One of the great minds of the 20th Century, Albert Einstein, made famous the "Gedankenexperiment" (translated in English as “thought experiment.”) A thought experiment involves the testing of a hypothesis, theory or principle by carefully thinking through all the possibilities and implications. In some cases, the experiment may not even be possible to run given current technologies. Many of Einstein’s great breakthroughs in physics originated in his thought experiments. Breakthroughs like the theories of relativity and special relativity emerged from his ability to tackle complex physical concepts using objects and scenarios from everyday life.
Later in his life Einstein recalled that at the age of 16 he formed the basis of his theory of special relativity in a thought experiment. The theory of special relativity concludes that the speed of light in a vacuum or a place where there is no matter or gas is always the same no matter how fast the observer of the light is moving. He said that in his thought experiment he imagined riding on a beam of light. Riding on a beam of light, he would be traveling at the speed of light, which is a little over 680 million miles per hour. He then thought that if there were a parallel beam of light next to him that the beam would be frozen because he was traveling at the same speed as the beam next to him. The concept is much like if you are in your car traveling down the highway at 65 mph and the car next to you is going the same speed it can look like the other car is standing still. But he reasoned that light cannot be frozen, and* no matter how fast you are going that light will always be traveling at 680 million miles per hour. In another thought experiment, Einstein proposed that if you were born as a twin, and abruptly just after being born, your twin sister was sent out into space on a rocket ship traveling through space near the speed of light for 70 years. Upon return, your twin sister would only be getting ready for high school while you have already lived the majority of your life. Einstein argued in his theory of special relativity the laws of physics apply equally to all observers and that the speed of light was independent of the speed of the observer.
Einstein proposed his theory of special relativity in 1905, and it took him ten more years and more Gedankenexperiments to develop his theory of general relativity. But Einstein was not the only person noted for thought experiments. The Scottish physicist, James Clerk Maxwell proposed a thought experiment that theoretically violates the Second Law of Thermodynamics. The Second Law of Thermodynamics is a fancy way of saying that things fall apart over time. They go from order to disorder, like an ice sculpture of an eagle will become a puddle of water over time as it melts, or you can think of putting a warm can of Coke into an ice chest filled with ice. Over time the ice melts and the Coke gets colder. Maxwell proposed a thought experiment, now known as Maxwell’s Demon, that challenged the Second Law of Thermodynamics. Maxwell’s Demon suggests that the equivalent to breaking the Second Law of Thermodynamics would be putting our warm cokes in the ice chest and finding out later that the Cokes got warmer and the ice got colder. Sticking with our Coke analogy I will provide an interpretation of Maxwell’s Demon. In his thought experiment, Maxwell imagined a demon inside the ice chest could open and close a tiny door on a can of Coke so fast that he could let individual water molecules in or out of the Coke can. Being a physicist, Maxwell knew that in an ice chest full of ice and water that some of the water molecules will be a little warmer than others. He proposed that the demon could see when warmer water molecules came near the coke can and open the tiny door and let the warmer molecules into the can. By the same token, the demon could let cooler water molecules out of the can. Over time the can of Coke would get warmer thus violating the Second Law of Thermodynamics. Now, employing a demon to break the Second Law of Thermodynamics cannot truly happen, but Maxwell needed to challenge the law. Laws in science and in life need to be challenged to test their integrity and validity. It follows that the thought experiment offers a fantastic way to do it. Keep in mind, the thought experiment should not be purely the domain of physicists, but rather everyone should try to push themselves into deeper thinking. Really use your imagination and experience of the natural world to challenge your theories, hypotheses and principles.