Yes, the kind from off-planet.
Have they visited this planet? Not bloody likely… For one, it is immensely arrogant to believe that a species with technology allowing it to travel distances spanning light years just to land here look around, and leave. Think of our solar system visits. The exploring we’ve done with our probes is enough that we’d have found any civilizations The Mars rovers have driven on the surface of Mars extensively. If there were any nominally intelligent life we’d have seen it. But not even a tumbleweed, but that’s not the point. If E.T. came to look at us up close, he’d probably try initiating contact. But there’s more to it then no direct contact.
Distance: The nearest star outside our solar system is Proxima Centauri, at about 4.2 light years. And it is a red dwarf. Not very life sustaining planet happy… Let’s go with the sister stars, shall we? Only a difference of .21 light years there. The star system is known as Alpha Centauri by most, though more appropriately as Rigil Kentaurus. The two remaining stars could, theoretically, have planets that sustain life kicking around in orbit. We’ll be optimistic and say yes, they do, and yes, they are more advanced then us, technologically. We’ll not get into alien sexual prowess.
So, at a distance of 4.41 light years, they would first need to get very exact measurements of the Earth’s movement. Next, they’ll need fuel. They’ll need a fair amount for acceleration and small course corrections, however, the near perfect vacuum of space poses very little drag, so keeping the forward speed at what they want will cost minimal fuel. We will assume they have suspended animation chambers to cut down on supplies. The on board computer is advanced enough to keep everything on course until they reach this inconspicuous marble that has been sending out transmissions for so long. They’re in for a long trip.
Speed and time dilation: The speed of light (hereafter, c) in a perfect vacuum is 186,282 miles per second. So far, quantum physics has not way of accelerating matter to, or beyond this speed. Even more, it is speculated that such an act would convert the matter to pure energy, as in Einstein’s famous equation, E=MC2.
In other words, how the hell do you propel something to the c? First, we’ll look at current propulsion systems we have that work in space.
Rocket- Dominantly used. This involves the combustion of explosively flammable materials, typically liquid hydrogen with oxygen present to allow for burning. Cons- Heavy and volatile. Burns well above Mach, but far less than the c.
Gyroscope- Used more for steering, for which it is highly effective.
“Aerosol” type propellant- Used on astronauts performing space walks. Sub-Mach, slow acceleration.
Now what of nuclear reactions? Well we don’t have them as propulsion, but we’ll assume the aliens do.
Fission- Results in hydrogen atoms being split. Immensely powerful, emits photons (which make up light) in massive quantities. The downside- Most photon emition is gamma type, which has a very short wavelength, causing it to have tremendous energy. This causes it to be extremely deadly, causing radiation poisoning in all known life. This is because the radiation causes the water contained in cells to flash-evaporate, bursting the cell and spewing toxins therein through the body. This requires that the alien crew be in hibernation before the main engines are online. Still, there is not enough thrust from particles that do achieve c to attain c in a reasonable amount of time.
Fusion- When two atoms combine to form one larger atom. Starts with hydrogen. This method has the advantage of not releasing immense amounts of lethal radiation. It also provides more thrust than fission. It will likely yield higher speeds given the same amount of time. Downside- Requires immense pressure to sustain. Jupiter, the largest planet in the solar system, needs five times the mass it posses to sustain fusion. Heat also becomes problematic. The center of fusion reactions is in the millions of degrees Fahrenheit This temperature is far in excess of the boiling point of even Tungsten, the element with the highest boiling point, at 10,031 Fahrenheit when under one atmosphere of pressure. The pressure required to sustain fusion is beyond the strength of any known substances, as it is in the millions of atmospheres, at minimum. Still lacks the necessary amount of power coming from particles traveling at c.
