Page 13 of 19 FirstFirst ... 31112131415 ... LastLast
Results 361 to 390 of 545

Thread: What do you think is the most likely explanation for the Fermi paradox?

  1. #361
    Join Date
    Mar 2004
    Posts
    18,329
    Quote Originally Posted by cosmocrazy View Post
    Absolutely, which is why it can be quite worrying knowing that we have already unintentionally leaked signals that could be detected and acted upon by a far more advanced species that have little to no concern about our survival. The threat maybe there and maybe there is nothing we can do about it already.

    Personally, I don't believe a more advanced species would feel the need to wipe us out. But again I'm making this assumption based on human nature and human logic.
    I call this the "paranoid species" argument. Our leaked signals generally aren't that big an issue - most would be undetectable at interstellar distances, except maybe some high powered radar. Intentional METI transmissions would be a bigger issue, but very little of that has been attempted. In any case, there would need to be a receiver at just the right place listening at just the right time.

    My key issue, though, is that if aliens want us dead, they could and should have done it a long time ago. Paranoid aliens could have blasted the planet long ago. There's no reason for them to wait until a species can develop spaceflight and be a potential danger to them.

    So I expect there either are no nearby aliens with interstellar technology, or if they exist, they aren't paranoid. I don't think it's at all likely that aliens could be near us, could destroy us, but wouldn't know about us - telescopes and space probes are just so much easier than causing destruction at interstellar distances.

    "The problem with quotes on the Internet is that it is hard to verify their authenticity." — Abraham Lincoln

    I say there is an invisible elf in my backyard. How do you prove that I am wrong?

    The Leif Ericson Cruiser

  2. #362
    Join Date
    Apr 2007
    Location
    Nowhere (middle)
    Posts
    36,941
    Maybe they already did attack. We think the Theia impact was caused by a small planet. Was it? Was it.... really?

    (Note: yes.)
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

  3. #363
    Join Date
    Oct 2009
    Location
    a long way away
    Posts
    10,674
    Here is an article on some new work on this: https://www.quantamagazine.org/galax...adox-20190307/

    I had always assumed that it was just because "space is big; I mean really, really big" but apparently the motion of stars makes that less of a problem: "Hart and others calculated that a single space-faring species could populate the galaxy within a few million years, and maybe even as quickly as 650,000 years."

    In their new paper, Carroll-Nellenback, Wright and their collaborators Adam Frank of Rochester and Caleb Scharf of Columbia University sought to examine the paradox without making untestable assumptions. They modeled the spread of a “settlement front” across the galaxy, and found that its speed would be strongly affected by the motions of stars, which previous work — including Sagan and Newman’s — treated as static objects. The settlement front could cross the entire galaxy based just on the motions of stars, regardless of the power of propulsion systems. “There is lots of time for exponential growth basically leading to every system being settled,” Carroll-Nellenback said
    But:

    the fact that no interstellar visitors are here now — what Hart called “Fact A” — does not mean they do not exist, the authors say. While some civilizations might expand and become interstellar, not all of them last forever. On top of that, not every star is a choice destination, and not every planet is habitable. There’s also what Frank calls “the Aurora effect,” after Kim Stanley Robinson’s novel Aurora, in which settlers arrive at a habitable planet on which they nonetheless cannot survive.

  4. #364
    Join Date
    Apr 2007
    Location
    Nowhere (middle)
    Posts
    36,941
    Quote Originally Posted by Strange View Post
    Here is an article on some new work on this: https://www.quantamagazine.org/galax...adox-20190307/

    I had always assumed that it was just because "space is big; I mean really, really big" but apparently the motion of stars makes that less of a problem: "Hart and others calculated that a single space-faring species could populate the galaxy within a few million years, and maybe even as quickly as 650,000 years."
    I'm of the opinion, as I've often reiterated, that an interstellar society would not need planets per se. However, I'm also convinced that not all civilizations that spread to the stars, will necessarily spread to every star either. Their priorities and goals might change away from expansion across the millennia of development and evolution, both natural and self-directed.

    regardless of the power of propulsion systems.
    ...Well, then how would they get to these moving stars? Gotta have some kind of propulsion.
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

  5. #365
    Join Date
    Oct 2009
    Location
    a long way away
    Posts
    10,674
    Quote Originally Posted by Noclevername View Post
    ...Well, then how would they get to these moving stars? Gotta have some kind of propulsion.
    It doesn't say they don't need any propulsion, just that they don't need near-lightspeed propulsion to cross the vast distances.

