Relativity I: What Does it Mean to be Relative?

Hello, and welcome back to MPC! Last week, we discussed one of the mind-bending phenomena that relativity will help us explain. This week, we are going to be delving into relativity a little bit more. In order to understand what relativity is all about, though, we need to understand where its name comes from. Specifically, we need to understand what it means to be relative to something.

When you hear the word relative, your relatives may come to mind. Indeed, a family tree is a perfect example of what it means to be relative to someone/something. For example, let’s consider your father. What do you call your father? Dad, of course! However, what does your cousin call your father? Uncle! This right here is the core concept of relativity: the same thing/person can be a different thing/person to different people. When you use the phrase “relative to + [person] the [object] is [description],” you are saying that, from person’s perspective, the object is description. So, in the aforementioned case, we would say that relative to you, your father is your father, but relative to your cousin, your father is his uncle.

Here’s another example: To you, your sister is your sister, but to your son, your sister is his aunt. This is illustrated below (with one of my amazing drawings).

Relativity1.png

Everything is relative in a family tree

Let’s talk about a more scientific example of being relative: distance. You and your friend are standing 10 meters away from a cookie. All of a sudden, your friend walks forward 2 meters. Now, he is standing 8 meters away from the cookie, and you are still standing 10 meters away from the cookie.

Relativity2.png

Your friend is closer to the cookie than you are now

What is the point of this story? Well, just like the family tree, how far away the cookie is is relative to the person that you ask. Relative to you (from your perspective), the cookie is 10 meters away, but relative to your friend (from his perspective), the cookie is 8 meters away.

Now, let’s discuss one final example: speed. This one is a little more difficult to grasp, but if you take the time to understand it, relativity will be a cinch! So, you are sitting in your car, next to your friend. Your friend starts driving away. After 1 hour, your friend is 100 km away, after 2 hours, he is 200 km away:

relativity3

Your friend is speeding away from you!

What do you, sitting in your car, think your friend’s speed is? Well, every 1 hour, he seems to be traveling 100 km away from you. Using our speed/velocity formula, this means that your friend is traveling at 100 km/h.

Pretty awesome, right? Not really, that was kind of boring. But things get a bit more interesting once you start moving too. Let’s say, just as your friend starts moving at 100 km/h, you start moving at 90 km/h:

Relativity4.png

Don’t let your friend get away!

What do you now think your friend’s speed is? Although your friend traveled 100 km in the first hour, to you (who has traveled 90 km), it appears as if he has only traveled 10 km (he is only 10 km ahead of you). In the second hour your friend has driven 200 km in total, but relative to you, he has only driven 20 km. Using our speed/velocity formula again, to you, your friend seems to only be traveling at 10 km/h. So relative to you when you were stationary, your friend was moving 100 km/h, but relative to you when you are moving at 90 km/h, your friend is moving at 10 km/h. This is his relative speed. An easy way to calculate relative speed in this scenario is to subtract your speed from your friend’s speed: 100 km/h – 90 km/h = 10 km/h.

What happens if you start moving at 100 km/h too? Using our formula, your friend’s speed, relative to you, is 100 km – 100 km, or…0 km/h? Of course it is! There must have been a time when you were sitting in a car, and there was a car driving right next to you. If this car seemed stationary (with regards to your position, for your mind can tell that both your car and that car were moving by judging how the trees and other objects around you were moving; if there was nothing around you, though, that car would have seemed completely still), it was moving at the same speed as your car:

Relativity5.png

Relative to you, your friend is motionless

That is all for today! Next week, we will talk about how the idea of relative speed led to the discovery of something called special relativity. See you then!

(featured image: http://www.bhmpics.com/walls/fall_autumn_foliage_trees-wide.jpg)

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