How fast does sound travel? 3. How fast does sound travel? The speed at which sound propagates (or travels from its source) is directly influenced by both the medium through which it travels and the factors affecting the medium, such as altitude, humidity and temperature for gases like air. Bad news for Star Wars fansthere is no sound in the vacuum of space because there are too few molecules to propagate a wave. It is important to note that sound speed in air is determined by the air itself. It is not dependent upon the sounds amplitude, frequency or wavelength. To calculate the speed of sound in dry air at sea level, use the following formula: V = 331.4 + 0.6Tc V = velocity (m/s), Tc = temperature in Celsius. Use the interactive Speed of Sound calculator below to see the effect of temperature on sound's velocity. For comparison, the speed of light in a vacuum is 299,792,458 meters per second or 186,000 miles per second (669,600,000 mph), which is roughly 870,000 times faster than the speed of sound.

The difference between the speed of light and the speed of sound is why you see lightening before you hear it (unless you are struck by it, in which case it may be simultaneous!). As a comparison with sound in other mediums, the speed of sound in helium at 0°C is approximately 972 meters per second (m/s), or around 3 times as fast as in air. Sounds travels even faster in liquids and solids because of their greater density (although standard measurement is slightly more complicated because of it is effected by the shape of the material and also the fact that both longitudinal and transverse waves may propagate in solids). Sound in 20° C water travels an average of 1482 m/s, while sound in aluminum travels at 6420 m/s. Modern navies depends heavily on being able to predict the speed of sound in varying water conditions for their SONAR echo-location systems. If you have ever lived in an apartment with shared walls and a party next door, you are no doubt aware that sound waves can transfer from one medium to another and back againin this case from air to the rigid wallboard and back to air again. The speed with which it travels changes accordingly with the shift in mediums.

A wide variety of sound isolation products and construction techniques attempt to mitigate this sort of transfer. For further sound-abatement information, see Auralex's Acoustics 101 . It is important to realize that while all sound in the equivalent conditions travels at the same speed, whether louder or softer, higher or lower, the speed or force of the individual molecules bumping into one another increases with amplitude and/or frequency. For further study, see Hyperphysics->Speed of Sound An Acoustics Primer, Chapter 3 URL: ~emusic/acoustics/speed.htm Copyright 2004 Prof. Jeffrey Hass Center for Electronic and Computer Music, School of Music Indiana University, Bloomington, Indiana What is the speed of sound in a vacuum ? Is it faster or slower than its speed in air? | How Things Fly Ask an Explainer Q: What is the speed of sound in a vacuum ? Is it faster or slower than its speed in air? A: The speed of sound in a vacuum is zero meters per second, as sound cannot travel in a vacuum. Sound is a wave, which means it spreads through the vibration of particles in a medium, such as water or air.

Since a vacuum is empty space, there is no medium for the sound to travel through. The speed of sound in air mainly depends on the temperature of the air. On average, it is about 343 meters per second (1,125 feet per second), which is faster than the speed of sound in a vacuum. thermodynamics - What is the speed of sound in space? - Physics Stack Exchange What is the speed of sound in space? up vote 73 down vote favorite 19 Given that space is not a perfect vacuum, what is the speed of sound therein? Google was not very helpful in this regard, as the only answer I found was 300 km/s, from Astronomy Cafe , which is not a source I'd be willing to cite. ;) 3 The question is whether "sound" can even be defined in space (or a very very low pressure environment). – Faq Jan 29 '15 at 11:29 10 @LoveLearning The answer to that question is "We'll call it 'sound' if it can be transmitted coherently in that environment" and the condition for that is "wavelength much longer than mean-free path".

So, low enough frequency sounds can exist. – dmckee♦ Jan 29 '15 at 13:52 6 "In space, no one can hear you scream". – Gavin Coates Jan 30 '15 at 12:49 16 With regard to hearing a scream in space, that's not possible. The highest possible sound frequency in a gaseous medium has a wavelength roughly equal to the mean free path. In interplanetary space near Earth, the mean free path is about one astronomical unit and the speed of sound is on the order of 10 to 100 km/s. That corresponds to a frequency of about one cycle per month. That is many, many octaves below the frequency of a scream. – David Hammen Jan 30 '15 at 15:26 2 @DavidHammen - it depends on who/what is doing the screaming.

