# Electromagnetic waves use spacetime as a medium?

#### Werapon Pat

Joined Jan 14, 2018
35
So that's why black holes can absorb light eventhough it doesn't have mass because it can bend spacetime, but that's just my oppinion
what do you think?

#### nsaspook

Joined Aug 27, 2009
10,427
Electromagnetic waves as photons are still matter so they exist with the properties of matter (energy being one of many) but don't have rest mass(a figure of speech that physicists use to describe something about a photon). The trick is they don't rest so the idea of rest mass doesn't really apply to them. They do have energy and momentum thus they have equivalent 'relativistic mass' that a black hole can trap.

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#### MrAl

Joined Jun 17, 2014
9,563
So that's why black holes can absorb light eventhough it doesn't have mass because it can bend spacetime, but that's just my oppinion
what do you think?
Hi,

That is an interesting question and i have to ask in return, what made you think of this question? This could lead to a more interesting exploration of what happens when light travels.

Unfortunately in physics we often see a workaday solution which comes from a simple observation and is usually self satisfying and unfortunately self defining and although complete in itself it does not get to the root of the phenomenon.
A simple example is in early physics books which describe a 'force', such as:
"A force is a push or a pull"
and this may be very satisfying for many individuals because of common experience, until we ask what is a push or pull. Then the answer is, of course, "A force".
So a force is push or a pull, and a push or a pull is a force, end of discussion. Well, at least for the time being, until someone realizes that we still dont know what the root cause is, we just know the effects.

A better example is inertia. The standard textbook explanation is:
"Inertia is the property of an object that is moving to keep moving in the same direction or of an object that is still to remain still unless acted on by an external force".

That's a wonderful definition of inertia, and that helps us solve almost every problem we encounter in engineering for example. The only problem is, that is also a description of an effect we are seeing, not a description of what is causing this incredible phenomenon. People have been thinking about this for years now.

So what happens when a photon passes close to a large object like a planet. Does the mass of a moving photon get 'attracted' to the planet and thus move closer, or does the already bent spacetime cause the photon to follow a path other than a straight line?
Keep in mind that spacetime is already bent before the photon even gets there. If that were a transmission line with an electron passing from one end to the other, it would have no choice but to follow the bend.

So what theory do you want to use? The bending of spacetime, or the gravitational field? Pick your poison . Interesting though, the photon is not at rest.

#### Alec_t

Joined Sep 17, 2013
13,160
Interesting though, the photon is not at rest.
Playing devil's advocate here. So what happens to a photon hitting a block of light-absorbing material, say carbon? Does it travel indefinitely within the block?

#### MrAl

Joined Jun 17, 2014
9,563
Playing devil's advocate here. So what happens to a photon hitting a block of light-absorbing material, say carbon? Does it travel indefinitely within the block?
Hi,

Not sure why you are asking this, maybe you could elaborate just a little? Thanks.

I would say the wave function breaks down and it generates heat energy.

#### Alec_t

Joined Sep 17, 2013
13,160
I asked because of the statement "the photon is not at rest". Just wondering what happened if a photon was 'brought to rest' by hitting something solid. I agree with the conversion to heat energy, but is there an instant when the photon still exists but is momentarily at rest?

#### MrAl

Joined Jun 17, 2014
9,563
I asked because of the statement "the photon is not at rest". Just wondering what happened if a photon was 'brought to rest' by hitting something solid. I agree with the conversion to heat energy, but is there an instant when the photon still exists but is momentarily at rest?
Hi,

I would think that if it came to rest then it had already transferred all of it's momentum to the medium, and that would mean that it just isnt there anymore. This might be too casual of a conversation to talk about this in great detail though. Usually this stuff has to be melded with some experiment or something. In electronics we often have to break the time line down into very small pieces that are actually more theoretical than anything else, like 0+ and 0- and just plain 0 time.

I think the wave function view makes it look like a sort of chirp, like a traveling pea pod. Once it interacts with something it then becomes a probability function of some kind which shows how it breaks down. I dont think i can perform this calculation myself though as it's been a long time even since i used the Schrodinger Equation.

