c in a vacuum?

[jajrussel]

I tend to get a little confused sometimes when people start talking about c being constant.

 

I thought that c is said to be constant in a vacuum, allowing it in any other states to go slower.

 

So this led me to believe that c had to be a calculated number since it seems to me that actually finding a vacuum where nothing interacts with the photon might be difficult.

 

Yet people constantly talk of photon, partial interaction as if while interacting the photon is in a vacuum state therefore its speed is c.

 

So is c a calculated speed, or do people have a fuzzy way of viewing vacuum?

 

To remove some possible confusion when I say calculated I mean by measuring as precisely as possible then removing mathematically what you can not possibly remove any other way from the controlled experiment that may be affecting the photons speed.

Edited December 25, 2014 by jajrussel

[elfmotat]

c is a fundamental constant related to the geometry of spacetime. The fact that light travels at c in a vacuum is just a consequence of photons having zero mass. Other particles have zero mass as well, and they also propagate at c. I'm not quite sure what you mean by a "calculated speed," so perhaps you could expand on that.

[jajrussel]

c is a fundamental constant related to the geometry of spacetime. The fact that light travels at c in a vacuum is just a consequence of photons having zero mass. Other particles have zero mass as well, and they also propagate at c. I'm not quite sure what you mean by a "calculated speed," so perhaps you could expand on that.

I did expand on it in an edit to my original post when I realized that the term calculated speed might be confusing, but I cannot figure out how to copy it here, so you just need to scroll up and read the bottom portion.

 

I read something to the effect in another thread that special relativity assures that something with zero mass can have no change in velocity magnitude. When I looked up velocity magnitude I read that it basically ment speed, so I am assuming this means that c is c under all conditions. If this is so what is the need for the term vacuum applied to c?

 

Also, what is happening during gravitational lensing? I thought that a change of direction ment a change in acceleration. In this case since the photon can't speed up it had to be slowing down until it reached a point where it could return to its normal state of c.

 

Apparently,I have it all wrong? This is actually a question not just a statement though as a statement it may be true.

 

One more question. In terms is physics when you have something going as fast as it can physically go is it wrong to assume, or say that if it maintains that speed then it is under constant acceleration? I ask because because though it may not be changing direction it would seem that at that point it would still need a reason to change position even at a constant rate, and I can't think of a reason why this particular thought would conflict with Newton's laws of motion.

Edited December 25, 2014 by jajrussel

[MigL]

In the absence of any interaction the speed limit is c .

With interactions, the index of refraction determines the speed limit, which will naturally be always less than, or equal to , c .

In some materials massive particles can travel faster than light does in that material, see Cerenkov radiation.

 

Gravitational lensing means that light is following a 'curved' geodesic, not that it's speeding up or slowing down.

 

Massive particles will maintain their speed according to Newton's law without any force ( or resultant acceleration, which can be a speeding up or slowing down ). But as you approach the limiting value of c , a disproportionate amount of force needs to be applied to speed up ( reaching infinite at c ).

Massless particles cannot speed up or slow down, no matter the amount of force applied; they always travel at c .

[swansont]

So is c a calculated speed, or do people have a fuzzy way of viewing vacuum?

 

c is now a defined value, whose value was chosen based on measurements made back when the meter was a defined value.

 

One could measure c in a gas with varying pressures, and extrapolate that to zero. (such as http://adsabs.harvard.edu/abs/1935ApJ....82...26M) I suspect that the errors here were small compared to other errors in the experiment.

 

See also http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/measure_c.html

[jajrussel]

I believe I read somewhere else that when a photon interacts it ceases to exist then it exist then it interacts again then it ceases to exist then it exist again. That the interaction takes time resulting in the apparent slowing down. Is this what you mean by the refraction index? I'm sorry if I got the question wrong I didn't quote your post and this little box I am writing in won't allow me to see anything but what I am writing in the little box. I am reasonably sure you used the word refraction, but I am not sure about the word index.

 

The reason I want to refer to a photon as being under constant acceleration is that I read once that in vector math a change of position could constitute a change of direction, so I am wondering why a change of position can not be viewed as a form of acceleration. This would seem to allow that gravitational lensing could occur without a seeming need for a photon to slowdown. Then you could call it a geodesic path, or you could call it a curved path the photon is simply changing position at a constant rate.

 

Once I make this little box go away then I can re-read your post then try to understand it better. Thanks.

[jajrussel]

In the absence of any interaction the speed limit is c .

With interactions, the index of refraction determines the speed limit, which will naturally be always less than, or equal to , c .

In some materials massive particles can travel faster than light does in that material, see Cerenkov radiation.

 

Gravitational lensing means that light is following a 'curved' geodesic, not that it's speeding up or slowing down.

 

Massive particles will maintain their speed according to Newton's law without any force ( or resultant acceleration, which can be a speeding up or slowing down ). But as you approach the limiting value of c , a disproportionate amount of force needs to be applied to speed up ( reaching infinite at c ).

