h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

A particle seems easier to understand.

Is a wave the pattern that some particles/physical things make sometimes when they move? eg a wave in the ocean, or the way land moves in an earthquake.

(and if so, what makes something a wave as opposed to merely being random movement?)

But are waves also patterns of the movement of non-physical things? eg sound (or is sound moving a physical thing called air (oxygen, nitrogen etc molecules)?)

Is the ohysical/non-physical aspect of this post an unnecessary distraction?

I'll just add something quickly to this question regarding physical / non-physical and allow others to answer further.

Waves are associated with a type of "oscillatory movement" of the constituent parts of matter, within matter. Sound is "oscillatory movement" of air within air. Water waves are "oscillatory movements" of water within water and so forth. Associated with "movement" is energy (energy of motion) and associated with "oscillation" is the "springiness" of matter (which itself is associated with "potential" energy). So waves in this sense are associated with the coordinated "movement" of energy through matter, hence we can say it has a physical sense and a physical meaning (linked with matter).

However, "scientists" also refer to "light waves", and in this sense I would like to know what others say regarding whether this is a "physical" or a "non-physical" wave. [Light is said to be an oscillation of electric and magnetic "field" which is not associated with "physical matter"].

Actually .... although light is not associated with matter, maybe it is in an Einstein relativistic sense (and hence it is a "physical wave"). It is said that light has zero "rest mass", but it does have "relativistic mass" - associated with the energy of movement (speed of light). That is to say the "oscillating electric and magnetic" field of light, is itself associated with mass and hence "matter". So in this sense it would be a "physical wave". Anyway I'll allow others to add their views / expertise.

E=mc2 is the relativistic mass of light.

Where to start with this one?

Firstly, the physical/non-physical distinction is very important. Waves can broadly be classified into mechanical waves, in which a medium is moved (including water waves and sound waves) and electromagnetic waves, which are treated very differently and have a different set of laws and equations governing them. From a scientific viewpoint, they are effectively two very different things with the same name.

Generally speaking, scientists aren't terribly interested in mechanical waves, because there's not a lot to be found out about them apart from a number of prosaic mathematical formulae governing how they move.

Electromagnetic waves, on the other hand, were of great interest in the 19th and 20th Centuries (and, to a lesser extent, still today) because they transfer energy without using particles, and are consequently the entire foundation for our entire electrical and communication infrastructure. The discovery that particles and waves weren't necessarily two distinct things was the foundation for quantum physics, which underpins every silicon chip on the face of the Earth today.

B

>>From a scientific viewpoint, they are effectively two very different things with the same name.



>>because they transfer energy without using particles,

How? Or why? I guess my question now is 'what is an electromagnetic wave?'

Another big question.

It is a packet of energy that sometimes travels in a wave shape but sometimes also behaves like a particle - a photon - depending on the circumstances. Also known as electromagnetic radiation. They are produced by electrical fields and chemical reactions.

Much of an electromagnetic wave's properties are tied in with its frequency. We distinguish different frequencies by calling them (from low to high frequency radio waves, microwaves, infra-red, visible light, ultra-violet, X-Rays and Gamma Rays, but this disguises the fact that they are all essentially the same thing. They all travel at the 'speed of light', for example. This means that the wavelength changes inversely with the frequency - radio waves have the lowest frequency but longest wavelength.

In passing, the amplitude ('height', if you must) of the wave is related to how much energy it is carrying.

Generally speaking, the higher-frequency waves are more penetrating and more damaging to living tissue, which is why sunbeds produce low-frequency UV light, but the X-ray attendant needs to stand behind a lead shield.

B

OK, so I've come in late - I hope not too late

Let's try and forget wave-particle duality and quantum physics to begin with shall we.

What is a wave? Well Stone Art was spot on in post 2 to begin.

A wave is described as an oscillatory motion within a medium that transfers energy in one direction. The actual particles (or whatever the wave consists of) do not themselves move in that direction. This latter point is key.

In a compression wave such as a sound wave they move backwards and forwards in the same and opposite direction to the transmission direction of the energy.

In a transverse wave they move perpendicular (at right angles) to the direction of energy progression.
A Mexican Wave is a very good example of this. People stand up and down, but the wave goes across this direction of oscillation.
Waves in water also do this.

