The Instigator
likespeace
Con (against)
Winning
15 Points
The Contender
philochristos
Pro (for)
Losing
5 Points

It is unlikely that any extraterrestrials have ever visited earth.

Do you like this debate?NoYes+2
Add this debate to Google Add this debate to Delicious Add this debate to FaceBook Add this debate to Digg  
Post Voting Period
The voting period for this debate has ended.
after 6 votes the winner is...
likespeace
Voting Style: Open Point System: 7 Point
Started: 1/2/2013 Category: Science
Updated: 4 years ago Status: Post Voting Period
Viewed: 1,997 times Debate No: 28834
Debate Rounds (3)
Comments (9)
Votes (6)

 

likespeace

Con

Pro will claim that it is unlikely that any extraterrestrials have ever visited earth using this 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.

Pro will have the burden of proof in this debate. I only need to debunk his 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."

To simulate the original debate structure (Pro, Con, Pro, Con w/o new evidence)--

Round 1: Con (this). Pro states his case.
Round 2: Con/Pro debate
Round 3: Only Con speaks (but no new evidence)

Motivation: While Pro's argument is interesting, I don't believe that Con made the best rebuttal, and look forward to a debate that more closely examines the truth of the matter.
philochristos

Pro

Thanks for the challenge, likespeace. Good luck!

In addition to what Con has already laid out, I add the following: I will leave my last post empty so that we will both have an equal number of posts.

I'm just cutting and pasting this opening from my last debate on this same topic.

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......
Debate Round No. 1
likespeace

Con

First, thanks to my opponent for accepting this challenge, and I look forward to a good debate. :)


The Argument Itself

Pro's burden of proof is steeper than it may first appear. He must actually show it's very likely that any visiting ETs would live nearby and very unlikely that there are any ETs nearby capable of visiting our planet.

When you combine premises that indicate uncertainty--such as with the words "probably" or "unlikely"--that uncertainty is compounded in the conclusion.

1. If any extraterrestrials have ever visited earth, then they probably they 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.

The problem with uncertainty becomes clear when you try to quantify these terms--

1a. A nearby ET visitor is more likely (51%)

1b. A faraway ET visitor is more likely (49%)

2a. Nearby ET visitors don't exist (51%)

2b. Nearby ET visitors do exist (49%)

If it's only 49% likely ET visitors would come from far away and 24% likely nearby ET visitors exist, it's folly to conclude that it's unlikely we've been visited, even if you agree 51% with both of his premises.

49% + 24% adds up to something much greater than 49%!

In fact, according to probability theory, you must agree roughly 71% with each of Pro's premises, to judge his conclusion likely to be sound. (71% x 71% = 50%).

 

The First Premise: If any extraterrestrials have ever visited earth, then they probably live within 50 light years of earth.

> The closest star to earth is about 4.2 light years away.. it would be quite the undertaking for anybody to travel even to one nearby.

I accept light speed as a limitation on communication, and some fraction of light speed between 0.30 and 0.90 as a limitation on travel speeds (using technologies such as nuclear pulse, fusion, or antimatter[1]. As such, that trip would take 5 to 15 years.

> Given that there are hundreds of billions of stars in the galaxy, the odds are against anybody just luckily running into the earth.

This claim is unsubstantiated. First, the density of life in our university is unknown--there could be any number of homeworlds, races, planets, moons, asteroids, outposts, or ships. Second, a space-faring race could and likely would extend its reach using machines and remote sensors. Third, it would be hubris to claim we were the first to develop space flight; other races may have begun their search for life hundreds of millions of years ago.

> 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.

Aliens don't need to know that life on Earth exists to make the trip worthwhile; it's enough to know that the possibility of intelligent life exceeds a certain threshold. Alien races more advanced than us will be able to make strong educated guesses about which stars orbiting the galactic center, and which planets orbiting those stars, are likely to support the evolution of intelligent life. For example, the presence of liquid water may be a strong indicator, and that's only possible for planets within certain orbits.

> Earth produced the first radio communication in the 1890's. 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 receiving those signals...

Even if aliens were to passively await a radio signal from us rather than actively investigating planets likely to contain life, the alien civilization as a whole needn't know we exist in order for our race to have been visited by some aliens.

For example, the alien civilization might be 500 light years away, a sensor in an asteroid belt detected our transmission, and a patrol ship 20 light years away came to investigate.

Finally, it's possible all life on this planet or mammalian life or human life was seeded or mutated by an alien race. Such a race would know precisely where we are.

 

The Second Premise: It is unlikely that any extraterrestrials capable of traveling to earth live within 50 light of earth.

> But if there were civilizations within 50 light years who had radiotechnology long before us, then SETI would've found them.

That is factually incorrect.

First, aliens may have had radiotechnology long before us, but stopped using it over 50 years ago.

