How to explain gravitational lensing to a layman
This question is about how to explain gravitational lensing to a layman, not about exact theoretical correctness.
I am accustomed to explaining optical refraction in terms of wavefronts and the fact that light moves at different speeds in different media. For example, I explain an optical lens by saying “A plane wave that is incident normal to the flat surface of a plano-convex lens will propagate slower inside the lens. This means that the portions of the wavefront that exit the lens first, near the edges, will end up ahead of the portions that exit near the middle of the lens. This results in the wavefronts downstream from the lens having a concave curvature. Because the wave propagates perpendicular to the wavefront, it converges to a point.”
I would like to use an analogous explanation to describe the effects of a gravitational lens. Although the local speed of light is invariant in a vacuum, gravity causes the local frequency and wavelength of light in an initially flat, monochromatic, wavefront to vary with radial distance from a gravitating mass. When the light wave has passed far enough downstream from the gravitating mass, the frequency and wavelength return to their original values, but the wavefront is curved because of the changes experienced by different parts of the wavefront while passing through the gravitational field of the mass: the parts of the wavefront near the middle are delayed relative to those near the edges.
I've been trying to refine this explanation, but am running into a complication that, thus far, is beyond my skills. I understand time dilation in terms of gravitational redshift and blueshift. The complication is that both the wavelength and frequency of light (from the perspective of a distant observer) would seem to be affected by the gravitational field. It's not obvious to me how wavelength is affected by gravity. Locally, the wavelength change should balance the frequency change in such a way that the speed of light is c. This suggests that the apparent wavelength as judged by a distant observer should also change.
I don't know how to explain this in an intuitively satisfying way, probably because I don't understand well enough how a distant observer can remotely measure the length of something that is deep in a gravitational well. Any help will be greatly appreciated.
general-relativity gravity gravitational-lensing
add a comment |
This question is about how to explain gravitational lensing to a layman, not about exact theoretical correctness.
I am accustomed to explaining optical refraction in terms of wavefronts and the fact that light moves at different speeds in different media. For example, I explain an optical lens by saying “A plane wave that is incident normal to the flat surface of a plano-convex lens will propagate slower inside the lens. This means that the portions of the wavefront that exit the lens first, near the edges, will end up ahead of the portions that exit near the middle of the lens. This results in the wavefronts downstream from the lens having a concave curvature. Because the wave propagates perpendicular to the wavefront, it converges to a point.”
I would like to use an analogous explanation to describe the effects of a gravitational lens. Although the local speed of light is invariant in a vacuum, gravity causes the local frequency and wavelength of light in an initially flat, monochromatic, wavefront to vary with radial distance from a gravitating mass. When the light wave has passed far enough downstream from the gravitating mass, the frequency and wavelength return to their original values, but the wavefront is curved because of the changes experienced by different parts of the wavefront while passing through the gravitational field of the mass: the parts of the wavefront near the middle are delayed relative to those near the edges.
I've been trying to refine this explanation, but am running into a complication that, thus far, is beyond my skills. I understand time dilation in terms of gravitational redshift and blueshift. The complication is that both the wavelength and frequency of light (from the perspective of a distant observer) would seem to be affected by the gravitational field. It's not obvious to me how wavelength is affected by gravity. Locally, the wavelength change should balance the frequency change in such a way that the speed of light is c. This suggests that the apparent wavelength as judged by a distant observer should also change.
I don't know how to explain this in an intuitively satisfying way, probably because I don't understand well enough how a distant observer can remotely measure the length of something that is deep in a gravitational well. Any help will be greatly appreciated.
general-relativity gravity gravitational-lensing
Can't you just say that light is deflected by gravity just like everything else? Getting into frequency is just overcomplicating and it's actually not very relevant once you get into the math. Gravitational lensing doesn't depend on the frequency.
– Javier
2 hours ago
The way you’re trying to think about strikes me as way too complicated for a layman. I would just tell someone that the path of the photons gets bent by the gravitating mass, so that they “focus” as if passing through a lens.
