Waves in Space!!!

[misskitty]

My physics class is studying waves at the moment. I was reading something in my book that I thought was rather interesting. :rolleyes:

Mechanical waves don't travel very well through space because space is nearly a vaccum. However, electromagnetic waves travel well through space. There are some kinds of elecrtomagnetic waves that can not escape the gravity of black holes, such as light waves. :bugeye:

I was wondering why. Why can electromagnetic waves travle through space with relative ease, yet mechanical waves cannot? I know mechanical waves need an elastic medium to travel through, but isn't space a medium too? If it isn't why not? :uhh:

Just some food for thought.
:wink:

[Crosson]

Why can electromagnetic waves travle through space with relative ease, yet mechanical waves cannot?

Mechanical waves propagate through matter. Light propagates through the electromagnetic field (which extends through all of space).

[misskitty]

Okay, could I have a bit of expansion on that idea? Not that it wasn't a good explanation, because it was! I just need a little more information.

Isn't space matter? kinda confused...

[Lord Flasheart]

Space is space. Mechanical/acoustic waves cannot travel through a lack of material medium.

I'll avoid an obvious cliche.

[dextercioby]

Light waves propagate through matter as well.Let's not induce confusion.

Let's keep it very simple and say that,classically,waves are divided into 2 big categories:
*mechanical &
*electromagnetic waves.

Mechanical waves cannot propagate in (classical) vacuum,because they are determined by the vibrations of the constituents of the medium in which they propagate,whether the vibration of air molecules,plasma,atoms in a crystal lattice,water layers,and so on & so forth.If it's (classical) vacuum,then there are no particles to vibrate,ergo no waves to propagate.

The electromagnetic waves are simply c/n travelling perturbations of a classical electromagnetic field.For vacuum,"n=1" and the waves propagate at "c".


I know that this was something rather simplistic,but i tried to keep away mathematical details.

Daniel.

[misskitty]

Hi Daniel. Good to have you here. :smile:

Let me see if I have this right; because space is so close to being a vacuum, mechanical waves don't have anything substanctial enough to act as a medium for them travel through?

I don't want to sound stupid or ignorant, but we haven't quite reached this yet. So I'm going to ask: could someone explain what an electromagnetic field and wave is? I could possibly deduce an electromagnetic field is a field that has some kind of electrical force that is also attracted by a magnectic force to create a field....but I think I'm waaayyy of base here.

[dextercioby]

Hi Daniel. Good to have you here. :smile:

Good to have you here. :smile:

Let me see if I have this right; because space is so close to being a vacuum, mechanical waves don't have anything substanctial enough to act as a medium for them travel through?

If you're talking about interstelar/intergalactic space,then u hit right on the head.

I don't want to sound stupid or ignorant, but we haven't quite reached this yet. So I'm going to ask: could someone explain what an electromagnetic field and wave is?

It's not that simple to define unambiguously the concept of EM field.U'd have to start with the definitions of electric & magnetic field respectively,then formulate the laws of classical electromagnetism (a.k.a. Maxwell's equations) and just then to interpret the latter as to come up with a definition for the EM field.Once u've done all that,u can define an electromagnetic wave as being propagating electromagnetic field at large distance from the sources ...

I could possibly deduce an electromagnetic field is a field that has some kind of electrical force that is also attracted by a magnectic force to create a field....but I think I'm waaayyy of base here.

The force with which a classical em field acts on a charged particle is called the Lorentz force...

Daniel.

[misskitty]

Ok that makes sense. Why is it called a Lorentz force? What does it do?

[dextercioby]

Because Hendrik Antoon Lorentz discovered it...:bugeye: It determines the movement of an electric charge in EM field...


Daniel.

[misskitty]

Ah, name it after the person who found it...we like to do that :wink:.

So what kinds of things do waves in an electromagnetic field do? Do the same properties apply to EM waves that apply to mechanical waves? :rollseyes:

[dextercioby]

Well,there are a bunch of properties that all waves share:diffraction,interference,polarization and other.The distinction is that EM waves are transversal,while most of the matter waves are longitudinal.To be sincere,it's really difficult to try & find common aspects to such different phenomena.It would be better for me to concentrate only one type of wave at a time.

Anyway,things are really interesting,however,qualitative descriptions are not satisfactory (to me at least).

Daniel.

[misskitty]

We can cover one thing at a time. :wink:

So let's start with EM waves. They share a bunch of properties; refraction, diffraction, interferrance etc. What happens when EM waves interact with one another in space? Do their amplitudes combine to make a resulant wave equal to their sum? Do they continue in their original direction movement? What do you mean by EM waves are transversal and not longitudinal? Their particle patterns of movement are perpendicular to their diection of motion?

