Freaky Fzx Friday - Speed of Gravity

Einstein did thought experiments.  It was 1905 and horses were faster than cars.  Aeroplanes were in size 3 Pampers and could puddle jump almost as well as my children.  Even bicycles were still faster than cars.

"Mile-a-Minute Murphy"

Faster than any car in 1899

Steam trains were the fastest human transport in H.G. Wells' 1897 The War of the Worlds.  Bicycles and horses also starred alongside the martial Martians in that excellent story.

So Albert Einstein had to imagine.  He boarded a brain train near the speed of light and put together his ideas on special relativity in 1905.  The constancy of the speed of light was one of the postulates on which he based his theory.

One implications of the special theory is that no thing (with mass) can travel at the speed of light.  And nothing, not even light or information can travel faster than the speed of light.

Imagine you're hanging out in the high school cafeteria with your buddies.  What teenager you know wouldn't ask the question "which travels faster, light or gravitational force?"

That was the question posed earlier in the week by a student who was discussing it with his friends.  "Imagine the sun suddenly disappeared.  The sun is like four light-minutes away, right?  Would we feel it first or see it first?"

not to scale

See those tendrils coming off the sun?  We don't see them until 8 minutes later.  And we feel the effects in our electrical grid a pair of days after that.

Measurable gravitational forces require large masses and short distances but light travels at unimaginable speeds.  And we can't exactly hammer the sun into a different position to see what happens.  Real experiments to answer our question are difficult.

But if the sun disappears, would we see it before it affected the earth?  That's a tough question that requires a thought experiment.  Newton imagined zero lag - instantaneous effect and infinite speed.  But Einstein changed things just a bit.

Since nothing travels faster than the speed of light, most modern scientists have been of the opinion that the earth won't know the sun disappeared until we see it.  In other words, gravity travels at the speed of light and the race ends in a tie.  It's still a bit controversial since a few scientists don't believe that our experiments have borne out this answer.  Whatever your conclusion, remember that

Thought experiments are wonderful.  Keep thinking.