No. A material particle can never reach the speed of light. But as it
approaches the speed of light, its mass increases. How much it increases, it depends on its speed. If you want to know how much it increases, there is a calculator
here. Try inputting different velocities, and you will notice that the effect is not linear: at 200000 km/s (light speed being 299792.458 km/s) the mass increase factor is only 1.34, but as you approach the speed of light, the effect becomes bigger and bigger (1.81 at 250000 km/s, 3.94 at 290000 km/s, 13.76 at 299000 km/s, 40.27 at 299700 km/s and so on).
Calculations show that, if a material particle could reach the speed of light, its mass would become infinite, but in order to get there, it would need an infinite amount of energy, because the more mass it has, the more energy it is needed to accelerate it further.
No, a photon has zero mass. You can multiply zero by any number you want, and you'll always get zero.
Yes. If we could mount a clock on a photon, that clock would remain still. Conversely, if we could ride a photon ourselves, we would experience the entire history of the universe, from the present to the end, in a single instant.
Right.
No, for the reason I explained before. If it starts with zero mass, you can multiply it by any factor you want and it'll always be zero.
Photons don't have momentum, but gravity still influences them because gravity curves space. If you have two masses (like two stars in a binary system) they can orbit each other, or even fall into each other, because each mass curves its surrounding space, and each mass is influenced by the curvature caused by the other. But if you have one star and a photon, the photon will still be deviated by the star because the mass of the star still curves space.
And if the star is sufficiently massive to be able to collapse below a certain radius (the exact formula is
r = (2*G*m)/c^2, where G is the gravitational constant, m is the mass of the object and c is the speed of light in a vacuum ), a region will form where space is so curved that it closes back into itself, so that anything entering that region will be unable to escape (and if it has a mass, it will eventually crash into the collapsed star). That is a black hole.
What matters is the speed of light in a vacuum, which is always the same no matter the environment one is in to measure it. That is what we call
c.
In reality, saying that "light slows down when it passes through a different medium" is not precise. If you could see a photon while it travels through a piece of glass, you would see it moving in the vacuum between the atoms at a speed of exactly c. Then, when it collides with an atom, the atom would absorb it. After a short interval, that atom would emit another photon, which would again travel at c in the vacuum between the atoms, until it touches another atom, which absorbs it, then emits another photon after a short time... and so on. If you add up all intervals between an absorption and an emission, you get the impression that photons have slowed down, but they haven't. The photons that come out of a piece of glass are not the same photons that went in.
No, here you're just confused. Light has no momentum. As you say, momentum is mass*velocity, but photons have 0 mass, so 0*c means that they have a momentum of 0 kg*m/s.
Energy can turn into mass and viceversa, but for that, it's not necessary for light to have mass. In fact, energy and mass turn into each other every time a reaction (chemical or nuclear) takes place. If you burn something and you weigh (with a normal, kitchen-grade scale) the products before the reaction and after the reaction, you would be under the impression that the weight has not changed, but only because the scale you used was not precise enough to notice the change. Use a scale that is precise enough and you'll notice that after the reaction, a tiny amount of mass will be missing. How much? Exactly the equivalent of the energy that was liberated by the fire, which can be calculated with the famous formula: E=m*c^2 (or, if you want to calculate mass, m=E/(c^2) ).
This, by the way, is also why matter and antimatter annihilate: because the reaction between matter and antimatter is so energetic that the entire mass of the reagents is consumed when energy is liberated (in that specific case, as gamma rays).
No, as I explained, light doesn't need to have mass for that. Stars and planets have mass, so they curve space, so anything (it doesn't matter what it is) passing nearby will naturally follow a curved trajectory.