Like other electromagnetic particles, light has a specific velocity at which it travels. Further, it is presumed to be the fastest theoretical speed of any particle. The speed of light–referred to as “c” in a vacuum–is roughly 300,000,000 m/s. Since it moves so fast, most people never observe light as a moving particle or something that takes time to see. However, in the scale of the universe, the constant “c” can be useful to describe fast distances.
A light year is the distance that light travels in a year at its constant speed “c.” This is not only useful for breaking down/assigning meaning to vast distances, but it is also useful to think about in terms of our observation of the universe. If something is “x” light years away, the light we observe from it on earth is “x” years old. Essentially, we are looking into the past.
Here is a diagram putting the speed of light into context with our solar system and galaxy. It is taken from a Quora post.
At this very moment, the Voyager 1 spacecraft is hurtling through space at about 61,500 km/h each and every hour, carrying it ever further from our Solar system which it left 18 years ago. But despite this staggering speed, it would still take Voyager more than 75,000 years to reach the nearest star, Alpha Centauri. By 2025 the Parker Space Probe is expected to travel briefly at around 692,000 km/h as it swings in close to our sun, at this blistering speed it would still be around 7,000 years before it reached our nearest cosmic neighbor. Our best be then will be radio signals and other forms of light, which still take an astonishing four years to reach this closest star despite traveling at the universal speed limit. Humanity has been pumping radio waves into the sky since the turn of the century, although it was with the advent of higher powered broadcasts, especially TV, in the 1920’s that Earth truly lit up from a cosmic perspective. These radio signals have travelled 100 light years in the time since they were emitted, an astonishing 950 trillion kilometers or 570 trillion miles, but this represents only 1/1000 of our Galaxy’s diameter, or about 14,000 of our Galaxy’s 100 billion stars.
As you can see, the odds of anybody having noticed our early shouts don’t seem great. The Universe may simply be too big for us to ever properly leave an impact on. However, just because Humanity is too young for our signals to travelled very far at all doesn’t mean that everyone else is as late to the game as us. As long as the SETI continues, there’s always a chance that tomorrow will be the day a message finally reaches us, and the long isolation of distance is ended, and until then all we can do is keep exploring and keep crying out.
The Cosmic Calendar is a concept presented by famous astronomer Carl Sagan. In this diagram, the 13.8 billion year history of the universe is condensed into a single calendar year with the Big Bang happening on January 1st and recorded humanity being comprised in the last day of the last month of the last minute in which we exist.
This concept is so interesting to me as our history as humanity is only in the last few minutes but in reality it’s been hundreds of thousands of years since we’ve been here. I often think about the quote that says “too late to explore the Earth, but too early to explore the Universe,” (which comes from a meme, mind you) because it’s sooooo true. We have the math and such to date us back, but not the technology to actually move at the speed of light to explore the things we are discovering. It’s also so interesting because all of this progress in technology has been done in relatively such little time compared to the universe!
The Power of Ten video puts into perspective the miniscule size of not only individuals on Earth but of the Solar System entirely. At only 3 minutes into the video, we were already far beyond the Solar System with so much time remaining. Not only was I struck by the immense scale of the universe, but I was even more fascinated by the level of detail and intricacy at even the smallest units of measurement we know. The same building blocks apply to everything for billions of light years across the whole of the universe. The distance between galaxies also was startling to me as its hard to imagine the size of the empty chasms of space that separate us from other galaxies. The increasing rate of expansion detected in the universe also indicates that one day it might be impossible to even see some of the galaxies visible today. The other scary thought is the fragility of Earth at the mercy of such a large universe. The complexity at the atomic level and the specific forces required for us to live comfortably are truly a miracle.
As a child, I spent a considerable time looking at the Moon, and at one point, I may have even believed it was my friend. From the backseat of many late-night car rides, it “followed me”. In fact, during this period, my mornings consisted of watching Bear in the Big Blue House and the character – Luna -. It is safe to say that in the early 2000s, I felt close to the Moon. I will go a step further and share that my favourite movie and book in the Twilight series is New Moon. Thus, when I discovered that during this past Saturday’s New Moon was the closest to Earth since the Middle Ages, I knew I had to share.
Since the Moon does not have a perfectly circular orbit, it does not maintain an equal distance to Earth as it orbits. That said, New Moons at this proximity have occurred about three times in the past 2000 years. The last was in 1030, according to astronomers’ algorithms. Of course, the most recent was yesterday. The next will be far past our life spans.