Anti-matter- The most powerful energy source available. When matter and anti-matter come into contact, they annihilate, becoming pure energy. This results in an immense energy release for tiny amounts of mass consumed. No pressure is needed. Almost all particles that are ejected are in the form of energy traveling at or near c, with any extra being the result of an imbalance of the matter to anti-matter ratio. Very low radiation emition on dangerous levels. Downside- Extremely dangerous to contain. Requires constant magnetic fields to keep away from matter, else it annihilates prematurely or in the wrong area. Even one gram (think paper clip) would destroy the ship and an area the size of a small city. This requires that more mass be devoted to the ship in the form of redundancy for the anti-matter containment. This would likely get more attention than life-support. Also, due to the immense energy release, there becomes a problem with containing the reaction and directing it. Getting the required fifty-fifty ratio of matter to anti-matter is also of exceeding importance. Any extra matter becomes rapidly moving, incredibly dangerous missiles. Any extra anti-matter will be propelled, likely contacting the ship.
So now we can see that anti-matter is about the only choice. Next, time dilation Time dilation is proven to exist. The closer to c you come, the faster time moves for those not going the speed you are going. In other words, you slow down in relation to everyone else. This is because light cannot move faster than c while in a perfect vacuum. If your ship has headlights and you travel at one fourth c, the light coming from the headlights will appear to be traveling at c in relation to you. A motionless viewer would likewise see the light moving at c in relation to them. If the spectator watched you move about the ship, he would see you moving around inside your ship at three fourths your normal speed. At twice c, you’ll be moving at half speed, and everyone else will appear to be going twice as fast from your perspective. So, when you achieve c… No once really knows what time will flow like then. Theoretically, you could have time stop for you… And your ship would hurtle on until something slowed it down. Unless the alien civilization planned for this occurrence, you’d miss Earth. Still, you’re entire research operation would be fucked from this event either way. As would be Earth, if the aliens had planned on this. Still, this is one theory. This is kind of moot, however, as even anti-matter probably is unable to achieve c. Very close, but unlikely to get there, probably not even half of c.
Acceleration: And now the biggest bitch of traveling at or near c. Getting there without destroying the craft and killing the occupants.
I am going to do this off of known scientific principles, and as a true stasis field has no explanation within science, we will be using a type of hibernation similar to torpor that slugs enter. In this state, which can theoretically be used on humans, a creature’s oxygen levels rapidly drop to a certain range. This range will kick in a response that puts the subject into a state similar to torpor of snails, where there is next to no signs of life. In this state, oxygen needs are dropped to near zero, as are water and nutrient needs. As a bonus, the body becomes less susceptible to damage, as demonstrated when this procedure was performed on lab animals. A syringe was inserted into the brains of a control group of animals (squirrels, if I remember correctly) that were merely sedated. Then, a group of animals under this torpor-like state (same species) had the syringes inserted into the same area of brain. They were revived and after a set time all animals were euthanized and dissected. The control group all showed dead brain cells around the injury, where as those that had been in torpor showed little to none, despite being conscious for as long as the control group. This state has been induced in many animals, all of which have shown no adverse effects. Humans have yet to be tested. We will assume these aliens can achieve the state of torpor, be it an innate ability, or the result of technology. This protects from a lot of damage.
However, there is still a limit to what their bodies can sustain before being ripped asunder. If they have had their heart torn off by acceleration while in torpor, they would either be dead before revival, or soon there-after. So, there is a limit of extended g-force of about twenty g’s. At this rate, it will take roughly 321 days to hit c. Sustained g-force like that would require constant medical attention and the ability to cut that acceleration down if even one body shows signs of dangerous damage being caused. Breaking will also be the same. Total time for acceleration and subsequent deceleration: 642 days, possibly longer due to stress on the aliens.
Possible atmospheric complications: These aliens may even be killed by what’s in our air. No, not water soluble bad guys. I’m more referring to stuff like nitrogen, CO2, etc. This is fairly unlikely to cause problems, however.
Amateur astronomers: There are a lot of these guys. There is a good chance that a dude with a good telescope would see these aliens first.
SETI: That would be the Search for Extra-Terrestrial Intelligence. They have radio telescopes all over the world, listening in for any communications being sent out. And advanced civilization capable of sending visitors here would have started broadcasting long ago. We would have heard it. Good bye inhabited Alpha Centauri…
Space is big. And very empty. The odds of intelligent life being within five light years of Earth? Atrocious. They would have to be about four and a half years away from radio communications. Based off SETI’s years of listening alone, I’d say we’re alone in this arm of the galaxy, possibly even this entire half.
Space is a lonely place.