<p>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.</p>

<p>First, what would it take for life to travel to Earth? Well, the <a href="https://en.wikipedia.org/wiki/List_of_potentially_habitable_exoplanets" rel="nofollow noreferrer">closest <em>potentially</em> habitable planet</a> 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.</p>

<p>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.</p>

<p>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.</p>

<p>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.</p>

<p>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.</p>

<p>Finally, the lack of definitive evidence makes this possibility negligible.</p>

CC BY-SA 3.0
2018-02-24T14:24:24.773Z
*[Luyten b](https://en.wikipedia.org/wiki/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.
StephenG - Help Ukraine
CC BY-SA 3.0
2018-02-24T16:34:22.193Z
Interesting! thank you for your answer it was very informative
Matthew
CC BY-SA 3.0
2018-02-24T19:59:18.727Z
@StephenG Right, I should've been clearer. Fixed.
Sir Cumference
CC BY-SA 3.0
2018-02-25T22:26:31.483Z
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.
Allure
CC BY-SA 3.0
2018-02-25T22:40:36.033Z
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.
ProfRob
CC BY-SA 3.0
2018-02-25T22:40:38.483Z
@Allure Perhaps. Though my answer didn't only talk about the distances. It mentioned how slim the chance of life is on nearby exoplanets, and that's just life in general, not particularly sentient beings. To that end, life intelligent enough to travel dozens of light years in a reasonable time is significantly less probable. And for such a civilization to be certain that Earth harbors life and be determined to visit us, then disappear without proof is, as you can imagine, even less likely. I don't think I have to stress any further how unlikely an alien visit would be.
Sir Cumference
CC BY-SA 3.0
2018-02-25T22:42:55.880Z
We have NO IDEA how probable/common intelligent life is. Even if it requires particular conditions to emerge, that does not stop it colonising the Galaxy.
ProfRob
CC BY-SA 3.0
2018-02-25T22:43:43.323Z
And BTW, we don't know whether there are habitable planets around Alpha Centauri. So the statements about the distance to the nearest habitable planets is incorrect.  Nearest *known* habitable planets perhaps.
ProfRob
CC BY-SA 3.0
2018-02-25T22:49:36.067Z
@RobJeffries By what standard are you considering intelligent? Here on Earth, with billions of species of different lifeforms, only one is capable of space travel, and not even to a great distance as of yet. Not to mention, we still haven't found many planets relatively closeby that could harbor life of any form. Do you think that the existence of life forms that intelligent among nearby planets is likely?
Sir Cumference
CC BY-SA 3.0
2018-02-25T22:51:32.833Z
@RobJeffries "Even if it requires particular conditions to emerge, that does not stop it colonising the Galaxy." Even if aliens came from somewhere across the galaxy, are we actually supposed to assume traveled the distance to Earth and just left randomly, when humanity has no strong evidence of their arrival?
Sir Cumference
CC BY-SA 3.0
2018-02-26T01:41:30.900Z
True that about time dilation. My brain wasn't working at the time, sorry! About colonizing the galaxy, I think the point is that we don't know. There're just too many unknowns. For all we know some alien spaceship could've stopped by, unfolded a big solar sail, replenished their batteries, and moved on. If that happened in 1000 AD, we'd probably not even have noticed. We simply don't know what the aliens' motivations might be. We can say a lot of competing hypotheses are plausible, but can't 
say anything conclusive.
Allure
CC BY-SA 3.0
2018-02-24T14:21:05.583Z
2018-02-24T20:00:54.627Z
2018-02-24T20:00:54.627Z
Sir Cumference
<p>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).</p>

<p>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.</p>

CC BY-SA 3.0
2018-02-25T22:24:14.433Z
2018-02-26T09:51:35.090Z
2018-02-26T09:51:35.090Z
Allure
<p>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.<br>
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 .</p>

