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Theory of Relativity
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0:11
So where do we start with something as big and as complicated as the Theory of Relativity? I'm sure some of you wouldn't even know what it is outside of its name, which is fine. But I'm sure you do know the man who came up with the idea, Albert Einstein. Einstein is revered as one of the smartest people to ever live, and he helped shape how we perceive both our world and our universe.
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So it might surprise you that this very brilliant man once started off as nothing more than a patent clerk. No, really, he did, and that's part of the origin story to the Theory of Relativity. Because one day, after doing his work at the patent office, he went on a trolley car to go home. And he would do this day after day after day. This is important because while he was on that car, he would think about the universe at large. He would ask himself questions and try
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to figure out the answers as best he could with the information he had. And one day, he was going away from a clock tower when he asked what would happen if the car he was on was going away from the clock tower...at the speed of light. This may seem like an odd question to ask, but lightspeed travel is something that scientists are honestly trying to achieve right now, and these questions were truly the building blocks of this really happening. Anyway,
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back to the clock tower. Einstein theorized, as well as realized, that if he was moving the speed of light (which if you don't know is 299,792,458 meters per second), the hands on the clock tower (meaning the minute hand and the hour hand) would quite literally appear to stop in place. But, he also knew that while he himself was traveling at the speed of light and seeing everything stop more or less, everyone who was at the clock tower,
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and seeing things in "normal time" would not see them stop. The clock tower and its hands would keep ticking along as if nothing wrong. Yet in this experiment, for Albert Einstein, time had literally slowed down, and it was at this moment that the "light bulb" went off in his head. Because it was through this experiment that he realized that if you go faster and faster through space, you're actually causing time to go slower around you. But how was this possible if time was
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quite literally a constant force in the universe? To try and answer this, Einstein would look to some of the other fathers of science to try and figure out the missing points in his equation. For example, he looked at the three laws of motion via Sir Isaac Newton. Newton notes that while objects do move at a certain speed, their values are never an absolute. Mainly because every speed we go at is based on a force imparted on something, or relative to something
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else. Such as how a car can go 65 miles per hour on a highway...but that's only because the ground and friction ALLOW it to do so. No friction on the road? You're not going that speed. Thus why he notes that every speed has to have "in respect to" another force or object that is allowing or perceiving that object's speed. However, in contrast, there is James Clark Maxwell, the father of electromagnetism, who notes that of all the things in the universe,
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it is light that is fixed. And as noted, light goes 299,792,458 meters per second. That will never change. That speed is another constant force in the universe. Anyone, anywhere in the world, or even anywhere in the universe will be able to determine that the speed of light is the same, it won't change, and that's part of the reason why the universe works like it does, because the speed of light is constant, right? But therein lies the problem, or at least,
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Einstein realized that this was a problem. Because Newton said that no speed in the universe could be an absolute. But then Maxwell counters this by saving the speed of light is ALWAYS a constant. Which means that these two very universal and very accepted pieces of science are at a contradiction. Which is something you never want in the world of science, trust me. If you're still not getting the full picture of why this is a problem, here's another thought
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experiment from Einstein to help explain it. Imagine you are at a train station, and you are standing out on the platform when a storm comes. Then, out of the blue, two lightning bolts strike on either side of you. Because of your position in the middle of these lightning bolts, you perceive them at the exact same time, and the light reaches you at that same time. Everything sounds good, right? But here's the twist in the equation as it were. Because now I
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want you to imagine that you are now on the train passing by the station. And you're watching as someone else is having lightning striking on either side of them. Because you are moving, and thus not still like the person on the platform, the lightning bolts would send their light to you at different rates relatively to your train speed (for sake of argument the train is going the speed of light) and your position to the lightning bolts.
