# Light Speed and Sound Barrier

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Question:

When lightning strikes afar off, we see the light almost instantly and the sound waves race at different speeds to us with the shortest wavelengths first (crackle sound) and then the rest in a rolling sound as each frequency reaches the ear one after another with lower and lower pitch until the largest wavelength finally finishes the race. This makes sense because Speed=Distance/Time. The shortest wavelength travels less distance to the ear because it’s journey is closer to a straight line. The largest wavelength travels more distance because it must ascend and descend a good distance before making much forward motion. If the electro-magnetic spectrum includes large wavelength radio waves, medium wavelength visible light, and very short wavelength gamma rays, then why does it not follow that light must travel at varying speeds and not constant. If gamma rays and radio waves both were moving at 186,000mi/sec wouldn’t the radio waves be slower than the gamma rays since Speed=Distance/Time. The radio waves takes a longer journey than the gamma ray because the wavelengths are the photons ascending and descending and not moving purely forward at 186,000mi/sec. If we do observe all of the spectrum being constant at the same speed, wouldn’t that mean that radio waves move faster than gamma rays to make up for the greater ascension and descension? If all light is constant in speed then isn’t all light moving faster than the speed of light because 186,000mi/s (I assume) is the speed of light from point A to point B in a straight line in a vacuum? Light must be traveling faster than that figure to make up the extra distance, right?

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Hello, and welcome to the forums!

4 hours ago, Jordan The Wonderer said:

The shortest wavelength travels less distance to the ear because it’s journey is closer to a straight line. The largest wavelength travels more distance because it must ascend and descend a good distance before making much forward motion.

...not sure what you mean?  "ascend and descend"?  Sound waves travel in straight lines.  (Well, okay, they can bend due to diffraction, refraction, and such, but they go in straight lines through a uniform medium.)  The speed of sound in air is pretty much constant across frequencies.  My understanding is that there is some variation, but in the audible frequencies it's very small, less than 1 part in 1000-- in other words, not anywhere near enough to account for the long drawn-out rumble of thunder.

Bear in mind that when you hear thunder, you're not just hearing sound that's coming straight from the source.  Sound reflects.  The sound that comes to your ears can come straight from the lightning bolt, sure-- but you're also getting echoes as the thunder bounces off of terrain and such, resulting in many, many potential pathways of varying lengths.  My guess would be that the initial "CRACK!" is the direct-from-the-bolt sound, and the long drawn-out rumble would be the varying-path-length echoes.  Note that although sound's frequency doesn't make much difference to its speed, it does make a big difference to absorption.  High-frequency sounds get absorbed much more readily than low-frequency ones; they don't reflect as well.  So I'd guess that the low drawn-out rumble is simply a result of the higher frequencies getting filtered out as the sound waves ricochet around.

4 hours ago, Jordan The Wonderer said:

If the electro-magnetic spectrum includes large wavelength radio waves, medium wavelength visible light, and very short wavelength gamma rays, then why does it not follow that light must travel at varying speeds and not constant.

Because the speed of light in a vacuum is a constant.  Light travels in straight lines at constant speed.  This does not vary with wavelength.

4 hours ago, Jordan The Wonderer said:

The radio waves takes a longer journey than the gamma ray because the wavelengths are the photons ascending and descending and not moving purely forward at 186,000mi/sec.

...again, not sure where you're getting this?  They do move "purely forward".  The photons aren't "ascending and descending", they're simply moving in a straight line, which is what light does.

Note that the way light is drawn in cartoony diagrams often shows a wiggly line, and it's described as a "wave" ... but that doesn't portray the way photons move.  They don't move in a wiggle.  They go straight.  The "wave" refers to the the varying amplitude of the electric and magnetic fields along the light's flight path.  Those fields are indeed directed perpendicular to the direction of travel-- but please don't confuse that with the motion of the light itself.

4 hours ago, Jordan The Wonderer said:

186,000mi/s (I assume) is the speed of light from point A to point B in a straight line in a vacuum?

Yes.  (To within three significant figures, anyway.)

4 hours ago, Jordan The Wonderer said:

isn’t all light moving faster than the speed of light

Nope.  It's moving exactly at the speed of light.

