The Instigator
philochristos
Pro (for)
Winning
8 Points
The Contender
rross
Con (against)
Losing
4 Points

It is unlikely that extraterrestrials have ever visited earth

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Post Voting Period
The voting period for this debate has ended.
after 3 votes the winner is...
philochristos
Voting Style: Open Point System: 7 Point
Started: 12/25/2012 Category: Miscellaneous
Updated: 4 years ago Status: Post Voting Period
Viewed: 1,439 times Debate No: 28622
Debate Rounds (3)
Comments (2)
Votes (3)

 

philochristos

Pro

I am going to defend the claim that it is unlikely that any extraterrestrials have ever visited earth. I will have the burden of proof in this debate. My opponent only needs to debunk my arguments.

By "extraterrestrials," I mean living intelligent beings who originate somewhere in the universe other than earth.

By "living," i mean living in the biological sense, so I'm not talking about angels, demons, ghosts, spirits, or gods.

By "intelligent," I mean conscious, sentient, etc. I do not mean "smart."

I set the character limit to 6000 because people seem to like shorter debates better which makes them more likely to vote.

I set the voting period to 1 week in hopes of getting a lot of votes.

First round is for acceptance, well wishes, and things like that. I will post my argument in round 2.

No new evidence in the last round because that wouldn't be fair to me since I wouldn't be able to respond.

Please do not accept the debate unless you feel fairly confident that you will be able to finish it.

May the odds be ever in your favor.
rross

Con

Hello philochristos.

Thank you for putting up this debate. I can't wait to see what you have to say.
Debate Round No. 1
philochristos

Pro

Thank you, rross, for accepting this debate and giving me a hearing. And thank you audience for coming to tonight's debates. I know that many of you would like to believe aliens have visited earth because the very thought of it is exciting, and it stirs the imagination. It makes movies more interesting if you can entertain the idea. I hope this debate will not leave you too disappointed.

This is my argument in a nutshell:

1. If any extraterrestrials have ever visited earth, then they probably live within 50 light years of earth.
2. It is unlikely that any extraterrestrials capable of traveling to earth live within 50 light of earth.
3. Therefore, it is unlikely that any extraterrestrials have ever visited earth.

This argument is logically valid, so the only question is whether the premises are true. Let's discuss them one at a time.

1. If any extraterrestrials have ever visited earth, then they probably live within 50 light years of earth.

Not even considering the size of the whole universe, our galaxy is very large. It is about 100,000 light years in diameter, and there are anywhere from 200 to 400 billion stars in our galaxy.[1] Roughly half of the stars are in binary systems where two stars rotate around each other in close proximity.[2] The closest star to earth is about 4.2 light years away.[3] Since these stars and systems are spaced so far apart, it would be quite the undertaking for anybody to travel even to one nearby.

The only reason anybody would come to earth is either if they already thought there was something very unique about earth or if they just happened upon earth while exploring the galaxy. Given that there are hundreds of billions of stars in the galaxy, the odds are against anybody just luckily running into the earth. It is far more likely that if anybody comes to the earth, it is because they already know the earth is has intelligent life on it. That's really the only thing that would single us out and make such a major undertaking worth it.

Earth produced the first radio communication in the 1890's.[4] That means the farthest any intelligent signal could've traveled from earth so far is only 110 light years away. Supposing some civilization 110 light years away is just now recieving those signals and discovering that there's life in this part of the galaxy, it would take them another 110 years to get here if they travel at the speed of light. Assuming our earliest radio waves could be detected in other places in the galaxy, and assuming light speed travel is possible, and assuming the aliens left immediately upon receiving the signals, the farthest anybody could be from us in order to get here today is 55 light years away. That would allow 55 years for our earliest signals to get to them and another 55 years for them to arrive here.

According to Einstein's special theory of relativity, no matter can travel at the speed of light.[5] It is doubtful anybody could even come close. It is also doubtful that the earliest signals produced on earth would be strong enough to be detected 50 light years away. That means if any aliens have ever visited earth, they would have to live well within 50 light years of us.

2. It is unlikely that any extraterrestrials capable of traveling to earth live within 50 light of earth.

Any civilization advanced enough to undertake a trip to earth would have to be vastly more technologically advanced than we are since we are incapable of making such a trip. That means they probably invented radio technology long before we did. But if there were any such civilizations within 50 light years of us, we would already know about it. SETI has been searching the skies for the last 40 years or so for signs of extraterrestrial life.[6] So far, they have turned up nothing. But if there were civilizations within 50 light years who had radiotechnology long before us, then SETI would've found them. Since SETI hasn't found them, they likely aren't out there.

