(July 7, 2019 at 4:01 pm)Rhondazvous Wrote: I’ve been laboring under the impression that photons are massless. But I’m now reading Einstein’s Universe by Nigel Calder in which he claims that Einstein said light is heavy
In some ways, it does seem that light must have mass. How could gravity bend light or trap light in a black hole if it had no mass? At the same time light doesn’t have the second property of matter. It doesn’t take up space. It wouldn’t be able to pass through solid objects such as glass, plastic. Or ice if it took up space. Only in intense concentration (laser) does light appear to take up space and move material objects out of its way.
So, are we making a difference between mass and matter? Between photons and light?
No, Einstein did NOT say that light is heavy. Even through the equation E=mc^2, it is not massive because the *correct* and most general equation is E^2 = m^2 c^4 +p^2 c^2 where p is the momentum. In the case of light, m=0 and E=pc.
Now, in general relativity *all* forms of energy contribute to the gravitational field, so mass is not directly required. Conversely, light *does* move along a different path than it could if it were massive (a null path--massive paricles do not travel along null paths).
Next, the classical ideas of matter have to be modified because of what we know about the atomic realm. The reason 'classical matter' takes up space is because of something known as the Pauli exclusion principle. It applies to any 'particle' with 'half-integer spin'. Such particles are also known as fermions. For example, electrons and protons are fermions.
But then, so are neutrinos, which pass through matter quite easily. To further complicate things, electrons are thought to be point particles: they technically do NOT take up space. But they do repel each other which forces them apart. This repulsion is partily due to the fact that they are fermions. This is why most ordinary matter 'takes up space'.
There are also particles called bosons that do NOT obey the Pauli exclusion principle. While fermions don't like to be together, bosons very much do. And guess what? Photons are bosons, not fermions. And yes, there are bosons which have mass. The Higg's boson actually has a mass about the same as an atom of Cesium.
The upshot? Classical ideas about mass and matter have to be modified considerably when faced with the realities of the atomic and subatomic realms. Things just don't work at that level based on classical mechanics.