Second, SETI scans a subset of radio communications. "Virtually all radio SETI experiments have looked for.. narrow-band signals. If E.T. is a decent (or at least competent) engineer, he'll use narrow-band signals as beacons to get our attention.[2]"

Thus, a more accurate statement would be, "If there were civilizations within 50 light years who still use radiotechnology and wanted to be found, we'd have found them by now!"

Of course, an alien race would be entirely justified in wanting encounters with other aliens to be on their own terms. They may be hiding their signals in ambient noise (wide band, steganography), shielding them (as the atmosphere sometimes does), or directing them (a dish antenna aims signals in one direction), amongst other options.

Simpler yet, they may be using a different electromagnetic wavelength.

Alternatively, their homeworld may be too far away for us to detect, but their scout ships or drones or sensors may be close enough that they can listen to us.

 

Conclusion

My opponent has the burden of proof to convince you each of his premises are 71% certain, and I have poked quite a few holes in each of his premises.

 

Sources

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

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

philochristos

Pro

Con has convinced me that my argument is not very good, so in my judgment, he has won this debate. But just to keep it interesting, I'll give him a little feedback.

First, I think his probability assessment is flawed. I've been reading about probability when applied to modus tollens, and it's quite a bit more complicated than simply multiplying the probability of each premise to get the probability of the conclusion. However, he is right that I need better than 51% probability for each premise, and I suspect 71% is close enough.

It is true that alien races could use remote probes and such to locate life-permitting planets, but if those probes discovered life or planets with a high probability of having life, they'd have to send that information back to the home planet, and then there would have to be a voyage undertaken. So I don't think this solves any problem for Con as far as time and distance is concerned.

Moreover, Con suggests the possibility that advanced aliens might've stopped using radio communication. If so, then it's hard to imagine how they would communicate with their probes. Besides, the mere possibility that some advanced civilization no longer uses radio technology doesn't do much damage to the probability that they still use it. All "possibility" does is remove certainty.

While it's true that the life-density of the galaxy is unknown, all we need for my argument to work is for the intelligent-enough-to for-interstellar-travel-life-density of the galaxy within 50 light years or so to be low or non-existent. And we can tell that it probably is from SETI (depending on how thorough SETI's searches are, and Con doesn't seem to think they're very thorough).

That's about all I have to say. Thank you, likespeace.
Debate Round No. 2
likespeace

Con

While Pro judges I have won the debate, I address his points below, and invite voters to judge for themselves.


The Argument Itself

> it's quite a bit more complicated than simply multiplying the probability of each premise to get the probability of the conclusion. However, he is right that I need better than 51% probability for each premise, and I suspect 71% is close enough.

I concede the math I presented was unintentionally faulty and apologize. As I've pledged not to introduce new arguments this round, I won't produce the replacement formula here, but will note that we both agree that to support Pro's conclusion, you must have a high confidence in each of the premises, in the ballpark of 70%.


The First Premise: If any extraterrestrials have ever visited earth, then they probably live within 50 light years of earth.

> It is true that alien races could use remote probes and such to locate life-permitting planets.. but.. I don't think this solves any problem for Con as far as time and distance is concerned.

I agree the sensors were superfluous to my argument, since you stated we could assume they could detect our radio waves if in range. The sensors would've helped to amplify and direct our signals towards alien starships.

> they'd have to send that information back to the home planet

Rather, only to the nearest alien starship! I find the notion compelling that aliens who toiled to design and build interstellar craft would send at least some of those ships to other solar systems. A fraction of those starships would be closer to us and thus able to detect and visit us even if their home planet remained outside that 50-light-year window.

> While it's true that the life-density of the galaxy is unknown, all we need for my argument to work is for the intelligent-enough-to for-interstellar-travel-life-density of the galaxy within 50 light years or so to be low or non-existent.

I raised the issue of life-density because, as it increases, so too does the likelihood that Earth had a visitor whose home is more than fifty light years away. The more aliens out there, the more time they had, and the better their ability to identify stars and planets capable of detecting life.. the more likely it is that faraway aliens have visited Earth.

I do agree, "Visitors are more likely to have a home 0-50 light years away than 50-100 light years away."

However, I contend you didn't show "Any visitors would very likely have a home within 50 light years."

I extend all other arguments.


The Second Premise: It is unlikely that any extraterrestrials capable of traveling to earth live within 50 light of earth.

> Con suggests the possibility that advanced aliens might've stopped using radio communication. If so, then it's hard to imagine how they would communicate with their probes.

I did propose an alternative--that they are using other portions of the EM spectrum. Non-radio wavelengths include microwave, infrared, visible light, ultraviolet light, x-rays, and gamma rays. Radio is such a small fraction of the total EM spectrum, that without further evidence, I'd say radio is a possibility and otherwise is a probability.