CC BY-SA 3.0
2018-02-24T14:13:24.397Z
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.
userLTK
CC BY-SA 3.0
2018-02-26T10:16:09.580Z
I'm voting to close this question as off-topic because it is nothing about astronomy.
Rory Alsop
CC BY-SA 3.0
2018-02-26T10:48:19.670Z
You should rather ask him to prove that.  Then it can be easier to dismantle that theory.
Alchimista
CC BY-SA 3.0
2018-02-26T15:37:02.117Z
Just tell him you **are** an alien, and now he must die.
Carl Witthoft
CC BY-SA 3.0
2018-02-26T20:32:35.477Z
Well, Enrico Fermi (one of the greatest scientific thinkers of all time) was astounded that aliens apparently have *not* visited the Earth.
Fattie
CC BY-SA 3.0
2018-02-26T20:33:54.157Z
A simple answer to your question is to play friend the episode of **Cosmos** which deals with exactly this.  Carl Sagan is widely regarding as probably the greatest and clearest scientific "explainer" of all time.  His presentation on why (unfortunately) it's very unlikely aliens have never visited, would be as good a place as any to begin.
Fattie
<extra-terrestrial>
How to prove to my friend that aliens have never visited earth
Matthew
# How to prove to my friend that aliens have never visited earth

(Asked by: username_0 on 2018-02-24)

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 .

**Tags:** extra-terrestrial
> **Comments:**
> **username_1**: 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.  
> **username_2**: I'm voting to close this question as off-topic because it is nothing about astronomy.  
> **username_3**: You should rather ask him to prove that.  Then it can be easier to dismantle that theory.  
> **username_4**: Just tell him you **are** an alien, and now he must die.  
> **username_5**: Well, Enrico Fermi (one of the greatest scientific thinkers of all time) was astounded that aliens apparently have *not* visited the Earth.  
> **username_5**: A simple answer to your question is to play friend the episode of **Cosmos** which deals with exactly this.  Carl Sagan is widely regarding as probably the greatest and clearest scientific "explainer" of all time.  His presentation on why (unfortunately) it's very unlikely aliens have never visited, would be as good a place as any to begin.  

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## 2 answers

### Answer by username_6 on 2018-02-24. Score: 2 | Accepted answer

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](https://en.wikipedia.org/wiki/List_of_potentially_habitable_exoplanets) 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.

> **Comments:**
> **username_7**: *[Luyten b](https://en.wikipedia.org/wiki/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.  
> **username_0**: Interesting! thank you for your answer it was very informative  
> **username_6**: @StephenG Right, I should've been clearer. Fixed.  
> **username_8**: 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.  
> **username_9**: 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.  
> **username_6**: @Allure Perhaps. Though my answer didn't only talk about the distances. It mentioned how slim the chance of life is on nearby exoplanets, and that's just life in general, not particularly sentient beings. To that end, life intelligent enough to travel dozens of light years in a reasonable time is significantly less probable. And for such a civilization to be certain that Earth harbors life and be determined to visit us, then disappear without proof is, as you can imagine, even less likely. I don't think I have to stress any further how unlikely an alien visit would be.  
> **username_9**: We have NO IDEA how probable/common intelligent life is. Even if it requires particular conditions to emerge, that does not stop it colonising the Galaxy.  
> **username_9**: And BTW, we don't know whether there are habitable planets around Alpha Centauri. So the statements about the distance to the nearest habitable planets is incorrect.  Nearest *known* habitable planets perhaps.  
> **username_6**: @RobJeffries By what standard are you considering intelligent? Here on Earth, with billions of species of different lifeforms, only one is capable of space travel, and not even to a great distance as of yet. Not to mention, we still haven't found many planets relatively closeby that could harbor life of any form. Do you think that the existence of life forms that intelligent among nearby planets is likely?  
> **username_6**: @RobJeffries "Even if it requires particular conditions to emerge, that does not stop it colonising the Galaxy." Even if aliens came from somewhere across the galaxy, are we actually supposed to assume traveled the distance to Earth and just left randomly, when humanity has no strong evidence of their arrival?  
> **username_8**: True that about time dilation. My brain wasn't working at the time, sorry! About colonizing the galaxy, I think the point is that we don't know. There're just too many unknowns. For all we know some alien spaceship could've stopped by, unfolded a big solar sail, replenished their batteries, and moved on. If that happened in 1000 AD, we'd probably not even have noticed. We simply don't know what the aliens' motivations might be. We can say a lot of competing hypotheses are plausible, but can't  say anything conclusive.  

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### Answer by username_8 on 2018-02-25. Score: 4

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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