Let's talk about little grey men from another planet and why I don't believe we've ever been visited before or likely ever will be. This is all speaking scientifically, and leaving all biblical prophesy out of the conversation.
First though, I do believe there is a possibility that life of some kind could possibly exist somewhere else in this universe, yet the bible doesn't speak of any - but that doesn't mean it doesn't exist. Perhaps God did create life elsewhere, but chose not to tell us about it, and made them [life] so far away that neither party could ever visit or detect each other. But that is all I'm going to say about it in this article.
Now, scientists at NASA have recently discovered (2009) the most Earth-like planet, revolving around a sun-like star in the constellation Cygnus, called Kepler 22b. Kepler is believed to be a planet 2.4 times the size of Earth and a temperature of 72°F. That's a good start to being a planet that could possibly sustain life of some sort. Whether that life be microorganisms or a type of intelligent life not unlike mankind. Kepler 22b is about 600 light years away.
For the sake of argument, let's just assume that Kepler 22b has the right temperatures, surface density, weather, atmosphere, etc... to support life, even intelligent life - as smart or smarter than mankind. It's 600 light years away and there is no practical or legal way anyone or anything could get there in a reasonable about of time and make the trip worthwhile. We're also talking about a one-way trip here too.
What do I mean by legal you may ask? By legal, I mean the laws of physics determine the speed and time needed to get to Kepler 22b from Earth or vice a versa. Kepler 22b is 600 light years away, and do you realize how far that really is? Well, it's 3,527,099,888,526,008.5 or 3.5 quadrillion miles away. Or 3.5×10 to the 15th power. Now we already established that it's 600 light years away, which means that it takes light 600 years to get there. In case you didn't know, light travels in the vacuum of space at 186,000 miles per second or in miles per hour, 671 million mph.
The laws of physics say that nothing can travel faster than the speed of light, despite what science fiction movies depict. So according to these laws, we must follow.
Obviously we cannot travel or even come remotely close to moving at the speed of light. But if we could, we'd still not be able to make a meaningful trip to Kepler 22b because no one lives for 600 years.
The fastest man-made object ever built was the Helios space probe sent to study the sun in the mid 1970's. It actually had help from the sun in order to achieve its record breaking speed. Objects in orbit of a planet, such as Earth, travel in circular patterns and remain at a constant speed. For instance, once the space shuttle achieves orbit, and runs out of fuel, save for the trip home, it travels at a constant speed of 17,000 to 18,000 mph. Helios entered an orbit around the sun with an elliptical orbit. Meaning at times, it was far from the sun and would get much closer and then get farther away again. As the probe got closer to the sun, the suns gravity caused it to speed up. Helios achieved a speed of 150,000 mph, the fastest man-made object ever sent into space. In comparison, Voyager 1 is traveling at 38,000 mph and took over 30 years to reach the edge of our solar system. In a few more years, it is expected to enter into interstellar space, meaning the area defined as the solar system, the area governed by the gravity of the sun.
Back to our hypothetical argument that Kepler 22b can support life and we're going to travel there using the fastest spacecraft we can build. We'd also be using the gravity of the sun or other solar body to slingshot our craft even faster, to 150,000 mph. At that speed, it would take 23,513,999,256.84, or 23.5 billion years to get there. For comparison, the estimated age of the universe is between 12-14 billion years. So it would take longer to get there than the universe has existed.
So far all we talked about was the mathematical means of travel. Now we need to take into account, the craft which would take us there, either at the speed of light or much, much slower.
Our best technology is all rocket and nuclear powered and cannot achieve anything close to the speed of light. All the power and thrust of the rocket boosters that put the space shuttle in orbit can only get it to less than 20,000 mph. At that speed, it'll take an even longer time to get to Kepler 22b than it would at the speed Helios achieved.
But for fun, let's say we could travel at light speed, we still have to figure out a way to survive the trip ourselves. We need to live to be well over 600 years old to get there alive and be effective, functioning humans. We'd need provisions, food, water, air to breath for the whole trip. There will be no support for the trip, no one to help if there was trouble. We'd need to be able to carry enough food to last the trip and beyond. Where would we store it? The craft would have to be enormous just for food storage purposes alone.
What about the dangers of space flight? Space debris, such as micrometeoroids or full sized meteoroids? If one of those hits the craft, the trip is likely over. Cosmic rays, such as deadly gamma rays from quasars and other types of celestial bodies, can kill living organisms upon contact. Some type of shielding would have to be devised.
What about navigation? Someone has to plan the route there, account for all known objects on the way, and navigate around them, plus calculate where they'll be by the time you get to them, not where they are when devising the plan on Earth. Then you have to consider the unknown things out there and someone or a computer smart enough needs to be awake and monitoring for these things the whole trip.
What about propulsion? What could we come up with, that is plentiful enough on Earth that could provide the necessary amount of energy to propel a craft at faster than light speed, yet be light enough to carry out of Earth orbit, and have a mass small enough to fit in a reasonably sized craft? So far, nothing. You can say nuclear energy, yes, because atoms are infinitesimally small. But we don't have the knowledge or technology to harness enough of the power to make it usable.
Then there is probably the least talked about problem with space travel, the psychological part. How will people react and handle the utter isolation of space travel? Knowing that they are getting farther and farther from our only home and going somewhere where everything is totally unknown. The isolation of deep space, knowing that there is no one else who can help when/if trouble arises would be maddening to probably everyone put into such a position.
All these things and more we'd have to figure out and overcome to travel anywhere outside, and even inside our solar system. Likewise, little grey men from Kepler 22b would also have to figure out all of these problems if they wanted to travel away from their home planet.
Those, essentially are the facts of space travel and why we will in all probability never achieve, nor any other life form in the universe.
Now if one wants to theorize about shortcuts in the space-time continuum, such as wormholes, that's a whole other topic, one which is total theory, that I do not subscribe to. While fascinating to theorize, I don't believe they exist and the problems with that are just exponentially more than with regular space travel.
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