Great...now I'm addicted to learning about how physics effects the universe! Lol. :biggrin:

[dextercioby]

We can cover one thing at a time. :wink:

So let's start with EM waves. They share a bunch of properties; refraction, diffraction, interferrance etc. What happens when EM waves interact with one another in space?

That "interaction",at classical level,is called "interference".Simply add the vectors electric & magnetic.

Do their amplitudes combine to make a resulant wave equal to their sum? Do they continue in their original direction movement?

I'm sure u'll learn in school that the wave interference is either distructive,or constructive.

What do you mean by EM waves are transversal and not longitudinal?

The plane in which the electric & magnetic vectors oscillate is perpendicular to the direction of propagation.


Their particle patterns of movement are perpendicular to their diection of motion?

No,there's no particle at classical level when describing a wave.

Daniel.

[misskitty]

So EM waves also have constructive and destructive interferrance. Ok. To get their resultant wave you have to add their electric componants together and their magnetic componats, right?

The microwaves discovered by Weinberg and Salaam ( I think it was them, :redface:) were electromagnetic waves right?

[dextercioby]

Weinberg,Salam (sic!) and Glashow had nothing to do with microwave radiation.I think u're referring to Penzias & Wilson who discovered the backgroud microwave radiation in 1964 (i'll have to check,though,it's been a while since reading Weinberg's (the same as above :tongue2:) book:"The first three minutes").

Daniel.

[dextercioby]

****,i'm getting old!!:yuck: 1965

Penzias,Arno A.,Wilson,Robert W.,Astrophys.J.,142,419 (1965)

Shared the Nobel in 1978 with Piotr Kapitza.

Daniel.

[misskitty]

You are NOT Getting OLD!!!! :tongue2: :biggrin:

That's who I was referring to. I couldn't remember who it was. I figured if I got it wrong then someone would correct me. Thanks. :smile:

[dextercioby]

I know i looked young in that picture,but i don't look like that anymore.:tongue2:

Daniel.

P.S.Don't mention it,it's not a great feeling to correct someone.:rolleyes:

[misskitty]

I know the feeling,:redface:.

Rturnign to our original topic of discussion: so what did the discovery of the background radiation waves mean for science?

[dextercioby]

It meant that,once,there was a big agglomeration of radiation & matter and that our universe was really hot (literally :wink:) a long time ago.It supported the idea of a Big Bang.

Daniel.

[misskitty]

And as time progressed the matter in the universe cooled down to what it is now. So, if this is true, could we also speculate that the universe could cool to absolute zero? If that is true, where would all the energy go? Because when something cools, its entropy decreases and the entropy of the surroundings increases. So if the universe was to continue cooling where would the energy go?

[selfAdjoint]

And as time progressed the matter in the universe cooled down to what it is now. So, if this is true, could we also speculate that the universe could cool to absolute zero? If that is true, where would all the energy go? Because when something cools, its entropy decreases and the entropy of the surroundings increases. So if the universe was to continue cooling where would the energy go?

Precisely; so it can't cool to absolute zero everywhere, but the zones where it's above that will get smaller and farther apart. Then when everything has evolved into widely separated black holes, which carry entropy proportional to their mass, the black holes will evaporate and leave the universe filled with Hawking radiation in (eventual) thermal equilibrium. This is the "heat death" predicted by nineteenth century thermodynamicists from general principles, not absolute zero but no temperature differences at all so no free energy, so no work can be done.

[misskitty]

This may be the wrong question to ask because it contradicts my other thread discussing heat death, but how could the universe reach thermal equilibrium if there are millions, perhaps even billions of natural processes, that continually increase and decrease the energy of the universe?

Thermal equilibrium would mean the entropy of the universe is stable, but how could that be?

[dextercioby]

The universe is a closed system in which energy is conserved.That "cooling" that u speak about is nothing else but a drop in the temperature of the equilibrium radiation that fills it.As the universe extends,the energy being constant,the matter+radiation will tend to an equilibrium (so called "thermal death of the universe"),but the density of particles will be very,very small.

Daniel.

P.S.Read Weinberg's book.

[misskitty]

Alright, makes sense I think.

Getting back to waves, how much energy can an EM wave carry?

P.S. I don't have the book. It doesn't help me any if I don't know what the title is. :smile: Where can I find it?

[dextercioby]

This may be the wrong question to ask because it contradicts my other thread discussing heat death, but how could the universe reach thermal equilibrium if there are millions, perhaps even billions of natural processes, that continually increase and decrease the energy of the universe?