  6. #366
    Join Date
    Apr 2007
    Location
    Nowhere (middle)
    Posts
    36,941
    Quote Originally Posted by Strange View Post
    It doesn't say they don't need any propulsion, just that they don't need near-lightspeed propulsion to cross the vast distances.
    I guess I don't understand what "regardless of the power" of that propulsion means. There's got to be some set level of power to come up with such a specific time frame to move across the Galaxy.

    ADDED: Ah, I see. The article's author said this, but that term is not used in the abstract itself.
    Last edited by Noclevername; 2019-May-31 at 10:19 AM.
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

  7. #367
    Join Date
    Apr 2005
    Posts
    2,474
    Quote Originally Posted by Strange View Post
    Here is an article on some new work on this: https://www.quantamagazine.org/galax...adox-20190307/

    I had always assumed that it was just because "space is big; I mean really, really big" but apparently the motion of stars makes that less of a problem: "Hart and others calculated that a single space-faring species could populate the galaxy within a few million years, and maybe even as quickly as 650,000 years."



    But:
    I've made a similar point on previous threads. We have several close-ish approaches scheduled for the next few millennia, including one that could be within 1 light-year.

    Also, the galactic habitable zone would've started in the inner regions of the disk. The stars are closer together there, and close approaches more frequent than in our neighbourhood.

    The first civilisations to evolve would be faced with say one light-year to the next star instead of five (the average out here).

  8. #368
    Join Date
    Oct 2009
    Location
    a long way away
    Posts
    10,674
    Quote Originally Posted by kzb View Post
    Also, the galactic habitable zone would've started in the inner regions of the disk.
    Why is that?

  9. #369
    Join Date
    Mar 2008
    Posts
    5,583
    Quote Originally Posted by kzb View Post
    I've made a similar point on previous threads. We have several close-ish approaches scheduled for the next few millennia, including one that could be within 1 light-year.

    Also, the galactic habitable zone would've started in the inner regions of the disk. The stars are closer together there, and close approaches more frequent than in our neighbourhood.

    The first civilisations to evolve would be faced with say one light-year to the next star instead of five (the average out here).
    Wouldn't it also be more active with debris and such? My thinking being that a planet capable of supporting and developing life may be more prone to destructive bombardment? Thus making it less likely that life has enough time to evolve into a intelligent technological species. Again i'm just basing this on the Earth and what we know about our own history and evolution.

  10. #370
    Join Date
    Apr 2005
    Posts
    2,474
    Quote Originally Posted by cosmocrazy View Post
    Wouldn't it also be more active with debris and such? My thinking being that a planet capable of supporting and developing life may be more prone to destructive bombardment? Thus making it less likely that life has enough time to evolve into a intelligent technological species. Again i'm just basing this on the Earth and what we know about our own history and evolution.
    I think the dangers of high stellar density are taken into account when defining the GHZ. A few kpc radius around the centre of the galaxy was not included in some models, precisely because the rate of encounters and also of supernovae is/was too high.

    I'm not sure that "debris" is thought to be a danger in the inner galactic disk any more than it is here. The rate of encounters would presumably disrupt Oort clouds and lead to more cometary impacts but I don't know if that is a show stopper.

  11. #371
    Join Date
    Apr 2007
    Location
    Nowhere (middle)
    Posts
    36,941
    https://astrobiology.nasa.gov/news/g...bitable-zones/

    For one thing, the Sun is composed of the right amount of “metals.” (Astronomers refer to all elements heavier than hydrogen and helium as “metals.”) Moreover, the Sun’s circular orbit about the galactic center is just right; through a combination of factors it manages to keep out of the way of the Galaxy’s dangerous spiral arms. Our Solar System is also far enough away from the galactic center to not have to worry about disruptive gravitational forces or too much radiation.
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

  12. #372
    Join Date
    Apr 2007
    Location
    Nowhere (middle)
    Posts
    36,941
    Quote Originally Posted by Noclevername View Post
    I interpret the statement as meaning another series of uncommon Goldilocks factors: A safe and stable galactic orbit, plus being in an area just close enough to supernovae to be salted with metallicity and heavy elements without being sterilized by them.
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

  13. #373
    Join Date
    Sep 2003
    Posts
    12,834
    Quote Originally Posted by Noclevername View Post
    Quote Originally Posted by article
    Moreover, the Sun’s circular orbit about the galactic center is just right; through a combination of factors it manages to keep out of the way of the Galaxy’s dangerous spiral arms.
    Is that correct? The arms, IIRC, are density waves, like a heavy traffic on the highway that slows everyone down. Is this a traffic jam we somehow managed to avoid, for once?
    We know time flies, we just can't see its wings.

