33 replies to this topic

[Egann]

This thread is all about discussing the likelihood of Aliens, because while I can tolerate some stupidity from my science fiction, I despise it when I think astronomers and cosmologists who should know better get this one wrong. This is just my opinion based on the quite limited information I have at hand, but I find these arguments rather convincing.

Life in general is relatively uncommon throughout the universe, and intelligent, civilization-building life is quite possibly unique to humans throughout the entire universe. The reason is that life should be reasonably easy to detect close in, and that as alien civilizations come anywhere close to interstellar travel they become impossible to hide.

The basic jist is that places like SETI are so focused on being able to decode a particular message that they miss how obvious cumulative effects are. Effects like, thanks to telecommunications, Earth is brighter in the sun in the radio spectrum. Any civilization with basic radio astronomy can detect that and would probably draw a big "WTF?" circle around it. Now apply this logic in reverse; ignoring the time-delay factors, we can detect ourselves within a space of several thousand light years. Now let's throw civilizations into the matter with the Kardashev scale.

Level 1: A civilization that is able to use all the energy output of a planet
Level 2: A civilization that is able to use all the energy output of a star
Level 3: A civilization that is able to use all the energy output of a galaxy

I have problems with this scale, but it's the staple of the Sci Fi department, so let's use it anyway. On this scale, our civilization falls roughly as a 0.7, we are barely able to send little robotic rovers to our neighboring planets, and we can detect ourselves within a radius of several thousand light years. It's a fair assumption, then, that the increased energy output of a greater civilization means that we could detect them at a much greater distance.

Conclusion: ignoring time delays, we can already detect any level 3 civilization in the visible universe, and could probably detect a level 2 civilization most anywhere else in the Milky Way in the form of stuff we have absolutely no explanation for. The most reasonable conclusion is that civilizations like this probably do not yet exist.


Discuss.

[ganonlord6000]

There is no doubt in my mind that life does indeed exist on other planets. However, with our current technology it would take centuries to get to a civilization on our level.

My theory is that, at least in our galaxy, most planets are either at our level or far behind us. It is also possible that a) our planet is to primitive to communicate or even detect some of the advanced races or b) they know we're here and what we're doing, and they're just ignoring us until the time is right.

[Selena]

This thread is all about discussing the likelihood of Aliens, because while I can tolerate some stupidity from my science fiction, I despise it when I think astronomers and cosmologists who should know better get this one wrong. This is just my opinion based on the quite limited information I have at hand, but I find these arguments rather convincing.

Life in general is relatively uncommon throughout the universe, and intelligent, civilization-building life is quite possibly unique to humans throughout the entire universe. The reason is that life should be reasonably easy to detect close in, and that as alien civilizations come anywhere close to interstellar travel they become impossible to hide.

The first red flag is that you - a young dude without a basic science degree - are accusing professional astronomers of being wrong. Please step back and rethink that. It does not make you or your argument look good at all.

"Relatively" uncommon doesn't mean much on a large scale. Solid matter is "relatively" uncommon in the universe, but it's still everywhere. We do not know how rare life is. We haven't even properly explored our own solar system - or our own planet - much less the systems beyond. We are in the dawn of space exploration. We know very little about the galaxy around us, and we are constantly being surprised with new information that continues to tear apart the notion that we're somehow unique. Basically, if you're fairly certain of anything in the universe (or science), then get rid of your preconceptions and keep asking questions.

Civilized life is easy to detect at close range, yes. But nothing is in close range, so that's a moot point.

Interstellar travel does not make something impossible to hide. What is the basis for this assumption? The idea that everyone and their space dog is going to be using warp drives, wormholes, or other fictional modes of interstellar travel? A spaceship is a giant hunk of metal. It only emits significant heat signatures when it burns engines. Otherwise it's not going to be much more obvious than an asteroid - at least not until you get close enough to pick up transmissions. FTL or near-FTL speed is not required for interstellar travel. You don't need to go super quick, for example, if you've somehow turned off that troublesome "aging" thing. Or if you've found a way to suspend yourself on long trips.

And even if space vessels were traveling at near-FTL speeds, they remain very small. And because they are very small, they are difficult to detect at any range further than our solar system.