Thermal equilibrium would mean the entropy of the universe is stable, but how could that be?

H (capital eta) theorem due to Ludwig Boltzmann is the greatest achivement in classical statistics.It basically asserts that all nonequilibrium processes evolve towards equilibrium.And in this "evolvement",the entropy will increase.

Think the universe as a whole.

Daniel.

[misskitty]

Alright, I think that sort of makes sense.

[dextercioby]

Everything will make sense,as soon as u'll know more from each branch of physics.:wink:


Daniel.

[misskitty]

This like you need to know the little pieces before you can know the big pieces but you've got to learn about the big pieces to understand the little pieces. Isn't it? :bugeye:

[dextercioby]

I wouldn't go for this circular logic,sorry.Those "little pieces" need to be thoroughly understood before jumping to more complicated matters.Else,u'd be trying to build a castle on sand.

Daniel.

[misskitty]

Thought so. Ok. Try to stick with the little stuff. Can do.

[RoboSapien]

http://www.fulvics.com/lighttheory/experiments.htm

[Reshma]

Mechanical waves don't travel very well through space because space is nearly a vaccum. However, electromagnetic waves travel well through space. There are some kinds of elecrtomagnetic waves that can not escape the gravity of black holes, such as light waves. :bugeye:

I was wondering why. Why can electromagnetic waves travle through space with relative ease, yet mechanical waves cannot? I know mechanical waves need an elastic medium to travel through, but isn't space a medium too? If it isn't why not? :uhh:



Although your question has been covered quite well in thread, I thought I might just give you a little more details.

Let us start with the basics here:

a. A wave is an oscillation or disturbance in space which transfers energy from one point to another without the transport of matter.

b. Depending on the method of propagation through space, waves can be either mechanical waves or electromagnetic waves (EM waves).
(i) Mechanical waves – a medium is required for propagation. Some examples are ocean waves and sound.
(ii) Electromagnetic waves – these waves are made up of electric and magnetic fields whose strengths oscillate at the same frequency and phase. The fields are perpendicular to each other as well as the direction of propagation of the wave and no medium is necessary for propagation. Light is an EM wave.

c. If the oscillation or disturbance is in the direction of wave propagation, then the wave is longitudinal. On the other hand, transverse waves oscillate perpendicular to the direction of wave travel. EM waves are
transverse waves while sound is a longitudinal wave.

Points of maximum disturbance are known as crests for transverse waves and compressions for longitudinal waves. Similarly, troughs and rarefactions are points of minimum disturbance for transverse waves and longitudinal waves respectively.

Some other wave properties:
Supersition:
The total displacement or disturbance at a point through which multiple waves cross is the vector sum of the individual displacements due to each wave at that point.

Empirically, it is found that waves in the same physical location do not affect one another and simply pass through each other unchanged. Therefore,total disturbance at that point is just the sum of the disturbances of the individual waves.

Both interference and diffraction are phenomenon in which superposition plays a big role.

a. Interference – When waves from coherent sources cross in a particular region, superposition occurs which reinforces waves at some points and diminishes them at others.

b. Diffraction – Waves can spread into unexpected areas when they pass through an opening or round an obstacle. This phenomenon is known as diffraction.

Hope this helps... :biggrin:

[misskitty]

http://www.fulvics.com/lighttheory/experiments.htm


:bugeye: OOOOhhh, a link! Cool.

[misskitty]

Reshma, I've got one thing to say to you....You're Good! :biggrin:

Ok, so since I have a big test on waves tomorrow and I need to do well, I was wondering if I could throw some clarification questions at all you wonderful posters? I hope that's ok. :redface:

[1,1,2,3,5,8,13,21]

My physics class is studying waves at the moment. I was reading something in my book that I thought was rather interesting. :rolleyes:

Mechanical waves don't travel very well through space because space is nearly a vaccum. However, electromagnetic waves travel well through space. There are some kinds of elecrtomagnetic waves that can not escape the gravity of black holes, such as light waves. :bugeye:

I was wondering why. Why can electromagnetic waves travle through space with relative ease, yet mechanical waves cannot? I know mechanical waves need an elastic medium to travel through, but isn't space a medium too? If it isn't why not? :uhh:

Just some food for thought.
:wink:

Look at it this way, yes there is matter in space, so you would think mecanical waves would travel in space, correct? Not so. the speed of sound in air (not so dense) is around 600 mph. it is faster in water(water is denser), and faster in solids than water (guess what is denser, solid or water) (if you guessed water, you failed in life). so let's say that the more stuff, the faster the sound (a mechanical wave) travels. now let's take the density down to 1 atom/meter^3. this is space at its most empty. will sound travel at all? if you answered yes, hit yourself and enroll at a head-start class for 2 year olds. if you answered no, you are thinking quite well, pat your self on the back. did that help? if no, hit yourself.

hope you don't hurt too bad.