  14. #374
    Join Date
    Apr 2007
    Location
    Nowhere (middle)
    Posts
    36,941
    Quote Originally Posted by George View Post
    Is that correct? The arms, IIRC, are density waves, like a heavy traffic on the highway that slows everyone down. Is this a traffic jam we somehow managed to avoid, for once?
    Not always, it seems. The timing of the End-Permian Extinction suggests (according to John Gribbin) correlation, if not causality, with our passage through a Spiral Arm. But if so, why not more often? Timing is everything!
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

  15. #375
    Join Date
    Apr 2007
    Location
    Nowhere (middle)
    Posts
    36,941
    https://en.wikipedia.org/wiki/Galact...tic_morphology

    Various morphological features of galaxies can affect their potential for habitability. Spiral arms, for example, are the location of star formation, but they contain numerous giant molecular clouds and a high density of stars that can perturb a star's Oort cloud, sending avalanches of comets and asteroids toward any planets further in.[20] In addition, the high density of stars and rate of massive star formation can expose any stars orbiting within the spiral arms for too long to supernova explosions, reducing their prospects for the survival and development of life.[20] Considering these factors, the Sun is advantageously placed within the galaxy because, in addition to being outside a spiral arm, it orbits near the corotation radius, maximizing the interval between spiral-arm crossings.
    https://en.wikipedia.org/wiki/Corotation_circle
    The corotation circle is the circle around the galactic center of a spiral galaxy, where the stars move at the same speed as the spiral arms. The radius of this circle is called corotation radius. Inside the circle the stars move faster and outside they move slower than the spiral arms.
    The Sun is located near the corotation circle of the Milky Way.
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

  16. #376
    Join Date
    Apr 2007
    Location
    Nowhere (middle)
    Posts
    36,941
    Seriously, it seems to have taken so many Fortunate Coincidences to make us what we are that it almost makes me believe in the Anthropic Principle. Except of course, I realize that the Universe is a huge trial and error laboratory, playing with infinite variables over very long time scales. Roll enough dice enough times, and you will eventually come up with sevens a million times in a row. Makes me wonder what other numbers and patterns are out there.

    The existence and evolution of the human mind is at the far end of a very long probability curve. We are all Teela Brown.
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

  17. #377
    Join Date
    Apr 2005
    Posts
    2,474
    Quote Originally Posted by Noclevername View Post
    I interpret the statement as meaning another series of uncommon Goldilocks factors: A safe and stable galactic orbit, plus being in an area just close enough to supernovae to be salted with metallicity and heavy elements without being sterilized by them.
    We've been in and out of spiral arms several times.
    Also there was the crossing the galactic plane business, that was said to be hazardous. That was debunked.

  18. #378
    Join Date
    Apr 2007
    Location
    Nowhere (middle)
    Posts
    36,941
    Quote Originally Posted by kzb View Post
    We've been in and out of spiral arms several times.
    Yes, as previously pointed out.

    But the point was, we cross spiral arms much less often than most Milky Way stars.
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

  19. #379
    Join Date
    Apr 2005
    Posts
    2,474
    Quote Originally Posted by Strange View Post
    Why is that?
    The average metallicity of the Milky Way disk decreases with radius from the centre. The metallicity increases with time with the density of supernovae.

    The inner disk reached the correct metallicity for terrestrial planets earlier than the outer disk.

  20. #380
    Join Date
    Apr 2005
    Posts
    2,474
    Quote Originally Posted by Noclevername View Post
    Yes, as previously pointed out.

    But the point was, we cross spiral arms much less often than most Milky Way stars.
    Have there not been studies on extinction events, trying to correlate them with being in a spiral arm? I have a vague memory these failed, or were later debunked.

