Waves that go through each other

[Dalo]

I find the idea that waves transport energy and not matter very mysterious. There is of course the case of electricity running through wires, but I find somehow this phenomenon easier to understand.

The perfect example of the principle "energy and not matter" is given by the example of two transversal waves running along a string. They seem to pass through each other without disturbing each other, reaching the end of the string and come back running again.

 

I never understood why such waves were not simply considered as two standing waves, the place where they meet being the place of their collision and their recoil. In other words, why assume that they go through each other while it would be much easier to assume that each wave simply traces its path back after being stopped by the other?

Are there deeper motivations and reasons behind the view that they do go though each other?

[swansont]

That the string doesn't go anywhere shows you that matter is not being transported.

Waves obey the rule of superposition. Amplitudes add.

1 hour ago, Dalo said:

 I never understood why such waves were not simply considered as two standing waves, the place where they meet being the place of their collision and their recoil. In other words, why assume that they go through each other while it would be much easier to assume that each wave simply traces its path back after being stopped by the other?

We don't consider/assume that because we see that it's wrong. Traveling waves exist, and if waves of different amplitudes move toward each other, we see that they do not reflect.

[Dalo]

31 minutes ago, swansont said:

We don't consider/assume that because we see that it's wrong. Traveling waves exist, and if waves of different amplitudes move toward each other, we see that they do not reflect.

What do you mean by "not reflect"?

I suppose you mean that if wave A is stronger than wave B, when they meet, we see that wave A and wave B both continue on their path, instead of being thrown back.

I would like to point out that it still does not necessarily mean that they go through each other. Maybe both inherit the strength of the wave they collide with.

Edited November 24, 2017 by Dalo

[swansont]

3 hours ago, Dalo said:

What do you mean by "not reflect"?

You described reflection. It doesn't do that.

3 hours ago, Dalo said:

I suppose you mean that if wave A is stronger than wave B, when they meet, we see that wave A and wave B both continue on their path, instead of being thrown back.

I would like to point out that it still does not necessarily mean that they go through each other. Maybe both inherit the strength of the wave they collide with.

If they have a different wavelength, it would have to somehow inheret that, too. So it's not reflection.

[Dalo]

1 minute ago, swansont said:

You described reflection. It doesn't do that.

If they have a different wavelength, it would have to somehow inheret that, too. So it's not reflection.

I must say that I was thinking not about light waves, but of waves on a string. Like when two people move a rope each from one end, and create two opposite waves?

[studiot]

15 hours ago, Dalo said:

I find the idea that waves transport energy and not matter very mysterious.

Before I waste any time on this do you seriously want to find out or do you just want to expound your theories on a subject you admittedly know little about?

There are many types of waves.

All waves are disturbances of the Queen's peace in some way.

Some waves transport energy.

Some waves transport matter.

Some waves follow linear mathematical laws.

Some waves follow non linear mathematical laws.

Waves can have one two or three dimensional structures (ignoring fancy theories like string theory)
Within those structures their may have different geometries so the first thing to do when asking what happens if two (or more) wave interact is to examine their respective structures eg what happens when a plane wave meets a spherical wave?

An interesting experiment is the study of Lecher Lines.

https://www.google.co.uk/search?source=hp&ei=AksZWojYPMvZgAaj-4nwBA&q=lecher+lines&oq=lecher+lines&gs_l=psy-ab.3..0.1160.3289.0.3488.12.12.0.0.0.0.192.1335.1j10.11.0....0...1c.1.64.psy-ab..1.11.1326...46j0i131k1j0i131i46k1j46i131k1j0i46k1j0i10k1j0i22i30k1j0i22i10i30k1.0.bGoieGes9IM

 

[swansont]

10 hours ago, Dalo said:

I must say that I was thinking not about light waves, but of waves on a string. Like when two people move a rope each from one end, and create two opposite waves?

The concept of reflection can apply to other types of waves. If you affix a rope to a fixed object, or a rope of different density, and you send a traveling wave down it, it will reflect.

 

Here's one with a slinky and what looks to be a hose.

Notice how the pulse inverts at the boundary. That's indicative of reflection. The transmitted pulse does not invert. Another reason to reject the notion that opposite-direction waves bounce off of each other and reflect, but instead pass through one another.

[Dalo]

3 hours ago, studiot said:

Before I waste any time on this do you seriously want to find out or do you just want to expound your theories on a subject you admittedly know little about?

I saw the example of two waves starting at opposing ends of a string in a video course by Richard Wolfson, a course which was based on his textbook on Physics.

In the video, which is copyright protected, you can clearly see both waves moving towards each other, passing though each other without any visible effect, and continuing on their path. His explanation was that those waves were transporting energy, not matter. And that is why, according to him, they could pass through each other. That is what I find mysterious.

Swansont's video is I am afraid not really representative of this example.

Edited November 25, 2017 by Dalo

[Strange]

Can you provide a link to this video?

[Dalo]

4 hours ago, Strange said:

Can you provide a link to this video?

If you mean Wolfson's, then no, I cannot. You have to buy the course. The title is

Physiscs and our Universe: How it All Works. By Richard Wolfson.

I think he uses the same example in his textbook: Essential University Physics

 

Here is a screen shot of a part of 14.5 Interference

 

Edited November 25, 2017 by Dalo

[swansont]

10 hours ago, Dalo said:

I saw the example of two waves starting at opposing ends of a string in a video course by Richard Wolfson, a course which was based on his textbook on Physics.

In the video, which is copyright protected, you can clearly see both waves moving towards each other, passing though each other without any visible effect, and continuing on their path. His explanation was that those waves were transporting energy, not matter. And that is why, according to him, they could pass through each other. That is what I find mysterious.

Swansont's video is I am afraid not really representative of this example.

It's meant as a rebuttal, to show what reflection looks like. Waves passing through each other does not look like reflection.  As I explained before.