There’s an old famous story regarding the discovery of gravity that involves an apple falling on Sir Issac Newton’s head. Now, I know that this is not exactly how Sir Issac Newton had his epiphany but the fact remains that this discovery pretty much changed how we look at the world.
It may sound intuitive now to think that there’s a force pulling everything towards the center of the Earth but there was a time when this was not so obvious.
However, the theory of gravitation has gone through many changes over the years. Just like most ideas in Science, the fundamental principle of gravitation itself has changed.
From a force pulling masses towards each other, gravitation is now considered a dent in the space-time fabric. In a few years, maybe even this idea will sound obvious and intuitive to future generations.
Aside from attracting bodies to each other, there’s another thing that gravity is capable of doing that might not seem obvious at first glance. It can bend light!
The Propagation of Light
Now, talking about light is a tricky business. Light exhibits properties of both waves and particles. For the sake of simplicity, I will only be referring to light as a stream of little packets of energy called photons.
It is well documented that light tends to travel in a straight line. You pick up a flashlight, switch it on, and you will get a straight stream of light. You put a mirror in front of that flashlight and try to deflect the light beam, and it will do so but again, in a straight line. So, is it possible to bend light like a straw?
The answer is a resounding yes! And is much easier than you think it is.
Go to a water tap in your house and turn it on so that the flow of water looks smooth. Get a laser pen and try to pass the beam through the water at an angle. What do you notice?
Credits: Buffalo State College
The light bends with the water! And there you go, you’ve just bent light to your will.
Now, imagine this happening in space. Think of the possibilities it could present for new discoveries. The fact is, this phenomenon does occur in space and at a much, much larger scale than possible on Earth. In the example given, the bending of light occurs due to total internal reflection but in space, this phenomenon utilizes the effects of gravity.
Let’s take a look into how that happens.
The Effects of Gravity on Light
If you know anything about gravity, you understand that gravity acts upon and due to mass. The heavier an object is in terms of mass, the more gravity you feel around it. This might make you wonder. If gravity only acts on objects with mass, does light have mass too?
Now, this is where the business gets tricky.
The fundamental particle of light, the photon, itself does not have mass but it does possess energy and momentum. And according to Einstein’s most famous equation,
mass, and energy are interchangeable. Where there exists energy, there exists mass.
In technical terms, we would say light has no rest mass but it does have relativistic mass.
In simpler terms, photons possess mass as long as they are moving.
I understand this is a bit difficult to wrap your head around but for the time being, take my word for it. You can follow the link given at the end of the blog to learn more about this bizarre concept.
Now that we’ve established that light possesses mass (sort of), we can safely state that it is also affected by gravity. However, this effect is not visible on Earth. Earth’s gravity is not nearly strong enough to bend light appreciably but there are objects in the universe that are so heavy that they are able to change the path of light around them. And this is how gravitational lensing works.
Gravitational Lenses
A huge amount of matter, like a cluster of galaxies or even a black hole, creates a gravitational field that distorts light coming from distant galaxies behind it. This creates a kind of giant magnifying glass in space. This effect is well observed by the Hubble telescope and aides in studying galaxies that are too far away for our current instruments to capture.
Another interesting use of gravitational lensing is in the mapping of dark matter.
Credits: ESA/Hubble and NASA
If the concentration of matter is at the center of the cluster of galaxies, the lensing produced is almost symmetric and we get a circle of light around the cluster known as Einstein ring. But if the distribution of matter is uneven and does not have a center, the lensing is asymmetric and multiple lensed images are formed. This gives a clue to observers about the distribution of matter in the cluster of galaxies. Data suggests that most of the matter in galaxy clusters is not visible and is actually dark matter.
From the distribution of lensed images, astronomers can map the distribution of visible and dark matter as well.
To learn about gravitational lensing in detail, readers are encouraged to follow the links below: