>>51108031>What actually is Gravity"We dont know" is a valid answer. It is thought to be the bending of spacetime due to the influence of Energy and Momentum (so technically speaking, not mass or matter specifically).This is the theory of General Relativity, and while on large scale it works beautifully well, it does not work on the very small scales where we have another very good and working theory, Quantum Mechanics and the Standard Model of Particle Physics.
So, we know that Quantum Mechanics exist, and that quantum scale systems behave as predicted by Quantum Mechanics. We know that Gravity exists, and we know that Gravity behaves like it does according to General Relativity. It would be natural to then combine the two models and create the "Theory of Everything", which would explain how the entire universe works.
There lies the issue. When we try to do this, everything breaks down. Quantum Mechanics, the Standard Model, and General Relativity do not work together, but separate they do work beautifully well. This is a sign that something is wrong with one, or all, of the models, and we do not know what. Theories like the String Theory are attempts at unifying the two, but those models have their own issues, particularly with string theory that for it to work we would have to assume 10 spatial dimensions instead of our observable 3. Physicists do not like making assumptions like those unless they can be measured and verified.
>Nature of Mass and EnergyYes, black holes have mass, and no, light does not have mass, nor does it have matter. Let me explain.
So, it turns out that Matter is not a fundamental building block of existence, but that it is instead merely another form Energy can take. Mass is Energy, in a literal sense.
This is where we get the famous E=mc^2, telling us that Mass ("m" in the equation) is directly equivalent to Energy ("E" in the equation). The "c" is the speed of light, which is a universal unchanging constant.
This is where we get into Nuclear Fission and Fusion as well, although this is a bit of a tangent. It turns out that when we split a uranium atom, the halves of the split atom combined weight less than the whole uranium atom. Same happens when we fuse Hydrogen into Helium: The Hydrogen atoms alone had more mass than the helium atom does. The difference in mass turns to energy, in the form of light (Gamma and X-ray Radiation), and momentum in the form of the kinetic energy of the atom or the particles that were born from the reaction (Alpha and Beta radiation).
E=mc^2 only describes "stationary" systems though, and for objects that move we need to expand it by adding a Momentum part, turning the equation into E=mc^2 + pc.
"p" here means Momentum, and "c" is the same speed of light as above.
So, since light AKA Photons do not have mass, the equation describing their energy becomes just E=pc, meaning that it is entirely defined by their momentum. For photons, p = hk, where "h" is the Planck Constant, and "k" is a number describing the (inverse) wavelength of the photon. Photon momentum and by extension photon energy is thus inversely related to it's wavelength - Longer Wavelengths (Radiowaves, Microwaves, Red Light) have less energy than Shorter Wavelengths (Gamma Rays, X-Rays, Blue Light).
Because Gravity interacts not with just Mass-Energy, but with Momentum, and Photons have Momentum, it follows that Photons are affected by Gravity despite being massless. This also means that photons do indeed exert a very very tiny Gravitic pull, despite having no mass and no matter. This is how you get the idea of the kugelblitz, theoretical prediction of the General Relativity; a black hole formed from concentrated photons.
From light having Momentum also follows that Photons can push things. This is where ideas such as Laser Sails come from.
You could think of Mass and Light as being just two sides of the same coin, that is Energy, with Energy being the fundamental building block of existence for STUFF as you know it.
Matter on the other hand is its own thing. I'll explain more about matter in the next post.
