Nieminen Juha wrote:
> 
> Simen Kvaal <sim### [at] studentmatnatuiono> wrote:
> : Light has mass too! According to Einstein, light has no resting mass, but
> : relativistic mass.
> 
>   I wouldn't say that light actually has mass.
>   AFAIK light has no mass. Period.

  I heard an astronomer say on an educational program just 2 days ago
that light does indeed have mass. The surface of the earth is bombarded
with roughly the equivalent of 2 pounds of light per day. This is of
course heresay and would not hold up in a court of law. The program
that I heard this on a is called "Universe: The Infinite Frontier"
which airs on a local public broadcast station (PBS).

-- 
Ken Tyler
1100+ Povray, Graphics, 3D Rendering, and Raytracing Links:
http://home.pacbell.net/tylereng/index.html

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Simen Kvaal <sim### [at] studentmatnatuiono> wrote:
: Light has mass. Period!! It's not just a mathematical trick. Light *has*
: relativistic mass, and so have you. When travelling faster and faster, you
: gain weight. You cannot travel at the speed of light, because you'd have
: infinite mass:

: m = m0 / sqrt(1 - (v*v/c*c))

: If m0 is your resting mass, v is your velocity and c is the speed of light,
: then relativistic mass (aka REAL mass) is m. You can see from the equation
: that when your speed approaches the speed of light, your mass diverges into
: infinity. These are the equations Einstein found, and they have been
: *empirically* verified!

  Actually it was Lorentz who first proposed the relativistic transformation
of distances with the factor 1/sqrt(1-v*v/c*c). Einstein always refers to
the Lorentz transformations.

: Light has energy, right? Then, light must have mass. It cannot have resting
: mass, because light doesn't rest.

  Well, the actual reason why it cannot have resting mass is because it
wouldn't travel at speed c if it had.

  Now tell me something:
  When light travels through matter (for example water), it travels at a
speed which is less than c. Because of this, the factor 1/sqrt(1-v*v/c*c)
gets a non-infinite value, which multiplied with the rest mass of the
light gives us 0. This would mean that the energy of the light travelling
through water would be 0.
  How is this possible?

:>reason why light bends near massive objects is because the space is curved
:>there and the light tends to move along the geodesic lines of the space
:>(which are actually the shortest way from one point to another). The
:>massive object doesn't attract the light, it's just bending the space.
:>

: Correct, afaik.

  Is the bending of the light caused by the curvature of the space,
the mass of the light (which the massive object attracts), or both?
  If both, then it would seem that the light would not stay in the
geodesic line because the massive object is attracting it out of that line.

: Okay. I was wrong there. But light has mass! The solar winds was just a
: (very) bad example. You can verify that photons actually may cause pressure!
: This is the empirical prof of the mass of light. You cannot have pressure
: without mass.

  How can I verify that photons may cause pressure?

-- 
main(i,_){for(_?--i,main(i+2,"FhhQHFIJD|FQTITFN]zRFHhhTBFHhhTBFysdB"[i]
):5;i&&_>1;printf("%s",_-70?_&1?"[]":" ":(_=0,"\n")),_/=2);} /*- Warp -*/

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On Thu, 30 Sep 1999 10:39:37 +0200, Simen Kvaal wrote:
>E = f * k
>
>where f is the frequency and k is a constant (which i cannot remember; it's
>three years since i learnt this). Put this into the "emc" and you get:

Planck's Constant, usually denoted with an h.  6.63e-34, if my calculator 
is correct. :)

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On 30 Sep 1999 09:44:34 -0400, Nieminen Juha wrote:
>  When light travels through matter (for example water), it travels at a
>speed which is less than c. Because of this, the factor 1/sqrt(1-v*v/c*c)
>gets a non-infinite value, which multiplied with the rest mass of the
>light gives us 0. This would mean that the energy of the light travelling
>through water would be 0.
>  How is this possible?

The explanation I always got is that the photon is being absorbed and
re-emitted each time it hits a water molecule, but it is traveling at
c in the interstices between molecules.

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Ron Parker <par### [at] fwicom> wrote:
: The explanation I always got is that the photon is being absorbed and
: re-emitted each time it hits a water molecule, but it is traveling at
: c in the interstices between molecules.

  The speed of the photon must slow down at least somewhere so that
the average speed could be less than c.
  If the photon is absorbed by a molecule, why does the photon leave the
molecule in the exact same direction as it entered it?

-- 
main(i,_){for(_?--i,main(i+2,"FhhQHFIJD|FQTITFN]zRFHhhTBFHhhTBFysdB"[i]
):5;i&&_>1;printf("%s",_-70?_&1?"[]":" ":(_=0,"\n")),_/=2);} /*- Warp -*/

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Nieminen Juha skrev i meldingen <37f36942@news.povray.org>...
>Simen Kvaal <sim### [at] studentmatnatuiono> wrote:
>: Light has mass. Period!! It's not just a mathematical trick. Light *has*
>: relativistic mass, and so have you. When travelling faster and faster,
you
>: gain weight. You cannot travel at the speed of light, because you'd have
>: infinite mass:
>
>: m = m0 / sqrt(1 - (v*v/c*c))
>
>: If m0 is your resting mass, v is your velocity and c is the speed of
light,
>: then relativistic mass (aka REAL mass) is m. You can see from the
equation
>: that when your speed approaches the speed of light, your mass diverges
into
>: infinity. These are the equations Einstein found, and they have been
>: *empirically* verified!
>
>  Actually it was Lorentz who first proposed the relativistic
transformation
>of distances with the factor 1/sqrt(1-v*v/c*c). Einstein always refers to
>the Lorentz transformations.
>

Okay, i have no problem with that. But that doesn't mean that light doesn't
have mass? Do you agree when I say that light has mass?

>: Light has energy, right? Then, light must have mass. It cannot have
resting
>: mass, because light doesn't rest.
>
>  Well, the actual reason why it cannot have resting mass is because it
>wouldn't travel at speed c if it had.
>

That's just what I was trying to say, but in other words. (Mine was
unclear.) If light travels at the speed of light, it would have infinite
mass, which it clearly doesn't. (I thing I'd get a headache if I turned on a
lamp in my room if it did...)

>  Now tell me something:
>  When light travels through matter (for example water), it travels at a
>speed which is less than c. Because of this, the factor 1/sqrt(1-v*v/c*c)
>gets a non-infinite value, which multiplied with the rest mass of the
>light gives us 0. This would mean that the energy of the light travelling
>through water would be 0.
>  How is this possible?

Hard question that one. I kept wondering, but now I got the solution! The
equation for m that I gave:
m = m0 / sqrt(1 - (v*v/c*c))
cannot be used. AFAIK is photons travelling at the speed of light a special
case.This equation is to be used with bodies travelling at speed v
*relative* to the observer, but light *always* travels at speed c *relative*
to the observer. Actually, the factor 1/sqrt(1-v*v/c*c) doesn't give meaning
to bodies (photons) travelling at the speed of light. It is exactly the
same! It is just another view of the photons not having resting mass, but
relativistic mass only. It's not really hard to accept that if light travels
at the speed of light (naturally) and *only* that, we must in some way have
a special case. (It is the light itself that makes the equations possible.)

Thus, the energy of light travelling through water is not 0. When the light
goes out in the vacuum again, it's speed also increases. Thus, it is
impossible that light loses energy on the way in and gains on the way out.
(Where should the photons take their energy from?) What actually happens, is
that the wavelength og the light changes with refraction. You can see this
if you send white thru a prism: Different wavelengths refracts differently
and produces a spectre on the other side. (There is a path for POV-ray to
simulate this, i think.)

In physics I learnt that when a wave on water hits a shallower area, the
wavelength and speed must change in order to preserve energy. You can see
waves on water refract like light, hitting a diagonal, shallow area. It's
exactly the same with light: speed and wavelength changes during refraction
in order to preserve energy. The rules for index of refraction
(sin(alpha1)/alpha1=sin(alpha2)/alpha2) is also true for waves on water!