:-O – Bob Jarvis Jan 31 '15 at 3:07 By popular demand (considering two to be popular — thanks @Rod Vance and @Love Learning), I'll expand a bit on my comment to @Kieran Hunt's answer: Thermal equilibrium As I said in the comment, the notion of sound in space plays a very significant role in cosmology: When the Universe was very young, dark matter, normal ("baryonic") matter, and light (photons) was in thermal equilibrium, i.e. they shared the same (average) energy per particle, or temperature. This temperature was so high, that neutral atoms couldn't form; no matter what, the electron would be knocked off by a photon (or another particle). The photons themselves couldn't travel very far, before hitting a free electron. Speed of sound in the primordial soup Everything was very smooth, no galaxies or anything like that had formed. Stuff was still slightly clumpy, though, and the clumps grew in size due to gravity.

But as a clump grows, pressure from baryons and photons increase, counteracting the collapse, and pushing baryons and photons outwards, while the dark matter tends to stay at the center of the overdensity, since it doesn't care about pressure. This creates oscillations, or sound waves with tremendously long wavelengths, and with velocities of the order of half the speed of light (here you cannot use the formula given by @Kieran Hunt, since the pressure of photons cannot be neglected, instead you use the more general $$c = \sqrt{\gamma \frac{p}{\rho}},$$ where $p$ and $\rho$ are the pressure and density of the gas; this formula reduces to @Kieran Hunt's for an ideal gas). Recombination As the Universe expanded, it gradually cooled down. At an age of 379,000 years it had reached a temperature of ~3000 K, and protons and electron could now combine to form neutral atoms. This is called the "Epoch of Recombination", though they hadn't previously been combined.

Decoupling Hence, suddenly (the cosmological "suddenly" — I think it was over a course of ~10,000 years) the free electrons disappeared, and photons could stream freely, diffusing away and relieving the overdensity of its pressure. The photons are said to decouple from the baryons. Cosmic microwave background The radiation that decoupled has ever since redshifted due to the expansion of the Universe, and since the Universe has now expanded ~1100 times, we see the light (called the cosmic microwave background, or CMB) not with a temperature of 3000 K (which was the temperature of the Universe at the time of decoupling), but a temperature of (3000 K)/1100 = 2.73 K, which is the temperature that @Kieran Hunt refers to in his answer.

Baryon acoustic oscillations These overdensities, or baryon acoustic oscillations (BAOs), exist on much larger scales than galaxies, but galaxies tend to clump on these scales, which has ever since expanded and now has a characteristic scale of ~100 $h^{-1}$Mpc, or 465 million lightyears. Measuring how the inter-clump distance change with time provides a way of understanding the expansion history, and acceleration, of the Universe, independent of other methods such as supernovae and the CMB. And beautifully, the methods all agree . +1 This is a really nice discussion – Kieran Hunt Jan 29 '15 at 13:06 slightly off-topic, but I feel that I have to take a course in astroparticle-physics :) – F. Ha Feb 28 '16 at 15:30 Don't we all… :) Do you mean in order to understand the answer, or just in general?

– pela Feb 28 '16 at 16:28 up vote 42 down vote From the ideal gas law , we know: $$ v_\textrm{sound} = \sqrt{\frac{\gamma k_\textrm{B} T}{m}} $$ Assuming that interstellar space is heated uniformly by the CMB , it will have a temperature of $2.73\textrm{K}$. We know that most of this medium comprises protons and neutral hydrogen atoms at a density of about 1 atom/cc . This means that $\gamma = 5/3$, and $m = 1.66\times 10^{-27}\textrm{kg}$, giving a value for $v_\textrm{sound}$ of $192\textrm{m s}^{-1}$. However, this is not propagated efficiently in a vacuum. In the extremely high vacuum of outer space, the mean free path is millions of kilometres , so any particle lucky enough* to be in contact with the sound producing object would have to travel light-seconds before being able to impart that information in a secondary collision.

*Which for the density given, would only be about 50 hydrogen atoms if you clapped your hands - very low sound power! -Edit- As has quite rightly been pointed out in the comments, the interstellar medium is not that cold. At the moment, our solar system is moving through a cloud of gas at approximately 6000K . At this temperature, the speed of sound would be approximately $9000\textrm{m s}^{-1}$. See Kyle's answer for a table of values for $v_\textrm{sound}$ that can be found in different environments in space, or pela's for information on how early universe sound waves became responsible for modern-day large scale structure. 28 Argh, you beat it to me by seconds. Well, let me just add that sound in space plays a very significant role in cosmology: Just before recombination, 380.000 years after Big Bang, the speed of sound was approximately half the speed of light. When light and matter decoupled, the sound waves remained "frozen" in space, meaning that galaxies tend to form in clumps that are separated by this wavelength.

