What speed do quasars eject matter out the galaxy?

  1. At what speed do quasars eject matter out the galaxy. I heard, I don't if I'm remembering correctly, that quasars can eject matter faster then the speed of light and I am very sure i got this information from the show "the universe" a couple years ago.
  2. jcsd
  3. It is possible that you heard such utter crap on a popularization TV show, but it is not true. Nothing travels faster than light. These shows are often mentioned here on this forum, with great derision.
  4. as crappy as popular science shows typically are, i doubt the OP heard such drivel. my guess is that the show mentioned something about superluminal motion, which is an optical illusion - after all, nothing can travel faster than light itself - and he probably just misunderstood or misinterpreted what was being said.
  5. One would hope so, but I think you may be underestimating the level of inanity of these shows. I watch a lot of them because they have terrific graphics and with the notable exception of Brian Cox they have good presenters, and most of what they present is good stuff. BUT ... I often find even seemingly serious physicists (e.g. Sean Carroll) spouting the most ridiculous nonsense.
  6. yeah many of them are more than misleading, often to the point of insanity
  7. Jonathan Scott

    Jonathan Scott 1,360
    Gold Member

    I expect what you heard is that quasars can eject matter which appears to move somewhat faster than light, in that features in the jets apparently move across the sky at a speed which would imply faster-than-light motion at the observed distance.

    The standard explanation for this is that the jet is in fact moving at a narrow angle towards us, so as the knot moves it is getting rapidly closer to us. This means we see the new position sooner than we see the old position, so it is actually taking much more time than we see to move a given distance, and the "superluminal" motion is therefore an illusion.

    It is however surprising how common this effect seems to be, in that we would expect jets to be randomly in all directions. Again, there is a standard explanation related to the fact that jets coming towards us will normally appear much brighter and hence be more likely to be observed.

    There is also a possibility that in some jets the moving features might be illusions too, in the same way for example that rotating a helix about its axis causes it to appear to be moving along the axis, but I think there are many cases where it is clear that this is not the explanation.

    Some quasars have visible pairs of jets in opposite directions. If apparent superluminal motion were observed in both jets, that would take a lot of explaining, as it would suggest that the relevant quasar was in fact much closer than previously thought. As far as I know, this has not yet happened. (The Wikipedia article on superluminal motion says "Superluminal motion is often seen in two opposing jets, one moving away and one toward Earth" which I think is misleading; it is only the one moving towards the Earth which is superluminal, as otherwise this would rule out the standard explanation).
  8. Say what ??? This would imply that it moved from the old position to the new position FTL.
  9. Jonathan Scott

    Jonathan Scott 1,360
    Gold Member

    I mean that you see the new position being reached sooner after it happens (in Earth frame) than you see the old position being reached, not that you see them in reverse order.
  10. Typically jets move at about 10% of the speed of light but they can be much faster.

    As several others have said, if the jet is angled almost towards us, there is an optical illusion effect that makes it look as though the speed is greater than the speed of light if the radial change of distance is ignored. There's a series of graphics illustrating it in slides 19 to 25 here:

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