4 ms·

If a star is moving at nearly the speed of light relative to us, doesn't that also mean that we're moving at nearly the speed of light relative to them? If tha

If a star is moving at nearly the speed of light relative to us, doesn't that also mean that we're moving at nearly the speed of light relative to them? If that's so, wouldn't that mean that the Earth is a "hypervelocity planet" in the article's terminology.. at least relative to some other objects in the universe?

NikolaeVarius 6y ago
Yes.
JoeAltmaier 6y ago
Well, every cosmic ray makes you hypervelocity too I guess. But to be meaningful you want to talk about speed relative to your local galactic frame or some such. Most of the mass around us is moving relatively slowly to one another. That makes these fast stars an anomoly. And it means their (relative) effect is going to be interesting.
tectonic 6y ago
That's right. From their perspective, a hypervelocity galaxy is moving past them.
simonh 6y ago
The difference is these stars experienced a massive acceleration, while we didn’t, so it’s not actually a symmetric relationship.
- savant_penguin 6y ago
  From the point of view of the star didn't we accelerate though? The twin paradox never made sense to me. I've never understood the asymmetry
  - enkid 6y ago
    Acceleration isn't relative, velocity is. An object undergoes acceleration when a force is applied to it. Objects "know" when they are being accelerated, just like how you know you're being pulled down by gravity. The same can't be safe for velocity.
  - MichaelApproved 6y ago
    When you accelerate away from me in an elevator, only one of us experiences additional forces during that acceleration. Only your body feels heavier as the elevator speeds up. Here’s a minute physics video that shows how acceleration explains the twins paradox (the “common” explanation) https://youtu.be/0iJZ_QGMLD0 https://youtu.be/0iJZ_QGMLD0 After you watch minute physics video, you might feel like you understand. Nope, can’t have any of that. Now, you must watch this Fermi lab video that says the common explanation is “not fundamentally correct” https://youtu.be/noaGNuQCW8A https://youtu.be/noaGNuQCW8A
    - wruza 6y ago
      Fermilab video is hard to grasp intuitively to me, but one important thing it did was it cleared that confusing misconception of acceleration, which was induced by [too many] other videos. After watching these incorrect videos it was still unclear why acceleration even matters, because with 1g accel in space you can reach subluminal speed in a matter of days, and then, at the distant location, turn back with just 2x long 1g again, and then “brake” with 1g again to land on earth. While the twin on earth experienced that 1g all the time. One may even pick a distance and a (probably hyperbolic-y) route with rotations so that all of the journey would consist of a constant 1g for a flying twin, exactly as a sitting one experiences. Put them both into opaque boxes, knock senseless for a couple of hours at launch, and neither of them would even tell who is where, until opened.
    - savant_penguin 6y ago
      Nice video from fermilab I still don't get it though
      - tsimionescu 6y ago
        I think the most important point if you want an intuitive explanation is that the observer who is in the same place at the beginning as at the end has aged more. If you were to ask which of the twins is older at the time the second twin reaches the far away star, the answer is that the question doesn't make sense. They are in different places, so there is no way to compute a fixed time. Imagine each twin is broadcasting a video signal of their face for the entire duration of the trip. The twin on earth puts a big red dot on the screen the moment that the traveling twin had reached the distant star in their frame of reference; while the twin on the ship would start putting a big red dot on their video the moment they reach the star in their own. Because the speed of light is limited, over the whole duration of the departure trip, for both twins, the latest image they see of their sibling will be younger than themselves. When the twin on the ship reaches the star and starts emitting the red dot, they will have aged L/gamma * v years. By the time the twin on Earth sees this image, they will have aged L/v + L/c years = L(c+v)/c*v years, so they are [L(c+v)/c*v] / (L/gamma * v) = gamma(c+v)/c years older. The twin on Earth will start emitting the red dot after L/v earth years, or L/gamma*v ship years. This signal will meet the ship after L/(gamma*(c - v)) ship years from the moment it was sent - so in total, L/gamma*v + L/gamma*(c-v) = L*c/gamma*v*(c-v) ship years. So, they are [L*c/gamma*v*(c-v)] / (L/gamma*v) = c/(c-v) ship years older than their sibling, which is gamma*c/(c-v) earth years. While both see that they are older than their sibling at the time they first see their sibling's red dot, there is an asymmetry coming from the fact that one of them is at the same position as they were initially, while the other one is at a different position, relative to the star. Equivalently, there is an asymmetry that is caused by their different definitions of "the moment the ship reaches the star".
  - ganafagol 6y ago
    The asymmetry comes from only one of the twins experiencing a force causing it to accellerate at the point where it turns around to come back.
jessriedel 6y ago
These sorts of things are generally measured with respect to the galactic rest frame or the rest frame of the CMB. Since the vast majority of stars are moving much slower, hypervelocity stars are not on the same footing.
robscallsign 6y ago
yes, but it means that everything around us - the sun, the solar system, the Galaxy are also "hypervelocity" relative to any such object, thus making our own "hypervelocity" label not that special.
hibbelig 6y ago
It's like wrong way driving: If everyone else is moving in the same direction / the same speed, only you are different, then maybe you're driving the wrong way / fast.

