This post is a collection of quotes from Eugene Khutoryansky's relativity videos on YouTube. I recommend that everyone should watch the videos linked below and other physics videos on Eugene's channel. There are 37 quotes divided into 10 sections.
Click here to view Eugene's channel
1. Two central principles of Einstein's relativity
2. Time and moving near the speed of light
3. Disagreement on simultaneity
4. Simultaneity and length contraction
5. Length contraction near the speed of light
6. Length contraction at the speed of light
7. Energy, mass and the speed of light
8. Relativistic mass and rest mass
9. Traveling across the universe and back
10. Cause of gravity
1. Two central principles of Einstein's relativity
"Einstein's theory of relativity is based on two principles. The first principle is that if you have two objects and nothing else then it is not possible to tell which object is moving and which object is standing still... In general, any observer can believe that they are standing still and that the rest of the universe is moving around them." (Albert Einstein's Theory of Relativity, 2011)
"The second principle is the speed of light is the same for all observers." (Albert Einstein's Theory of Relativity, 2011)
2. Time and moving near the speed of light
"Suppose Adam fires a laser at the ground, which bounces off a mirror and Adam measures the time it takes for the light to return to the spaceship. From Adam's perspective, the laser light goes straight up and down. From Sarah's perspective, the laser light follows a 'V' shape path. The 'V' shape path is longer than the straight up and down path." (Albert Einstein's Theory of Relativity, 2011)
"Since the speed of light is the same for all observers, from Sarah's perspective it takes a longer amount of time for the laser light to return to the spaceship. But if Sarah looks at Adam's clock, she will see that Adam's clock read less time in between when he fired the laser and when he received the reflected light back." (Albert Einstein's Theory of Relativity, 2011)
"The closer that a spaceship approaches the speed of light, the slower the time inside the ship will flow. If the speed of the spaceship was equal to the speed of light, then time inside the ship would stop altogether. However, no matter how fast the spaceship travels, Adam will never notice that time for him is going slower." (Albert Einstein's Theory of Relativity, 2011)
"Everything is slower by the exact same amount, including the speed of his thoughts, so time for Adam appears to be flowing normally... Adam will think that everyone else's clocks are moving slowly." (Albert Einstein's Theory of Relativity, 2011)
3. Disagreement on simultaneity
"Suppose there are three ships moving together and Adam fires a laser to the two other ships. From Adam's perspective, all three ships are standing still and the two other ships will receive the laser light at the same time." (Albert Einstein's Theory of Relativity, 2011)
"Sarah must see the light from both lasers move at the same speed. Therefore, Sarah will see the left ship receive the laser light before the right ship. But we know that the clocks on the two ships read the same time when they each receive the laser light. This means that Sarah must see the clock on the left spaceship running ahead of the clock on the right spaceship." (Albert Einstein's Theory of Relativity, 2011)
"According to Einstein's theory of relativity, different observers will disagree on whether or not two events happened at the same time and no observer is more correct than any other." (Nature of Time and Simultaneity, 2015)
"If two events happened at both the same time and place, then everyone will agree that the two events happened together." (Nature of Time and Simultaneity, 2015)
"Even after the time necessary for light to travel is taken into account, different observers will still disagree about whether or not two different events happened at the same time." (Nature of Time and Simultaneity, 2015)
4. Simultaneity and length contraction
"Suppose that Adam sends a command for all the ships to fire their engines at the same time. From Adam's point of view, all the ships accelerate together and the distance between them stays the same." (Albert Einstein's Theory of Relativity, 2011)
"From Sarah's point of view, time on the left ship is running ahead of time on the right ship. Sarah will see the left ship accelerate first and the right ship accelerate last and the distance between the ships will shrink." (Albert Einstein's Theory of Relativity, 2011)
"Each spaceship can be thought of as being made up of smaller spaceships, with the distance between them shrinking as they move faster." (Albert Einstein's Theory of Relativity, 2011)
5. Length contraction near the speed of light
"The closer a spaceship approaches the speed of light, the shorter it becomes." (Albert Einstein's Theory of Relativity, 2011)
"Everything inside the spaceship has gotten shorter by the exact same amount, including Adam himself and all his rulers, so everything inside the spaceship will look normal to Adam." (Albert Einstein's Theory of Relativity, 2011)
