> In 2018, S2 made its closest approach to the black hole, giving us a chance to observe an effect of relativity known as gravitational redshift.
The reality seems more complicated because the star speeds up at the closest approach. When it speeds up, it is subject to time dilation. Time slows down more as it speeds up, and so any electromagnetic waves emitted by the body will have a lower frequency.
The effect of the speed up is not separable from the fact that it has dunked deeper into the gravitational field of the black hole, which also contributes time dilation.
> If you shine a beam of light into the sky, the light doesn't slow down, but gravity does take away some of its energy.
The red shift that we see is purely a time dilation effect. So that is to say, if there were, say, a 101.5 Mhz radio station on that star, we would see that at a lower Mhz figure purely due to the oscillator of that station appearing to be slower due to time dilation. Someone riding that star, clapping their hands once per second might look like they are clapping once every 1.2 seconds.
The beam of light which is conveying to us the events from a source cannot alter the frequency of those events, even if it changes speed along the way. If we see 1000 events per second, then it means the source is generating 1000 events per second, according to our frame of reference. It cannot be that the source is generating 1200, but then the light somehow subtracts from that due to losing energy while escaping gravity.
The reality seems more complicated because the star speeds up at the closest approach. When it speeds up, it is subject to time dilation. Time slows down more as it speeds up, and so any electromagnetic waves emitted by the body will have a lower frequency.
The effect of the speed up is not separable from the fact that it has dunked deeper into the gravitational field of the black hole, which also contributes time dilation.
> If you shine a beam of light into the sky, the light doesn't slow down, but gravity does take away some of its energy.
The red shift that we see is purely a time dilation effect. So that is to say, if there were, say, a 101.5 Mhz radio station on that star, we would see that at a lower Mhz figure purely due to the oscillator of that station appearing to be slower due to time dilation. Someone riding that star, clapping their hands once per second might look like they are clapping once every 1.2 seconds.
The beam of light which is conveying to us the events from a source cannot alter the frequency of those events, even if it changes speed along the way. If we see 1000 events per second, then it means the source is generating 1000 events per second, according to our frame of reference. It cannot be that the source is generating 1200, but then the light somehow subtracts from that due to losing energy while escaping gravity.