Light near a gravitational field.

[geordief]

When a beam of light approaches a Black Hole "head on" would  a stationary* observer between it and the Black Hole notice any  change in it as it  approaches?

 

If it cannot go any faster than c  but is being pulled into the Black Hole , does the beam  become more energetic ?

 

*For the observer to be stationary near a Black Hole   ,he must,I suppose  be accelerating towards the beam of light  and so would that explain the increased energy between him and the oncoming beam of light ?(if indeed there is an increase in energy)

 

 

Edited November 16, 2017 by geordief

[Janus]

He would see gravitational Doppler shift in the light.  Light, like anything else, has to account for the difference in gravitational potential when moving through a gravitational field,  Light does this by changing its frequency.

[geordief]

4 minutes ago, Janus said:

He would see gravitational Doppler shift in the light.  Light, like anything else, has to account for the difference in gravitational potential when moving through a gravitational field,  Light does this by changing its frequency.

So the same applies to a beam of light emerging from a gravitational well? It loses energy (its frequency is lowered)  but maintains a constant  speed?

[Janus]

Just now, geordief said:

So the same applies to a beam of light emerging from a gravitational well? It loses energy (its frequency is lowered)  but maintains a constant  speed?

yes

[geordief]

Just now, Janus said:

yes

thanks

[swansont]

Pound and Rebka demonstrated this in a tower, showing there was a frequency shift of light as it moved up or down in a gravitational potential.

[geordief]

6 minutes ago, swansont said:

Pound and Rebka demonstrated this in a tower, showing there was a frequency shift of light as it moved up or down in a gravitational potential.

 Is there any sense in which the frequency of light can be seen as another  measure of the light's "speed"?

You can say the frequency "slows down",can't you ? In that sense  can one say that the speed  of light has no  limit** or  am I misunderstanding  the phenomenon  as an up/down motion when it really isn't there? 

 

**or a different limit

Edited November 16, 2017 by geordief

[swansont]

1 hour ago, geordief said:

 Is there any sense in which the frequency of light can be seen as another  measure of the light's "speed"?

In a vacuum the product of wavelength and frequency will give you c. Regardless of the frequency.

1 hour ago, geordief said:

You can say the frequency "slows down",can't you ? In that sense  can one say that the speed  of light has no  limit** or  am I misunderstanding  the phenomenon  as an up/down motion when it really isn't there? 

 

**or a different limit

It travels at c. 

[Country Boy]

If you are thinking of the wave property of light as an "up down motion" in space then, yes, you are misunderstanding.  The "wave" is an "increase, decrease" of the electro-magnetic field strength.

[Itoero]

The corona of a black hole is assumed to have relativistic electrons which scatter lower energy photons from the thermal spectrum  to a higher energy (increase in frequency).https://en.wikipedia.org/wiki/Compton_scattering#Inverse_Compton_scattering

This might imply light gets blueshifted when it approaches a black hole.

[geordief]

1 hour ago, HallsofIvy said:

If you are thinking of the wave property of light as an "up down motion" in space then, yes, you are misunderstanding.  The "wave" is an "increase, decrease" of the electro-magnetic field strength.

Yes that was my half misunderstanding.That whole area is a bit outside comfort zone.

1 hour ago, swansont said:

In a vacuum the product of wavelength and frequency will give you c. Regardless of the frequency.

 

That is interesting. Does that  deconstruct c into spatial and temporal components?

[swansont]

57 minutes ago, geordief said:

 That is interesting. Does that  deconstruct c into spatial and temporal components?

I'm not sure what that would mean. It's a speed.  

[geordief]

1 hour ago, swansont said:

I'm not sure what that would mean. It's a speed.  

Yes ,after I posted  I realized it wasn't saying or asking  much as a question. (just trivially true?)

 

Even so I was not really clear on the  wavelength x frequency =speed (c) formula so hopefully I am now.

[Truden]

Excuse my layman's question, but what do you mean by [ligt] "approaches"?
There is no such thing as light approaching.
You're either hit by the light and you see it, or you don't see it until it reaches you.
You cannot see it approaching you. 

[swansont]

3 minutes ago, Truden said:

Excuse my layman's question, but what do you mean by [ligt] "approaches"?
There is no such thing as light approaching.
You're either hit by the light and you see it, or you don't see it until it reaches you.
You cannot see it approaching you. 

But you could, in principle, make measurements along the path of the light. As the light gets closer to the black hole, the frequency will change.

There is nothing wrong with saying "as the light approaches the black hole"

[Truden]

Quote

There is nothing wrong with saying "as the light approaches the black hole"

 

 

OK, that's why I asked to be excused.
It just doesn't sound right to me.
To my understanding, light cannot be observed in motion. We can only register its arrival. But again, I'm not familiar with the physics terminology.

Edited November 17, 2017 by Truden

[swansont]

8 minutes ago, Truden said:

OK, that's why I asked to be excused.
It just doesn't sound right to me.
To my understanding, light cannot be observed in motion. We can only register its arrival. But again, I'm not familiar with the physics terminology.

Even so, there's nothing wrong with saying that light approaches a black hole, and asking what happens to it.

Just like there's nothing wrong with saying the light that a sodium lamp emits is yellow, near 589 nm (there are actually two lines), even if you aren't looking at the light.

[Country Boy]

But there is something wrong with asking if an observer would notice a change in the light "as it approaches".  While the light is approaching the observer he cannot yet observe it!  If "as it approaches" refers to approaching the black hole, again, the observer won't notice any "change", he only sees the light at one instant.