As the rays of the sun pierce through the clouds they look like giant spokes of sunshine illuminating the sky. Those beams of sunlight diverge and pan out in straight lines from their glowing and glorious origin. Light, it appears, travels in straight lines. The tube of light coming out of an electric torch also suggests that light darts out from its source as straight as an arrow. We’ve all learnt at school that light travels in a straight line. But is it always the case? Can light travel along a curvy path? Can a ray of light go round a bend? We are not talking about using mirrors to change the direction of a beam of light or using curved mirrors to see round corners. This is about actually changing the path of a ray of light from the ‘normal’ straight-as-an-arrow path to a curvy, bendy one. Is that possible?
It turns out that it is. At first sight, this might be paradoxical for how can one manipulate something which is immaterial? How can one twist or bend light when we can’t even touch it? To answer this, it helps to understand what light is. If we consider light to be a wave (just like sound is a wave) then, yes, it is hard to imagine this light wave being manipulated without the use of mirrors or other devices. If, on the other hand, we picture light as a stream of small, miniscule particles flowing through space (somewhat like dust particles suspended in air) then we can assume that they can be subject to some force acting on them. Just like blowing on those dust particles can push them away, exerting a certain force on those light particles can cause them to react in a certain way.
So what is it then? What is light? A wave or particle? Does it propagate through space like a series of undulations and vibrations like a wave of water would? Or is light made up of a collection of particles? So far, the best answer we have is this: it is both! Depending on how it is being observed, sometimes light behaves as if it were a wave and sometimes as if it were made up of particles. What it is exactly? We have no name for that yet but we might as well call it a wavicle for lack of a better word which combines both the wave and particle nature of light.
When light behaves as a wave then we can subject it to things which are usually associated with waves. We can cause it to be diffracted, that is, to spread out as it propagates when going through a small gap or hole. We can cause it to refract, that is, change its direction of motion when it passes from one medium to another. Notice how light bends when it passes from air to water, for example. This is the reason why a plastic drinking straw in a glass of water might appear bent when, in fact, it is completely straight. Lastly, we can cause light to reflect as it bounces off a reflective surface such as a mirror.
When light behaves as if it is made up of particles then it can be subject to other effects. One of them is the bending of a ray of light by the force of gravity. That’s right. Gravity, that ubiquitous force we are all so familiar with (especially when we’re about to fall), can and does have an effect on light as well. Gravity is the force that acts between any two masses causing them to attract each other no matter how far apart they are. The closer they are, the stronger the attractive force between them will be. Also, the more massive those bodies are, the stronger the attractive force between them will be. Anything that possesses mass will attract another mass. The force with which they will attract each other depends on their respective masses and their distance apart. This is known as the law of gravitational attraction and was formulated some 325 years ago by Sir Isaac Newton, the legendary English scientist.
For an entity to possess mass, it does not have to be made up of matter. It can be completely immaterial and intangible. There is no contradiction here. It is true that in our day-to-day interactions with matter and stuff, things like food, furniture and fabrics, everything that we manipulate has mass and is material in nature, whether they are in solid, liquid or gaseous form. So, in that sense, it is almost logical to think of matter and mass as being synonymous for one does seem to imply the other. However, it is not always the case. As another giant in the scientific realm has shown, namely the great Albert Einstein, mass and energy are equivalent. He came up with his famous equation: E = mc2. Everything that possesses mass, also possesses an equivalent amount of energy given by the equation. Here, the m stands for mass, E is for energy and c denotes a rather large number. It is about 300 000 000 m/s and represents the speed of light in vacuum. When that number is multiplied by itself (hence the c raised to the power of 2), the outcome is even larger. Which is another way of saying that from a tiny amount of mass, one can, potentially, yield a humongous amount of energy. Conversely, mass can be derived from energy. In that case, a lot of energy is required to produce a relatively small amount of mass. That mass can be in the form of matter or it can remain in its immaterial form. What matters is, that the mass thus created is equivalent to the energy used to produce it.
Light is a form of energy. The name we give to that energy is called electromagnetic energy. This energy can be dissipated in the form of waves, as we’ve mentioned already. One property that all waves has is this thing called frequency. To put it simply, frequency has to do with how much the wave is oscillating or vibrating or undulating in a given amount of time. The more it does so, the higher its frequency. Waves with low frequency take a long time (relatively speaking) to oscillate. When we refer to sound waves, then low frequency sound waves are detected as low pitched sounds and high frequency are high pitched sounds. So, here, frequency is related to pitch or musical note being played. When we refer to light waves, frequency is related to colour. High frequency light is blue and low frequency light is red. There are other colours of light which we can’t see with our naked eyes. We just don’t call them colours but have names like infrared or ultraviolet (UV) or X-rays. Infrared light has a lower frequency than red light while UV light has a higher frequency than blue light.
There is another interesting aspect about the frequency of a wave, be it a light or sound wave. The frequency of a wave is also related to how much energy the wave possesses. The higher the frequency, the more energy it has. I suppose you can see, already, where I am heading with this. Because frequency is related to energy and because energy and mass are equivalent, then there is a link between frequency and mass! In fact, this is not far from the truth. We can definitely associate the frequency of a wave with its mass. However, the caveat is that we should be careful as to why we would do that. Would it really make sense to think of the mass of a sound wave, for example? Would it really help explain how a wave behaves if we took into consideration its mass? Not necessarily. But, in some cases, this link between frequency and mass can be helpful and insightful. When it comes to light waves, taking into consideration the link between frequency and mass can help explain some of the seemingly odd behaviour of light.
