Massless Energy & Nothing.

[lindagarrette]

 

Does that make sense? So this remains true to the fact that light is a constant speed within a vacuum as it does in a denser medium (such as water).

I understood this the first time I heard it and I understand very little about physics.

 

Damien

All along the light spectrum (and beyond) the speed is equal to the wavelength times the frequency. All waves arrive at the same time. If the light contains all wavelengths, we see it as white.

 

The variable speed of light through different media is essential for us to construct lenses.

[Damo2600]

Sorry Linda,

 

I don't understand if you are agreeing or disagreeing. I'm not that . Could you please rephrase your reply. I understand that light in vacuo has shorter and longer wavelengths this is why there is a spectrum right. So in water the frequency is shortened so that there are still shorter waves and longer waves. The wavelengths have been scaled to travel in that medium. We still see the entire spectrum however the light is slower. I maintain that the photon is travelling at a constant wave speed as was described in my previous post however the wave length has shortened.

 

You have wavelengths. Right? You have the speed of light (which is what we see). Right? But we need to also consider the speed that photons are travelling along the waves. Right? I'm not sure the correct term I described it as wave speed. Can anyone help me out?

 

We see light travelling in a straight line we do not see how fast the light is travelling along the waves.

 

Damien

[Damo2600]

Oh I just thought of something by tightening up the wave lengths uniformly the light would travel at the same wave speed however there would be no red or blue shift. The basic point is that light does not travel in straight lines so you need to consider the wave speed.

 

We cannot merely consider the time it takes to get from point A to point B.

 

Damien

[paultrr]

I'll state it a little more clearly the wave speed is EXACTLY the same.

The time it takes to travel from A to B is different.

How can this be that light has slowed down?

 

(Frozen light is NOT the same principle.)

 

Can I be more obvious?

 

Damien

 

The wave speed is not the same in different mediums. C is only constant in the medium of the vacuum. Its not constant in all mediums. Einstein himself never considered C to be constant in anything but a vacuum. All experiments have shown over time that C does vary from medium to medium. Lorentz invariance only holds for the vacuum itself. In fact, if one alters the vacuum dielectric properties C changes. There are specific conditions of the vacuum specified for C to be invariant globally or locally.

 

Is the speed of light constant?

My concern is about the constancy of c (the speed of light); as light goes into a denser medium than the medium it was previously travelling in, the light slows down and it bends as a cause of this. This seemed contradicting to me that the speed of light is constant and yet its speed varies in different mediums. Why isn't c constant in all mediums and does this variation of c in different mediums mean in anyway that time flows differently in different mediums?

 

You are correct that talk of the speed of light can be misleading. When we discuss the constancy of c we mean the speed of light in a vacuum. Since interstellar space is very empty and much like a vacuum, astronomers usually don't have to consider the speed of light in different media.

 

However, the speed of light is not constant as it moves from medium to medium. When light enters a denser medium (like from air to glass) the speed and wavelength of the light wave decrease while the frequency stays the same. How much light slows down depends on the new medium's index of refraction, n. (The speed of light in a medium with index n is c/n.) The index of refraction is determined by the electric and magnetic properties of the medium. For air, n is 1.0003, for ice, n is 1.31, and for diamond, n is 2.417. The bending of the light you mentioned upon entering a denser medium is how lenses work.

 

Although the speed of light is no longer constant when we think about different media, we do know that light always travels fastest in a vacuum. Nothing can reach speeds faster than c (well that's not exactly true, see here for more details). Thus from our equation v=c/n, n must always be greater than 1. Light moves slower through denser media because more particles get in its way. Each time the light bumps into a particle of the medium, the light gets absorbed which causes the particle to vibrate a little and then the light gets re-emitted. This process causes a time delay in the light's movement so the more particles there are (the more dense the medium), then the more the light will be slowed down.

 

July 2004, Sabrina Stierwalt

 

From: http://curious.astro.cornell.edu/question.php?number=630 ASK AN ASTRONOMER

 

Which is about as close to what you where saying. But notice the speed of light does change in each medium.

[paultrr]

The speed of light in a vacuum is exactly equal to 299,792,458 metres per second. The velocity of light is dependent not only on the dielectric permittivity, e,in free space(vacuum) designated as e(0); but also on the magnetic permeability of a medium, m,which for the vacuum is designated as m(0).

 

The propagation velocity for electromagnetic waves, c, is related to e and m according to the following equation,

 

1/c^2 = msub0esub0

 

c = 1/[msub0esub0]^1/2

 

for any medium in question. In these equations a 1 equals the velocity of light in the vacuum. All other cases where the answer is less than 1 or greater than 1 translate to C having a different velocity in that medium. Fill in the blanks if you wish since the common values for different materials can be had even online to get you're answer. C varies from medium to medium and some of the modern exploration on this includes speculation that even the vacuum may vary somewhat itself from place to placeor across time. One time period not even argued on is during inflation these properties for the vacuum where not the same as we globally observe today. C as we define it did vary during that time period. At the present the majority evidence would suggest that C has remained constant since that period meaning our vacuum has remained stable since then.

