Strangest Facts of Physics: Time Travel to the Past

Is it Possible? 

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.

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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.

Roy Kerr:

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Wormholes


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?

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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.

Kurt Gödel:

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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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Strangest Facts of Physics: Time Travel to the Future

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.

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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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Strangest Facts of Physics: We Are Empty

Let me ask you a question. Have you ever really thought about our universe? Have you ever asked yourself how did our universe start? Or how about what is time? Or even is there something more in our universe besides the matter and energy that we can measure?

If you did, and even tried to look for the answers, I bet the conclusion that you made about the universe is that it's very strange...

There's a saying that reality is stranger than fiction. I agree with it 100%, as I've never heard, read or seen something, that was more amazing and jaw dropping, as the mysterious facts about our universe. So let's check out some of the strangest facts of physics.

Let's start by a simple, yet fascinating fact: a human body is made of 99.9999999999999 % empty space. The atoms that make our bodies, as well as everything around you, are made mostly of empty space.

So how come, do we see all the matter around us and not only empty space? And let's make a step further: how come we can't walk through walls? The answer to these questions is that atoms are not all there is. Every atom is made up of smaller particles - protons, neutrons and electrons. Protons and neutrons make up the nucleus of an atom, where's electrons are attracted to the nucleus with an electromagnetic force. This electromagnetic force (transmitted by particles of light called photons), can be both attractive and repulsive. This electromagnetic force is what causes the repulsion of the atoms, when many of them are put close together. That's exactly why we can't go through walls, as the atoms of the walls push away the atoms in our body.

Composition of an atom:

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Also, atoms can reflect and absorb light. We actually see objects, when a light ray reaches an object and reflects from it to our eyes. Naturally light can only reflect from atoms and not from empty space, thus we can't see empty space. Also there are billions of billions of atoms literally everywhere, as well as billions of billions photons bouncing of them. All of this light makes up the illusion that all objects around us are made of continuous matter.

Photon radiated  from an atom:

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When you really think about it, it's really bizarre. I mean, what we see around us is only an illusion. I guess the universe is really more than meets the eye.

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