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Alien life

  1. Jan 31, 2009 #1
    What do you guys reckon to the odds of us finding alien life?

    The fermi paradox:
    'If there are all these billions of planets in the universe that are capable of supporting life, and millions of intelligent species out there, then how come none has visited earth?'

    I reckon there exists intelligent life, but the universe is simply too large for anyone to travel the distances required to find other planets in distant galaxies. I do think we may find alien life on some planets within ~100 light years of earth once we gain the technology to reach these exoplanets. I don't think these planets would be capable of harboring intelligent life, but maybe some lower life forms!
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  3. Jan 31, 2009 #2


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    Given our current understanding of the laws of physics, intergalactic travel is more or less impossible (read: impractical). Unless there is some way to circumvent the light speed barrier, then it seems very unlikely that intelligent species would ever reach out very far in the galaxy. The fact is, what kind of a society is going to finance a (presumably very costly) expedition the results of which will not be heard of for centuries? One can argue all day about how perhaps these aliens have longer lives and perhaps their society considers this a worthy investment, but the fact is there are many many many factors stacked up against the probability of anything resembling a galactic empire or even inter-stellar society.
  4. Jan 31, 2009 #3
    The Drake equation
    predicts the chance of intelligent life in the Milky Way at 2.31 times using current knowledge. Primitive life forms may be everywhere, including in our own solar system.

    Just 100 years ago people would have said going to the moon would have been impossible so there is hope. We live in exponential times.
  5. Jan 31, 2009 #4
    I see, but shouldn't these primitive lifeforms have evolved ? They may have been around for billions of years. It took 4billion years here on earth to get from bacteria to humans...maybe other planets are not capable of complex life forms? maybe due to fluctuations in temperature or lack of carbon
  6. Jan 31, 2009 #5


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    Although intergalactic travel seems unlikely without speeds above c, interstellar travel is quite likely. This is a part of the Fermi paradox; if other intelligent life exists in this galaxy, why aren't they here? Given no lightspeed, and advances no quicker than our current pace, we will be all over this galaxy in less than 100,000 yrs.

    If another civilization exists within this galaxy, it would have to be less than 1,000,000 yrs old, or they would have collonized Earth before we arose. This is a very narrow window, and makes it unlikely that such a civilization currently exists.
    Last edited: Jan 31, 2009
  7. Jan 31, 2009 #6


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    Will we really? Technologically, I certainly agree with you. Socially however, as I pointed out, do you really think we would fund such expeditions that we have no chance of hearing back from in our lifetimes? Are we going to spend trillions of dollars simply to send people out in space to never return?

    The only way this seems possible is if every single social crisis on Earth is solved and there's literally nothing better to do.
  8. Jan 31, 2009 #7
    Thats a very interesting view LURCH, thanks for sharing.
  9. Jan 31, 2009 #8

    D H

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    The Drake equation is not scientific. It is a kludge that incorporates time in only two places, the star formation rate and the lifetime of a civilization. It ignores the incredibly long time span between the formation of multicellular life and the formation of intelligent life, and the much , much longer span between the formation of primitive life and complex (but still monocellular) life. It ignores that stars themselves have a limited lifespan. It took 1/2 of our own sun's lifespan for communicative intelligent life to arise.

    The Fermi paradox is one of the indicators that something is awry with the Drake equation (or with the very overoptimistic numbers typically used within it). That 2.31 extant communicative species in our galaxy means an untold number of failed communicative species in the past. One of them would have spread.

    My personal opinion: The "pessimistic" interpretation of Drake's equation in the wiki article, 0.05 extant civilizations in our galaxy, is extremely optimistic.
  10. Jan 31, 2009 #9
    Space travel is not just about exploration though, it is essential for survival of any advanced species if they were to live pass the life time of their star. When the Sun goes into red giant stage, we had better have somewhere else to go by then (although one can just get into a space colony of some sort without travelling too far I supposed...)
  11. Jan 31, 2009 #10

    Ivan Seeking

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    How do we know that none have? The assumption is a bit paradoxical given all of the ET claims. :biggrin:

    A walk on the wild side
    Journal of the British Interplanetary Society, Vol 58, pp. 43-50, 2005.
  12. Jan 31, 2009 #11
    Maybe Earth was colonized and that is where life came from. Any # of setbacks could have wiped out the history books.

