# Does light slow down (relativistic) near a black hole?

• Buzzworks
In summary, light always travels at a constant velocity of c in a vacuum, regardless of the observer's location. However, near a black hole, time slows down so much that light cannot escape, making it appear to travel slower to an outside observer. This is due to the curvature of space caused by the black hole's mass. Despite this, the speed of light remains constant in all frames of reference.
Buzzworks
Does photons actually slow down near a black hole?

Whenever I look up these info, I always read the mention of red shifting.

The problem is we don't have a detector that is immune from the effects of relativity to directly observe these events in real time.

However, does current theories show light slowing near black hole, if only there's a way for an outside observer to directly observe light near a black hole in real time?

No, light, in vacuum, always has velocity c. What happens is that its energy changes which, for light, does not depend upon velocity but upon frequency and wave length.

time slows down near a black hole
everything including light is affected equally

Light has to travel at the speed of light, by definition. Its speed is fixed.

Its not the light the changes at a black hole, its time, time slows down.
In a black hole time slows down so much that light even at its very high speed cannot get anywhere.

The red shifting is due to the dopplier effect, its to do with the body generating the light you are seeing, either moving away from your are towards you.

As light cannot go any other speed that the speed of light, if a body is moving towards you the excess energy of it moving towards you increases the frequency of the light you see.

I think the shifting of light from objects moving towards or away from you is not the same effect as light not being able to escape from a black hole.

On a black home time is going so slow that light would go backwards, that means the light would 'shine' on the surface of the black hole, and no light would or could be emmited.

At a certain point away from the black hole where the gravity is not slowing down time as much the light would be able to stand still (the inverse speed of time = the speed of light) which would represent the event horizen of the black hole. and after that point an outside observer would be able to see light radiated.

I watched a doco on the SUN recently and it said that the light from the center of the sun can take 10,000 Earth years to reach the surface, then another 8 minutes to reach earth.

So the photons we get from the sun are not 8 minutes old, but 10,000 years and 8 minutes.. :)

But the light itself, because it travels at the speed of light experiences no time, therefore no distance. They just start to exist and then cease to exist for them, even if its an infinite time between the photon being created and destroyed.

Thanks a lot guys! I think I got my question wrong. I'm aware that c is always the same everywhere.

So if I understand it now correctly, to an observer far away from a black hole, light will appear to travel slower near a black hole, but an observer near a black hole, observing the same light, that light travels at c relative to him/her.

sort of, but not quite, no matter where you are, or from what "frame of reference" you are in, the speed of light will look the same to you.

So if you are far away from a black hole and light travels past that BH to you, you do not see any different in the speed of the light. No matter where you are.

Gravitational lensing, or the 'bending of light" around a massive object is not because the light itself changes, it is because the space around the massive object (a black hole, or galaxy) has changed its shape.

So if the space itself is not a straight line, the photons of light have to travel on the curve of real space/time.

So the path is a bit longer from the source of the light to your eyes, and the path is longer the effect would appear that the photons took a longer time to get to you.

So the speed of light, any light, as long as you can see it, will be traveling at a fixed speed, no matter where you are, how fast you are going, or the rate of the progression of time.

As time is the bit that changes, actually the time changes near large mass, it runs slower.

So on Earth light will go 300,000km per second, but if your second is 10 times longer than my second then the speed of light will be a constant 300,000,0 Km per Second.

Its the same speed, but time is longer with high mass like a black hole.
Its the same speed, but measured with different time frames of reference. Time is ticking much slower on a black hole. So slow, that light can't escape because it takes more time than it has to escape.

But in a vacuum, no matter how you look at it, light is going only one speed.
Even if your own time if different than that of the source of the light, or what that light passes close too, it does not change, but time does.

Darryl said:
sort of, but not quite, no matter where you are, or from what "frame of reference" you are in, the speed of light will look the same to you.
Locally. But speed of light "elsewhere" can appear to fluctuate, so crapworks' recap is quite valid.

Check out Alcubierre Drive, if you aren't familiar with it already. It relies on the fact that speed of light is fixed only in local space.

crapworks said:
Thanks a lot guys! I think I got my question wrong. I'm aware that c is always the same everywhere.

So if I understand it now correctly, to an observer far away from a black hole, light will appear to travel slower near a black hole, but an observer near a black hole, observing the same light, that light travels at c relative to him/her.

Imagine a very long rod extending from somewhere near an event horizon with an observer and mirror and mirror at each end. If the observer at the top shines a light downwards it may take thousands of years for the reflection to return to the top observer, while if the observer at the bottom of the rod shines a light upwards he might see the reflection return in seconds. So in this sense light really is slower or faster in a gravitational field if the light is not local to the observer, but locally it always measured to be c.

