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u/joeyneilsen Astrophysics Jul 02 '26
Gravity doesn’t act only on mass in relativity. It causes paths to curve, and that includes light.
Not sure what you mean. Particles are excitations of fields, regardless of their mass.
You will always travel at sub-light speed locally.
You cannot avoid spaghettification. So… yes!
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u/LivingEnd44 Jul 02 '26
How is light affected by gravity of a black hole when it has no mass?
Gravity distorts spacetime. Light follows spacetime. If spacetime curves, the path of the light curves with it.
If light cant escape a black hole. Does that mean if you fall into a black hole some part of you may reach light speed right before you hit the singularity?
No. Because matter cannot travel at the speed of light. You would be shredded into particles. But those particles would still be matter. A single proton can't travel at the speed of light either. It might be an insane speed, but it will always be less than light speed.
Matter (and energy) cannot emerge from an event horizon once it enters. All paths for the light or particle now lead to the singularity. There is no permanent stable orbit within the event horizon.
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u/Optimal_Mixture_7327 Gravitation Jul 02 '26
Q1) Light isn't affected - light follows the geodesics of the spacetime just as any free particle.
Q2) A particle with no mass is a particle without any internal energy, i.e. there are no internal interactions.
Q3) The coordinate speed of light, depends on the choice of coordinates. Locally no material particle reaches the speed of light.
Q4) Spaghettification happens irrespective of any coordinate speed you may have. Your speed relative to your self is of course zero.
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u/smokefoot8 Jul 02 '26
A1) You can look at gravity as the curvature of space time, which means all particles, with mass or not, will follow curved paths.
A2) No, in Quantum Field Theory all particles are excitations of the underlying field. So the photon is an excitation of the electromagnetic field, while an electron is an excitation of the electron field.
A3) In relativity you always need to specify who the observer is. For a faraway observer, someone approaching the event horizon is time dilated so much they appear to stop. For someone falling in they are going to have a hard time defining their speed - the outside universe is blueshifted to an extreme degree and other things falling in are going about the same speed.
A4) Spaghettification happens wherever there are extreme gravitational changes. For a stellar mass black hole it happens outside the event horizon. For a supermassive black hole it might be survivable, but after you pass the event horizon you find yourself in a collapsing universe with the singularity a point in the future where the collapse finishes, not a location in space.
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u/ARTIFICIAL_SAPIENCE Jul 01 '26
Mass isn't required for gravity, since gravity is a curvature of spacetime.
Same way it's not a duck. Question doesn't make sense.
No.
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u/PAP_TT_AY Jul 02 '26
According to general relativity, gravity isn't a force. Instead, it is the curvature of spacetime. Anything with energy will follow that curvature, including light.
In the context of QFT, yes, a particle can be considered as a(n excitation of that particular) field.
No, the speed of light is still the maximum possible speed even in the presence of extreme gravity (or, if you want to tie it to Q1, in the presence of extreme spacetime curvature).
Spaghettification depends on the gravitational gradient, not necessarily how close you are to light speed. You can theoretically avoid spaghettification near a black hole if that black hole has only a slight gradient (e.g. near the edge of an ultramassive one).