How can we prevent alien attack?
We couldn't. If aliens were to attack us, then we would be powerless to stop OR hide. The best solution would be to use diplomacy and even then there is no guarantee.
Attack – if aliens were to make a full on total war attack against us, we would be screwed. There is no way that we could prevent it because their technology would be on a level far and away better than ours. Heck, they probably would not even to be seen by the human eye. We would not be able to stop the attack in any way shape or form. The bets that we could do would be to lessen the effects of the attack. We would be next door to powerless to prevent the missiles incoming OR use deception to reduce the attack. Which leads me too...
Hide – another way to prevent the attack would be through hiding. Since 1880, we have been pumping out radio transmissions like crazy. These transmissions have been going through the universe since. AS a result we have been basically waving a flag and shouting "WE ARE HERE!" to the alien community since. AS a result we would not be able to hide out place in the universe. We would be easily identifiable.
Even if we remove this, they know where to attack by studying the galaxy and looking for habitable life as we have been. They would have studying the universe and making the same enquiries as we have (As they are space bound, it is natural to assume that they would be as curious as we are of life on other planets and making the same conclusions as we have). They would therefore be on the active hunt for us.
The only way that we could hide our presence in the universe would be to erect a Dyson sphere that would reduce our ability to be seen. That is a whole heap more complicated than can be written here.
Diplomacy – Negotiation would be hard. What language do they speak? What do they want? What would we be willing to give them? If they wanted total all out warfare then no amount of talk is going to help. If they wanted to enslave us then, again, diplomacy would be next door to irrelevant. If they wanted natural resources (Iron, silicon, etc.) then there would be batter ways to get it. The only way that we would be able to negotiate with them to avoid confrontation would be if we had something that they wanted, and there is very little chance of that.
In 2010, Stephen Hawking suggested that aliens might be "rapacious marauders, roaming the cosmos in search of resources to plunder and planets to conquer and colonize"; analogous to the fate of Native peoples encountering Christopher Columbus. All this notwithstanding, an alien attack is not something I worry about. (Stephen Hawking Is Still Afraid of Aliens)
Space travel is an expensive venture. Whatever aliens find lacking on their home planet-i.e., natural resources, including reproductive ones-are likely available on planets much closer to them.
Even NASA is not concerned with an alien attack. Rather, the main concern of their Office of Planetary Protection is that terrestrial microorganisms may "hitch a ride" on spacecraft and thus contaminate planets with habitable environments. (NASA Office of Planetary Protection)
Lastly, any civilization capable of interplanetary travel is obviously far more technologically advanced than us. Were there a chance planet Earth was about to be attacked, there would be no defense against a technology we as yet cannot perceive.
I witnessed a strange craft in the night sky, it was massive and silent and I would say if this thing meant the human race any harm, we would have been stuffed like a turkey along time ago. Why should anyone believe we would be in danger of attack by anyone who is so advanced in their technology as that. If we became a threat they could squash us like flies, which if you were of an advanced race, wouldn't you hit the threat and leave those who were peaceful alone, and so the only ones who would need to worry would be the armed forces who thought they could kick alien butt, the rest of us who would like to get to know our cosmic neighbours, would in my humble opinion, would be OK.
In our region of space, which is highly inhabited, worlds are not permitted to outright militarily conquer other worlds. It creates too much instability, which is bad for trade. Worlds conquer other worlds via more subtle, ingenious means. Kind of like how the CIA would try to take over and control a country.
According to the Allies of Humanity Briefings, which I firmly belief has the best analysis of first contact situations with alien life, there are three requirements for a world to maintain its freedom and sovereignty in the universe, which is abundant with highly competitive intelligent life:
- They must be extremely discrete. You don't want to advertise who you are, what you have, or where you live. Alien societies that survive, in the long run, tend to hide as much as possible and, when this is not possible, maintain as much discretion as they can.
- They must be self-sufficient. Alien worlds that trade for the goods, resources and the technology they need inevitably become controlled and highly influenced by others as they become dependent. Entire worlds are thusly overtaken without a single shot being fired.
- They must be united. Species that fight among themselves are easy pickings to others. Divide and conquer, right? Imagine if an alien species came to Earth and decided to play our most powerful nations against each other. Wouldn't be good for us. Imagine if they then decided that one of these countries should come out on top, under their control, and gave them they necessary technology to achieve this. What a disaster.
How many of these requirements are we currently fulfilling?
We're the very opposite of discrete as we broadcast everything into space, even information about how to find our solar system.
We're self-sufficient for now (all our goods and resources come from Earth), but there's a real risk of that changing. Look at how greedy we are for technology. How quickly would governments or corporations sign away our freedom in exchange for advanced technology? What if we become dependent on technology we can't replicate? We'd be completely under their control.
We're hardly united. We keep fighting amongst ourselves whereas we should be working together to defend our borders to space. I'm not talking about an oppressive world government. Just more cooperation amongst humans. We're a single species and civilization, after all - at least vis-a-vis other alien powers.
I'd like to add an important point: we need, on a worldwide level, to become very educated about the realities of first contact, which is happening now. Humans everywhere need to understand the challenges and opportunities inherent in this very difficult circumstance.
Otherwise, none of the above three requirements will happen. Governments and corporations and powerful individuals will foolishly and selfishly ruin our chances of emerging as a free and independent race in the cosmos. It's up to us to ensure this does not happen.
Check out the Allies of Humanity Briefings, the Declaration of Human Sovereignty, and the book Life in the Universe to find out more about how we can behave intelligently as a species to ensure our world does not get conquered.
Right now, nothing - since we haven't detected anything at near light speed decelerating out near the Oort cloud.
If some aliens decide to waste their entire species for many lifetimes developing the means to travel here just to attack us ... they'd probably have died on their own planet long before reaching the level of being able to, so again - we do nothing. (It's not as if there are real means of traveling 100 lightyears in a few months - they'd be as limited by the speed of light as anything else in the universe that's made of matter.)