    I doubt if would take 100,000 years to travel the galaxy. Because of exponential growth of knowledge, it could increase millions of times in the next 100 years. Unfortunately I think the H-bomb, a meteor or over population will bring us down.
  13. Feb 1, 2009 #12


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    I have two issues with this explanation. First, if space travel is merely a means to the end of survival, only when the native stellar system becomes unstable is it necessary and therefore there is no reason to expand beyond a single solar system at a time. Second, assuming a relatively average star and if we take the evolution rate of life on earth to be something as normal (a huge assumption), then the lifespan of these civilizations are on the order of 5 billion life years. This is way way way more than is predicted by even the most optimistic proponents of the drake equation.
  14. Feb 1, 2009 #13


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    We are assuming no way is found around the light barrier. No matter how much you know, if you can only move sub-c then you are intrinsically limited in your ability to colonize the galaxy.
  15. Feb 1, 2009 #14
    I am thinking we will find a way around the light speed barrier at some point but let's say we don't. If well into the future we saw a problem coming with the Earth's ability to support life we would have some options. Most would agree future technology would allow us to locate potential planets for us to move to. A 1G continuous acceleration rocket could take us from one edge of the Milky Way to the other in just 12 years travel time. Our likely destination would most likely be much less distance. Earth time may be many thousands of years but who cares if it is dying. It would be a one way trip to save the human race.
  16. Feb 1, 2009 #15

    Vanadium 50

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    That number in Wikipedia is innumerate poppycock. You can't take a bunch of order of magnitude estimates, multiply them together and get a number with three significant figures.
  17. Feb 1, 2009 #16


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  18. Feb 1, 2009 #17
    Are you kidding me? The milky way is 100 000 light years in diameter.

    Anyway whos to say that life isnt common. We know it is possible, aka us. Another advanced life form out there mightve sent autonomous space craft all over the galaxy seeding life.
  19. Feb 1, 2009 #18
    I've been a lurker here for a few weeks... but WOW, this thread prompted me to register. A lot of incorrect information here.

    This is pure conjecture, totally lacking in any grounding in reality. Why would they have "colonized Earth" before we arose? Why would they do such a thing? Do you mean to tell me a species is going to travel 1000, 5000, 10,000, or 50,000 light years across the galaxy to live on our modest planet? They're going to skip over every other planet between here and there? How do you know that Earth's climate is even hospitable to those civilizations? Perhaps nitrogen is toxic to them. Wouldn't it be easier to colonize a planet with little existing atmosphere, to have a clean slate for "building" the proper atmosphere for the species? Surely there is a planet closer than thousands of light years away.

    In short, your prediction that any alien civilization would have colonized Earth by now if they are older than 1,000,000 years old is flat-out absurd.

    This is complete nonsense. "Current knowledge" still puts big ranges on some of the variables in the Drake Equation, and some variables have to be based on pure conjecture. For example, there is no way to scientifically calculate how likely an alien civilization is to have the desire to communicate. There is also no way to calculate the average lifetime of a communicative civilization. The Drake Equation estimates a range between 1 (us,) and hundreds of thousands.


    In whose frame of reference? It would take a minimum of approximately 100,000 years, regardless of acceleration, to travel across the galaxy from the point of reference on Earth. By the time you reached your destination, Earth would have went around the sun about 100,000 times, that is to say.

    Now, if you mean from the point of reference inside the accelerating rocket, I don't know the math of Einstein's relativity enough to calculate it.
  20. Feb 1, 2009 #19


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    He's only assuming here that the Earth is a desirable place for life.

    Where'd you get a hundred thousand from?
  21. Feb 1, 2009 #20


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    Back to the Fermi paradox and the chatting about numbers of years and light years:

    On Earth it seems like similar traditions and technologies were developed independently by humans at around the same time. Perhaps geography was limiting or helpful in different cases, but nonetheless, there seems to be a clear path of development. Why wouldn't life be the same way? After the star systems and planets were created, is there a clear path of development from:

    nonlife conditions > conditions conducive to life > conditions supporting of living organisms
  22. Feb 1, 2009 #21
    I don't see so much bad information but a lot of opinions based on speculation. Speculation is all we have in some cases. However, we do understand length contraction, time dilation and SR well enough to come up with real #'s.