When we consider light as being made up of particles, then the relationship between frequency and mass becomes meaningful. The particles of light are called photons. A photon of blue light (high frequency) has more mass than a photon of red light (low frequency). Now, as with everything else that has mass, photons are also subject to the force of gravity. If a beam of light is passing close to a massive object, they will attract each other with their mutual force of gravity. It is similar to a ball whizzing past you as it gradually falls towards the ground along a curved path. The ball is attracted towards the Earth and so is the beam of light passing near a massive star. The Earth is not massive enough to attract or divert light’s flight path towards it. The Sun, which is about 333 000 times more massive than Earth, can cause light to deflect from its path as it passes near it. This deflection (albeit tiny) in the path of light rays coming from distant galaxies was observed for the first time during a solar eclipse. The eminent British astronomer, Sir Arthur Eddington, who conducted this observation in May 1919, showed that the deflection of light was as predicted by Einstein’s theory of General Relativity published some four years ago. This was a tremendous achievement both in theoretical and practical physics at that time. Most of all, it was the definite proof that Einstein’s theory was (and still is) the most precise one about gravity.
The deflection of light rays is absolutely possible. Light can travel along a curved path; it doesn’t always travel in a straight line. We might not observe this here on Earth but it certainly does take place near massive objects like stars and galaxies. This phenomenon, of light bending as it passes near such massive bodies, has been given a name. It is called gravitational lensing. What that means is that, those stars and galaxies act as though they are lenses which distort the light coming from even farther celestial objects. What happens then is that they change how we, here on Earth, observe those distant objects. They appear to be in a different location from where they actually are. This is because, from our ‘normal’ perception that light travels in a straight line, we automatically place those distant objects in their expected position based on the light we receive from them. We have to take into consideration any gravitational lensing effect so as to correct that position and properly place those objects in their right location.
The more precise description of why light changes its path when going past massive objects has to do with geometry, not force. Again we call upon the one and only Albert Einstein for an explanation on this. According to his theory of General Relativity, gravity is interpreted in terms of the curvature of spacetime. In comparison, Sir Isaac Newton’s interpretation of gravity was in terms of forces. They are both correct in their interpretation except that Einstein’s interpretation is more precise than Newton’s. Consider this little gem of a number which is so common in mathematics: π. Pi is a number which cannot be written down. It has an infinite number of digits after the decimal place. If we divide 5 by 2 then we have 2.5. This is a number with only 1 digit after the decimal place. Pi can be represented as 3.1415926 (up to 7 decimal places). Or, it can be represented as this: 3.14159265358979323846 (up to 20 decimal places). They are both correct, for all practical purposes, but one is more precise than the other. In a similar way, interpreting gravity in terms of geometry as opposed to force is more precise but for most practical purposes we can still apply Newton’s interpretation.
When we speak of the geometry of spacetime, we mean that spacetime has a structure that can be subject to deformations and distortions. Those distortions arise because of the presence of mass (or, equivalently, energy). The more mass there is, the greater the distortion. This is not unlike the dent caused on a cushion. The more massive person will cause a greater dent than a less massive one. So what happens when spacetime gets distorted by the presence of mass? Well, think of a falling apple. Ever since Newton came up with his theory of gravitation, we’ve interpreted gravity as this force acting between masses. The Earth attracts the apple with a certain force and the apple thus falls to the ground. Another way to think about this is to consider the spacetime between the apple and the Earth as curved or dent in a certain way that, the only path the apple can follow (once it’s off the branch) is to move along that dented region of spacetime until it reaches the ground. Here, no forces are involved explicitly, only the path or geometry of spacetime directing the apple to the ground. The source of this curved spacetime is the Earth. By distorting spacetime around it, the Earth therefore causes other masses to follow that path of the curved spacetime towards it. This is what we interpret as the force of gravity. This geometrical description of gravity by Einstein is very different from Newton’s interpretation but it doesn’t diminish the significance of Newton’s work.
This very curvature of spacetime is what causes light rays to bend as they pass nearby massive objects. Prior to Einstein’s theory of General Relativity, such bizarre behaviour of bent light beams would have been impossible. Using Newton’s theory of gravitation would not give the right explanation. But Einstein didn’t stop there. There was more to spacetime than it just being flexible and curvy. Spacetime could, at least in principle, be so dented and distorted that it pierces the very ‘fabric’ of spacetime! There could be a hole in spacetime! This hole would be caused by a very massive object or an object with extremely high density (lots of mass packed in a miniscule volume). Such an object would be so dense that it would not only curve spacetime around it but also pierce it! And any massive body unfortunate enough to venture close to that dense object will inevitably follow a path that leads it straight into that hole never to emerge again! It would be a sure death for any such body with mass. Since even light has mass, it will also be subjected to follow a suicidal path along the highly distorted spacetime near the superdense object. As such, not even light can escape from this ominous celestial monster. We have a name for such a beast that engulfs everything in its vicinity. We call it a black hole. As its name suggests, it is a hole in spacetime and it’s black because, well, nothing, not even light can escape once they’ve plunged right into its guts.
Light is not so ethereal in that sense. Even though it is only energy and not material in nature, it does interact with the universe, sometimes in strange ways. It can fall under the spell of gravity just like any other entity with mass. Whether we observe it as waves or as photons, its behaviour has intrigued scientists for as long as there have been scientists. Lasers are but one simple yet genius application of the science of light. Without lasers we cannot imagine any of the modern game consoles or BluRay players to function. Without lasers we cannot envision a certain type of eye surgery. Without lasers we cannot add the spectacular effects of colours and illumination in certain concerts and ceremonies. Light, in all its form and glory, is certainly an interesting phenomenon of the physical world. Physics, with all its ingenuity and simplicity, can shed some light (no pun intended) about the nature of light.