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? :uhh:

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 :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:

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.:tongue2:

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? :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...

 

[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.