– G. Smith
2 hours ago
@Javier, I understand that all frequencies of light are affected identically by a gravitational field, just as all frequencies of light are affected identically by a lens made of a material having no spectral dispersion. I also know it's valuable to explain (and understand) things from multiple perspectives. So, I'd like to be able to explain how gravity affects a propagating plane light wave, rather than just say that gravity bends light rays.
– S. McGrew
2 hours ago
How about "light takes the (well, a) shortest path", so gravity acts like a lens, and several paths are locally shortest.
– JEB
2 hours ago
add a comment |
This question is about how to explain gravitational lensing to a layman, not about exact theoretical correctness.
I am accustomed to explaining optical refraction in terms of wavefronts and the fact that light moves at different speeds in different media. For example, I explain an optical lens by saying “A plane wave that is incident normal to the flat surface of a plano-convex lens will propagate slower inside the lens. This means that the portions of the wavefront that exit the lens first, near the edges, will end up ahead of the portions that exit near the middle of the lens. This results in the wavefronts downstream from the lens having a concave curvature. Because the wave propagates perpendicular to the wavefront, it converges to a point.”
I would like to use an analogous explanation to describe the effects of a gravitational lens. Although the local speed of light is invariant in a vacuum, gravity causes the local frequency and wavelength of light in an initially flat, monochromatic, wavefront to vary with radial distance from a gravitating mass. When the light wave has passed far enough downstream from the gravitating mass, the frequency and wavelength return to their original values, but the wavefront is curved because of the changes experienced by different parts of the wavefront while passing through the gravitational field of the mass: the parts of the wavefront near the middle are delayed relative to those near the edges.
I've been trying to refine this explanation, but am running into a complication that, thus far, is beyond my skills. I understand time dilation in terms of gravitational redshift and blueshift. The complication is that both the wavelength and frequency of light (from the perspective of a distant observer) would seem to be affected by the gravitational field. It's not obvious to me how wavelength is affected by gravity. Locally, the wavelength change should balance the frequency change in such a way that the speed of light is c. This suggests that the apparent wavelength as judged by a distant observer should also change.
I don't know how to explain this in an intuitively satisfying way, probably because I don't understand well enough how a distant observer can remotely measure the length of something that is deep in a gravitational well. Any help will be greatly appreciated.
general-relativity gravity gravitational-lensing
This question is about how to explain gravitational lensing to a layman, not about exact theoretical correctness.
I am accustomed to explaining optical refraction in terms of wavefronts and the fact that light moves at different speeds in different media. For example, I explain an optical lens by saying “A plane wave that is incident normal to the flat surface of a plano-convex lens will propagate slower inside the lens. This means that the portions of the wavefront that exit the lens first, near the edges, will end up ahead of the portions that exit near the middle of the lens. This results in the wavefronts downstream from the lens having a concave curvature. Because the wave propagates perpendicular to the wavefront, it converges to a point.”
I would like to use an analogous explanation to describe the effects of a gravitational lens. Although the local speed of light is invariant in a vacuum, gravity causes the local frequency and wavelength of light in an initially flat, monochromatic, wavefront to vary with radial distance from a gravitating mass. When the light wave has passed far enough downstream from the gravitating mass, the frequency and wavelength return to their original values, but the wavefront is curved because of the changes experienced by different parts of the wavefront while passing through the gravitational field of the mass: the parts of the wavefront near the middle are delayed relative to those near the edges.
I've been trying to refine this explanation, but am running into a complication that, thus far, is beyond my skills. I understand time dilation in terms of gravitational redshift and blueshift. The complication is that both the wavelength and frequency of light (from the perspective of a distant observer) would seem to be affected by the gravitational field. It's not obvious to me how wavelength is affected by gravity. Locally, the wavelength change should balance the frequency change in such a way that the speed of light is c. This suggests that the apparent wavelength as judged by a distant observer should also change.
I don't know how to explain this in an intuitively satisfying way, probably because I don't understand well enough how a distant observer can remotely measure the length of something that is deep in a gravitational well. Any help will be greatly appreciated.
general-relativity gravity gravitational-lensing
general-relativity gravity gravitational-lensing
asked 2 hours ago
S. McGrew
6,77021027
6,77021027
Can't you just say that light is deflected by gravity just like everything else? Getting into frequency is just overcomplicating and it's actually not very relevant once you get into the math. Gravitational lensing doesn't depend on the frequency.