Remember that just a few decades ago, certain scientists - including Sagan himself, to an extent - thought extrasolar planets were incredibly rare. We've now identified hundreds of them, some in habitable zones, and this technology is still very new. As in, less than 20 years. And the number of planets we're detecting increases almost exponentially as techniques and technologies improve. It is now estimated that each star has an average of at least one planet. This would have blown everyone's minds in the 1970's.



Sci-fi's chief sin is not the inclusion of aliens or speculative technology; it's reducing the scale of the universe. By the time you can detect anything you think should be obvious at any substantial range, you're detecting it thousands or millions of years in the past. If not longer. Any form of communication, any form of travel, any type of observation equipment -- all of it is subject to delay. Until we wizard up some sort of instant interstellar communications network, every distant colony or vessel will effectively be blind until something stumbles on top of it.

So, no, there is no indication that we're alone. There is no indication that we're the only civilization in the universe. There is no indication that there aren't more advanced civilizations out there. We have been observing the universe for a very short time, and, if there are other space-faring species out there, it is unlikely that they're any closer than a thousand lightyears - if not further. We can't yet see them, either due to their history or our inadequate technology, and they are seeing a Earth in a time without machinery or radio transmissions.

We might be the most advanced. We might not be. It's impossible to tell.

[Sir Deimos]

Here's a question. How are we to detect energy signatures of level 2 or level 3 civilizations? We couldn't detect radio waves without first discovering the technology.

[Egann]

The first red flag is that you - a young dude without a basic science degree - are accusing professional astronomers of being wrong. Please step back and rethink that. It does not make you or your argument look good at all.

It's not like I am a single voice saying one thing against each and every professional in the field; this argument is basically taken from The Anthropic Cosmologic Principle, which was written by Frank Tipler and John Barrow, who are both professional cosmologists, physicists, and mathemeticians. I have to admit that Tipler is kinda fringe and is distantly associated with Intelligent Design, but both of them hold professorships at respected universities.

And just because I happen to find this argument convincing doesn't mean I agree with Barrow and Tipler on everything. They have several sections on Von Neumann machines exploring the galaxy I strongly disagree with. It just means that, from my point of view, I understand and appreciate the logic of this argument more than Hawking mystically saying "there could be life in (insert wierd place here)" in an indulgent juvenile fantasy.

Basically, in this context we have two groups. Group A is incredibly optimistic despite a lack of any real form of evidence in the affirmative direction. Group B is open-mindedly skeptical and, despite the logical impossibility of proving a negative, has a rational argument behind their position. And yet Group A is mainstream and Group B is fringe. Does this really not strike you as backwards?

Civilized life is easy to detect at close range, yes. But nothing is in close range, so that's a moot point.

Interstellar travel does not make something impossible to hide. What is the basis for this assumption? The idea that everyone and their space dog is going to be using warp drives, wormholes, or other fictional modes of interstellar travel? A spaceship is a giant hunk of metal. It only emits significant heat signatures when it burns engines. Otherwise it's not going to be much more obvious than an asteroid - at least not until you get close enough to pick up transmissions. FTL or near-FTL speed is not required for interstellar travel. You don't need to go super quick, for example, if you've somehow turned off that troublesome "aging" thing. Or if you've found a way to suspend yourself on long trips.

And even if space vessels were traveling at near-FTL speeds, they remain very small. And because they are very small, they are difficult to detect at any range further than our solar system.

Remember that just a few decades ago, certain scientists - including Sagan himself, to an extent - thought extrasolar planets were incredibly rare. We've now identified hundreds of them, some in habitable zones, and this technology is still very new. As in, less than 20 years. And the number of planets we're detecting increases almost exponentially as techniques and technologies improve. It is now estimated that each star has an average of at least one planet. This would have blown everyone's minds in the 1970's.

There's actually a solid reason you can't hide a civilization in space. Heat radiators.

According to the Carnot cycle, no heat engine can run more efficiently than a Carnot engine, which is roughly 1/3rd efficient, so we should see three times the actual energy use of a civilization bled into space as excess heat.

Now remember how I mentioned that humanity makes Earth brighter than the sun in the radio spectrum? That's not even all of our communications: just the wireless ones. And we're only a 0.7 civilization on the Kardashev scale.

Granted, stars in general and the sun in particular are a lot brighter in the infra red spectrum than in radio, so heat radiators would get more drowned out than our radio broadcasts. But still, big civilizations should stick out from the background in at least one and probably several big ways. Not only are we not detecting any energy signitures from FTL travel, but we're also not seeing any telecommunications or any energy activity at all.

