24 ms·

We wrote about this in Orbital Index recently (https://orbitalindex.com https://orbitalindex.com) and so I did some background research: Loeb's paper [1], menti

We wrote about this in Orbital Index recently (https://orbitalindex.com https://orbitalindex.com) and so I did some background research: Loeb's paper [1], mentioned in the article, found that hypervelocity stars accelerated by massive black hole binaries could hypothetically reach as much as *half the speed of light*. This is not something you want to be anywhere near. Relatedly, Chinese astronomers recently published research on 591 high-velocity stars in our galaxy [2, 3], and 43 of these were hypervelocity stars, moving at relativistic speeds and destined to eventually escape the Milky Way entirely. They used China's LAMOST optical telescope and ESA's Gaia and doubled the number of known high-velocity stars. Apparently this is useful for measuring the mass of the Milky Way. [1] https://arxiv.org/abs/1411.5030 https://arxiv.org/abs/1411.5030 [2] http://english.cas.cn/newsroom/research_news/phys/202012/t20201228_260875.shtml http://english.cas.cn/newsroom/research_news/phys/202012/t20... [3] https://iopscience.iop.org/article/10.3847/1538-4365/abc16e https://iopscience.iop.org/article/10.3847/1538-4365/abc16e P.S. HN needs a better way to make links.

jessriedel 5y ago
What fraction of c qualifies as "relativistic" in this case?
- ineedasername 5y ago
  Generally it's whatever speed that forces you to account for relativity for you desired precision. However, I've seen 10% referenced as a lower limit.
  - jessriedel 5y ago
    That's generally true, but it's not meaningful in this case. If I say "One in 10,000 stars in the Milky way is a hypervelocity star moving at relativistic speeds", there is no context to provide a threshold for the magnitude of relativistic corrections that are important. (And if it's just about escape velocity, they should just say that. The Milky Way's escape velocity is 550 km/s = 0.002 c, for which relativistic corrections are extremely tiny: \gamma ~= 1.0000017.)
    - ineedasername 5y ago
      Yes, absolutely-- the general claim lacks enough specificity to fully evaluate it. Should there be a need for extremely high precision, even somewhat "slow" speeds would be considered relativistic in terms of the need to take such corrections into account.
kingsuper20 5y ago
> This is not something you want to be anywhere near. Oh jeez. First I find out about Gliese 710, now this.
- taneq 5y ago
  Sounds dramatic, but on my scale of existential dread, nowhere near up there with simply being trapped in a human body where there's no offsite backups and you (or anyone else close to you) can just get an aneurysm and die at any moment. I'm not sure I'm helping. >.>
  - fsloth 5y ago
    I don't think death of the individual is something to be dreaded. I suggest meditating on your own death until the thought no longer causes anxiety. Death of the individual is a completely natural part of the cycle of life and should not be dreaded beyond the fact that it hurts like hell when it happens to someone you love.
    - deleted 5y ago
      [deleted]
    - colordrops 5y ago
      It seems like it hurts like hell when it happens to you in most cases as well, unless it's sudden.
      - fsloth 5y ago
        Moment of death might be full of horror and torment but there is no reason to make it worse by stressing about it your whole life. Worrying about death is like going to a movie, and being so sad throughout the movie it will end at some point that you fail to enjoy the movie.
        PicassoCTs 5y ago
        This seems to be just another form of denial of existential dread to me. Draped in the ever fashionable "I can reshape anything by wishing upon it" but to be honest, i rather be around someone who suffers through and faces his mortal fears, then somebody thinking around them with some barely disguised religious rituals. The later is much less likely to lash out, when his "protective" bubble against existentialist fear is penetrated. Its also indicative of a curious mindset, if you stare into your own emotional blindspot, to find out what you are made of. Of course, it can become a self-destructive behaviour, destroying ones ability to function, which suppressing the fear in the end is.
daniel-thompson 5y ago
What would life be like for an Earth-like planet that orbited a star like that? What about spaceflight (e.g. from a planet to a moon) within that solar system? And what would the rest of the universe look like to them?
- Thorondor 5y ago
  Life, including spaceflight, would be pretty much the same as it is for us. Since the entire solar system would be moving, the relative velocities of the planets and moons would be the same as in any other solar system. The rest of the universe would look quite different, though. Stars "ahead" of the solar system - in the path of its motion - would be noticeably blue-tinted and clumped together. Stars behind the system would be red-shifted. There are some great visualizations at [0]. [0] https://math.ucr.edu/home/baez/physics/Relativity/SR/Spaceship/spaceship.html https://math.ucr.edu/home/baez/physics/Relativity/SR/Spacesh...
  - swayvil 5y ago
    The rest of the universe might age very quickly, relatively speaking. Stars might pass through their lifecycle in a matter of weeks. Constellations might deform over days.
    - Semiapies 5y ago
      Not at just half the speed of light.
      - MichaelApproved 5y ago
        Not even at 99.999% the speed of light. You’d need to even faster.
    - catillac 5y ago
      There is a bad-but-good hard SF book about this general subject called Tau Zero. You’d definitely need to be going much faster than 0.5c to get anything close to the effects you’re describing, but that book is a fun read if you’re interested in the subject.
Keysh 5y ago
"Chinese astronomers recently published research on 591 high-velocity stars in our galaxy [2, 3], and 43 of these were hypervelocity stars, moving at relativistic speeds and destined to eventually escape the Milky Way entirely." All of those stars have velocities < 1000 km/s relative to the Milky Way, which is less than 1/3 of one percent of the speed of light. Calling that "relativistic" is just hyperbole.

