Relativity

Everyone has heard the name Albert Einstein. At the turn of the century, Einstein proposed that the speed of light was fixed in a vacuum regardless of the speed of the observer, and that nothing could travel faster than the speed of light.

This also meant that we had a physical barrier in the form of the amount of energy it would take to approach the speed of light. If we could build a ship in an attempt to approach light speed the amount of energy grows exponentially such that even if we could throw all the energy in the universe into the ship we could never reach the speed of light. 

Another side effect of relativity is time dilation. if you had a twin named Gary and Gary remains on earth and you jump in a rocket that traveled at 0.999999991 times the speed of light (186,282 miles per second or 670,615,200 mph) around the earth for a year then when you land your rocket in your twins’s driveway he will have aged a year however you will only be less than a day older. You clock on the ship even reads only a day as well.

Additionally, length contracts as you approach the speed of light as well. The length of the object in the direction of travel actually shortens which really can make one's mind spin.

In the past time, travel was the food for the imagination especially when it came to the possibility of space travel and visiting other worlds. There were many different loopholes that people attempted to use however inevitably barriers arose that would prevent it.

Today we still dream about travel to other stars or worlds but we are still a very long way off from having a reasonable shot at doing this any time soon. Most likely, other worlds would be visited by machines we build using artificial intelligence and using them to probe other worlds first.

Travel to distant worlds may well be mankind's most insurmountable barrier. The distances are so vast between worlds. Our closest neighbor is Alpha Centauri which is 4.37 light years or 25,689,538,124,784 miles. That is almost 26 quadrillion miles, traveling even 600,000 mph (10 times our current capability), it would take us 4,884 years to get there. It is quite obvious before visiting other star systems we will need significant technological advances before it will be feasible.