Sci-fi's chief sin is not the inclusion of aliens or speculative technology; it's reducing the scale of the universe. By the time you can detect anything you think should be obvious at any substantial range, you're detecting it thousands or millions of years in the past. If not longer. Any form of communication, any form of travel, any type of observation equipment -- all of it is subject to delay. Until we wizard up some sort of instant interstellar communications network, every distant colony or vessel will effectively be blind until something stumbles on top of it.
So, no, there is no indication that we're alone. There is no indication that we're the only civilization in the universe. There is no indication that there aren't more advanced civilizations out there. We have been observing the universe for a very short time, and, if there are other space-faring species out there, it is unlikely that they're any closer than a thousand lightyears - if not further. We can't yet see them, either due to their history or our inadequate technology, and they are seeing a Earth in a time without machinery or radio transmissions.

We might be the most advanced. We might not be. It's impossible to tell.

Again, my complaint is not what sci-fi does. Sci-fi has always tended to ignore the facts and involve too much Handwavium and Technobabble and way too many optimistic assumptions. That's just what writers usually do, and while I try to hold myself to a higher standard than that for the sake of readability, I don't fault myself or others too much on this matter.

No, the problem is best summarized by that good old quote from Red Dwarf.

Lister: What do you believe in, then? Do you believe in God?
Rimmer: God? Certainly not! What a preposterous thought! I believe in aliens, Lister.
Lister: Oh, right, fine, something sensible at last.

When you get right down to it, the "do aliens exist" matter doesn't really have evidence around it. Back in the 20th century, this could be justified by the technology being new and us not knowing what we were looking at, but as time progresses, the evidence in favor of extraterrestrial life has not kept in step with our knowledge of the universe. It stubbornly remains in the realm of relatively baseless conjecture.

Baseless conjecture is fine for fiction, but it's should be recognized as conjecture and not treated as a part of some scientific canon.

Here's a question. How are we to detect energy signatures of level 2 or level 3 civilizations? We couldn't detect radio waves without first discovering the technology.

Hardly. Detection is actually rather forgiving on this scale. To make sense of the energy signatures we would detect, our theoretical physics needs to be roughly on par with their applied technology, which is a really big difference. But if that wasn't a wide enough difference, detecting their presence in the form of an unexplained anomaly only requires a detection device and a basic model for how the universe works.

Edited by Egann, 01 August 2012 - 11:28 AM.

[Showsni]

Either there's stuff out there that's so far away/long ago it really doesn't matter anyway, or there's nothing out there, or there's something we jsut haven't been looking for in the right way, or in exactly the right place... Basically, I have no real ideas on the whole subject. Certainly there's the possibility aliens are out there (I mean, what about medieval level of civilisations a few light years away? How would we be able to check for them, beyond flagging their planet as "hospitable zone"?), but then again maybe they aren't. Does it really matter?

I guess until we find anything the only impact on us is how much money and time gets spent on the search. And if that's also producing useful results for other fields, I don't think there's any harm in it.

Edited by Showsni, 01 August 2012 - 02:54 PM.

[Selena]

Basically, in this context we have two groups. Group A is incredibly optimistic despite a lack of any real form of evidence in the affirmative direction. Group B is open-mindedly skeptical and, despite the logical impossibility of proving a negative, has a rational argument behind their position. And yet Group A is mainstream and Group B is fringe. Does this really not strike you as backwards?

I don't know. Religious people have always considered blind optimism with no evidence to be a valid justification for their beliefs. [/ba-dum-tish!]

But your argument doesn't sound open to possibilities with a dose of healthy skepticism. It's tilting strongly to "there are no aliens, full stop." At this stage, we don't even have any real observations to be skeptical of. We barely understand our own planet, much less the rest of space. Group A is no less rational than Group B. The only difference between them is a "glass half full" or "glass half empty" view on the situation. There is plenty of potential for there to be other life in the galaxy. There is plenty of potential for that life to still be in a more basic state, no where near civilization. We do not know. We will not be prepared to know for quite some time.

And again, the more cynical scientists - and even some of the optimists - also doubted the existence of there being a significant number of exoplanets in the galaxy. They were wrong. New technology opens eyes and unlocks possibilities.

There's actually a solid reason you can't hide a civilization in space. Heat radiators. [...]

Yes, all of this is technically true. But it still misses the point and focuses more on splitting hairs.