The distance between these clumps expands with the general expansion of the Universe (and is now ~465 million lightyears), and provides a standard measure of length. – pela Jan 29 '15 at 9:33 8 -1. This is not a good answer. Nothing in space is that cold. The interplanetary medium is in the tens of thousands of kelvins. The interstellar medium varies from ten of kelvin in molecular clouds to tens of millions of kelvins. The intergalactic medium is extremely hot, again in the tens of millions of kelvins. The widely varying temperature and makeup (molecular hydrogen vs ionized plasma) means the speed of sound in space varies considerably. – David Hammen Jan 29 '15 at 10:46 5 I suppose 6000K is the AVERAGE temperature, otherwise we would be boiling... – algiogia Jan 29 '15 at 13:47 8 @yo' He is right. You can see it very simply in this way: what happens if you drop a blazing hot metal ball in the sea? The sea does not boil.

To go back to reality, the ball is space: very hot, but with very low mass (very few atoms around). The Earth then is the sea: low temperature, but huge. Thus Earth is not boiling. – Svalorzen Jan 30 '15 at 11:07 6 @yo' - The "but" is very simple. The medium may well be very hot, BUT because it's so very, very tenuous, there is hardly any heat transfer from it to a macroscopic object. For a macroscopic object in space, radiative heat transfer (heat from the sun, cooling toward empty space) completely dominates over heat transfer from the hot but almost non-existent medium. – David Hammen Jan 30 '15 at 14:53 Just want to bring up that most answers seem to be taking "space" to be a nice uniform medium. However, even within our own galaxy, conditions vary wildly.

Here are the most common environments in the Milky Way: Molecular Clouds, $\rho\sim 10^4\,{\rm atom}/{\rm cm}^3$, $T\sim 10\,{\rm K}$ Cold Neutral Medium, $\rho\sim 20\,{\rm atom}/{\rm cm}^3$, $T\sim 100\,{\rm K}$ Warm Neutral Medium, $\rho\sim 0.5\,{\rm atom}/{\rm cm}^3$, $T\sim 10^4\,{\rm K}$ Warm Ionized Medium, $\rho\sim 0.5\,{\rm atom}/{\rm cm}^3$, $T\sim 8000\,{\rm K}$ HII Region, $\rho\sim 1000\,{\rm atom}/{\rm cm}^3$, $T\sim 8000\,{\rm K}$ Hot Ionized Medium, $\rho\sim 10^{-3}\,{\rm atom}/{\rm cm}^3$, $T\sim \;{>}10^6\,{\rm K}$ The

 sound speed is proportional to $\sqrt{T}$.

Given that the temperature varies over about 7 orders of magnitude (maximum at about $10^7\,{\rm K}$, minimum at about $3\,{\rm K}$), the sound speed varies by at least a factor of $1000$. The sound speed in a warm region is on the order of $10\,{\rm km}/{\rm s}$. Trivia: the sound speed plays a crucial role in many astrophysical processes. This speed defines the time it takes for a pressure wave to propagate a given distance. One place this is a key time scale is in gravitational collapse. If the sound crossing time for a gas cloud exceeds the gravitational free fall time (time for a gravity-driven disturbance to propagate), pressure is unable to resist gravitational collapse and the cloud is headed toward the creation of a more compact object (denser cloud, or if conditions are right, a star). More trivia: space is a very poor carrier (non carrier) of high frequency sounds because the highest frequency pressure wave that can be transmitted has a wavelength of about the mean free path (MFP) of gas particles. The MFP in space is large, so the frequency limit is low. 2 +1.

This is the answer to this question. The hot intracluster medium can be even hotter than the items on your list, up to $10^8$ kelvin. A high metallicity molecular cloud is not ionized and can contain some fairly massive compounds. You can easily add another order of magnitude to that factor of 1000. – David Hammen Jan 30 '15 at 12:04 Even though sound travels faster in space than in a terrestrial atmosphere, the vacuum of space isn't generally regarded as carrying sound well. Is that because pressure waves in space will be primarily reflected when they hit solid objects, or because they'll be converted to heat when they hit solid objects, or because they get converted to heat in transit? – supercat Jan 30 '15 at 18:10 1 @supercat What solid objects? Space is very empty, on average! Space is a very poor carrier (non carrier) of high frequency sounds because the highest frequency pressure wave that can be transmitted has a wavelength of ~the mean free path of gas particles. The MFP in space is large, so the frequency limit is LOW.

