What is the force which accelerates light to its speed?

[nizmo]

Simple question i guess, but one i dont have an answer for...

[petebro]

perhaps we need to look at the transport mode allowing the waves to transit

the photon has some bearence on allwing gh to travel through space it may not be the ultimate conclusion , but it certainly is the transport.

[nizmo]

I don't really understand... Im not much of a scientist, just a curious person

 

Does light have mass?

[petebro]

Lets look at light as an energy source , the emmissions are radiation , the transport is the photon or the etha , light waves transit through photons at various wavelengths and temperature and do have sub atomic mass . Therefore force is heat energy high levels of radiation banding into spectrums of energy emmission transitting through space .Sub atomic radioactive energy rays .

[iNow]

Light does NOT accelerate.

 

Light, by definition, is always traveling at the same velocity.

It is never stopped.

It is never slowed down (although, sometimes it may appear so to us when it is absorbed and re-emitted, but that's not important here).

 

When light exists, it travels at c. No acceleration, hence, no force to accelerate it.

 

Also, light by definition is without mass. If it had a non-zero mass it would not be able to travel "at the speed of light."

 

 

Petebro - Your posts are just word salad and don't make sense. Please try to avoid further confusing people already uncertain.

Nizmo - Just ignore what he's said. It's meaningless dribble.

Edited May 6, 2009 by iNow

[Sisyphus]

The photons which make up light do not accelerate. They come into existence moving at C, which is the only speed they ever move at. They do not have mass, but they do have momentum proportional to the frequency of the light. Anything with mass moving at C would have infinite kinetic energy, which is one reason it is impossible for matter to move at the speed of light.

[nizmo]

Thanks iNow and Sisyphus i see what you're saying.

 

1) If light doesnt have mass, then howcome it is affected by gravity?

 

2) Howcome light travels faster in a vacuum than in an atmosphere?

[gre]

Thanks iNow and Sisyphus i see what you're saying.

 

1) If light doesnt have mass, then howcome it is affected by gravity?

 

 

Because photons follow the curved space-time created by earth's gravity.

 

Here's another question for someone else..

 

Do higher energy photons curve more, less, or the same as lower energy photons (in earth's gravitational field)?

 

 

2) Howcome light travels faster in a vacuum than in an atmosphere?

 

Photons still travel at the speed of light in earth's atmosphere, but they're absorbed and emitted by molecules in the atmosphere at a slower rate than in free space.

Edited May 5, 2009 by gre

[timo]

Do higher energy photons curve more, less, or the same as lower energy photons (in earth's gravitational field)?

On typical scales (visible light in the influence of a planet or star) the path is independent of the energy.

[gre]

But a 400 GeV photon might behave differently in a planets gravitational field than say a 3 eV photon? Or no?

[timo]

Unlikely. The point is: The most primitive form for an equation of motion of a particle through a gravitational field (more precisely: through a space-time) is the geodesic equation. In this equation for the case of light, the energy of the light does not come in. However, the application of the geodesic equation will certainly be limited. The geodesic equation inherently comes with a few assumptions that might break down at some point, like the object being point-like and not having any influence in space-time itself.

It is a bit like the statement that in vacuum all objects fall equally fast towards the center of a an attracting planet (provided an equal starting distance, of course). If you verify this the it will work great for a feather, a block of lead and a car. It will not work well anymore for a sun because the inherent assumption that the attraction of the falling object on the attracting planet is negligible does not hold.

 

400 GeV is less than the mass of a single polymer so you can probably forget the influence on the gravitational field of a planet. 3 eV is a point-sized wavelength compared to stellar scales.

[gre]

It is a bit like the statement that in vacuum all objects fall equally fast towards the center of a an attracting planet (provided an equal starting distance, of course). If you verify this the it will work great for a feather, a block of lead and a car. It will not work well anymore for a sun because the inherent assumption that the attraction of the falling object on the attracting planet is negligible does not hold.

 

400 GeV is less than the mass of a single polymer so you can probably forget the influence on the gravitational field of a planet. 3 eV is a point-sized wavelength compared to stellar scales.

 

 

Interesting. So at what point will massive objects fall at different velocities towards the center of a planet? Can this be determined? Would a car and a mountain sized mass in earth's orbit still fall at the same rate towards the center of the earth?

 

Is there a known point which photons (wrt their energy) will start to behave differently in a gravitational field?

