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Physics Question  
User currently offlineSeb146 From United States of America, joined Nov 1999, 11796 posts, RR: 15
Posted (7 years 2 months 1 week 16 hours ago) and read 1222 times:

My partner and I drove to the Bay area and back last week. I drove most of the way back at night. I was pondering something through the mountain passes: Scientists say nothing we have discovered is faster than the speed of light. If the speed of light is whatever it is and an airplane goes 600 miles per hour, does that mean the warning beacons and landing lights (when they are on) are actually going light speed plus 600MPH, or faster than the speed of light? Following that logic, light CAN go faster than the speed of light, right? Why not develop technology to harness that speed for spacecraft?


Life in the wall is a drag.
22 replies: All unread, jump to last
 
User currently offlineMolykote From United States of America, joined Aug 2005, 1343 posts, RR: 15
Reply 1, posted (7 years 2 months 1 week 16 hours ago) and read 1215 times:
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Quoting Seb146 (Thread starter):
My partner and I drove to the Bay area and back last week. I drove most of the way back at night. I was pondering something through the mountain passes: Scientists say nothing we have discovered is faster than the speed of light. If the speed of light is whatever it is and an airplane goes 600 miles per hour, does that mean the warning beacons and landing lights (when they are on) are actually going light speed plus 600MPH, or faster than the speed of light? Following that logic, light CAN go faster than the speed of light, right? Why not develop technology to harness that speed for spacecraft?

For a short answer to your "light + 600mph" question read up on basic relativity. Anything you read (or google) will likely branch out and cover your other questions and trains of thought as well.



Speedtape - The aspirin of aviation!
User currently offlineCastleIsland From , joined Dec 1969, posts, RR:
Reply 2, posted (7 years 2 months 1 week 16 hours ago) and read 1209 times:

Light itself is a wave. It is emitted at the speed of light at discrete locations that are distant from one another only in infinitesimally small increments. So, the light itself is not moving at 600 mph plus the speed of light, rather, it is emitted at the speed of light from a fixed position (i.e., velocity of the light source = 0) over and over again. These fixed positions are so close to one another, that their distance from one another cannot be measured on any simple real-world scale. Together, they create a seamless beam of light from an approaching source.

User currently offlineMDorBust From , joined Dec 1969, posts, RR:
Reply 3, posted (7 years 2 months 1 week 16 hours ago) and read 1209 times:

Incoming Doppler Shift discussion in 5...............4.......3...2.1

User currently offlineCastleIsland From , joined Dec 1969, posts, RR:
Reply 4, posted (7 years 2 months 1 week 16 hours ago) and read 1199 times:

Quoting MDorBust (Reply 3):
Incoming Doppler Shift discussion in 5...............4.......3...2.1

I'll bite.  Wink

Since the speed of light is a constant (in theory), any change in frequency must result in an equal and opposite (in magnitude) change in wavelength. Hence, the light that moves toward you and away from you at identical velocities appears more blue when approaching, and more red when receeding. This effect is more readily heard than seen by the human senses, although stars can provide some nice Doppler Effect examples.


User currently offlineSeb146 From United States of America, joined Nov 1999, 11796 posts, RR: 15
Reply 5, posted (7 years 2 months 1 week 16 hours ago) and read 1197 times:

Quoting CastleIsland (Reply 2):
So, the light itself is not moving at 600 mph plus the speed of light, rather, it is emitted at the speed of light from a fixed position

But the "fixed position" is moving. Let's say an F-15 is moving at 600MPH. If the bulb or diode on that F-15 emitting the light is moving at 600MPH and the peak wave of the light pulse is being released from the bulb or diode at light speed, with the "fixed position" also moving, the wave would be moving at light speed+600MPH? Scientificaly speaking, the difference does not make much difference. I know that sounds strange. Why is light measured in a different fashion than objects? If science can "clock" light at 670,616,629MPH and can "clock" an airplane at 600MPH what is the difference? Why would science not work towards building a space craft capable of moving faster than light using that theory?

Molykote: I am looking relativity right now. Thank you.



Life in the wall is a drag.
User currently offlineCastleIsland From , joined Dec 1969, posts, RR:
Reply 6, posted (7 years 2 months 1 week 16 hours ago) and read 1191 times:

Quoting Seb146 (Reply 5):
But the "fixed position" is moving.

It is moving on a macroscopic scale, as it were. The light is emitted so often, at intervals that are so close to one another that, for all intents and purposes, the light source is NOT MOVING at the instant the light is emitted.


