When it comes to physics, the first person that comes to our heads is Albert Einstein. The famous picture of this colossal figure of science taken in 1951 shows the charismatic and humorous side of the genius. In a sense, this picture captures the whole essence of Einstein as a scientists - a brave risk taker and a rebel among the genii of physics.
Everybody these days knows about the successful theories of Einstein. However, not many people know about the most important and definitely the hardest theory that Einstein never finished - the theory of everything. This grand theory would be the greatest achievement of theoretical physics, as it would combine all the known laws of physics, explain all the forces and behaviour of particles and large objects such as planets, galaxies and the universe itself.
So let's check out this great documentary, that tells a story of Einstein's unfinished theory of everything. It contains great actors as well as interesting biographical facts. Enjoy.
Thanks forQuantumLeep365for uploading the full documentary
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Isaac Newton, inspired by the falling apple in his yard during the Great Plague, created his greatest work – the law of gravity. This wonderful and simple law predicted the movements of planets, stars and even galaxies. Even though, Newton’s calculations could precisely predict movement of any stellar object, the great genius had no idea how gravity actually worked.
More than 3 centuries later perhaps the greatest physicists ever, Albert Einstein, created his famous special and general theories of relativity. Even though Newton's law of gravity was very precise, there were some cases when movement of the planets and galaxies could bot be explained by this simple law. However, Einstein's general relativity combined with Newton's laws could explained almost all the movements of the planets, stars and galaxies. The great thing about these theories is that they can be used to find out more about the universe. For example by observing the rotation of the outer stars in the galaxy, scientists could calculate the total mass of the galaxy. This is where the whole strangeness started. When calculating, the mass of various galaxies scientists realised that those galaxies move in such a way as if there was more mass in them than we can observe. This meant that either there was something wrong with current theories and calculations or there was some invisible matter, which interacted only gravitationally. And since Einstein's theories have been tested and proved dozens of times, scientists came to the conclusion that there is some kind of strange invisible matter called dark matter. This is our strange fact of today.
Even more shocking is that there is almost 6 times more dark matter than ordinary matter in our universe. This means, that even now, there's invisible dark matter in your room, which cannot be seen or detected in any way, except gravitationally.
Last time we looked at time travel to the future, which is indeed real, but how about time travel to the past? Is it possible?
This is where opinions of scientists split up. Some believe that time travel to the past is possible, though we don't have the technology yet, whereas others believe that it isn't possible due to various paradoxes.
Nevertheless, time travel to the past, at least theoretically seems to be possible. And that is the strange fact of today.
Seeing the Past
So let's look at some ways to travel back to the past. First of all, we see past all around us, as the light from any object takes time to travel to our eyes. Even though light is extraordinary fast, it takes very long periods of time to travel from distant stars. This means, that if we were to look through a telescope, we would see distant stars as they were billions of years ago. In a sense, this is a form of time travel, as we can see the past. But is there any real way to not only see the past but also travel to it?
Black Holes
The possibility might hide deep in the far reaches of the space. Mysterious massive objects known as black holes might hold the key to time travel. But what is a black hole? When a very massive star runs out of fuel to burn, it collapses. If the star is massive enough, there is simply nothing that can hold the gravitational collapse. Thus such a star collapses rapidly down to a singularity (infinitely dense and small region of space). As the gravitational pull of such a singularity is extraordinary strong, even light can't escape the gravitational pull near the star.
From special relativity we know that the faster an object moves the slower time runs for it. If an object reaches light speed, time completely stops running for it. Now following this logic, some scientists believe, that if something moved faster than light, time would run backwards. So this gives an idea how we could travel through time. If we were to fall down to a black hole, at some point we would end up travelling faster than light.
However, there's one "little" problem. The gravitation of the black hole would simply tear our bodies apart. So travelling back to time using this kind of black holes is out of the question.
