# Thread: Light speed question (based on my reading so far)

1. So I'm a dozen or so pages into Einstein's Mirror and the start is all about light and time and what not...

The authors keep talking about time and distance and how far and fast light travels etc etc.

So, nothing travels faster than light but... does light always HAVE to travel at that speed or is it possible (even if just in theory, of course) to slow down the speed of light and if light is able to slow down does it cease to be light and become some other form of energy/particle/wave/whatever?

2.

3. Yes it can slow down depending what medium it is traveling through, usually not very much however. the constant is through a vacuum.

Light slows down some through say water, or glass & various other mediums. It slows down considerably through Bose-Einstein condensate.

None of these change what light is. its still light.

edit: I would also have to do more research but i believe the wavelength affects speed of light as well, such as infared will travel slower than say ultraviolet due to wavelength on the electromagnetic spectrum. Could be wrong however.

4. Originally Posted by GoldenRatio
Yes it can slow down depending what medium it is traveling through, usually not very much however. the constant is through a vacuum.

Light slows down some through say water, or glass & various other mediums. It slows down considerably through Bose-Einstein condensate.

None of these change what light is. its still light.

edit: I would also have to do more research but i believe the wavelength affects speed of light as well, such as infared will travel slower than say ultraviolet due to wavelength on the electromagnetic spectrum. Could be wrong however.
I love your big, fissured, beautiful brain!

Originally Posted by GoldenRatio
Yes it can slow down depending what medium it is traveling through, usually not very much however
Does it degrade faster when it slows down?

Originally Posted by GoldenRatio
It slows down considerably through Bose-Einstein condensate.
I'm not up to this chapter yet, so I have no idea what this is.

BTW, another question, if light travels fastest in a vacuum how does it move? I know it's a wave but, I mean what gives it its push? on earth we use gravity and our muscles for movement but how does light actually start it's journey without any gravity in a vacuum to give it its start?

5. Originally Posted by grmpysmrf
Originally Posted by GoldenRatio
Yes it can slow down depending what medium it is traveling through, usually not very much however. the constant is through a vacuum.

Light slows down some through say water, or glass & various other mediums. It slows down considerably through Bose-Einstein condensate.

None of these change what light is. its still light.

edit: I would also have to do more research but i believe the wavelength affects speed of light as well, such as infared will travel slower than say ultraviolet due to wavelength on the electromagnetic spectrum. Could be wrong however.
I love your big, fissured, beautiful brain!

Originally Posted by GoldenRatio
Yes it can slow down depending what medium it is traveling through, usually not very much however
Does it degrade faster when it slows down?

Originally Posted by GoldenRatio
It slows down considerably through Bose-Einstein condensate.
I'm not up to this chapter yet, so I have no idea what this is.

BTW, another question, if light travels fastest in a vacuum how does it move? I know it's a wave but, I mean what gives it its push? on earth we use gravity and our muscles for movement but how does light actually start it's journey without any gravity in a vacuum to give it its start?
This is a question that keeps coming up in all science forums. So it does become a bit tedious after a few dozen times. If you measure the speed of light through a medium other than in a vacuum, it will take more time and appear to be traveling slower. However if you examine light at the photon level, the photon will always travel at the speed of light. What happens as light passes through a transparent medium is that the photons will get absorbed by the electrons of the atoms and then re-admitted. Depending on how many absorptions and re-admissions the photon goes through as it passes any given distance through the medium will determine how much time it takes passing through the medium. But the photon when it's in motion always travels at the speed of light. Their is no accelerating to get up to the speed of light and there is not anything that can be called a push. When electrons change energy states to a higher level they will emit a photon or photons until they reach the lowest stable energy state they can be at in the atom.

As to whether the different wavelengths of light travel at different speeds is still a subject for debate. But I think the consensus is still in favor of that not being the case.

6. Originally Posted by grmpysmrf
Does it degrade faster when it slows down?
It doesn't degrade, so it can't degrade faster

BTW, another question, if light travels fastest in a vacuum how does it move? I know it's a wave but, I mean what gives it its push? on earth we use gravity and our muscles for movement but how does light actually start it's journey without any gravity in a vacuum to give it its start?
Photons are massless and have to travel at light speed. Nothing gives them a "push".