#### nsaspook

Joined Aug 27, 2009
10,427
I asked because of the statement "the photon is not at rest". Just wondering what happened if a photon was 'brought to rest' by hitting something solid. I agree with the conversion to heat energy, but is there an instant when the photon still exists but is momentarily at rest?
So, light isn’t being “stopped” it’s “imprinting” on some of the electrons in the crystal that are in very, very carefully prepared states. This imprint isn’t light (so it doesn’t have to move), it’s just excited electrons. That imprint lasts for as much as a minute; slowly accruing errors and fading. After some amount of time that imprint is turned back into light, and it exits the crystal at exactly the speed you’d expect. What makes the experiment most exciting is that this experiment has proven to be an extremely long term method for storing quantum information, which has traditionally been a major hurdle. Normally a quantum computer (such as they are) has to get all of its work done in a fraction of a second.

#### xox

Joined Sep 8, 2017
789
Playing devil's advocate here. So what happens to a photon hitting a block of light-absorbing material, say carbon? Does it travel indefinitely within the block?
The photons would most likely be converted to electrons via the photoelectric effect. And depending on the isotrope of carbon in question those electrons would either generate electricity or heat. Or maybe you were thinking something else?

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#### MrAl

Joined Jun 17, 2014
9,563
Hello,

I think it first generates a current and that current then generates heat. The current may not be the same as we normally think of current (as more or less following a single path) but since the electrons move we might still think of it as a current.

#### xox

Joined Sep 8, 2017
789
I think it first generates a current and that current then generates heat. The current may not be the same as we normally think of current (as more or less following a single path) but since the electrons move we might still think of it as a current.
Basically what I was saying WRT the PE effect. Light doesn't carry momentum in the classical sense of
the word. When it impinges on a substance different things can happen. Sometimes only electricity flows but at other times
heat can be generated. There is even the possibility of a nearly complete conversion to heat which should result in an equal but opposite force appearing on the surface of the substance in question.

#### joeyd999

Joined Jun 6, 2011
4,603
...that a black hole can trap.
Ha! So, what happens to the photon that gets "trapped"????

From an observer outside the event horizon, it asymptotically approaches the event horizon but never penetrates. It's wavelength drops (again asymptotically and with respect to the outside observer) to zero. Poof! Is it gone?

But what does the photon see? Or, more realistically, what does an observer see who is contained within the event horizon?

I asked two questions weeks ago:

Here are two more games for you:
1. You're an astronaut who just fell past the event horizon of a black hole. Look up: what do you see?
2. Forget about speghettification. How long till your atoms hit the singularity?
No one has yet given a good answer(s).

#### MrAl

Joined Jun 17, 2014
9,563
Basically what I was saying WRT the PE effect. Light doesn't carry momentum in the classical sense of
the word. When it impinges on a substance different things can happen. Sometimes only electricity flows but at other times
heat can be generated. There is even the possibility of a nearly complete conversion to heat which should result in an equal but opposite force appearing on the surface of the substance in question.
When heat is generated that excites the electrons so the electrons move faster. That could be interpreted as an increase in current, although the current direction in this case could be random not the usual current like how we usually think of current flow.

#### nsaspook

Joined Aug 27, 2009
10,427
Ha! So, what happens to the photon that gets "trapped"????

From an observer outside the event horizon, it asymptotically approaches the event horizon but never penetrates. It's wavelength drops (again asymptotically and with respect to the outside observer) to zero. Poof! Is it gone?

But what does the photon see? Or, more realistically, what does an observer see who is contained within the event horizon?

I asked two questions weeks ago:

No one has yet given a good answer(s).
The truth is we really don't know for sure.
http://math.ucr.edu/home/baez/physics/Relativity/BlackHoles/fall_in.html

So for black holes larger than about 1000 solar masses I could probably fall in alive, and for still larger ones I might not even notice the tidal forces until I'm through the horizon and doomed.

https://archive.org/details/GravitationMisnerThorneWheeler

#### xox

Joined Sep 8, 2017
789
When heat is generated that excites the electrons so the electrons move faster. That could be interpreted as an increase in current, although the current direction in this case could be random not the usual current like how we usually think of current flow.
True but of course current has no real meaning unless you are talking about flowing from one definite point to another (and heat is mostly vibrational anyway).