But don't worry, there might be another type of black holes. In 1962 New Zealand mathematician Roy Kerr, proposed a working theory of a rotating black hole. According to Kerr such a black hole would be spinning, thus wouldn't collapse to a singularity. In theory, it would be possible to travel near such a black hole without getting torn apart. So at least in theory, it might be possible to travel back to past, although we have no proof of such black holes existing.
Every sci-fi fan probably knows what wormholes are and how they work. But could they exist in reality? Could we be travelling back in time like those guys in Star Trek?
First let's look at how wormholes actually work. The idea is that a wormhole is a tunnel connecting two points in space-time. A wormhole that connects two points in space-time in a specific way, so that one point is in the past, might be used for time travel.
The problem is that, even though such wormholes seem theoretically possible, in reality they would be very unstable and probably wouldn't exist for a long time.
Closed Timelike Curves
It's amazing, but ideas of time travel to the past span back more than half a century ago. Back in the 1940s Austrian mathematician Kurt Gödel was working with the solutions of Einstein's general theory of relativity. In 1949 Gödel discovered interesting solutions of the equations of the general theory of relativity. These solutions showed that there might exists objects called closed timelike curves, which would allow time travel. A CTC is a world line that a particle or a person can follow which ends at the same space-time point (the same position and time) as it started.
Unfortunately, CTC's seem to exist only if space-time has a special form (it should have holes sort of like a doughnut shape).
Cosmic Strings
Some scientists believe that strange objects called cosmic strings (not to be confused with strings in string theory) might exist. These massive and extraordinary thin objects might help in time travel. If two such strings could be brought close together close timelike curves might be created.
As always, this has some problems. First of all, these strings might not even exist. Furthermore, energy required to manipulate such strings to create CTC's would be enormous.
Paradoxes
So as you can see there are many great problems with time travel to the past. Naturally, there are scientists who do not believe in time travel to the past. To defend their position they often use famous paradoxes, which show that time travel might not be logical. The most important and popular paradox is the so called grandfather paradox. It goes like this: let us say you go to the past and for some reason decide to kill your grandfather before you were born. Would it be possible? I mean if you'd kill your grandfather in the past, you would never been born and thus wouldn't travel back in time and kill your grandfather, which is not logical. Of course some say, that there would always be something stopping you from killing your grandfather (which doesn't sound realistic to me). Others believe, that you would kill your grandfather in a parallel universe, which wouldn't cause a paradox.
Conclusion
Even though many scientists don't believe in time travel to the past, it is still theoretically possible. We don't have a sufficient technology to test it, however such a technology might be available in the future. And even though time travel to the past seems unlikely, only time will tell...
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When I was a child and was only starting to learn physics, one thing for me was stranger than anything else. You might guess it was the strangeness of quantum mechanic. But actually (even though quantum mechanics is very very strange and fascinating) the thing that really caught my attention was time travel.
Time travel is one of the most popular subjects of science fiction. Starting with Star Trek ending with newer shows like Lost most of the sci-fi shows have a fare share of time travel. But have you ever wondered if time travel is possible? Well the answer is yes! And that is our strange fact of today.
First let's look at time travel to the future. Even though Einstein's special theory of relativity is a part of high school course, not many people know that time travel to the future is actually a scientifically proven fact. It's not only proven, it's a big part of communication technology, as even GPS services wouldn't work without the precise calculations from the special and general theories of relativity. But what does this have to do with time travel?
Einstein back in 1905, when his special theory of relativity was created, showed, that time runs slower for fast moving bodies. To get a grasp of how this works, let's check out this mechanism called photon clock. It consists of two mirrors with a photon (particle of light) trapped between them. Every time the photon hits the mirror a unit of time passes.
When the clock is not moving, the photon just bounces up and down vertically. However, if the clock is moving, as you can see in the image, the photon needs to go diagonally from the upper mirror to the lower one and vice versa. It's clear that when the clock is moving, the photon has to travel a greater distance from one mirror to another. In other words, time passes slower for a moving photon clock. But the interesting thing is that this is true not only from photon clocks, but for any moving objects, as Einstein showed in his theories. Even the photons in the atoms of our bodies must travel a longer distance between the nuclei and electrons when our bodies are moving.
