Energy-Interfered waves

1. Jul 26, 2011

thebiggerbang

If I make two mechanical transverse or EM wave interfere destructively at each and every point, Will it be stored as potential energy forever? I am right to assume that this energy will never show up as I am making the waves interfere destructively forever. Also, energy can't be destroyed.

what will happen to the energy of the system?

Pardon me for my unscientific language! :)

Last edited: Jul 26, 2011
2. Jul 26, 2011

nasu

How do you propose to do this?

3. Jul 26, 2011

xts

Consider Young's double-slit experiment. In some places waves interferes destructively, and there the energy density is null. But at other places waves interfere constructively, there energy density is four times higher than if only one slit is open. Total energy measured at the screen is the same as a sum of energies when just one slit is open.

4. Jul 26, 2011

Studiot

Last edited by a moderator: Apr 26, 2017
5. Jul 26, 2011

y33t

Energy is not destroyed in this case, it is dissipated in the medium.

6. Jul 26, 2011

nasu

Sorry, I did not look at the poster's background. I thought that he imagined some way of satisfying the condition he describes.
A good start would be to understand that you cannot have the waves interfering destructively in each and every point unless you have no waves at all.
I think that the link is a great start.

7. Jul 26, 2011

chrisbaird

Mathematically, you can have two waves interfering destructively at every point in a given region. So where does the energy go? The answer is that there must be more to the story. The energy goes to a different region. This is the principle behind http://en.wikipedia.org/wiki/Thin-film_interference" [Broken]. If conditions are right, the reflected wave off the front and the reflected wave off the back surfaces of a soap bubble can interfere destructively at every point. Therefore no energy can be reflected, and all the energy must be transmitted. This effect is very useful in making frequency-selective mirrors by adding coatings. But do the two reflected waves actually physically travel back a ways in space in the reflected direction and then destroy each other? Not really. Rather, they destroy each other before they even have a chance to get going, so they never have a chance to carry any energy away.

Last edited by a moderator: May 5, 2017
8. Jul 26, 2011

nasu

"In a given region" is not quite the same as "in each and every point".
Other than this, I think the problem was already discussed on the forum, including the case of a thin-film.

9. Jul 26, 2011

Studiot

Is a good answer for waves that have a material medium such as the noise cancelling enclosures that work by generating opposite phase sounds to those emitted by say a generator that it is required to quieten.
The generator produces real sound energy input as does the noise cancelling loudspeaker. This energy is dissipated as heat in the air, which ends up slightly warmer than it would otherwise be.

So no, biggerbang, the energy does not increase the potential energy of the system (medium) it goes the way of all energy in the end it degrades to heat.

However the OP also asked about non material media so I will hand over to you phD boffins to answer.

10. Jul 26, 2011

y33t

Assume there is wave1 (W1) and wave2 (W2) seperated by a finite distance. These waves are exactly opposite to each other so that when they interact vector components will cancel each other out and there is nothing to observe. But when at time t (interaction moment) waves collide into each other, total energy should be dissipated in the medium they are in. There was some stuff to observe before the interaction moment (which are individual waves traveling to each other) but not after the interaction. There is no such thing as negative energy in this case thus energy originating from interference of these two waves should be dissipated in the medium.

Which makes me ask that dissipation will occur as heat (since all components cancel out) but heat is also a electromagnetic radiation.. It will definitely differ if it's a mechanical wave or an em wave.

Last edited by a moderator: May 5, 2017
11. Jul 27, 2011

thebiggerbang

Wow! Never expected so many people to reply! I'm proposing this hypothetical experiment, maybe with a load of ghastly assumptions, but still.

I take a huge circular string in vacuum (dissipation of energy, you get ruled out!) and then if possible, say I create two disturbances with the same frequency, exactly out of phase and the circumference is a integral multiple of the wavelength of the wave, where will the energy that I put in go? I agree there will be a few visible troughs and crests due to the delay in the creation of the second wave. Hope I made some sense!

Also, thanks a lot for liking that amazing article, studiot!

12. Jul 27, 2011

thebiggerbang

After reading the article again, I think I have got my answer pertaining to mechanical waves!! At the point of creation of the second disturbance, two waves will be created and there will be both destructive and constructive interference, not leading to any loss of energy!

Thanks :)

13. Jul 27, 2011

chrisbaird

Heat is a macroscopic statistical description of the kinetic energy of random atomic motion. Strictly speaking, infrared radiation is not heat, but is a standard form of electromagnetic radiation. Infrared has frequencies that are particularly adept at being emitted by atoms because of the thermal motion. When a wave is absorbed by a medium, the speed of the atoms in their random thermal motion increases and we say that the energy of wave has been converted to heat. Infrared radiation is emitted by the atoms in their thermal motion, but it always is, so that is nothing unique to this problem.

14. Jul 27, 2011

y33t

Infrared radiation is not heat but when it interacts with matter there will occur 3 phenomena;

Absorbsion
Reflection
Scattering

Absorbed part of incident wave will penetrate into material and increase kinetic energy of molecules/atoms inside. How much energy will depend on the intensity, polarization, frequency of the wave and also dispersive characteristics of the medium as well as taking into account isotropy/anisotropy.

Radiation is radiation doesn't matter which part of the spectrum it's originating from. Of course neglecting some special metamaterials, energy transferred generally increases with frequency.

I believe constructing two systems with the mentioned specifications (completely destructive interference) can be achieved. It all yields to control of E and H fields, specifically vector signs which bases on nothing but current directions.

At moment t (interaction moment) summation of energy of these 2 waves should be dissipated in the medium, do you agree this ?

15. Jul 27, 2011

thebiggerbang

It has to go somewhere or the other! Well if the system is in vacuum, won't radiation be the only way of dissipation of energy? Hmmm...gotta think over this!

Another question : If two EM waves interfere with each other destructively does that permanently destroy the evidence for future observations that they ever existed? Or can we by some method /optical phenomena recover the two waves?

16. Jul 27, 2011

y33t

Theoretically you can but in practice considering today's technology it might not be possible. It will depend on the time elapsed from interaction moment to observation moment. Interference will leave a radiation behind and unless this is absorbed by a material, it is possible to tell they existed. %100 recovery might not be possible depending on the conditions but it is quite possible to recover some information about two waves.

17. Jul 27, 2011

thebiggerbang

If so, can we sense some application of this phenomena in encryption or such stuff?

18. Jul 28, 2011

y33t

What kind of encryption scheme are you talking about ? You have waves and an unknown radiation left from interaction. Which part of the phenomena are you planning on using as a component of an encryption scheme ?