Common sense tells us that time travel is nonsense; however, common sense is not always the best guide. Some other objections (paradoxes) reinforce the theory that time travel is nonsense, like if we could travel through time, imagine what would happen to a time traveller if he travelled back in time and killed his own dead grandparents at birth. In theory the time traveller will therefore never be born, so the journey could never have been made in the first place; but if the journey never occurred then the grandmother would be born which means the time traveller would have been born and could make the journey and so on. But there is a twist to all this, which makes time travel a possible, at least theorically.
Is Time Travel Possible?
Imagine being able to travel in time, back and forth. What would you do then? Before we prove that time travel is possible, we have first to deal with the above paradoxes (time traveller killing his grandparents). One of the answers would be that everything that has happened or must happen, including the time traveller’s attempt to kill his grandparents, cannot be altered and so nothing will change the course of history. In other words, the time traveller will be prevented from trying to kill his grandparents and will never achieve the murder, so that the time or (history) will remain intact. Other possible answers could be that: when the time traveller kills his grandparents he immediately creates a new quantum universe, in essence a parallel universe where the young grandparents never existed and where the time traveller is never born, and thus the original universe still remains.
According to Einstein, time is like a river, which is moving aimlessly around stars and galaxies, speeding up and slowing down as it passed around massive bodies. Einstein thinks that one second on earth is not one second on Mars. If we travel at speeds close to the speed of light, time would slow down, once we reach the speed of light time would stop, then when the velocity becomes greater than the speed of light time moves backwards and the traveller has entered the realms of negative time. This means that if you were to travel into outer space and return, moving close to the speed of light, you could travel thousands of years into the Earth's future. And if you travel faster than the speed of light then you will even be able to travel not only to its future but its past, may be our entire life is a blink of an eye to an alien traveling close to the speed of light.
Velocity close to the speed of light = Time slows down
Velocity the same as the speed of light = Time stops
Velocity greater than the speed of light = Time moves backwards
Now the only question is, can we travel the speed of light.
Can we travel the speed of light?
It was said before that man will never achieve the speed of sound. However now astronauts in a space shuttle can travel more than 20 times faster than the speed of sound. Could that be the same with the speed of light? It's important to note that the speed of sound is 343 meters per second (1,125 ft/s), while the speed of light is often approximated as 300,000 kilometres per second (186,000 miles/ second). So we might to try a little bit harder for this one.
According to the theory of relativity, the speed of light is the fastest at which anything can travel. However there is a surprise with this latest discovery; scientists believed for decades that there is nothing faster than light moving through a vacuum, a speed of 186,000 miles per second. But in an experiment physicists sent a pulse of laser light through cesium vapor so quickly that it left the chamber before it had even finished entering. The pulse traveled 310 times the distance it would have covered if the chamber had contained a vacuum. Researchers say it is the most convincing demonstration yet that the speed of light can be pushed beyond known boundaries, at least under certain laboratory circumstances. The experiment shows that the generally held misconception that "nothing can travel faster than the speed of light" is wrong.
Chemical engines like those used by NASA space shuttles for chemical propulsion, or Ion propulsion (used recently by NASA as well) are a very fast way to travel but will not help us by any mean reach the speed of light. Antimatter propellants are a good idea, using only a few billionths of a gram of antimatter, a 400 ton spacecraft could travel to Mars and back in four months, with a one month stay on the surface. Good but not enough.
Laser sails is also a project which relies on the principle that light has momentum. Because of this, when light strikes an object, a slight force is applied. If enough light is concentrated on a very large area, this force becomes great enough to be used for propulsion. A recent design proposal involved shining a 10 million gigawatt laser through a thousand kilometer lens onto a thousand kilometer sail. This system could send a thousand ton spacecraft with crew to Alpha Centauri (nearest star outside our solar system) within 10 years. Very impressive but is year from achieving, and also doesn’t come close to the speed of light.
We can travel faster than the speed of light for sure (in some cases), in fact we did (by making the speed of light slower). Danish physicists performed an experiment where they slowed light down to only 38 miles per hour or about 57 kilometers per hour. They did this by sending a beam through a material made of sodium atoms cooled to near absolute zero (-273°C or -460°F). They achieved this low temperature by using lasers to slow down the atoms, through a special method used in quantum mechanics called the Bose-Einstein condensate (well, that’s like the story of “if you can’t run faster than a tiger, try to slow the tiger down) good idea but doesn’t help, because when we discuss going faster than the speed of light we are really talking about exceeding the speed of light in vacuum c (299792458 m/s).
Scientists believe there is a way around it; the theory is: a controller based on Earth is monitoring a space-ship moving away at a speed 0.8c. According to the theory of relativity he will observe a time dilation affecting the clocks on the ship and slowing them down by a factor of 0.6, even after he has taken into account the Doppler shift of signals coming from the space-ship. If he works out the distance moved by the ship divided by the time elapsed as measured by the on-board clocks, he will get an answer of 4/3 c. This means that the occupants of the ship are traversing the distances between stars at effective speeds greater than the speed of light when measured with their clocks. From the point of view of the occupants, it is the distance between the stars which is contracted by a factor of 0.6 and they also agree that they are covering the known distances between stars at 4/3 c.
This is a real effect which in principle could be used by space travelers to cover very large distances in their lifetimes. If they accelerate at a constant acceleration equal to the acceleration due to gravity on Earth, they would not only have a perfect artificial gravity on their ship, but would also be able to cross the galaxy in only about 12 years of their own proper time. However, this is not true FTL travel. The effective speed calculated used the distance in one reference frame and the time in another. This is not the real speed. Only the occupants of the ship benefit from this effective speed. The controller will not see them traveling large distances in his lifetime.
Another theory is a space drive (Warp Drive) would be a mechanism for warping space-time in such a way that an object could move faster than light. Miguel Alcubierre was working out a space-time geometry which describes such a warp drive. The warp in space-time makes it possible for an object to go FTL (faster than light) while remaining on a time-like curve. The wrap drive would mean then a hyper-fast travel within general relativity which explains how an object moves at speeds greater than the speed of light. The basic idea behind the Alcubierre warp drive is to greatly expand space-time behind the ship, and greatly contract it in front of the ship, The expanded space-time region would push the ship forward, while the contracted region would pull it in the same direction, resulting in somewhat of a galactic surf ship.
There are some interesting properties of the Alcubierre warp drive. First is that a ship traveling at the center of the warped space would be at rest with the surrounding space-time area, so that while the ship may be traveling at faster than light speeds to an outside observer, there would be no relativistic mass increase or time dilation effects. The ship will also not be experiencing any acceleration effects or tidal forces. The main catch is the same one that may stop us making large wormholes, to make it work you would need exotic matter with negative energy density. Even if such exotic matter can exist it is not clear how it could be deployed to make the warp drive work.
A famous proposition for global traveling faster than the speed of light is to use wormholes. It’s said that the fastest way from A to B is a straight line, well not anymore, wormholes are shortcuts through space-time from one place in the universe to another which would permit you to go from one end to the other in a shorter time than it would take light passing by the usual route. Wormholes are a feature of classical general relativity but to create them you have to change the topology of space-time. That might be possible in quantum gravity.
To keep a wormhole open, regions of negative energy would be needed. Misner and Thorne have suggested using the Casimir effect on a grand scale to generate the negative energy while Visser has proposed a solution involving cosmic strings. These are very speculative ideas which may simply not be possible. Exotic matter with negative energy may not exist in the form required.
Thorne has found that if wormholes can be created then they can be used to construct closed time-like loops in space-time which would imply the possibility of time travel. It has been suggested that the multiverse interpretation of quantum mechanics gets you out of trouble by allowing time to evolve differently if you succeed in going back to a previous time. Hawking says that wormholes would simply be unstable and therefore unusable. The subject remains a fertile area for thought experiments which help clarify what is and what is not possible according to known and suggested laws on physics.
Traveling the speed of light or faster than the speed of light seems impossible from the point of view of many theories and common sense, but on the other hand there are other theories supporting traveling faster than the speed of light, and that’s what we will hold onto, because humans are always optimistic and that’s how we achieve the unthinkable.
Here is a list of other technologies that are so powerful, conquering them would mean conquering the world:
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