Fibonacci :tongue2:

[misskitty]

Thankfully I didn't need to hit myself. :smile: So yes I am thinking. :tongue2:

I understand that waves are motions of distrubance and they travel faster through denser materials because as they vibrate the material, the material vibrates the atoms next to it faster since they are right there and not 10cm away or what not. What's the point of a wave?

[1,1,2,3,5,8,13,21]

Thankfully I didn't need to hit myself. :smile: So yes I am thinking. :tongue2:

congratulations! i never did enjoy hitting myself, it sucks. think of all the brain cells! i need those to think! or do I? do i think? uhh...


Fibonacci

[misskitty]

:smile: They usually come in handy. :wink:

What are some examples of longitudinal and transversal waves? I know sound waves are an example of longitudinal waves, but what are some others?

[1,1,2,3,5,8,13,21]

:smile: They usually come in handy. :wink:

What are some examples of longitudinal and transversal waves? I know sound waves are an example of longitudinal waves, but what are some others?

don't forget surface waves, like tsunami waves! transverse would be taping a slinky to a wall and moving it like so -> or <- as in forward or back. this creates compressions and rarefractions. go to www.howstuffworks.com . it is as great as great itself

Fibonacci

[Reshma]

:smile: They usually come in handy. :wink:

What are some examples of longitudinal and transversal waves? I know sound waves are an example of longitudinal waves, but what are some others?

Common types of mechanical waves include sound or acoustic waves, ocean waves, and earthquake or seismic waves. In order for compressional waves to propagate, there must be a medium, i.e. matter must exist in the intervening space. For our purposes, we use the term matter to mean that atoms must exist in the intervening space.

Common types of electromagnetic waves include visible light, infrared, and ultraviolet radiation, among others. The transmission of electromagnetic waves does not require a medium and electromagnetic waves are able to travel through vacuums. Unlike mechanical waves such as sound, electromagnetic waves can travel successfully across the near emptiness of outer space.

In transverse waves, the components of the medium oscillate in a direction perpendicular to the direction of propagation of the wave through the medium. Example: The waves in stretched strings.
In longitudinal waves, the components of the medium oscillate in a direction parallel to the direction of propagation of the wave through the medium. Example: Sound waves in columns of air.

[Reshma]

BTW, thanks for your compliment :angel:

[dextercioby]

:angel:= o:) :wink-wink:

Daniel.

[RoboSapien]

...
(ii) Electromagnetic waves – these waves are made up of electric and magnetic fields whose strengths oscillate at the same frequency and phase. The fields are perpendicular to each other as well as the direction of propagation of the wave and no medium is necessary for propagation. Light is an EM wave... :uhh:

Who proved that ? Any links about it.

How come nor magnetism or Electric fields affect these EM waves ?

Naa, I dont believe U.

[kaos]

waves in space?????

so thats how the silver surfer surfs........

[dextercioby]

:uhh:

Who proved that ? Any links about it.

How come nor magnetism or Electric fields affect these EM waves ?

Naa, I dont believe U.

You gotta be joking,right...? :surprised

Daniel.

[Ouabache]

P.S. I don't have the book. It doesn't help me any if I don't know what the title is. :smile: Where can I find it?

dextercioby gave the title of the book by Weinberg


(i'll have to check,though,it's been a while since reading Weinberg's book:"The first three minutes").
e.g. ---> http://www.sciencedaily.com/cgi-bin/apf4/amazon_products_feed.cgi?Operation=ItemLookup&ItemId=0465024378

It is interesting, the two people you mentioned Weinberg, Salam and a third fellow Glashow. They were awarded a Nobel Prize in 1979 for their contribution to elementary particle physics.

...Sheldon L. Glashow.., ..Abdus Salam.., and ..Steven Weinberg.., for their contributions to the theory of the unified weak and electromagnetic interaction between elementary particles, including inter alla the prediction of the weak neutral current.
ref: --> http://nobelprize.org/physics/laureates/1979/press.html

[Ouabache]

...I think u're referring to Penzias & Wilson who discovered the backgroud microwave radiation in 1964 ...
Penzias,Arno A.,Wilson,Robert W.,Astrophys.J.,142,419 (1965)
Shared the Nobel in 1978 with Piotr Kapitza.