Beyond our nearness being simply astonishing, I also experienced an overwhelming sense of gratitude at the revelation. I do not know if I will ever touch the surface of the Moon, but yesterday I savoured the fact that I was the closest to it than I had ever been. Even when studying objects in our solar system, I often find it difficult to grapple with how they are both near and far. Yesterday, I simply closed my eyes and made peace with how close but equally distant the Moon was to my heart.
Our brains struggle to comprehend the magnitude of our universe. We make comparisons based on that which we can see, yet it is totally inadequate to what reality holds. For example, the average person is just under 2 meters tall. But their size of the Earth when compared to that is substantial. The mean diameter of our planet is 12,742 km (Universe Today). That’s nearly 6400 times in magnitude. (That means in one day, someone at the equator travels a speed of over 330 miles per hour, since the equatorial bulge makes its diameter even longer than the mean.)
The distance from our Sun to Pluto is much larger. At its farthest point, the distance is 50x the distance from the Earth to the Sun (Nasa). The distance to the near the edge of our solar system (the outer edge of the Oort Cloud), is 2000 times (NASA). At a speed of 100 mph, it would take us about 106,000 centuries to make it to that point. One light-year is around 1016 meters. If you think that is a long distance, the Milky Way Galaxy is 100,000 light-years across(Universe Today). But the universe contains many galaxies, spread out amongst the system. The nearest one is 2.5 million light-years away (Astronomy). The universe is such a large expanse our brains can not comprehend its immensity. This video (Powers of Ten) gives a small glimpse into it, by changing the power to which 10 meters is given, yet it can only do so much!
We take it for granted that the speed of light is the universal speed limit; nothing can go faster than light. The knowledge of light-years is crucial to understanding the images that we see in the night sky. These images are not of the celestial bodies as they currently exist, but as they once were, as light has usually taken thousands of years to reach us. There is, however, one phenomenon that exceeds even the speed of light: the expansion of the universe, which was observed to be far faster than the speed of light shortly after the Big Bang.
This article by BBC Sky at Night Magazine explains the phenomenon. The speed of light is indeed the universal speed limit; however, the universe itself is not bound by this speed limit. The article describes the universe as a “backdrop,” (BBC) rather than a “material thing.” (BBC) Material things within the Universe can not exceed the speed of light, but the backdrop they are on can. This is an extremely abstract concept, but it helps us understand the expansion of the universe.
3*10^8 m/s. Wow. It’s practically impossible to conceptualize something moving 300 thousand Kilometers every single second, but believe it or not photons are doing this as we speak. To put this blistering pace into perspective, the distance from the North Pole to the South Pole is about 20,000 kilometers. Not even close to the length a photon can travel in just a second. It takes light only 66 milliseconds to travel the length of the entire globe! Naturally, it is seen as an incredibly valuable goal for humans to be able to travel at light speed. With current understanding of Physics, using Einstein’s equation of special relativity, it is not possible to do this. Mass, energy, and the constant of light speed are all related to one another, and it would require nearly infinite energy to propel an object with the mass of even a proton to lightspeed. Photons are just so small and energized that they can not compare to even the smallest other particles known to humans.
The image above is an artist’s illustration of everything in the observable universe. I find this image very interesting as it shows the scale increasing starting with our solar system and going out to galaxies, superclusters, our local group and further. This makes me realize how small we truly are in comparison to the universe we are truly just a speck of dust. Furthermore, this currently all we can see because this is all the light that has reached us so far, the universe can be much larger than even this. Currently we can see about 14 billion light years away from the Earth, as time passes we will be able to see more and more of the universe. However, the universe is also constantly expanding so we won’t ever be able to truly tell the full scale of it. I believe that it is important for all of us to understand our place as a miniscule part of everything around us so that we always keep looking forward and expanding the possibilities of all that is left to explore.
People say time traveling doesnt exist. They are skeptical we are able to break every law of physics we grew up learning. But, if we really think about it, we have a time machine right in front of us: a telescope.
What is a light year? Don’t be confused with the word ‘year’ being in this term, because a light year is a measure of distance rather than time. Because our Universe is so large, when we look through a telescope at stars and planets and other objects far away from us, we are really looking back in time. It’s a tricky concept to wrap your mind around at first. When we look at distant objects in our universe, we do not see the objects as they are the moment we look at them, but rather we see them as they were when that specific object released its light. This light then travels the distance (the light years) to our eyes, which takes a great amount of time, hence us looking at the object in its past not in its present state.
So next time you doubt the idea of time travel, think back to the idea of light years, and how we are able to look in the past with our very own eyes.
I’ve included a infographic which helps visualize how far some of the closest objects to us are in terms of light year distances.