To add comments or start new threads please go to the full version of: Light,mass,velocity And Kinetic Energy
PhysForum Science, Physics and Technology Discussion Forums > Relativity, Quantum Mechanics and New Theories > Relativity, Quantum Mechanics, New Theories

scorpion9
I have a little question about light/photons.
What makes them behave differently from other objects in space?
Does light have kinetic energy?
i guess it has because it has mass and it can propel objects(solar sails, light engine),it has "impact" on objects.

if it has kinetic energy then why doesnt it decay/decrease over time?

also as the law of physics says that all motion has counter effect egg. when i shoot a gun, a bullet flies in one direction and recoil with same energy goes in another direction.

does this same thing work with photons?
if a substance emits a photon, does it obtain the same momentum in the opposite direction. If it does, shouldnt it fly away with almost the same speed as photon (depending on the emitters mass) as light.

another Q:
what happens if a light starts bouncing between two mirrors in vacuum ?
since light isnt slowing down/absorbing, it should push the mirrors away from each other infinitively, constantly accelerating. Is this hypothesis true ?



BigDumbWeirdo
QUOTE (scorpion9+Feb 23 2008, 11:54 AM)
I have a little question about light/photons.
What makes them behave differently from other objects in space?
Does light have kinetic energy?
i guess it has because it has mass and it can propel objects(solar sails, light engine),it has "impact" on objects.

Light does not have mass. It has energy. This is what 'pushes' against a solar sail.

QUOTE
if it has kinetic energy then why doesnt it decay/decrease over time?

Light is mass-less, thus allowing it to move at c. This gives it sort of an equivalence to inertia, which basically means it would take energy to slow it down. Currently, there is no known way to infuse energy into light in such a way that it slows down, rather that becoming a higher wavelength or amplitude.
The slowing of light mentioned on science sites all over the Internet is the slowing of the propagation of light, not the light itself. This means that it takes a certain amount of light longer to pass through certain materials, but that individual photons are still moving at their own speed. It's like the difference between racing down the center of a racetrack and bouncing off the retaining walls. When you're bouncing off the retaining walls, you might be doing the same speed between them that you would otherwise be doing down the track, but it still takes you noticably longer to finish the race, because you've travelled so much more distance.

QUOTE (->
QUOTE
if it has kinetic energy then why doesnt it decay/decrease over time?

Light is mass-less, thus allowing it to move at c. This gives it sort of an equivalence to inertia, which basically means it would take energy to slow it down. Currently, there is no known way to infuse energy into light in such a way that it slows down, rather that becoming a higher wavelength or amplitude.
The slowing of light mentioned on science sites all over the Internet is the slowing of the propagation of light, not the light itself. This means that it takes a certain amount of light longer to pass through certain materials, but that individual photons are still moving at their own speed. It's like the difference between racing down the center of a racetrack and bouncing off the retaining walls. When you're bouncing off the retaining walls, you might be doing the same speed between them that you would otherwise be doing down the track, but it still takes you noticably longer to finish the race, because you've travelled so much more distance.

also as the law of physics says that all motion has counter effect egg. when i shoot a gun, a bullet flies in one direction and recoil with same energy goes in another direction.

does this same thing work with photons? if a substance emits a photon, does it obtain the same momentum in the opposite direction. If it does, shouldnt it fly away with almost the same speed as photon (depending on the emitters mass) as light.

Photons are emitted, not 'shot.' Because they have no mass, there is no counter effect to their emission. Here's why:
When you fire a gun in a 0-g environment, you get pushed back as the bullet travels forward. Now, you weigh a lot more than the bullet, so you fly back gently, while the bullet races forward. The energy expenditure in the firing of the bullet is finite, and thus only has so much energy to put into pushing you back. Naturally, it takes more energy to move a more massive object.
When a particle emits a photon however, there is no explosion. The photon simply appears at the same location as the particle and moves away, thus, there is no kickback.


QUOTE
another Q:
what happens if a light starts bouncing between two mirrors in vacuum ?
since light isnt slowing down/absorbing, it should push the mirrors away from each other infinitively, constantly accelerating. Is this hypothesis true ?