4 hours ago, Jordan The Wonderer said:

Light must be traveling faster than that figure to make up the extra distance, right?

No, because there isn't any "extra distance".  Light moves in straight lines.   (Yes, it can be bent by refraction, diffraction, etc. but that's not what you're talking about, here.)

Incidentally, on a related note:  If you'd like to consider what happens when different wavelengths of light do travel at different speeds, just look at a rainbow.    Light moves at a constant speed in a vacuum, but when it's moving through a medium such as water, glass, etc., it goes at a slower speed determined by the medium's index of refraction.  And it's a fairly common property of most refractive media that the index of refraction varies by wavelength, a property called dispersion.  (The amount it varies depends on the material.)

That's what causes rainbows (either from rain droplets, or via prisms or the like):  the different wavelengths of light get bent by different amounts, thus separating "white" light into its components.  It's also what causes chromatic aberration, the bane of refractive telescopes and camera optics, causing unwanted rainbow-colored fringes in the captured images.  Optics designers have to do some clever engineering to try to minimize the effect.  One way to do this is to try to use materials with as low a dispersion as possible, which can reduce but generally not eliminate the effect.  Another technique is to use compound optics:  have a concave lens mated to a convex lens, using two different materials with different amounts of dispersion (the idea being for one of them to undo the dispersion of the other).

All of that pertains to light going through a medium (glass, water, etc.), though.  In a vacuum, there's no such effect; all wavelengths travel at the same speed.

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6 hours ago, Jordan The Wonderer said:

Question:

When lightning strikes afar off, we see the light almost instantly and the sound waves race at different speeds to us with the shortest wavelengths first (crackle sound) and then the rest in a rolling sound as each frequency reaches the ear one after another with lower and lower pitch until the largest wavelength finally finishes the race.

Nope. The initial crack is the shock wave, explosively expanding air from the lightning channel that reaches you in a direct line (Edit: the shock wave, not the channel of course ;-)). Maybe multiple shock waves at different sound levels if the channel branches. It can branch out of sight inside the cloud as well. The rolling otoh are echoes and reflections from ground, objects, clouds or even distant parts from the lightning's channel. It is not dependent on the frequency but the time it takes for the sound to travel from source to ear.

Quote

This makes sense because Speed=Distance/Time. The shortest wavelength travels less distance to the ear because it’s journey is closer to a straight line.

No. See above. Speed of sound only very slightly depends on frequency (air is regarded non-dispersive) and it surely does not travel in a curve.

Quote

The largest wavelength travels more distance because it must ascend and descend a good distance before making much forward motion.

The farther away travels longer distance and mixes with reflections from closer sounds. Sound wavelengths in air travel all at the same speed. Btw. the thunder frequencies aren't that wide spread, from low infrasound to a few hundred hertz. No standard pitch :-)

Edited by Green Baron
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6 hours ago, Jordan The Wonderer said:

The largest wavelength travels more distance because it must ascend and descend a good distance before making much forward motion.

Jordan,
You're mistaking sound and light as transverse waves. They're actually longitudinal waves. Sound is alternating compression and rarefaction (high and low pressure), while light is alternating electrostatic and electromagnetic fields.
HTHs,
-Slashy

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1 hour ago, GoSlash27 said:

You're mistaking sound and light as transverse waves. They're actually longitudinal waves. Sound is alternating compression and rarefaction (high and low pressure), while light is alternating electrostatic and electromagnetic fields.

To be clear, sound (in a gas) is a longitudinal wave and light is a transverse wave (that's why you can polarize light but not sound). Light consists of an electric field at right angles to a magnetic field, with both of them perpendicular to the direction of travel.

All of that is moot, though, with respect to the OP's question. Both sound and light travel in straight lines, and whether they're longitudinal or transverse is irrelevant to their propagation speed.