Point 1: Detecting things is not easy

Space is big. Vessels and planets are not. The heat emissions of a space vessel or civilized planet are still infinitesimal to the point of non-detection, at least at a distance. And that's when something is at a predictable standstill.

If a vessel is moving at a high rate of speed, then it probably won't be noticed at all - or it'll be dismissed as an error. Scientists only declare a distant body to "exist" if they can observe it more than once and with routine predictability. The old "Wow!" signal was abnormal, but it never repeated itself, and so it was dismissed. If an object that small is in a state of rapid transit, especially if it's outside the system, you've moved well beyond "needle in a haystack" chances of ever acknowledging it. And if the vessel is next to a foreign star or other large body, you're not going to detect it at all.

As for planets, some of them naturally have incredibly high heat signatures, so that is not an automatic "pay attention!" notice. And even detecting those planets has often been challenge for our observers.

FTL is not required for interstellar travel, and, as far as modern science is still concerned, outright impossible. So failing to detect these signatures is not an issue.


Problem 2: Distance and Lag

Because, again, space is frickin' big, being able to detect abnormally high radio signals doesn't necessarily mean anything. In fact, if you think we should be detecting a civilized planet using our current technology right now, you assume one of the following:

1. These civilizations are roughly as advanced, or more advanced, than we are.
2. They're somewhere right next door (any distance less than 1000 lightyears).
3. If you aim to detect a civilization further out than that, then we are to assume they achieved detectable prominence a long time ago.

And nobody's detecting Earth at the moment, because we only started using radio communication en masse within the last century -- those signals haven't gone anywhere substantial yet.

Time delays and distance cannot be ignored.

If we are the most advanced race in the observable galaxy, which is a possibility, then we aren't going to detect anything for a while. If comparable civilizations are a great distance away, then we're still not going to detect anything for a while. Just because a galactic empire is not apparent on this side of the Milky Way (yay) does not mean other civilizations are not out there.


There is no evidence for it. There is no evidence against it. It's a complete unknown, and to think that we're even remotely in a position to declare "no, there are no aliens" is both absurd and arrogant. Our technology is not that impressive, there aren't a whole lot of people working on planetary detection, space is huge, a good chunk of it is blocked from observation by various obstructions, and distance makes long-range detection of anything inaccurate due to time delay.

If you look at any map of the known galaxy, you will see that we've only observed and logged a tiny sliver of the Milky Way.

[J-Roc]

The thing I find funny is that with all of the television and itnernet and everything that any alien civilization that was passing by would be able to find out everything they would ever want to know about us.

No wonder we haven't been officially visited, and probably never will be.



This show makes me not want to be anywhere near this planet, either

[ganonlord6000]

@ The Dude

Providing their own civilization isn't any different in terms of shows? But I agree. Shows like that are probably part of the reason why other civilizations haven't tried to make contact with our current civilization. Now several thousand years ago is something else, and they're probably keeping a safe distance from Earth for the time being until our race matures.

[J-Roc]

@ The Dude

Providing their own civilization isn't any different in terms of shows? But I agree. Shows like that are probably part of the reason why other civilizations haven't tried to make contact with our current civilization. Now several thousand years ago is something else, and they're probably keeping a safe distance from Earth for the time being until our race matures.

Not only that but we broadcast it and shoot time capsules out into space. So, not only do we come across as stupid, but we also come across as proud of what we do (ie. murdering eachother en masse and destroying our own home world for what would appear to aliens as no reason at all, but "profit" in our eyes). Violent, fucked up, and proud of it.

[ganonlord6000]

@ The Dude

Providing their own civilization isn't any different in terms of shows? But I agree. Shows like that are probably part of the reason why other civilizations haven't tried to make contact with our current civilization. Now several thousand years ago is something else, and they're probably keeping a safe distance from Earth for the time being until our race matures.

Not only that but we broadcast it and shoot time capsules out into space. So, not only do we come across as stupid, but we also come across as proud of what we do (ie. murdering eachother en masse and destroying our own home world for what would appear to aliens as no reason at all, but "profit" in our eyes). Violent, fucked up, and proud of it.

True. We do have some complete idiots (sometimes hilarious) and total sickos on this planet, don't we? Besides, who says the aliens are any different? Some of them could be just like us if not worse. I wouldn't blame them if they are currently ignoring us, though. I'm sure they don't have a death wish.