User currently offlineTRVYYZ From Canada, joined Oct 2004, 1375 posts, RR: 10
Reply 7, posted (7 years 2 months 1 week 16 hours ago) and read 1190 times:

Speed of light remains the same, but only the distance of that light is changing at 600 miles/hr. one way to look at it.
You could use the doppler eqn for precise calculation, similar to sound, i guess  Smile


User currently offlineMolykote From United States of America, joined Aug 2005, 1343 posts, RR: 15
Reply 8, posted (7 years 2 months 1 week 16 hours ago) and read 1185 times:
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Quoting Seb146 (Reply 5):
Molykote: I am looking relativity right now. Thank you.

You're welcome.

This link might be a good one for you:
http://science.howstuffworks.com/relativity7.htm

I've been awake for about 19 hours and I have to get some sleep before I do another 12. I'll see where this thread ends up later. Enjoy the reading.



Speedtape - The aspirin of aviation!
User currently offlineSeb146 From United States of America, joined Nov 1999, 11796 posts, RR: 15
Reply 9, posted (7 years 2 months 1 week 15 hours ago) and read 1182 times:

Quoting TRVYYZ (Reply 7):
Speed of light remains the same, but only the distance of that light is changing at 600 miles/hr.

Okay, two things: 1. Composition of velocities to me says if a rocket traveling at 2/3 the speed of light fires a missle, the missle would be traveling at 12/13 the speed of light. So, wouldn't the missile have gained speed firing from the rocket?

2. From reading, I thought the speed of light was constant only in a vacuum, and therefore, would render this entire thread moot, since the only real vaccums we have to test any of this are in labs.



Life in the wall is a drag.
User currently offlineMolykote From United States of America, joined Aug 2005, 1343 posts, RR: 15
Reply 10, posted (7 years 2 months 1 week 15 hours ago) and read 1177 times:
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Before this thread gets too long I'll make one more contribution.

If you're looking for a great book to explain a basic understanding of relativity and a few other scientific theories, buy this (it's only a few dollars):
http://search.barnesandnoble.com/boo...isbninquiry.asp?r=1&ean=0880292512

This book uses well written text to explain basic scientific concepts as well as nearly any lecture I've attended. It's easy and enjoyable to read as well.



Speedtape - The aspirin of aviation!
User currently offlineCastleIsland From , joined Dec 1969, posts, RR:
Reply 11, posted (7 years 2 months 1 week 15 hours ago) and read 1173 times:

Quoting Seb146 (Reply 5):
Why would science not work towards building a space craft capable of moving faster than light using that theory?

Because the theory is flawed. The flaw is, you are adding 600 mph to the speed of light, which cannot be done. The light is emitted at the speed of light from an effectively STATIONARY POINT. The reason the source is effectively stationary is because the light is being emitted so rapidly (i.e., at intervals that are so small) as to dwarf the actual velocity of the light source.


User currently offlineTRVYYZ From Canada, joined Oct 2004, 1375 posts, RR: 10
Reply 12, posted (7 years 2 months 1 week 15 hours ago) and read 1167 times:

Quoting Seb146 (Reply 9):
Composition of velocities to me says if a rocket traveling at 2/3 the speed of light fires a missle, the missle would be traveling at 12/13 the speed of light. So, wouldn't the missile have gained speed firing from the rocket?

I'm not an expert in mechanics, but with whatever experience I have and intutively guessing, I would say, Yes

Now the real question is what is stopping the rocket from achieving the speed of light?
Didn't think this deep, but you may have a point but at the same time I don't think the rocket and airplane examples are the same, since the velocity of the plane doesn't add up to the light source.

[Edited 2007-10-15 11:11:58]

User currently offlineCastleIsland From , joined Dec 1969, posts, RR:
Reply 13, posted (7 years 2 months 1 week 15 hours ago) and read 1163 times:

Quoting Seb146 (Reply 9):
I thought the speed of light was constant only in a vacuum, and therefore, would render this entire thread moot

It is correct that the speed of light is only constant in a vacuum, but that does not at all render this thread moot. Science cannot really deal with being exact; it relies on models that approximate reality. In some cases, these approximations are so accurate that they can be considered nearly perfect. In other words, we know we are not getting EXACTLY the right answer, but if we are only off by, say, 0.000000000001%, is that not good enough?

The answer is: in most cases, it is good enough, but not in all cases.

For example, the ideal gas law, pV = nRT is, as it suggests, for an ideal gas. There is no such thing, but the ideal gas law is still used as the basis for all sorts of engineering calculations that give perfectly legitimate results. An uncertainty analysis should always be done to approximate how far we are from the "correct" answer, and what effect that error has on our work.

