# How light bends to black hole?

According to Einestine,
E=mc2
So light dosent hav weight.but i m heard of gravitational lensing....if light doesnt hav weight,den how can it influenced by the gravity?i m confussed...

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tiny-tim
Homework Helper
hi spg89!

light has momentum …

a spaceship with an extremely large sail would be able to point it towards the nearest star, and the momentum of the light hitting it (on one side only, of course) would give the same force as ordinary wind

(i'm not talking about the "solar wind" itself, which is made of ordinary particles, not light, and doesn't go very far from the star)

FtlIsAwesome
Gold Member
An object doesn't need mass to affected by gravity. It needs mass to cause gravity. So a massless photon will be affected by the gravity of an object that has mass.

Nabeshin
It needs mass to cause gravity. So a massless photon will be affected by the gravity of an object that has mass.
Not true. All you need to create "gravity" is a nonzero contribution to the stress-energy-momentum tensor. Photons definitely have momentum, so they do contribute.

Everything is "affected" by gravity, simply because free particles, no matter what they are, move on geodesics, and gravity alters the form of those geodesics.

how photon have momentum?cause,at light speed(for light itself),nothing has weight...then how can it be possible?

Even if photon has mass,according to E=mc2...everything is energy...

Even if photon has mass,according to E=mc2...everything is energy...
E=mc2 is NOT, I repeat NOT, the correct equation. The m in that equation is rest mass, and for photons that is zero.

The correct equation is E2 = m2c4+p2c2 where p is the momentum. It can be shown that the momentum of a photon is h/$$\lambda$$, where $$\lambda$$ is the wavelength of the photon and h is Planck's constant.

I hope this has cleared things up.

yeah...thnx man..