Yes it is. 186,282 miles per second in free space
No, it seems not.
Special Relativity. The speed of light really is absolute. No particle moves faster in relation to it. Ignorant people think it can be done but it really can't. Its like saying an electron can be attracted to an electron. It just can't be done.
Very thought-provoking question. I feel like there are probably other undiscovered particles that have the potential to travel faster than the speed of light.
Look at the link shown in the description.
No. It can be slowed down in water and air. In a perfect vacuum, it is 136 282 mi/s, but elsewhere, it is modulated by what it is moving through.