Edited to add: The only way this "thread would be rendered moot" is if the difference between the speed of light in a vacuum and [600 mph + the speed of light in a vacuum] is less than the difference between the speed of light in a vacumm and the actual speed of light as observed outside of a vacuum. If the former is less than the latter, then our tolerance limits are based on the difference between the speed of light as considered constant and the speed of light that occurs in "the real world," and your 600 mph difference would be smaller than the error introduced by our model (i.e., vacuum vs. the real world). I doubt this is the case, however, so the thread is "unlikely to be rendered moot."   

[Edited 2007-10-15 11:12:50]

User currently offlineD L X From United States of America, joined May 1999, 11569 posts, RR: 52
Reply 14, posted (7 years 2 months 1 week 15 hours ago) and read 1160 times:

No need for Doppler (although it is freakishly cool stuff).

Think of it this way: you're travelling around the world (for say, 80 days), and at regular intervals (say each hour at :35 past) you send a postcard to your friends back home. After a few hundred, you're so efficient, that you manage to send each postcard without losing a step. Over the course of 80 days, you eventually get to your destination, having traveled at some speed. How fast are the postcards going? (Hint: do you have enough information to answer that question?)

The point is, the speed of the postcards is not sped up because you happened to be moving when you sent them. The postcards travel at the speed that postcards travel at. Same for light -- same idea, different medium. The speed that light travels does not increase because the object emitting the light was moving. Light travels at its own speed regardless of how fast the object that emitted the light was moving.

Quoting Seb146 (Reply 9):
I thought the speed of light was constant only in a vacuum

Not quite: the speed of light is only 3 x 10^8 m/s in a vacuum, but it is constant in other media also -- just slower. I think it's constant at something like 2 x 10^8 m/s in water. (It's just like how the speed of sound is different based on the material the sound is going through. It is much faster in solids than in liquids, and much faster in liquids than it is in air.)

There's a really cool effect completely analogous to a sonic boom that you see in nuclear reactors that are insulated by water. Particles from the reactor leave the reaction at speeds near 3e8 m/s in the reactor, then hit the water at faster than light travels through water. So, what happens? the light waves back up on each other and build into a mega wave, just like a sonic boom. Just like you can hear (and feel) a sonic boom, you can see this "light boom" as a glowing green-blue light.

This is why reactors glow.   Ain't that awesome?

[Edited 2007-10-15 11:11:15]


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User currently offlineSeb146 From United States of America, joined Nov 1999, 11796 posts, RR: 15
Reply 15, posted (7 years 2 months 6 days 17 hours ago) and read 1048 times:

Quoting Molykote (Reply 8):
This link might be a good one for you:
http://science.howstuffworks.com/rel...7.htm

Interesting site. The section on Doppler Effect was easy for me to understand. I never got that before, but I like the example they use. Thanks!

So, here is another question: If an object is traveling at 600,000MPH and turns on a light, wouldn't the light relative to the person inside the object be traveling at 670,616,629 MPH PLUS 600,000MPH while on the ground, the light would still be traveling at 670,616,629MPH? So why not harness the energy emitted from light to help boost space craft speeds?



Life in the wall is a drag.
User currently offlineManuCH From Switzerland, joined Jun 2005, 3012 posts, RR: 46
Reply 16, posted (7 years 2 months 6 days 16 hours ago) and read 1042 times:
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Quoting Seb146 (Reply 15):
So, here is another question: If an object is traveling at 600,000MPH and turns on a light, wouldn't the light relative to the person inside the object be traveling at 670,616,629 MPH PLUS 600,000MPH while on the ground, the light would still be traveling at 670,616,629MPH?

No, because...

Quoting D L X (Reply 14):
The speed that light travels does not increase because the object emitting the light was moving. Light travels at its own speed regardless of how fast the object that emitted the light was moving.



Never trust a statistic you didn't fake yourself
User currently offlineSeb146 From United States of America, joined Nov 1999, 11796 posts, RR: 15
Reply 17, posted (7 years 2 months 6 days 16 hours ago) and read 1023 times:

Quoting ManuCH (Reply 16):
No, because...

Quoting D L X (Reply 14):
The speed that light travels does not increase because the object emitting the light was moving. Light travels at its own speed regardless of how fast the object that emitted the light was moving.

That does not make sense to me. I understand the actual frequency of the light is a constant, but if we put light on a larger scale using the rocket/missile example, why does that not work for light? Just to keep numbers simple, if a rocket were traveling at 600,000MPH and shot a missile at 670,000,000MPH, then the missile relative to the rocket would be traveling at 670,600,000MPH because the rocket is alread traveling at 600,000MPH regardless of what we see. It should be the same for light regardless of the frequency of light, right?



