## How can light not lose momentum when traveling through glass

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### How can light not lose momentum when traveling through glass

The speed of light measured within a media (glass, etc.) is slower than in a vacuum. Is there no power or speed lost by Resistance?
BMcD
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You're thinking of light as if it has mass. It doesn't.

Sparky
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OK, Pls help me understand-does light have energy? How can you split a photon (via mirror), and have the energy split(?) into polorized vertical and horizontal if it has no mass? Why does the famous Quantum M. experiment prove that light is either a wave or a particle, depending on apon which "answer" you search for, (wave vs. particle)? How do we reconsile the existance of energy if there is no mass? PS-if light has no mass, how does it have energy to sunburn, to photosythesize? How can there be energy w/o mass?

,
BMcD
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I know. It can be confusing. Read this first:

http://math.ucr.edu/home/baez/physics/R ... _mass.html

... then let us know if you still have questions.

Sparky
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Thanks! Having no mass would totally explain it.
BMcD
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Light has no mass, but it does have momentum and can exert pressure on a surface. Hence, solar sails are feasible.

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### Re: How can light not lose momentum when traveling through g

This just gives an idea how powerful those celestial objects are. Traveling through the space, if there is obscured by the interstellar clouds, there is really nothing to weaken the light. Its intensity, however, would decrease inversely proportional to the spare of the distance. For us to see the objects that far, yes, they have to be immensely bright.
silvesterstuck
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### Re: How can light not lose momentum when traveling through g

So, what would happen to the momentum of light if you changed it's wavelength?

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### Re: How can light not lose momentum when traveling through g

$\Large E = \frac{hc}{\lambda}$

$\Large p = \frac{h}{\lambda}$

where $\lambda$ is wavelength, h & c are constants and E & p are energy and momentum.

Inside a medium, the situation is hugely more tricky. The index of refraction is an effective theory that corresponds to the actual situation, which involves a series of interactions between the photon and the medium. Within the medium, individual photons still travel at the speed of light as they move between atoms, however the effective velocity is lower.

There are very few treatments on this in the popular literature. Feynman's popular book "QED" is about the only one.

Lincoln
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### Re: How can light not lose momentum when traveling through g

I'm missing something here.

The intensity of light (and therefore all EM phenomena) follows the inverse square law -- power dissipates more rapidly as distance from the source increases.

And yet momentum remains constant...?

What happens when the power from the point source is infinitesimally small? Is there a practical boundary where propagation stops (there must be, otherwise that implies all EM travels infinitely, requiring infinite energy). At this hypothetical boundary, where power is near zero, what happens to momentum then?

psionic11
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### Re: How can light not lose momentum when traveling through g

At any one moment, a source emits a specific number of photons. Each of those photons carry an unchanging amount of energy.

The inverse square law is nothing more than those same number of photons hitting an ever-expanding area. If you have 100 photons, they will go through a certain area when a sphere has a radius of 1 meter. They go through an area four times larger when the sphere has a radius of 2 meters. Thus each unit area gets hit by fewer photons.

Each photon is unchanged, with unchanged momentum. The inverse square law just reflects the fact that fewer photons hit a unit of area.

Lincoln
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### Re: How can light not lose momentum when traveling through g

Excellent, thank you. A photon then is a discrete package of energy, a fixed quantity that travels with fixed momentum in a certain direction at a fixed frequency. Correct so far?

The trajectory only changes when it interacts with another particle or field, during which interaction there is an interchange of energy, resulting in changes of energy levels, frequencies and trajectories of all involved particles in the interaction, correct? Then does the individual, fixed momentum of a photon remain constant throughout this interchange, or is it more accurate to say "global momentum" of all involved particles is preserved?