This simply means that the faster some object moves, the slower time runs for it. This gives rise to the famous twin paradox. It goes like this. Let's say there are two twins. One of them decides that life on Earth is boring, so he jumps on a fast rocket and flies away for let us say 50 years at a speed of 75% of light speed. Then after 50 years, he get's back to Earth, and is amazed, as his twin brother appears to be 25 years older than him.What happened was that time ran slower for the brother, who was in the rocket, which basically made him travel to the future, as time on Earth was running faster.
A video about Twin Paradox:
You might be wondering does this have any proof? The answer is yes. Two super precise atomic clocks measured time, while one of them was in a plane flying around the world, whereas the other clock was stationary on the ground. The difference in the clocks, when the plane completed its journey, was a very very small (nanoseconds), but it agreed with Einstein's theories.
So time travel to the future is really possible, though our current technology is not advanced enough to make use of it. Rockets, travelling at almost the speed of light would be required to travel to actually travel to the future. Also going back from the fancy future would be much more difficult (some believe even impossible), but more about that next time.
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Here I have a nice video lecture dedicated to the century since the great discoveries of Albert Einstein. It's mostly about Einstein's biographical facts, and his greatest discoveries - General and Special theories of relativity. It contains a lot of interesting and not known to the wide community facts from a variety of sources including Einstein's memoirs.
The video quality is not perfect, but the audio is good. So feel free to enjoy, and if you want to download this video you can find it here.
When it comes to GR, it's easy to find good books, which explain you the basics and the non-mathematical side of the subject, however, it's really hard to find a book, which would explain the basics of mathematics of GR in a easy to understand way. Thus I would like to share my experience of reading a book called "Firs Course in General Relativity" by Bernard Schultz.
As the books half title says - "Clarity, readability, and rigor combine in the second edition of this widely used textbook to provide the first step into general relativity for undergraduate students with a minimal
background in mathematics." This is truely the case, because the book is written in a easy to understand way.
The book contais theory, including special relativity in algebraic form and geometric form and general relativity basics. Furthermore, it has some exercises with solved examples.
From my own experience I can say that it's one of the best books to get the basics of both SR and GR. The difficulty level is not over the top like in the most of similar books (I'm a fresher in Physics, and my knowledge in maths and physics is basically enough to read and understand this book). Also, it's a great book to refresh your knowledge of SR and GR.
Ever since the beginning of human kind, humans were fascinated by the concept of time.
It’s probably mostly due to the fact, that time was always a very important thing. Even prehistoric people had their system of measuring time, which was basically watching the stars, the sun and the moon. Later civilizations discovered various clocks, starting with simple sand or sun clocks, ending with complex water clocks. In Middle Ages mechanical clocks were discovered.
As we can see, civilizations throughout the history believed in two different time models. The first one and possibly the earliest was cyclic time model. Most of the ancient civilizations measured time by observing the sun, the moon and the change of the seasons, which naturally led them to think that time runs in cycles. For example in ancient India years were named and once the 60 names are finished, the next year starts with the first name again. Ancient Aztecs also had special calendars divided in cycles. After every cycle, they believed, a new similar cycle starts once again. It’s interesting to notice that the famous Mayan calendar also measured time in cycles. So it’s not true that it ends in 2012, actually only one of the cycles will end at 2012 and then a new cycle will begin.
Another way of thinking about the run of time was linear time model. It’s the most familiar model to us. Perhaps, the best comparison about time was given by Einstein, who said that time is like a river. This river, by the way, can fork, have whirlpools and run in very interesting ways.
The biggest question, when talking about time, is, of course, what exactly time is. What is that magical river, which brings us to the future? It seems that this question is harder to answer than you might think. Let’s see how people throughout various periods of history answered this question.
