[Script Info]
Title: 
[Events]
Format: Layer, Start, End, Style, Name, MarginL, MarginR, MarginV, Effect, Text
Dialogue: 0,0:00:06.62,0:00:10.51,Default,,0000,0000,0000,,Humans have been fascinated\Nwith speed for ages.
Dialogue: 0,0:00:10.51,0:00:14.75,Default,,0000,0000,0000,,The history of human progress\Nis one of ever-increasing velocity,
Dialogue: 0,0:00:14.75,0:00:18.61,Default,,0000,0000,0000,,and one of the most important achievements\Nin this historical race
Dialogue: 0,0:00:18.61,0:00:21.50,Default,,0000,0000,0000,,was the breaking of the sound barrier.
Dialogue: 0,0:00:21.50,0:00:24.87,Default,,0000,0000,0000,,Not long after the first\Nsuccessful airplane flights,
Dialogue: 0,0:00:24.87,0:00:29.98,Default,,0000,0000,0000,,pilots were eager to push\Ntheir planes to go faster and faster.
Dialogue: 0,0:00:29.98,0:00:32.38,Default,,0000,0000,0000,,But as they did so, increased turbulence
Dialogue: 0,0:00:32.38,0:00:37.69,Default,,0000,0000,0000,,and large forces on the plane\Nprevented them from accelerating further.
Dialogue: 0,0:00:37.69,0:00:41.65,Default,,0000,0000,0000,,Some tried to circumvent\Nthe problem through risky dives,
Dialogue: 0,0:00:41.65,0:00:44.08,Default,,0000,0000,0000,,often with tragic results.
Dialogue: 0,0:00:44.08,0:00:47.55,Default,,0000,0000,0000,,Finally, in 1947, design improvements,
Dialogue: 0,0:00:47.55,0:00:52.30,Default,,0000,0000,0000,,such as a movable horizontal stabilizer,\Nthe all-moving tail,
Dialogue: 0,0:00:52.30,0:00:55.52,Default,,0000,0000,0000,,allowed an American military pilot\Nnamed Chuck Yeager
Dialogue: 0,0:00:55.52,0:01:03.72,Default,,0000,0000,0000,,to fly the Bell X-1 aircraft at 1127 km/h,
Dialogue: 0,0:01:03.72,0:01:06.92,Default,,0000,0000,0000,,becoming the first person\Nto break the sound barrier
Dialogue: 0,0:01:06.92,0:01:09.72,Default,,0000,0000,0000,,and travel faster than the speed of sound.
Dialogue: 0,0:01:09.72,0:01:13.93,Default,,0000,0000,0000,,The Bell X-1 was the first of many\Nsupersonic aircraft to follow,
Dialogue: 0,0:01:13.93,0:01:17.91,Default,,0000,0000,0000,,with later designs reaching\Nspeeds over Mach 3.
Dialogue: 0,0:01:17.91,0:01:21.57,Default,,0000,0000,0000,,Aircraft traveling at supersonic speed\Ncreate a shock wave
Dialogue: 0,0:01:21.57,0:01:25.68,Default,,0000,0000,0000,,with a thunder-like noise\Nknown as a sonic boom,
Dialogue: 0,0:01:25.68,0:01:29.18,Default,,0000,0000,0000,,which can cause distress to people\Nand animals below
Dialogue: 0,0:01:29.18,0:01:31.07,Default,,0000,0000,0000,,or even damage buildings.
Dialogue: 0,0:01:31.07,0:01:32.07,Default,,0000,0000,0000,,For this reason,
Dialogue: 0,0:01:32.11,0:01:35.34,Default,,0000,0000,0000,,scientists around the world\Nhave been looking at sonic booms,
Dialogue: 0,0:01:35.34,0:01:37.79,Default,,0000,0000,0000,,trying to predict their path\Nin the atmosphere,
Dialogue: 0,0:01:37.79,0:01:42.19,Default,,0000,0000,0000,,where they will land,\Nand how loud they will be.
Dialogue: 0,0:01:42.19,0:01:45.31,Default,,0000,0000,0000,,To better understand\Nhow scientists study sonic booms,
Dialogue: 0,0:01:45.31,0:01:48.30,Default,,0000,0000,0000,,let's start with some basics of sound.
Dialogue: 0,0:01:48.30,0:01:51.93,Default,,0000,0000,0000,,Imagine throwing a small stone\Nin a still pond.
Dialogue: 0,0:01:51.93,0:01:53.18,Default,,0000,0000,0000,,What do you see?
Dialogue: 0,0:01:53.18,0:01:55.88,Default,,0000,0000,0000,,The stone causes waves\Nto travel in the water
Dialogue: 0,0:01:55.88,0:01:58.67,Default,,0000,0000,0000,,at the same speed in every direction.
Dialogue: 0,0:01:58.67,0:02:02.89,Default,,0000,0000,0000,,These circles that keep growing in radius\Nare called wave fronts.
Dialogue: 0,0:02:02.89,0:02:05.90,Default,,0000,0000,0000,,Similarly, even though we cannot see it,
Dialogue: 0,0:02:05.90,0:02:09.31,Default,,0000,0000,0000,,a stationary sound source,\Nlike a home stereo,
Dialogue: 0,0:02:09.31,0:02:12.20,Default,,0000,0000,0000,,creates sound waves traveling outward.
Dialogue: 0,0:02:12.20,0:02:14.33,Default,,0000,0000,0000,,The speed of the waves depends on factors
Dialogue: 0,0:02:14.33,0:02:18.11,Default,,0000,0000,0000,,like the altitude and temperature\Nof the air they move through.
Dialogue: 0,0:02:18.11,0:02:24.46,Default,,0000,0000,0000,,At sea level, sound travels\Nat about 1225 km/h.
Dialogue: 0,0:02:24.46,0:02:27.29,Default,,0000,0000,0000,,But instead of circles\Non a two-dimensional surface,
Dialogue: 0,0:02:27.29,0:02:30.73,Default,,0000,0000,0000,,the wave fronts\Nare now concentric spheres,
Dialogue: 0,0:02:30.73,0:02:35.90,Default,,0000,0000,0000,,with the sound traveling along rays\Nperpendicular to these waves.
Dialogue: 0,0:02:35.90,0:02:40.08,Default,,0000,0000,0000,,Now imagine a moving sound source,\Nsuch as a train whistle.
Dialogue: 0,0:02:40.08,0:02:43.03,Default,,0000,0000,0000,,As the source keeps moving\Nin a certain direction,
Dialogue: 0,0:02:43.03,0:02:47.57,Default,,0000,0000,0000,,the successive waves in front of it\Nwill become bunched closer together.
Dialogue: 0,0:02:47.57,0:02:52.64,Default,,0000,0000,0000,,This greater wave frequency is the cause\Nof the famous Doppler effect,
Dialogue: 0,0:02:52.64,0:02:55.73,Default,,0000,0000,0000,,where approaching objects\Nsound higher pitched.
Dialogue: 0,0:02:55.73,0:02:59.93,Default,,0000,0000,0000,,But as long as the source is moving\Nslower than the sound waves themselves,
Dialogue: 0,0:02:59.93,0:03:02.76,Default,,0000,0000,0000,,they will remain nested within each other.
Dialogue: 0,0:03:02.76,0:03:07.77,Default,,0000,0000,0000,,It's when an object goes supersonic,\Nmoving faster than the sound it makes,
Dialogue: 0,0:03:07.77,0:03:10.60,Default,,0000,0000,0000,,that the picture changes dramatically.
Dialogue: 0,0:03:10.60,0:03:13.20,Default,,0000,0000,0000,,As it overtakes sound waves\Nit has emitted,
Dialogue: 0,0:03:13.20,0:03:15.70,Default,,0000,0000,0000,,while generating new ones from\Nits current position,
Dialogue: 0,0:03:15.70,0:03:19.82,Default,,0000,0000,0000,,the waves are forced together,\Nforming a Mach cone.
Dialogue: 0,0:03:19.82,0:03:22.81,Default,,0000,0000,0000,,No sound is heard\Nas it approaches an observer
Dialogue: 0,0:03:22.81,0:03:27.89,Default,,0000,0000,0000,,because the object is traveling faster\Nthan the sound it produces.
Dialogue: 0,0:03:27.89,0:03:33.05,Default,,0000,0000,0000,,Only after the object has passed\Nwill the observer hear the sonic boom.
Dialogue: 0,0:03:33.05,0:03:37.01,Default,,0000,0000,0000,,Where the Mach cone meets the ground,\Nit forms a hyperbola,
Dialogue: 0,0:03:37.01,0:03:41.31,Default,,0000,0000,0000,,leaving a trail known as the boom carpet\Nas it travels forward.
