# F a transmiter for arguments sake, fitted in a satalite orbiting the earth

#### wolram

Gold Member
im not sure how to pose this question, my ist try.
if a transmiter for arguments sake, fitted in a satalite orbiting the earth sent a 1mgh sgnal say for 10 secs and an identicle transmiter some distance away transmited an identicle signal but 180 deg phase differance would they cancel each other at point of meeting, if ansewer is yes then is this true for all waves ?

Last edited by a moderator:
Related Other Physics Topics News on Phys.org

#### Labguy

Re: currious

Originally posted by wolram
im not sure how to pose this question, my ist try.
if a transmiter for arguments sake, fitted in a satalite orbiting the earth sent a 1mgh sgnal say for 10 secs and an identicle transmiter some distance away transmited an identicle signal but 180 deg phase differance would they cancel each other at point of meeting, if ansewer is yes then is this true for all waves ?
No, each would continue on, independent of the other.

#### russ_watters

Mentor
Re: Re: currious

Originally posted by Labguy
No, each would continue on, independent of the other.
Why not? Light waves are certainly capable of interfering with each other. I think they WOULD cancel each other out.

And yes, interference works for all waves. Light, sound waves, waves on the ocean, waves on a string, etc.

#### Labguy

Re: Re: Re: currious

Originally posted by russ_watters
Why not? Light waves are certainly capable of interfering with each other. I think they WOULD cancel each other out.

And yes, interference works for all waves. Light, sound waves, waves on the ocean, waves on a string, etc.
The odds of photons (or any EMR) transmitted miles away from each other actually interacting is "astronomically" small. Imagine how our radios would sound if they did. Also, the "folded" optical paths in some telescope designs would see what? Black?

Gold Member

#### Labguy

Originally posted by wolram
My "elaboration" is easy. There would be no "point of meeting", and the odds of photon interaction would be virtually nil.

#### wolram

Gold Member
now i see distance is major factor, in a confined space there could be interferance but how far away from each other would trasmiters have to be to guarantee no interferance? transmiters rated at 100W.

#### russ_watters

Mentor
Re: Re: Re: Re: currious

Originally posted by Labguy
The odds of photons (or any EMR) transmitted miles away from each other actually interacting is "astronomically" small. Imagine how our radios would sound if they did. Also, the "folded" optical paths in some telescope designs would see what? Black?
It was a hypothetical, Labguy. Of course the possibility is small. Maybe a better hypothetical would be using a laser. That way you have a LOT of photons traveling in the same direction. Shine another laser in the same direction out of phase and they WOULD cancel out.

now i see distance is major factor, in a confined space there could be interferance but how far away from each other would trasmiters have to be to guarantee no interferance? transmiters rated at 100W.
Distance isn't really much of an issue. The issue is can you make the beams cross out of phase? Its a piece of cake with sound, since the wavelengths are pretty long, but its tough for light since the wavelenghts are very small.

#### Integral

Staff Emeritus
Gold Member
Isn't this idea the basis for the GPS system? You recive signals from 3 seperate satilites, compare the phases and determine your postion.

Yes, the signals would interfer but, each signal would continue uneffected after the point or points of interference. At any point is space where the phase of the signals differ by 180deg there will be distructive interference, at points where the signals are in phase the signals will reinforce each other.

#### Tyger

There is something rather similar

I read in the Feynman lectures, but I couldn't find it in the index. I beleive it's called the Hadley_Twiss_Brown Effect (delightfully British name) that says that if two light sources are located a small angular distance apart at the receiver, even though they may be far apart in space, the example used a street lamp and a star, the combined light will tend to be in phase. It's strictly a quantum mechanical effect due to the fact that photons are bosons.

#### Phobos

Staff Emeritus
Gold Member
off to the Physics forum this goes!

#### wolram

Gold Member
so, if 2 laserbeams one 180 degrees out of phase with the other are focused at a point one wave lengh in front of a perfect reflector, would there be no reflected light?

#### russ_watters

Mentor
Originally posted by Integral
Isn't this idea the basis for the GPS system? You recive signals from 3 seperate satilites, compare the phases and determine your postion.

Yes, the signals would interfer but, each signal would continue uneffected after the point or points of interference. At any point is space where the phase of the signals differ by 180deg there will be distructive interference, at points where the signals are in phase the signals will reinforce each other.
I don't think GPS uses actual interference patterns, but instead compares coded signals. Similar, but not quite the same.

And your second part is correct. I didn't mean to apply that the waves would cancel and then cease to exist.

### Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving