What is the cutoff speed-of-sound for humans to distinguish direction in water?

[misskitty]

Waves in Space!

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

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.

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?

Just some food for thought.

 

[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 traveling 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.

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.

Good to have you here.

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... 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 .

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.

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.

 

[dextercioby]

We can cover one thing at a time.

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 components together and their magnetic componats, right?

The microwaves discovered by Weinberg and Salaam ( I think it was them, ) 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 ) book:"The first three minutes").

Daniel.

 

[dextercioby]

****,i'm getting old! 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!

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.

 

[dextercioby]

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

Daniel.

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

 

[misskitty]

I know the feeling,.

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 ) 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. 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.


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?

 

[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.

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?

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...

 

[misskitty]

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

OOOOhhh, a link! Cool.

 

[misskitty]

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

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.

 

[1123581321]

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

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.

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?

Just some food for thought.

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

 

[misskitty]

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

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?

 

[1123581321]

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

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]

They usually come in handy.

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

 

[1123581321]

They usually come in handy.

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]

They usually come in handy.

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

 

[dextercioby]

= :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...

Who proved that ? Any links about it.

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

Naa, I don't believe U.

 

[kaos]

waves in space?

so that's how the silver surfer surfs...

 

[dextercioby]

Who proved that ? Any links about it.

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

Naa, I don't believe U.

You got to be joking,right...?

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. 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.

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.

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 don't 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.