Comment on The speed of light
yesman@lemmy.world 3 days ago
I think it’s neat that Newton is taught first. As in: gravity is a function of mass. Because that works in so many scenarios.
But then you learn that gravity bends light and that photons have no mass.
So… Gravity isn’t a force, it’s more like going downhill… in the dimension of time.
spacehulk@lemmy.zip 2 days ago
If they have no mass, how do they push solar sails?
ttayh@lemmy.zip 2 days ago
They have momentum, but not mass (ignore the other explanation, like yes, E=sqrt(m0^2c^4 + (pc)^2), but so what? m0=0 for photons)
As you can see, momentum, p, is p=E/c, and we know that the energy for light is proportional to its frequency, f, E=hf (h is Plank’s constant). So, p=hf/c. When light is absorbed by a material momentum (and energy) conservation apply and it imparts p onto the object. If light is reflected it imparts 2*p, showing this is left as an exercize to the reader
BlackLaZoR@lemmy.world 2 days ago
They have mass. Everything that has energy has mass. They don’t have inertial mass but it’s just part of the equation
Zink@programming.dev 2 days ago
I think you are stating that backwards.
You can definitely say that everything with mass has energy. And yeah the two are kind of interchangeable.
But that does not mean that a photon HAS mass. It just means that we can calculate how much mass its energy is equivalent to.
porous_grey_matter@lemmy.ml 2 days ago
They have momentum, not mass, in relativistic physics you need something more complex than p=mv to describe momentum.
BlackLaZoR@lemmy.world 2 days ago
Energy=mass always. Theoretically you can make a black hole out of light, or you can turn that photon energy into inertial mass by running light in a closed loop.