– Kyle Oman Jan 30 '15 at 19:34 3 @supercat you're confusing two things here. Speed of sounds is one thing. Frequencies that can be carried by a fluid is another. The frequencies that can be carried by the ISM are much lower than the lower limit of human hearing. That doesn't mean the sounds aren't meaningful, or that they don't exist. They just have low frequencies. – Kyle Oman Jan 30 '15 at 20:21 2 @hobbs: As Kyle reminded me, the concept of "impedance" in a sound-transmission medium is only meaningful at frequencies which are low relative to the frequency of particle interactions. For a tuning fork vibrating at 440Hz to transmit any meaningful information about its frequency, it must be hit by a lot more than 440 particles per second [regular sampling at 880 would suffice; I'm not sure how to describe the information conveyed by random samples].

– supercat Jan 31 '15 at 16:45 up vote 10 down vote I know this question is technically already answered, but there were several things missing from the answers that I thought should be mentioned (I am writing a review paper comparing different regions of space so I had these numbers at hand already as well). The speed of sound in space has multiple meanings because space is not a vacuum (though the number density of Earth's magnetosphere can be ~6-12 orders of magnitude more tenuous than the best vacuums produced in labs), it is full of ionized particles, neutral and charged dust . In the interplanetary medium or IPM, there are five relevant speeds that can all be considered a type of sound in a way, because each are related to the speed of information transfer in the medium. Classical idea of sound speed When one discusses the speed of sound , they generally refer to the common form of $C_{s}^{2} = \partial P/\partial \rho$, where $P$ is the thermal pressure and $\rho$ is the mass density.

In a plasma, this takes the slightly altered form of: $$ C_{s}^{2} = \frac{ k_{B} \left( Z_{i} \ \gamma_{e} \ T_{e} + \gamma_{i} \ T_{i} \right) }{ m_{i} + m_{e} } $$ where $k_{B}$ is Boltzmann's constant , $Z_{s}$ is the charge state of species $s$, $\gamma_{s}$ is the adiabatic or polytrope index of species $s$, $m_{i}$ is the mass of species $s$, and $T_{s}$ is the average temperature of species $s$. In a tenuous plasma, like that found in the IPM, it is often assumed that $\gamma_{e}$ = 1 (i.e., isothermal) and $\gamma_{e}$ = 2 or 3, or that $\gamma_{e}$ = 1 and $T_{e} \gg T_{i}$. The above form of the sound speed is known as the ion-acoustic sound speed because it is the phase speed at which linear ion-acoustic waves propagate.

Thus, $C_{s}$ is a legitimate type of sound speed in space. In the IPM, $C_{s}$ ~ 13 - 240 km/s [e.g., Refs. 12; 33; 34]. Speed of magnetic fields The cryptic title is eluding to what is known as the Alfvén speed, which is defined as: $$ V_{A} = \frac{ B_{o} }{ \sqrt{ \mu_{o} \ \rho } } $$ where $B_{o}$ is the magnitude of quasi-static, ambient magnetic field, $\mu_{o}$ is the permeability of free space , and $\rho$ is the plasma mass density (which is roughly equivalent to the ion mass density unless it's a pair plasma). This speed is typically associated with transverse Alfvén waves , but the speed is relevant to information transfer in plasmas, thus why I included it here. In the IPM, $V_{A}$ ~ 4 - 220 km/s [e.g., Refs. 10; 12; 33; 34].

Speed of magnetized sound waves In a magnetized fluid like a plasma, there are fluctuations that are compressive whereby they compress the magnetic field in phase with the density. These are known as magnetosonic or fast mode waves. The full MHD definition of the phase speed for a fast mode wave is given by: $$ 2 \ V_{f}^{2} = \left( C_{s}^{2} + V_{A}^{2} \right) + \sqrt{ \left( C_{s}^{2} + V_{A}^{2} \right)^{2} + 4 \ C_{s}^{2} \ V_{A}^{2} \ \sin^{2}{\theta} } $$ where $\theta$ is the angle of propagation with respect to $\mathbf{B}_{o}$. $V_{f}$ is the relevant speed for shock waves in weakly collisional and collisionless plasmas. It is also a type of sound speed, thus the name magnetosonic. In the IPM, $V_{f}$ ~ 17 - 300 km/s [e.g., Refs.

10; 12; 33; 34]. Side Note There is also a slow mode wave, which differs in polarization and the relative phase between the magnetic and density fluctuations. It is called slow because it has a smaller phase speed than the fast mode in the same medium. Thermal speeds The last two speeds that are relevant are the thermal speeds of the electrons and ions. The one-dimensional rms speed is given by: $$ V_{Ts}^{rms} = \sqrt{\frac{ k_{B} \ T_{s} }{ m_{s} }} $$ where the definitions are the same as in previous sections and $s$ can be $e$(electrons) or $i$(ions). Generally we use the three dimensional most probable speed, which is given by: $$ V_{Ts}^{mps} = \sqrt{\frac{ 2 \ k_{B} \ T_{s} }{ m_{s} }} $$ In the IPM, the electron [e.g., Refs.