Many ancient civilizations believed that time, like all the nature, is controlled by the gods. The Vedas, the earliest texts on Indian philosophy and Hindu philosophy dating back to the late 2nd millennium BC, describe ancient Hindu cosmology, in which the universe goes through repeated cycles of creation, destruction and rebirth, with each cycle lasting 4,320,000 years. Ancient Greeks believed that that time, as well as their destiny was controlled by divine deities for the Mount of Olympus. However, ancient Greek philosophers had more sophisticated ideas. Some of them believed that time is infinite – the universe, as well as time had no begging, nor will it have an end. However, there were philosophers, like Zeno, who didn’t believe in any kind of change. Zeno, in his famous paradox (Zeno’s paradox) argued that any kind of change or movement is impossible. This lead to the conclusion, that time is only an illusion.
Philosopher and theologians argued that both universe and time had a beginning, and will have an end. They believed, that time started at the beginning of the universe.
Immanuel Kant, a famous Prussian philosopher, had his own ideas. He believed that time was only an idea like numbers, or dimensions. Kant thought of time as a fundamental part of an abstract conceptual framework, together with space and number, within which we sequence events, quantify their duration, and compare the motions of objects.
Immanuel Kant
Isaac Newton shared similar ideas. He believed that time is like an arrow, which always goes only one way. He believed that time was universal – it was running at the same rate for all observers.
All of these ideas caused two distinct viewpoints of time among philosophers and scientists. One view is that time is a fundamental part of the structure of the universe. It’s like another dimension, in which events occur like in normal special dimensions. Other point of view is that time is not a part of the structure of the universe. Time, according to this point of view, is not an event, nor a dimension or a thing.
Isacc Newton
The big breakthrough in understanding the nature of time came in the beginning of the XX century. Einstein, with his famous theories (Special and General Theories of relativity) showed that time is not universal. The rate of the clock depends how fast you are moving. Simply speaking, time flow depends on various conditions, which include the velocity of the body. Later in his General Relativity Theory Einstein showed that time also slows down in a gravitational field. These two very important conclusions lead scientists to believing that time actually is a part of the fundamental structure of the universe. Also when Einstein showed that time travel (at least to the future is possible) it became clear that the old view of time as nor a dimension or a thing was wrong.
Now, a century later, we know many wonderful things about time. We know that time is highly connected with the form of Space Time continuum. For instance, if you would spend some time orbiting a huge star, the gravitational field of which is very strong, you would age slower than let us say your twin brother who is spending his days on Earth. We also know that the speed of time flow is connected to the velocity of the body. Another great discovery is the so called arrow of time. It’s clear that time goes from the pas to the future. This makes time a unique dimension. We can freely movie in any of the three special dimensions, but when it comes to time we are prisoners. Scientists believe that the entropy of the universe is the so called arrow of time. In this case entropy can be viewed as a measure of disorder. According to thermodynamics, the entropy of a closed system tends to increase. And since universe is a closed system, entropy in the universe increases steadily. Thus, for instance, a broken cup of coffee, newer comes back to the table and fixes itself as it was traveling back in time.
Albert Einstein
Only one thing is really certain – time is connected to the geometrical properties of the space-time continuum. The newest discoveries of astrophysics gave us many mysteries. Mysterious Dark Matter and Dark Energy are great examples, that we still don’t know many secrets of the Universe and time itself. For instance some scientists believe that time is highly connected to the accelerating expansion of the Universe, which is supposed to be caused by the Dark Energy. This Dark Energy, according to scientists, is constantly changing the form of the Space-time continuum. This means that in order to understand more about the Space-time continuum, the Universe and the time itself we need to understand what Dark Energy is. Even at this moment, due to the Dark Energy, the time itself can be speeding up. All in all, the mysteries of the Dark Energy and Dark Matter hold many secrets of time.
Our generation is a really lucky generation, because we know more about Universe and time than any other generation throughout the history. So let’s hope we will find out the secrets of the universe sooner or later. After all, who wouldn’t like to find out what exactly is the magical river of time.
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