> SETI (depending on how thorough SETI's searches are, and Con doesn't seem to think they're very thorough)

According to the quote I posted before, SETI doesn't seem to think SETI is good at finding alien races that don't want to be found! This refutes your argument that "If there were life nearby, we would know about it due to SETI."

I can't say I'm very confident in either the existance or non-existance of nearby life.

I extend all other arguments.


Conclusion

I don't believe Pro has made his case, that each of his premises are very likely to be true. Thus, his burden of proof is not met, and so his conclusion doesn't logically follow. Still, Pro's argument was interesting and well-written, and this was a fun debate that raised more questions than it answered. I thank him for sharing and discussing it!
philochristos

Pro

And that concludes tonight's debate. Thank you for coming. Please vote.
Debate Round No. 3
9 comments have been posted on this debate. Showing 1 through 9 records.
Posted by charleslb 4 years ago
charleslb
philochristos: "Charleslb, it sounds like you're saying that because there are a lot of false reports of ET's, that there therefore aren't any true ones. I don't think that follows, although it is good reason to be skeptical of particular claims."

Not quite. What I'm saying is that all "evidence" whatsoever of otherworldly visitation ought to be considered false, misinterpreted, or utterly fake because an extraterrestrial civilization that could send explorers across the vast distances of space to our humble little third rock from the sun would have the unimaginable technological sophistication to go all about our world without leaving the slightest scientifically investigatable trace. Mm-hmm, it follows that any alleged traces of ETs having visited us are ipso facto the product of quite earthbound causes, even hoaxers, and ergo constitute grounds for eye-rolling rather than belief that aliens are among us. Yes, when it comes to any so-called evidence that's been asserted to establish the reality of little green or gray men, to do the trick there must in turn be ample evidence supporting the automatically dubious contention that the evidence in question is indeed authentic evidence of what it purports to prove, otherwise the standard and intelligent assumption is always that it's rubbish.
Posted by philochristos 4 years ago
philochristos
"Props for not outright forfeiting"

Outright forfeits are a pet peeve of mine.
Posted by philochristos 4 years ago
philochristos
1dustpelt, actually I did concede. I said, "Con has convinced me that my argument is not very good, so in my judgment, he has won this debate." But thanks for the counter. It looks like Bull_Diesel meant to vote for Con but accidentally voted for me. I tried to send him a message, but he's got his settings to where I can't.
Posted by philochristos 4 years ago
philochristos
Charleslb, it sounds like you're saying that because there are a lot of false reports of ET's, that there therefore aren't any true ones. I don't think that follows, although it is good reason to be skeptical of particular claims.
Posted by charleslb 4 years ago
charleslb
I might also observe that somewhat paradoxically all of the yet-to-be-discredited alleged evidence of extraterrestrial visitation that has thus far been provided by so-called UFOlogists is actually, in itself, an argument against believing that alien astronauts are frequenting the earth. That is, beings hailing from any civilization with the level of technology to do so, to travel the vast interstellar distance to our little ole planet would also ipso facto be so scientifically sophisticated they'd be quite capable of visiting and surveilling us without ever being detected or leaving any signs or evidence, whatsoever, of their presence. Also, if somehow visually observed their craft would certainly not resemble anything stereotypically or conventionally recognizable as a spaceship. Ergo, all of the "evidence" and sightings generated by all of the cases and close encounters that buoy up the faith of UFO believers are, on the contrary, proof that no ETs were involved in these incidents. Mm-hmm, anything substantive that seems to support the view that little gray men are paying calls to our world actually substantiates skepticism. And therefore UFO enthusiasts are left in the lame and intellectually indefensible position of having to assert that the best evidence for the claim that aliens are among us is the total absence of genuine evidence. Well, if they're going to fall back on that one then of course the burden is completely on them to demonstrate why the absence of evidence is evidence of what they believe and not a damning indication that what they believe is utter rubbish. Have they met this burden? Not that I'm aware of, and so I must conclude that both the absence of evidence, as well as the "evidence" of conventional spacemen/craft, leaves us in the position of not being able to hold that aliens have visited earth. I suppose that this will have to remain the intellectually legitimate position until some ET reveals himself and says take me to your leader.
Posted by likespeace 4 years ago
likespeace
> "I assume nobody forced Pro to agree to those terms..."

The origin of this debate was another debate, where Pro proposed it and won. I liked its construction and at first found it convincing, but then on a road trip I happened to spot a "UFO" (in the strict, non-alien sense). As I began discussing his premises, I found what I perceived to be problems. I thought it'd be fun, for both of us as truth-seekers, to re-open the debate. :)

http://www.debate.org...
Posted by philochristos 4 years ago
philochristos
Bull, you can submit your vote again, and it'll change.
Posted by Bull_Diesel 4 years ago
Bull_Diesel
Shoot, sorry, I missed the agreed with Con after the debate button. My bad guys
Posted by likespeace 4 years ago
likespeace
I wanted to post a couple links for the benefit of my opponent and other readers. Please ignore these for the purposes of judging the debate--

"Modus Tollens Probabilized"
http://www.math.utk.edu...
If I could rewrite the math, I'd write "('A) = P(B|A) * P('B)". While the result is the same, 71%, this appears to be a more accurate formulation.

