The Speed of Light and the Speed of Sound

Many people often make comments about something being as fast as the speed of light. I don't think most of them realize just how fast that is!

Until about 150 years ago, a person riding a horse at about 30 mph was as fast as anyone had ever experienced. WHOA! Thirty miles per hour ... a breakneck speed! Then when railroads developed, some went as fast as 60 miles per hour. Since a person walking moves at around 3 mph, and running about 10 mph, and riding a bicycle as much as 15 mph, these were considered incredible speeds, because no one ever in history had ever moved that fast!

In the early decades of the Twentieth Century, automobiles and airplanes were invented and flourished, and speeds as high as 200 mph were being accomplished by thrill seekers! In the 1940s, military aircraft were flying at 350 mph.

And then the jet engine was invented, and it became easy to fly at 500 mph, which is around how fast you have flown if you have ever flown in an airliner. Keep in mind that this was already more than 100 times faster than you can walk!

Chuck Yeager was the first human to survive a flight where an aircraft went faster than the speed of sound, at around 700 mph. Before he did that, most people thought it would forever be impossible for man to ever fly faster than the speed of sound! Once he did it, and survived, it wasn't long (less than 20 years) that many military aircraft were supersonic, flying faster than the speed of sound. Eventually, the Concorde supersonic airliner was crossing the Atlantic, and many people paid a lot to buy a ticket to fly at about twice the speed of sound, around 1400 mph.

This is probably about it, regarding how fast anyone on or near the Earth will ever travel. We certainly have the available technology to go faster, but the fuel consumption is incredibly high, and it is hard to carry enough fuel along for anything longer than a test flight, without making the aircraft too heavy to fly!

If and when we go out into space, higher speeds are possible. First, there is not the atmosphere where the huge drag of air resistance must constantly be overcome, just to keep a constant forward speed. Jet airliners already take many thousands of horsepower, just to not slow down due to air resistance! Also, in space, it is possible to fly without constantly needing to develop LIFT to keep the aircraft from falling straight down. More thousands of horsepower is needed for this in every airliner. This all explains why an airliner that is to fly from New York to Los Angeles, 3000 miles, must start out with around 10,000 gallons of jet fuel. There is some extra loaded, in case of an emergency, but at least 6,000 gallons would be used for that one trip. Where car drivers now complain about a gas mileage of 20 miles per gallon, such an airliner uses up two gallons per mile, or 0.5 miles per gallon!

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In space, things are really different. Once a spacecraft is headed to Saturn, for example, there is no air resistance to slow it down or to require extra fuel to be carried or used, and there is no large planet like the Earth to be pulling it down, so it pretty much can coast! It turns out that it makes sense to send such a spacecraft off with a pretty high initial speed, because it takes around 25,000 mph to be able to escape the gravity of the Earth (called escape velocity). (If we only want to sent a spacecraft into orbit, as with communication satellites and such, the final speed can be much less, just under 18,000 mph.

We could never accomplish such speeds on Earth. Even when we try to send small objects at very high speed, about the best we usually do is to send high power rifle bullets at around 2,000 mph, not even close to how fast satellites move in orbit around the Earth!

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We really have no way to get our brains to comprehend just how fast 2,000 mph or 18,000 mph actually is! We can see a jet airplane go by at 400 mph in an airshow, or maybe even hear one make a sonic boom at around 700 mph, and those speeds seem nearly impossible to grasp to our minds. Higher speeds almost might as well be INFINITY!

About 50 years ago, Isaac Azimov wrote an interesting book called One, Two, Three ... Infinity. He was commenting on some remote African tribes which had words for one, and for two and for three, but for ANY quantity greater than that, the simply had a word for many! From their point of view, four might as well be 7,000,000, because it was such a large number to be meaningless to them. When we confront some number that is too large for us to deal with, we call it infinity. Pretty much the same thing!

I guess I am saying that speeds above around the speed of sound, around 700 mph, might as well be infinity to us, as we cannot properly comprehend any differences in higher numbers. I am not sure that we humans are as super smart as we think we are! Almost anyone who has seen the night sky out far from a city where all the stars can be seen, invariably says that they see millions of stars. And, there is no one who would argue with them, because it certainly seems to be millions that are visible. But somebody actually counted them! (Actually, a lot of people have!) At any one time, on the clearest night possible, a person can see almost exactly 2,000 stars! But our brains choose to overestimate and believe it to be millions! Interesting!


We should note here that the speed of sound is NOT an absolute number! First, the speed of sound IN AIR depends on the air temperature. At 68°F, it is 1126 feet per second (768 mph). At even 59°F, it has reduced to 1116 f/s (761 mph). At 30,000 feet altitude, where commercial airliners usually fly, the air temperature is around -48°F and the speed of sound is down to 995 f/s (678 mph). So, referring to the speed of sound does not always mean the same thing, because you have to tell what temperature the air is!

It gets worse! The speed of sound depends on just what it is passing through! For example, if instead of air, it is Hydrogen gas, the speed of sound is nearly four times as fast, at 4,165 f/s (2840 mph)! In water, it is even faster, at 4,794 f/s (3,269 mph), in most types of wood, twice as fast as that, and in metals, even faster! In iron or steel, the speed of sound is around 15 times as fast as in air, 16,410 f/s (11,189 mph).