Sidenote: Bob Wilson and Arno Penzias discovered this radiation quite by accident. They were not looking for it. They were using a microwave antenna at Bell Labs and no matter which direction they pointed the antenna, they noticed constant background noise. They wanted to eliminate this noise, because it interfered with their experiments. They even went to the extent of cleaning pigeon sh#t out of the antenna in attempt to eliminate the noise. :rofl:

Bob Dicke and Jim Peebles at Princeton Univ (only 30mi from Bell Labs) were actually looking for cosmic background radiation also using a microwave horn antenna. Wilson called up Princeton and asked Dicke and Peebles if they could solve their problem. The Princeton researchers drove to Bell Labs, looked at their data and explained to them what they had found (background radiation of the universe)

For their discovery, Wilson & Penzias were awarded a Nobel prize, while Dicke & Peebles didn't even get a mention. :cry:

The information described above was taken from interviews I watched between Dicke, Peebles and Wilson, on the PBS airing of "Stephen Hawking's Universe"

[RoboSapien]

Warning : This is not a joke, U r Obliged to answer the question Or I will unsubscribe this thread. The fact that this question was ignored proves that there is something seriously wrong with this theory of EM Waves.

Who proved that ? Any links about it.

How come nor magnetism or Electric fields affect these EM waves ?

Naa, I dont believe That.

[Crosson]

Who proved that ? Any links about it.

James Clerk Maxwell proved that light was an electromagnetic wave in about 1865, experimental verification came from Hertz a few decades later.

Maxwell's equations describe the geometry of the electromagnetic field near a charge and current distribution. It is very simple to manipulate maxwell's equations to show that electric and magnetic fields satisfy the same equation a waves on a string.

Heres the kicker: Based on the strength of the electric and magnetic fields, Maxwell calculated the speed of these EM waves to be 3.00 * 10^8 m/s, which agreed with the previously determined value for the speed of light. The conclusion was immediate.



How come nor magnetism or Electric fields affect these EM waves ?

Because the field interacts primarily with charges, and secondarily with itself. Still, magnetic and electric fields can affect light waves.

[Reshma]

Any links about it.

You want links? Why don't you Google on 'EM waves'?
Anyway,
http://electron9.phys.utk.edu/optics421/modules/m1/emwaves.htm
If you like it.

[misskitty]

Warm greetings to my friends,

I'm so happy to be back on PF! Sorry for my absense, I had some personal family business I needed to take care of. Now I'm back!! :biggrin:

You guys sure did say qutie a bit while I was away. Looks like I have a bit of catching up to do with my posting. :rolleyes: Ah, well, no matter I'[m happy to do it. Besides we have an excellent discussion going here! :smile:

So without further ado, lets get started with some of this info. I'm glad to be back here with all of you. :wink:

[misskitty]

don't forget surface waves, like tsunami waves! transverse would be taping a slinky to a wall and moving it like so -> or <- as in forward or back. this creates compressions and rarefractions. go to www.howstuffworks.com . it is as great as great itself

Fibonacci

Tsunamis huh :rolleyes: . Never really though of them. Thats a good example. We had talked about the slinky in my physics class. Who knew there was so much physics in such a simple, but cool :cool: , toy?

Cool link by the way. Amazing the stuff you can find on the web, isn't it?

[misskitty]

Common types of mechanical waves include sound or acoustic waves, ocean waves, and earthquake or seismic waves. In order for compressional waves to propagate, there must be a medium, i.e. matter must exist in the intervening space. For our purposes, we use the term matter to mean that atoms must exist in the intervening space.

Acoustic waves like the ones from a guitar or in an auditorium or any other kind of instrument? Ocean waves, that would be like the tsunami. The seismic waves, aren't they measured using logarithmic functions? I don't know much about them, just that they are measured on a seismegraph. Matter acts as interferrance right?

Common types of electromagnetic waves include visible light, infrared, and ultraviolet radiation, among others. The transmission of electromagnetic waves does not require a medium and electromagnetic waves are able to travel through vacuums. Unlike mechanical waves such as sound, electromagnetic waves can travel successfully across the near emptiness of outer space.

Ah, the kind of waves that cook ya if ya sit out in 'em for too long. :cool:

In transverse waves, the components of the medium oscillate in a direction perpendicular to the direction of propagation of the wave through the medium. Example: The waves in stretched strings.
In longitudinal waves, the components of the medium oscillate in a direction parallel to the direction of propagation of the wave through the medium. Example: Sound waves in columns of air.