– Javier
2 hours ago
The way you’re trying to think about strikes me as way too complicated for a layman. I would just tell someone that the path of the photons gets bent by the gravitating mass, so that they “focus” as if passing through a lens.
– G. Smith
2 hours ago
@Javier, I understand that all frequencies of light are affected identically by a gravitational field, just as all frequencies of light are affected identically by a lens made of a material having no spectral dispersion. I also know it's valuable to explain (and understand) things from multiple perspectives. So, I'd like to be able to explain how gravity affects a propagating plane light wave, rather than just say that gravity bends light rays.
– S. McGrew
2 hours ago
How about "light takes the (well, a) shortest path", so gravity acts like a lens, and several paths are locally shortest.
– JEB
2 hours ago
add a comment |
Can't you just say that light is deflected by gravity just like everything else? Getting into frequency is just overcomplicating and it's actually not very relevant once you get into the math. Gravitational lensing doesn't depend on the frequency.
– Javier
2 hours ago
The way you’re trying to think about strikes me as way too complicated for a layman. I would just tell someone that the path of the photons gets bent by the gravitating mass, so that they “focus” as if passing through a lens.
– G. Smith
2 hours ago
@Javier, I understand that all frequencies of light are affected identically by a gravitational field, just as all frequencies of light are affected identically by a lens made of a material having no spectral dispersion. I also know it's valuable to explain (and understand) things from multiple perspectives. So, I'd like to be able to explain how gravity affects a propagating plane light wave, rather than just say that gravity bends light rays.
– S. McGrew
2 hours ago
How about "light takes the (well, a) shortest path", so gravity acts like a lens, and several paths are locally shortest.
– JEB
2 hours ago
Can't you just say that light is deflected by gravity just like everything else? Getting into frequency is just overcomplicating and it's actually not very relevant once you get into the math. Gravitational lensing doesn't depend on the frequency.
– Javier
2 hours ago
Can't you just say that light is deflected by gravity just like everything else? Getting into frequency is just overcomplicating and it's actually not very relevant once you get into the math. Gravitational lensing doesn't depend on the frequency.
– Javier
2 hours ago
The way you’re trying to think about strikes me as way too complicated for a layman. I would just tell someone that the path of the photons gets bent by the gravitating mass, so that they “focus” as if passing through a lens.
– G. Smith
2 hours ago
The way you’re trying to think about strikes me as way too complicated for a layman. I would just tell someone that the path of the photons gets bent by the gravitating mass, so that they “focus” as if passing through a lens.
– G. Smith
2 hours ago
@Javier, I understand that all frequencies of light are affected identically by a gravitational field, just as all frequencies of light are affected identically by a lens made of a material having no spectral dispersion. I also know it's valuable to explain (and understand) things from multiple perspectives. So, I'd like to be able to explain how gravity affects a propagating plane light wave, rather than just say that gravity bends light rays.
– S. McGrew
2 hours ago
@Javier, I understand that all frequencies of light are affected identically by a gravitational field, just as all frequencies of light are affected identically by a lens made of a material having no spectral dispersion. I also know it's valuable to explain (and understand) things from multiple perspectives. So, I'd like to be able to explain how gravity affects a propagating plane light wave, rather than just say that gravity bends light rays.
– S. McGrew
2 hours ago
How about "light takes the (well, a) shortest path", so gravity acts like a lens, and several paths are locally shortest.
– JEB
2 hours ago
How about "light takes the (well, a) shortest path", so gravity acts like a lens, and several paths are locally shortest.
– JEB
2 hours ago
add a comment |
1 Answer
1
active
oldest
votes
I think the distinction that you are making, with concerns about the intermediate blueshifts, redshifts, and time dilation, is too technical for someone whose education is outside of physics altogether. I would start with something simpler and add complexity as the responses of your audience suggest appropriate directions to go.