These are probably physics 101 questions. I should read up first before nattering on with basic questions that any number of texts answer easily enough. The apparent conflicts in concepts are more likely gaps in my understanding rather than fundamental mis-steps in premises.

psionic11
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### Re: How can light not lose momentum when traveling through g

Yes to everything. When a photon interacts with something, it is absorbed and another photon is re-emitted, most likely with a different frequency/energy/momentum. There are some subtle variations that a pro or a picky person might point to, but I'm ignoring those, since what I'm saying here covers 99+% of the situations.

The photon out is not the same energy as the photon out most of the time, simply because it is a different photon.

These aren't physics 101 questions. Maybe 201 or 310. Light as a particle is not touched upon in 101.

Lincoln
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### Re: How can light not lose momentum when traveling through g

The idea that light has no mass is complete nonsense. If Gravity has an effect on light, then light has an effect on gravity. They absolutely must be related to each other or light could never have an influence on mass.
since mass influences light, light influences mass. The person that shoots this down had better have a good way of explaining, because any contradiction would mean that action is not equal and opposite. (please dont nitpick on my use of equal and opposite rather than equal but opposite. It's a habit.)
legosbulock
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### Re: How can light not lose momentum when traveling through g

The intensity of light (and therefore all EM phenomena) follows the inverse square law -- power dissipates more rapidly as distance from the source increases.

can you elaborate?
legosbulock
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### Re:

Sparky wrote:I know. It can be confusing. Read this first:

http://math.ucr.edu/home/baez/physics/R ... _mass.html

... then let us know if you still have questions.

This link is not working? Is there a way to find this information?

Watson
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### Re: How can light not lose momentum when traveling through g

Sparky, kick yourself... hard! of course light has mass or it couldnt be influenced by gravity. Light still follows the same principal of everything having an equal/oppposite reaction. if it didn't it could never be bent by a gravitational pull.
legosbulock
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### Re: How can light not lose momentum when traveling through g

Didnt mean that to sound caustic, but I shouldn't have to chase websites to make your argument for you. Show me a sound or logical principal, and I will chase that. I dbody n't care what the moderators say, you keep posting. (be nice) and I will try to help you sort the logic. I cant think of a single advance in science that didn't start with somebody trying something the other guy didnt. keep trying, but keep learning.
legosbulock
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### Re: How can light not lose momentum when traveling through g

You are welcome to believe what you like, but you're wrong. Light doesn't have mass.

Further, you are thinking in terms of gravitational mass, which is not a priori the same as inertial mass. It has been experimentally shown to be the same and in relativity it is defined to be the same.

However the fact that the inertial mass of a photon is zero has been experimentally demonstrated to be true to astounding precision.

P.S. I'm not a mod, but I do know a thing or two about physics.

Lincoln
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### Re: How can light not lose momentum when traveling through g

Sorry Lincoln, what you are saying is a mathematical impossibilty. Let me explain. The value of any given property must be somewhere between non-existant and all encompassing. To create a value reference, we will use zero and one. Zero being that it doesnt exist, and one being that it is all encompassing. If an object were to be infinite along any givin axis( say the x axis), nothing else could exist along that axis, there simply wouldn't be room for it. Conversly, if an object had a REAL zero for that that property, and axis, it wouldn't even exist along that dimension (axis). My point being, if Light has absolutely no mass( in any fashion) it t would have absolutely no effect on anything else we can detect, but also that we couldnt detect it ourselves in the first place because it wouldn't exist in a way we could even observe. Summary, Light absolutely must have mass, even if it's a trivial amount. (to help clarify my point, if light didn't exist along the gravitational axis, we wouldn't even know, because we couldn't share the same axis, and couldn't observe it.) (more specifically, if light had zero mass, we wouldn't know it existed in the first place except for a flat world, round world experiment I have never seen or heard of
legosbulock
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### Re: How can light not lose momentum when traveling through g

Basically, what I'm saying is that if we can measure it's value in any way, it has value, and mathematically, that value can never truly be zero. Is this making any sense to you? Please dont shoot me down with a bunch of crap about what's proven and what's not. I don't care about things that could be wrong, or hypothetical BS that people made up because there was a big freakin hole in their theory and had to invent a mathematical explanation. Of course their theory will fit, they invented it to fit. Anyway I guess I'm on a rant and it's time to hang up. Night guys and gals.
legosbulock
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### Re: How can light not lose momentum when traveling through g

I don't much care if you believe me or not. Experimental facts are experimental facts. I am sorry if this does not conform with your intuition.

