If Light has No Mass, Why is it Affected by Gravity?

What is gravity?

Two renowned scientists conducted research on the workings of gravity, with Sir Isaac Newton being the foremost among them. Newton's explanation of gravity posits that all objects in the universe possess mass, and as such, there will always be an attraction between any two objects. This attraction is known as gravitational force, and it varies according to the mass of the object. An object with a greater mass will exert a stronger gravitational pull on a lighter object, and the force decreases as the distance between the two objects increases. This decrease in force is inversely proportional to the distance between the objects raised to the power of two. Newton's theory of gravity has been proven to be accurate, as evidenced by the Earth's orbit around the Sun, the Moon's orbit around the Earth, and the workings of the universe as a whole. However, the question arises as to why the Earth does not collide with the Sun, given the latter's strong gravitational force. The answer lies in the relevant factors of distance and angular momentum. Despite this, scientists have been confounded by the nature of the force that binds objects separated by vast distances, such as the Sun and the Earth. This is where Albert Einstein's contributions to the field of physics come into play. As per Einstein’s assertions, gravity cannot be considered a force. However, this proposition appears to lack coherence as the occurrence of events cannot be accounted for in the absence of gravity. The enigmatic entity responsible for its manifestation remains unclear. Einstein's elucidation of gravity, although initially incomprehensible to the masses, was eventually comprehended a few years later. This revelation propelled him to the status of a prominent scientist on the global stage.

Let us now examine Einstein's gravitational model and its operational mechanisms. Through thought experiments, Einstein discovered the truth, which was later confirmed through mathematical equations. One day, while sitting in his office, Einstein observed a man cleaning the windows of a nearby building and began to contemplate what would happen if that person fell down. Although it may seem strange, Einstein describes this as the happiest thought of his life. His musings led him to the realization that a person falling from the top of a building and one floating in space would not feel any gravitational force. The phenomenon of falling down as a free fall without any reference point is akin to floating in space. According to physics, there is no difference between a person falling from the top of a building and one who is floating in space. Neither of them would feel any gravitational force. Therefore, empty space weightlessness is equivalent to free fall on Earth.

Let us proceed to the subsequent stage. At present, I find myself suspended in space within the confines of the lift, accompanied by a floating ball. A motor engine has been installed at the base of the lift, and upon activating it, I initiate an upward acceleration of the lift at a rate of 9.8 meters per second squared. This value corresponds to the gravitational force exerted by Earth, equivalent to 1 kilogram. Consequently, when I accelerate, the ball that was previously floating alongside me descends to the floor of the lift, while I am also compelled towards the bottom. The sensation of gravity experienced within the accelerating lift in space mirrors that which is felt on Earth. Thus, any action achievable with the aid of gravity on Earth can also be accomplished within this lift, as I perceive no discernible distinction between the two environments. Hence, free fall on Earth is tantamount to unrestricted movement in space. In other words, the acceleration of 9.8 meters per second squared in space engenders gravity, akin to that experienced on Earth.

Now, situated within the lift in space, I deactivate the engine, causing the lift to remain stationary. What would occur if I were to hurl the ball in this state? When the ball is thrown, it follows a linear trajectory. However, if it is thrown within an accelerating lift, it will traverse a curved path and ultimately collide with the floor. Thus far, everything has proceeded as anticipated. Now, I transition from employing a ball to utilizing a beam of light. What would transpire if I were to illuminate the accelerating lift with a torch? It is widely known that light perpetually travels in a straight line at a constant velocity. When a ball is thrown on Earth, it follows a curved trajectory before descending to the ground. Similarly, if the ball is thrown within an accelerating lift in space, it will also traverse a curved path. Does this imply that light itself will possess the ability to undergo bending? Can light be subjected to bending despite its lack of mass and its consistent movement along a straight path? According to Einstein, light indeed has the capacity to bend, provided there exists a phenomenon known as space curvature. The means by which we can ascertain this curvature is as follows: the acceleration experienced by an elevator, which is merely 9.8 meters per second squared, does not offer sufficient opportunity for light to bend. However, if we were to alter the acceleration rate to 98 meters per second squared, in accordance with Einstein's prediction, the degree of light bending resulting from this acceleration would be minuscule, equivalent to the size of an electron. This implies that as we increase the force of gravity or acceleration, light can undergo increasingly significant bending.

Now, how can we substantiate this claim? During that era, constructing an engine capable of such acceleration and implementing it was deemed impossible. In fact, even the invention of a rocket had not yet come to fruition. However, we do possess the force of gravity. The sun, in particular, possesses the most potent gravitational pull. In comparison to Earth, its gravity is 28 times stronger when considering objects in close proximity, owing to its significantly larger mass, which is 330,000 times greater. If we were to calculate the consequences of this gravity, the outcome would be light bending to an extent of 1.75 arc seconds. One arc second corresponds to 360 degrees in a circle, and within one degree, there are 60 minutes. Thus, 1.75 arc seconds, when broken down into 60 seconds per minute, is exceedingly minute.

In 1915, Einstein published his theory, accompanied by equations. This theory left everyone perplexed upon its introduction. What is the meaning behind this? What is he attempting to convey? Initially, the concept was not comprehended by anyone. However, four years later, Arthur Eddington, a preeminent astronomer from England, embarked on an experiment with his team to investigate this theory. At that juncture, they possessed precise knowledge of the location of certain stars, and if Einstein's theory was accurate, the star behind the sun should have been visible elsewhere due to the bending of light, which would have been perceptible to the human eye. They awaited a total solar eclipse as the sun's radiance made it arduous to observe nearby stars. On the day of the solar eclipse, Arthur Eddington's team commenced taking photographs. To their astonishment, the star appeared in the images in an unexpected location. This was due to the fact that gravity had bent light itself, as space-time had been contorted. For instance, if I travel directly from one location to the North Pole, I perceive it as a straight line, but to an external observer, it appears as a curved path. Similarly, light always travels in a straight line, but due to its curved path, it also bends. Therefore, the shortest path of light can be a curved one.

One may question whether space only bends at the bottom of a massive object, but also bends at the top. The two-dimensional form of space-time curvature is comprehensible, whereas the three-dimensional form is intricate. Einstein was hailed as the greatest scientist, and this thought experiment is not a simple one; it is the most significant thought in human history. It not only elucidated gravity but also demonstrated how the entire universe operates. This is known as the General Theory of Relativity. However, how does time factor into space-time curvature? How does space-time elucidate time dilation? The Special Theory of Relativity provides an explanation.