>
>:>reason why light bends near massive objects is because the space is
curved
>:>there and the light tends to move along the geodesic lines of the space
>:>(which are actually the shortest way from one point to another). The
>:>massive object doesn't attract the light, it's just bending the space.
>:>
>
>: Correct, afaik.
>
>  Is the bending of the light caused by the curvature of the space,
>the mass of the light (which the massive object attracts), or both?
>  If both, then it would seem that the light would not stay in the
>geodesic line because the massive object is attracting it out of that line.
>


This is a very advanced topic, and I know absolutely nothing about it. In
fact, I think it is two sides of the same case. But I don't know at all.

>: Okay. I was wrong there. But light has mass! The solar winds was just a
>: (very) bad example. You can verify that photons actually may cause
pressure!
>: This is the empirical prof of the mass of light. You cannot have pressure
>: without mass.
>
>  How can I verify that photons may cause pressure?
>

This is a simple experiment:
1. Take a glass jar (empty).
2. Take two small sheets of aluminium foil and create a kind of windmill.
4. Use a fire and darken every other side of the mill's wings.
3. Fasten it to a thin thread and hang it into the glass jar.
4. Put in sunlight.

(I'll post a rendered picture in a minute)

You can now see that the windmill turns from the heat of the sunlight! The
black sides absorb much more energy (pressure from the light) and the mill
starts turning.

This is an experiment we did in physics class some years ago, and it works!
This shows that photons cause pressure.

(It should not be very difficult to accept, though. Mass traveling at a
given velocity does indeed create pressure. Soap-bubbles, wind, the shower,
rain, et.c.)

>--
>main(i,_){for(_?--i,main(i+2,"FhhQHFIJD|FQTITFN]zRFHhhTBFHhhTBFysdB"[i]
>):5;i&&_>1;printf("%s",_-70?_&1?"[]":" ":(_=0,"\n")),_/=2);} /*- Warp -*/

What does this code do? I've tried it with Borland, but I get some error
messages. "Cannot call main from main" et.c.

Simen.

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On 30 Sep 1999 10:29:58 -0400, Nieminen Juha wrote:
>Ron Parker <par### [at] fwicom> wrote:
>  The speed of the photon must slow down at least somewhere so that
>the average speed could be less than c.

In the period of time between absorption and re-emission, the photon 
doesn't exist.  While it exists, its speed is exactly c.  While it 
doesn't, it doesn't have a speed but you can make a convincing argument
for assuming a value of zero.  The time during which it doesn't exist
is small but nonzero, and it happens many many times while the photon 
is traveling in a medium.

>  If the photon is absorbed by a molecule, why does the photon leave the
>molecule in the exact same direction as it entered it?

This is something I don't know offhand.  I suppose that the ability to do 
that over a given frequency range is what makes it a transparent molecule 
rather than an opaque one, however it comes by that ability.  Something to
do with dielectric constants and such, but I changed majors before we got
into any of the heavy stuff.

It's also possible that it was an oversimplistic abstraction given to 
first-year physics students. :)

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Look in binaries.images for an illustration to the experiment i mentioned.

Simen.

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Simen Kvaal <sim### [at] studentmatnatuiono> wrote in message
news:37f37cd8@news.povray.org...
> Look in binaries.images for an illustration to the experiment i mentioned.

I've got one of these specially made actually.  It's a specially blown glass
"bubble" in a shape similar to a light bulb... (bulb at the top, a pin goes
up through the middle point upwards and the mill sits down on top of it on
the point) it is pressure sealed and thus there is not very much resistance
inside against the "mills".

If I leave it in the sun it gets up to about 120 RPM.  I find it quite
interesting to observe :)

My opinion, light creates pressure, therefore photons have mass (which makes
sense when you think about how light reacts with things like liquids, or
gravitational lens etc).  But I'm not well enough advanced to be able to
prove any of that yet.

BTW, this should be moved to p.o-t

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Lance Birch wrote:
> 
> BTW, this should be moved to p.o-t

I agree on that 8)

Besides, I tried to point at that there's more to colour then the physical
apprach and see what happens here... :o

Remco

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