Related question to sound; why do things echo in caves and rooms that have nothing in them? Doesn't sound have an easier time traveling through a solid medium because the molcules vibrate easier and they make the neighboring molecules vibrate quickly too?

[misskitty]

BTW, thanks for your compliment :angel:

Your welcome. Its true, you rock. :biggrin:

[misskitty]

Sidenote: Bob Wilson and Arno Penzias discovered this radiation quite by accident. They were not looking for it. They were using a microwave antenna at Bell Labs and no matter which direction they pointed the antenna, they noticed constant background noise. They wanted to eliminate this noise, because it interfered with their experiments. They even went to the extent of cleaning pigeon sh#t out of the antenna in attempt to eliminate the noise. :rofl:

So wait, if they weren't looking for the backround radiation waves, what were they looking for? Did they contact Dicke and Peebles and let them know what they found? :bugeye:

Bob Dicke and Jim Peebles at Princeton Univ (only 30mi from Bell Labs) were actually looking for cosmic background radiation also using a microwave horn antenna. Wilson called up Princeton and asked Dicke and Peebles if they could solve their problem. The Princeton researchers drove to Bell Labs, looked at their data and explained to them what they had found (background radiation of the universe)

For their discovery, Wilson & Penzias were awarded a Nobel prize, while Dicke & Peebles didn't even get a mention. :cry:

That seems little acinine, why didn't they get mentioned? One would have thought they would have at least gotten a mention.

The information described above was taken from interviews I watched between Dicke, Peebles and Wilson, on the PBS airing of "Stephen Hawking's Universe"

They have some wicked good shows. :smile:

[misskitty]

James Clerk Maxwell proved that light was an electromagnetic wave in about 1865, experimental verification came from Hertz a few decades later.

Hertz? Wasn't the unit of frequency named after him?

Maxwell's equations describe the geometry of the electromagnetic field near a charge and current distribution. It is very simple to manipulate maxwell's equations to show that electric and magnetic fields satisfy the same equation a waves on a string.

Heres the kicker: Based on the strength of the electric and magnetic fields, Maxwell calculated the speed of these EM waves to be 3.00 * 10^8 m/s, which agreed with the previously determined value for the speed of light. The conclusion was immediate.Because the field interacts primarily with charges, and secondarily with itself. Still, magnetic and electric fields can affect light waves.

So wait, what exactly does that mean? How did Maxwell come up with the equations? Were they based on an actualy experiment he conducted?

Ahh, so much information!! :biggrin:

[misskitty]

Warning : This is not a joke, U r Obliged to answer the question Or I will unsubscribe this thread. The fact that this question was ignored proves that there is something seriously wrong with this theory of EM Waves.

Who proved that ? Any links about it.

How come nor magnetism or Electric fields affect these EM waves ?

Naa, I dont believe That.

Robo, take it easy. Don't leave the thread, please. Your contributions in the thread have been good. :smile:

It's actually not a bad question. Why don't magnetism or electric fields affect these waves?

[misskitty]

Here's a question I just thought of. We were discussing resonance in my physics class today. We defined it, but I don't think the definition was very comprehendable. Honestly it was rather confusing.

Anyway, I know it has to do with something wanting to vibrate at a certain fundamental frequency. I know mechanical waves are affected by resonance because sound resonates. Does resonance occur with EM waves too? If so, what happens?

[whozum]

Resonance occurs in mechanical waves depending on the source of the wave. For example, hitting a pair of tongs will produce a different frequency depending on where you hit the tongs. Whereas hitting a wooden table would produce pretty much the same frequency regardless.

The frequency produced by such a wave is the resonant frequency, defined as the "standard frequency of the wave created by vibration".

I'm not sure if EM waves have resonant frequencies, but I know electronic circuits have resonant frequencies based on the impedance and inductance of the circuit, but thats a completely different area.

[misskitty]

Why does the frequency change, using your example of the tongs, depending upon when you hit the tongs? I don't mean different surfaces you strike them on, you can include that should you like too, but if you strike them on a table at the end of the tongs and then again, but closer to where you are holding them.

I'm not sure if what I'm asking makes sense....

[whozum]

Hit your hand against a table. Doesn't make a very nice ring does it. The frequency is pretty low and the wave is so dampened that the sound kind of sucks. Tongs on the other hand are metal and usually have long 'fingers'. The longer the finger, the higher the frequency. I'm not really sure how to describe how this happens, but theres alot more energy released when you hit a farther point. Higher energy waves have are frequency waves, since energy is proportional to frequency.