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7 hours ago, Jordan The Wonderer said:

Question:

When lightning strikes afar off, we see the light almost instantly and the sound waves race at different speeds to us with the shortest wavelengths first (crackle sound) and then the rest in a rolling sound as each frequency reaches the ear one after another with lower and lower pitch until the largest wavelength finally finishes the race. This makes sense because Speed=Distance/Time. The shortest wavelength travels less distance to the ear because it’s journey is closer to a straight line. The largest wavelength travels more distance because it must ascend and descend a good distance before making much forward motion. If the electro-magnetic spectrum includes large wavelength radio waves, medium wavelength visible light, and very short wavelength gamma rays, then why does it not follow that light must travel at varying speeds and not constant. If gamma rays and radio waves both were moving at 186,000mi/sec wouldn’t the radio waves be slower than the gamma rays since Speed=Distance/Time. The radio waves takes a longer journey than the gamma ray because the wavelengths are the photons ascending and descending and not moving purely forward at 186,000mi/sec. If we do observe all of the spectrum being constant at the same speed, wouldn’t that mean that radio waves move faster than gamma rays to make up for the greater ascension and descension? If all light is constant in speed then isn’t all light moving faster than the speed of light because 186,000mi/s (I assume) is the speed of light from point A to point B in a straight line in a vacuum? Light must be traveling faster than that figure to make up the extra distance, right?

When lightning strikes the air around the discharge explodes, that is what you should hear, but unless you are very close thats probably no what you actually hear.

Light travels relatively close to the speed of light in a vacuum. You don't live and breath in a vacuum and in a thunderstorm there  is alot of stuff going on in the atmosphere. So that unless you are on the clean side of a storm looking into it the light you see is going to be scattered. The higher the wavelength the more likely it is to be scattered.

With sound there is interference, as the highest frequency of sound travels though the atmosphere it gets interfered with, the highest wavelengths get cancelled first and the lower wavelengths bounce off of structures in the atmosphere and rumble. If you are far enough away you may not hear the initial strike. This is because the earth is curved and there is alot of interference close to the ground and the sound you hear are bouncing off of very large structures (the rumble). The high frequency sounds have been interfered with and absorbed by structures close to the strike.

C = speed of light, " Its exact value is 299,792,458 metres per second ' wikipedia. It is the free-form field propagation speed in our universe. Any field that is not bound will propagate at this speed. This includes the Higg's field, gravitational field, and light.

Light never changes speed in a vacuum. If you travel toward light you always will measure light at that speed. However the light will be more energetic. But unlike matter where KE = 1/2 mv2 , light energy = hv. As you speed up the frequency increases and as you move the other direction it decreases (red shifts).

C is essential to the fundamental speed of the universe. If we set C = 1 unit, h = 2 pi unit, and the gravitational constant, it is possible to derive a unit of length and time (known as quantum length and time) which describe the smallest possible times and and length of things in the universe.

E = hv  where v = C/g  (g = wavelength)

E = hc/g

from de Broglie hypothesis we get the momentum (p) of a photon as being g = h/p

planks constant in circular system can be used to define angular momentum (w, Radians per sec) as h = h/2pi. The energy of a photon can be describe more basically as E = hw

If you could imagine some future time when humans come into contact with other aliens (assumming they don't destroy us) if they adopted plancks units as did we, then upon that meeting we would share the same system of measurement even if the two peoples never met.

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Physics classes often use water waves to explain wave behaviour for sound and light.

This can do terrible damage if your physics teacher doesn't make it abundantly clear that:

• this is to illustrate the concept of wave motion and the sorts of effects it has (reflection, refraction, interference etc.)
• the wave that you see is not an accurate representation of what is actually causing the wave. Rather, the water wave  - that sinusoidal wave shape - is a great visual indicator of the phase of each part of the wave over time:
• the wave function of light indicates its vibrating electromagnetic field. The crest of the wave is peak field strength in one direction, the trough is peak field strength in the other. (edit: or maybe minimum field strength... yes, probably better that way for most intents and purposes)
• the wave function of sound indicates how compressed the air is. So the "crest" would be maximum compression, and the trough would be maximum dilation. A more accurate representation of sound would be: .. .  .   .  . . .. . .  .   .   but it is a pain to draw...
• the wave function of water waves indicates where in the circular motion of water each individual molecule is at. Water doesn't compress well, so it tries to get out of the way of whatever is pushing behind it. The easiest place to get out of the way is into the air, so the pressure is eased in that direction. The crest of the wave is the point that all of the water in the column below has moved up out of the way, and is exerting peak pressure downwards due to the weight of that column.
• the wave function of AC electricity is very similar to sound, if you replace "compression" with "electric pressure" a.k.a. "voltage".