Massless particles cannot speed up or slow down, no matter the amount of force applied; they always travel at c .

I have a tendency to focus too entirely on my own thoughts sometimes, and it happened here. I should have been paying more attention to what you wrote. Instead I rambled on in senseless confusion. I am sorry?

 

If the refractive index controls the speed limit, and massless particles cannot speed up or slow down what exactly is the refractive index presenting?

 

If you read an article it usually says something like light moves slower through water when explaining the refractive index. As a massless particle which speed is said constant this doesn't make sense to me unless I assume that c is the speed of light in a vacuum, and that it's speed can be slower in a more dense medium, and that in that same medium it's speed will be constant throughout that medium because it is interacting at a fairly constant rate.

 

Now if the massless particles speed has to be c the only explanation that does make sense to me is that over a given distance the particle is constantly changing direction, and the rate of change is determined by the medium through which it is passing. This would make the slower speed be to be an illusion, and if that is what it is why not just say that is what it is then give the reason why.

 

Even in this post you wrote;

 

In some materials massive particles can travel faster than light does in that material, see Cerenkov radiation.

 

Is that exactly what is happening, or are the massive particles just crossing the finish line first because they are taking the more direct route?

 

I still need to see Cerenkov radiation. Thank you for pointing me on that direction.

 

As for geodesics, I understand the desire for the term more than I do the need. Maybe as I learn more my understanding will change.

Edited December 30, 2014 by jajrussel

[Endy0816]

 

If you read an article it usually says something like light moves slower through water when explaining the refractive index. As a massless particle which speed is said constant this doesn't make sense to me unless I assume that c is the speed of light in a vacuum, and that it's speed can be slower in a more dense medium, and that in that same medium it's speed will be constant throughout that medium because it is interacting at a fairly constant rate.

 

Yeah, Light's speed in a vacuum should be what is said to be constant(an unchanging numerical value). We tend to erroneously call c the speed of light and leave off the "in a vacuum" part of the definition.

 

Might want to think of light's velocity in terms of c/n(speed of light in a vacuum / refractive index). Keeping in mind that information transfer will always be at c or less.

Edited December 30, 2014 by Endy0816

[jajrussel]

c is now a defined value, whose value was chosen based on measurements made back when the meter was a defined value.

 

One could measure c in a gas with varying pressures, and extrapolate that to zero. (such as http://adsabs.harvard.edu/abs/1935ApJ....82...26M) I suspect that the errors here were small compared to other errors in the experiment.

 

See also http://math.ucr.edu/home/baez/physics/Rrelativity/SpeedOfLight/measure_c.html

In the first link they created an environment free of mass particles to act as the medium for photons travel through?

 

Is it safe to say that in its own medium a photon will travel at c? Or, would this be dependent on the mediums energy? I am not sure I have asked the question right.

 

Does radiation have a density based on its energy, and would this density effect a photon of lower energy value passing through it?

 

The second link eventually led me to an article about the moon spinning around my head which I can not even begin to explain, but it was making sense while I read it and led to a thought of could the first link experiment have been designed smaller using a laser mounted on a spinning shaft inside a box with a sensor to represent the moon, or maybe a laser mounted on one side of the box with a mirror on the spinning shaft and a sensor mounted somewhere on the box that would allow extrapolation to calculate c?

[swansont]

In the first link they created an environment free of mass particles to act as the medium for photons travel through?

 

 

No, that's impossible to do. There is no perfect physical vacuum. You can measure the speed as a function of density (which is related to the index of refraction) and extrapolate to zero.

 

Is it safe to say that in its own medium a photon will travel at c? Or, would this be dependent on the mediums energy? I am not sure I have asked the question right.

 

Does radiation have a density based on its energy, and would this density effect a photon of lower energy value passing through it?

 

 

Light needs no medium, and is not a medium. I'm not aware of any effect on the speed of light from intensity, unless there is an interaction with the medium itself which changes the index. But that's a change in n, not c.

[jajrussel]

Will two laser beams interfere with each other?

Why does a laser get wider with distance?

It is my understanding that this will happen even in space.

What causes the laser to get out of phase?

If a black hole is emitting a stream of light will the light from a star on the other side of the stream pass through it?

[swansont]

Will two laser beams interfere with each other?

Why does a laser get wider with distance?

It is my understanding that this will happen even in space.

What causes the laser to get out of phase?

If a black hole is emitting a stream of light will the light from a star on the other side of the stream pass through it?

 

Lasers can interfere under the right circumstances.

 

Light generally spreads out. Non-diverging beams are only an ideal case.

 

The temporal coherence of a laser is limited. The frequency is not exact — there will be some width to it, so a phase difference will accumulate over distance, and time. http://en.wikipedia.org/wiki/Coherence_(physics)

 

Black holes don't emit light.

[jajrussel]

Black holes don't emit light.

 

I need to find out why I thought they did?

But, to your statement I assume you mean in streams, or do you mean it as specifically as you stated?

 

Okay, I see what you mean. Quasars are not a part of the black hole, but are a part of the system. The stream is made up in part of mass particles as opposed to pure radiation, so the question is invalidated.

Edited January 21, 2015 by jajrussel

[swansont]

Black holes don't emit light.

 

I need to find out why I thought they did?

But, to your statement I assume you mean in streams, or do you mean it as specifically as you stated?

 

Okay, I see what you mean. Quasars are not a part of the black hole, but are a part of the system. The stream is made up in part of mass particles as opposed to pure radiation, so the question is invalidated.

 

Areas outside of a black hole can give off light, but black holes themselves don't, by the very definition of a black hole.

[jajrussel]

The questions may sound silly, but I was thinking about spectrums projected on a screen when I was a kid, and was wondering if two projections were made to cross would they interfere, but then I realized that they are moving through atmosphere, and wondered what would happen in a vacuum. Would the vacuum effect the spectrum? Then there is still the question of would they interfere? As in would the projected spectrums be different than they would be if they were not made to cross?

[mothythewso]

I started to say, as was stated above, that because of virtual particles, there is no "true" vacuum. And I wondered if a real photon can interact with a virtual anti-photon and annihilate if they should somehow "meet"? But first, I decided to research virtual photons (having been a former biology major and not wishing to make a fool of myself), and now my head hurts! Is there an explanation using words of no more than five syllables?

[jajrussel]

I started to say, as was stated above, that because of virtual particles, there is no "true" vacuum. And I wondered if a real photon can interact with a virtual anti-photon and annihilate if they should somehow "meet"? But first, I decided to research virtual photons (having been a former biology major and not wishing to make a fool of myself), and now my head hurts! Is there an explanation using words of no more than five syllables?

Are you in the right thread? There is no need for five syllable words a simple yes or no will do.

 

On the other hand if you are on the right thread five or more syllable words may be necessary because smaller words don't seem to be working.

 

Actually your statement, ment for me or not, pointed me in an interesting direction that may help to shed some light on what I have been thinking about though it is much more technical than I would have liked.

 

A perfect vacuum,I do understand is unattainable. Assuming empty space is a perfect vacuum having zero density. What I am thinking is that light increases density, or put another way enough energy in the form of light increases spacial density even though it has no mass.

 

It was suggested in another thread that that light did not need a medium to move through, but that said light moving through this non medium might have a space time curvature effect(hence gravity). I was thinking that there might be a simple way of testing assuming that light moving at cross purposes might cause ripples in this space time curvature and show up as a visual effect. Though I admit I am not expecting to see ripples, but merely something different than what I would have seen without the light at cross purposes.

 

I was wondering if crossing light spectrums might do the trick?

Edited January 22, 2015 by jajrussel

[swansont]

The questions may sound silly, but I was thinking about spectrums projected on a screen when I was a kid, and was wondering if two projections were made to cross would they interfere, but then I realized that they are moving through atmosphere, and wondered what would happen in a vacuum. Would the vacuum effect the spectrum? Then there is still the question of would they interfere? As in would the projected spectrums be different than they would be if they were not made to cross?

 

The vacuum would remove a small amount of refraction from the atmosphere, but the interference would depend on the coherence of the light. Not likely you would see anything, as projectors are generally not coherent sources.

[mothythewso]

After my head stopped hurting, I recalled that although virtual particles are very short-lived, their life span increases with the mass of the specific particle. It follows that virtual photons should be the geezers of virtual-particlehood. I started wondering about neutrinos, that the neutrino is its own anti-particle. Is there a virtual neutrino? Why isn't the speed of neutrino propagation the standard max speed of the physical universe, n vs. c ? What I'm saying (poorly) the max speed is light in a vacuum, but there's no "real" vacuum because of virtual particles, plus all the detritus of empty space. Photons aren't totally non-interacting, and what would happen if a photon should run into a virtual photon pair, could that happen considering virtual photon pairs are relatively long-lived. The neutrino , as I understand it, is the most inert of all particles, it should be less affected by the vacuum or anything in it, its speed should more representative, on the whole, of the max speed of the universe.

[Strange]

After my head stopped hurting, I recalled that although virtual particles are very short-lived, their life span increases with the mass of the specific particle.

 

Lifetime is the inverse of energy: more energetic virtual particles exist for less time.

[mothythewso]

My bad, brain-typing finger disconnect.

[swansont]

"Vacuum" has multiple definitions, and you can't swap them. One vacuum is just a region of lower pressure – a vacuum chamber, for instance, and you quantify the quality of the vacuum by measuring the pressure. Another vacuum is the absence of all matter (what I referred to as a physical vacuum). A third definition is the absence of anything at all. There is no physical vacuum; that's an idealized extrapolation. The idea of a vacuum being nothing at all is long deprecated. There's really no point in discussing it, IMO. But you can't swap those two situations and make a consistent argument, since the situations are very different.

[studiot]

The speed of light is a measured quantity.

There is no theoretical derivation of the figure, although it is the ratio of other measured quantities.

 

Here are the results of the first hundred years since we have been able to measure it.