I’d like to comment on each of the points 1, 2, 3 in a question on quora.com -

If light has no mass, why is it affected by gravity?

1. As you say, light bends by the effect of gravity.

Prof. Max Tegmark of MIT (Massachusetts Institute of Technology in the USA) hypothesizes in his Mathematical Universe Hypothesis that the physical universe is not merely described by mathematics but IS mathematics. His categorization of the universe has four levels, with level 4 being altogether different equations or mathematical structures.

These “altogether different mathematical structures” are, in this article you’re reading, proposed to be two-dimensional Mobius strips that are formed by the binary digits of 1 and 0, and are joined as pairs into figure-8 Klein bottles which are mathematically immersed in the 3rd dimension. Then the photon (the fundamental unit of electromagnetism) and graviton (the hypothetical unit of gravitation) are created by, respectively, trillions of Mobius strips and trillions of figure-8 Klein bottles. These photons and gravitons interact via Vector-Tensor-Scalar Geometry to produce a united space-time (in Einstein’s term, a Unified Field) and every form of mass, including the Higgs boson and dark matter. They also interact via Wick Rotation to form the time in space-time.

Like the “lock and key” mechanism in biological organisms of molecules engaging with cells’ receptors, gravity may deflect light because the latter’s photons are a key fitting into the former’s graviton-locks. This makes sense if trillions of Mobius strips make up a photon, and trillions of figure-8 Klein bottles make a graviton. Photons and gravitons fit together because Mobius strips and figure-8 Klein bottles fit together – the second topological shape is also called a Mobius Doublet since it’s a joining of two strips.

2. Light has no mass; thanks to this it can travel at the speed at which it does.

The relationship of light to mass can be illustrated with a SIMPLE diagram which explains the vector-tensor-scalar geometry mentioned in my second paragraph.

I found a dictionary more valuable than any textbook for defining the words vector, tensor, and scalar. A vector is a quantity which possesses both magnitude and direction. Two such quantities acting on a point may be represented by two adjoining sides of a parallelogram, so that the resultant diagonal^ also represents the vectors. The two sides and diagonal thus illustrate the graviton's spin 2 and the photon's spin 1. The resultant diagonal represents the interaction of the sides/vectors (1÷2 = the spin ½ of every matter particle). Tensor calculus changes the coordinates of the sides and diagonal into the coordinates of a single point (the scalar) on the diagonal. This scalar point is associated with particles of spin 0. If the energy (mass) produced previously by the photon-graviton reaction happens to be 125 GeV/c^2, its union with spin 0 produces the Higgs boson - and relates the Higgs boson/field to the supposedly unrelated graviton/gravitational field (together with the latter's constant interaction with the photon/electromagnetic field).

^ The resultant diagonal of those two sides can be pictured as a boat being driven in, say, the vertical direction across a river while simultaneously being pushed horizontally by the river’s fast-flowing current. Referring to the mass of the scalar Higgs boson on the diagonal, what does GeV/c² mean? GeV/c² originates with Einstein's famous formula E=mc² solved for m (m=E/c², or mass equals the Energy of 125 Giga - billion - electron Volts divided by the speed of light squared). The short version is that the mass of the Higgs particle is 125 GeV.

3. Light travels in a straight line.

Electromagnetic waves – light is one type of electromagnetism, as are radio waves and X-rays etc. - are described by science as transverse, as are water waves. If a stone is dropped into a pool of calm water, many circular waves soon cover the surface of the water, and the water appears to be moving outwards from where the stone was dropped in. Actually, the particles of water simply rise then fall – it's the wave motion that moves outward. Similarly, there is little movement of photons - they simply rise and fall like corks bobbing up and down in the ocean’s waves. Energy is transferred from photons in one place to photons in the next place (photons fill space-time as well as, according to vector-tensor-scalar geometry, residing in every form of mass) by travelling space-time disturbances, and this wave motion is what is referred to as an electromagnetic wave.

If there is little movement of photons (and gravitons*), the universe could not be expanding (or contracting) but its space and time is static. The Big Bang has impressive points … leading to the idea that it’s a necessary stepping-stone. For example, the Big Bang’s supposed origin from quantum fluctuations is reminiscent of bits switching between 1 and 0.

* Einstein's equations say gravitational fields carry enough information about electromagnetism to allow Maxwell's equations to be restated in terms of these gravitational fields. This was discovered by the mathematical physicist George Yuri Rainich in 1925. Like an electromagnetic wave really being the exciting of photons in mass and space-time, “gravitational wave” is actually a convenient way of referring to the excitation of gravitons in space-time, and in objects possessing mass, by travelling space-time disturbances.