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But kinetic energy comes from the mass and the velocity of an object, and since the rock's velocity didn't change when it gave off photons, Einstein concluded that its mass must have changed.
In fact, he found that the mass of the rock changed by an amount equal to the energy of the photons, if you multiplied it by the speed of light squared. In other words, he found the following:
"Energy of the photon = (Change in mass of the rock) x (speed of light)2
"What this means is that when a photon leaves the rock, it actually changes the mass of the rock.
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"Then, Einstein stepped in with a clever argument that settled the debate.
"At the time, Einstein had been preoccupied with the idea of relativity, the study of how the laws of physics apply to things that are moving relative to one another.
It was known back t...
"But what exactly does that mean? How are mass and energy and light actually related to one another? And what does this relationship say about the fundamental nature of ourselves and the universe?
"For most of us, mass is the stuff we're made out of. If something has mass, it g...
The net effect is that you, in the spaceship, would measure the energy of the photons to be different than if you measured it when you're floating next to the rock.
But since it's the same photons, something else must have changed.
"According to Einstein, what a...
Einstein teaches us that mass and energy are different forms of the same thing:
"If there's one physics equation most people know, it's probably E = mc2. It's the most famous equation in physics, likely because it's easy to remember. Its form is simple and elegant, alm...
"But physics formulas are not just math; they're supposed to describe something about the physical universe. And this is another reason why E = mc2 sticks in people's minds.
Here E stands for energy, m means mass, and c is the speed of light in a vacuum, or 299,792,458 meters...
Initially, they looked at the electron, which had just been discovered. Physicists noticed that when a charged particle (like the electron) moves, it makes a magnetic field.
This magnetic field then pushes back on the particle, making it harder to get the partic...
"For example, if you added energy to something, like a cup of water, you could think about it speeding up the little water molecules in the cup, but not changing the mass of the water.
After all, adding heat didn't change how many H20 molecules there were; it just made them w...
This fundamental limitation makes for some really strange effects when you consider how things look to someone standing on Earth and someone going really fast on a rocket ship.
"For example, Einstein considered the case of a rock in space giving off heat. That heat will come o...
"This was a pretty groundbreaking result, to say the least.
It threw out thousands of years of human intuition that told us that mass and energy were totally different things.
Instead, Einstein's equation says that the two things are related to each other and t...
"But if you were traveling past Earth on a speeding rocket ship, you would see something different. Einstein used the formulas of relativity to figure out that you would see the photons coming off the rock at a different frequency of light.
This is an effect called the relati...
For centuries, F = ma held the top spot as the most important physics equation in the world.
In this formula, F is the force that you apply to an object, m is the object's mass, and a is the acceleration, or how quickly the object starts to move.
If the object ...
This change in mass is the same (if you multiply it by the speed of light squared) as the energy of the photon emitted.
It seems that a little bit of the mass of the rock was transformed into energy, which then went off in the form of a photon (remember that photons don't hav...
"On the other hand, we tend to think of energy as something completely different. We associate energy with heat, light, fire, or motion.
It seems like something ephemeral that can flow or be transmitted. It gives you the power to do things and burn things. Like a magical quan...
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Albert Einstein did not think about symmetry when he wrote his first relativity papers in 1905. He was considering several seemingly unrelated puzzles and connecting the dots.
Einstein's special theory of relativity applies only to steady, unchanging motion through space-time, not accelerating motion like an object falling toward Earth.
Albert Einstein held that space and time together made up the universe's fabric that could stretch and compress. Einstein suggested that the shape of spacetime is what gives rise to the force of gravity. A concentration of mass, like the earth or sun, ben...
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