2; 3; 5; 7; 8; 14; 17-22; 24; 25; 27; 29-34] and ion [e.g., Refs. 1-6; 8-11; 13; 15-17; 19; 20; 23; 26-32] thermal speeds are $V_{Te}^{mps}$ ~ 1020 - 5170 km/s and $V_{Ti}^{mps}$ ~ 13 - 155 km/s, respectively. Summary There are several different types of sound-like speeds in space and each of them can produce similarly related phenomena. For instance, we often refer to Mach numbers associated with $C_{s}$, $V_{A}$, and $V_{f}$. In addition, there are several plasma instabilities that result from an effect that similar to Cerenkov radiation , whereby a beam of particle exceeds, for instance, the electron thermal speed.

In summary, in the regions outside of local magnetospheres but within the realm of our sun's influence, there is a wide range of sound speeds. References up vote 7 down vote Given the low density of gas, the speed of sound would be a direct function of the temperature of the gas ie the speed of the molecules/atoms. Since this varies from about 2.7K to millions of degrees near some stars, the speed of sound can change quite a bit. answered Jan 29 '15 at 9:03 user56903 up vote 2 down vote Direct measurement shows the speed is 1100 m/s. ESA's dart-like Gravity field and Ocean Circulation Explorer (GOCE) Earth Explorer used to orbit as close to Earth as possible - just 260 km up - to maximize its sensitivity to variations in Earth's gravity field. At that altitude, there is enough atmosphere to exert a small drag. The satellite had an aerodynamic shape and a small engine to keep it in orbit. The mission ended when the engine ran out of fuel. In 2011, the huge 9.1 Japanese Tohoku earthquake generated atmospheric disturbances. These deflected the satellite.

Density variations were also measured. Article and video here . Speed of Sound Speed of Sound Introduction In old westerns, cowboys often put their ear to a railway track to hear if a train is coming. This is because the speed of sound waves depends upon the material through which they are travelling. Sound travels 15 times faster through the metal railway tracks than through air, giving the cowboy an early warning of the train's arrival. How fast is sound? Have you ever been swimming in the sea when a boat goes past a long way off? You can hear the motor sooner and much more clearly underwater than above the water. Look at the table in Fig.1 to work out why. Material Figure 1. The speed of sound in different materials. Which of these materials does sound travel fastest in? Air Water A vacuum Sound travels fastest in solids. This is because the molecules in solids are close together, which makes it easier for the vibrations to pass from one molecule A molecule is a group of atoms bonded together.molecule to the next. The relationship between the speed Speed is a measure of how fast something is moving. It is calculated by dividing the distance travelled by the time taken.speed, frequency In general, the frequency of an event describes how often it occurs.

When talking about waves, the frequency is a measure of how many waves go past a fixed point in a given time.frequency, and wavelength The wavelength of a wave is the distance from one peak to the next, or from one trough to the next.wavelength of sound is the same as for other waves: We can use this equation to work out the wavelength of a particular sound wave, or a certain note. The frequency of the musical note middle C is 261 Hz. What is the speed of sound in air? (Look at the table above.) 330 m/s 5000 m/s The wavelength is found by dividing the speed by the frequency: The frequency of the musical note A is about 440 Hz. It has a higher pitch and a higher frequency than middle C. What is the wavelength of the musical note A? 0.75 m Shorter Measuring the speed of sound A common way to measure the speed of sound is to set up two microphones that are both attached to an electronic timer. A loud noise is made near the first microphone and the timer starts. The sound wave then travels towards the second microphone. When the wave reaches the second microphone, the timer stops.

If you know the distance between the two microphones you can work out the speed of sound. The distance between the two microphones is 20 m. Sound travels at 330 m/s in air. Click on the t in the equation above to rearrange it. Then use the equation to work out how long the sound takes to travel between the two microphones. How long does it take the sound to get from one microphone to the other? 6600 s Sound waves being emitted from a fast-moving plane Figure 4. A photograph of the sound waves coming from a plane travelling faster than the speed of sound. The animation in Fig.2 shows a plane flying slower than the speed of sound. You can see that all the circles (wavefronts) remain inside each other. The plane in Fig.3 is moving faster than the speed of sound. The wavefronts generated by this plane overlap and form a cone in which the waves pile on top of each other (interfere constructively). This leads to the high intensity sound of a sonic boom A sonic boom is the loud booming noise created when a moving object travels faster than the speed of sound. It is caused by constructive interference of overlapping sound waves.sonic boom.

