3 ms·

LIGO/Virgo/KAGRA found that speed of light in vacuum is affected by gravitational waves. This is verified experimentally to very high precision (1.2E-20m). If m

LIGO/Virgo/KAGRA found that speed of light in vacuum is affected by gravitational waves. This is verified experimentally to very high precision (1.2E-20m). If merge of neutron stars 200 megaparsec away can affect propagation of light here, on Earth, then why it cannot affect it near to the star?

ithkuil 2y ago
ok I think we're talking past each other. let's take a step back. Let's imagine two points in space A and B, that are let's say 10 light minutes distant from each other. A signal going straight from A to B will thus take 10 minutes. If point A sits in a strong gravitational field (e.g. it's orbiting a very heavy star), the signal will still take 10 light minutes to reach point B. (please tell me if you disagree with this assumption). Now, let's place another heavy star at the midpoint between points A and B. How long will it take for a photon emitted by A to reach B? Well, it won't reach it because it will hit the start that's in between. But another photon whose direction wasn't directly in the path from A to B will follow a longer path, be deflected around the star and reach point B. It will take longer than 10 minutes to reach point B because it will move along a longer path. Do you agree this is what would happen?
- cyberax 2y ago
  > How long will it take for a photon emitted by A to reach B? Well, it won't reach it because it will hit the start that's in between. Now imagine that it's not a star, but a black hole with a small radius to make arguments easier. You shoot a photon slightly off the axis, and it gets deflected. You can try to treat a photon as a moving object, and integrate the forces acting on it. Taking Lorentz transformations into account, of course. But the thing is, your calculations will be off, and the experimental results won't match your predictions. You will need to take into account that the lightspeed near massive objects is _slower_ for distant observers. Another example, suppose that you have a star surrounded by a massive cloud of fog. Somebody shoots a laser beam from one side of the fog bank to another, while you are far away from the star. The fog is there just to allow you to see the beam as it moves, it does not by itself slow the light. But you will actually see the light moving _slower_ than lightspeed! Or equivalently, you can take a clock that ticks every second. And if you lower that clock to the surface of a planet, you will see the clock ticking slower. And this is a very real effect, we have to correct for it in the GPS satellites.
  - ithkuil 2y ago
    Yes the clock will indeed tick slower deep in the gravity well and that's the reason why from the point of view of the observer outside the gravity well the photon will be red shifted! The speed of light is the same in both frames of reference. What you think is going affect the speed is actually the slowing of the proper time which effectively causes the photon to redshift.
    - cyberax 2y ago
      > The speed of light is the same in both frames of reference. What you think is going affect the speed is actually the slowing of the proper time which effectively causes the photon to redshift. No. You can drop a ruler onto the surface of Earth and measure from the Moon the time it takes the light to travel from one end of the ruler to the other. It will be slower than the lightspeed from your point of view. This is a real effect, we've measured it. However, it will be lightspeed from the point of view of an Earth observer.
      - ithkuil 2y ago
        Ok but that's exactly what it means when we say that clocks are ticking slower in a gravity well (after all you can implement a clock by bouncing light). And this effect falls out directly from the warping of space-time described by general relativity am I understanding correctly that you claim that the warping of space-time is just a mathematical trick and that the phenomena are better explained by just postulating they light slows down in a gravity well?
        cyberax 2y ago
        > am I understanding correctly that you claim that the warping of space-time is just a mathematical trick and that the phenomena are better explained by just postulating they light slows down in a gravity well? Light slows down in gravity wells because the space-time is "denser" near massive objects. This is not a mathematical trick, this is actually a real effect. It's also the reason for gravitational lensing, as the shortest path through a gravity well is not a straight line. Light can avoid the slowdown near the massive object, if it instead "goes around" it. The curved path is longer, but faster lightspeed along it compensates for the additional length.

ithkuil 2y ago

ok I think we're talking past each other. let's take a step back. Let's imagine two points in space A and B, that are let's say 10 light minutes distant from each other. A signal going straight from A to B will thus take 10 minutes. If point A sits in a strong gravitational field (e.g. it's orbiting a very heavy star), the signal will still take 10 light minutes to reach point B. (please tell me if you disagree with this assumption). Now, let's place another heavy star at the midpoint between points A and B. How long will it take for a photon emitted by A to reach B? Well, it won't reach it because it will hit the start that's in between. But another photon whose direction wasn't directly in the path from A to B will follow a longer path, be deflected around the star and reach point B. It will take longer than 10 minutes to reach point B because it will move along a longer path. Do you agree this is what would happen?

cyberax 2y ago
> How long will it take for a photon emitted by A to reach B? Well, it won't reach it because it will hit the start that's in between. Now imagine that it's not a star, but a black hole with a small radius to make arguments easier. You shoot a photon slightly off the axis, and it gets deflected. You can try to treat a photon as a moving object, and integrate the forces acting on it. Taking Lorentz transformations into account, of course. But the thing is, your calculations will be off, and the experimental results won't match your predictions. You will need to take into account that the lightspeed near massive objects is _slower_ for distant observers. Another example, suppose that you have a star surrounded by a massive cloud of fog. Somebody shoots a laser beam from one side of the fog bank to another, while you are far away from the star. The fog is there just to allow you to see the beam as it moves, it does not by itself slow the light. But you will actually see the light moving _slower_ than lightspeed! Or equivalently, you can take a clock that ticks every second. And if you lower that clock to the surface of a planet, you will see the clock ticking slower. And this is a very real effect, we have to correct for it in the GPS satellites.
- ithkuil 2y ago
  Yes the clock will indeed tick slower deep in the gravity well and that's the reason why from the point of view of the observer outside the gravity well the photon will be red shifted! The speed of light is the same in both frames of reference. What you think is going affect the speed is actually the slowing of the proper time which effectively causes the photon to redshift.
  - cyberax 2y ago
    > The speed of light is the same in both frames of reference. What you think is going affect the speed is actually the slowing of the proper time which effectively causes the photon to redshift. No. You can drop a ruler onto the surface of Earth and measure from the Moon the time it takes the light to travel from one end of the ruler to the other. It will be slower than the lightspeed from your point of view. This is a real effect, we've measured it. However, it will be lightspeed from the point of view of an Earth observer.
    - ithkuil 2y ago
      Ok but that's exactly what it means when we say that clocks are ticking slower in a gravity well (after all you can implement a clock by bouncing light). And this effect falls out directly from the warping of space-time described by general relativity am I understanding correctly that you claim that the warping of space-time is just a mathematical trick and that the phenomena are better explained by just postulating they light slows down in a gravity well?
      - cyberax 2y ago
        > am I understanding correctly that you claim that the warping of space-time is just a mathematical trick and that the phenomena are better explained by just postulating they light slows down in a gravity well? Light slows down in gravity wells because the space-time is "denser" near massive objects. This is not a mathematical trick, this is actually a real effect. It's also the reason for gravitational lensing, as the shortest path through a gravity well is not a straight line. Light can avoid the slowdown near the massive object, if it instead "goes around" it. The curved path is longer, but faster lightspeed along it compensates for the additional length.