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I Spacetime moving faster than speed of light?

  1. Jul 2, 2017 #1
    I was watching a seminar on black holes and one of the participants threw out as an aside that spacetime could travel faster than the speed of light - so within spacetime nothing could travel faster than spacetime but spacetime itself however was not bound by this rule so it could move faster than the speed of light. Is that correct? If so, is there some speed postulated that that even space-time could not move faster than?
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  3. Jul 2, 2017 #2


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    Sounds like nonsense to me. "Space-time" is a framework, not something solid that moves. However I often find that there are parts of cosmology I know nothing about so this could be one of those times. Also, "faster than c relative to WHAT?
  4. Jul 2, 2017 #3
    In the beginning, space inflated at a rate vastly exceeding the speed of light. This was perhaps mediated by a hypothetical particle called the inflaton. Since space isn't actually made of stuff to which laws like those governing baryonic matter apply, this could occur. I suspect that is to what the lecture was referring.
  5. Jul 2, 2017 #4


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    Yeah, that could be it, but I certainly would not apply the word "travel" to that. Recession is different than traveling, else we would have to say that the galaxies out at the outer reaches of our observable universe are "traveling" at about 3c, which would be ridiculous. They are receding at about 3c.

    debroglie, I'm not arguing with you at all here, I've added my comments to my agreement w/ you for the sake of the OP.
  6. Jul 2, 2017 #5
    It's hard to come up with proper language in English that describes the incredibly complicated things that go on or have gone on in the universe. I'm not well versed enough in the relevant mathematics to really say anything much with accuracy. I certainly agree, though. "Travel" implies things that aren't occurring, as far as I'm aware.
  7. Jul 2, 2017 #6


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    Distances between objects can increase or decrease faster than c. Is this "motion of space faster than light"? A matter of semantics I guess. It is not a motion of spacetime, because spacetime is the overall construct - it cannot move in time, it includes the time dimension already.
  8. Jul 2, 2017 #7
    Mind expanding stuff indeed. Yes, a thing expanding through itself seems a contradiction. Makes sense. I wish I understood the maths a bit better, apparently it all becomes much clearer.
  9. Jul 2, 2017 #8


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    People sometimes do try to explain black holes in this manner: for instance we have "The River Model of Black Holes", http://aapt.scitation.org/doi/10.1119/1.2830526 Probably the main reason why this approach is not very popular is that the concept of "the speed of space-time" is something that is not physically observable according to special relativity. So the theory is described in terms of concepts that don't have any observable physical basis.

    This is a rather serious criticism, and I seem to recall some comments by the author that he had difficulty getting his paper published, presumably for this very reason. If you overlook this significant problem with the presentation, though, the idea can be made to work correctly, in spite of the problems with the lack of an underlying physical basis.

    Probably my biggest concern with this approach is that if presented to a target audience unfamiliar with special relativity who do not realize that "the speed of space-time" is something that's not physically observable, they will be confused on some important points about what special relativity says. This is relevant because general relativity is built on top of special relativity as a foundation, though many casual science readers attempt (and mostly fail, IMO) to learn something about general relativity without first understanding special relativity :(.

    That said, this presentation does avoid some of the issues with the "time stops at the event horizon" approach to black holes, an approach that is also widely misunderstood and frequently leads to misconceptions about black holes and incorrect conclusions.

    Another concern I have is how popular this idea is among professionals. This would best be measured by it's impact rating. I strongly suspect that it's impact rating would be rather low, indicating a lack of interest in the approach by professionals, but I don't have any hard data on that - I know that there are databases that try to estimate the "impact factor" of published papers via tracking citations, but I'm not sure where to get the raw data.
  10. Jul 2, 2017 #9
    Thanks for all the feedback and the distinction between traveling and recession/expansion. I dug this up - https://www.universetoday.com/119068/how-can-space-travel-faster-than-the-speed-of-light/ It seems to put some flesh on the barebones statement of something moving faster than the speed of light. If any one has time to peruse and provide feedback as to whether the flesh makes a scientifically satisfactory meal it would be appreciated.
  11. Jul 2, 2017 #10


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    If you look at the units, you will see that they don't match up in this statement.
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