NikolaeVarius 6y ago

Yes.

JoeAltmaier 6y ago

Well, every cosmic ray makes you hypervelocity too I guess. But to be meaningful you want to talk about speed relative to your local galactic frame or some such. Most of the mass around us is moving relatively slowly to one another. That makes these fast stars an anomoly. And it means their (relative) effect is going to be interesting.

tectonic 6y ago

That's right. From their perspective, a hypervelocity galaxy is moving past them.

simonh 6y ago

The difference is these stars experienced a massive acceleration, while we didn’t, so it’s not actually a symmetric relationship.

savant_penguin 6y ago
From the point of view of the star didn't we accelerate though? The twin paradox never made sense to me. I've never understood the asymmetry
- enkid 6y ago
  Acceleration isn't relative, velocity is. An object undergoes acceleration when a force is applied to it. Objects "know" when they are being accelerated, just like how you know you're being pulled down by gravity. The same can't be safe for velocity.
- MichaelApproved 6y ago
  When you accelerate away from me in an elevator, only one of us experiences additional forces during that acceleration. Only your body feels heavier as the elevator speeds up. Here’s a minute physics video that shows how acceleration explains the twins paradox (the “common” explanation) https://youtu.be/0iJZ_QGMLD0 https://youtu.be/0iJZ_QGMLD0 After you watch minute physics video, you might feel like you understand. Nope, can’t have any of that. Now, you must watch this Fermi lab video that says the common explanation is “not fundamentally correct” https://youtu.be/noaGNuQCW8A https://youtu.be/noaGNuQCW8A
  - wruza 6y ago
    Fermilab video is hard to grasp intuitively to me, but one important thing it did was it cleared that confusing misconception of acceleration, which was induced by [too many] other videos. After watching these incorrect videos it was still unclear why acceleration even matters, because with 1g accel in space you can reach subluminal speed in a matter of days, and then, at the distant location, turn back with just 2x long 1g again, and then “brake” with 1g again to land on earth. While the twin on earth experienced that 1g all the time. One may even pick a distance and a (probably hyperbolic-y) route with rotations so that all of the journey would consist of a constant 1g for a flying twin, exactly as a sitting one experiences. Put them both into opaque boxes, knock senseless for a couple of hours at launch, and neither of them would even tell who is where, until opened.
  - savant_penguin 6y ago
    Nice video from fermilab I still don't get it though
    - tsimionescu 6y ago
      I think the most important point if you want an intuitive explanation is that the observer who is in the same place at the beginning as at the end has aged more. If you were to ask which of the twins is older at the time the second twin reaches the far away star, the answer is that the question doesn't make sense. They are in different places, so there is no way to compute a fixed time. Imagine each twin is broadcasting a video signal of their face for the entire duration of the trip. The twin on earth puts a big red dot on the screen the moment that the traveling twin had reached the distant star in their frame of reference; while the twin on the ship would start putting a big red dot on their video the moment they reach the star in their own. Because the speed of light is limited, over the whole duration of the departure trip, for both twins, the latest image they see of their sibling will be younger than themselves. When the twin on the ship reaches the star and starts emitting the red dot, they will have aged L/gamma * v years. By the time the twin on Earth sees this image, they will have aged L/v + L/c years = L(c+v)/c*v years, so they are [L(c+v)/c*v] / (L/gamma * v) = gamma(c+v)/c years older. The twin on Earth will start emitting the red dot after L/v earth years, or L/gamma*v ship years. This signal will meet the ship after L/(gamma*(c - v)) ship years from the moment it was sent - so in total, L/gamma*v + L/gamma*(c-v) = L*c/gamma*v*(c-v) ship years. So, they are [L*c/gamma*v*(c-v)] / (L/gamma*v) = c/(c-v) ship years older than their sibling, which is gamma*c/(c-v) earth years. While both see that they are older than their sibling at the time they first see their sibling's red dot, there is an asymmetry coming from the fact that one of them is at the same position as they were initially, while the other one is at a different position, relative to the star. Equivalently, there is an asymmetry that is caused by their different definitions of "the moment the ship reaches the star".
- ganafagol 6y ago
  The asymmetry comes from only one of the twins experiencing a force causing it to accellerate at the point where it turns around to come back.

jessriedel 6y ago

These sorts of things are generally measured with respect to the galactic rest frame or the rest frame of the CMB. Since the vast majority of stars are moving much slower, hypervelocity stars are not on the same footing.

robscallsign 6y ago

yes, but it means that everything around us - the sun, the solar system, the Galaxy are also "hypervelocity" relative to any such object, thus making our own "hypervelocity" label not that special.

hibbelig 6y ago

It's like wrong way driving: If everyone else is moving in the same direction / the same speed, only you are different, then maybe you're driving the wrong way / fast.