Misconceptions

SOME MISCONCEPTIONS IN PHYSICS

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Imagine learning for the first eighteen years of your life that the earth is flat. All through elementary school and high school you grow up hearing about the flat earth we live on and doing boring flat earth physics homework, and then if you’re lucky enough you get to college and psych! For the first time they show you a globe and say, “Sorry for lying. The earth is actually round”. Well, this is unfortunately exactly what we do with gravity. You probably learned that objects attract each other based on their mass, so you probably grew up thinking that light can’t possibly be affected by gravity because it is massless. I know I did. Well guess what? The source of gravity is not mass, it’s energy and momentum, which light certainly has. Of course, regular matter does too. So, not only does light get bent passing by a star or a planet or a black hole, but light attracts the planet or the star or the black hole in return. To be sure, it’s a very very small amount. But, that small amount is not zero. Anyway, the point is that Newton’s law of gravitation is just an approximation, good enough to get us to the moon. But it’s not perfect. General relativity is better.

Next comes special relativity. You probably have also learned that if a sheep is moving 2mph relative to a train and that train is moving 2mph at the same direction relative to the ground, then the sheep is moving 4mph relative to the ground. 

Special relativity

2mph + 2mph = 4mph   

Right?

Wrong!

Experiments in special relativity have confirmed that velocities don’t simply add together.  So the sheep will in fact be moving very very ever so slightly slower than 4 mph relative to the ground. The formula that correctly predicts this deviation from just adding velocities is:

The formula

 It’s not a very big effect, but then again, the earth looks pretty flat doesn’t it?

But it isn’t flat. If I walk 10,000km away from my cat, and you continue on walking  10,000km more, you’re not 20,000Km away from my cat. You’re just 12,750km away. In fact, the farthest on earth you can get from anything on earth is 12,750km. It’s the earthly distance limit, though we normally call it the diameter of the earth. Similarly, when you try to add two velocities together, there’s a cosmic speed limit of 300,000,000m/s, that is, the speed of light.

Earthly distance limit

 So, just because to our eyes the earth looks flat, velocities look like they simply add together, and light looks like it doesn’t attract gravitationally, is that an excuse to mislead ourselves about the true nature of things?