Cam waves travelling at same direction interfere? Can two waves travelling in same direction interfere?
 A: Yes! It might be hard to have two separate wave sources that produce waves going in exactly the same direction (if you think about it geometrically, one source might block the other), so the two waves might only have to be going in the same direction approximately in most realistic experiments.
Here's a way to make two waves of light go in the exact same direction. Make a cavity, which is essentially a pair of mirrors that can support a standing wave of light in between them. One of the mirrors is made to be slightly transparent, so that we can input some light of a fixed wavelength into the cavity. Some of it passes through the first mirror, reflects off of the second mirror, then reflects off of the first mirror again, etc. Next, input some more light of the same wavelength into the cavity. If you time things right, the second beam of light can be going in the same direction as the first, so they can be made to interfere while going in the same direction (they will interfere regardless of the timing).
This whole experiment can probably be done with a string with partially fixed points. I'd imagine a guitar string fixed at two ends and almost fully fixed in the middle. Each side supports the same set of fixed wavelengths to make standing waves. If you lightly pluck one side, it will set a standing wave on that side with a small amplitude. Then, if you strongly pluck the other side, it will create a large-amplitude standing wave on the other side. If the middle point is slightly loose, some of the large-amplitude wave will leak onto the other side, so the two waves on that side will now interfere, even if they're traveling in the same direction.
