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I come from a physics background where I don't believe something unless the evidence is shown to me, however I have a friend who believes Aliens have visited earth , which he believes based on conspiracy theories.
My question is: is there any solid scientific reasoning as to why the conspiracy theories aren't true. As for him I need to give him evidence as to why they don't exist , and not the other way around .

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    $\begingroup$ It's impossible to prove a negative in this case. That is, proving aliens never visited is impossible. What you can do is show that there's no solid evidence that they did, that is, he might say "the pyramids are proof" and you can say "no, they could have been built this way", but neither argument = proof. $\endgroup$ – userLTK Feb 24 '18 at 14:13
  • $\begingroup$ I agree with @userLTK: lack of evidence is not evidence of lack. You can try to debunk specific alien conspiracy theories, but that doesn't prove aliens have never visited Earth. And, for all we know, maybe they have (our planet's been around for a long time). $\endgroup$ – user21 Feb 24 '18 at 16:21
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    $\begingroup$ "Extraordinary claims require extraordinary evidence". It is not up to you to prove that aliens have not visited earth, it is up to your friend to prove that they have. Don't jump into this pitfall. $\endgroup$ – user1569 Feb 24 '18 at 16:24
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    $\begingroup$ I agree with previous comments, see Russel's teapot theory, and replace "religion" with "aliens visited earth" $\endgroup$ – qq jkztd Feb 26 '18 at 9:53
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    $\begingroup$ I'm voting to close this question as off-topic because it is nothing about astronomy. $\endgroup$ – Rory Alsop Feb 26 '18 at 10:16
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If your friend believes in these conspiracy theories he's not likely to listen to scientific reasoning, but I'll give it a shot. Obviously there's no direct evidence, but there are convincing reasons to rule out this possibility.

First, what would it take for life to travel to Earth? Well, the closest potentially habitable planet is 4.2 light years away from us. That means it would take light, the fastest thing in the entire universe, over 4 years to travel that distance. That's an incomprehensibly vast distance.

Note, though, that the chance of such intelligent life on Proxima b is quite slim, since it faces stellar wind pressures over 2,000 times what we experience on Earth. Thus, it likely has had its atmosphere blown away.

Okay, so how about the next potentially habitable planets? The next closest one, Luyten b, is a more likely candidate for life; it seems to have a similar size and potentially similar temperature to Earth. However, it's three times farther than Proxima b. So for starter's, we'd be making two assumptions: that life exists on Luyten b, and that it is capable of traveling across an unfathomable distance like that.

If Luyten b doesn't satisfy both of those qualities, then that's not good. The subsequent exoplanets on that list have major problems that make habitability extremely unlikely, and the only good candidate shows up at around 39 light years from us — another 3 times farther.

Even if we assume that any of these planets can support life capable of traveling to Earth, we raise the second question: why would they bother? There would be no certainty that they'd find anything on Earth. There'd be no certainty that they'd survive on Earth. Unless they have some method of magically confirming life, that entire trip seems pointless.

Finally, the lack of definitive evidence makes this possibility negligible.

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    $\begingroup$ Luyten b, is actually quite similar to Earth Note this means "roughly similar size, maybe temperature", not anything else. We know basically very, very little about this planet. $\endgroup$ – StephenG Feb 24 '18 at 14:24
  • $\begingroup$ Interesting! thank you for your answer it was very informative $\endgroup$ – Matthew Feb 24 '18 at 16:34
  • $\begingroup$ @StephenG Right, I should've been clearer. Fixed. $\endgroup$ – Sir Cumference Feb 24 '18 at 19:59
  • $\begingroup$ I don't agree with this answer I'm afraid. 4 light years is "only" four years away with sufficiently advanced technology. There're already plans to send probes to Alpha Centauri, so it's definitely conceivable. Over intergalactic distances there might be doubt, but interstellar shouldn't be hard for an advanced civilization like the aliens presumably are. $\endgroup$ – Allure Feb 25 '18 at 22:26
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    $\begingroup$ In addition to what @Allure says, if you can travel at a good fraction of the speed of light then it doesn't take 4 years according to the traveller - only according to us. So the second paragraph needs a complete rework. The plausibility of covering these distances is in fact the whole basis of the Fermi paradox. $\endgroup$ – Rob Jeffries Feb 25 '18 at 22:40
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I don't think you can prove that aliens have never visited Earth. It's theoretically provable if one had a time machine, but without one, we can't really prove that aliens didn't visit Earth (say) 2 billion years ago. The physical evidence would be long gone (especially with plate tectonics).

What you can say is that the burden of proof is on the person making the assertion. The null hypothesis is that aliens have never visited Earth. If your friend asserts that aliens have visited then the onus is on him to provide proof. If you can refute all the proofs he brings up, you can argue the null hypothesis remains the one to believe.

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