Lincoln
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### Re: How can light not lose momentum when traveling through g

Fair enough sir. I guess I'll drop the issue for now. Be well.
legosbulock
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### Re: How can light not lose momentum when traveling through g

legosbulock wrote:of course light has mass or it couldnt be influenced by gravity.

I googled this out of curiosity, the answer I found may be of interest to you:

strong gravitational fields may not be able to interact with light but they do change the shape of space-time. Light is responding to the curvature in space caused by gravity not the because gravity is 'pulling' on it.

So gravity doesn't influence light directly, it changes the shape of space itself, and since light travels in space, it effectively changes the shape of the path that light is taking.

flannel jesus
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### Re: How can light not lose momentum when traveling through g

@ legosbulock

It may help you if you regard the terms "mass" and "energy" for what they truly are. HUMAN concepts. We do our best to understand the world that appears around us by labeling things for a better understanding.

No one is sure what Mass or Gravity is. They are names to explain a phenomenon we observe through experimentation.

If you start to look at literally EVERYTHING in this way maybe you will better understand our lack of understanding and the scientific theories made to date and why people hold these beliefs in high regard over other beliefs.

I advise you to check out ANY lecture by Richard Feynman and how he talks about nature and our understanding of it. (You'll find an array of videos on Youtube and I would personally recommend anyone to watch them regardless of mathematical knowledge - although it does help you to appreciate things more obviously if you have some grasp of mathematics)

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### Re: How can light not lose momentum when traveling through g

flannel jesus wrote:
legosbulock wrote:of course light has mass or it couldnt be influenced by gravity.

I googled this out of curiosity, the answer I found may be of interest to you:

strong gravitational fields may not be able to interact with light but they do change the shape of space-time. Light is responding to the curvature in space caused by gravity not the because gravity is 'pulling' on it.

So gravity doesn't influence light directly, it changes the shape of space itself, and since light travels in space, it effectively changes the shape of the path that light is taking.

Space-time is a graph with space time coordinates. Apart from mathematical treatment, what physical reasoning explains their interdependence? How can space vanish and turn into time? Unless it is clearly explained, it cannot be science.
For space to contract or expand or change shape, it has to be some entity. Where is the direct proof?
Vilas Tamhane
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### Re: How can light not lose momentum when traveling through g

Space-time is a graph with space time coordinates. Apart from mathematical treatment, what physical reasoning explains their interdependence? How can space vanish and turn into time? Unless it is clearly explained, it cannot be science.
For space to contract or expand or change shape, it has to be some entity. Where is the direct proof?

Space doesn't vanish and turn into time. They coexist as spacetime. And it is not an entity, but more a medium to exist within.

If it were clearly explained we wouldn't need science, would we?

Watson
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### Re: How can light not lose momentum when traveling through g

Vilas Tamhane wrote:For space to contract or expand or change shape, it has to be some entity. Where is the direct proof?

Gravitational lensing, is that enough proof?

dragslaye
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### Re: How can light not lose momentum when traveling through g

It may help you if you regard the terms "mass" and "energy" for what they truly are. HUMAN concepts. We do our best to understand the world that appears around us by labeling things for a better understanding.

No one is sure what Mass or Gravity is. They are names to explain a phenomenon we observe through experimentation.

If you start to look at literally EVERYTHING in this way maybe you will better understand our lack of understanding and the scientific theories made to date and why people hold these beliefs in high regard over other beliefs.

