Freaky Fzx Friday - Light Ain't No Baseball

Imagine that my aging arm is capable of 100 mph just before another dislocation.



You're sitting on your stoop watching me as I travel past you at 50 mph and I throw my amazing fastball toward the front of the bus.  What speed do you measure for the ball?  I see it travel at 100 mph.

It has to be greater than 100 mph since the bus is already moving so you just add the two speeds and get 150 mph forward.

Now I'm at the front of the bus, distracting the driver, and I see a misbehaving student in the back seat.  I take my fastball and send it in her direction.  What speed does she measure?  100 mph.  What speed do you measure as you record the whole thing with your iPhone to forward to the authorities and the media?

50 mph backward.

It turns out that the same principle works with sound waves in my enclosed bus.  You would just add or subtract the velocities, depending on the situation.

Later, in another bus, with different bus driver and a laser gun, we perform the same experiment and you're still waiting for the bus.

Proper Gangsta Grip for Accuracy

I always measure that light to travel at 186,000 miles per second.

The new bus is moving 100,000 miles per second.  What speed do I measure for the light beam?  186,000 miles per second.  What speed do you measure as I pass on the corner at 100,000 miles per second?

Nope.  It's NOT 286,000 miles per second.  You measure 186,000 mph.  It is NOT the sum of the speeds this time.

Now let's do it again but in a different direction.  I still measure 186,000 miles per second.  What speed do you measure?  (And how many style points do I get?)

Crap.  You still measure 186,000 miles per second!  What's going on?  

Light ain't not baseball.  Light will always be measured to have the exact same speed no matter the speed of the source or observer.  We cannot add or subtract velocities for light.  Einstein used this fact as the basis for his Special Theory of Relativity.