Question about gravity and time

[ParanoiA]

Much thanks for going to all the trouble Timo.

 

The keyword is gravitational redshift.

 

But isn't the shift of color due to differences in gravity as opposed to differences in time? To be clear, my hypothetical involves no exaggerated change in gravity - no change in gravity at all in fact from our "actual" physical universe. Only the calibration of time is exaggerated here.

 

Not sure if calibration is the right word. In our actual universe, you don't notice any difference in gravity when you climb down into a 20 foot hole. In my hypothetical, this is exactly the same. However, in our actual universe you also won't notice any difference in time when you climb down into that hole, but in my hypothetical the difference in time is staggering as you climb down the hole. The relationship between time and gravity is what I've changed in my hypothetical universe.

 

 

 

Also, I'm trying to make sense of that article you linked.

 

In physics, light or other forms of electromagnetic radiation of a certain wavelength originating from a source placed in a region of stronger gravitational field (and which could be said to have climbed "uphill" out of a gravity well) will be found to be of longer wavelength when received by an observer in a region of weaker gravitational field.

 

If I'm interpreting this correctly, this seems to say that if I'm standing at the surface looking down in the well, I will see a color shift toward red down there due to longer wavelengths.

 

Light that has passed "downhill" into a region of stronger gravity shows a corresponding increase in energy, and is said to be gravitationally blueshifted.

 

And this seems to me to say that if I'm standing in the well, looking up to the surface, I will notice a color shift toward blue up there, due to shorter wavelengths.

 

If I have that right, then why am I not getting this part:

 

The receiving end of the light transmission must be located at a higher gravitational potential in order for gravitational redshift to be observed. In other words, the observer must be standing "uphill" from the source. If the observer is at a lower gravitational potential than the source, a gravitational blueshift can be observed instead.

 

Ok, now we're talking gravitational potential, instead of field. So maybe I'm confused because of that consequence. I don't know. But this seems to me to contradict the statements above...that I must be standing *in* the well (higher gravitational potential ?) for redshift to be observed - but then restates this to say the observer must be standing "uphill" from the source (the source being in the well).

 

Clearly I'm not getting that.

[swansont]

But isn't the shift of color due to differences in gravity as opposed to differences in time? To be clear, my hypothetical involves no exaggerated change in gravity - no change in gravity at all in fact from our "actual" physical universe. Only the calibration of time is exaggerated here.

 

Not sure if calibration is the right word. In our actual universe, you don't notice any difference in gravity when you climb down into a 20 foot hole. In my hypothetical, this is exactly the same. However, in our actual universe you also won't notice any difference in time when you climb down into that hole, but in my hypothetical the difference in time is staggering as you climb down the hole. The relationship between time and gravity is what I've changed in my hypothetical universe.

 

It's not due to a change in the strength of gravity (the wiki article is awkwardly worded; this holds for a constant g as well). It's due to the position in the field, which is the gravitational potential. That just means the hole is pretty deep if g is small, or if the hole is shallow, g must be very large. The dilation is gh/c^2 for constant g.

 

The time dilation and redshift are the same thing. The oscillation frequency changes. If you have a clock in a potential well, that means the phase accumulates more slowly. If you look at the oscillator itself, it shifts to a lower frequency.

 

Also, I'm trying to make sense of that article you linked.

 

 

 

If I'm interpreting this correctly, this seems to say that if I'm standing at the surface looking down in the well, I will see a color shift toward red down there due to longer wavelengths.

 

 

 

And this seems to me to say that if I'm standing in the well, looking up to the surface, I will notice a color shift toward blue up there, due to shorter wavelengths.

 

If I have that right, then why am I not getting this part:

 

 

 

Ok, now we're talking gravitational potential, instead of field. So maybe I'm confused because of that consequence. I don't know. But this seems to me to contradict the statements above...that I must be standing *in* the well (higher gravitational potential ?) for redshift to be observed - but then restates this to say the observer must be standing "uphill" from the source (the source being in the well).

 

Clearly I'm not getting that.

 

As I said before, "field" doesn't matter — it's always the potential. You can't see light when it's at the other end of the well. You can only see it when it hits your eye. For two points (source and observer), one in the well and one outside of it: When the light source is in a well, it will be redshifted. When the observer is in the well, the light is blueshifted.