Shulou(Shulou.com)11/24 Report--

In 1801, Thomas Young performed a simple double-slit experiment showing that light behaves like waves. So the dominant theory of light at the time was that it was a wave. The problem is that people think waves have to travel through some medium.

It is well known that light can travel in a vacuum because you can see starlight. However, outer space is thought to be empty and empty. So scientists believe that the only way light waves can travel in a vacuum is if there is a medium throughout the universe that they call "ether."

This ether theory was the standard theory of physics in the 19th century. In 1887, Albert Michelson and Edward Morley came up with an idea to test the existence of ether. The background ether is thought to be stationary, but since the earth is in motion, it is thought to affect the velocity of particles (or waves). If the wave travels in the same direction as the Earth, the wave velocity should be higher in the direction of the Earth velocity.

To test this hypothesis, Michelson and Morley designed a device called an interferometer. The device splits a beam of light into two and reflects it off a mirror, moving it in different directions and eventually hitting the same target. The idea is that two beams of light follow different paths through the ether, traveling the same distance but moving at different speeds. Therefore, when they hit the final target screen, these beams will be slightly out of phase with each other, creating an interference pattern.

But the results of this experiment were amazing. Experiments have shown that there is no difference in the speed of the two beams of light. No matter which path the beam takes, the light seems to move at exactly the same speed. This seriously jeopardized the ether theory, and no one knew why, and no one thought of another theory to explain it. It was called "the most failed experiment ever."

Einstein began to intervene in 1873 when James Clerk Maxwell proposed that light was an electromagnetic wave. He even calculated its speed, which is about 300,000 kilometers per second. Einstein understood this when he proposed a thought experiment: imagine chasing a beam of light while traveling at the speed of light. What would he see? He wrote in his notes: "If I could catch up with this beam of light, I should observe a scene in which light acts like a static electromagnetic field, although it oscillates in space. "In other words, Einstein thought he should see stationary light waves.

However, that was impossible. Einstein knew that such a static field would violate Maxwell's equations of electromagnetism. Any wave in an electromagnetic field must travel at the speed of light and cannot stand still. Furthermore, Einstein argued that if someone travels in a non-accelerating train that approaches the speed of light, that person cannot know how fast he is traveling without windows. The laws of physics for a person traveling at a fixed speed should be the same as for a person standing still.

So he proposed two hypotheses and tried to figure out what physics would look like if both were true. One of the assumptions is that the laws of physics are the same for all inertial reference frames. Suppose two is that the speed of light in vacuum is constant for all inertial reference frames.

The first assumption is almost common sense and has been assumed for hundreds of years. The second hypothesis, however, is revolution. Light moves in a vacuum, and we always measure its speed at 300,000 kilometers per second, which means Einstein never sees a stationary oscillating field. This was the only way Einstein thought Maxwell's equations could be reconciled with the principle of relativity.

But this solution seems to have a fatal flaw. Imagine a person standing next to a railway line emitting a beam of light as a train whizzes past at 30,000 kilometers per second. A man beside the railway would measure the speed of the beam at the standard 300,000 kilometers per second, but a man on the train would see light traveling at 270,000 kilometers per second. Einstein concluded that if the speed of light appeared to be the same in all inertial frames, how could he correct the observer on the train to measure a beam of light at 300,000 kilometers per second?

Einstein pondered this question for nearly a year. Suddenly, one morning in May 1905, Einstein saw the solution. The solution to his thought experiment is that a person traveling on a train must experience time differently than an observer in relative motion! It was a revolutionary moment, a reversal of centuries of classical physics pioneered by Galileo and Newton, in which time was fixed and absolute in the universe.

Einstein showed that time is relative and differs in different frames of reference. There is no absolute frame of reference provided by aether theory, so the concept of aether is no longer needed. This understanding also led to other conclusions of special relativity: scaling effects, kinetic mass, mass-energy equations.

This article comes from Weixin Official Accounts: Vientiane Experience (ID: UR4351), by Eugene Wang

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