To understand waves water waves this is all you really need.

Simple waves are described by the mathematical formulae

x=R x.Rsin(y.z)
or x = R.cos(y.z)

(sin (sine) and cos (cosine) are the same thing as in trigonometry http://en.wikipedia.org/wiki/Trigonometry)
x = coordinate along x-axis on a graph. y = wavelength constant, z = angle
R = amplitude
(wavelength equals the distance between two maxima or minima of the oscillation)

I wouldn't have bothered with the maths but since you've been told that a different set of equations governs (say) water waves and light waves then I thought I would. That's not true that I am aware of - these equations describe any simple transverse wave perfectly.


So what other properties to waves have other than transmitting energy along an axis whilst displaying motion of individual particles in a different direction?

Waves display these properties also.

1. Refraction.

Change the medium in which it is travelling and the wave deflects in a different direction. This can be seen by shining light from air through (say) water or a prism.
It was also _originally_ observed with water waves. This can be seen in natural surf when the water depth changes suddenly or you can set it up in a tank with two different depths and just look at the change in direction.
http://www.smkbud4.edu.my/Data/sites/vschool/phy/wave/refraction.htm

2. Diffraction.

Send waves through a barrier that has a slit in it of similar width to the wavelength and what you see is the same wave the other side but now on a circular front. Sort of like the spherical wave progression you get when you chuck a stone in a still pond.

You can also diffract around an object such as a post in the water if again it has similar width to the wavelength.


3. Interference.

Constructive interference.
Waves coming from different angles that are _in phase_ (ie. the maxima match up with the maxima and minima match up with the minima) will accentuate each other to form a wave that has the sum of the two amplitudes at the maxima and at the minima. I.e. they add together.

Desctructive interferences.
Waves coming from different angles that are 90 degrees out of phase with one another (i.e. maxima and minima simultaneously arrive at one point) will cancel one another out and no oscillatory motion will occur.


So from the latter two you can set up the classic interference pattern so beloved of physics. Set up a water wave in a tank and put a barrier with _two_ appropriately sized slits and you will get two diffracted waves that both constructively and destructivel interfere and so give the interference patterns you can see in this link.

http://www.smkbud4.edu.my/Data/sites/vschool/phy/wave/diffraction.htm

Right so that's about it for waves.

That's what they do.





So the physicists (or more correctly scientists - I'm not sure the boundaries between physics, chemistry etc. had been fixed yet) had worked out hundreds of years ago.

At some point people like Newton started noticing that light from the Sun did the same things as water waves.
It refracted through tanks of water, just as water waves refracted when the sea bed got shallow all of a sudden.
Newton (I think originally?) noticed that it also refracted through prisms and for the first time outside of a natural rainbow it was discovered artificially that white light is made of light of different wavelengths since depending on the wavelength the waves refracted at different angles - giving the characteristic visible spectrum as we know it (and presumably demystifying the rainbow for everyone).

So light was displaying wavelike properties.

In the 17th century it was discovered that light also diffracted through very small slits. More evidence that light is a wave. Diffraction patterns could start to be made by carefully constructed double slit experiments using the pinhole effect to make sunlight a coherent and monochromatic as possible.

So there you have it. Light diffracts, refracts and interferes. It's a wave.

Particles behave differently. They have mass, mometum, exist in one place at one time etc.


But the in the 19th century someone had the temerity to show that the newly discovered elementary particles electrons _also_ could be made to behave as waves if you did the right experiment.

Then someone found that light could be made to behave like particles.

Then it gets complicated and is I think beyond what the original question asked.

So what does a wave consist of?

Well water waves - easy. Water molecules oscillating up and down as energy is transmitted at 90 degrees to the angle of oscillation.

Light?
Well - this is still not really known. It is true that you have an oscillating electric field at right angles to the direction of travel of the wave. It is also true that you have an oscillating magnetic field at right angles to both the direction of travel of the wave _and_ at right angles to the electric field.
But what does the wave consist of? What does it travel through?
Noone knows for sure - but there is strong evidence for zero-mass boson particle exchange generating photon waves (light waves) - look at Richard Feynman's quantum electrodynamics if you like but I know I don't follow it particularly.