I advise you to check out ANY lecture by Richard Feynman and how he talks about nature and our understanding of it. (You'll find an array of videos on Youtube and I would personally recommend anyone to watch them regardless of mathematical knowledge - although it does help you to appreciate things more obviously if you have some grasp of mathematics)

You correctly say that mass and energy are human concepts. Somewhere I read, ‘we do not know what things are, we know things by their properties.

Newton discovered gravity but we do not know why mass should exert force and that too only on mass.

Physics is a science in which theory is proposed. This theory explains in logical words, mechanism how things work. Mathematics is just a tool to calculate and verify a theory. Unlike religion, science welcomes criticism. That is why it has progressed so much

If science rejects criticism then it is not science. It is religion. I am afraid, to some extent this has happened. SR is a good example.

Legosbulock has a point to make which was not taken in true spirit of science. What he wants to say is that bending of light is not possible unless photon has mass.

Isn’t it strange that photon has momentum but not mass? In fact Einstein derived momentum form energy-mass relationship. If mass is energy then energy is mass. E=mc^2 and so p=mc=(E/c^2)c=hf/c. However if we admit that photon has mass then we contradict mass/velocity relationship of SR. Therefore we admit what is convenient.
Present explanation of bending of light is that photons merely follow straight line in space which happens to be curved due to gravity. Should we believe this statement?

If space has curvature then, gravitational object being round, it must bend in circles. Material object will fall into these curvatures if mass-velocity is suitable, otherwise due to inertia they will break the curvature and go away from the object.

In the case of light, it has no mass and no inertia and so as soon as it enters these curvatures, it should become satellite of gravitational object. Space curvature is not defined properly. It is actually not only a spatial phenomenon but it is made mysterious by including time as a coordinate. I don’t know even little about GR and so I can’t visualize space-time curvature. I don’t know if it is possible for experts, if not then we expect big holes in the theory.
Vilas Tamhane
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### Re: How can light not lose momentum when traveling through g

Hi all,

I am probably wrong, but my best visualization of why light bends around a Gravitational Mass is due to Time Dilation.

Time is slower nearer to a mass, as predicted by GR and proven via our GPS system. We do see time on Earth is a bit slower nearer the surface, than out.. say.. a few 1000 miles into space.

Likewise.. time does becomes slower the nearer one approaches our massive Sun. This is part of the Curvature of Space-time.

The best (visual) example I can come up with for the Gravitational path bending of Massless Light is this:

Imagine a wheel chair. It has two wheels of course and a passenger. Imagine the passenger in the chair is shifting in the seat between the wheels in a wavelike fashion, as in rocking back and forth between the wheels. The passenger is maintaining a constant velocity by equal but alternating attention to which ever wheel is closest and would therefore travel normally in a curvy, but generally straight line as averaged.

Next.. think of each wheel as being like a "Clock" and imagine what would happen to the trajectory of the wheelchair if one wheel, say the right side (closer to a mass), was slightly slower than the left side. The path of the wheelchair would curve, on the average, towards the slower side.. of course.

Now mix the two ideas above.. that the side of a photon closest to the sun is traveling slightly slower than the side of the photon further from the sun, keeping in mind that the photon is also a frequency wave with measurable peak-peak thickness.

In this fashion, a photon doesn't require any mass. It just needs to exist, alternating as a wave does, in a medium where Time is a bit slower on one side of it's line of travel than the other side.. where "Side" is the peak-peak width of it's wavelength.

Thus a Photon is traveling through a Temporal Gradient, or Curved Space-time, (if you will) and that Temporal Gradient bends it's trajectory.

Any complaints? (lol)

As far as why light doesn't lose momentum in a glass medium, well momentum is a function of mass. I am more curious why coherent light remains coherent traveling through a fiber optic. Given that it is conceded that it interacts with the glass molecules, is absorbed and re-emitted and presumably always re-emitted in the same direction with the correct timing as to maintain coherence with it's fellow photons. Just seems like light should become diffused very quickly in a glass medium if it's interacting so much with the glass.

Any thoughts anyone?

Best regards,
Dave :^)

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