Life in the wall is a drag.
User currently offlineCastleIsland From , joined Dec 1969, posts, RR:
Reply 18, posted (7 years 2 months 6 days 16 hours ago) and read 1020 times:

Quoting Seb146 (Reply 17):
actual frequency of the light is a constant

No, the speed, or velocity, of light is a constant.

Quoting Seb146 (Reply 17):
but if we put light on a larger scale using the rocket/missile example, why does that not work for light?

Because light is a wave, not a particle.


User currently offlineSprout5199 From United States of America, joined Feb 2005, 1855 posts, RR: 2
Reply 19, posted (7 years 2 months 6 days 16 hours ago) and read 1016 times:

Quoting Seb146 (Thread starter):
If the speed of light is whatever it is and an airplane goes 600 miles per hour, does that mean the warning beacons and landing lights (when they are on) are actually going light speed plus 600MPH, or faster than the speed of light?

Only if it is on a conveyor belt going the same speed.(just had to throw that in)

And what about the white aft light? is it going 600 mph slower?

Dan in Jupiter


User currently offlineManuCH From Switzerland, joined Jun 2005, 3012 posts, RR: 46
Reply 20, posted (7 years 2 months 6 days 16 hours ago) and read 1009 times:
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Quoting Seb146 (Reply 17):
That does not make sense to me. I understand the actual frequency of the light is a constant, but if we put light on a larger scale using the rocket/missile example, why does that not work for light?

As CastleIsland pointed out, light is a wave. The theory of throwing stuff doesn't work with waves, only with particles. You don't "throw" light, you only "emit" it, and moving while emitting a wave doesn't change its speed.

Another example just came to my mind, but I'm not sure it's exactly the same thing: take an airplane, and splash it into the sea while travelling at 600 mph, at an angle almost parallel to the water surface. The water waves probably won't travel at 600 mph ... or will they?  Confused  Wink



Never trust a statistic you didn't fake yourself
User currently offlineD L X From United States of America, joined May 1999, 11569 posts, RR: 52
Reply 21, posted (7 years 2 months 6 days 13 hours ago) and read 978 times:

Quoting Seb146 (Reply 17):
That does not make sense to me. I understand the actual frequency of the light is a constant, but if we put light on a larger scale using the rocket/missile example, why does that not work for light?

Well, the biggest reason it doesn't make sense to you is that you (nor I) have ever traveled anywhere near the speed of light. In fact, the *frequency* of the light is what is NOT constant. But just to help you out, you have to think waves, not objects.

Go to an airport, or especially an airshow with high performance jets. If you were standing on a spot, and a plane was approaching you at high speed, think carefully: when would you HEAR the plane? Do you hear the plane sooner when the plane is moving fast versus moving slow or standing still? The answer is no.

Take a still plane. If the plane is 1000 feet away, you hear the plane about 1 second after it makes a noise. This is because the speed of sound is about 1000 feet per second through air on the earth's surface. Now, if the plane is moving at near the speed of sound, let's say 900 feet per second at an airshow, does the speed of sound increase to 1900 feet per second? If it did, you would hear the sound the plane made when it was 1000 feet away in about HALF a second, while the plane wouldn't fly over you until just over a second after it was 1000 feet away. But that's not what actually happens, right? Instead, when a high performance plane flying 900 feet per second flies over your head, you hear it about a TENTH of a second before it flies over. Why? Because the sound is just barely outpacing the plane. The sound is going 1000 feet per second, while the plane is going 900. In fact, you can fairly accurately tell how fast a plane is flying towards you by timing how long it takes from your being able to hear the plane until the plane flies over your head. (A plane taking off is going around 200 feet per second. It should make sense to you now that if you were standing off the end of the runway, you'll hear its sound about 5 seconds before it flies over your head.) Also think about it: can you hear a plane travelling faster than the speed of sound? Not until after it passes you.

Why is this so? Because sound is a wave, not a particle. No matter how fast the plane is going, its sound goes 1000 feet per second (in air near the earth's surface). The exact same thing is true for light. No matter how fast the object that emits light is going, the light goes the same speed (in a vacuum).

[Edited 2007-10-16 13:08:02]


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User currently offlineMakeMinesLAX From United States of America, joined Dec 2003, 566 posts, RR: 0
Reply 22, posted (7 years 2 months 5 days 13 hours ago) and read 908 times:

A number of you have hit the nail on the head, but to simplify:

For sound and light, speed is constant for the given conditions (density/uniformity of medium, etc.). If the emission source is moving, it is the frequency which changes.

The Doppler effect, as it relates to sound emanating from moving objects, is the correct analog for your light-related example. The red-shift exhibited by receding stars is the best example involving light.

To sum it up in an obvious way, sound never travels faster than the speed of sound (same for light).


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