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The speed of light (mander.xyz)
submitted 5 months ago by to c/science_memes@mander.xyz
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[–] 32 points 5 months ago (7 children)

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.

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  • [–] 5 points 5 months ago (6 children)

    If they have no mass, how do they push solar sails?

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  • [–] 17 points 5 months ago*

    They have momentum, but not mass (ignore the other explanation, like yes, E=sqrt(m0^2 c^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

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  • [+] -6 points 5 months ago (4 children)

    They have mass. Everything that has energy has mass. They don't have inertial mass but it's just part of the equation

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  • [–] 4 points 5 months 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.

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  • [–] 3 points 5 months ago (2 children)

    They have momentum, not mass, in relativistic physics you need something more complex than p=mv to describe momentum.

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  • [–] -4 points 5 months ago (1 child)

    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.

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