 

The velocity of electromagnetic wave propagation is described by the speed of light in a material. The speed of light in a material that is not a vacuum is always slower than the speed of light in vacuum and its only in the vacuum itself that C is properly considered as invariant. There are also cases known where C can be higher where the above properties of a material or even a vacuum have been altered(ie a Casmir vacuum state and acceleration mediums). Einstein's equations under GR utilized the stress energy tensor which also incoporates the above as just one aspect of that tensor too. C is seen as only constant in the vaccum and only if the vacuum in question has a certain dielectric permittivity and magnetic permeability. If either of those values for a specific vacuum state or material change then the velocity of light will change. This is taught commonly in physics, in electronics, and appears in different articles on the subject across the board. C is only constant in a vacuum. That was what Einstein postulated and what is taught across the board by accepted standards.

[paultrr]

Oh I just thought of something by tightening up the wave lengths uniformly the light would travel at the same wave speed however there would be no red or blue shift. The basic point is that light does not travel in straight lines so you need to consider the wave speed.

 

We cannot merely consider the time it takes to get from point A to point B.

 

Damien

 

Yes, its far more than a point A to point B thing. The term or usage of constant when it comes to C is misleading somewhat. We general think of something as constant if it never changes period which is not actually the case with C from material to material. But, Einstein's point was that from all measurments C remains constant in a vacuum which brings up Lorentz invariance in the process whereby all reference frames in a vacuum measure the same speed of light. One could also say that in air all frames measure the same speed also or in water as long as the medium in question does not itself vary at least globally. Its not a violation of relativity to know that C varies from one medium to another. Its just one of those things where something we call a constant is only such when the medium in question remains the same.

[BlameTheEx]

Energy has mass - it is implied in E=MC2 for two reasons:

 

1) The formula was based on the assumption that matter consists of standing waves travelling at the speed of light. ALL energy travels at C. It is just that some energy travels in circles.

 

2) E=MC2 can be rearranged as M=E/C2. Mass is directly proportional to energy regardless of its nature.

 

 

There is only confusion when the concept of REST mass comes into play. This concept is different from mass. A photon clearly has no rest mass because it has no rest energy however it DOES have energy and it DOES have mass.

 

In this I am defining mass as the ability to be attracted by gravity, the creation of a gravitational field (however small) and momentum.

 

 

We know a photon has momentum because it has been measured.

 

We know a photon can be attracted by gravity because the sun alters the direction of photons passing close to it.- again a measured effect.

 

We can deduce that photons create a gravitational field from the above 2 facts and the need to conserve momentum. If the sun pulls a photon then the photon must be pulling at the Sun. In effect the photon is pulling the sun with its gravitational field. More that in effect - the photon must actually have a gravitational field.

 

To argue that the photon is just following a straight path in gravity warped space would also be correct but that is just a different way of describing the same thing. A bit like the arguments over whether photons are waves or particles.

 

Personal I would suggest that in physics when you find different ways of describing something, and they all work, you are probably looking at something fundamental.

[Damo2600]

Thanks Blame the Ex,

 

I was sure someone out there could explain the difficulty I was having.

 

"To argue that the photon is just following a straight path in gravity warped space would also be correct but that is just a different way of describing the same thing."

 

So when my body is falling due to gravity we can say "My body is just following a straight path in gravity warped space ."

 

This does not however mean that my body has no mass. So the following equation is correct:

 

m = sqrt{E^2/c^4 - p^2/c^2}.

 

When p = c

 

m = E/c2 (I get it now)

 

Therefore to substitute the value:

 

E = pc

 

is saying that E equals the speed of light times the speed of light. Where did this understanding come from?

 

Because if we substitute it into Einstien's equation:

 

E = mc^2

 

pc = mc^2

 

pc/c^2 = mc^2/c^2

 

You cancel out the c^2, leaving:

 

0 = m

 

So E cannot equal c^2. Is this right?

 

(I will blame the ex if you can't answer this - lol)

 

Damien

[BlameTheEx]

Damo

My apologies but I have not quite grasped your maths.

Perhaps you explain m=sqt{E^2/c^4-p^2/c^2} in a previous post that I have not yet read.

 

E does not equal C^2 unless you define m as 1.

Starting with pc/c^2=mc^2/c^2

we can simplify (by cancelling out the c's) to p/c= m.

