'Mach c'? Scientists observe sound traveling faster than the speed of light

[alkemical]

'Mach c'? Scientists observe sound traveling faster than the speed of light


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In this schematic of the acoustical test system, the scientists could create superluminal group velocity of sound waves, as well as negative group velocity. In the latter case, the peak of the output pulse traveling through the loop filter exited the filter before the peak of the input pulse had reached the beginning of the filter. Image credit: Bill Robertson, et al.
For the first time, scientists have experimentally demonstrated that sound pulses can travel at velocities faster than the speed of light, c. William Robertson’s team from Middle Tennessee State University also showed that the group velocity of sound waves can become infinite, and even negative.

Past experiments have demonstrated that the group velocities of other materials’ components—such as optical, microwave, and electrical pulses—can exceed the speed of light. But while the individual spectral components of these pulses have velocities very close to c, the components of sound waves are almost six orders of magnitude slower than light (compare 340 m/s to 300,000,000 m/s).

“All of the interest in fast (and slow) wave velocity for all types of waves (optical, electrical, and acoustic) was initially to gain a fundamental understanding of the characteristics of wave propagation,” Robertson told PhysOrg.com. “Phase manipulation can change the phase relationship between these materials’ components. Using sound to create a group velocity that exceeds the speed of light is significant here because it dramatically illustrates this point, due to the large difference between the speeds of sound and light.”

The experiment was conducted by two undergrads, an area high school teacher and two high school students, who received funding by an NSF STEP (Science, technology, engineering, math Talent Enhancement Program) grant. The grant aims to increase recruitment and retention of students to these subjects.

In their experiment, the researchers achieved superluminal sound velocity by rephasing the spectral components of the sound pulses, which later recombine to form an identical-looking part of the pulse much further along within the pulse. So it’s not the actual sound waves that exceed c, but the waves’ “group velocity,” or the “length of the sample divided by the time taken for the peak of a pulse to traverse the sample.”

“The sound-faster-than-light result will not be a surprise to the folks who work closely in this area because they recognize that the group velocity (the velocity that the peak of a pulse moves) is not merely connected to the velocity of all of the frequencies that superpose to create that pulse,” explained Robertson, “but rather to the manner in which a material or a filter changes the phase relationship between these components. By appropriate phase manipulation (rephasing) the group velocity can be increased or decreased.”

To rephase the spectral components, the sound waves were sent through an asymmetric loop filter on a waveguide of PVC pipe, about 8 m long. The 0.65-meter loop split the sound waves into two unequal path lengths, resulting in destructive interference and standing wave resonances that together created transmission dips at regular frequencies.

Due to anomalous dispersion (which changes the wave speed), sound pulses traveling through the loop filter arrived at the exit sooner than pulses traveling straight through the PVC. With this experiment, the group velocity could actually reach an infinitely small amount of time, although the individual spectral components still travel at the speed of sound.

“We also achieved what is known as a ‘negative group velocity,’ a situation in which the peak of the output pulse exits the filter before the peak of the input pulse has reached the beginning of the filter,” explained Robertson. “Using the definition for speed as being equal to distance divided by time, we measured a negative time and thus realized a negative velocity.”

[alkemical]

It might not seem that a negative velocity would exceed the speed of light, but in this case, Robertson said, the speed of the pulse is actually much faster than c.

“Consider the pulse speed in a slightly less dramatic case,” Robertson said. “Say the peak of the output pulse exits the filter at exactly the same time as the input pulse reaches the beginning. In this less dramatic case, the transit time is zero and the speed (distance divided by zero) is infinite. So we were beyond infinite! (‘To infinity and beyond,’ to steal a line from Toy Story.) In our experiment, we measured a negative transit time corresponding to a negative group velocity of -52 m/s.”

Although such results may at first appear to violate special relativity (Einstein’s law that no material object can exceed the speed of light), the actual significance of these experiments is a little different. These types of superluminal phenomena, Robertson et al. explain, violate neither causality nor special relativity, nor do they enable information to travel faster than c. In fact, theoretical work had predicted that the superluminal speed of the group velocity of sound waves should exist.

“The key to understanding this seeming paradox is that no wave energy exceeded the speed of light,” said Robertson. “Because we were passing the pulse through a filter, the sped-up pulse was much smaller (by more than a factor of 10) than the input pulse. Essentially, the pulse that made it through the filter was an exact (but smaller) replica of the input pulse. This replica is carved from the leading edge of the input pulse. At all times, the net energy of the wave crossing the filter region was equal to, or less than, the energy that would have arrived if the input pulse had been traveling in a straight pipe instead of through the filter.”

Is this phenomenon simply the result of a clever set-up, or can it actually occur in the real world? According to the scientists, the interference that occurs in the loop filter is directly analogous to the “comb filtering” effect in architectural acoustics, where sound interference occurs between sound directly from a source and that reflected by a hard surface.

