Freethought & Rationalism Archive

Answerer · 07-02-2002, 06:12 PM

Quote:

Originally posted by Kachana:

Given that light can only travel at c, would it be more like a car that goes 60mph when you hit the gas, and then turns into something else when you hit the brakes? Or do photons stop for a bit when they are absorbed?

Well Kachana, if light travelling in a vaccum doesn't have a constant value no matter what happens, you will see the breakdown of all laws of physics and the world that you are living in will not behaving in the way it is now. Even experiment show the constancy of light's speed to be a fact.Light will not speed up or slow down(unless through a medium) but rather the spacetime of observer will be affected.

Answerer · 07-02-2002, 06:18 PM

Quote:

Originally posted by Kenny:


It is true that photons have momentum, but this does not mean they have mass. See my first post here to see how that it possible. It is sometimes said that photons have no “rest mass,” but do have “effective mass” as a result of their motion, but many physicists do not like the concept of rest mass preferring to view rest mass as simply mass and what you have called “effective mass” as merely a consequence of an object’s relativistic momentum. You can associate a sort of mass with photons. The energy of a photon is given by E = hf (where h is Plank’s constant and f is the frequency of the photon). You can also say since E = mc^2, mc^2 = hf, m = hf/c^2. But, this is already fudging things a little (the m in E = mc^2 properly refers to “rest mass” or just plain mass depending on how you want to look at things, not “effective mass” associated with motion) and this mass is a pseudo-mass at best.

God Bless,
Kenny

[ July 02, 2002: Message edited by: Kenny ]

If I'm not wrong, in modern physics, there is never a clear definitions of mass as not even modern physicists, themselves understand its full concept. Since energy is a form of mass, as long as photon carry energy, it is regarded as having a mass(not rest one, of course). Anyway, do you have any idea of what rest mass is make up of? Gravitons?

John Page · 07-02-2002, 07:06 PM

Quote:

Originally posted by Answerer:
If I'm not wrong, in modern physics, there is never a clear definitions of mass....

Glad to hear this. Aren't we just drawing analogies between stuff we experience more directly (balls, string etc.) and what happens at more basic levels of existence?

Isn't mass an effect rather than a substantial property of an object? Taking a cognitive view, isn't solidity (concreteness) just a phenomenon detected when "effects" can't be intermingled in the same space? I have a mental image that a neutrino is a very small particle (small ball or ball-like thing) but it seems to be able to dodge some pretty substantial "matter".

Anyone know of any sources that follow up this theme?

Cheers, John

Friar Bellows · 07-02-2002, 11:12 PM

Quote:

Originally posted by John Page:
Anyone know of any sources that follow up this theme?

I'm not sure I follow your theme, but if you're looking for a book devoted to exploring the concept of mass try:

Concepts of Mass in Contemporary Physics and Philosophy
by Max Jammer
Princeton U. P., Princeton, N.J., 2000. 180 pp. $39.50 hc
ISBN 0-691-01017-X

Here's a <a href="http://www.physicstoday.org/pt/vol-53/iss-12/p67b.html" target="_blank">review</a>.

Here it is at <a href="http://www.amazon.com/exec/obidos/ASIN/069101017x/002-6047557-8800059" target="_blank">amazon.com</a>.

KontinMonet · 07-03-2002, 08:55 AM

Quote:

Originally posted by beausoleil:


If I may...

<a href="http://www.wikipedia.com/wiki/refractive+index" target="_blank">http://www.wikipedia.com/wiki/refractive+index</a>

From which:
On an atomic level, the slowing of light as it passes through a material may be considered as a continuous process of absorption and emission of photons as they interact with the atoms of the material. Between each atom, the photons travel at c, as in a vacuum. As they impinge on the atoms, they are absorbed and near-instantly re-emitted, creating a slight delay at each atom which (on a large enough scale) seems to be an overall reduction in the speed of the photons.

But, at an atomic level, why should a photon be re-emitted almost always in the same direction? Does the atom 'remember' the photon's momentum?

Shadowy Man · 07-03-2002, 09:48 AM

This was my basic argument. Also, if the photon is absorbed by an atom, it should do so at only discrete wavelengths, as the energy states of the atom are quantized.

I think this is more of a breakdown in trying to use the "particle" aspect of light to describe this. It is probably better to use the "wave" nature of light when describing passage through a medium. Then you can discuss the interaction of the oscillating EM wave with the polarizability of the material, etc. etc.

Personally, I'm a one atom, one photon kind of guy.

Edit: Reading that link, I prefer the argument made after the absorption/emission argument:

Quote:

The absorption and emission process can be thought of as the electric field of a photon creating an oscillating force on the charges of each atom (primarily the electrons). This oscillation of charges itself causes the radiation of an electromagnetic field, which is slightly out-of-phase compared to that of the original photon, leading to a slight retardation of the field and an apparent delay in the photon's travel.

[ July 03, 2002: Message edited by: shadowy_man ]

[ July 03, 2002: Message edited by: shadowy_man ]

KontinMonet · 07-03-2002, 01:06 PM

Quote:

Originally posted by shadowy_man:

Edit: Reading that link, I prefer the argument made after the absorption/emission argument:

The absorption and emission process can be thought of as the electric field of a photon creating an oscillating force on the charges of each atom (primarily the electrons). This oscillation of charges itself causes the radiation of an electromagnetic field, which is slightly out-of-phase compared to that of the original photon, leading to a slight retardation of the field and an apparent delay in the photon's travel.

Presumably, the same goes for particles such as electrons. If there is no adequate explanation of the vectored propagation of light (or electrons) by considering them as particles, why are we not taught all this stuff using wave functions?

Shadowy Man · 07-03-2002, 01:32 PM

It is probably just an equivalent wavelength issue. Electrons, having mass, will have a de Broglie wavelength equivalent to a high energy photon (although I'm not in a position to whip out my calculator and do the math), and I bet you probably can use a nice particle-photon approach to the propagation of x-rays through materials.

But I hold judgment until I do the math...

Mr. Sammi · 07-04-2002, 11:42 AM

Accelerating has an inherent implication of additional impetus. Light is the last stage of all particle theory. This means whatever mass/energy existed before the light stage, is now all converted to quanta, whose interaction with the medium through its heat energy content causes it to "glow". There is no possibility of acceleration simply because there can be no additional impetus. One simply cannot give the quanta more of anything. (except shade OR a good home).

The propogation effect is an interesting effect.

Sammi Na Boodie ()

John Page · 07-04-2002, 03:32 PM

Quote:

Originally posted by Friar Bellows:
Concepts of Mass in Contemporary Physics and Philosophy

Thanks, that sound like what I need.

Cheers, John

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07-03-2002, 01:32 PM	#38
Shadowy Man Veteran Member Join Date: Jun 2002 Location: A Shadowy Planet Posts: 7,585	It is probably just an equivalent wavelength issue. Electrons, having mass, will have a de Broglie wavelength equivalent to a high energy photon (although I'm not in a position to whip out my calculator and do the math), and I bet you probably can use a nice particle-photon approach to the propagation of x-rays through materials. But I hold judgment until I do the math...

07-04-2002, 11:42 AM	#39
Mr. Sammi Banned Join Date: Jun 2002 Location: Montr�al Posts: 367	Accelerating has an inherent implication of additional impetus. Light is the last stage of all particle theory. This means whatever mass/energy existed before the light stage, is now all converted to quanta, whose interaction with the medium through its heat energy content causes it to "glow". There is no possibility of acceleration simply because there can be no additional impetus. One simply cannot give the quanta more of anything. (except shade OR a good home). The propogation effect is an interesting effect. Sammi Na Boodie ()

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