Conclusion

In summary, any aliens that have visited earth would have to live within 50 light years of us because (1) the trip would be a major undertaking and would require quite a bit of motivation to undertake, (2) the discovery of intelligent life is about the only thing interesting enough to warrant a trip to earth as opposed to any of the other billions of solar systems in the galaxy, and (3) even traveling at the speed of light, which is not feasible, ET's that discovered our existence and left immediately could only have been traveling for half the time our earliest radio signals left earth.

But any civilization capable of such travel would be far more adanvced than us, and it is unlikely there are any civilizations more advanced than us within 50 light years because if there were, we'd know it. It follows that it's unlikely any extraterrestrials have ever visited earth.



[1] http://en.wikipedia.org...

[2] http://www.astro.cornell.edu...

[3] http://www.astro.wisc.edu...

[4] http://www.clemson.edu...

[5] http://curious.astro.cornell.edu...

[6] http://www.seti.org...





rross

Con

I thank my opponent for his coherent and beautifully-phrased argument. What a pleasure it was to read.

To meet the burden of proof, my opponent needs to show that:
1. It is possible to assess the likelihood of extraterrestrials visiting the Earth.
2. Such a visit is less rather than more likely.

My opponent has indeed made statements about the likelihood of a visit. However, these statements are based on speculation rather than evidence. He cannot assess the likelihood of a extraterrestrial visit to Earth because of our complete absence of knowledge about extraterrestrials.

In the absence of evidence, my opponent has made assumptions about extraterrestrial form, behavior capabilities and even motivation. These assumptions are heavily influenced by his knowledge of terrestrial life. However, the only thing we know about extraterrestrials is that they are not from Earth, by definition. We know nothing further.

Specifically:

assumption 1: we can guess at extraterrestrials' decision-making processes

Pro speculated: "The only reason anybody would come to earth is either if they already thought there was something very unique about earth of if they just happened upon earth while exploring the galaxy."
Even supposing that extraterrestrials have minds like ours (and why would they?), it really is outrageous to assume knowledge of their motivation

assumption 2: extraterrestrials would have visited Earth only while humans were alive
The Earth is 4.5 billion years old [1]. Humans have been on it for, at most, 7 million years [2], only 0.16% of the time. Therefore, if an extraterrestrial visit occurred, it is much more likely to have been before humans evolved.

assumption3: extraterrestrials are made of matter
Life forms on Earth are composed of matter. They cannot travel at the speed of light. It would be rash to make assumptions about extraterrestrials based only on our observations of terrestrial life forms, however.
Of course, I can't imagine a sentient life form that is not composed of matter. Happily, though, the possibilities of the universe are not constrained by my lack of imagination.

assumptions 4 and 5: extraterrestrials live on planets and are roughly the same size scale as us
My opponent has assumed that extraterrestrials, like us, would have evolved bound to a planet and that they would be roughly the same size as terrestrial life forms. This would mean that, to get to Earth, they would need to "undertake a trip" and that the trip would involve complex technology. Given that we know absolutely nothing about extraterrestrials, these are extraordinary assumptions. I don't see why extraterrestrials need to be bound to planets like us, or why they might not be naturally nomadic, or even several light years wide.

assumption 6: we would be able to perceive extraterrestrials, perhaps by SETI
My opponent claims that any intelligent extraterrestrials nearby "probably invented radio technology long before we did." Why? Radio technology has been useful to humans because it supplements our natural limitations. I can't see how you can conclude that it would be useful or even interesting to extraterrestrials.


conclusion
I don't know if there are sentient extraterrestrials or not. I haven't the faintest idea, and nor does anyone else (ruling out conspiracy theories). Furthermore, should they exist, I can't speculate as to their form and qualities, let alone motivation.

In such a knowledge vacuum, no conclusions can be drawn. We can't perceive any evidence of an extraterrestrial visit to Earth. That's as far as we can go. We don't know what evidence of such a visit would look like, or if there would even be any evidence.

Therefore, any assessment of the likelihood of a visit is not evidence-based. Any opinions on the matter are interesting only in terms of what they say about the person making them. Like a Rorschach test.
Debate Round No. 2
philochristos

Pro

This argument occured to me a few years ago, and although I've shared it with various people, this is the first time I've really subjected it to scrutiny from a person attempting to debunk it. I appreciate that my opponent has not dropped the ball.

In the last round, I gave a syllogism to summarize my argument. Then I defended each premise individually. My opponent did not question the validity of my argument. Her critique concentrates solely on the truth of the premises. Nothing she said gave us any reason to think my premises were false. Rather, if her critiques are valid, that would just mean I have failed to show that my premises are true. So her critiques are undercutting defeaters rather than rebutting defeaters.[1]

Since she did not question the validy of my argument, we have only to look at how she attempted to undercut the premises.