Thanks Darryl, K^2, and Yuiop! Never heard of those illustrations before!

I'll take it that our detection technology is quite limited as of yet, especially in Darryl's explanation. I'll also take it that all these explanations are theoretical because no one has actually seen light slow down.

Perhaps if someone were to invent a special type of telescope that could observe light traveling in any direction (not just towards the telescope) in real time, thus immune to the effects of relativity. Then we'll actually see a light beam slow down whenever it's going near a strong gravity field if current theories prove to be correct.

yuiop said:
Imagine a very long rod extending from somewhere near an event horizon with an observer and mirror and mirror at each end. If the observer at the top shines a light downwards it may take thousands of years for the reflection to return to the top observer, while if the observer at the bottom of the rod shines a light upwards he might see the reflection return in seconds. So in this sense light really is slower or faster in a gravitational field if the light is not local to the observer, but locally it always measured to be c.

So why does the same rationale not apply to all gravitational redshift - the explanation of wavelenght change is never attributed to a change in photon velocity - yet the idea of light not being able to escape from a black hole seems to imply that as the gravitational field gets stronger (e.g., if mass were added to a G source gradually) the photon speed is gradually slowed eventually to the point of no escape. I suppose the answer is that a local observer in any hi G field will always measure the same photon velocity because of the slowed time of the measuring clock - but is there a way that a distant observer can distinquish the difference - if not - then from a cosmological time perspective does it make any differrence whether we say a G field slows light or changes the photon frequency?

crapworks said:
Thanks Darryl, K^2, and Yuiop! Never heard of those illustrations before!

I'll take it that our detection technology is quite limited as of yet, especially in Darryl's explanation. I'll also take it that all these explanations are theoretical because no one has actually seen light slow down.

Perhaps if someone were to invent a special type of telescope that could observe light traveling in any direction (not just towards the telescope) in real time, thus immune to the effects of relativity. Then we'll actually see a light beam slow down whenever it's going near a strong gravity field if current theories prove to be correct.

No they are not theoretical. Light slow down has actually been measured by sending radar signals to Venus and back (and Mercury). This is known as Shapiro time delay. See http://en.wikipedia.org/wiki/Shapiro_delay

yogi said:
- but is there a way that a distant observer can distinquish the difference - if not - then from a cosmological time perspective does it make any differrence whether we say a G field slows light or changes the photon frequency?

Yes, the difference can be distinguished. If you have a variety of light sources at a location (A) all with different frequencies that all emit a signal simultaneously, then they all arrive at some receptor B at the same time. Clearly the frequency of light has no effect on its speed, so a frequency change can not cause a delay in a radar signals. Measured time delays in Shapiro time delay type experiments are not explained by a change in photon frequency.

Last edited:
"Locally. But speed of light "elsewhere" can appear to fluctuate, so crapworks' recap is quite valid."

I think you may observe fluctuations in the speed of light from the same source, but I would explain that by the two photons taking different paths to get to your detector.

If you get two cars, going from point A to point B, both at 100Mph, one car goes in a striagh line and the other car travels in a large arc or curve.

The from your observations at point B the two cars will arrive at different times, they speed is the same. but one had to travel on a longer path.

If you cannot 'see' the paths the two cars traveled on, you would have to assume from your observation of the arrival time of the cars that one car was traveling faster than the other.

The only way you can tell that the two cars took different paths to your detector would be if you knew that the two cars traveled at exactly the same speed. Then you can conclude that the two cars must of traveled different paths and distance to get to you.

So allthough you can observe some light photons arriving at your detector later than others (from the same source) that is because one photon or some of the photons had to travel a longer path than the other.

Making it appear that one is going either slower or faster than the other, even if that is not the case.

I still believe that regardless of the rate of time in space, if a photon changes its speed from that of the speed of light it ceases to be light anymore.

yuiop said:
Yes, the difference can be distinguished. If you have a variety of light sources at a location (A) all with different frequencies that all emit a signal simultaneously, then they all arrive at some receptor B at the same time. Clearly the frequency of light has no effect on its speed, so a frequency change can not cause a delay in a radar signals. Measured time delays in Shapiro time delay type experiments are not explained by a change in photon frequency.
If it was true that frequency affected the speed of light, then we would expect all the high frequency radiation from a Supernova explosion to arrive first at out telescopes and then all the low frequency radiation would arrive some time later (or vice versa), but this does not happen in actual observations.

yuiop said:
If it was true that frequency affected the speed of light, then we would expect all the high frequency radiation from a Supernova explosion to arrive first at out telescopes and then all the low frequency radiation would arrive some time later (or vice versa), but this does not happen in actual observations.

