We have a rough idea of the speed of light but we do not have an exact number. There can be a bit of variation. As Einstein said time is relative. Quantum entanglement is faster than the speed of light. Not only can two particles anywhere in the universe communicate,, anywhere in history they can communicate. So partials that are 13 billion years apart can communicate with each other in real time. ... With a quantum computer you could communicate with everything everywhere in the universe in real time. ...
Off topic perhaps, but looking into this area is a separate interest of mine. I understand why you have written what you have, but I think this common way of thinking is wrong. Let me explain.
Quantum theory postulates a wave function and equates the quantum particle (e.g. photon or proton) with the wave function. So if one starts with entangled photons and sends these to separate ends of the universe, quantum theory states that these particles are in a state of superposition until the measurement occurs. And it is at that point that the particles "collapse" into reality. The process of measuring one particle on the "left" side of the universe, forces the particle on the "right" side to become the complementary particle. So if the spin on the "left" side is found to be "up", than this information has to be transmitted instantaneously, at faster-than-light speed, to the "right" side such that the other photon can be found to be spin "down." This is what Einstein objected to, calling it "spooky action at a distance."
A key piece of evidence that led to this theory is the double slit experiment, where particles, e.g. photons or protons, fired at a double slit demonstrate an interference pattern such that it is a wave that interferes with itself, **even if the particles are sent through one at a time.
So formulas derived from this quantum theory are very accurate at predicting outcomes from experimental evidence. The problem is the INTERPRETATION of quantum theory. This
page from Wikipedia lists 13 different interpretations. The most commonly taught one is the Copenhagen interpretation, which is behind what Diamond7 has written (though I think he has a few important parts incorrect after faster-than-light information transmission). My favorite is the De Broglie-Bohm theory, also called pilot wave theory. Veritasium had a eye-opening video about this
here.
In pilot wave theory, the state of the two particles was already determined as soon as they were separated. Like taking a pair of mittens or gloves, putting the each in a box, and mailing them to separate ends of the country. Each box contains either the left glove or the right. Until the box is opened, one would not know which was which. But as soon as the first box is opened, the state of the second box is known -- as if this information was instantly transmitted at great distance. Now this idea that each box holds the state of the glove (or particle) even before measurement is called a "hidden variable", and the idea that there actually was a state before measurement is called "realism". And the idea that position has meaning is called "locality."
Dr. John Bell wrote a paper in 1964 where he proposed what became known as "Bell's Inequality" which attacked the idea of hidden variables. It is widely quoted that Bell's theorem rules out hidden variables. But if one pays close attention to the details, it may be learned that Bell's theorem only rules out LOCAL hidden variables. It says nothing about GLOBAL hidden variables. And so, with pilot wave theory, as shown by Veritasium's video(
here) one can have a global wave that carries individual photons, replicating the double slit interference pattern. This global wave is the global hidden variable.
In summary:
- Quantum theory accurately predicts experiment outcomes, but the interpretation is debated
- The most common interpretation is the Copenhagen interpretation which I think is wrong and leads to problems (not discussed here).
- De Broglie-Bohm theory is an alternate theory that explains the facts, and includes realism and allows for global hidden variables.
- Bell's rule excludes local global hidden variables, but does not rule out global hidden variables.
I'm not a physicist and I likely got stuff wrong here. Sabina Hossenfelder does a much better job
here.
KT