1. If any extraterrestrials have ever visited earth, then they probably live within 50 light years of earth.

The crux of Con's critique is that my argument depended on a lot of unwarranted assumptions. Let's look at them one at a time.

assumption 1: we can guess at extraterrestrials' decision-making processes

Actually, we don't need to look at their decision-making process at all. We know, by the law of excluded middle, that if they came here, they either did it for a reason or they did not do it for a reason. If there was no reason, then the odds are against them coming here because there are hundreds of billions of solar systems in the galaxy. If they did it for a reason, then there had to be something that distinguished earth from everywhere else. The only thing that distinguishes earth is that there is intelligent life on earth.

assumption 2: extraterrestrials would have visited Earth only while humans were alive

Actually, I did not assume this; rather, I gave an argument for it. I argued that the only reason aliens would single earth out is because there is intelligent life on earth and the aliens knew about it.

assumption3: extraterrestrials are made of matter

This was not an assumption. This was a stipulation of the debate. If you'll recall in the first round, I said that by "extraterrestrials," i meant "living intelligent beings who originate somewhere in the universe other than earth," and that by "living," I meant "living in the biological sense, so I'm not talking about angels, demons, ghosts, spirits, or gods." If an alien is living in the biological sense, then they are made of matter because life, in the biological sense, is composed of complex chemistry.

assumptions 4 and 5: extraterrestrials live on planets and are roughly the same size scale as us

Although it's true that I was imaginging aliens who lived on planets and were roughly the same scale as us, it's really not relevant to my argument. Pro suggests that perhaps they could've been nomads. But again, they either wandered here aimlessly, which the odds are against, or they came here on purpose because they knew there was intelligent life here.

Con also suggests that perhaps they could even be several light years wide. Recall that my resolution was that it is unlikely that extraterrestrials have ever visited earth. While one might entertain the mere possibility of a biological entity being several light years wide, just because something is possible doesn't mean it's reasonable to believe. But in this case, Con's suggestion is not even possible. No biological entity could be several light years wide because the gravity would crush it, making chemistry impossible. Think about it. An organism that large would be bigger than the biggest star ever discovered. And how could the earth survive a visit from such a thing?

Besides that, the size of an alien is irrelevant to my argument.

2. It is unlikely that any extraterrestrials capable of traveling to earth live within 50 light of earth.

Only one of the assumptions was relevant to my second premise:

assumption 6: we would be able to perceive extraterrestrials, perhaps by SETI


This was not an assumption. I actually gave an argument for it. The argument is that any civilization capable of traveling to other solar systems would be more advanced than ours, and since we have already discovered radio technology, they have probably discovered it long before us. And if they have discovered it, and if they live within 50 light years of earth, then SETI would've discovered them already.

Con's only objection is that we have no reason to suspect aliens would have any use for radio technology. At best, all Con can say is that it's merely possible that visiting aliens don't use radio technology, but as I said before, just because something is possible doesn't mean it's reasonable to believe. Remember that I am only arguing that it's unlikely that extraterrestrials have ever visited earth, not that it's impossible. Given the fact that we have gotten a great deal of use out of radio technology, we have a strong inductive reason to believe aliens would, too. Radio technology allows us to communicate over long distances wirelessly, as well as operate machinery over long distances. That would be very useful to any advanced civilization, and their advancement would be greatly impeded if they did not have it.

Conclusion

Keep in mind the resolution: It is unlikely that extraterrestrials have ever visited earth. I am not saying it is impossible, so it is not enough to defeat my argument for my opponent to give us a far-fetched scenario that is merely possible.

I want to thank my opponent once against for having this debate with me which has allowed me to subject my argument to scrutiny. I've always wondered how people might object.

I also want to thank the readers for coming to tonight's debate. I hope you found it interesting and that it made you think. I also ask that you vote, though I will not be so crass as to ask you to vote for me. Vote for who you think won.


[1] This web page explains the distinction: http://www.iep.utm.edu...
rross

Con


I thank my opponent for his link to the article on defeaters in epistemology [1]. This article, especially the parts about justified belief, is particularly relevant to this debate. In fact, it is the discussion of propositional defeaters (the good kind) that are the most helpful for my main objection to the resolution.


Let’s imagine that Philochristos had argued that aliens had never visited Earth. This, incidentally, is my own belief. This belief is justified on the grounds that there is no evidence of such a visit. However, let’s imagine that we had knowledge of billions of nomadic extraterrestrials who had visited most of the planets in our galaxy. In the context of this new knowledge, our belief would no longer be justified, although we might still be right by chance.