When I explain about gravitational lensing, I use an approach like this:
The equivalence principle predicts that light should also fall in a gravitational field. (And experiments on Earth confirm this, but the details are tricky.) So if a distant star/galaxy and a nearby star/galaxy are along the same line of sight, light from the distant object that would have missed us can get bent back towards us by the nearby star's gravity, similar the way that a magnifying lens makes parallel light rays converge. I usually end up drawing some variant of the below:
[source]
Beware that most people's experience with magnifying lenses is to look through the lens at a virtual image. Looking through a magnifier with enough distance to see the inverted real image is a surprise for many adults, and putting your eye at the focus is just plain strange-looking. So you may have some intuition failures to cope with if you use this approach.
If they get that gravity can bend light and make images of things appear in the "wrong" place, I ask them to remember a time they walked through a parking lot and noticed a dent on a shiny car. You don't actually see the dent. What you see is the reflection in the car of the car's surroundings. The reflection in the body of the car is distorted a little, because a car isn't flat like a mirror. And where there is a dent, there is more and different distortion. Sometimes you'll see the same object reflected more than once, from different sides of the dent. Most people have seen this enough times that they have a pretty good idea of the size and shape of the dent just from this reflected-image information: walking up to the car and touching the dent is not usually a surprise. And in the same way, an image like this one
[source]
lets you say things about the distribution of matter in the foreground galaxy cluster that's doing the lensing.
This little spiel has served me well.
As I was posting this, I saw your comment that the wavefront explanation for lensing is what you're really hoping to explain, and that you suspect the reason it's hard to come up with a layman's explanation is that your own understanding is incomplete. I still think (as I did when I started writing this answer) that's too complicated for most non-physicists, but maybe that's because I don't completely understand it either. Maybe you'd like to hash out your technical understanding of that issue, perhaps in a separate question, before coming back to produce a non-technical explanation?
– rob♦
2 hours ago
1
Not a bad idea. I'll try to formulate that question tomorrow.
– S. McGrew
2 hours ago
I've been doing some digging, and have found several papers and presentations that offer an effective refractive index of the vacuum, proportional to the gravitational potential. Calculation of light propagation based on that refractive yields correct values for the deflection of light rays by a gravitational field. I reckon it would work equally well from a wave perspective. Still, I'm bothered by the fact that refractive index affects only the wavelength of light, while gravity also affects light frequency. Maybe if I don't look at that issue it will go away--
– S. McGrew
53 mins ago
add a comment |
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I think the distinction that you are making, with concerns about the intermediate blueshifts, redshifts, and time dilation, is too technical for someone whose education is outside of physics altogether. I would start with something simpler and add complexity as the responses of your audience suggest appropriate directions to go.
When I explain about gravitational lensing, I use an approach like this:
The equivalence principle predicts that light should also fall in a gravitational field. (And experiments on Earth confirm this, but the details are tricky.) So if a distant star/galaxy and a nearby star/galaxy are along the same line of sight, light from the distant object that would have missed us can get bent back towards us by the nearby star's gravity, similar the way that a magnifying lens makes parallel light rays converge. I usually end up drawing some variant of the below:
[source]
Beware that most people's experience with magnifying lenses is to look through the lens at a virtual image. Looking through a magnifier with enough distance to see the inverted real image is a surprise for many adults, and putting your eye at the focus is just plain strange-looking. So you may have some intuition failures to cope with if you use this approach.
If they get that gravity can bend light and make images of things appear in the "wrong" place, I ask them to remember a time they walked through a parking lot and noticed a dent on a shiny car. You don't actually see the dent. What you see is the reflection in the car of the car's surroundings. The reflection in the body of the car is distorted a little, because a car isn't flat like a mirror. And where there is a dent, there is more and different distortion. Sometimes you'll see the same object reflected more than once, from different sides of the dent. Most people have seen this enough times that they have a pretty good idea of the size and shape of the dent just from this reflected-image information: walking up to the car and touching the dent is not usually a surprise. And in the same way, an image like this one
[source]
lets you say things about the distribution of matter in the foreground galaxy cluster that's doing the lensing.
