Massless photon

[Gareth56]

If a photon is massless how can the intense gravity of a black hole have any affect it?

 

Ta

[swansont]

Because the notion that gravity only affects massive objects is a Newtonian one, but a better description of gravity is General Relativity. In that, gravity is due to/manifests itself as the curvature of space.

[Mr Skeptic]

And in any case you can consider photons to have (relativistic) mass since they have momentum and energy. Their rest mass is zero, however.

[ajb]

And in any case you can consider photons to have (relativistic) mass since they have momentum and energy. Their rest mass is zero, however.

 

I do not think that relativistic mass is a useful concept in this situation.

 

As swansont has stated, gravity is best thought of a the local geometry of space-time. In particular, that geometry need not be flat.

 

Now it turns out that photons travel along the next best thing to a straight line on a space-time: a null geodesic. To be a little more careful, we are thinking of photons as test particles, that is they do not effect the space-time themselves.

 

The energy-momentum is what acts as a source of gravity. Although photons are massless they carry energy-momentum and can act as a source of gravity.

[Mr Skeptic]

Even Newtonian gravity predicted the deflection of light by gravity (though only half as much as General Relativity predicted).