Summary Sound waves travel fastest in solids and slowest in gases. The speed of sound can be measured in a laboratory using microphones and an electronic timer. A sonic boom is created when a moving object travels faster than the speed of sound. Exercises 1. During a thunderstorm, you see a flash of lightning. Several seconds later, you hear the associated thunder. Why is this? Light travels more slowly than sound. The lightning was made before the thunder. The lightning is nearer than the thunder. Sound travels at the same speed as light. Sound travels more slowly than light. 2. Select the correct material to complete the paragraph below. Sound waves travel slowest through a and fastest through a . Sound waves travel at an intermediate speed through a . Sound does not pass through a at all. 3. A sound wave has a frequency of 500 Hz. In air, the speed of this wave is 330 m/s. What is the wavelength of this sound wave? cm 4. The same sound wave travels through a solid at a speed of 4000 m/s. What is the wavelength of the wave now? m 5.

A sound wave traveling in water has a speed of 1500 m/s and a wavelength of 6 cm. What is the frequency of this sound wave? Hz 6. This sound wave passes into air. Its frequency does not change, but its speed changes to 330 m/s. What is the wavelength of the wave in air? cm A method of measuring the speed of sound in air involves using two microphones both connected to an electronic timer. The timer measures how long the sound takes to travel between the two microphones. The experiment is repeated 10 times and the results are shown in the table below. Experiment number How Fast Does Light Travel? | The Speed of Light How Fast Does Light Travel? | The Speed of Light By Nola Taylor Redd, Space.com Contributor | May 22, 2012 08:37pm ET MORE Einstein's theory of special relativity sets of the speed of light, 186,000 miles per second (300 million meters per second). But some scientists are exploring the possibility that this cosmic speed limit changes.

Credit: Iscatel | Shutterstock The speed of light in a vacuum is 186,282 miles per second (299,792 kilometers per second), and in theory nothing can travel faster than light. In miles per hour, light speed is, well, a lot: about 670,616,629 mph. If you could travel at the speed of light, you could go around the Earth 7.5 times in one second. Early scientists, unable to perceive light’s motion, thought it must travel instantaneously. Over time, however, measurements of the motion of these wave-like particles became more and more precise. Thanks to the work of Albert Einstein and others, we now understand light speed to be a theoretical limit: light speed — a constant called "c" — is thought to be not acheivable by anything with mass, for reasons explained below. That doesn’t stop sci-fi writers, and even some very serious scientists, from imagining alternative theories that would allow for some awfully fast trips around the universe.

Speed of light: History of the theory The first known discourse on the speed of light comes from the ancient Greek philosopher Aristotle, who penned his disagreement with another Greek scientist, Empedocles. Empedocles argued that because light moved, it must take time to travel. Aristotle, believing light to travel instantaneously, disagreed. In 1667, the Italian astronomer Galileo stood two people on a hill at a distance of less than a mile, each holding a shielded lantern. One uncovered his lantern; when the second saw the flash, he uncovered his, as well. By observing how long it took for the light to be seen by the first lantern-holder (and factoring out reaction times), he thought he could calculate the speed of light. Unfortunately, Galileo's distances were too small to see a difference, so he could only determine that light traveled at least ten times faster than sound. In the 1670s, Danish astronomer Ole Römer used eclipses of Jupiter 's moon, Io, as a chronometer for the speed of light.

Over the course of several months, as Io passed behind the giant gas planet, Römer found that the eclipses came later than calculations anticipated, although over the course of several months, they drew closer to the predictions. He determined that light took time to travel from Io to Earth. The eclipses lagged the most when Jupiter and Earth were farthest apart, and were on schedule as they were closer. He concluded that light took ten to eleven minutes to travel from the sun to Earth, an overestimate since it in fact takes 8 minutes and 19 seconds. But at last scientists had a number to work with — his calculation presented a speed of 125,000 miles per second (200,000 km/s). In 1728, English physicist James Bradley based his calculations on the change in the apparent position of the stars due Earth's travels around the sun. He put the speed of light at 185,000 miles per second (301,000 km/s), accurate to within about 1 percent. Two attempts in the mid-1800s brought the problem back to Earth.

French physicist Hippolyte Fizeau set a beam of light on a rapidly-rotating toothed wheel, with a mirror set up 5 miles away to reflect it back to its source. Varying the speed of the wheel allowed Fizeau to calculate how long it took for the light to travel out of the hole, to the adjacent mirror, and back through the gap. Another French physicist, Leon Foucault, used a rotating mirror rather than a wheel. The two independent methods each came within about 1,000 miles per second of the speed of light measured today. Prussian-born Albert Michelson, who grew up in the United States, attempted to replicate Focault's method in 1879, but used a longer distance, as well as extremely high-quality mirrors and lenses. His result of 186,355 miles per second (299,910 km/s) was accepted as the most accurate measurement of the speed of light for 40 years, when Michelson remeasured it. Einstein and special relativity In 1905, Albert Einstein wrote his first paper on special relativity .