#### bogosort

Joined Sep 24, 2011
696
At the scale of interactions between a photon and electron, I don't think it's appropriate to apply the notions of heat and electric current, which are macroscopic aggragate/statistical concepts. The interaction of a photon and a bound electron is a quantum phenomena (few degrees of freedom), with essentially four possible outcomes: nothing (transparent), complete absorption (opaque), absorption and lower-frequency photon emmittence (Compton scattering), or pair production (electron-positron). Which outcome occurs depends on the particular energy value of the photon and the binding energy of the electron.

xox

#### bogosort

Joined Sep 24, 2011
696
In response to the OP, I wouldn't call spacetime a medium -- spacetime is not a physical thing, rather it's a mathematical framework, like a coordinate system. We can't measure spacetime, but we can measure the distances between physical things, which gives us a metric, which we use to model the geometry of our universe.

If it helps to think of space as some kind of medium, we may say that space is made of quantum fields, and the medium of electromagnetic waves is the electromagnetic field. Likewise, the medium of gravity is the gravity field, and since it's a universal field, it imposes a geometry. Spacetime is just a handy coordinate system for this geometry.

#### xox

Joined Sep 8, 2017
789
Ha! So, what happens to the photon that gets "trapped"????

From an observer outside the event horizon, it asymptotically approaches the event horizon but never penetrates. It's wavelength drops (again asymptotically and with respect to the outside observer) to zero. Poof! Is it gone?

But what does the photon see? Or, more realistically, what does an observer see who is contained within the event horizon?

I asked two questions weeks ago:

No one has yet given a good answer(s).
A black hole often evokes the image of a mathematical profundity, a singularity within the fabric of spacetime that sucks all things nearby to the doom of the fifth dimension. In reality it is just a star. And it's effect on the curvature of spacetime just results in a mass that occupies a much smaller space than usual. Light incident to the normal of it's spinning surface gets sucked up while the axes spew out light and other forms of energetic material.

An observer standing at the event horizon would first notice younger feet...followed by a strange burning sensation as they finally reached the surface! (A big enough black hole might forestall the moment indefinitely of course.) I wonder would it feel cool all the way down?

#### xox

Joined Sep 8, 2017
789
In response to the OP, I wouldn't call spacetime a medium -- spacetime is not a physical thing, rather it's a mathematical framework, like a coordinate system. We can't measure spacetime, but we can measure the distances between physical things, which gives us a metric, which we use to model the geometry of our universe.

If it helps to think of space as some kind of medium, we may say that space is made of quantum fields, and the medium of electromagnetic waves is the electromagnetic field. Likewise, the medium of gravity is the gravity field, and since it's a universal field, it imposes a geometry. Spacetime is just a handy coordinate system for this geometry.
Wrong! If space were not a medium then accelerated reference frames would be no different from inertial ones. But the aether reacts to changes in motion and thus accelerations lead to gravitational (and even electromagnetic) effects.

#### joeyd999

Joined Jun 6, 2011
4,603
And it's effect on the curvature of spacetime just results in a mass that occupies a much smaller space than usual.
Careful here. I've noticed much discussion above about "distance" and "time" in the context of relativistic phenomena. In these realms, there is much argument to be had between individual observers! To claim "smaller" or "bigger" or "faster" or "slower" is only valid with respect to a single observer (or to multiple observers present within the same frame of reference). The significance of this point cannot be understated or ignored.

Light incident to the normal of it's spinning surface gets sucked up while the axes spew out light and other forms of energetic material.
For the sake of simplicity, can we limit the discussion to non-rotating black holes? While I assume that non-rotating black holes are unlikely to exist, the math just gets unreasonably complicated what with space-time getting all swept up in the rotation and making life difficult for us laymen. For my thought experiments, I'd also like to assume that photons, observers, etc., are travelling perpendicular to the event horizon. This should be good enough for government work.

An observer standing at the event horizon would first notice younger feet...followed by a strange burning sensation as they finally reached the surface! (A big enough black hole might forestall the moment indefinitely of course.) I wonder would it feel cool all the way down?
I am trying to avoid discussion of the physical effects of proximity to a singularity (whether inside or outside of the event horizon). I am well aware that biological entities would encounter difficulty near the event horizon of all but the largest black holes. My questions were more of the line: How does your perception of space/time change as you journey from beyond the event horizon to your final destination at the singularity?

Here is a short part of my (very long) answer:

I think the universe abhors infinities more than physicists do. The curvature of space/time around massive objects is the universe's successful attempt at avoiding infinities. If one can divorce himself from the concept of distance/time, and consider only c as a universal constant (regardless of your position (including within an event horizon)/velocity/acceleration through space), I think the nature of space/time becomes quite clear both near an event horizon and within it.

Again, look up (or down). What do you see?