Relativity Simplified With No Math. Einstein Thought Experiment.

in 1894, a high school teacher suggested to one of his precocious pupils that he should leave, because he was unhappy. The teenager took that advice and never came back.

Later, he tried to apply to a prestigious university, but failed the entrance exam. Later in his life, when he tried to get his dream job as a professor, no university would hire him. He had to settle for a lowly job as a clerk at a patent office.

History does not remember the name of the teacher, or the names of the universities that rejected him for a job,

but it will never forget that teenager, because he went on to not only revolutionized physics, but changed the way we view reality itself.

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In 1999, Time magazine named him man of the century. Today his name is synonymous with "genius." I'm talking, of course, about Albert Einstein.

Yet this entire revolution in physics started with a simple thought experiment, conjured up in the prolific imagination of a teenager before he even graduated from high school.

What was this simple thought experiment? And how did it lead to probably the biggest revolution in physics since Isaac Newton? 


Einstein's theory of special relativity is convention today, but to understand how revolutionary it was for its time, it's helpful to look at what the conventional understanding of physics was during the time of Einstein's teenage years.

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First, in 1801 Thomas Young had conducted a simple double slit experiment that showed that light behaved like a wave. So the predominant theory about light at the time was that it was a wave.

The problem is that a wave, it was thought, had to move through some sort of medium. Something has to be there to make the wave - similar to how waves on an ocean need water to create a wave.

But light was known to travel through outer space, obvious because you can see starlight. Yet, outer space was believed to be empty, containing nothing. And it could be easily demonstrated the light can indeed travel in a vacuum.

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So scientists thought that the only way light waves could travel through the vacuum was if there was some kind of medium that pervaded space and the entire cosmos. They called this substance the "luminiferous Ather."

And this theory of Ather was the standard theory of physics for most of the 19th century. later in that same century, in 1887, two scientist by the name of Albert Michelson and Edward Morley, came up with an idea to test the existence of the Ather.

The background Ather was believed to be unmoving and static, but because the earth was moving it was thought that it would affect the speed of particles (or waves),

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if the wave was traveling in the same direction as the earth. The speed of the wave should be higher in the direction of the speed of the earth.

This would be similar to how a boat moves faster if it's moving with the flow of the current, than if it is moving against the current.

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To test this hypothesis, Mickelson and Morley designed a device that split a beam of light, and bounced it off mirrors so that it moved in different directions, and finally hit the same target.

The idea was that if two beams travel the same distance along different paths through the Ather, they should move at different speeds.

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And therefore, when they hit the final target screen, those light beams would be slightly out of phase with each other, which would create an interference pattern.

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The results of this test were astonishing. They showed that there was no difference in the speed of light of the two measurements. No matter which path the beam took, light seemed to be moving at precisely the same speed.

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This seriously jeopardized the Ather theory, at least for light. No one could make sense of this, or come up with an alternate theory to explain it. It was labeled "the greatest failed experiment of all time."

This is where Albert Einstein comes in. The term relativity had been around even before Albert Einstein.

But it was thought of in a completely different way. The term had originated with Galileo Galilei. He and Isaac Newton had demonstrated relativity.

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So for example, if you're walking on a moving train, and someone's stationary in the ground is watching, your speed, relative to that observer, will be the sum of the speed of the train and your walking speed. This makes logical sense.

But something seemed wrong with this classical interpretation of relativity as it applied to light.

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Just prior to Einstein, in 1873, it had been recently proposed by James Clerk Maxwell that light was an electromagnetic wave. And he had calculated its speed, which was approximately 186,000 miles per second.

Einstein knew this. And he came up with a thought experiment as a sixteen-year-old. His thought was to imagine that he was chasing a beam of light while traveling at the speed of light himself. 

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What would he see? 

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If young Albert could catch up to the beam, he writes in his notes, "I should observe such a beam of light as an electromagnetic field at rest, though spatially oscillating." 

In other words, Einstein thought that he should see a stationary wave of light. Yet, that was impossible. Einstein knew that such stationary fields would violate equations of electromagnetism developed by James Clerk Maxwell 20 years earlier. 

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The laws were quite strict. Any ripples in the electromagnetic field have to move at the speed of light, and cannot stand still. There are no exceptions.

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In addition, Einstein reasoned that if someone was traveling on a non- accelerating train at close to the speed of light, there would be no way for that person to know how fast he was going, if there were no windows.

