Relativity: Alternate Realities

Before I start, I just want to say that I found this website that lets you experience the vast distances of outer space and it is honestly so fascinating. Check it out! http://joshworth.com/dev/pixelspace/pixelspace_solarsystem.html

I live in a constant state of ‘existential crisis’. This is the fuel to my unending thirst to know more and not just about the ‘what’s of the world but more importantly, the ‘why’s. I have progressed from the tiniest of curiosities of an eager child but at this stage of life and probably for the rest of my life, I have been stuck and will be stuck at wondering about the ‘meaning of life’ and I’ll go on to ask my eternal ‘why’s which question even the best answers out there. Unlike some, I do have realistic expectations from the human made, ungodly (yet incredibly useful and fascinating) subject that is physics. So, by logical progression, we move on to my biggest logical question- The nature of reality itself.

A fairly well-treaded path, to find the nature of reality and to understand it to its depths (maybe even try to describe it if we really want to be super ambitious), we must start with defining ‘realities’ in the universe. What I mean by this is to identify absolute, universal truths i.e. facts that are indisputable regardless of the perspective they are being observed from. At first glance, this seems like a pretty easy task- the speed of sound is 330 ms-1, the earth revolves around the sun in an elliptical orbit, a basketball is bigger than a tennis ball and my left pinky is about 3 cm long. Good observations, but if it was so easy then there wouldn’t be any fuss.

The problem with absolute truths is that the things we use to describe objects, be it speed or position, change with each perspective. What we see, we see from our own perspective which might be stationary, moving at a certain velocity or accelerating. This change in perception is called relativity- simply because our observations are relative to our perspective.

Imagine this- you’re a police detective and your partner just got murdered in a public place with a lot of suspects. You want to solve the case, an extremely difficult but relieving task. How do you go about it? You cut all ties to any of your preconceptions, not unlike a scientist, and you go and take the statements of everyone who was even remotely involved in his life and try piece together a narrative. You question this narrative until it holds true regardless of what you throw at it. You try to know what really happened. You try to find the absolute truth. This is a difficult task but sooner or later you have a rough idea of what might have happened. You are essentially using the scientific method and the concept of relativity to weed out the details which differ in each perspective and try to piece together the similarities in each account.

While we do eventually want to move on to the concepts of special and general relativity in this blog, without the basic foundation of what relativity is, it is going to be difficult to explain. Relativity is a way of thinking about things that help you to evaluate how universal a given truth actually is. Like in the example above, it is basically the compilation of two simple questions that may have mind boggling answers:

1. What changes do you observe in an object as you change your own perspective?

2. What remains constant in an object regardless of your state of motion?

Let’s start with the principle of relativity. This concept, unlike all the others associated with this branch of physics, can in fact be explained in a sentence. If you aren’t familiar with modern physics, that is extremely rare. The Principle of Relativity, at its base, says that motion is relative i.e. motion is always described with respect to some other object in space-time. If you think about it, this makes a lot of sense and again, unlike most things in modern physics, you can and you do observe this everyday, say when you’re cycling to the neighborhood park or when your dad drops you to school at 7 am in the morning.

When you’re standing on a footpath next to a busy highway, cars swoosh by at speeds where you can only see a blur of color and a gush of cold air. When you’re in a car on the highway and a different car overtakes yours, it looks like the car is ‘walking’ past, while the streetlights and pedestrians in the background seem to swoosh by in, again, a blur. When you’re in one of those amusement park rides where the machine tosses you around in circles like how a cowboy twirls his rope before throwing it, the rider right in front of you seems to be absolutely stationary while all evidence apart from that seems to point to the contrary.

The reason for this confusing mess (that somehow doesn’t seem to worry adult minds, a shame for science, a blessing for mankind) is our own perspective. When we look at something while we are in motion (or the other body is, or both), our perspective changes. We use our senses to measure our ‘relative speed of approach/separation’ when we look at an object, if that is easier to understand.

When you’re in a car which is moving at 40 kilometers per hour towards the north, you are moving at 40 kmph relative to the ‘stationary’ objects around you i.e. the trees, the streetlamps etc. If another car is moving at 60 kmph beside your own car – to you it would seem like you are stationary and that car is moving at 20 kmph. The car is moving at 20 kmph relative to you. When you are moving in circles hanging from a series of suspiciously thin ropes, so is the person right in front of you and they move with the exact same velocity as you so the difference in the velocity is zero. The distance travelled every second towards/away from your respective initial positions remains zero because both you and the person in front of you move the same distance every second.