[misskitty]

Ah, no it does make sense. So is it safe to say the higher the frequency the more energy is carried on the wave? I was pondering what reasoning I used on my recent physics test. :rolleyes:

[Ouabache]

So wait, if they weren't looking for the backround radiation waves, what were they looking for? Did they contact Dicke and Peebles and let them know what they found? :bugeye:
They were working for AT&T and experimenting with microwaves, presumably to learn more about them and to use them for telephone communications, say point-to-point on land or to transmit and recieve microwaves via satellites.

That seems little acinine, why didn't they get mentioned? One would have thought they would have at least gotten a mention.
Well they are certainly mentioned today in historical perspective of how this discovery unfolded. However the Nobel Prize committee didn't include them in their award. We would have to do a little more digging to find out if they were mentioned when Wilson and Penzias published their findings.
If you're interested in learning more on this story, there is a good read at ---> http://www.princeton.edu/~paw/archive_new/PAW00-01/03-1025/features2.html
See part that begins "Thirty-five years ago, the Princeton Gravity Group, including professors Robert Dicke, Jim Peebles and David Wilkinson, were already building an experiment to detect the distant CMB (cosmic microwave background)....."

They have some wicked good shows. :smile:
That's for sure!! I've watched the Stephen Hawkings Universe several times (i have his book by same name). It never ceases to amaze me what this fellow comes out with, under such adversity.. Brian Greene's The Elegant Universe is another favorite. An astrophysics classic (i sent away for this one), Carl Sagan's COSMOS .. I've found quite a few more good science videos at the public library. The NOVA series is excellent as is Alan Alda's Scientific American Frontiers :wink:

[Ouabache]

Here's a question I just thought of. We were discussing resonance in my physics class today. We defined it, but I don't think the definition was very comprehendable. Honestly it was rather confusing.

Anyway, I know it has to do with something wanting to vibrate at a certain fundamental frequency. I know mechanical waves are affected by resonance because sound resonates. Does resonance occur with EM waves too? If so, what happens?

Sometimes it is hard to form a clear concept, from a definition. Examples are always much better. :tongue2:

When you swing on a swing in a playground, did you ever wonder why it swings at a certain rate? As you bend your knees and swing higher, you increase the amplitude of this system but you do not change the velocity. The swing and you move at a natural frequency. The system is resonant. If you try to swing faster or slower, you find that you cannot. That is how a pendulum works.

see ---> http://hyperphysics.phy-astr.gsu.edu/hbase/sound/reson.html#c2

If you are familiar with musical instruments, a clarinet is a good example for resonance of acoustic waves. Each key that is held down creates a cylindrical cavity (of a specific length) that is open at one end and blocked at the other. As you try to play a low note a lot of sqawking sound will occur until you find a certain note that will resonate for that length of pipe. Other pitches will also resonate for the same length of pipe and sound higher, those are harmonic frequencies and are related to the lowest (fundamental) note. A church pipe organ is certainly another good example of resonance.

see ---> http://www.umanitoba.ca/faculties/arts/linguistics/russell/138/sec4/resonanc.htm

Electromagnetic waves also exhibit resonance. In a radio, you can tune the dial to a certain frequency, to hear your favorite station. However the reception may be poor. You can improve the reception by making an antenna with the correct geometry (length of its elements), such that it resonates at the same frequency as the radio waves travelling through air. If you made the antenna a bit longer or shorter, the antenna would no longer resonate at that frequency and reception would get weaker. (there is a bit more to antennas than that, but this illustrates the point)

If you're curious about antennas a good read can be found at --->
http://www.qsl.net/g3yrc/antenna%20basics.htm
they have a nice diagram of the electromagnetic spectrum from power line frequencies all the way up to cosmic radiation.

Lots of shapes resonate, including bridges. A famous instance was the "Tacoma Narrows" suspension bridge in Washington state. It was noticed that as the winds blew, this bridge would begin to swing. When the wind reached 42mph, the bridge began to oscillate at its resonant frequency, creating both transverse (side to side) and longitudinal (length wise) oscillations. Eventually the amplitude became so great that the entire suspension collapsed.

For some vivid photos, see ---> http://www.lib.washington.edu/specialcoll/tnb/

[Reshma]

Hi Misskitty, good to have you back. :smile:

Acoustic waves like the ones from a guitar or in an auditorium or any other kind of instrument? Ocean waves, that would be like the tsunami. The seismic waves, aren't they measured using logarithmic functions? I don't know much about them, just that they are measured on a seismegraph. Matter acts as interferrance right?