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So as you can see, both laws were right, and yet, both laws were wrong depending on the position and situation you are in. For scientists and thinkers like Einstein, this is a problem, because that kind of flexibility isn't what you would want in a scientific principle. This is where the Theory of Relativity comes in. Because Einstein had to figure out a way to make his theory make sense while not breaking either law by Maxwell or Newton. Because if Newtons
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laws were wrong, then physics wasn't what we thought they were. But if Maxwell was wrong, and the speed of light isn't a constant, then something much different is happening with light in the universe, and we would have a whole bunch of other questions to have. Breaking either would be a problem, and that's when Einstein realized that with a small non-destructive tweak to the Newton laws, but principles could still be true in the wider
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world that we live in. He called what he created "Time Dilation". Proving via his theory that time itself can be stretched or scrunched depending on the speed of the person moving through space. What's more, he realized that not unlike space itself, time deserved to be not just a principle of the universe, but in fact deserved to be its own dimension. But as Einstein later theorized, this shouldn't be a separate dimension, but rather a dimension in which both space and
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time would exist as one. Thus was born his belief in "spacetime". Where space and time were together in such a way that all the events of the universe happened because they were combined. This also helped him paint a different picture of what the universe was doing in terms of its formation and orbits. Because most people believe that the force of gravity and other elements help keep solar systems and galaxies together. But for Einstein, he felt he had the better explanation.
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Mainly in that he felt that spacetime itself was being warped by massive objects like the sun, and that the Earth, other planets, other stars, and the other things in galaxies and systems were just trapped in this "valley". A radical theory to be sure, but Einstein truly believed that this would explain much of what was just "accepted" in the field of space science. If you're having trouble picturing that, how's this for a visual? Imagine yourself
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on a trampoline. Now imagine that you put a bowling ball onto it. What would it do? Simple, it would sink to the middle, right? Now, with the bowling ball in the middle, what would happen if you put a marble on the edge of the trampoline? The answer is it would go into the "valley" that was created by the bowling ball. Due to the sun having the biggest "valley", it is the center of our system, and we all rotate around it. But, all the other planets
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in our solar system (and by extension all of the ones in space itself) have their own mini valleys, and that's why the moons of various planets (including our own) rotate around them. But wait, this gets deeper if you can believe it. Because this Theory of Relativity applies not just to the distortions caused by planets, but the masses and gravities of said planets. Because as you move through space, the time is relative to your time,
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right? Well that would mean that with planets that have heavier gravities that they experience time a lot slower than ones with lighter gravities. This was actually a major plot point in the film Interstellar. In that movie, some astronauts went to a world that was on the edge of a black hole, and thus gravity and light were much different. The black hole put a lot of strain on the planet, and by that token, light had a different effect on it because of its mass,
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gravity and "speed". To the extent that a single hour on this planet was 7 years back on Earth! But let's ask the real question here, while it's nice to know that there are ways to perceive time outside the norm, does it really affect us in our daily lives? Well, yes and no, it's a complicated question to be sure, so allow me to answer it as best I can. While it's true that there are ways to "distort" time by going faster in terms of speed,
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the way you're doing it is so small in its dilation that it honestly won't matter to you. But there are ways to notice how time is distorted if you pay attention enough. Such as how there are clocks that are always a few seconds apart from one another despite them being "locked in" on the "correct" time. Some are based on mathematical figures, while others are dictated by other forces. So thus, there is a disparity depending on how you "receive" your time.
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And if you really want to be creeped out, because of how the Earth works in its time, it's technically true that your head is older than your feet because your feet are closer to the core of the planet where you're being "pulled" at a faster rate. E = MC Squared is the now legendary formula that helps show off the Theory of Relativity, but no matter how you spell it out, it's now a key part of our understanding of space, time,
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spacetime, and the universe itself. Even though it does have kind of a big flaw. What is that flaw? It's simple, think about the name of this principle. The THEORY of Relativity. Remember, at its core this is a thought experiment. One that is predicated on having a manmade object traveling at the speed of light. We can do that right now, and it's unlikely that we'll be able to do that for at least the next few decades, if not the next couple hundred of years.
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So until we are able to get into the realms of lightspeed travel, we're honestly not going to be able to know if Einstein was right. And there are many accredited scientists who legitimately wonder if Einstein is right at all. And what that would mean for the universal principles we believe in if we do find out that this theory...is wrong. However, that's not stopping others from taking this notion and seeing how far they can go with
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it. How so? Well, remember the idea of spacetime and how things like the Earth and the sun are distorting the spacetime around them? Scientists are using that concept to see if they can make a ship that can go beyond the normal speeds of our own technology. Basically, they're trying to make the Warp Engine from Star Trek. But in this instance, the "Warp" part of the engine is to warp spacetime around the craft so that it can go incredible speeds.
