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  • Hi! so you want a simple guide to the theory of relativity? Great!

  • Youve come to the right place!

  • Part 1: Classical Relativity

  • Let’s start with some absolutes that we need to accept.

  • One, there is no such thing as absolute motion or absolute rest.

  • Objects move relative to each other.

  • For example look at that truck with the two kids playing catch. The truck is moving at

  • a constant speed of 50 miles per hour but as the kids are also moving with it at 50

  • miles per hour they could easily think they are standing still. When the children throw

  • the ball at 10 miles per hour they observe that the ball they have thrown is moving at

  • 10 miles per hour.

  • But their little brother who is not on the truck and is standing still has observed that

  • the ball has moved at 60 miles per hour. He has added both the speed of the truck and

  • the speed of the ball together.

  • But is the little brother really standing still? No! The earth is constantly in motion,

  • the sun is constantly in motion and the galaxy is constantly in motion. Therefore nothing

  • is ever at absolute rest or absolute motion, things just move relative to each other.

  • This is classical relativity.

  • Part 2: Special theory of relativity - Time Dilation

  • The second thing we need to accept is that the speed of light is the same for all observers.

  • So let’s imagine a beam of light being reflected between two mirrors.

  • Let’s replicate this set up so we have another beam of light being reflected between another set of mirrors

  • but this time the mirrors are moving close to the speed of light.

  • We can see in the moving set of mirrors that the beam of light is moving a greater distance.

  • Now, lets put a couple of people in this scenario. Meet Alex, he sits inside the moving set of

  • mirrors and Bill sits inside the stationary set of mirrors.

  • The speed of light must be the same for both Alex and Bill and their light beams must hit

  • the bottom mirror after the same amount of time has passed. But as Alex’s light beam

  • is covering a longer distance how is that possible?

  • Well, the only way for that to work is if time for Alex slows down. Look at the clock

  • hand, notice how Alex’s clock hand is moving much more slowly compared to Bill's.

  • Time for Alex has slowed which allows his light beam to travel a greater distance.

  • Look at it this way, speed equals distance divided by time. As Alex’s distance is more,

  • the value of his time must be more or rather Alex’s one minute actually turns out to

  • be longer than Bill’s one minute when you compare the two.

  • Time has slowed down for Alex. This is called time dilation.

  • This is quite a hard concept to grasp because it seems to go against what we experience

  • on a practical level but to understand the theory of relativity you must stop thinking

  • of time as a fixed unit for everyone. Time is relative.

  • Confused?? Don’t worry if you don’t grasp all of this immediately. Remember it was the

  • most brilliant scientist of the 20th century who put forward this theory so feel free to

  • rewatch this part of the video as many times as you need.

  • Part 3: Special theory of relativity - Length contraction.

  • In the previous section we spoke about how the speed of light remains the same for Alex

  • and Bill because while the distance Alex’s light beam travels is more his time has dilated.

  • However, distance and time don’t increase by the exact same amount. In fact, time doesn’t

  • actually dilate enough to account for the speed to remaining the same for both boys,

  • something else happens - length contraction.

  • When objects are moving close to the speed of light, not only does time slow down but

  • the objects itself contracts. This is called length contraction.

  • So in this scenario, Alex and his mirrors are actually smaller than we thought and therefore

  • the distance the light beam has to travel is actually smaller than we thought.

  • In conclusion time dilation and length contraction work together to ensure that the speed of

  • light remains the same for both Alex and Bill.

  • Einstein preferred to think of space and time not as two separate things but one entity;

  • he called it spacetime.

  • Part 4: Time travel

  • Time slows down for objects travelling close to the speed of light; and time stops all

  • together for objects travelling at the speed of light; it then follows that time must go

  • backwards if objects travel faster than the speed of light.

  • So far it has been seen that no object can travel at the speed of light. On approaching

  • the speed of light objects start to increase in mass rather than speed and it would take

  • an infinite amount of energy for an object to achieve the speed of light. This is why

  • we cannot travel back in time, we have simply not been able to reach a greater speed than

  • the speed of light.

  • Part 5: General theory of relativity.

  • Einstein realized that the special theory of relativity which we explained previously

  • only worked for circumstances when things moved at constant speeds through the Universe.

  • What about when objects sped up? What about the effect of gravity? And come to think of

  • it, what is gravitational pull after all?

  • Einstein theorized that objects warped space time around it, causing it to become curved

  • and as a result objects experienced gravitational attraction to each other. This warping of

  • spacetime explains how objects behave as they move through space.

  • This is the general theory of relativity.

  • Part 6: How do we know it’s true?

  • How do we know that the theory of relativity is true? Well countless experiments have been

  • carried out and Einstein’s equations hold true. Modern GPS uses Einstein’s theory

  • of relativity to pinpoint our locations with impressive accuracy.

  • In 1971 an experiment was conducted with four super accurate atomic clocks which were set

  • to the exact same time as a reference clock. The clocks were placed aboard commercial flights

  • and the planes flew twice around the world, first eastward and then westward. When the

  • clocks returned and were compared to the reference clock, the clocks were no longer all synchronised

  • and the differences were consistent with the predictions of special and general relativity.

  • So that concludes our explanation of the theory of relativity. Bear in mind this is a very

  • simplified explanation but it should give you a basic understanding of the core ideas

  • so you can study further on your own.

  • We hope you enjoyed this video and be sure to subscribe to LondonCityGirl to see other

  • interesting videos!

Hi! so you want a simple guide to the theory of relativity? Great!

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B1 中級

相対性理論を7分で説明 (Theory of relativity explained in 7 mins)

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    VoiceTube に公開 2021 年 01 月 14 日
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