A hundred forty eight million kilometers away from us—a distance we call an astronomical unit—lies our Sun. To give this large distance more meaning, the Sun, in all its wondrous glory, is large enough to fit over a million Earths inside of it, but even so, it only appears as a small circle of light in our sky. Knowing that things appear smaller the further away they are, we then know that the Sun is quite a distance away. And yet, we are trapped in its orbit.
How could this be? How are we stuck orbiting the Sun, in a motion similar to a ball on the end of a string being swung around, when there’s no legitimate “string” between us?
The answer to this question is…gravity!
Gravity is an invisible attractive force that not only explains how we remain in orbit, but can also explain a multitude of other scenarios. Take for example Neptune, the outermost planet in our solar system, lies at a distance away from the Sun that is thirty astronomical units; and still, it remains trapped in an orbit around it!.
To go even further (literally), in the center of our galaxy, at a distance of twenty-six thousand light years, lies a supermassive black hole called Sagittarius A*. To put this distance in perspective, it is around one and a half billion astronomical units! Even at this absurdly large distance, our Sun is bound in an orbit around this black hole because of gravity.
A little weird, isn’t it? I mean, the concept of gravity is probably nothing new to you, the first thing you learn about it is that it’s what keeps us bound to our planet! Yet, it’s such an odd thing to think about; an invisible force that keeps our feet planted on Earth is the same force that binds Earth to the Sun and the Sun to Sagittarius A*.
Something to note is that this force isn’t just present in these large scales, but gravity actually presents itself everywhere. Newton’s Law of Universal Gravitation describes the gravity between two things as a force that is proportional to the product of the masses of each thing, and which gets weaker the farther away the particles are—with the force only approaching zero at infinity. For the force to be “proportional” to the product of the masses essentially tells us that the force is much stronger for objects with larger mass—which explains why even though Sagittarius A* is thousands of light years away, our Sun is still bound in its orbit.
This fascinating law applies to the majority of things you can think of, from large planetary scales to the miniature world of particles, but are there any other “invisible” forces that govern how objects interact with one another?
The answer to that question lies on your refrigerator door.
The concept of magnetism was first observed in things called permanent magnets—objects that would exert forces on other permanent magnets, as well as on unmagnetized iron pieces. This type of interaction was first explained using magnetic poles, where some bar shaped permanent magnet has one end called a north pole, which points north if the magnet were free to rotate, and the other end called a south pole, which points south. And, as per a saying that you’ve probably heard before, opposites attract—that is, the opposite poles are attracted to one another.
There must be some deeper explanation of this, gravity can be reasoned as the interaction of masses with other masses, and the aforementioned miniature world of particles.
Magnetic fields—that is, fields that exist in the space around magnets in which other objects are influenced—are now known to be caused by the movement of electric charges. In fact, the magnetic interactions between objects are actually “fundamentally due to interactions between moving electrons in the atoms of the objects” (Young & Freedman, 2019).
We can describe the magnetic field created around a positive charge using something called the right-hand rule—if take your right hand and point your thumb in the same direction as the direction that the positive charge is traveling, and then you curl your fingers, the curl of your fingers give you the direction that the magnetic field points all the way around the moving charge—if it were a negative charge, the magnetic field around it would point in the opposite direction as your fingers.
Interesting, isn’t it? Not only have we figured out ways to describe why objects interact in the way they do, we’ve also dug deeper into trying to understand and explain why those descriptions exist—and with every new finding, comes a multitude of new questions!
These forces of attraction are a magnificent example of the strange mysteries of our universe that we try to explain. Such mundane, everyday experiences are given this touch of magic; take notice of the way your arms swing at your side when you relax your muscles or the way that you can make a nail spin by bringing a magnet nearby, and revel in the interesting and invisible forces at work.
Sources
https://www.psu.edu/news/research/story/black-hole-center-milky-way-resembles-football
https://science.nasa.gov/sun/facts/#hds-sidebar-nav-2
https://www.nasa.gov/image-article/supermassive-black-hole-sagittarius/
https://coolcosmos.ipac.caltech.edu/ask/5-How-large-is-the-Sun-compared-to-Earth #:~:text=The%20Sun%20is%20864%2C400%20miles,can%20fit%20inside%20of%20it.
https://science.nasa.gov/neptune/neptune-facts/#:~:text=Size%20and%20Distance,-With%20an%20equatorial&text=From%20an%20average%20distance%20of,from%20the%20Sun%20to%20Neptune.
https://www.space.com/sagittarius-a
https://ece.northeastern.edu/fac-ece/nian/mom/magfields.html#:~:text=Magnetic%20fields%20are%20produced%20by,is%20created%20around%20each%20atom.
https://www.nde-ed.org/Physics/Magnetism/fieldcreation.xhtml