Are you still making p=c? if so you get m=1 not m=0.

m=1 is consistent with your E=c^2 but m=0 would require p=0.

[Damo2600]

Hi Blame the Ex,

 

Sorry you are absolutely correct :Alien: . I stated in a previous post that my maths skills are not the best.

 

Someone posted in a #37 of this thread the following link. This link states that light can be seen as having relative mass as well as an invariant mass of zero. I have no idea where they got this equation so I was asking yourself or anyone else.

 

http://math.ucr.edu/home/baez/physi...light_mass.html

 

E = pc

 

m = sqrt{E^2/c^4 - p^2/c^2}.

 

m = pc/c^2 - p^2/c^2

 

So in this equation m = 1 - 1 = 0

 

Or m = E/c^2 - 1

 

If I place E = pc into Einstien's equation you get:

 

c^2/c^2 = mc^2/c^2

 

Which gives m = 1 as you stated.

 

So you have two equations giving two seperate results.

 

If you substitute E = mc^2 into the equation:

 

m = sqrt{E^2/c^4 - p^2/c^2}.

 

m = sqrt{(mc^2)^2/c^4 - p^2/c^2}.

 

m = mc^2/c^2 - p^2/c^2

 

m = m - 1

 

We have this strange third result. The - 1 in each result is causing some problem.

 

I don't know. Can you look at the link and make heads or tails of it?

 

Damien

 

(edit: it appears they have added minus one to the Einstien equation so you get a zero result - that's as far as I can tell)

[WebFeet]

The formula was based on the assumption that matter consists of standing waves travelling at the speed of light. ALL energy travels at C. It is just that some energy travels in circles.

 

We can deduce that photons create a gravitational field from the above 2 facts and the need to conserve momentum. If the sun pulls a photon then the photon must be pulling at the Sun. In effect the photon is pulling the sun with its gravitational field. More that in effect - the photon must actually have a gravitational field.

There must be a single cause of gravity.

If the photon creates a gravitational force, a photon being energy, and all mass consisting entirely of energy, would it be consistant to say that energy/photon is the cause of gravity ?

 

This would mean that G - gravitational constant - must be able to be derived from the energy/photons radiating from a mass.

[Damo2600]

Hi Web Feet,

 

In answer to your question basically no. The energy within an atom does not consist of photons alone. There are other particles beside the photon on a sub atomic level. However I have spoken to one person who has studied physics for an extremely long time and has a theory that is in a round about way similar to what you are saying.. He hasn't proven his theory so I won't state it as being a fact. It's not a flat out no because we don't actually know what a proton, electron or neutron actually are. So perhaps on some level what you are saying may have some truth to it. However it's is a present time no though until we are able to find out more about what particles really are. If we ever do.

 

Damien

[Qfwfq]

It's not a flat out no because we don't actually know what a proton, electron or neutron actually are.

Do we know what a table or a chair actually are?

 

We know quite a lot about the structure of nucleons and of other hadrons.

[WebFeet]

So perhaps on some level what you are saying may have some truth to it. However it's is a present time no though until we are able to find out more about what particles really are. If we ever do.

Doesn't that make it less of a NO and more of a MAYBE

 

After all, until we can look inside an atom without having to smash it to bits, as you say, we'll never really know.

[Qfwfq]

After all, until we can look inside an atom without having to smash it to bits, as you say, we'll never really know.

But we have looked into atoms without smashing them into bits. Not all methods of investigation are destructive, starting with spectroscopy and Rutherford, f'rinstance.

[Damo2600]

Hi Paul,

 

After you posted the explanation by Sabrina Stierwalt I took a mental note to look into it further. Now the following site gives a explanation on 'the wave nature of light'.

 

http://wine1.sb.fsu.edu/chm1045/notes/Struct/Wave/Struct01.htm

 

It gives the equation to work out the speed of light:

 

Speed of the wave = (distance between peaks) * (frequency)

 

= (wavelength) * (frequency)

 

Now this formula gives the value of the speed of the wave from point A to point B (i.e. if the wave length is shortened light will take longer to move between the two points. Right?). Since light travelling through a denser medium doesn't change the frequency. The speed of the wave has changed due to the *wavelength only* as I did state. This does not give the speed of the photon travelling along the wave it only gives a one dimesional speed. Right? So I would be correct in stating that the speed of light is constant and a shorter wave length will slow down light. That is, since the wave speed and speed of the wave(in a one dimesional line) are different, I assume I'm correct.

 

Can anyone verify this? (although I think I just did)

 

So the speed of a photon is invariant. Unless someone has something to add that contradicts the above calculation...

 

Thanks

Damien

[Tormod]

I can recommend the Particle Adventure site for more info about the different particles:

 

http://particleadventure.org/particleadventure/