Let's say sender is A and I'm B.
1 - A sends frequency "f" foton (light wave) to B, "f" does not matter to time
2 - A sends several fotons in row to B, time between fotons is not identical

I'm not now sure am i correct or totally wrong. But if we see light spectrum from light, redshift is that all colors are moving to red end. So that part can change. But all colors arrive at same time (even they have different frequencies).

Still from other point of view. Single light wave always comes in same time when sent at same time. But if sending a two waves, time difference between waves can be different.

So light travels at same speed, but frequency between fotons can change between A & B. As if A sends X fotons per sec, and B has longer sec (example 1.003 sec relative to A), B receives X fotons per 1.003 sec.

Everyone says that light stays the same speed but i thought when molecules get extremely cold it becomes a Bose-Einstein Condensate. Light through a condensate slows down to the point where it is observable by humans. Black holes are calculated to be colder the bigger they are... Is it possible that light moves slower in a black hole because it creates a natural condensate?

paulino1013 said:
Everyone says that light stays the same speed but i thought when molecules get extremely cold it becomes a Bose-Einstein Condensate. Light through a condensate slows down to the point where it is observable by humans. Black holes are calculated to be colder the bigger they are... Is it possible that light moves slower in a black hole because it creates a natural condensate?
Not everyone says that light "stays the same speed"; for a distant observer, light progresses slower near a black hole than further away from it.

Apart of that, the speed of light in vacuum is not known to be affected by temperature; and indeed, that concept doesn't make much sense in vacuum!

In a vacuum no, you are right. I was thinking about within the black hole itself, my mistake...

I was thinking the black hole is all particles. All super cold particles that gets colder the heavier it gets and light in that is probably moving at a crawl. But we will never know for certain. If i fly into a black hole, i wouldn't know if I was right or wrong for long.

paulino1013 said:
In a vacuum no, you are right. I was thinking about within the black hole itself, my mistake...
Light slows down outside the black hole, so whatever you think the temperature inside the bh is, it is not relevant to the speed of light outside. Temperature affects the rate of some things such as chemical reactions but has no effect on other things like a good clock. This is the difference between temperature slow down and gravitational time dilation. Time dilation slows down everything equally.

Now when I say "Light slows down outside the black hole" this is not strictly true without specifying that I mean as measured in Schwarzschild coordinates by a distant observer, just as when another person says "The speed of light is always constant" is not telling the whole truth and nothing but the truth, because it is only true for a local observer in a vacuum.

Now if a photon is orbiting a black hole at the photon sphere radius, the time measured for the photon to complete an orbit according to a distant observer is 1.732 times greater than the orbital time measured by a local observer, so the distant observer concludes that the speed of light near a black hole must be relatively slower than the speed of light far from a black hole.

Does photons actually slow down near a black hole?

"Light slows down outside the black hole" this is not strictly true without specifying that I mean as measured in Schwarzschild coordinates by a distant observer, just as when another person says "The speed of light is always constant" is not telling the whole truth and nothing but the truth, because it is only true for a local observer in a vacuum.

Those [correct] observations are based on the fact that in flat spacetime, no gravity, the speed of light is always observed as 'c' locally or distant. So even in a curved spacetime, LOCALLY spacetime can be considered flat, and all appear normal; but with curved spacetime over distances and times, distant observations will give generally different values of 'c'. As noted already, none of this depends on the energy [frequency] of the light...all frequencies travel at 'c' locally...

## 1. Does light slow down near a black hole?

Yes, light does slow down near a black hole due to the immense gravitational pull of the black hole. This effect is known as gravitational time dilation, where time appears to slow down for an outside observer as an object gets closer to a massive object like a black hole.

## 2. Is this slowdown in light speed due to the black hole's gravity?

Yes, the slowdown of light near a black hole is a result of the black hole's intense gravity. The strong gravitational pull of the black hole causes space-time to warp, which affects the speed of light.

## 3. Does light always slow down near a black hole?

Yes, the slowdown of light near a black hole is a fundamental aspect of Einstein's theory of general relativity. This effect is observed for any object, including light, that gets close enough to a black hole.

## 4. How close do you have to be to a black hole for light to significantly slow down?

The closer an object, including light, gets to a black hole, the more its speed will slow down. However, the exact distance at which light significantly slows down can vary depending on the mass of the black hole and the speed of the light. Generally, the closer you are to the event horizon (the point of no return for a black hole), the more significant the effects of gravitational time dilation will be.

## 5. Can light escape a black hole's gravitational pull?

No, light cannot escape a black hole's gravitational pull once it crosses the event horizon. This is because the escape velocity (the speed needed to break free from a massive object's gravitational pull) at the event horizon is faster than the speed of light. Therefore, even light cannot escape the intense gravity of a black hole once it passes the event horizon.

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