Quoting from my opponent’s own source:


“The generic idea is that a person S knows p only if there is no true proposition, d, such that if S were to believe d (or d were added to S’s evidence for p), S would no longer be justified in believing p. In other words, the existence of certain unpossessed evidence prevents a person from actually knowing p if this unpossessed evidence would result in a loss of justification were the person to acquire the evidence, be aware of it, or recognize it. So according to defeasibility theories, it’s a true proposition that does the defeating, not a believed proposition.”


Of course, my opponent did not argue that extraterrestrials have never visited Earth. He argued that such a visit “is unlikely”. A much bolder claim.


In round 2, he presented a syllogism, each step guessing at the probability of extraterrestrial behavior. I’m not at all certain that a syllogism so stuffed with vague probabilities and likelihoods would be valid, but I will concede the point to my opponent because it doesn’t matter here.


Surpassing all this is the question: can my opponent assess the likelihood of any aspect of extraterrestrial behavior? No, he cannot. He is entitled to any kind of belief about extraterrestrials, of course. However, in the absence of knowledge of extraterrestrial behavior, it is impossible to make any scientific assessment of the likelihood of their having visited Earth. My opponent cannot do it, and therefore he cannot meet the burden of proof in this debate.


***


When my opponent talks about “undercutting defeaters”, I think he is referring to my observation that his extraterrestrial fictions are inspired by his experience of terrestrial life. Of course they are. But this is irrelevant to the central argument. Even if his fantasy of extraterrestrials was quite unlike those on Earth, the main argument would still hold: that any claim about likelihood is belief, not fact.


Unlike my opponent, I have never sat around with friends speculating about extraterrestrials. I’m not really into science fiction. Such a non-preference might be almost offensive on a site like this, I know, and I apologize. The reason I mention it is because when I countered my opponent’s extraterrestrial fictions with my own suggestions, the effect was regrettably raw. His space fantasies are far, far more compelling than mine. However, fiction doesn’t transform into fact just because it’s really good.


***


This is one of those situations when it's difficult to be rational. I know that if I were at a party, holding a glass of wine, listening to my opponent's theory, I would be smiling and quite genuinely agreeing with him. This would be partly due to my opponent's charm and persuasive abilities, I'm sure, but also because this is a thinking bias that comes naturally to humans. I have never seen extraterrestrials, I don't believe in extraterrestrials or spaceships or any of the rest of it, and it seems a tiny, tiny jump to claim that a visit from extraterrestrials is "unlikely". But it's not a tiny jump. It's going from belief to false fact. This debate is as fun as a party, but it's not a party. We have to stick to reason and evidence, no matter how tedious it might be.


***


Thank you, Philochristos, for creating this debate and for your impeccable conduct throughout, and thank you to anyone who has read this far.


I will not be so brutish as to try and drum up votes or in any way stress the competitive side of this otherwise pleasant occasion.


We all agree, I’m sure, that the only victory worth bothering about is the one belonging to Reason herself.


***


1. http://www.iep.utm.edu...


Debate Round No. 3
2 comments have been posted on this debate. Showing 1 through 2 records.
Posted by likespeace 4 years ago
likespeace
This debate has been resurrected! May the truth of the matter prevail. :)
Posted by rross 4 years ago
rross
Sorry! Forgot references!
1.http://www.nature.com...
2.http://www.nhm.ac.uk...
3 votes have been placed for this debate. Showing 1 through 3 records.
Vote Placed by bladerunner060 4 years ago
bladerunner060
philochristosrrossTied
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Reasons for voting decision: While I believe Pro is probably correct, I believe that the arguments used were not persuasive, and were effectively dealt with by Con's.
Vote Placed by utahjoker 4 years ago
utahjoker
philochristosrrossTied
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Total points awarded:50 
Reasons for voting decision: better reasoning and more reliable sources
Vote Placed by johnlubba 4 years ago
johnlubba
philochristosrrossTied
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Total points awarded:31 
Reasons for voting decision: Ok, Now I have read the debate I am ready to cast my vote. Firstly I would like to say, I have had the pleasure of reading material from both sides on other debates on this site and think both of the debaters are very civil and portray very good conduct. Plus Rross gave me my fisrt ever vote here, so she has a sentimental place in my mind. I won the debate. by 1 vote. I was so pleased. Ok, However I did agree with Rross to begin with on this debate, but as it progressed I found she did very little to rebut some of Pro's premises, and she focued much of the debate on what Pro thought ET's might look like. instead of attacking the points Pro mentioned, Such as distance and time it would take to travel. Overall I found Con did very little to dismantle Pro's arguments. I will add that Pro made a few more spelling and grammar errors. Thanks to both. :)