The theory of relativity arises from a simple assumption that the speed of light is constant as viewed from any frame of reference. That is, an observer travelling towards a light beam would measure the same speed of light as an observer who is moving away from it.
One of the consequences is that two events can never happen simultaneously. For two stationary observers, an event occuring at time t, will appear to have occured at time t', where t-t' depends on how long light takes to travel to each observer.
So what about moving reference frames?
According to the theory, there is no special reference frame. A person on a plane crusing at a constant speed can never know if he is in motion unless they look out the window. And for all they know, the plane could be stationary and the earth could be moving underneath them. A clock that ticks every second on a plane would also take a second to tick if it was on the ground. However each observer would see a different ticking rate if he looked at the other person's clock:
If the person on the ground makes a measurement of the tick of the clock on the plane, he would first see the clock beginning to tick, but the plane would have moved by the time the tick has finished, so the observer sees a longer tick duration. Time runs slower on the plane.
Another way of looking at this is to imagine two spaceships (A and B) moving away from earth in opposite directions near the speed of light (0.9 C, where C is the speed of light). What would the speed of the spaceship A be as seen from spaceship B? 1.8 C? but we know that faster that C is not possible. The answer is actually 0.996 C. That means that time runs slower in the spaceships relative to eachother, and since distance is speed multiplied by time, the distance between the spaceships also shrinks.
Now to space travel:
If humans ever developed near the speed of light space travel, there would still be a couple of major problems to deal with. The distance to the closest star is about 2 light years. It takes light 2 years to reach it. So from an observer on earth, a spaceship travelling near the speed of light would take slightly more than 2 years to reach it. For the passengers however, time runs slower and depending on how close you are to the speed of light, the time can be reduced indefinitely, but it can never reach zero.
The diameter of the Milky Way galaxy is about 150,000 light years. The closest galaxy, Andromeda, is 2.5 million light years away.
Sending probes it also difficult. We would have to wait 300,000 light years to be able to send and receive a signal across the galaxy, not to mention the signal would have to be very strong. So travelling across the galaxy is like a one-way trip. Don't expect to hear from the passengers soon.
So why would an intelligent race want to travel this far...for resources? unlikely. Intelligent beings capable of building near the speed of light spaceships would be able to manufacture anything they want out of raw matter. Exploration? The cost seems too great. Achieving relativistic speed requires enormous amounts of energy. The most likely scenario is receiving radio signals. But depending on the distances, the signals need to be very strong...such as the signal from pulsars. Pulsars naturally produce powerful radiation at regular intervals. But to be able to control such a system would require lots of energy...perhaps beyond the capabilities of any intelligent being.
But even if we do receive signals, there would be no means of communication within a single human lifespan. The best case scenario would probably be the fact of knowing that other intelligent life exists...nothing more.
Hope you liked the article. Post your questions below. I will be posting a much more detailed (and mathematical) article on relativity soon. Time to take my orange pill now...
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