Kids, don't try this at home! But when I was a kid, a friend and I took hammers to a local railroad track. We found a section that was a solid piece, welded together without any expansion joints. WE CAREFULLY CHECKED FOR ANY ONCOMING TRAINS! We took turns hitting the rail with the hammer, with the other person about a block away. It was really cool! We would hear the sound through the track almost instantly when we saw him hit it, and then, half a second later, we heard the sound that came through the air! Science in action! (I understand that our Government has gotten much more red about many things, and that they now consider it to be some sort of Federal crime for anyone to even step onto a railroad right-of-way!)

The point of all this is that the term, the speed of sound, is not a very specific number, and in science, it is considered to be of little value, except for aircraft that have to fly really fast! Question: Since the speed of sound depends on the material it goes through, and science likes ABSOLUTE things, what is the speed of sound in a vacuum, where there is no material? Like on the Moon, where there is no air? The answer, of course, is ZERO, because sound cannot travel unless it has some material to pass through. It is actually just a mechanical vibration of a material, and without material, there can be no sound.


Speed of Light

Light is rather different than sound. Yes, it is a sort of vibration, but that vibration is called electromagnetic radiation, electricity and magnetism. No actual material is needed for light to travel through. So there actually IS a speed of light in a vacuum, and it has been measured really accurately. It is 983,600,000 f/s, or 186,300 miles/second, or 671,000,000 miles per hour, or 299,800 kilometers per second. Science likes numbers like this because they are absolute, they do not change, and we think they are true anywhere in the Universe.

But do you see the speed? Instead of about 1,000 feet per second, this is about a million times faster than that, and we have already noted that we have a hard time comprehending even the speed of sound with our brains!

You actually see examples of this all the time. When you watch a Fourth of July fireworks display, you SEE a bright flash of a bomb (almost instantly) but then it is more than half a second later that you hear the loud sound of it. Say you were about 1,000 feet away from where the bomb goes off. You can use the numbers above to figure out that the light from it would get to you about 0.000001 second later (almost instantly), but the sound would take about one full second to get to you.

Another common example, which is essentially the same, is lightning and thunder. Lightning is an incredibly powerful electrical discharge which creates a lot of heat and light in the process. The light is the flash of lightning, which travels to you almost instantly, at the speed of light. Again, you can calculate that for a lightning stroke a mile away from you. The intense heat of a lightning stroke causes the air along the stroke to immediately get super-hot, and it expands (according to the Ideal Gas Law). This sudden expansion causes a shock wave to occur in the air, which is what sound is! This is how thunder gets created. The important part here is that it is a sound wave, so it travels at the speed of sound. Since the speed of sound is around 1100 f/s and a mile is 5,280 feet, it takes the thunder roughly five seconds to travel a mile. This why we can see a lightning stroke and immediately start to count seconds to find how far away it was. Since the light got to us so quickly, we do not need to worry about that amount of time. But if we then hear thunder 10 seconds after a lightning stroke, it had occurred approximately two miles away from you.


With the speed of light being a million times faster than the speed of sound, which we have difficulty comprehending, the speed of light is really a meaningless number to us, except regarding scientific investigations. It is essentially the equivalent to infinity, in our normal experience!

However, within a scientific realm, there are some interesting consequences.

The Earth is around 93 million miles from the Sun. The light from the Sun travels at 186,000 miles per second. This means that the light takes around 500 seconds to get to Earth, around 8 minutes. If the Sun were suddenly go out, we wouldn't know it for about 8 minutes! This situation is vaguely related to Einstein's Theory of Relativity. Exactly WHEN did the Sun go out? When it ACTUALLY happened, or when we saw it happen? Well, each is true, depending on your point-of-view, being on the Sun or on the Earth. So the statement is RELATIVE to where you are and how you are examining things.

All of the stars are similar to our Sun, but millions of times farther away. When we would try to describe the distances in feet or miles or meters, the numbers are HUGE! And so a different "yardstick" is often used, the distance light would travel in a year, called a light-year. It is NOT an amount of time, but instead is a distance. Light only takes 8 MINUTES to get to us from the Sun, 93,000,000 miles away. Light from the nearest star we know about takes over 4 YEARS to get to us, and so we say its distance is 4.3 light years away. Sounds like a little number, but it describes an enormous distance.

If you hear anyone describing some amount of time in "light years", you could feel free to correct them by mentioning that it is a distance and not a time! But you might use tact in doing so!

Modern Physics believes that the Universe is around 15 billion years old. That is a conclusion based on an idea that light from the very most distant things that powerful telescopes can see appears to have been traveling for 15 billion years! And THAT is based on a couple of assumptions: (1) that the spectrum of very distant objects are shifted toward the red end (called a red-shift!) by an amount which is proportional to the speed that the object is moving away from us (a similar situation to the way the pitch of a train whistle changes as it goes by you) by a formula called the Doppler Shift, or, more correctly, the Relativistic Doppler Effect; and (2) that the speed versus red shift is consistent, a condition that scientists call linear.

These assumptions and conclusions give the speed (velocity) which the object appears to be moving away from us. Then another assumption is necessary, another formula that has a factor called the Hubble Constant, which converts such velocities into actual distances. Unfortunately, the Hubble Constant is not yet known very accurately, and so all values for the size of the Universe are pretty approximate.


This presentation was first placed on the Internet in August 2004.

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C Johnson, Theoretical Physicist, Physics Degree from Univ of Chicago