"From Adam's point of view, his ship is standing still and it is the rest of the universe that is moving. Therefore Adam will think its the rest of the universe that has gotten shorter." (Albert Einstein's Theory of Relativity, 2011)
6. Length contraction at the speed of light
"From light's point of view, the journey took no time because the entire universe has shrunk to absolute zero length and the two planets have therefore always been at the same location." (At the speed of light, what would you see? 2015)
"From light's point of view there is no 'journey', because every point along its path is located at exactly the same position in space and they all pass by at exactly the same moment in time." (At the speed of light, what would you see? 2015)
7. Energy, mass and the speed of light
"Suppose two ships throw identical balls at each other. The balls bounce off each other and return to each ship. From Sarah's point of view, time on Adam's ship is moving slowly. From Sarah's point of view, Adam throws his ball much more slowly." (Albert Einstein's Theory of Relativity, 2011)
"From Sarah's point of view, Adam's ball and everything on Adam's ship must have more [relativistic] mass. Otherwise, Adam's ball would not have had enough momentum to cause her ball to bounce back. This is the meaning of E = MC^2." (Albert Einstein's Theory of Relativity, 2011)
"As an object moves with more energy, its [relativistic] mass increases. This is why nothing can travel faster than the speed of light." (Albert Einstein's Theory of Relativity, 2011)
"As an object gets close to the speed of light, its [relativistic] mass becomes so big that an infinite amount of energy would be required to move it faster." (Albert Einstein's Theory of Relativity, 2011)
8. Relativistic mass and rest mass
"It is true that the gravitational mass of an object does not change. That is, the gravitational attraction of an object does not increase due to the fact that the object is moving close to the speed of light." (Relativistic Mass and Energy, 2015)
"What does change is the more an object's speed approaches the speed of light, the greater the force necessary to increase the object's speed even further." (Relativistic Mass and Energy, 2015)
"Another way to describe this phenomena is to introduce the concept of relativistic mass, and to say that the relativistic mass of the object increases." (Relativistic Mass and Energy, 2015)
"The relativistic mass can never be used to calculate the gravitational attraction of an object." (Relativistic Mass and Energy, 2015)
"The mass that an object has when it is at rest is what we call the object's 'rest mass' and its is the rest mass that we use to determine gravity, even when the object is in motion." (Relativistic Mass and Energy, 2015)
9. Traveling across the universe and back
"From Sarah's point of view, Adam will travel the length of the entire universe while Adam's clock moves forward by only a few seconds... To Adam, the length of the universe passes in a few seconds because the length of the entire universe has shrunk to almost zero." (Albert Einstein's Theory of Relativity, 2011)
"For Adam to return back to Earth, he will eventually need to fire his rockets to turn around.... In order to believe that his spaceship is standing still, he needs to also believe that there is now a gravitational field present that is cancelling out the force from his rockets." (Twin Paradox in General Relativity, 2015)
"It is only during the very brief moment when Adam is firing his rockets to turn around that he believes this external gravitational field is present." (Twin Paradox in General Relativity, 2015)
"During the very brief moment when he fires his engines, he sees time on Earth flowing so extremely fast that this far more than compensates for the fact that he sees time on Earth flowing slowly during the rest of his journey." (Twin Paradox in General Relativity, 2015)
"When Adam returns to Earth, they will both agree that more time has passed for Sarah, but they will disagree on the reason why." (Twin Paradox in General Relativity, 2015)
10. Cause of gravity
"The mass of the Earth warps spacetime. This causes the clocks near the Earth to run slower than clocks far away." (Gravitational Time Dilation causes gravitational 'attraction', 2016)
"Suppose we have an object that is initially at rest relative to the Earth. This object will move through time. But different parts of the object will want to move through time at different rates. As the object continues traveling through spacetime in a straight line, the object starts moving towards the Earth." (Gravitational Time Dilation causes gravitational 'attraction', 2016)
"Even if the Earth is not moving through space, it would still be moving through time. The path that the Earth takes through space and time is what we will call its 'world line'." (Gravitational Time Dilation causes gravitational 'attraction', 2016)
"The full picture is not accurately represented with the visualization that is typically given of the Earth causing an indentation in a rubber sheet. Aside from the fact that this image falsely implies that objects are more likely to be attracted to the South Pole, this visualization is incomplete in that it does not show time as one of the four dimensions." (Gravitational Time Dilation causes gravitational 'attraction', 2016)
