Waves and Dimensions

Shnoz (Sophie) · #1 30-01-2009, 04:11 PM

I'm just starting a year 11 physics class, and I think I've already come across a mistake in the textbook.

The textbook said that transverse and longitudinal waves can travel in one and three dimensions, but longitudinal waves could not travel in two dimensions, only transverse waves could.
Is this right?

If it is, what's the reasoning behind it. Why can longitudinal waves travel in one and three dimensions, but not two?

Neither my physics teacher nor any of my peers could work this one out any further than that if what the textbook says is true then due to sound waves being longitudinal, Flatlanders must be deaf.

renormalised (Carl) · #2 30-01-2009, 04:49 PM

Longitudinal waves can propagate through a medium as they move parallel to the direction of the wavefront...like sound, as they displace the medium they travel through in the direction of travel. Transverse waves move perpendicular to the direction of propagation, so cannot transmit through a medium like a gas or liquid (there's no way to generate motion in that direction) ...they can move across its surface, like a pond ripple. They can transmit through solids, though (think of earthquakes or vibrations in a pot when you bang it).

Here's a couple of site that'll help you out....

http://hyperphysics.phy-astr.gsu.edu...nd/tralon.html

http://www.matter.org.uk/Schools/Con...ransverse.html

http://www.kettering.edu/~drussell/Demos/waves/wavemotion.html

Looking at how waves propagate through a medium, I think I can see what the textbook is getting at. A longitudinal wave can pass through a 1 dimensional surface as it can readily propagate the wavefront by displacing the medium in the direction of travel. It can also do this through 3 dimensions. However, in 2 dimensions a wave can move up and down and/or side to side, with the motion of the wave being perpendicular to the direction of propagation. Hence no net displacement in the direction of propagation. So, flatlanders live in a very silent world

They could infer something was happening if they measured the displacement through a 1 dimensional surface, but all they'd be getting is a series of points moving past their measuring equipment and no information being carried. However, if one of us were to lift "Prof. Flatland Scientist" out of his/her space and into ours, he/she would hear him/herself scream for the first time!!

That could be disturbing as the flatlander would have no experience of hearing sounds and the mere thought of hearing a noise would probably be (initially, at least) rather frightening. To him/her, they'd appear to be coming from everywhere, including from inside of him/her!!!

Then again, being from a silent world, they may not have the ability to produce or hear sounds, so the experience would be a moot point.

Shnoz (Sophie) · #3 30-01-2009, 05:44 PM

Hey, that's cool. I think I get it now. Too bad about the Flatlanders, but my classmates will be happy.

renormalised (Carl) · #4 30-01-2009, 05:52 PM

Actually, much of the physics of a 2 dimensional world would be similar to ours, yet there would be important differences. Especially at the atomic and subatomic levels.

Archy (George) · #5 21-02-2009, 08:42 AM

.....They could infer something was happening if they measured the displacement through a 1 dimensional surface...[/quote]

Can you point to one real life example of a one dimensional surface?http://www.iceinspace.com.au/forum/..../helpsign3.gif

renormalised (Carl) · #6 21-02-2009, 10:01 AM

Quote:

Originally Posted by Archy

Can you point to one real life example of a one dimensional surface?http://www.iceinspace.com.au/forum/..../helpsign3.gif

It's not technically a one dimensional surface but any line is, by definition, one dimensional. What I should have said was a line, as a surface in nearly all cases has both length and breadth, which makes it two dimensional. That was a mistake of semantics on my part....one of those things that happens

It'd be strange watching something in one dimension propagating, especially in Flatland, because of their unique perspective on things.

theodog (Jeff) · #7 21-02-2009, 10:27 AM

Quote:

Originally Posted by renormalised

It'd be strange watching something in one dimension propagating, especially in Flatland, because of their unique perspective on things.

In my mind Flatland is 2D. I think a better term would be Lineland.

Waves propagating through Lineland would be compressional and as "space" would all move as the wave passed I doubt if the inhabitants would experience it. Unless they built something akin to a gravitational wave detector.

xelasnave · #8 21-02-2009, 11:33 AM

I understand currently physics says a wave needs no medium and I find that difficult to imagine. How can waves travel with no medium.
alex

renormalised (Carl) · #9 21-02-2009, 11:50 AM

A wave needs no external medium to propagate because the wave itself acts as the medium.

wasyoungonce (Brendan) · #10 21-02-2009, 12:14 PM

Quote:

Originally Posted by renormalised

A wave needs no external medium to propagate because the wave itself acts as the medium.

If no medium is required is thus the wave a particle instead?

renormalised (Carl) · #11 21-02-2009, 01:09 PM

Both....simultaneously. Don't you know your quantum physics

xelasnave · #12 21-02-2009, 04:25 PM

Thanks for that.
alex

Shnoz (Sophie) · #13 22-02-2009, 07:03 PM

Ah, but it depends whether we're talking about mechanical or electromagnetic waves. (Which it sounds like we're talking about electromagnetic waves).

