Einstein says photon aka light acts like particle but is not because it does not have mass. Particles at speed of light reach masslessness so photons must just be stretched out so far they exhibit masslessness. Space holds photon whole but the speed stretches photon until poof gone to some other energy.

photons at rest have no mass

but photons have an inertial mass that is affected by gravity

that is why the doppler effect holds true and light moving away from you will have the red shift and light moving toward you will have the blue shift

how can something be massless but have an internal mass? I know massive gravity from blackholes can pull light in. How does gravity act on photons to pull them?

Has science stopped a photon? I have not heard of this. If we could stop photons we probably would be able to figure out how to accelerate things near light speed.

photons do not have mass, but they do have momentum. The proper, general equation to use is E2 = m2c4 + p2c2 So in the case of a photon, m=0 so E = pc or p = E/c. On the other hand, for a particle with mass m at rest (i.e., p = 0), you get back the famous E = mc2.

This equation often enters theoretical work in X-ray and Gamma-ray astrophysics, for example in Compton scattering where photons are treated as particles colliding with electrons.

"A particle like a photon is never at rest and always moves at the speed of light; thus it is massless," says Dr. Michael S. Turner, chair of the Department of Astrophysics at the University of Chicago.

What about experimental evidence? Experiments don't determine exact quantities because of small errors inherent in making measurements. We have, however, put an upper limit on the photon rest mass. In 1994, the Charge Composition Explorer spacecraft measured the Earth's magnetic field and physicists used this data to define an upper limit of 0.0000000000000006 electron volts for the mass of photons, with a high certainty in the results.

This number is close to zero; it is equivalent to 0.00000000000000000000039 times the mass of an electron (the lightest particle), says Turner.

-nasa

so if a photon is stopped it miracously has no energy? What happens to it does it break all the rules of physics. You gave equation and if photon stops p=0 and E=0 and the photon does not exist within the bounds of science?

so if a photon is stopped it miracously has no energy? What happens to it does it break all the rules of physics. You gave equation and if photon stops p=0 and E=0 and the photon does not exist within the bounds of science?

now you say it has mass you need to pick one side lol. So now plug that in your equation for m. I will solve it after my shower. Your number is zero like as it approaches zero from the left in direction of negative infinity.

a photon is ALSO an electromagnetic wave thus it has frequency (wavelength)

if you stop it then it has a frequency of zero and quits being a photon

but that is a theoretical upper limit

there is no lower limit but zero

if you shoot a beam of light at a surface the photons will either be absorbed or reflected

when the photons are absorbed their is a measurable momentum imparted to the object implying a inertial mass

when they are reflected there is zero inertial mass

this guy says it better than I can


The concept of the rest mass derives from special relativity. The rest mass is the mass of a particle (in our case the photon) as measured by an observer who sees the particle still and with zero speed. In other words, the particle is at rest as far as this observer is concerned. Thus comes the term REST mass. But according to special relativity, light ALWAYS travels with the light speed c, and is NEVER at rest. And so it has zero REST mass.

The mass of a particle which travels with a velocity v is given by the following equation:



where m0 is the rest mass of the particle, v its velocity and c the speed of light. It may appear that because photons have zero rest mass (m0=0), their mass is zero too. A closer look at the equation will show that this is not the case because photons travel with the velocity c and the equation collapses to an undefined form (m=0/0) The conclusion is that this equation applies only to sub-lightspeed particles and NOT to photons. But relativity also tells us that every particle with mass m is equal to an energy E given by the famous equation:

E=mc2

where m the mass of the particle, c the speed of light and E the energy that equals to the mass m. This equation also works backwards.An amount of energy E equals to a mass m. Photons however may not have rest mass, but they DO have energy. The energy of a photon is given by the equation:

E=hf

where h is a constant (Planck's constant) and f the frequency of the photon (don't forget that the photon is an electromagnetic WAVE and has a frequency). If we combine these two equations, we come up with the following,which gives us the mass of a photon

mc2 = hf => m=hf/c2

which means that though photons don't have rest mass, they do have energy and thus they have mass. The photons are wave particles. This means that they act as waves and as particles as well. This is the duality of the nature of light (and of every particles). And so as particles they have mass, and as waves they have frequency. The pressure they exert is due to the particle nature of light. It's easy now to understand the mechanism that causes this pressure.

-Warren F. Davis, Ph.D. Physics, President, Davis Associates, Inc., Newton, MA USA

thus a large enough concentration of photons absorbed could possibly move the object a little?

absolutely

that is the concept of the "solar sail"

Oh and curious why does reflection go outside the properties of physics, chemistry and momentum. If two objects collide I was told a force from the second object would be exerted on first and vice.

I guess I stumped you?

i don't see how this conflicts with reflection and absorption.

Yeah, close enough. "acts like" is the key phrase there! Ain't nothing wrong with something 'acting like' a particle, and not being a particle.




"Reach" ??? This is a very confused way of expressing this. I think you mean to say "only massless particles can reach the speed of light" and that is true. Photons are massless. They have momentum, and 'relativistic mass', which is a phrase connected to a measurement of energy - not true mass.



I don't mean to be rude, but I'm discounting this as having any meaning until its explained a bit better.