The big difference with water waves compared to every other wave is we can see it at each part of the cycle at once. That leads to the misconception that waves are actually sinusoidal in shape... but they aren't.

Edited by Plusck
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I guess you're thinking that somehow waves actually have a wavy way to reach you. Let's get this sorted.

First of all, why "waves" ?

Waves means there's a change, there's a rate, there's a range. Surface water waves convenes the variant height of water surface. Sound waves convenes the variant density of air.

So what does EM waves do ? They change the electric and magnetic field strength.

Perhaps you got your thinking by seeing water waves. But there's a few things :

- The water does not necessarily moves across with it.

- The only thing you see is the movement of water height variation, not a flow of water.

You can try by putting colorations in water and see whether they actually moves as fast as the visible waves. (spoiler : it isn't.)

So, no matter what waves you have, it doesn't move something else that's not represented by it.

"but light, photons are waves, so... ?"

Here's the thing : photons are not waves. They represent the energy carried in the waves. Which somehow have all the characteristics of a particle. Even if they're like the water in water waves, they don't necessarily move with the "wave". But photons aren't like water, they really are a bundle of energy contained in the waves. Wave energy have nothing to do with how quick they move at all either, they have more to what the wave can do, what it can impart. A tsunami moves as fast as the usual waves you get on the shore, you know.

Edited by YNM
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4 hours ago, Snark said:

[deletia]

Note that the way light is drawn in cartoony diagrams often shows a wiggly line, and it's described as a "wave" ... but that doesn't portray the way photons move.  They don't move in a wiggle.  They go straight.  The "wave" refers to the the varying amplitude of the electric and magnetic fields along the light's flight path.  Those fields are indeed directed perpendicular to the direction of travel-- but please don't confuse that with the motion of the light itself.

[more deleted]

One thing that the "cartoony" diagram gets right is that photon can be found in all the positions on the wave: this is fundamental to limits of lenses (it comes up more in microscopes than telescopes, but should also come up in cameras).  Still, the photon shouldn't be thought of as moving from one bit of the wave to the next (even if it is a single photon): it will always have a velocity in the direction it is traveling (the beam, not the "wave").

There are similar problems with classic "atom" model that kids are exposed to.  The electrons may be found in bands around the nucleus, and they may have angular momentum, but don't think of them as moving from positions around the atom to another.  Electrons and photons don't do that.  They simply act like quantum objects and are pretty weird.

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8 hours ago, Snark said:

Hello, and welcome to the forums!

...not sure what you mean?  "ascend and descend"?  Sound waves travel in straight lines.  (Well, okay, they can bend due to diffraction, refraction, and such, but they go in straight lines through a uniform medium.)  The speed of sound in air is pretty much constant across frequencies.  My understanding is that there is some variation, but in the audible frequencies it's very small, less than 1 part in 1000-- in other words, not anywhere near enough to account for the long drawn-out rumble of thunder.

Bear in mind that when you hear thunder, you're not just hearing sound that's coming straight from the source.  Sound reflects.  The sound that comes to your ears can come straight from the lightning bolt, sure-- but you're also getting echoes as the thunder bounces off of terrain and such, resulting in many, many potential pathways of varying lengths.  My guess would be that the initial "CRACK!" is the direct-from-the-bolt sound, and the long drawn-out rumble would be the varying-path-length echoes.  Note that although sound's frequency doesn't make much difference to its speed, it does make a big difference to absorption.  High-frequency sounds get absorbed much more readily than low-frequency ones; they don't reflect as well.  So I'd guess that the low drawn-out rumble is simply a result of the higher frequencies getting filtered out as the sound waves ricochet around.

This, an gunshot work the same way it sounds different close up than far away, yes its not as load but you also get echoes who draws it out.
For explosions this is far more noticeable as its far loader, you are less than one meter from the source of sound then firing an gun, that would kill you with typical high explosives at one meter, the explosion is also faster but not sure this matter here. Now very close lighting sounds like close high explosives and is scary.

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