of the Universe.

There are two interesting observations we can make from this thought experiment.

First, Einstein explained gravity in a wholly new way. It is not just a tweak of Newton’s theory, it completely replaces it. Because of the way Einstein explained gravity, it is no different from the centrifugal force of acceleration generated by a turn. People in cars feel it, even though there is no “force field” around them that generates it, and satellites in orbit feel it. The centrifugal force of outward acceleration generated by their circular motion exactly balances the centripetal force of the earth’s pull, and that’s why the satellite neither falls nor spins away into space. The idea of “gravitational field,” then, is somewhat of a misnomer. It actually refers to the curvature of space, and not to a field of force. And finally, we can see that the force you feel on your posterior as you sit on a chair is not the earth’s pull, it’s the force of your own butt pushing against the chair. If the chair were removed, you would suddenly enter a free-fall condition, and for a brief period of time, you would feel neither gravity nor weight as you moved along the natural path toward the center of the earth — and then the sensation of weight would return as you encountered the floor.

Second, the use of thought experiments depends mainly on the clarity of thinking of the person who uses them. Einstein used them as inspirations, not proofs. He developed an elaborate mathematical theory, and rested his arguments on the mathematical formulations, not on the conceptual scenario of the thought experiment. Which implies that the use of thought experiments in Philosophy is not wrong, but should not be the only type of argument we make. Rather, one must go on from the thought experiment to work out the implications of the argument it makes. Either such elaborations will lead to a reductio ad absurdam, or a Q.E.D.

Next time, more thought experiments from Einstein and the World of Physics.