“The superluminal acoustic effect we have described is likely a ubiquitous but imperceptible phenomenon in the everyday world,” the scientists conclude.

Citation: Robertson, W., Pappafotis, J., Flannigan, P., Cathey, J., Cathey, B., and Klaus, C. “Sound beyond the speed of light: Measurement of negative group velocity in an acoustic loop filter.” Applied Physics Letters 90, 014102 (2007).

By Lisa Zyga, Copyright 2006 PhysOrg.com.
All rights reserved. This material may not be published, broadcast, rewritten or redistributed in whole or part without the express written permission of PhysOrg.com.

[Billy Clyde Puckett]

I could have told you that. When my wife screams at me when I come home drunk I hear it before I see the flashlight in my eyes.

Seriously though, We are on the verge of some great scientific break throughs. It will be fun to see them play out.

[-Slap-]

Originally Posted by Big Guy

I could have told you that. When my wife screams at me when I come home drunk I hear it before I see the flashlight in my eyes.

I knew this post - in some form - was coming.

[alkemical]

No doubt Big Guy - this stuff really holds my interest.

[PLOWHORSE]

"If youre traveling at the speed of light, and you turn on your headlights...What Happpens?" --Steven Wright

[cutthemdown]

I'm looking foward to Helium-3 being used in fusion for electricity. I don't know much about it but it sounds remarkable.

Helium-3 undergoes the following aneutronic fusion reaction, among others, although this is the one most promising for power generation:

D + ³He → ⁴He (3.7 MeV) + p (14.7 MeV)

The appeal of helium-3 fusion stems from the nature of its reaction products. Most proposed fusion processes for power generation produce energetic neutrons which render reactor components radioactive with their bombardment, and power generation must occur through thermal means. In contrast, helium-3 itself is non-radioactive. The lone high-energy proton produced can be contained using electric and magnetic fields, which results in direct electricity generation.
However, since both reactants need to be mixed together to fuse, side reactions (D + D and ³He + ³He) will occur, the first of which is not aneutronic. Therefore in practice this reaction is unlikely to ever be completely 'clean'. Also, the temperatures required for D + ³He fusion are much higher than those of conventional D + T fusion, so it is unlikely that this type of fusion will be achieved before the problems with conventional fusion are worked out.

[Rock Chalk]

Originally Posted by PLOWHORSE

"If youre traveling at the speed of light, and you turn on your headlights...What Happpens?" --Steven Wright

Einstein proved that the speed of light is the same regardless of how fast you are moving. It is relative to your current speed. So, the answer to Steven Wright (who is a comic genius btw) is, the headlights come on.

[Rock Chalk]

Originally Posted by cutthemdown

I'm looking foward to Helium-3 being used in fusion for electricity. I don't know much about it but it sounds remarkable.

Helium-3 undergoes the following aneutronic fusion reaction, among others, although this is the one most promising for power generation:

D + ³He → ⁴He (3.7 MeV) + p (14.7 MeV)

The appeal of helium-3 fusion stems from the nature of its reaction products. Most proposed fusion processes for power generation produce energetic neutrons which render reactor components radioactive with their bombardment, and power generation must occur through thermal means. In contrast, helium-3 itself is non-radioactive. The lone high-energy proton produced can be contained using electric and magnetic fields, which results in direct electricity generation.
However, since both reactants need to be mixed together to fuse, side reactions (D + D and ³He + ³He) will occur, the first of which is not aneutronic. Therefore in practice this reaction is unlikely to ever be completely 'clean'. Also, the temperatures required for D + ³He fusion are much higher than those of conventional D + T fusion, so it is unlikely that this type of fusion will be achieved before the problems with conventional fusion are worked out.

I think it needs 300 million degrees to cause fusion compared to tritium which requires only 100 million degrees (or something like that). Either way, the radioactive material has a half life of 12 years in teh standard method compared to fission's radioactive by products half life of like a million years.

The problems with fusion aren't really in the physics but the engineering and computing. 40 years, 40 billion dollars and not a single kilowatt of net energy produced and likely 50 more years before the problem is figured out and 50-100 more years before it is commercially available.

Call me a fusion skeptic. Sounds great, but they have shown little in the way of progress to justify the time and money spent so far.

[Rock Chalk]

Oh and Josh, didnt this sound faster than light thing happen like 2 or 3 years ago? Seems like I remember them doing this.

[orangeatheist]

So, to make this relevant to me, is this saying that by 2063 we're gonna see something like this:



and make first contact with the Vulcans?

[watermock]

I'm still working on my ice cube cold fusion theories.

[fdf]

Originally Posted by cutthemdown

I'm looking foward to Helium-3 being used in fusion for electricity. I don't know much about it but it sounds remarkable.