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Something that wouldn't have even been thought of if not for Einstein and his Theory of Relativity. To help wrap this all up, you need to ask why this theory matters at all. Beyond the potential spacecraft, or even how it may or may not help us understand the universe, why does it matter? The answer is that it is a question that deserves to be answered. The Laws of Motion and the Speed of Light seem like very strong constant things, and yet they directly
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contradict each other in key ways. How is that possible when they're both proven to be true? Einstein's thought experiments may seem like the musings of a man trying to make his mark in the scientific world, but that's exactly what happened to Newton, Maxwell, and others across time. They had a thought, a notion, a theory, and they took it as far as they could go in order to see where it led them. And for Einstein, it led him to the
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Theory of Relativity, which helped him start a very long and prosperous career in science.
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The Theory of Special Relativity”
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The theory of special relativity explains how space and time are linked for the objects moving at a constant velocity. This ‘constant velocity’ is also the main difference between special and general relativity. When we talk about special relativity we take into account the objects moving with constant velocity, for example, a car moving on a straight road with a constant velocity; whereas in general relativity we talk about motion which is not constant velocity motion.
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Motion in which the observer could be speeding up or slowing down or going around a bed, which we call the accelerated motion and also known as generalised motion(like, motion of planets or any other general motion). The move from constant velocity motion to accelerated motion, it turns out, brings in an understanding of force of gravity. Einstein realised that by including generalised motion that would include accelerated motion;
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he could get an understanding of the force of gravity in terms of warps and curves.
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Now lets talk about some very important corollaries of special relativity. One of the most famous equation in physics comes from special relativity. The equation—E=m means—“Energy equals mass times the speed of light squared”. It shows that energy and mass are interchangeable: they are different forms of the same thing. If mass is somehow converted into energy, it shows how much energy would reside inside that mass: quiet a lot (This equation is one
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of the demonstration for why an atomic bomb is so powerful, once its mass is converted into energy or explosion). This equation also shows that mass increases with speed, which effectively put a speed limit on how fast things can move in the universe.Simply we can say that the speed of light(c) is the fastest velocity at which any object can travel. As an object moves, its mass also increases. Near the speed of light, the mass is so high that it reaches infinity,
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and would require infinite energy to move it(as we have discussed earlier), thus capping how fast an object can move. You all might think that how light travels at that speed? The only reason why light speed moves at this speed is because it consists of photons, which have a rest mass of zero! As an object approaches speed of light, its mass becomes infinite and also the body contracts in the direction of motion. This cosmic speed limit has been a subject of much discussion in
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physics, and also in science fiction, as people think how to travel through vast distances. In fact, Einstein realized that the answer is both: space “contracts” and time “dilates”(or slows). Why? If I am sitting in my room on my study table, you would say that I am not moving but in fact, I am moving just not through space but time. Time and space are interchangeable or we can say they are two sides of a coin. So when we move through space, some of the motion can be
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thought of as being “diverted” into motion through time( and vice-versa), in much the same way as a car travelling north-west diverts some of its northwards motion towards west. Space and time are interrelated somewhat like x and y-axis as shown in figure (Below), the only difference is; that we cannot move ‘only’ through space(we move through time also), we always move through time, when we are at rest we move only through time but when we move through space also,
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some of our motion through time is diverted towards space and so the time is less that it should be or we can say time is “dilated”. Thus, the dimension of space and time effect each other, and both space and time are thus relative concepts . This revolutionary idea also helps in better understanding of simultaneity but we will come to that later.