This little spiel has served me well.
As I was posting this, I saw your comment that the wavefront explanation for lensing is what you're really hoping to explain, and that you suspect the reason it's hard to come up with a layman's explanation is that your own understanding is incomplete. I still think (as I did when I started writing this answer) that's too complicated for most non-physicists, but maybe that's because I don't completely understand it either. Maybe you'd like to hash out your technical understanding of that issue, perhaps in a separate question, before coming back to produce a non-technical explanation?
– rob♦
2 hours ago
1
Not a bad idea. I'll try to formulate that question tomorrow.
– S. McGrew
2 hours ago
I've been doing some digging, and have found several papers and presentations that offer an effective refractive index of the vacuum, proportional to the gravitational potential. Calculation of light propagation based on that refractive yields correct values for the deflection of light rays by a gravitational field. I reckon it would work equally well from a wave perspective. Still, I'm bothered by the fact that refractive index affects only the wavelength of light, while gravity also affects light frequency. Maybe if I don't look at that issue it will go away--
– S. McGrew
53 mins ago
add a comment |
I think the distinction that you are making, with concerns about the intermediate blueshifts, redshifts, and time dilation, is too technical for someone whose education is outside of physics altogether. I would start with something simpler and add complexity as the responses of your audience suggest appropriate directions to go.
When I explain about gravitational lensing, I use an approach like this:
The equivalence principle predicts that light should also fall in a gravitational field. (And experiments on Earth confirm this, but the details are tricky.) So if a distant star/galaxy and a nearby star/galaxy are along the same line of sight, light from the distant object that would have missed us can get bent back towards us by the nearby star's gravity, similar the way that a magnifying lens makes parallel light rays converge. I usually end up drawing some variant of the below:
[source]
Beware that most people's experience with magnifying lenses is to look through the lens at a virtual image. Looking through a magnifier with enough distance to see the inverted real image is a surprise for many adults, and putting your eye at the focus is just plain strange-looking. So you may have some intuition failures to cope with if you use this approach.
If they get that gravity can bend light and make images of things appear in the "wrong" place, I ask them to remember a time they walked through a parking lot and noticed a dent on a shiny car. You don't actually see the dent. What you see is the reflection in the car of the car's surroundings. The reflection in the body of the car is distorted a little, because a car isn't flat like a mirror. And where there is a dent, there is more and different distortion. Sometimes you'll see the same object reflected more than once, from different sides of the dent. Most people have seen this enough times that they have a pretty good idea of the size and shape of the dent just from this reflected-image information: walking up to the car and touching the dent is not usually a surprise. And in the same way, an image like this one
[source]
lets you say things about the distribution of matter in the foreground galaxy cluster that's doing the lensing.
This little spiel has served me well.
As I was posting this, I saw your comment that the wavefront explanation for lensing is what you're really hoping to explain, and that you suspect the reason it's hard to come up with a layman's explanation is that your own understanding is incomplete. I still think (as I did when I started writing this answer) that's too complicated for most non-physicists, but maybe that's because I don't completely understand it either. Maybe you'd like to hash out your technical understanding of that issue, perhaps in a separate question, before coming back to produce a non-technical explanation?
– rob♦
2 hours ago
1
Not a bad idea. I'll try to formulate that question tomorrow.
– S. McGrew
2 hours ago
I've been doing some digging, and have found several papers and presentations that offer an effective refractive index of the vacuum, proportional to the gravitational potential. Calculation of light propagation based on that refractive yields correct values for the deflection of light rays by a gravitational field. I reckon it would work equally well from a wave perspective. Still, I'm bothered by the fact that refractive index affects only the wavelength of light, while gravity also affects light frequency. Maybe if I don't look at that issue it will go away--
– S. McGrew
53 mins ago
add a comment |
I think the distinction that you are making, with concerns about the intermediate blueshifts, redshifts, and time dilation, is too technical for someone whose education is outside of physics altogether. I would start with something simpler and add complexity as the responses of your audience suggest appropriate directions to go.
