If the gravity of a star can change the trajectory of a photon, is the speed of the photon reduced at the moment of departure from the stellar's surface? Can modern science calculate the measure of change?
59471
How do I solve the speed of light in gravitational field?  Should I just add gravitational acceleration in speed of light?  $$c'=c_0+g(r)t~?$$
83030
In a question  Ron Maimon comments that "relativistic mass makes gravity, not rest mass."  If so, does that mean that the faster that stars orbit the galaxy the larger the relativistic mass of the galaxy therefore more gravity therefore the stars orbit even faster therefore the gravity gets even more therefore the stars keep on getting faster ...  So the speed of stars and mass of galaxy would be correlated with age and history of the galaxy or galaxy clusters and that's why calculations of galaxies' masses don't add up.
6070
A thought experiment: Imagine the Sun is suddenly removed. We wouldn't notice a difference for 8 minutes, because that's how long light takes to get from the Sun's surface to Earth.   However, what about the Sun's gravitational effect? If gravity propagates at the speed of light, for 8 minutes the Earth will continue to follow an orbit around nothing. If however, gravity is due to a distortion of spacetime, this distortion will cease to exist as soon as the mass is removed, thus the Earth will leave through the orbit tangent, so we could observe the Sun's disappearance more quickly.  What is the state of the research around such a thought experiment? Can this be inferred from observation?
1557
I know a photon has zero rest mass, but it does have plenty of energy. Since energy and mass are equivalent does this mean that a photon (or more practically, a light beam) exerts a gravitational pull on other objects? If so, does it depend on the frequency of the photon?
103983
The red-shift of the light of a star in a galaxy or that of a galaxy in a cluster of galaxies is generally interpreted as how fast the star or the galaxy is moving, i.e. it is interpreted in a purely special-relativistic way. However, general relativity predicts that a light produced in a gravitational field gets red-shifted when comes out of the field. I wonder why the excess red-shift (over the Hubble red-shift) of stars and galaxies is only interpreted as how fast the object moves, not that how strong the ambient gravitational field is.
70183
Using classical mechanics, the formula for gravitational attraction is   $$F = G\frac{m_1m_2}{r^2}.$$   This formula does not work for photons, and we need to use Einstein's theory of gravity to account for that, as the photon is massless. What if, instead of using the mass of the photon, we rather used the inertia of it?  The momentum of a photon is     $$p=\frac{h}{\lambda}.$$  Thus, using that, we can calculate its supposed 'mass' (inertia) as momentum is also equal to mass times velocity. So 'mass' of a photon is     $$m=\frac{p}{v}=\frac{h}{\lambda  c}.$$   If we use this mass as the mass of a photon, can we use Newton's equation for gravitational attraction?
50612
So, I'm traveling to another star 100 light years away in my spaceship. This ship has a solar sail pushed by a laser beamed from my home star system, so can achieve a velocity close to c. It's also got a robust parachute to slow down with.  I understand that if I measure light coming from my origin star, it will always still seem to be streaming past me at light speed (but will red shift as my speed increases). Light coming from my destination star also travels past me at light speed, and will become increasingly blue shifted as I gain speed.  I also understand that an observer checking on my speed at my origin or destination will always find it to be less than c.  However, will I perceive that in terms of the time it apparently takes me to reach my destination, my speed was greater than c? In other words, will it seem to take less than 100 years to reach the destination? 10 years on my watch, say. Or 1 year. Or a week?  Ie, as far as I'm concerned, while light keeps zipping past me at light speed, do I continue to accelerate unabated to an arbitrary apparent speed?  If not, how do I notice my continued acceleration being prevented?
41963
What does general relativity say about the relative velocities of objects that are far away from one another? In particular:--  Can distant galaxies be moving away from us at speeds faster than $c$? Can cosmological redshifts be analyzed as Doppler shifts? Can I apply a Lorentz transformation in general relativity?
43352
Since electromagnetic energy is carried by photons and moves in forms of waves, does it mean that a single photon when propagating through space doesn't follow the straight path but instead always moves up and down, up and down like a wave. If so another question arises the speed of propagation of light in vacuum is fixed meaning that it will always take the same amount of time for it to travel from point A to point B, but if a photon always moves up and down it will also mean that it travels longer distance than the distance between A and B and so it ill travel faster than light propagates, is it even possible, could you please clarify these concepts for me?
2349
I'm a big fan of the podcast Astronomy Cast and a while back I was listening to a Q&amp;A episode they did. A listener sent in a question that I found fascinating and have been wondering about ever since.   From the show :      Arunus Gidgowdusk from Lithuania asks: "If you took a one kilogram mass and accelerated it close to the speed of light would it form into a black hole? Would it stay a black hole if you then decreased the speed?"   Dr. Gay, an astrophysicist and one of the hosts, explained that she'd asked a number of her colleagues and that none of them could provide a satisfactory answer. I asked her more recently on Facebook if anyone had come forward with one and she said they had not.  So I thought maybe this would be a good place to ask.
8426
I've heard that gravitational waves travel at the speed of light, and have some parallels to electromagnetic waves.  EM waves slow down as they pass through matter (speed of light in glass is slower than in vacuum, for instance).  Do gravitational waves also slow down as they pass through matter?  If so, are there any effects like  when matter passes through a medium at a speed greater than the velocity of gravity in that medium?  Do large masses like stars or Jupiter act as ball lenses for gravitational waves?
25639
Does the mass of both the parent object, and the child object affect the speed at which the child object orbits the parent object?  I thought it didn't (something like $T^2 \approx R^3$) until I saw an planet on the iphone exoplanet app, that is closer to it's star than a planet in another system, yet takes longer to complete one orbit. Both of the planets were a similar mass, as were the stars.
15651
The speed of light is the same in all inertial frames.  Does it change from a non-inertial frame to another? Can it be zero?  If it is not constant in non-inertial frames, is it still bounded from above?
57356
I am following Carroll's GR book. He explain that it is convention to parameterize geodesics of photons by a parameter $\lambda$ such that  $$p^\mu ~=~ \frac{d x^{\mu}}{d \lambda}.\tag{3.62}$$  But this is the definition of 4-velocity for a massive particle in the case of $\lambda=\tau$ equal to proper time. My question then, is $$u^\mu = p^\mu$$ for all massless particles? I ask because it's well-known that proper time $\tau$ freezes for massless particles.
87985
In electromagnetism, Maxwell's equations predict that electromagnetic disturbances travel at the speed $$c = \frac{1}{\sqrt{\mu_0 \epsilon_0}}.$$ Does general relativity predict that gravitational waves travel at the same speed or is this "put in by hand", that is, is there a wave equation with constants that presumably include the gravitational constant $G$ and other constants?
55655
This question has been touched tangentially by  and  but I do not think they reach the core of my question.  When I read that many other theories, e.g. the Standard Model, have been constructed to be Lorentz invariant it appears that electromagnetism has some of preponderant role in the universe compared to other theories. I suspect that the answer is that this is not the case, and that the space-time Speed of Light (with upper-case) is really more of a universal constant that is not necessarily related to the speed of the electromagnetic waves or of photons.  There are many other related questions:    would it really be devastating for relativity that we find out in the future that photons have a tiny mass and move slower than ... uhh... the speed of light? which "c" is applicable in different situations? I mean, possibly the space-time constant should be used in $E=mc^2\gamma$, it would be the  velocity of gravitational waves, etc..., but in other situations such as the Bohr radius or Compton wavelength I am not sure.   Well, any answer will be appreciated. Please tell me if you know any reference to bibliography that address this issue.
17748
While I was hobby-reading about quantum mechanics I came across Heisenberg's theory. But while I was trying to understand it I thought of this: if the speed of light (as well as the momentum of a photon) is well-defined, then if in an experiment the position of the photon was found, the researchers would know the exact position of the photon, but also its momentum which is constant. Is this an exception from the rule, or am I just ignorant?  PS, 12 grade physics education, please try to explain in simple words