7. if photons are massless then what makes them curve in gravity?

8. Originally Posted by 514void
if photons are massless then what makes them curve in gravity?
Gravity is the curvature of space-time (that is what creates the thing we perceive as the force of gravity). Photons, being massless, follow what are called "null geodesic" in space-time.

But even Newton showed that light is affected by gravity (although by the wrong amount) despite being massless.

9. Not only mass causes, or is influenced, by gravity. Anything with energy or momentum also causes and is affected by gravity.Photons are massless but they do possess energy.

10. Originally Posted by 514void
if photons are massless then what makes them curve in gravity?
I blame Newton for questions such as this.

The trajectory in four-dimensional spacetime of an object in a gravitational field is a straight line and does not depend on the material properties of the object. What this means is that a point object behaves the same whether it is in a gravitational field or not. While energy-momentum is a source of gravitation, it isn't affected by it (except for the tidal effect on extended objects). This is due to the equivalence principle.

Even in Newtonian theory, one has:

Therefore:

In other words, acceleration (which governs the trajectory of the object) depends only on the mass of the gravitational source and doesn't depend on the object at all (provided its effects as a source of gravitation can be neglected) .

11. Originally Posted by KJW
I blame Newton for questions such as this.
That is a bit unfair because ...

Even in Newtonian theory, one has ...
Maybe we should blame science writers.

12. Originally Posted by Strange
Originally Posted by KJW
I blame Newton for questions such as this.
That is a bit unfair
Only because Newton didn't know any better. The difference between general relativity and Newtonian gravitation in the domain where Newtonian gravitation is applicable is a change in what it means to accelerate, with the result that gravity is not a force. This is a qualitative difference that prevents Newtonian gravitation from being an approximation of general relativity even in the weak field limit.

13. Originally Posted by Strange
Originally Posted by grmpysmrf
Does it degrade faster when it slows down?
It doesn't degrade, so it can't degrade faster .
So light lasts forever? I thought at some point it fizzles out. (for lack of a better phrase)
Then how come the entire universe isn't illuminated? I think I remember hearing there is a spot in the universe that is so cold and dark because no heat/light has been there, how can that be if light exist forever? I'd think there would be no part of the universe not touched by light at at least some point.

14. Originally Posted by Bad Robot

This is a question that keeps coming up in all science forums. So it does become a bit tedious after a few dozen times. If you measure the speed of light through a medium other than in a vacuum, it will take more time and appear to be traveling slower. However if you examine light at the photon level, the photon will always travel at the speed of light. What happens as light passes through a transparent medium is that the photons will get absorbed by the electrons of the atoms and then re-admitted. Depending on how many absorptions and re-admissions the photon goes through as it passes any given distance through the medium will determine how much time it takes passing through the medium. But the photon when it's in motion always travels at the speed of light. Their is no accelerating to get up to the speed of light and there is not anything that can be called a push. When electrons change energy states to a higher level they will emit a photon or photons until they reach the lowest stable energy state they can be at in the atom.

As to whether the different wavelengths of light travel at different speeds is still a subject for debate. But I think the consensus is still in favor of that not being the case.
Agreed, Guess I should of explained my response better but correct. It "appears" to slow down, but on photon level it does not.

Originally Posted by grmpysmrf
So light lasts forever? I thought at some point it fizzles out. (for lack of a better phrase)
Then how come the entire universe isn't illuminated? I think I remember hearing there is a spot in the universe that is so cold and dark because no heat/light has been there, how can that be if light exist forever? I'd think there would be no part of the universe not touched by light at at least some point.
Light gets absorbed, very easily. Far as the "spot in the universe" without light I say hokum without sources.

15. Originally Posted by GoldenRatio
Light gets absorbed, very easily.
So light does become something else, or at least part of something else?

Originally Posted by GoldenRatio
Far as the "spot in the universe" without light I say hokum without sources.
No longer relevant since you shot down the light is every where question. but I did try to look up hokum just for clarification but all I could find was music references.

16. Originally Posted by grmpysmrf
Originally Posted by GoldenRatio
Light gets absorbed, very easily.
So light does become something else, or at least part of something else? .
I...I guess? If you want to look at it that way...Not sure if i am following your train of thought.