Like the examples above where speed is relative to your state of motion, certain systems look different too, relative to your perspective. Let’s first take the earth individually. You look at Earth from say the ISS, the voyager 1(the farthest manmade object from earth), or even the Moon or Mars, you will see that the earth rotates on an axis. Each day marks one rotation. This is a more universal truth than the others that follow.

Now, let’s take the solar system. If you look at the system from the earth, at first you will see that the sun revolves around the earth every year. That is not as stupid as it might sound. It is a logical deduction from the information we have because in truth, *relative* to the earth, the sun actually does revolve around the earth. This is a fact that is essentially indisputable considering the fact that our perspective is from the earth. However, if we observe the movement of other celestial bodies from the earth we can determine a more universal truth that someone standing at the edge of the solar system might also agree to- that it is the earth that revolves around the sun every 365.25 days (the extra 6 hours each year is what adds up to a leap day every 4 years).

We learnt all of this in elementary school, at least till the part that the earth circles the sun. Something we now should consider is that the sun itself also is orbiting the supermassive black-hole, Sagittarius A*, which resides at the center of the milky way. Think about it for a second, and you start to wonder about how the earth then ‘actually’ moves. You wonder how you might perceive it if you were ‘stationary’ (you still won’t ever truly be stationary, the word has no meaning) or rather that you had a perspective that doesn’t move along with the solar system. By imagining this perspective, we are again moving to a more universal truth, which is that the Earth actually moves in a spiral, ever trying to catch up to the sun, which itself circles around the galactic center. We can go on like this forever, talking about how the milky way moves and further but this is enough to demonstrate how the motions of things differ and how exactly we can move towards truths that are essentially truer than others, by looking at the ever bigger picture.

How the Earth and Sun move

Unfortunately, the explanation above demonstrates the principle of relativity only in the tiniest but it will help build you build a mindset that will help you better understand it from here on out. We are at a position where we have understood the concept of relativity, which is why I am going to use that word when I want you to focus on a specific perspective. The principle of relativity basically asserts that yes, while some truths are more universal than others, it is wrong to say that the less universal truths are incorrect. In the case of the solar system, we are often dealing with incomplete truths, which are altered as we get more information, but there are some situations which are more ambiguous.

I take this example from Brain Greene’s The Elegant universe, as I do many other concepts introduced in this post, because I am unable to think of a better example. Take two cosmonauts, George and Gracie, untethered and floating in the darkness of outer space. It is important to consider that there are a limited number of celestial bodies in the background, which too are way too far away to compare positions to periodically from the naked eye. In short, it is impossible for our space walkers to determine if they are in motion or not just by looking at their surroundings as we often do when we are in a car on a smooth road travelling at a constant speed here on earth.

Let’s take George’s perspective. George sees that every second, Gracie seems to be a certain, definable distance closer to him than the second before. This distance is always the same for a given interval of time so we can conclude that their relative speed of approach is constant. Soon, he can see that Gracie’s space-suit reads NASA, not unlike his. As Gracie passes by him, she waves at him to which he responds with a similar wave. After Gracie passes by the point where the distance between the two cosmonauts would be at a minimum, George sees that Gracie seems to be farther away from him than she was the second before. Gracie keeps moving in the same direction as she was moving in the start of this thought experiment, which is now away from George unlike before.

Now let’s take Gracie’s perspective. Gracie sees that every second, George seems to be a certain, definable distance closer to her than the second before. This distance is always the same for a given interval of time so we can conclude that their relative speed of approach is constant. Soon, she can see that George’s space-suit reads NASA, not unlike hers. As George passes by her, she waves at him to which he responds with a similar wave. After George passes by the point where the distance between the two cosmonauts would be at a minimum, Gracie sees that George seems to be farther away from her than he was the second before. George keeps moving in the same direction as he was moving in the start of this thought experiment, which is now away from Gracie unlike before.

You might’ve noticed that I wrote the same thing twice and changed the names. I did that to demonstrate exactly how similar the two perspectives are, and without any real ‘stationary’ thing to compare our objects to, there is no perspective here that is more universal than the other. We see this phenomenon and our brain asks- but what is the truth? What it sometimes can’t digest is that both of these perspectives are as true as the other. They’re both true and neither of them are true all at the same time. The statements we can make with absolute certainty is that:

1. George is moving relative to Gracie

2. Gracie is moving relative to George

Finally, after trying everything, physicists had to deduce that motion itself is relative and we broke into a better understood yet more confusing world. In conclusion, the sun does move around the earth and the earth does move around the sun and these two things are indisputable facts when the perspectives are stated explicitly.