The term "Acoustics" is the branch of physics which studies sound, mechanical waves in gases, liquids, and solids. So any kind of study of sound waves be it music, seismology etc. falls under acoustics. For more check out this link: http://en.wikipedia.org/wiki/Acoustics.

Seismic waves are waves that travel through the earth(waves can be due to any natural phenomenon viz. earthquake, volcanoes etc.) A seismograph is basically a device which can detect and measure the intensity of seismic waves. If you are interested in learning how a seismograph works check out this link: http://en.wikipedia.org/wiki/Seismograph

Related question to sound; why do things echo in caves and rooms that have nothing in them? Doesn't sound have an easier time traveling through a solid medium because the molcules vibrate easier and they make the neighboring molecules vibrate quickly too?
Echoes are a diferent phenomenon altogether.
An "echo" occurs when sound reflects back to the listener from rocks or other hard surfaces, especially flat vertical surfaces. The human ear cannot differentiate between two sounds if they occur together within a time interval of 1/17th of a second(the exact same reason why you cannot understand anything when ten people yap at the same time!) :wink:

When echoes overlap in time, or when multiple echoes are so closely spaced in time that human ears cannot resolve individual echoes, the effect is called "reverberation". Reverberation is defined as the persistence of sound in a closed, or partially enclosed space after the source of sound has stopped.

[Reshma]

Lastly, yes! Denser the medium of propagation-->faster the speed of sound. The speed of sound depends upon the type of medium and its state(solid, liquid,gases).

Examples on Different speeds of sound:
Gases:
331m/s in air at 0 degree celsius
343m/s in air at 20 degrees "
319m/s in argon
Liquids:
1207m/s in ethyl alcohol
1497m/s in distilled water
1531m/s in sea-water
Solids:
6420m/s in aluminium
5790m/s in stainless steel

As you can see here, the speed increases as the density of the medium increases.

[Reshma]

Now, I am bit dubious about the speeds in the solid media since my text isn't so accurate. Nevertheless, I hope I solved atleast some of your doubts...:biggrin:

[Ouabache]

Lastly, yes! Denser the medium of propagation-->faster the speed of sound. The speed of sound depends upon the type of medium and its state(solid, liquid,gases).

Examples on Different speeds of sound:
Gases:
331m/s in air at 0 degree celsius
343m/s in air at 20 degrees "
Liquids:
1207m/s in ethyl alcohol
1497m/s in distilled water
1531m/s in sea-water

As you can see here, the speed increases as the density of the medium increases.

When I scubadive in sea-water, I cannot discern the direction a sound is coming from. It seems to come from every direction simultaneously. Could the information you have given here, allude to a reason, why humans have trouble determining direction of sound under water? ? :rolleyes:

[turbo]

When I scubadive in sea-water, I cannot discern the direction a sound is coming from. It seems to come from every direction simultaneously. Could the information you have given here, allude to a reason, why humans have trouble determining direction of sound under water? ? :rolleyes:I'm pretty sure you already know this :rolleyes: but we humans have a natural ability to determine the direction from which a sound is emitted, and part of the ability comes from the perception of relative loudness from one ear to the other and part of it comes from an ability to sense a delay between one ear and the other. The very rapid propagation of sound underwater takes much of the delay away, and robs us of some of the perception of direction. Dolphins and whales do not have this problem. :wink:

[Ouabache]

This wave phenomena is great!!

I'm pretty sure you already know this :rolleyes: but we humans have a natural ability to determine the direction from which a sound is emitted, and part of the ability comes from the perception of relative loudness from one ear to the other and part of it comes from an ability to sense a delay between one ear and the other. The very rapid propagation of sound underwater takes much of the delay away, and robs us of some of the perception of direction. Dolphins and whales do not have this problem. :wink:

Turbo: you're right, I am aware of this and glad that you posted your reasoning on how we perceive direction in air. Triangulation from two recieving antennas (our ears) is an efficient system that our brain handles really well.. Amazing actually :bugeye:

It is also a good deduction, that the rapid propagation of sound underwater is the reason why we lose direction perception. But how fast is too fast?

Is there some way we could find the cutoff speed-of-sound, beyond which we cannot distinguish direction? :confused:

Here is the information I have found so far.
In air, sound reaches one of our ears 30μsec before the other.
ref----> http://library.thinkquest.org/28170/36.html and Reshma has told us that the speed of sound in sea-water is 1531m/s . The distance between our ears (as I measure with a ruler) is approximately 7 inches (0.178m).
Also from Reshma's data, sound waves propagate 1531/343 = 4.46 times faster in the ocean than in air.