Mechanical waves do need a medium, all they do is transfer energy(very basically put)

.

Electromagnetic waves are different. They don't need a medium because they transfer energy in the form of electrical and magnetic waves that propagate perpendicular to each other.
It's pretty cool what happens. Say a chemcial reaction makes an electron fall closer to its nucleus in an atom. Because the electron moves, it generates a magnetic field. The magnetic field generates an electrical field, which in turn generates another magnetic field, and it just keeps going. And that's what light is made out of.

I'm pretty sure that's what happens. I kind of inferred most of this from stuff that I've learnt, so if someone could tell me if I'm right or wrong that would be cool.

bojan · #14 23-02-2009, 09:38 AM

You are basically right.
What is important to note here is because moving electron carries electrical charge, when it moves this produces the change of electric field, which then generates changing magnetic field, which generates changing electric field.. etc.
Static electric or magnetic field will not generate the other one, so there will be no EM wave to propagate.

EDIT:
About the media.. since EM field is the property of space, we can safely say that the media for EM waves is space itself.

Shnoz (Sophie) · #15 23-02-2009, 10:21 AM

Cool, I thought it made sense.

I've also heard that the wave and photon theories of light are completely separate. Is there still no way that photons can possibly fit into the wave theory?

It seems somewhat confusing to have one thing showing the properties of two entirely different objects. I think light's having identity problems.

bojan · #16 23-02-2009, 10:54 AM

Light is no different from other particles/waves in a sense that it shows duality (wave - particle).

The Maxwell's equations (they describe the behaviour of the elecromagnetic field) are only the part of the whole story about photon.
The whole (? well.. the most complete) picture of the photon is as the quantum mechanics and the Standard Model teaches us..... and the waves do not exclude particles (and vice versa)

Quote:

Originally Posted by Shnoz

Cool, I thought it made sense.

I've also heard that the wave and photon theories of light are completely separate. Is there still no way that photons can possibly fit into the wave theory?

It seems somewhat confusing to have one thing showing the properties of two entirely different objects. I think light's having identity problems.

xelasnave · #17 23-02-2009, 10:21 PM

So how many particles make up the section of one wave lenght of EME... or if we take say light as a particle what particles and how many does it take...

alex

renormalised (Carl) · #18 23-02-2009, 11:06 PM

Quote:

Originally Posted by xelasnave

So how many particles make up the section of one wave lenght of EME... or if we take say light as a particle what particles and how many does it take...

alex

One.....

xelasnave · #19 24-02-2009, 10:29 AM

One wave length may represent a section of the visual spectrum ( say we are looking a visual spectrum) which may contain many colours for example... so I ask ...will the colours in the spectrum be represented by one particle each...
I find the complexity that must be associated with light to be contained in one particle hard to imagine and understand ....anyways even if each wave length of all the EME spectrums in the Universe there will be certainly a lot of particles set aside for the job of moving EME around the Universe.

I expect if worked with as a particle it is seen that each particle has no mass... I am sure that is the way it is...or is it???

AND if we treat light as a particle what do we call the particle? A photon I expect but is there a term to in effect say "at this point we deal with a particle (which has the quality of duality) so we call it...???? "

Sorry if my questions are all over the place but light is not easy for me to understand.

alex

alex

bojan · #20 24-02-2009, 10:52 AM

Quote:

Originally Posted by renormalised

One.....

Exactly! :-)

The problem is with our brains (as hardware) and how it is programmed (software)..

The brains as an organ evolved over billions of years as a response to immediate environment, and it has couple of basic functions, two of them being the most important: To eat, and to avoid to be eaten...
Those tasks (and most of other ones, that are vital to a survival of particular species) are hard-wired, like ROM, they are not learned but inherited.
Human brain is a huge overkill from the point of view of those basic tasks: while still retaining the hard-wired parts, by a series of harmless accidental mutations, some parts of our brains became capable of learning and, what is more important, of abstract thinking (RAM).
However, this capability, to be fully utilized, needs the training in early stages of people's life, otherwise the hard-wired part (ROM) may take over and disable the RAM part.
And even then, it is VERY hard to most individuals to jump out of pre-programmed
routines and to start to process information in ways that are not taking our perception of our environment into account. This lack of ability to think by no means make those individuals incapable of life.. because to survive in human environment, it is not important to understand quantum mechanics.

The difficulties we are facing in our attempts to understand the quantum mechanics are the direct consequence of this pre-programming…
So we have to stop trying to visualize the micro-cosmos, because this will get us nowhere. Much better way is to become familiar with the proper tools for that task: and this is math.
So, again: Photon manifests itself sometimes as a wave, sometimes as a particle…..
And final question: what is a photon, really?
Answer: photon is… a photon.

EDIT: Alex, welcome back

I was wandering what happened to you.. we missed you mate :-)