Photons, relativity and Gravity Q

Original post/question:......................

by the description of Einstien's (Albert, not his uncle Frank) special
theory, the bit about matter acquiring infinite mass as it reached the
speed
of light (I've probably not got this quite right). So, how come light
(photons or whatever) doesn't have infinite mass?

Ans (condensed): Well, so a photon (or whatever) doesn't have any mass. I
can accept this, sort of, but....

If a particle has no mass, then I would assume that it would have no energy
either (the old E=MC^2). This being the case, how come gravity appears to
bend light (not to mention the Swartzchild/event horizon thingy). Do
we have a gravity warp of space that light follows like a sloping road (more
problems!)? Or
does gravity directly effect this massless, energyless particle/wave?

Grifff

"Griff" wrote in message
u...
Original post/question:......................

by the description of Einstien's (Albert, not his uncle Frank)
special
theory, the bit about matter acquiring infinite mass as it reached
the
speed
of light (I've probably not got this quite right). So, how come
light
(photons or whatever) doesn't have infinite mass?

Ans (condensed): Well, so a photon (or whatever) doesn't have any
mass. I
can accept this, sort of, but....

If a particle has no mass, then I would assume that it would have no
energy
either (the old E=MC^2). This being the case, how come gravity
appears to
bend light (not to mention the Swartzchild/event horizon thingy). Do
we have a gravity warp of space that light follows like a sloping
road (more
problems!)? Or
does gravity directly effect this massless, energyless
particle/wave?

Grifff



A photon does have zero mass. A particle moving at c would have a
relativistic mass 'infinitely many times' its rest mass. For a photon
this is like zero * infinity, and it seems to give a finite value.
Relativistic momentum is given by

p = gamma * m * v,

which, if thought of as

p = (gamma * m) * v, (Eqn 1)

implies multiplying relativistic mass by velocity to find momentum.
The momentum of photons can be measured, and is a finite value. From
this relationship a photon can be said to have a relativistic mass
which is a finite value.

Total energy is given by

E = gamma * m * c^2 (Eqn 2)

which gives us a finite energy for photons. Energy of a photon can
also be found from the Energy - momentum relationship

E^2 = p^2 * c^2 + (m * c^2)^2,

which reduces to

E^2 = p^2 * c^2

for massless particles, or

E = p * c,

which agrees with the equations 1 & 2 above for total energy and
momentum where v=c. The energy of a photon can also be calculated
from

E = h * f,

where h is Planck's constant and f is the frequency of the light.

So, light (a photon) has no mass, though it does have a finite
relativistic mass, and finite momentum and energy.

-------------

I read that one can find a value for the bending of light by a massive
body by using Newtonian mechanics on a normal particle and taking a
limit as v - infinity. Or one can find a value by using the
principle of equivalence. These calculations, however, give only half
of the amount of bending observed. The other half can be attributed
to the bending of space by gravity. General relativity can be used to
calculate a deflection of light by a massive body. When people were
able to take this measurement accurately they found excellent
agreement to calculations.

j

As I've read, a photon has no "rest mass", being a relativistic
particle, but it has energy and is affected by gravity. Matter can't be
accelerated to light speed because it acquires mass as it is
accelerated, it takes even more energy to accelerate it further,
resulting in more mass- it would take an infinite amount of energy to go
light speed hence infinite mass. Light does not have infinite mass
because it is not matter, it's energy- and it was never accelerated but
came into existence moving at light speed.

kenneth mccoy wrote:
As I've read, a photon has no "rest mass", being a relativistic
particle, but it has energy and is affected by gravity. Matter can't be
accelerated to light speed because it acquires mass as it is
accelerated, it takes even more energy to accelerate it further,
resulting in more mass- it would take an infinite amount of energy to go
light speed hence infinite mass. Light does not have infinite mass
because it is not matter, it's energy- and it was never accelerated but
came into existence moving at light speed.

I sometimes wonder if the real limit to an object with mass trying to reach the
speed of light is it's mass becoming infinite or rather that before it reaches
the speed of light, the time in its frame slows to the point where it would take
it forever to get that last little bit of speed.. heh heh