When I explain about gravitational lensing, I use an approach like this:
The equivalence principle predicts that light should also fall in a gravitational field. (And experiments on Earth confirm this, but the details are tricky.) So if a distant star/galaxy and a nearby star/galaxy are along the same line of sight, light from the distant object that would have missed us can get bent back towards us by the nearby star's gravity, similar the way that a magnifying lens makes parallel light rays converge. I usually end up drawing some variant of the below:
[source]
Beware that most people's experience with magnifying lenses is to look through the lens at a virtual image. Looking through a magnifier with enough distance to see the inverted real image is a surprise for many adults, and putting your eye at the focus is just plain strange-looking. So you may have some intuition failures to cope with if you use this approach.
If they get that gravity can bend light and make images of things appear in the "wrong" place, I ask them to remember a time they walked through a parking lot and noticed a dent on a shiny car. You don't actually see the dent. What you see is the reflection in the car of the car's surroundings. The reflection in the body of the car is distorted a little, because a car isn't flat like a mirror. And where there is a dent, there is more and different distortion. Sometimes you'll see the same object reflected more than once, from different sides of the dent. Most people have seen this enough times that they have a pretty good idea of the size and shape of the dent just from this reflected-image information: walking up to the car and touching the dent is not usually a surprise. And in the same way, an image like this one
[source]
lets you say things about the distribution of matter in the foreground galaxy cluster that's doing the lensing.
This little spiel has served me well.
I think the distinction that you are making, with concerns about the intermediate blueshifts, redshifts, and time dilation, is too technical for someone whose education is outside of physics altogether. I would start with something simpler and add complexity as the responses of your audience suggest appropriate directions to go.
When I explain about gravitational lensing, I use an approach like this:
The equivalence principle predicts that light should also fall in a gravitational field. (And experiments on Earth confirm this, but the details are tricky.) So if a distant star/galaxy and a nearby star/galaxy are along the same line of sight, light from the distant object that would have missed us can get bent back towards us by the nearby star's gravity, similar the way that a magnifying lens makes parallel light rays converge. I usually end up drawing some variant of the below:
[source]
Beware that most people's experience with magnifying lenses is to look through the lens at a virtual image. Looking through a magnifier with enough distance to see the inverted real image is a surprise for many adults, and putting your eye at the focus is just plain strange-looking. So you may have some intuition failures to cope with if you use this approach.
If they get that gravity can bend light and make images of things appear in the "wrong" place, I ask them to remember a time they walked through a parking lot and noticed a dent on a shiny car. You don't actually see the dent. What you see is the reflection in the car of the car's surroundings. The reflection in the body of the car is distorted a little, because a car isn't flat like a mirror. And where there is a dent, there is more and different distortion. Sometimes you'll see the same object reflected more than once, from different sides of the dent. Most people have seen this enough times that they have a pretty good idea of the size and shape of the dent just from this reflected-image information: walking up to the car and touching the dent is not usually a surprise. And in the same way, an image like this one
[source]
lets you say things about the distribution of matter in the foreground galaxy cluster that's doing the lensing.
This little spiel has served me well.
answered 2 hours ago
rob♦
39.4k971162
39.4k971162
As I was posting this, I saw your comment that the wavefront explanation for lensing is what you're really hoping to explain, and that you suspect the reason it's hard to come up with a layman's explanation is that your own understanding is incomplete. I still think (as I did when I started writing this answer) that's too complicated for most non-physicists, but maybe that's because I don't completely understand it either. Maybe you'd like to hash out your technical understanding of that issue, perhaps in a separate question, before coming back to produce a non-technical explanation?
– rob♦
2 hours ago
1
Not a bad idea. I'll try to formulate that question tomorrow.