Can light be used for other energy? yeah, absolutely. Solar power & plants great example of light energy being converted.

If your trying to suggest something else however. I do not follow.

17. Originally Posted by grmpysmrf
Then how come the entire universe isn't illuminated?
That is a very, very good question. It puzzled people for hundreds of years. It was only answered when it was discovered that the universe is expanding:
Olbers' paradox - Wikipedia, the free encyclopedia

I think I remember hearing there is a spot in the universe that is so cold and dark because no heat/light has been there, how can that be if light exist forever? I'd think there would be no part of the universe not touched by light at at least some point.
I can't imagine what that was a reference to. It would be interesting to know, if you come across it again.

18. First problem i see with olbers paradox is "an infinite number of stars" there is a finite ammount that fluctuates daily with birth of new stars & death of old ones.

19. Originally Posted by GoldenRatio
First problem i see with olbers paradox is "an infinite number of stars" there is a finite ammount that fluctuates daily with birth of new stars & death of old ones.
At the time, the universe was thought to be infinite (as required by Newton's theory of gravitation).

20. Originally Posted by Strange
Originally Posted by grmpysmrf
Then how come the entire universe isn't illuminated?
That is a very, very good question.
Because I will never be at the level that you guys are at, I'm such a loser that this is what I strive to hear from you all <3

Originally Posted by Strange
Originally Posted by grmpysmrf
I think I remember hearing there is a spot in the universe that is so cold and dark because no heat/light has been there, how can that be if light exist forever? I'd think there would be no part of the universe not touched by light at at least some point.
I can't imagine what that was a reference to. It would be interesting to know, if you come across it again.
I think it was either this
Boomerang Nebula - Wikipedia, the free encyclopedia

or this
CMB cold spot - Wikipedia, the free encyclopedia

21. Originally Posted by GoldenRatio
Originally Posted by grmpysmrf
Originally Posted by GoldenRatio
Light gets absorbed, very easily.
So light does become something else, or at least part of something else? .
I...I guess? If you want to look at it that way...Not sure if i am following your train of thought.

Can light be used for other energy? yeah, absolutely. Solar power & plants great example of light energy being converted.

If your trying to suggest something else however. I do not follow.
Sorry.
I just mean that if light is going to be absorbed by something else isn't it now part of that "something else" instead light?

Maybe I'm misunderstanding what absorption is. If I rub lotion onto my skin my skin will absorb it. wouldn't it stand to reason that that lotion is now in my skin and can now be seen as skin?
or is that asinine?

22. Originally Posted by GoldenRatio
First problem i see with olbers paradox is "an infinite number of stars" there is a finite ammount that fluctuates daily with birth of new stars & death of old ones.
What you write kinda sounds like people, doesn't it? If we have an infinite amount of time and no threat of species killing accidents I suppose the potential for an infinite number of people exists based on the births and the deaths. Although it sounds more like existence rather than infinity.

23. Originally Posted by grmpysmrf
Because I will never be at the level that you guys are at, I'm such a loser that this is what I strive to hear from you all.
Please don't speak about yourself as being a loser. I am speaking for myself but I don't think any of the members wants to hear you saying that. We are not in a competition about who knows more and who's smarter. Each and every member brings things to the forum that makes it a better place for all of us. One thing about the tedious questions is, I keep finding better ways of providing an answer that's understandable in the least number of words. I like to say things as concisely as possible and this forum gives me the opportunity to hone my skills at doing that.

You will find that if you maintain an active membership for a few years or more that you will be able to speak with a great deal more confidence on a whole range of subjects that you currently are not able to now.

24. Originally Posted by grmpysmrf
I just mean that if light is going to be absorbed by something else isn't it now part of that "something else" instead light?
Normally, it will become heat. In some cases, such as photosynthesis, it can be used to create new chemicals so in that sense become part of it.

25. Originally Posted by Strange
Originally Posted by grmpysmrf
I just mean that if light is going to be absorbed by something else isn't it now part of that "something else" instead light?
Normally, it will become heat. In some cases, such as photosynthesis, it can be used to create new chemicals so in that sense become part of it.
In addition to what Strange said "Light" when being absorbed by non-transparent substances does get re-admitted, just not in the visible spectrum. Example, visible light gets absorbed by a rock, the rock feels warmer because it's re-radiating the visible photons as invisible inferred photons which we sense as heat.