This had been the classical view of relativity.

Why should the laws of physics be different for a person traveling at some fixed velocity, versus someone standing still? 

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This seemed untenable to Einstein. So he came up with two postulates, and tried to figure out what the physics would be like if the two postulates were true.

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Postulate one was that the laws of physics are the same for all inertial reference frames. This was part of classical relativity, pioneered by Galileo. 

Postulate two was that the speed of light in a vacuum is constant for all inertial reference frames. 

The first postulate is pretty much common sense, and had been assumed for hundreds of years.

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The second postulate, however, was the revolution.

It was a consequence of massless photons moving at a velocity "C" in a vacuum. You would always measure a lights beam velocity to be 186,000 miles per second.

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This meant that young Einstein would never see the stationary oscillating fields, because he could never catch the light beam. This was the only way that Einstein could see to reconcile Maxwell's equations with the principle of relativity.

But this solution seemed to have a fatal flaw. And Einstein later explained this problem with another thought experiment.

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Imagine firing a light beam along a railroad embankment just as the train roars by, in the same direction, at say, 2000 miles per second. Someone standing on the embankment would measure the light beam speed to be the standard 186,000 miles per second. But someone on the train would see it moving past at only 184,000 miles per second.

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If the speed of light was not constant, Maxwell's equations would have to somehow look different inside the rail car, Einstein concluded. And his first postulate, that the laws of physics must be the same for all frames of reference, would be violated.

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This apparent contradiction left Einstein spinning his wheels for almost a year. But then, on a morning in May 1905, he was walking to work with his best friend Michele Besso, an engineer he had known since his early student days.

The two men were talking about this dilemma. And suddenly Einstein saw the solution. He worked on it all night, and when they met up the next morning, Einstein told Michele, "Thank you, I've completely solved this problem."

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The solution to his thought experiment was that a person traveling on the train, must experience time differently, than the person on the embankment. Observers in relative motion experience time differently!

And this was the moment of the revolution. It completely overturned hundreds of years 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 varies in different frames of reference. There is no absolute frame of reference, that the Ather was theorized to provide.

Thus, the idea of the Ather was no longer needed.

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This one realization, that reality is not the same for different frames of reference, also led to other implications of special relativity; that fast moving objects appears shorter; that fast moving objects appear to have increased mass.

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And finally, the equivalence of mass and energy - this is the most famous equation in science – E=MC^2, that means, that mass and energy are equivalent.

So now the big question is,

How did Einstein come up with his most famous equation, based on his original two postulates? 


The math is rather complicated, but let's just look at it conceptually. There was a time when mass was always conserved. In any reaction whatever mass you put in, must be the mass you got out.

But if conservation of mass is interpreted as conservation of rest mass, this did not hold true in special relativity.

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Since different observers would disagree about what the energy of a system was, the mass and energy, taken together, must be conserved, not just the mass on its own.

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A train traveling close to the speed of light has a lot more energy than a train at rest. But a person riding on the non accelerating train, may not know that the train is moving.

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So this massive object is moving from the point of view of one observer, but at rest, as seen by another observer. One observer would see and measure zero energy of the object, and the other observer would measure a higher energy.

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It turns out that for the laws of physics, namely "conservation of energy," and "conservation of momentum" to be consistent in the two reference frames of two observers moving with respect to each other, there has to be an energy associated with a body at rest, not just a body in motion.

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And that is what E = MC^2 implies. The "M" in the equation is the mass at rest. All masses, even at rest, must have energy.

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Some people point out that much of the actual work for special relativity had already been done by the time Einstein presented it.

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The concepts of time dilation and simultaneity for moving objects, for example, were already in place. And the math had already been developed by people like Lorentz and Poincare.

Some have even called Einstein a plagiarist. There's no doubt that the revolution of Einstein was built on the shoulders of other great scientists. And Einstein may have been given a lot more credit than others who did prior work.

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At the same time, Einstein still deserves the accolades because he took the bits and pieces of the puzzle found by others, and put them all together into a whole new theoretical framework.

He rejected the idea of the Ather altogether, which other scientists had not done, and boldly proclaimed a new fundamental understanding of time and reality.

And the idea of mass and energy equivalence, via E = MC^2, is solely Einstein. Scientists would had done prior work like Thompson, Larmor, Lorentz, and Poincare had never implied such a bold proposition.

And just as in life, history tends to favor the bold.

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