In it, he established that light travels at the same speed no matter how fast the observer moves. Even using the most precise measurements possible, the speed of light remains the same for an observer standing still on the face of the Earth as it does for one traveling in a supersonic jet above its surface. Similarly, even though the Earth is orbiting the sun, which is itself moving around the Milky Way, which is a galaxy traveling through space, the measured speed of light coming from our sun would be the same whether one stood inside or outside of the galaxy to calculate it. Similarly, Einstein calculated that the speed of light doesn't vary with time or place. What is a light-year? The speed of light can cause the universe to seem to host a time machine. The distance light travels in the course of a year is called a light-year. A light-year is a measure of both time and distance. It is not as hard to understand as it seems. Think of it this way: Light travels from the moon to our eyes in about 1 second, which means the moon is about 1 light-second away. Sunlight take about 8 minutes to reach our eyes, so the sun is about 8 light-minutes away.

Light from the nearest star system, Alpha Centauri , is requires roughly 4.3 years to get here, so that star system is said to be 4.3 light-years away. Stars and other objects beyond our solar system lie anywhere from a few light-years to a few billion light-years away. Thus, when astronomers study objects that lie a light-year away or more, they are seeing it as existed at the time that light left it, not as it would appear if they stood near its surface today. In this sense, everything we see in the distant universe is, literally, history . This principle allows astronomers to see how the universe as it looked after the Big Bang, which took place about 13.7 billion years ago. Examining objects that are, say, 10 billion light-years away, we see them as they looked 10 billion years ago, relatively soon after the beginning of the universe, rather than how they appear today. Is the speed of light really constant? Light travels in waves, and, like sound, can be slowed depending on what it is traveling through. Nothing can outpace light in a vacuum.

However, if a region contains any matter, even dust, light can bend when it comes in contact with the particles, which results in a decrease in speed. Light traveling through Earth's atmosphere moves almost as fast as light in a vacuum, while light passing through a diamond is slowed to less than half that speed. Still, it travels through the gem at over 277 million mph (almost 124,000 km/s) — not a speed to scoff at. Can we travel faster than light? Science fiction loves to speculate about this, because “warp speed,” as faster-than-light travel is popularly known, would allow us to travel between stars in time frames otherwise impossibly long. And while it has not be proven to be impossible, the practicality of traveling faster than light renders the idea pretty farfetched. According to Einstein's general theory of relativity , as an object moves faster, its mass increases, while its length contracts. At the speed of light, such an object has an infinite mass, while its length is 0 — an impossibility. Thus, no object can reach the speed of light, the theory goes. That doesn’t stop theorists from proposing creative and competing theories.

The idea of warp speed is not impossible , some say, and perhaps in future generations people will hop between stars the way we travel between cities nowadays. One proposal would involve a spaceship that could fold a space-time bubble around itself in order to exceed the speed of light. Sounds great, in theory. Further Reading: What Is the Speed of Sound? | WIRED What Is the Speed of Sound? subscribe 6 months for $5 - plus a FREE Portable Phone Charger. Author: Rhett Allain. Rhett Allain Science Date of Publication: 10.17.12. Time of Publication: 8:22 am. 8:22 am What Is the Speed of Sound? Image: Red Bull Stratos This seems to be the most common discussion regarding the recent Red Bull Stratos Jump . Unless you have been living under a rock lately, you have probably seen the awesome jump from 128,000 feet. Here is a great summary video to get you pumped up. The official fastest free fall speed was reported as 373 m/s . AH HA! That is just barely over the speed of sound at 340 m/s.

My physics textbook says this is the speed of sound, so there. Well, not so fast. Sound is a pretty complicated thing. What Is a Sound Wave? First, let me talk about just sound in air. Of course you can have sound waves underwater (hello submarines) and even through solids. But think of air. At one level, air is made up of a whole bunch of tiny particles. Oh sure, it is really more complicated than just tiny air particles. It is mostly nitrogen gas (N2) with some oxygen. But in this model of sound waves, it is fine to think of them all as just small particles. What happens if you take a whole bunch of these particles and push them all at the same time? Well, the pushed particles will go a little ways, but they will collide with other air particles and push them. Those particles will collide with more and so on and so on. This is what we call a wave. The important thing to realize is that the air isn’t moving very far, but the compression is moving. Here is my attempt at a diagram showing this. Another great example of this is the wave in a football stadium. Here is an example in case you have no idea what I am talking about.