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The concept of time dilation is one of the most fascinating ideas in physics. A Strange conclusion from Einstein’s work is that time moves relative to the observer. An object in motion experiences time dilation, meaning that time moves more slowly when one is moving, than when one is at rest. Therefore, a person moving, ages more slowly than the one at rest. Merely, are able to say that “moving clocks run slow than those at rest”. How? Well as I have
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explained above that when we move through space some of our motion is diverted from time to space, and time moves slow. For a person moving, time would move normally for him but when observed from a different inertial frame which is at rest it would appear to be moving slowly. This becomes extremely apparent at speeds approaching speed of light. Imagine a 15 year old travelling at 99.5 percent the speed of light for five years( from his perspective). When he would get back
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to Earth, according to NASA, he would be only twenty years old. His twin on Earth, however, would be 65 years-old. This is also called “Twin Paradox”. So, when astronaut Scott Kelly spent nearly a year aboard the International space Station in 2015-2016, his twin brother Mark Kelly aged a little faster than Scott. To some extent, are able to say that the quicker or faster you go, the slower you age and therefore slimmer you're . The reason it is not noticeable in everyday
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life is that we move with velocity much smaller than speed of light. The nearer the speed of an object approaches the speed of light, the more warped lengths and time intervals tend to become. While this time dilation sounds much theoretical it does have practical applications as well. If you have a Global Positioning system (GPS) receiver in your car, the receiver attempts to find signals from at least three satellites to coordinate your position. The GPS sends out
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timed radio signals that the receiver listens to, triangulating its position based on travel time of signals. The challenge is, the atomic clocks on the GPS are moving and would therefore move slower and also move faster due to weaker gravity( because gravity also dilates time, we will discuss this aspect in general relativity), but overall they move faster than clocks on earth, creating timing issues.The clocks in our space are ticking faster, because the GPS satellites
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are above earth and experience weaker gravity.So even though the GPS satellites which are always moving, they experience a seven-microsecond of slowing every day because of their movement, the results of the weaker gravity causes clocks to tick quicker for about forty-five microseconds faster than the ground based clocks on Earth. So now ,when we add the two observations together, it leads to the GPS satellite clock ticking faster than a ground-based clocks (on Earth),
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by about Thirty-Eight microseconds everyday. So Engineers do and have to make the clocks on a GPS tick slower. The amount of ‘length contraction’ and ‘time dilation’ is given by Lorentz factor. The Lorentz factor is given by the equation : . The Lorentz factor or Lorentz term is the factor by which time, length, and relativistic mass change for an object while that object is moving. So if an object is moving with velocity c,
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we put its velocity in the equation which gives us the factor of how much it will be affected by its velocity. When this velocity v is equal to c, we can clearly observe from the equation that γ → ∞ (which means, γ will approach to infinity). So, we can assume that travelling at the speed approaching the speed of light would hypothetically enable the time travel into future, as time slows down for the speeding object in order to “prevent” the cosmic speed restriction
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of the speed of light. A corollary of all this is that, if it were possible to exceed the speed of light, then we could go back in time( it is shown in the TV series The Flash), although some scientists believe that some as yet undiscovered law of physics may intervene to prevent such paradoxes. Actually, special relativity does not forbids the existence of particles that can travel at speed faster than the light, and there's a theoretical
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sub-atomic particle known as Tachyon, which might spend its life travelling faster than the speed of light, however it's still hypothetical. Another corollary of the special relativity is ‘Simultaneity or Relativity of simultaneity’. It States that one persons interval of space is not the same as another person’s ,and time runs at different speed for different observer. Simply you can say that if two events occur simultaneously in an inertial frame then it
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is not necessary they will occur simultaneously in another inertial frame (the idea that, events that appear to happen at the same time for one person may happen at different time for another person). The idea of space time plays an important role in its understanding , we know space and time are interchangeable and how. The idea becomes even more clearer when we consider that our picture of moon is actually what the moon was like 1,1-4. seconds ago( the time light takes to reach
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earth from the moon), our picture of the Sun is actually how it looked 8 and-a-half minutes ago, and by the time we see an image of Alpha Centauri, our nearest star system, it is already 4.3 years old. We can therefore never know what the universe is like at this particular time, and the universe is clearly not a thing that extends just in space, but in space-time. Therefore we can say that time is experienced individually. There is no time, there are times!
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Due to the relativistic effects of the concept of time dilation, our idea and understanding of “now” is therefore something of an imaginary concept, one which we as humans have devised for ourselves, but for which nature itself has no real use.. Physicist do not regard time as “passing” or “flowing” and time is not a sequence of events which happen: the past, present and future are just there, laid out as space-time. The ‘Twin Paradox’ mentioned earlier
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is the perfect example for this. Therefore, as Einstein remarked, “For us physicists’, the distinction between past, present and future is only an illusion, however persistent”. Einstein’s theory of relativity introduced us with some of the most important underlying theories of modern physics which opened new doors of possibilities to explore for us.
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