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Taking this into consideration, it seems hopeless to travel between galaxies as we see in science fiction all the time. I mean, the nearest galaxy - Andromeda - is over 2 million light years away, so traveling slower than light means it would take much longer than 2 million years before a ship we sent over there arrived.
So how is it that something could cross the universe in a matter of days? How is it that we humans could theoretically get to Andromeda in a human lifetime?
In 1991, a cosmic ray detector in Utah registered a proton called the Oh My God Particle. Based on its enormous energy, they deduced its speed:
0.9999999999999999999999951 c (where c is the speed of light)In other words, it was traveling incredibly close to the speed of light without quite reaching that speed.
Here's the interesting part. When you plug that velocity into the equations of relativity, you find that the particle was traveling so fast that from its frame of reference it could travel to the nearest star in .43 milliseconds. Which may not seem all that weird until you remember that the nearest star is over 4 light years away (it takes 4 years for light to arrive here from there). So how does something traveling slower than the speed of light cross over 4 light years in a fraction of a second?
Well, it turns out that when you are moving, time passes slower than when you are still. It is such a tiny effect that you could not possibly notice it in day to day life, because even our fastest travels are nothing compared to the speed of light. But if you can accelerate something up close to the speed of light, the effect becomes quite dramatic. Time passes more slowly for the traveling mass and length contracts in the direction of motion (meaning the distance to your target becomes shorter - for distance is basically just a measure of how long it would take to get somewhere at a particular speed).
What this means is that if we sent a spaceship out at the speed of this particle, from our perspective watching it back on Earth, it would take about X years to travel X light years. It would not arrive at the nearest star until over 4 years past the launch date. However, from the perspective of the people onboard the ship, time would pass slower, distance would contract, and sure enough they'd reach the star in the same second they got up to full speed. They'd reach the nearest galaxy (billions of light-years away) in a few minutes. They could, in fact, travel the distance of the known universe in a matter of weeks.
Of course, we can't propel mass to those speeds, and if we did the ship would not be able to hold together and survive the first leg of the trip. But it is still mind-opening to plug in these numbers and think about the effects of relativity and to realize that time may not work like we generally think it does. We have no intuitive grasp of relativistic time because we are confined to such slow speeds, but in turn reality is by no means confined to our intuitions.