  21. #381
    Join Date
    Apr 2007
    Location
    Nowhere (middle)
    Posts
    36,941
    Quote Originally Posted by kzb View Post
    Have there not been studies on extinction events, trying to correlate them with being in a spiral arm? I have a vague memory these failed, or were later debunked.
    I have not found any recent research that supports either side. All the cites I have located are from 2014 or earlier.
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

  22. #382
    Join Date
    Apr 2007
    Location
    Nowhere (middle)
    Posts
    36,941
    OK, found this, May 15, 2018:

    https://www.cambridge.org/core/journ...342EE1CAA270E#

    High-density regions within the spiral arms are expected to have profound effects on passing stars. Understanding of the potential effects on the Earth and our Solar System is dependent on a robust model of arm passage dynamics. Using a novel combination of data, we derive a model of the timings of the Solar System through the spiral arms and the relationship to arm tracers such as methanol masers. This reveals that asteroid/comet impacts are significantly clustered near the spiral arms and within specific locations of an average arm structure. The end-Permian and end-Cretaceous extinctions emerge as being located within a small star-formation region in two different arms. The start of the Solar System, greater than 4.5 Ga, occurs in the same region in a third arm. The model complements geo-chemical data in determining the relative importance of extra-Solar events in the diversification and extinction of life on Earth.
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

  23. #383
    Join Date
    Apr 2005
    Posts
    2,474
    Quote Originally Posted by Noclevername View Post
    You can see the paper free here:

    http://eprints.lincoln.ac.uk/31796/

    Interesting, but I need a lot more convincing this is important.

    Point number one, the mechanism for extinctions is said to be impacts. But why would being in a spiral arm increase impacts ?

    Point number two, the timing of crossing spiral arms is heavily dependent on the figures. We are said to be catching up with spiral arms in our orbit. That means there is the subtraction of similar numbers problem. It would be easy to pick which numbers you want from the literature so as to come up with the required timing for extinctions.

  24. #384
    Join Date
    Apr 2007
    Location
    Nowhere (middle)
    Posts
    36,941
    Quote Originally Posted by kzb View Post
    Point number one, the mechanism for extinctions is said to be impacts. But why would being in a spiral arm increase impacts ?
    Stars get much closer together, and dust density goes much higher producing drag. These factors can disrupt the orbits of Oort cloud bodies. Also, new stars common in arms are more active and likely to SN.

    Point number two, the timing of crossing spiral arms is heavily dependent on the figures. We are said to be catching up with spiral arms in our orbit. That means there is the subtraction of similar numbers problem. It would be easy to pick which numbers you want from the literature so as to come up with the required timing for extinctions.
    You can always find a reason to doubt individual interpretations. Study the data instead.
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

  25. #385
    Join Date
    Apr 2005
    Posts
    2,474
    Quote Originally Posted by Noclevername View Post
    Stars get much closer together, and dust density goes much higher producing drag. These factors can disrupt the orbits of Oort cloud bodies. Also, new stars common in arms are more active and likely to SN.



    You can always find a reason to doubt individual interpretations. Study the data instead.
    I don't think the dust density can go that high. Yes to the SN density but the paper does not use that as its reason for extinctions.

    I did study the data and that was the conclusion I came to. You could select orbital period, orbital radius and pattern speeds from the literature to come up with the timings you want.

  26. #386
    Join Date
    Apr 2007
    Location
    Nowhere (middle)
    Posts
    36,941
    Quote Originally Posted by kzb View Post
    I don't think the dust density can go that high. Yes to the SN density but the paper does not use that as its reason for extinctions.
    And the gravitational disruptions by close passes of stars?
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

  27. #387
    Join Date
    Apr 2007
    Location
    Nowhere (middle)
    Posts
    36,941
    Quote Originally Posted by kzb View Post
    I don't think the dust density can go that high.
    What is the dust density in the spiral arms?
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

  28. #388
    Join Date
    Apr 2005
    Posts
    2,474
    Quote Originally Posted by Noclevername View Post
    What is the dust density in the spiral arms?
    If it was high enough to cause drag we wouldn't be able to see them.

  29. #389
    Join Date
    Apr 2005
    Posts
    2,474
    Quote Originally Posted by Noclevername View Post
    And the gravitational disruptions by close passes of stars?
    This is a more likely reason than drag from dust.

    But from memory I think the stellar density is hardly any greater in spiral arms then outside them, at the same galactic radius.

    They are brighter mostly because there are more young stars, not because the stellar density is vastly higher.

  30. #390
    Join Date
    Apr 2007
    Location
    Nowhere (middle)
    Posts
    36,941
    Quote Originally Posted by kzb View Post
    If it was high enough to cause drag we wouldn't be able to see them.
    Explain please?
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •