Why do people can go back the past when they travel faster than the speed of light? There are two important factors, which people should pay attention to. One factor is "faster"; the other one is "the speed of light". Why is the speed of traveling must faster than the speed of light? Why is the speed of light?
People know speed= space/time, so speed is relative with space and time. The change of speeds can have influence on space and time.
Speed is relative
The speed is not a absolute quantity. For instance, Jan is walking in a train, which is traveling at a speed of 15m/s. If Jan is walking at the same direction as the direction of the train at the speed of 1m/s, people will assume the speed of ground is zero, and Jan will travel at the speed of 16m/s. If Jan is walking at the opposite way of the direction of the train at the speed of 1m/s, Jan's speed will be 14m/s.
Speed is relative, so it is changeable.
However, the speed of light is never relative.
The speed of light is absolute
The speed of light is always a fixed value, which is 299,792,458m/s, and is a universal constant. For example, there is a light source at the top of a train, which is traveling at a speed of 15m/s. The speed of light is still 299,792,458m/s. Light speed is same to everyone.
How does it happen?
The speed of light does not change, so space and time must be changed. Follow the motion of the object; space and time need to be changed, relatively. Let have the same example. A train travels at the speed, which is a little bit less than the speed of the light. Jan is at this train and is watching her watch. The watch works normally. When she looks at outside, the view is twisted. If an observer is on the ground and hears the sound of click of the watch, the sound of clicks will be really slow(the effect of time). In the observer's view, the train is shorter than the previous size of the train because of the effect of the change of the space. It is also called as length contraction. People can make the conclusion that time goes slower as the speed approaches the speed of light.
The possible view of length contraction
Follow the logic chain. If people travel at the speed of light, the time will be zero. If people travel faster than the speed of light, the time should go back.
But it is only the hypothesis. No theories support a way to make a object faster than the speed of light. By now, people can only approaches the speed of light but can not reach it. Follow the next graph, it shows that infinity energy is required for speed up an object to the speed of light. It is impossible for human to make it happen.
Thanks for reading.
Reference: Wikipedia. (n.d.). Lorentz transformation. Retrieved April 11, 2016, from https://en.wikipedia.org/wiki/Lorentz_transformation
Wikipedia. (n.d.). Length contraction. Retrieved April 11, 2016, from https://en.wikipedia.org/wiki/Length_contraction
Robles, A. (2015, May 23). Defeating The Speed of Light. Retrieved April 5, 2016, from http://observationdeck.kinja.com/defeating-the-speed-of-light-1706575145
Images: http://observationdeck.kinja.com/defeating-the-speed-of-light-1706575145
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