Einstein's thought experiments

• 1. Visualizing Physics Through Thought Experiments

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  Albert Einstein's career was defined by his use of visualized thought experiments as a fundamental tool for understanding physical issues. He filled his papers with vivid practical details that made them quite different from the work of contemporaries like Hendrik Lorentz or James Clerk Maxwell. These mental scenarios allowed him to examine the implications of theories without needing new empirical data. A scientific thought experiment may examine laws using fictive particulars in an idealized environment. John D. Norton, a philosopher of science, noted that a good thought experiment is simply a good argument. Einstein used these logical arguments to stimulate readers' ability to apply their intuitions to complex scenarios. His work at the patent office stimulated him to see the physical ramifications of theoretical concepts. This approach became a hallmark of his entire professional life.

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• 2. The Pursuit of Light Beams

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  Late in life, Einstein recalled chasing beams of light when he was sixteen years old. He attended the Gymnasium in Aarau during late 1895 to early 1896. Various sources note that Einstein did not learn Maxwell's theory until 1898 while at university. A nineteenth-century aether theorist would have had no difficulties with this thought experiment. They would have regarded the frozen light scenario as merely a statement of fact since no one had ever traveled at such speeds. The youthful Einstein appears to have reacted out of an intuitive sense of wrongness rather than technical knowledge. He felt that the laws of optics should obey the principle of relativity. As he grew older, this early thought experiment acquired deeper levels of significance. It provided a powerful objection to emission theories of light which Einstein had worked on for several years prior to 1905. Norton has noted that Einstein's reminiscences were probably colored by a half-century of hindsight.

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• 3. Magnet Conductor Asymmetry

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  In the first paragraph of Einstein's 1905 work introducing special relativity, he writes about well-known experimental results obtained by Michael Faraday in 1831. These experiments describe what appeared to be two different phenomena: motional EMF and transformer EMF. James Clerk Maxwell drew attention to this fact in his 1861 paper On Physical Lines of Force. Although Einstein called the asymmetry well-known, there is no evidence that any of his contemporaries considered the distinction odd. Maxwell had repeatedly discussed Faraday's laws of induction without being bothered by the clear distinction between conductor-in-motion and magnet-in-motion. Yet Einstein's reflection on this experiment represented the decisive moment in his long path to special relativity. The equations describing the two scenarios are entirely different yet no measurement can distinguish whether the magnet or the conductor is moving. In a 1920 review on the Fundamental Ideas and Methods of the Theory of Relativity, Einstein related how disturbing he found this asymmetry. That decision ultimately led to his development of special relativity as a theory founded on two postulates.

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• 4. Relativity of Simultaneity Trains

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  Einstein translated the formal presentation of his 1905 paper into a thought experiment using a train, a railway embankment, and lightning flashes. Observer M stands on an embankment while observer M prime rides on a rapidly traveling train. At the precise moment that M and M prime coincide in their positions, lightning strikes points A and B equidistant from them. Light from these two flashes reach M at the same time, from which M concludes that the bolts were synchronous. Since M prime was equidistant from A and B when lightning struck, the fact that M prime receives light from B before light from A means that to M prime, the bolts were not synchronous. Instead, the bolt at B struck first. A routine supposition among historians of science is that Einstein discovered the relativity of simultaneity by thinking about how clocks could be synchronized by light signals. The Einstein synchronization convention was originally developed by telegraphers in the middle 19th century. However, all of the above is supposition. In later recollections, when Einstein was asked what inspired him, he would mention his riding a light beam and his magnet and conductor thought experiments. He never mentioned thought experiments about clocks and their synchronization.

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• 5. Falling Painters and Elevators

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  In his unpublished 1920 review, Einstein related the genesis of his thoughts on the equivalence principle. The realization startled Einstein and inspired him to begin an eight-year quest that led to general relativity. In most retellings of Einstein's story, the falling man is identified as a painter. In some accounts, Einstein was inspired after he witnessed a painter falling from the roof of a building adjacent to the patent office where he worked. This version leaves unanswered why Einstein might consider his observation of such an unfortunate accident to represent the happiest thought in his life. Einstein later refined his thought experiment to consider a man inside a large enclosed chest or elevator falling freely in space. While in free fall, the man would consider himself weightless and any loose objects would float alongside him. Then Einstein imagined a rope attached to the roof of the chamber being pulled with constant force. The chamber begins to move upwards with uniformly accelerated motion. Within the chamber, all of the man's perceptions are consistent with his being in a uniform gravitational field. Through this thought experiment, Einstein addressed an issue so well known that scientists rarely worried about it: Objects have gravitational mass which determines attraction and inertial mass which determines acceleration.

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• 6. Rotating Disks and Curved Space

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  By 1912, Einstein had reached an impasse in his kinematic development of general relativity. He realized that he needed to go beyond the mathematics that he knew and was familiar with. Stachel has identified Einstein's analysis of the rigid relativistic rotating disk as being key to this realization. An observer on the edge of a rotating disk experiences an apparent centrifugal force. By 1912, Einstein had become convinced of a close relationship between gravitation and pseudo-forces such as centrifugal force. In the accompanying illustration, A represents a circular disk of ten units diameter at rest in an inertial reference frame. The circumference of the disk is pi times the diameter. B represents a circular disk of ten units diameter that is spinning rapidly. According to a non-rotating observer, each ruler along the circumference is length-contracted along its line of motion. More rulers are required to cover the circumference while the number of rulers required to span the diameter remains unchanged. In later years, Einstein repeatedly stated that consideration of the rapidly rotating disk was of decisive importance to him because it showed that a gravitational field causes non-Euclidean arrangements of measuring rods. Einstein realized that he did not have the mathematical skills to describe the non-Euclidean view so he turned to his mathematician friend Marcel Grossmann for help.

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• 7. Quantum Challenges and EPR Paradox

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  In 1935, working with two younger colleagues, Einstein issued a final challenge to quantum mechanics. He enlisted the help of the forty-six-year-old Boris Podolsky who had moved to the institute from Caltech. He also enlisted the help of the twenty-six-year-old Nathan Rosen who did much of the math. The result of their collaboration was the four-page EPR paper which asked if Quantum-Mechanical Description of Physical Reality can be Considered Complete. Einstein considered that realism and localism were fundamental underpinnings of physics. His thought experiment involved two particles that collided or were created in such a way that they have properties which are correlated. Observation of the position of the first particle allows us to determine precisely the position of the second particle no matter how far the pair have separated. Likewise, measuring the momentum of the first particle allows us to determine precisely the momentum of the second particle. Bohr was stunned when he read Einstein's paper and spent more than six weeks framing his response. In 1964, John Stewart Bell made the groundbreaking discovery that Einstein's local realist world view made experimentally verifiable predictions that would conflict with those of quantum mechanics. Successive experiments improved the accuracy of observation and closed loopholes. To date, it is virtually certain that local realist theories have been falsified.

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