"How Far Radio Signals Have Travelled From Earth"
http://zidbits.com...
This shows SETI is not lazy.. they are simply bumping into the inverse square law, and limitations in our ability to discern weak signals. This is why, in my scenario, I placed an alien sensor in our solar system--to eavesdrop on our signals and/or amplify them.

"Next Generation Communications"
http://gsfctechnology.gsfc.nasa.gov...
This is a NASA article discussing an actual X-Ray communication system. The benefits include higher data transfer rates and less interference from space phenomena.

I didn't include these in the debate, since we agreed to "no new evidence".
6 votes have been placed for this debate. Showing 1 through 6 records.
Vote Placed by morgan2252 4 years ago
morgan2252
likespeacephilochristosTied
Agreed with before the debate:--Vote Checkmark0 points
Agreed with after the debate:Vote Checkmark--0 points
Who had better conduct:--Vote Checkmark1 point
Had better spelling and grammar:--Vote Checkmark1 point
Made more convincing arguments:Vote Checkmark--3 points
Used the most reliable sources:--Vote Checkmark2 points
Total points awarded:30 
Reasons for voting decision: Both sides have good conduct, good spelling and grammar, and cited their sources. However, Consa' rguments were definitely more convincing.
Vote Placed by RyuuKyuzo 4 years ago
RyuuKyuzo
likespeacephilochristosTied
Agreed with before the debate:--Vote Checkmark0 points
Agreed with after the debate:--Vote Checkmark0 points
Who had better conduct:-Vote Checkmark-1 point
Had better spelling and grammar:--Vote Checkmark1 point
Made more convincing arguments:Vote Checkmark--3 points
Used the most reliable sources:--Vote Checkmark2 points
Total points awarded:31 
Reasons for voting decision: Arguments to con for obvious reasons. Conduct to pro for his gracious concession.
Vote Placed by rross 4 years ago
rross
likespeacephilochristosTied
Agreed with before the debate:--Vote Checkmark0 points
Agreed with after the debate:--Vote Checkmark0 points
Who had better conduct:-Vote Checkmark-1 point
Had better spelling and grammar:--Vote Checkmark1 point
Made more convincing arguments:Vote Checkmark--3 points
Used the most reliable sources:--Vote Checkmark2 points
Total points awarded:31 
Reasons for voting decision: Pro conceded. Others would have bluffed on through, but he just coolly concedes and continues the discussion. Nice. Con's arguments: not bad :)
Vote Placed by DoctorDeku 4 years ago
DoctorDeku
likespeacephilochristosTied
Agreed with before the debate:--Vote Checkmark0 points
Agreed with after the debate:--Vote Checkmark0 points
Who had better conduct:--Vote Checkmark1 point
Had better spelling and grammar:--Vote Checkmark1 point
Made more convincing arguments:Vote Checkmark--3 points
Used the most reliable sources:--Vote Checkmark2 points
Total points awarded:30 
Reasons for voting decision: Concession by Pro. Props for not outright forfeiting :)
Vote Placed by 1dustpelt 4 years ago
1dustpelt
likespeacephilochristosTied
Agreed with before the debate:--Vote Checkmark0 points
Agreed with after the debate:--Vote Checkmark0 points
Who had better conduct:--Vote Checkmark1 point
Had better spelling and grammar:--Vote Checkmark1 point
Made more convincing arguments:Vote Checkmark--3 points
Used the most reliable sources:--Vote Checkmark2 points
Total points awarded:30 
Reasons for voting decision: Counter votebomb, they voted for the wrong guy. And Pro did not concede, that last round was part of the rules.
Vote Placed by Bull_Diesel 4 years ago
Bull_Diesel
likespeacephilochristosTied
Agreed with before the debate:--Vote Checkmark0 points
Agreed with after the debate:Vote Checkmark--0 points
Who had better conduct:-Vote Checkmark-1 point
Had better spelling and grammar:--Vote Checkmark1 point
Made more convincing arguments:--Vote Checkmark3 points
Used the most reliable sources:-Vote Checkmark-2 points
Total points awarded:03 
Reasons for voting decision: Nice debate. I'll give it to con because Pro conceded but I enjoyed the debate on both parts. It was kind-of a contrived resolution, but I assume nobody forced Pro to agree to those terms. I feel like the result of the debate would have been slightly different had the Resolution been mutually determined