Dialogue: 0,0:03:41.31,0:03:46.25,Default,,0000,0000,0000,,This makes it possible to determine\Nthe area affected by a sonic boom.
Dialogue: 0,0:03:46.25,0:03:49.30,Default,,0000,0000,0000,,What about figuring out how strong\Na sonic boom will be?
Dialogue: 0,0:03:49.30,0:03:52.87,Default,,0000,0000,0000,,This involves solving the famous\NNavier-Stokes equations
Dialogue: 0,0:03:52.87,0:03:56.26,Default,,0000,0000,0000,,to find the variation\Nof pressure in the air
Dialogue: 0,0:03:56.26,0:03:59.52,Default,,0000,0000,0000,,due to the supersonic aircraft\Nflying through it.
Dialogue: 0,0:03:59.52,0:04:03.85,Default,,0000,0000,0000,,This results in the pressure signature\Nknown as the N-wave.
Dialogue: 0,0:04:03.85,0:04:05.48,Default,,0000,0000,0000,,What does this shape mean?
Dialogue: 0,0:04:05.48,0:04:09.51,Default,,0000,0000,0000,,Well, the sonic boom occurs\Nwhen there is a sudden change in pressure,
Dialogue: 0,0:04:09.51,0:04:11.92,Default,,0000,0000,0000,,and the N-wave involves two booms:
Dialogue: 0,0:04:11.92,0:04:15.50,Default,,0000,0000,0000,,one for the initial pressure rise\Nat the aircraft's nose,
Dialogue: 0,0:04:15.50,0:04:18.35,Default,,0000,0000,0000,,and another for when the tail passes,
Dialogue: 0,0:04:18.35,0:04:21.02,Default,,0000,0000,0000,,and the pressure suddenly\Nreturns to normal.
Dialogue: 0,0:04:21.02,0:04:23.13,Default,,0000,0000,0000,,This causes a double boom,
Dialogue: 0,0:04:23.13,0:04:26.64,Default,,0000,0000,0000,,but it is usually heard as a single boom\Nby human ears.
Dialogue: 0,0:04:26.64,0:04:29.88,Default,,0000,0000,0000,,In practice, computer models\Nusing these principles
Dialogue: 0,0:04:29.88,0:04:34.02,Default,,0000,0000,0000,,can often predict the location\Nand intensity of sonic booms
Dialogue: 0,0:04:34.02,0:04:37.63,Default,,0000,0000,0000,,for given atmospheric conditions\Nand flight trajectories,
Dialogue: 0,0:04:37.63,0:04:40.74,Default,,0000,0000,0000,,and there is ongoing research\Nto mitigate their effects.
Dialogue: 0,0:04:40.74,0:04:45.81,Default,,0000,0000,0000,,In the meantime, supersonic flight\Nover land remains prohibited.
Dialogue: 0,0:04:45.81,0:04:48.57,Default,,0000,0000,0000,,So, are sonic booms a recent creation?
Dialogue: 0,0:04:48.57,0:04:50.09,Default,,0000,0000,0000,,Not exactly.
Dialogue: 0,0:04:50.09,0:04:52.52,Default,,0000,0000,0000,,While we try to find ways to silence them,
Dialogue: 0,0:04:52.52,0:04:56.04,Default,,0000,0000,0000,,a few other animals have been\Nusing sonic booms to their advantage.
Dialogue: 0,0:04:56.04,0:05:00.95,Default,,0000,0000,0000,,The gigantic Diplodocus may have been\Ncapable of cracking its tail
Dialogue: 0,0:05:00.100,0:05:07.94,Default,,0000,0000,0000,,faster than sound, at over 1200 km/h,\Npossibly to deter predators.
Dialogue: 0,0:05:07.94,0:05:12.44,Default,,0000,0000,0000,,Some types of shrimp can also create\Na similar shock wave underwater,
Dialogue: 0,0:05:12.44,0:05:16.16,Default,,0000,0000,0000,,stunning or even killing pray\Nat a distance
Dialogue: 0,0:05:16.16,0:05:19.73,Default,,0000,0000,0000,,with just a snap of their oversized claw.
Dialogue: 0,0:05:19.73,0:05:22.20,Default,,0000,0000,0000,,So while we humans\Nhave made great progress
Dialogue: 0,0:05:22.20,0:05:24.85,Default,,0000,0000,0000,,in our relentless pursuit of speed,
Dialogue: 0,0:05:24.85,0:05:27.41,Default,,0000,0000,0000,,it turns out that nature was there first.