26. Originally Posted by Bad Robot
Originally Posted by grmpysmrf
Because I will never be at the level that you guys are at, I'm such a loser that this is what I strive to hear from you all.
Please don't speak about yourself as being a loser. I am speaking for myself but I don't think any of the members wants to hear you saying that. We are not in a competition about who knows more and who's smarter. Each and every member brings things to the forum that makes it a better place for all of us. One thing about the tedious questions is, I keep finding better ways of providing an answer that's understandable in the least number of words. I like to say things as concisely as possible and this forum gives me the opportunity to hone my skills at doing that.

You will find that if you maintain an active membership for a few years or more that you will be able to speak with a great deal more confidence on a whole range of subjects that you currently are not able to now.
Thanks, Bad robot!
I was being hyperbolic about the loser thing. I was just using it to show how much I don't know around you all.
but noted, I'll be good! Thanks.

27. Originally Posted by Bad Robot
Originally Posted by Strange
Originally Posted by grmpysmrf
I just mean that if light is going to be absorbed by something else isn't it now part of that "something else" instead light?
Normally, it will become heat. In some cases, such as photosynthesis, it can be used to create new chemicals so in that sense become part of it.
In addition to what Strange said "Light" when being absorbed by non-transparent substances does get re-admitted, just not in the visible spectrum. Example, visible light gets absorbed by a rock, the rock feels warmer because it's re-radiating the visible photons as invisible inferred photons which we sense as heat.
So, simply put, it's either light or heat...? (Or both?)

28. Originally Posted by grmpysmrf
Sorry.
I just mean that if light is going to be absorbed by something else isn't it now part of that "something else" instead light?

Maybe I'm misunderstanding what absorption is. If I rub lotion onto my skin my skin will absorb it. wouldn't it stand to reason that that lotion is now in my skin and can now be seen as skin?
or is that asinine?
Okay, first you have to understand what light "is". It is electromagnetic radiation, Now, electromagnetic radiation consists of electric and magnetic waves, or basically changing electric and magnetic fields. When electric fields interact with charged particles and matter consists of charged particles. So when the changing electric field of light interacts with the charged particle of matter, it induces movement into them. This, in turn damps out the changing field. In essence, the light gives up its energy in moving the charges in the molecules of the matter. This increased movement of the molecules is what we interpret as "heat".

Now different substances, react to different frequencies of light in different ways. There are different types of natural modes of movement for different molecules and some frequencies are better at exciting these modes than others. As a result, different materials "absorb" and convert different frequencies of light into molecular movement better than others. Other frequencies can pass through the material or be reflected.

"White light" is really a mixture of all the frequencies of light our eyes can detect. When light hits something, some of the frequencies are converted to molecular motion (heat) and others are reflected. The reflected frequencies are what we see as the color of the object. A black object reflects none of the frequencies, and the white object reflects all of them, which is why a black object gets warmer when left out in the Sun than a white object does. (though a white object will still get warm because it is still absorbing some frequencies, including ones that we don't see.)

29. Originally Posted by grmpysmrf
Originally Posted by GoldenRatio
First problem i see with olbers paradox is "an infinite number of stars" there is a finite ammount that fluctuates daily with birth of new stars & death of old ones.
What you write kinda sounds like people, doesn't it? If we have an infinite amount of time and no threat of species killing accidents I suppose the potential for an infinite number of people exists based on the births and the deaths. Although it sounds more like existence rather than infinity.
Eh, theres some slight differences. For one star's dont have 10 offsprings just to get extra welfare. Humans breed for various reasons & we have an ever growing population that will continue to breed.

Stars however, have no consciousness, thus they dont just breed for the sake of having more offspring. Its all pure physics.

Originally Posted by Strange
Originally Posted by grmpysmrf
I just mean that if light is going to be absorbed by something else isn't it now part of that "something else" instead light?
Normally, it will become heat. In some cases, such as photosynthesis, it can be used to create new chemicals so in that sense become part of it.
Just to add onto this. The universe is full of the electromagnetic spectrum. If your eyes could perceive everything from radio waves to gamma rays. The universe would be supersaturated with "light"

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