What goes around the stadium? The people? No, they just move up and down. It is the disturbance that moves as the wave. The same is true for sound waves in air. OK, but that is just a simple model of sound. How fast is the sound wave in air? Although 340 m/s (760 mph) is a good first answer, it isn’t always true. Let’s look back a the wave of people in a football stadium. What would make this speed change? Two things could clearly make a difference. Suppose the stadium wasn’t full but instead about every other seat was occupied. This could change the speed of the sound wave. It isn’t quite clear if it would make it faster or slower, but I would guess faster since the person would be reacting to the previous person that was farther away. Another effect could be from the alertness level of the crowd. If people weren’t paying much attention, it could cause a longer reaction time and thus a lower wave speed. Actually, now I am curious. I wonder if stadium wave speeds are fairly constant for different stadiums and crowds. My guess is that they would all have similar speed values. Maybe this will be a future blog post.

OK, back to sound waves in air. What does this speed depend on? You could guess a few things. Just like the football crowd wave, the density of particles should matter. And what about the pressure in the air? That should matter too, right? Surprisingly (at least for me), a simple model for the speed of sound only varies with air temperature. Why? Well, as you get higher in the altitude (up to a point), the temperature decreases. The pressure and the density of air also decrease. The effects due to pressure and density essentially negate each other. Like I said, this over simplifies the whole issue. The Wikipedia page on the speed of sound has a lot more detail if you are interested. Speed of Sound vs. Altitude If you put this together, you can get a plot of the speed of sound as a function of altitude. Oh sure, it will change with weather and stuff, but still you can get a pretty basic model. Here is a plot of the speed of sound at different heights above sea level. At sea level, the value is right around the 340 m/s mark.

If you move up to 120,000 feet, the speed will drop down to around 200 m/s. Just from this data, you can see that Felix Baumgartner did indeed fall faster than the speed of sound. However, the question doesn’t really make sense. Did he fall faster than the speed of sound at sea level? Yes. Was he also going faster than the speed of sound for the altitude he was at? Well, it makes logical sense that if the speed of sound is greatest at sea level and he went faster than the speed of sound he would be going faster than the locals speed of sound. Speed vs. the Local Speed of Sound I don’t know if “local speed of sound” is an official term, but I like it. I am using it to mean the speed of sound at the current altitude. Here is a plot of the speed of Felix as he falls along with the plot of the local speed of sound at that same time. You will notice that from this numerical calculation, Felix was going faster than the local speed of sound for about 45 seconds.

You should also notice that this calculation has his maximum speed a little over the reported value of 373 m/s – hopefully I can fix this later when I compare my model to the real data – but it’s not too far off. Mach Number I guess I was right ( at least according to Wikipedia ). It has the definition of Mach number as the ratio of the speed of an object to the local speed of sound. Here is a plot of the speed of Felix as a function of altitude in terms of the Mach number (again, this is based on my not so perfect model). From this, he had a maximum speed of Mach 1.7 instead of the reported Mach 1.24. Of course this is very dependent on the actual speed of sound at that altitude. If the model is off a little bit on the speed of Felix as well as the speed of sound at that altitude (both using simple models), that could explain the discrepancy. Either way, there seems to be little doubt he went faster than the speed of sound. However, he didn’t break the speed of light. What? Yes. Here is an screen shot from MSNBC. I have trouble believing this is real, but I couldn’t find evidence that it was fake.

In case you didn’t click the image, it shows Felix Baumgartner after he stabilized his fall. The caption reads (and I kid you not): “FEARLESS FELIX” TRAVELED FASTER THAN THE SPEED OF LIGHT I understand that the MSNBC people were excited, but that is just crazy talk. Have you ever seen lightning and heard thunder? Do you hear them at the same time? No. You know why? Because the light from the event travels so much faster than the sound. The speed of light is crazy fast and you can’t really go faster than it anyway. But What About the Sonic Boom? Let’s start with Felix’s post jump press conference. Here is what he said ( full transcript available ): “I didn’t feel a sonic boom because I was so busy just trying to stabilize myself.” What is a sonic boom anyway? Well, it isn’t the sound an object makes when it transitions from going slower than the speed of sound to faster than the speed of sound. Instead, it is what stationary people would hear as an object moves by at supersonic speeds. Perhaps the best sonic boom analogy is a speed boat in the water. The boat makes waves as it moves, but it travels faster than the wave speed.

The result is a wake. Does the boat feel the wake? No. If you were on a dock as a speeding boat passed, then you would feel this wake.

Question: The speed of sound in a vacuum is?
Answer: Zero