# Quick clarification

If light has no mass how can it be affected by gravity?

Because it has momentum

But momentum is mass x velocity so how can it have momentum?

Because language can sometimes bring more problems than answers. Light does not obey the same laws as ordinary matter. To help show this I’ll use equations.

(this is the full version of the famous E=mc2
E=energy
P=momentum
c=speed of light
m=mass

So if we assume m=0 then we get E=Pc

We can find the energy of a photon by frequency x Planck’s constant (E=fh)

So if we know the energy and the speed of light (which is constant) then simple division will give us the momentum of the photon.

E/c=P

So gravity is related to momentum and not to mass?

Gravity is still determined by mass but in Relativity mass is determined by a stress-energy-momentum tensor. This tensor is what gives rise to the famous “curved spacetime”. If you would like to know about all of this and more in greater detail here is a fantastically well written Wikipedia article on General Relativity

# poem from general relativity.

from you, I learned that a double star is two
locked in mutual gravitation, ever-circling
the way I always want to be kissing your fingertips
burnt from stir fry

I am terribly in love with your fingertips

in context of the universe
I am young debris spinning
I make mistakes
(and also, with every last part of you, intricately built
tendons to toes
for me?)

do you know that you do that
in a wasteland of galaxies
these 11 dimensions caught between my reedy heartstrings

it’s you
it’s you
it’s you

String theory.

# Weinstein, The New Einstein?

guardian.co.uk

Alok Jha on Eric Weinstein, a New York hedge fund manager who may — may — have solved the biggest riddle in science: the bridge between general relativity and quantum mechanics:

In Weinstein’s theory, called Geometric Unity, he proposes a 14-dimensional “observerse” that has our familiar four-dimensional space-time continuum embedded within it. The interaction between the two is something like the relationship between the people in the stands and those on the pitch at a football stadium - the spectators (limited to their four-dimensional space) can see and are affected by the action on the pitch (representing all 14 dimensions) but are somewhat removed from it and cannot detect every detail.

Exciting. But who knows. Certainly not me.

More from Marcus du Sautoy of Oxford, where Weinstein will be presenting his findings.

# General Relativity Resource Post

This is here until the author de-anons. When that happens, I’ll post a link to her livejournal or wherever it is that she’ll post General Relativity. Until then, enjoy! And check out her Tumblr here!