Matthew O'Dowd, the Australian-born host of PBS Space-Time, insists that high-energy (short-wavelength) photons (or waves) of light will travel slightly more slowly through space than lower-energy, longer-wavelength ones; while Don Lincoln of Fermilab says the exact opposite....
Does anyone know the correct answer?
Do photons in the 'ultraviolet regime' travel slightly faster or slower than ones in the 'infrared' regime? .....
In standard general relativity, spacetime is considered to be smooth and continuous. That is, you can in theory divide it into smaller and smaller volumes without limit. Loop quantum gravity, LQG on the other hand (because it is a quantum theory after all), breaks this smoothness into a discrete structure where spacetime is a spin network of lines and nodes.
Loop quantum gravity predicts that the speed of light is not constant which violates one of the fundamental concepts of relativity. Specifically, higher energy photons would travel slower than lower energy photons.
Low frequency light, for example radio waves, have long wavelengths. For such a wavelength, travelling through spacetime will be fairly "smooth" since there is no microscopic structure of spacetime to contend with, since the scale for a radio wave is so much bigger than any loop quantum gravity scale. But on the opposite side, for example a gamma ray photon, has a very high frequency and a very small wavelength, one which exists on a scale somewhat closer to the scale where LQG (or any quantum theory of gravity) effects will become more noticeable.
So one could conclude that light of high energy and frequency, as it travels through spacetime interacts with these microscopic points in the spin network, so that the effect is a slowed down light ray, and as stated above, the converse is true for low energy long wavelength light, who's interaction with the same is negligible.
I'm inclined to think, that although there is this description of changing light-speed at different scales, one could still say that the local speed of light still remains unchanged. But there are various physicists that actually think that if LQG were accurate, and such spin networks existed, this would break the notion of the constant nature of the speed of light.
Answer 1 gives a solid explanation, but it can be made clearer and more approachable.
In simple terms, under Einstein's theory of general relativity, spacetime is smooth and continuous, and the speed of light is always constant. This means that in a vacuum, light of all kinds—whether it's high-energy light like ultraviolet or gamma rays or low-energy light like infrared or radio waves—travels at the same speed. That speed is about 299,792,458 meters per second.
However, loop quantum gravity (LQG) tries to combine quantum mechanics and general relativity. This theory suggests that, at extremely tiny scales (smaller than we can directly observe), spacetime might not be smooth. Instead, it might be made up of tiny, discrete building blocks, sort of like a fabric made of individual threads. This idea is very different from how we usually think about spacetime.
Now, if LQG is correct, it could mean that the speed of light might not always be constant, especially for very high-energy photons like gamma rays. The idea is that these high-energy photons, with their short wavelengths, might interact with this "grainy" structure of spacetime, and as a result, they could be slowed down a little. In contrast, low-energy photons with longer wavelengths, like radio waves, are too "big" to notice the fine structure of spacetime, so they would keep traveling at the usual speed.
In this way, it’s possible that higher-energy light could travel slightly slower than lower-energy light under certain conditions. However, this is still just a theory. There hasn't been any experimental proof to show that this actually happens in the real world. Scientists have looked for signs of this effect by studying things like gamma-ray bursts from faraway galaxies, but so far, the evidence isn’t clear.
Locally—meaning in the kinds of experiments we can do here on Earth—light always seems to travel at the same speed, no matter what. The difference in speed would only show up over very large distances or in extreme situations, according to these theories.
In short, while the idea that light’s speed could vary based on its energy is intriguing and a possibility in some quantum gravity theories, it hasn’t been proven yet. As far as we know from experiments, all light travels at the same speed in a vacuum.