– S. McGrew
2 hours ago
I've been doing some digging, and have found several papers and presentations that offer an effective refractive index of the vacuum, proportional to the gravitational potential. Calculation of light propagation based on that refractive yields correct values for the deflection of light rays by a gravitational field. I reckon it would work equally well from a wave perspective. Still, I'm bothered by the fact that refractive index affects only the wavelength of light, while gravity also affects light frequency. Maybe if I don't look at that issue it will go away--
– S. McGrew
53 mins ago
add a comment |
As I was posting this, I saw your comment that the wavefront explanation for lensing is what you're really hoping to explain, and that you suspect the reason it's hard to come up with a layman's explanation is that your own understanding is incomplete. I still think (as I did when I started writing this answer) that's too complicated for most non-physicists, but maybe that's because I don't completely understand it either. Maybe you'd like to hash out your technical understanding of that issue, perhaps in a separate question, before coming back to produce a non-technical explanation?
– rob♦
2 hours ago
1
Not a bad idea. I'll try to formulate that question tomorrow.
– S. McGrew
2 hours ago
I've been doing some digging, and have found several papers and presentations that offer an effective refractive index of the vacuum, proportional to the gravitational potential. Calculation of light propagation based on that refractive yields correct values for the deflection of light rays by a gravitational field. I reckon it would work equally well from a wave perspective. Still, I'm bothered by the fact that refractive index affects only the wavelength of light, while gravity also affects light frequency. Maybe if I don't look at that issue it will go away--
– S. McGrew
53 mins ago
As I was posting this, I saw your comment that the wavefront explanation for lensing is what you're really hoping to explain, and that you suspect the reason it's hard to come up with a layman's explanation is that your own understanding is incomplete. I still think (as I did when I started writing this answer) that's too complicated for most non-physicists, but maybe that's because I don't completely understand it either. Maybe you'd like to hash out your technical understanding of that issue, perhaps in a separate question, before coming back to produce a non-technical explanation?
– rob♦
2 hours ago
As I was posting this, I saw your comment that the wavefront explanation for lensing is what you're really hoping to explain, and that you suspect the reason it's hard to come up with a layman's explanation is that your own understanding is incomplete. I still think (as I did when I started writing this answer) that's too complicated for most non-physicists, but maybe that's because I don't completely understand it either. Maybe you'd like to hash out your technical understanding of that issue, perhaps in a separate question, before coming back to produce a non-technical explanation?
– rob♦
2 hours ago
1
1
Not a bad idea. I'll try to formulate that question tomorrow.
– S. McGrew
2 hours ago
Not a bad idea. I'll try to formulate that question tomorrow.
– S. McGrew
2 hours ago
I've been doing some digging, and have found several papers and presentations that offer an effective refractive index of the vacuum, proportional to the gravitational potential. Calculation of light propagation based on that refractive yields correct values for the deflection of light rays by a gravitational field. I reckon it would work equally well from a wave perspective. Still, I'm bothered by the fact that refractive index affects only the wavelength of light, while gravity also affects light frequency. Maybe if I don't look at that issue it will go away--
– S. McGrew
53 mins ago
I've been doing some digging, and have found several papers and presentations that offer an effective refractive index of the vacuum, proportional to the gravitational potential. Calculation of light propagation based on that refractive yields correct values for the deflection of light rays by a gravitational field. I reckon it would work equally well from a wave perspective. Still, I'm bothered by the fact that refractive index affects only the wavelength of light, while gravity also affects light frequency. Maybe if I don't look at that issue it will go away--
– S. McGrew
53 mins ago
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Can't you just say that light is deflected by gravity just like everything else? Getting into frequency is just overcomplicating and it's actually not very relevant once you get into the math. Gravitational lensing doesn't depend on the frequency.
– Javier
2 hours ago
The way you’re trying to think about strikes me as way too complicated for a layman. I would just tell someone that the path of the photons gets bent by the gravitating mass, so that they “focus” as if passing through a lens.
– G. Smith
2 hours ago
@Javier, I understand that all frequencies of light are affected identically by a gravitational field, just as all frequencies of light are affected identically by a lens made of a material having no spectral dispersion. I also know it's valuable to explain (and understand) things from multiple perspectives. So, I'd like to be able to explain how gravity affects a propagating plane light wave, rather than just say that gravity bends light rays.
– S. McGrew
2 hours ago
How about "light takes the (well, a) shortest path", so gravity acts like a lens, and several paths are locally shortest.
– JEB
2 hours ago