Not quite. As the light bounces back and forth, it looses a little bit of it's energy to the mirrors each time. Some of this energy will be translated into kinetic energy pushing against them. MOST of this energy will be translated into heat energy that warms the mirrors. Eventually, the photon will have expended all of it's energy, and simply will not be a photon anymore.
I hope this helps smile.gif

If anyone with a science background wishes to point out any errors I made, or clarify anything it seems I might have over-simplified, then feel free.
dougie_fresh_007
I have a little question about light/photons.
What makes them behave differently from other objects in space?

it doesnt really, its just really fast

Does light have kinetic energy?

no mass but has energy, not exactly kenetic

i guess it has because it has mass and it can propel objects(solar sails, light engine),it has "impact" on objects.

...corect um but light is energy w/no mass...wierd

if it has kinetic energy then why doesnt it decay/decrease over time?

does it? it doesnt

also as the law of physics says that all motion has counter effect egg. when i shoot a gun, a bullet flies in one direction and recoil with same energy goes in another direction.


does this same thing work with photons?
if a substance emits a photon, does it obtain the same momentum in the opposite direction. If it does, shouldnt it fly away with almost the same speed as photon (depending on the emitters mass) as light.

photon "push off" of the atom n the star that "created it"

another Q:
what happens if a light starts bouncing between two mirrors in vacuum ?
since light isnt slowing down/absorbing, it should push the mirrors away from each other infinitively, constantly accelerating. Is this hypothesis true ?

assuming 100% reflection ... as the mirrors move there is a doppler shift in frequency which is proportional to lights energy thus dropping the "force " applied by the photon
barakn
QUOTE (BigDumbWeirdo+Feb 23 2008, 05:12 PM)
Photons are emitted, not 'shot.' Because they have no mass, there is no counter effect to their emission. Here's why:
When you fire a gun in a 0-g environment, you get pushed back as the bullet travels forward. Now, you weigh a lot more than the bullet, so you fly back gently, while the bullet races forward. The energy expenditure in the firing of the bullet is finite, and thus only has so much energy to put into pushing you back. Naturally, it takes more energy to move a more massive object.
When a particle emits a photon however, there is no explosion. The photon simply appears at the same location as the particle and moves away, thus, there is no kickback.

blink.gif I was always taught that photons, while massless, have momentum (hence the ability to transfer momentum to a solar sail). Doesn't your recoilless model of emission violate conservation of momentum?
BigDumbWeirdo
QUOTE (barakn+Feb 23 2008, 01:08 PM)
blink.gif I was always taught that photons, while massless, have momentum (hence the ability to transfer momentum to a solar sail).  Doesn't your recoilless model of emission violate conservation of momentum?

I do believe you are right, sir.
I was attempting to explain things on a level with the questions being asked, and was not taking any significant time to double check myself.
(That's why I included that little request at the end.)
Thanks smile.gif
dougie_fresh_007
photon "push off" of the atom n the star that "created it
scorpion9
thanks for replying
got some answers and things got more confusing(i like it) biggrin.gif

I got another Q, its about light/photons and time.

does light travel outside the time and space. im asking it because when egg. a shuttle reaches the speed of light, it will contract, and time for the shuttle stops.
But a photon still is something and it always reaches the speed of light :S , why doesnt photon stop/disappear etc. Something strange should happen to it.
so is "time" effecting only bodies with mass.
AlphaNumeric
QUOTE (barakn+Feb 23 2008, 07:08 PM)
blink.gif I was always taught that photons, while massless, have momentum (hence the ability to transfer momentum to a solar sail). Doesn't your recoilless model of emission violate conservation of momentum?

Yes, you're right. I mark a 2nd year quantum mechanics course for my department and the first question on the first sheet was "Explain evidence for the existence of the photon and it having energy E=hf and momentum p = hc/f". Compton scattering provided the experimental evidence for photon momentum. smile.gif
PhysOrg scientific forums are totally dedicated to science, physics, and technology. Besides topical forums such as nanotechnology, quantum physics, silicon and III-V technology, applied physics, materials, space and others, you can also join our news and publications discussions. We also provide an off-topic forum category. If you need specific help on a scientific problem or have a question related to physics or technology, visit the PhysOrg Forums. Here you’ll find experts from various fields online every day.
To quit out of "lo-fi" mode and return to the regular forums, please click here.