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superchunk on 28 July 2008
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Retrasado said:

*sigh* another one of these crazy "aliens are visiting Earth" statements.

Look. Even if we aren't alone in the universe, there's no way Earth is being visited by aliens on a regular basis and I can prove it. First: the basics. 1) No star system is within 4 light-years of our own system. 2) it is highly unlikely that any civilization that has achieved a level of technology of our level or greater exists within 45 light-years. (We would have picked up their radio transmissions easily if they were there)

That being said, for the sake of our argument we will say that civilization with a level of technology slightly ahead of our own exists 20 light-years from Earth and has somehow managed to escape detection. In order for them to travel to our star system in a reasonable amount of time, let's say they have a spacecraft that weighs 5000 kilograms and is capable of traveling at 75% of the speed of light. (At this velocity it would take them 25 years to make the journey) In order to accelerate their spacecraft to it's maximum speed, they will have to expend E=1/2mv^2 = 5,000,000(g)*810,000,000(m/s)^2 =1.64025 x 10^24 joules of energy. (and they will expend an equal amount of energy to slow down at the end of the trip)

Now that's a LOT of energy. 1.64 x 10^24 joules would keep a 100-watt light bulb burning for 5.1968 x 10^15 years. (that's right. More than 5 QUADRILLION years!) Also note that a 5000 kg spacecraft is ludicrously small (the Space Shuttle routinely carries payloads of well over 20000 kg) and it is highly doubtable that such a craft would be able to provide the necessary provisions and equipment to survive a 25-year spaceflight.

As you can see, traveling to another planet in a reasonable amount of time would require a very significant investment and would not be undertaken at all unless there was a very good reason to do so; and even then, it would not be done frequently. Bottom line: Anyone who says Earth is being routinely visited by aliens is completely wrong.

What you say is a very sound theory when considering 'normal' traveling abilities. However, there are also plenty of theories, and sci fi shows love these, especially stargate, about wormholes or the ability to fold space and time to travel great distances in a fraction of that time. IF, and I realize this is a big if, that is attainable then species much further out could be traveling here and elsewhere. Personally, I think that if space travel outside of our solar system will ever happen it will be with some type of technology similar to this.