[Steel Samurai]

I would find it pretty hilarious if high alien culture was comprised mostly of shows like Jersey Shore.

[Toan]

Conclusion: ignoring time delays, we can...

Translation: Let's all pretend we live in a magic fairy-tale universe.

[arunma]

This thread is all about discussing the likelihood of Aliens, because while I can tolerate some stupidity from my science fiction, I despise it when I think astronomers and cosmologists who should know better get this one wrong. This is just my opinion based on the quite limited information I have at hand, but I find these arguments rather convincing.

The first red flag is that you - a young dude without a basic science degree - are accusing professional astronomers of being wrong. Please step back and rethink that. It does not make you or your argument look good at all.

Heh, you guys are putting me on the spot here. I feel like I'm supposed to step out and say, "well, as the professional astronomer..." followed by something smart.

So regarding life on other planets, awhile back there was the Drake equation, which predicted a fairly large number of civilizations in our own galaxy that could support life. Egann does correctly make the point that the universe is fairly radio quiet. You get radio emission from AGNs, pulsar wind nebulae, and other such sources. But stars don't emit any, and there's no diffuse radio background (at least not that I know of). So a civilization emitting radio waves would be a point source in the sky, something we can resolve with radio telescopes.

So yes, while I've never read much from the radio astronomers I'd totally believe the claim that our planet is brighter than the Sun in the radio. Of course, those emissions still die off according to an inverse square law, so how's a civilization elsewhere going to see us, or us them? After all, the sources of radio waves that we do see are really bright. The Crab Nebula, for example, is about 11 light years across. It's a pulsar wind nebula which accelerates electrons to energies higher than what even the LHC can achieve on earth, and these electrons emit radio synchrotron radiation. Because there are so many of them (due to the large size), we can detect it despite it being thousands of parsecs away. And from all the way here, it's way quieter than what you'd detect from a radio station. While we can rule out the existence of a level 3 civilization in our own galaxy, there's no evidence I can think of that denies the existence of one comparable to our own.

That, or they have a prime directive.

[SteveT]

All this talk of RF and stars has me curious. What's the breakdown for emissions frequencies for celestial bodies, arunma?

[J-Roc]

arumna, you have once again educated me.

[Sir Deimos]

Seconded. All that knowledge is a beautiful thing.

[Egann]

OK, I think we should do some basic math to establish what it is we are probably looking for in the first place. I think the results will be...interesting.

1) The Timeframe

The requirements for intelligent life are up to some debate. While I've seen lists upward of thirty factors, but I want to emphasize the metabolic one. Warm-bloodedness. On Earth, warm-bloodedness is about 200 million years old and intelligence could emerge at any time.

Assuming a similar time frame for other would be intelligent species, this means that, for the 200 million light years closest to Earth, intelligence has had both time to emerge and the light has had time to reach us. Obviously detection hardware is the sticking point, not the lag time for light to get where it's going. A sphere with a radius of 200 million light years
encompasses the Local Group many times over.

2) The Tech Level/ Dispersal

Let's be blunt; we aren't looking for civilizations on par with our own because there almost certainly are none. For 500 years humans have been developing at an incredible rate, and when you consider that development before that was measured in the millions of years level, being on-par with an alien civilization with a completely different development timeline? Incredibly unlikely.

So what are we looking for? Solar sails are theoretically capable of 0.1 C, and the Milky Way is 100,000 light years across, so given sufficient technology it is perfectly conceivable that a civilization could colonize the galaxy in one million years.

And intelligence could have emerged starting at least 200 million years ago. The math brings us to the inevitable conclusion that any sort of extraterrestrial intelligence we could contact would almost certainly already be a galaxy-wide power with a population in the hundreds of trillions. We are not looking to make contact with another astronomer on another burgeoning Earth on the other side of the galaxy. We are looking to accidentally bump into a traffic cop making his rounds perhaps ten or twelve light years away at most. Or happen across one of trillions of unmanned probes, or one of the bits of space junk so numerous it only makes sense to refer to them with scientific notation.

This is why I ignore lag. Considering the timescale, an insignificant civilization becomes a ubiquitous one in so short a time the transitional period isn't worth mentioning. And yet it is the one assumed for extraterrestrial contact.

That, or they have a prime directive.

While this makes sense, I must ask how they could enforce such a thing. Our population is in the billions and we are nowhere near like-minded enough to agree on a rule like that. A galactic civilization will have a population which starts with trillions and goes North. Every single individual agreeing on this matter constantly is highly unlikely.

