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On the other hand, they might be sensible enough to have banned nukes, so our weapons would be more powerful than theirs.
Also, it seems unlikely to me that life that evolved totally separately from us would be biologically similar enough for our diseases to transfer between us
Probably fungus could end up feasting on them like they are moving piles of organic material.
Not if they're something alien like silicon based life, or maybe their carbon chains are right handed (I think ours are left handed, if not, reverse that)
We almost certainly can't catch a disease from them in the same way we can't catch most animal and plant diseases. Each virus and bacterium relies on certain aspects of the host to survive and reproduce.
Fungal infections are more possible, because fungi just feed on decomposing organic matter. As long as they're carbon-based, they're probably edible. Same for parasites: as long as they're made of meat, they're edible. (Barring any strange chemical composition, like if for some reason they carry large amounts of what we would consider toxic metals.)
OP: you and many commenters are making some significant assumptions. There's a great short story available online: "The Road Not Taken" by American writer Harry Turtledove that considers how aliens might arrive without more advanced weapons.
I was thinking about that story a lot when all the hype about the room temperature (not) superconductor was going on: Had it been real, it was simple enough that we could have discovered it a couple centuries sooner and who knows what our technology would be like.
And it's a stretch, but it's good to point out that there's a lot of edge cases to consider. Even human big history is mostly open questions.
Yes to the first assumption - with a caveat. To cross the width of space, their technology must be far ahead of ours. Imagine a modern army fighting one from the 18th century, and you could imagine what a tank, a machinegun, modern artillery or whatever other aspects would do.
BUT: There is also the point that we would fight locally, while they would need quite long-winded logistics. OK, they could mine asteroids, and if they have all the means of production, we would be f-ed.
Regarding the disease, well, any disease needs a host, and it is formed by evolution over millions of years to work on that host platform. Just imagine the aliens have a different interpretation of DNA, maybe for pairs forming a codon instead of three, or just different encodings of amino acids for their codons, and nothing would work. And that would assume that they are similar enough to actually have DNA.
What they could do, though, if they are technologically far ahead, they could probably synthesize a virus that is basically made to kill us off the planet before we know it.
So better not piss off any aliens that made their way to us... Gray lives matter!
There’s an interesting caveat, looking at recent war history …..
War technology has always been a “battle” between offensive technology and defensive. However in recent years, offensive dominates. We may have reached the limits of physical materials such that there is no defense. Certainly a lot of todays fighting requires stopping the attack or making it miss, rather than trying to survive it.
There’s no reason to expect that shields can exist, so it’s quite possible we can hurt the aliens and their advantages are standoff and intelligent weapons, reconnaissance, targeting. Looking at recent wars, would we be like Iraq, appearing similar in destructive ability but overwhelmed, or more like Ukraine, appearing to have no chance but successfully fighting back? Ukraine’s advantage is partly economic: using cheap drones to destroy expensive or nonreplaceable tech. Imagine the Aliens as Russia, with all the apparent advantages in size, tech, experience ….. but they only have the weapons they came with and we can at least sting them, we can make them spend irreplaceable weapons on our much cheaper technology
Or think of Stargate SG-1. Humans technology was primitive, but the weapons could still kill. A combination of unwieldy political structure of the aliens and clever application of force managed to minimize the impact of the technology advantages. Then of course making the right friends enabled those humans to leapfrog ahead
SG-1 was an amazing series. I bet I was too young to really appreciate it the first time around.
People don't realise it, but current physics explains everything we've ever measured. We know there must be more from theoretical arguments, but it probably comes up very rarely, so in a lot of ways the alien engineers will be doing the exact same stuff as us. Most likely, bunkers and kinetic impactors will be just as effective as against other humans.
Strong AI and something resembling nanotech are the main things that should be possible, but that we can't do yet. Everything a living organism can do should be replicable. There's also a lot of things we just haven't spent the money on, which covers most space stuff.
To cross the width of space, their technology must be far ahead of ours. Imagine a modern army fighting one from the 18th century, and you could imagine what a tank, a machinegun, modern artillery or whatever other aspects would do. BUT: There is also the point that we would fight locally, while they would need quite long-winded logistics.
In regards to the long winded logostics (I read this from a second hand account and I'm sure this idea is from some scifi book) -a really fun idea is that if the aliens would send a war fleet our way and it takes say 50 years to get here... it is reasonable to assume that in those 50 years the aliens will do some technological breakthrough in both drive and weapon departments that by the time the fleet would get here a more advanced fleet would already be fighting the war - possibly even already won.
I think the idea can be found in Ender's Game. As the story progresses further into enemy territory, he 'controls' ships which are increasingly outdated because those are from older fleets sent decades ago.
Any vehicle that can cross interstellar space in a reasonable time also has enough energy to destroy a planet, simply by crashing into it.
So any alien race that can travel here could also drone strike earth and kill us all before we even saw it coming.
In fact, this makes it a prudent move to pre-emptively kill any civilization you notice, cause you can't ever know if they would do it to you, before it's too late. It's the principle of Mutually Assured Destruction, without the Mutual part.
As for diseases, we'd have to meet them in completely closed-off chambers.
And even then, it's possible that they emit strong x-rays to scan their surroundings, giving us radiation poisoning.
Or they use 200dB sonar, deafening us. Or the other way around, their bodies rapidly oxidize in an oxygen-rich environment.
So any alien race that can travel here could also drone strike earth and kill us all before we even saw it coming.
In fact, this makes it a prudent move to pre-emptively kill any civilization you notice, cause you can't ever know if they would do it to you, before it's too late.
This is basically dark forest theory.
It's also really hard to do stealth in space, and so interception would be easy enough to pull off.
Any vehicle that can cross interstellar space in a reasonable time also has enough energy to destroy a planet, simply by crashing into it.
Depends on the size of the craft. If it is only a few meters across, I think it would just burn up in the atmosphere before it even reached the ground.
If it burns up in the atmosphere with relativistic speed, it burns up the atmosphere.
True, but damage still depends on craft size. At the really small end you're basically just talking about a normal cosmic ray.
Maybe, maybe not. It could be that fast-moving projectiles and high explosives are still the cheapest way for them to cause damage, in which case they'd be fighting with somewhat better versions of the same kind of weapon. If they're not using the laws of physics we're familiar with and have an economy we're not familiar with it's anyone's guess.
If they don't have extra laws of physics (and there's great reason to think we know most of them) they're going to arrive in a small-ish craft after traveling many years or even centuries, and we would have a pretty significant resource advantage. The main way I can think of they could win would be by unleashing some kind of small, self-replicating weapon we can't effectively target. Anything else seems like it would be destroyed long before we're worn down.
Also how do we not catch a disease from them or vice versa?
Probably easily. Diseases generally have to be adapted to their host, and alien biology is bound to be very different. Even on the off-chance there's an exception, the aliens probably have some pretty advanced medicine as well. Invasive species ruining agriculture and our environment could be an issue, though!