 

 

Thanks

Edited May 6, 2009 by gre

[Sisyphus]

It is a bit like the statement that in vacuum all objects fall equally fast towards the center of a an attracting planet (provided an equal starting distance, of course). If you verify this the it will work great for a feather, a block of lead and a car. It will not work well anymore for a sun because the inherent assumption that the attraction of the falling object on the attracting planet is negligible does not hold.

 

Surely this isn't correct. The sun is accelerated by the Earth just as much as a feather would be at that distance. Sure, Earth-Sun comes together faster than Earth-feather, but that's because the Earth falls towards the Sun much faster than it falls towards a feather. Or is that what you meant?

[timo]

Interesting. So at what point will massive objects fall at different velocities towards the center of a planet? Can this be determined? Would a car and a mountain sized mass in earth's orbit still fall at the same rate towards the center of the earth?

Try it out. Newtonian Gravity is school physics.

 

Is there a known point which photons will start to behave differently in a gravitational field?

I would not know about a generally-agreed-upon scale - it will definitely not be a hard point like in phase transitions but a gradual failure of the approximations used. I would imagine you might run into problems when spacetime considerably changes over the distance of a wavelength, whatever "considerably changes" might effectively mean in this context. You might run into problems defining photons.

 

Surely this isn't correct. The sun is accelerated by the Earth just as much as a feather would be at that distance.Sure, Earth-Sun comes together faster than Earth-feather, but that's because the Earth falls towards the Sun much faster than it falls towards a feather. Or is that what you meant?

In short, yes: You approximate that earth accelerates with a=0 which is a good approximation for an attracting feather or an attracting car.

[gre]

Try it out. Newtonian Gravity is school physics.

 

Oops, I meant different velocities. I thought you were implying that mass at a certain size behaves differently in gravity .. I.e. won't fall at the same velocity as a smaller mass..

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So could you say photons contribute to gravity, but don't react to it?

 

I'm confused..

 

Here is quote from someone on another forum..

 

Massive particles are different from photons, but as they both contain energy, they both contribute to gravity, since it is energy that causes gravity (along with the other things I mentioned).

[swansont]

Photons follow the geodesic of the curved space. They are affected by gravity.

[gre]

But they don't create gravity or curve space-time by themselves?

[darshanchande]

No force is needed for the light to travel. Light travels by itself, for it's just like that

[Externet]

When light enters a different medium changes its propagation speed, right ?

...does it decelerates or accelerates at that transition ?

 

When light at c hits a mirror and bounces back at -c, there is an instant when its speed is zero. Is it?

[Klaynos]

When light enters a different medium changes its propagation speed, right ?

...does it decelerates or accelerates at that transition ?

 

When light at c hits a mirror and bounces back at -c, there is an instant when its speed is zero. Is it?

 

The individual photons travel at c.

 

The apparent change in speed is due to absorption and re-emission, the same can be said for reflection.

[proton]

Thanks iNow and Sisyphus i see what you're saying.

 

1) If light doesnt have mass, then howcome it is affected by gravity?

 

2) Howcome light travels faster in a vacuum than in an atmosphere?

 

In the comments above it was said that photons don’t have mass. What was meant by that is it has zero proper mass (aka rest mass) . However light does have inertial mass, since it carries momentum, it has passive gravitational mass, since it is acted on by gravity, and it has active gravitational mass, since it generates a gravitational field.

 

Feyman comments on this in the Feynman Lectures Vol on page 7-11. In the section entitled Gravitation and Relativity Feynman wrote

One feature of this new law is quite easy to understand is this: In Einstein relativity theory, anything which has energyas mass -- mass in the sense that it is attracted gravitationally. Even light, which has energy, has a "mass". When a light beam, which has energy in it, comes past the sun there is attraction on it by the sun.

Here Feynman is referring to the passive gravitational mass of light.

 

Re – Because photons follow the curved space-time created by earth's gravity.

 

Light is deflected in a gravitational field, not because of spacetime curvature, but due to the gravitational field it generates. Spacetime curvature is not a necessary condition for the deflection of light in a gravitational field. For example; if light is moving in a uniform gravitational field then it will be deflected even though there is no spacetime curvature in such a field.

 

Re – Does light have mass?

 

That depends on what one means by the term “mass”. A photon has non-zero inertial mass, passive gravitational mass and active gravitational mass. However it has zero proper mass.