Helium-3 undergoes the following aneutronic fusion reaction, among others, although this is the one most promising for power generation:

D + ³He → ⁴He (3.7 MeV) + p (14.7 MeV)

The appeal of helium-3 fusion stems from the nature of its reaction products. Most proposed fusion processes for power generation produce energetic neutrons which render reactor components radioactive with their bombardment, and power generation must occur through thermal means. In contrast, helium-3 itself is non-radioactive. The lone high-energy proton produced can be contained using electric and magnetic fields, which results in direct electricity generation.
However, since both reactants need to be mixed together to fuse, side reactions (D + D and ³He + ³He) will occur, the first of which is not aneutronic. Therefore in practice this reaction is unlikely to ever be completely 'clean'. Also, the temperatures required for D + ³He fusion are much higher than those of conventional D + T fusion, so it is unlikely that this type of fusion will be achieved before the problems with conventional fusion are worked out.

If you're interested in fusion, check out a recent talk by Dr. Robert Bussard (THE Dr. Bussard) called "should google go nuclear?" It is referred to in this link. He claims his company has achieved net output fusion with a simple (by fusion standards) device and that 200 million gets a prototype fusion plant running.

This guy is a former vice-chair of the Atomic Energy Commission and the inventor of the Bussard Ram Jet--a serious physicist.

Here's another link that discusses it:

http://www.rexresearch.com/bussard/bussard.htm

[Bronco_Beerslug]

Originally Posted by Alec

Einstein proved that the speed of light is the same regardless of how fast you are moving. It is relative to your current speed. So, the answer to Steven Wright (who is a comic genius btw) is, the headlights come on.

Really, is there an equasion for that?

[SoCalBronco]

Transparent aluminum?

That's the ticket, laddie.

[Bronco Billy]

Well, I certainly feel smarter now! I think I'll go out to Borders and buy Nanotechnology for Dummies to futher stimulate my brain! (Just kidding, the article was interesting.)

[Bronco Billy]

Originally Posted by Alec

Einstein proved that the speed of light is the same regardless of how fast you are moving. It is relative to your current speed. So, the answer to Steven Wright (who is a comic genius btw) is, the headlights come on.

Okay, so the headlights come on, but they wouldn't actually illuminate anything in front of you since the vehicle is already travelling the speed of light. Therefore, the headlights would be of no use. Correct?

[watermock]

The pointi is that even if you might be able to move waves faster than light, it has little to do with matter.

I don't like to discount anything, but waves and matter are well...two different matters.

[TheChamp24]

What the flying freakin f? Some of you guys are way geekier than I thought, lol.

[Taco John]

Originally Posted by Bronco_Beerslug

Really, is there an equasion for that?

I don't know about equation, but it's Einstein's addednum to Gallileo's general rule of relativity (ie. all uniform motion is relative). Einstein added that even at the speed of light, all uniform motion is relative.

[alkemical]

Originally Posted by Alec

Oh and Josh, didnt this sound faster than light thing happen like 2 or 3 years ago? Seems like I remember them doing this.

I dunno, it just caught up with me so it must move slower than I.

j/k - i'm not sure - and to be honest - i don't want to remember right now. Too much V.I.editor today.

Blargh

[alkemical]

This thread is awesome. Lots of good info.

Yes, yes i am a dork.

[Vegas_Bronco]

Random Tidbit: If you were to free fall from the earth's crust to the center of the earth it would take 36 hours - that's a lot of skydiving!

This earth is huge and it magnetic pull keeps our speed in check although I do wish they'd raise the speed limit again to 80-85 for modern vehicles. I can't wait to travel at the light speed of sound!

[alkemical]

Originally Posted by Vegas_Bronco

Random Tidbit: If you were to free fall from the earth's crust to the center of the earth it would take 36 hours - that's a lot of skydiving!

This earth is huge and it magnetic pull keeps our speed in check although I do wish they'd raise the speed limit again to 80-85 for modern vehicles. I can't wait to travel at the light speed of sound!

[Rock Chalk]

Originally Posted by Bronco Billy

Okay, so the headlights come on, but they wouldn't actually illuminate anything in front of you since the vehicle is already travelling the speed of light. Therefore, the headlights would be of no use. Correct?

No, light leaving the headlights from the drivers perspective leave away from you at the speed of light. So says Einstein. It defies logic, but so does relativity.

Einstein knew that all objects were in perpetual motion. We are rotating on the Earth, the Earth revolves around the sun, the sun eddies up and down (north and south) as it revolves around the galaxy and the galaxy itself is racing towards Andromeda. The Local Group of galaxies we live in is moving towards a larger super cluster of Galaxies and so on and so forth. As such, Einstein figured out that the speed of light was the only constant and that it remained the same no matter what the relative speed of the observer was. This has to do with time dilation I believe, (admittedly I could be wrong as to WHY we perceive it as moving the same regardless of our current velocity) as one travels faster time travels slower so your perceptions of what you see remain the same relative to how fast light travels.