Oh, and thank you for the professional opinion.

Edited by Egann, 03 August 2012 - 10:12 PM.

[Sir Deimos]

tl;dr.

From what I did read though, you've made a lot of assumptions. You can't do that. At all. Science is fact, assumption is not fact. You cannot use humanity as the blueprint for all intelligent life. Just because it took humanity X amount of time does NOT mean any alien civilization would also have developed in X amount of time. You take entirely too many assumptions, and quite frankly it is infuriating to read.

[arunma]

Man, I feel so smart...next year when I graduate I'm changing my user name to Dr. arunma.

All this talk of RF and stars has me curious. What's the breakdown for emissions frequencies for celestial bodies, arunma?

The sun's emission is almost entirely thermal, so the peak emission is in the optical. This incidentally is why life has evolved to see such a narrow band of radiation. The largest contributor is the blackbody spectrum with a 6000 K peak. It also bleeds into the UV and infrared. Now, other stars have different surface temperatures, and higher temperatures correspond to larger peak frequencies (and vice versa). So there are stars whose emission peaks are at frequencies closer to the UV or IR. The Sun also emits x-rays, but the flux is far less than the optical. You might think that the sun emits gamma rays since there are nuclear processes going on. However, these occur in the core, and those photons actually get converted to much lower energies before reaching the photosphere.

[J-Roc]

Man, I feel so smart...next year when I graduate I'm changing my user name to Dr. arunma.

Just wait till the hockey trivia thread, they'll probably promote me to King of the Universe.

[Sir Deimos]

Nah. We expect Canadians to know a lot of stuff about hockey.

[J-Roc]

Nah. We expect Canadians to know a lot of stuff about hockey.

Just like how we expect people from Jersey are all douches.

[Kisseena]

I'm not gonna say anything to this conversation, but I just have to do it.
I CAN'T RESIST.

[Egann]

YOU DID THAT IN SIXTY SECONDS?!?!

[Kisseena]

Oh no no no no.
That's draw something, an app.

You have an unlimited time for it. XD

[Mark]

ahh, a well discussed topic...

take a read - its called the 'Fermi Paradox' :
- http://en.wikipedia....i/Fermi_paradox


my take is to think that it is far from obvious that humans are mediocre.
to my knowledge it simply isnt easy to know how probable it is for first cells to have come into existance.
and being a substance dualist - I view human mentality as being an utterly outstanding phenomenon in the universe. (but is mentality rare? - No idea!)
I dont know, but I believe it isnt obvious.

what I can say is that I see the question of alien life to be a rather emotive kind of thing.
consider: Quarians have boobs!
SETI = search for extraterrestrial intellegence - not bacteria.....
and intellegence? not just any kind of other intellegence, but OUR kind of intellegence (!)

(And Frank Tipler is ..... something else....)

Edited by Mark, 05 August 2012 - 06:05 AM.

[J-Roc]

The CBC did a special on people who make fools of themselves on Facebook and in the doc they interviewed a social media expert from Harvard who said that we were essentially going to become the Borg soon. So I'm with you Mark, we're not so fantastic.

SOURCE:
http://www.cbc.ca/do...ok-follies.html

[arunma]

consider: Quarians have boobs!

You got my attention.

[SteveT]

Man, I feel so smart...next year when I graduate I'm changing my user name to Dr. arunma.

All this talk of RF and stars has me curious. What's the breakdown for emissions frequencies for celestial bodies, arunma?

The sun's emission is almost entirely thermal, so the peak emission is in the optical. This incidentally is why life has evolved to see such a narrow band of radiation. The largest contributor is the blackbody spectrum with a 6000 K peak. It also bleeds into the UV and infrared. Now, other stars have different surface temperatures, and higher temperatures correspond to larger peak frequencies (and vice versa). So there are stars whose emission peaks are at frequencies closer to the UV or IR. The Sun also emits x-rays, but the flux is far less than the optical. You might think that the sun emits gamma rays since there are nuclear processes going on. However, these occur in the core, and those photons actually get converted to much lower energies before reaching the photosphere.

When you put it like that, I just feel lazy for not looking up the equation myself. For whatever reason, I overlook the temperature ==> emission frequency relation. Thanks.

Edited by SteveT, 11 August 2012 - 08:20 PM.