jessriedel 5y ago

What fraction of c qualifies as "relativistic" in this case?

ineedasername 5y ago
Generally it's whatever speed that forces you to account for relativity for you desired precision. However, I've seen 10% referenced as a lower limit.
- jessriedel 5y ago
  That's generally true, but it's not meaningful in this case. If I say "One in 10,000 stars in the Milky way is a hypervelocity star moving at relativistic speeds", there is no context to provide a threshold for the magnitude of relativistic corrections that are important. (And if it's just about escape velocity, they should just say that. The Milky Way's escape velocity is 550 km/s = 0.002 c, for which relativistic corrections are extremely tiny: \gamma ~= 1.0000017.)
  - ineedasername 5y ago
    Yes, absolutely-- the general claim lacks enough specificity to fully evaluate it. Should there be a need for extremely high precision, even somewhat "slow" speeds would be considered relativistic in terms of the need to take such corrections into account.

kingsuper20 5y ago

> This is not something you want to be anywhere near. Oh jeez. First I find out about Gliese 710, now this.

taneq 5y ago
Sounds dramatic, but on my scale of existential dread, nowhere near up there with simply being trapped in a human body where there's no offsite backups and you (or anyone else close to you) can just get an aneurysm and die at any moment. I'm not sure I'm helping. >.>
- fsloth 5y ago
  I don't think death of the individual is something to be dreaded. I suggest meditating on your own death until the thought no longer causes anxiety. Death of the individual is a completely natural part of the cycle of life and should not be dreaded beyond the fact that it hurts like hell when it happens to someone you love.
  - deleted 5y ago
    [deleted]
  - colordrops 5y ago
    It seems like it hurts like hell when it happens to you in most cases as well, unless it's sudden.
    - fsloth 5y ago
      Moment of death might be full of horror and torment but there is no reason to make it worse by stressing about it your whole life. Worrying about death is like going to a movie, and being so sad throughout the movie it will end at some point that you fail to enjoy the movie.
      - PicassoCTs 5y ago
        This seems to be just another form of denial of existential dread to me. Draped in the ever fashionable "I can reshape anything by wishing upon it" but to be honest, i rather be around someone who suffers through and faces his mortal fears, then somebody thinking around them with some barely disguised religious rituals. The later is much less likely to lash out, when his "protective" bubble against existentialist fear is penetrated. Its also indicative of a curious mindset, if you stare into your own emotional blindspot, to find out what you are made of. Of course, it can become a self-destructive behaviour, destroying ones ability to function, which suppressing the fear in the end is.

daniel-thompson 5y ago

What would life be like for an Earth-like planet that orbited a star like that? What about spaceflight (e.g. from a planet to a moon) within that solar system? And what would the rest of the universe look like to them?

Thorondor 5y ago
Life, including spaceflight, would be pretty much the same as it is for us. Since the entire solar system would be moving, the relative velocities of the planets and moons would be the same as in any other solar system. The rest of the universe would look quite different, though. Stars "ahead" of the solar system - in the path of its motion - would be noticeably blue-tinted and clumped together. Stars behind the system would be red-shifted. There are some great visualizations at [0]. [0] https://math.ucr.edu/home/baez/physics/Relativity/SR/Spaceship/spaceship.html https://math.ucr.edu/home/baez/physics/Relativity/SR/Spacesh...
- swayvil 5y ago
  The rest of the universe might age very quickly, relatively speaking. Stars might pass through their lifecycle in a matter of weeks. Constellations might deform over days.
  - Semiapies 5y ago
    Not at just half the speed of light.
    - MichaelApproved 5y ago
      Not even at 99.999% the speed of light. You’d need to even faster.
  - catillac 5y ago
    There is a bad-but-good hard SF book about this general subject called Tau Zero. You’d definitely need to be going much faster than 0.5c to get anything close to the effects you’re describing, but that book is a fun read if you’re interested in the subject.

Keysh 5y ago

"Chinese astronomers recently published research on 591 high-velocity stars in our galaxy [2, 3], and 43 of these were hypervelocity stars, moving at relativistic speeds and destined to eventually escape the Milky Way entirely." All of those stars have velocities < 1000 km/s relative to the Milky Way, which is less than 1/3 of one percent of the speed of light. Calling that "relativistic" is just hyperbole.