# Relation between EMR & the force of gravity?

1. Jul 29, 2013

### MinnesotaState

We know that radio waves follow the curvature of the earth. Reason? Gravity (I think)

The only logical thing I can think of is what Johann Heinrich Winkler discovered in mid 1800's; that when electricity is given multiple paths to choose from, it invariably chooses the best conductor (least resistance)

So if the mass of earth is curving space-time at X curvature, then the path of least resistance will be X curvature ...assuming the radio wave was sent at X curvature ...then it should stay or want to stay at X curvature ...because it's lazy:P

2. Jul 29, 2013

### davenn

hi there

We don't, you thought
NO they don't, radio waves travel in straight lines.
radio waves, particularly those below ~ 1GHz are "refracted/reflected" around the earth by the Ionosphere. Specially notable on freq's 30MHz and below.
Frequencies above ~ 100 MHZ are also capable of long distance ( over the horizon ) propagation by tropospheric ducting

I have used all these methods and a couple more in my amateur radio activities on many different frequencies
Gravity has nothing to do with it :)

Dave

3. Jul 29, 2013

### WannabeNewton

Are you asking why electromagnetic waves follow null geodesics in space-time? You keep mentioning "radio waves follow the curvature of the earth" and "So if the mass of earth is curving space-time at X curvature" so I suspect that you are. Keep in mind that when you are on the surface of the Earth (as you are now), the effects of general relativity are utterly insignificant and electromagnetic waves will behave pretty much as usual.

4. Jul 29, 2013

### MinnesotaState

Thanks Dave

"refracted/reflected" around the earth by the Ionosphere"

I remember this now from "empire of the air."

I was confused about straight lines in curved space-time (below)

Since light always goes in a straight line relative to space-time, if it encounters a curvature in space-time it will continue going straight but since space-time is curved, "straight" will be curved as well.

5. Jul 29, 2013

### davenn

Gravity will affect light on a large scale ... the Abell 2218 galaxy cluster does a pretty good job with its gravity lensing

Dave