    A rocket at 1G continuous acceleration will go 100,000 ly in just 12 years according to the clocks inside the rocket. Over 100,000 years will pass on the clocks on Earth. If you want to end up at your destination stopped and not a splat it does take longer. You do not need to know the math well enough to calculate all this because someone put into a calculator that folks like me can ever use. :smile:
  23. Feb 1, 2009 #22

    D H

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    Of course it is. Any statement regarding alien life (intelligent or not), is pure conjecture. Saying that LURCH's statement has no grounding in reality is another thing. It is part and parcel of the Fermi paradox.

    This is an appeal by ridicule (a logical fallacy). Appeals to ridicule are often accompanied with a straw man argument, and that is exactly what you have done here. One argument in favor of the Fermi paradox is that an intelligent species capable of colonizing the stars will colonize every single planet capable of sustaining their life. They would not have come from their home planet to Earth, bypassing every other planet along the way. That is ridiculous, and that is not what LURCH said.

    One argument against the Fermi paradox is that a space-faring civilization might have rules against colonizing planets with intelligent life (which appears to be what LURCH was getting at), or even more stringent rules against colonizing planets with complex life, or even more stringent rules against colonizing planets with any life at all.

    Try again without resorting to insults and logical fallacies.

    The lower limit is a lot, lot less than one. Even a value of 10-7 would be consistent with the known fact that at least one communicative species (us) has appeared in our galaxy.

    In the reference frame of the occupants of the space vehicle, obviously. There is one key conceptual problem with N721YG's 12 year figure: It assumes the spacecraft continuously accelerates in the same direction. It will take nearly twice as long (proper time) if the spacecraft is to come to a stop.

    Learn it then. http://math.ucr.edu/home/baez/physics/Relativity/SR/rocket.html [Broken]
    Last edited by a moderator: May 4, 2017
  24. Feb 1, 2009 #23
    It's an absurd argument. With approximately 200 billion stars, even if only 50% had only one world capable of sustaining their life (with the help of terraforming, for example), that still leaves 100 billion planets to colonize. I don't see how that's even remotely possible in a mere 1 million years.

    If somebody is going to put forth such a laughable argument, don't get upset when it gets laughed at.

    I concede that point, with the caveat since we exist, it puts an artifical lower limit of 1 when the equation is used to attempt to solve "How many are there right now," and not the more general "How many in any given period."

    That wasn't made obvious. More than one person was confused by it his assertion. It has been cleared up now.

    Taking my first semester of calculus-based physics starting tomorrow. I'll learn it in due time.
  25. Feb 1, 2009 #24


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  26. Feb 1, 2009 #25

    D H

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    It is anything but an absurd argument. Stop using appeals to ridicule. You are not making a good start here.

    Various estimates for how long it would take to colonize the galaxy range from 1 to 10 million years. Here are a couple in the middle of that range:

    "Assuming a typical colony spacing of 10 light-years, a ship speed of 10 percent
    that of light, and a period of 400 years between the foundation of a colony and its
    sending out colonies of its own, the colonization wave front will expand at an
    average speed of 0.02 light-year a year. As the galaxy is 100,000 light-years
    across, it takes no more than about five million years to colonize it completely." Source: Crawford, I.A., "Where are They? Maybe we are alone in the galaxy after all", Scientific American, July 2000, 38-43, (2000). http://www.cc.nctu.edu.tw/~tseng327/nctu-origin/article/sc200007a.pdf [Broken]

    "A colonizing, space-faring civilization which exercises population control will colonize the Galaxy in about 5×106 years. This result assumes population growth rates and emigration rates appropriate to human experience and a 0.1c ship speed. The results are independent of the optimum population. Only with extreme assumptions is the colonization wave velocity less than 0.01 c." Source: Jones, E.M., "Colonization of the Galaxy", Icarus, Volume 28, Issue 3, July 1976, Pages 421-422 (1976). http://www.sciencedirect.com/scienc...serid=10&md5=6ae4f42af7c7090b33cddd257abf7f3a

    You are misconstruing what the Drake equation is (beyond being a complete kludge). It is in essence a single-chain conditional probability calculation. The end result is the mean number of extant communicative civilizations at any point in time.
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