How Fast Can We Go?
As a teacher, I get a lot of questions about if it is possible to travel faster than the speed of light by using the theory of special relativity. A very honest answer is that, according to the actual laws of special relativity, no matter how powerful a spaceship engine is, the most extreme speed in space is (and will always be limited to) the speed of light. Of course, this speed limit is quite high (exactly 299,792,458 meters per second) but, it is still a limit annoying enough to be ignored by a lot of science fiction stories. Still, there are three very strange, very interesting and very real effects related to extreme speed. They are briefly introduced below.
The Speed of Light - The Universal Speed Limit
Before introducing the three effects, one has to understand the main hypothesis of special relativity, the one on which all the theory was constructed by Einstein in 1905. His article “On the Electrodynamics of Moving Bodies” proposed that the speed of light has to be the same for every observer, no matter what.
For example, if an observer is moving behind a light ray at almost the speed of light, he will still observe that the distance between him and the light will increase of 299,792,458 meters each second. He will observe the same increase each second if he decides to suddenly stop moving or even if he starts to move away from the light at any possible speed. This strange hypothesis for high speed is different from the day-to-day experience of relative speed. For example, on the highway, when a driver observes a car ahead with a little more speed, the distance increases just a little each second. The difference in the observations comes from the fact that the speed on the highway is really slow compared to the speed of light (see table below).
Some speeds compared to the speed of light:
- 10 m/s —> Speed of human sprinter
- 100 m/s —> Speed of F1 racing car
- 343 m/s —-> Speed of sound
- 8,000 m/s —> Speed of orbiting space shuttle
- 220,000 m/s —> Speed of Sun orbiting the Milky Way
- 299,792,458 m/s —> Speed of light
See also: How Fast Earth Rotates
Consequences of This Universal Limit
One of the consequences of this hypothesis about the constant speed of light is that it is also the maximum possible speed for anything, anywhere, anytime. Therefore, it is not possible to travel faster than the speed of light.
The other consequences are the three effects that can be used to travel into the future, to shorten objects and to desynchronize the present. They are explained in the next page…
First Effect: Time Slows Down
When somebody or something is moving fast enough, their time begins to slow down.
However, they don’t feel anything different. Everything seems completely normal for them. Only other observers looking at them moving at high speed will notice the difference.
This strange slowing effect can be used to travel in the future. Let’s say your mother embarks on a spaceship and travels at very high speed for a year (of her slow time) and returns. If the spaceship speed is high enough (i.e. close to the speed of light), it is possible that meanwhile 50 years will pass for you on Earth. If this happens, your returning mother will be younger than you. She will have traveled in your future! This first effect is also called the time dilation effect: the more speed you have, the slower your time is.
Second Effect: Length Gets Shorter
When somebody or something is moving fast enough, they shorten in the direction of their movement.
Like for the first effect, they still don’t feel anything; everything still seems completely normal for them, and only other observers looking at them moving at high speed will notice the difference.
This contraction effect can be used to make a long object fit in a small place for a brief moment. Let’s say you have a long spaceship and a short spaceship garage, when the spaceship is moving fast enough it can contract enough to fit in the garage for a brief moment. If you want the spaceship to fit in the garage for a longer period, you just have to move the spaceship in a circle at very high speed. As long as it moves, the contraction effect continues and the spaceship fits it the garage! For obvious reasons, this second effect is also called the length contraction effect: the more speed you have, the shorter you appear to others.
Third Effect: Simultaneous Events Are Not Simultaneous
Two events, simultaneous for an immobile observer, are not necessarily simultaneous for a second observer moving fast enough.
The third effect is the strangest of all. It states that the chronology of events can change for objects moving at high speed. The general rule is that the events at the rear of a moving object tend to happen earlier than normally expected. For the application of this rule, the front is defined as the direction of the movement.
For example, consider two students raising their hands at the same time in a bus. When these two students look at each other, they agree on the fact that they are synchronized (because they are not moving one relative to the other). However, for an observer looking at the bus moving toward or away from him, the two students will be desynchronized. For a bus moving to the right, away from the observer, he will see the student on the left raise his hand first. For a bus moving to the left, toward the observer, he will see the student on the right raise his hand first.
This third effect is also called the relativity of simultaneity: as you get more speed, your reality desynchronizes relative to others.
The Big Picture
Globally, if you understand the three precedent effects well enough, you can apply them to any situation and make the speed of light constant for any observer involved in that situation, no matter what. The application of these strange effects (time dilation, length contraction, relative simultaneity) is called special relativity and this theory works for all the experiments done so far, even if a lot of persons are still trying very hard to find something – anything – that goes faster than the speed of light. Nobody has been successful so far…it is still not possible to travel faster than the speed of light.
Speed Limit: adapted from Wikimedia Commons/Ltljltlj
Space Shuttle: Wikimedia Commons/NASA
Moving Fast and Bus Moving: adapted from MSWord/Clipart