Car driving at the speed of light.......
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Car driving at the speed of light.......
Forgive me for asking but can someone help me prove somebody wrong.....
IMAGINE a car was travelling at the speed of light and it was dark. If the driver turned his headlights on would they light the way in front of him or can light not travel any faster than the speed of light?!
Similarly, if you were driving the same car at the speed a bullet leaves a gun and fired a gun forwards, would the bullet leave the barrel or would it dribble out?
IMAGINE a car was travelling at the speed of light and it was dark. If the driver turned his headlights on would they light the way in front of him or can light not travel any faster than the speed of light?!
Similarly, if you were driving the same car at the speed a bullet leaves a gun and fired a gun forwards, would the bullet leave the barrel or would it dribble out?
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Monocock
As an aircraft moves through the air it propagates a pressure wave outward in all directions. This pressure wave moves away from the aircraft at the speed of sound and if you are standing beneath the aircraft it is this wave passing over you that enables you to hear the aircraft. As the aircraft approaches the speed of sound the wave makes less progress forward relative to the movement of the aircraft and the waves begin to coalesce. At the speed of sound the waves make no forward progress and all lump together in a shockwave. In reality they make a little progress forwards as the lumping together increases the local air temperature and the speed of sound gets faster with increasing temperature but this is negligible for your interests. This is what causes the “sonic boom”. And also why you get less warning of the approach of a high speed aircraft passing over you at low level. Applying the same principle to light I would be reasonably confident in suggesting that at the speed of light the light from your headlights will make no forward progress, and that your path would not be illuminated (that said I am no physicist).
If you posted this on tech log you would probably attract the interest of some serious propeller heads (and get lots of answers that you might not understand). But might get more of a take up than it does here.
As an aircraft moves through the air it propagates a pressure wave outward in all directions. This pressure wave moves away from the aircraft at the speed of sound and if you are standing beneath the aircraft it is this wave passing over you that enables you to hear the aircraft. As the aircraft approaches the speed of sound the wave makes less progress forward relative to the movement of the aircraft and the waves begin to coalesce. At the speed of sound the waves make no forward progress and all lump together in a shockwave. In reality they make a little progress forwards as the lumping together increases the local air temperature and the speed of sound gets faster with increasing temperature but this is negligible for your interests. This is what causes the “sonic boom”. And also why you get less warning of the approach of a high speed aircraft passing over you at low level. Applying the same principle to light I would be reasonably confident in suggesting that at the speed of light the light from your headlights will make no forward progress, and that your path would not be illuminated (that said I am no physicist).
If you posted this on tech log you would probably attract the interest of some serious propeller heads (and get lots of answers that you might not understand). But might get more of a take up than it does here.
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If I recall this was all covered in full in the HP&L section of the PPL exams. As I understood it at the time, both cases have the same issue at their heart, relative speed. The car going at the speed of light would initially see where it was going, but the light/space friction co efficient, or drag, means that within a period of time as the light lost energy and the car continued forward, there would come a point at which the car would have travelled further than the light. Effectively it would pass its own light ends, leaving them behind.
The bullet would be the same but the friction or drag would be greater as the mass of the bullet will be more than the mass of a photon. Initially the bullet would leave the gun, well, like a speeding bullet, but would lose energy quickly, whilst the car continued to speed along. Eventually the bullet would crash through the car window. This is exactly why the POH of all supersonic fighters require bullets to be fired at an airspeed slower than the muzzle velocity of the guns, or, for an immediate change of direction after firing. In other words a change in velocity.
A similar analogy which is easier to comprehend is the old adage, never pee into a head wind. Same thing really.
I hope that is clear.
The bullet would be the same but the friction or drag would be greater as the mass of the bullet will be more than the mass of a photon. Initially the bullet would leave the gun, well, like a speeding bullet, but would lose energy quickly, whilst the car continued to speed along. Eventually the bullet would crash through the car window. This is exactly why the POH of all supersonic fighters require bullets to be fired at an airspeed slower than the muzzle velocity of the guns, or, for an immediate change of direction after firing. In other words a change in velocity.
A similar analogy which is easier to comprehend is the old adage, never pee into a head wind. Same thing really.
I hope that is clear.
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Headlights: no, they wouldn't light the way ahead.
More alarming is the fact that anyone in front of the car would see the headlights at the exact same instant the car ran them over
More alarming is the fact that anyone in front of the car would see the headlights at the exact same instant the car ran them over
Last edited by rustle; 7th Feb 2003 at 14:35.
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First of all, I'm not certain on this, if I'm wrong I'll quite happilly bow to greater knowledge.
You've asked two seperate questions - one about the speed of light, and one about the speed of a bullet. And Rote 8 has added the speed of sound into the equation. Here's the way I understand it.
The speed of light is measured relative to space. If you are moving through space at the speed of light, and you turn your headlights on, then the light from your headlights would move through space at the speed of light too - so they would not light the way in front of you.
The speed of sound is measured relative to the medium the sound is moving in, usually air. Mach 1 is defined as that speed where your TAS (i.e. your speed relative to the air) is equal to the local speed of sound. So if you are travelling at Mach 1, the sound you produce will move at the same speed as you, and therefore will not propogate in front of you. However, notice the subtle difference between this and the speed of light - the speed of light is relative to space, whereas the speed of sound is relative to the air. Thus, with a strong tail-wind, an aircraft at Mach 1, and its sound-wave, will have a higher ground-speed than in still air (even if the temperature, and therefore the local speed of sound, were the same).
The bullet is different yet again, because the speed of a bullet is relative to the gun. So if you move your gun at the speed of a bullet, then fire it, I believe the bullet would be moving at twice the speed of a bullet. (Although, as Ludwig points out, it would slow down fairly quickly.)
I have no idea what this has to do with Private Flying, and I'm sure BRL will move it to somewhere where more educated minds can tell me I'm wrong! (Not that us private pilots aren't an educated bunch, of course!)
FFF
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You've asked two seperate questions - one about the speed of light, and one about the speed of a bullet. And Rote 8 has added the speed of sound into the equation. Here's the way I understand it.
The speed of light is measured relative to space. If you are moving through space at the speed of light, and you turn your headlights on, then the light from your headlights would move through space at the speed of light too - so they would not light the way in front of you.
The speed of sound is measured relative to the medium the sound is moving in, usually air. Mach 1 is defined as that speed where your TAS (i.e. your speed relative to the air) is equal to the local speed of sound. So if you are travelling at Mach 1, the sound you produce will move at the same speed as you, and therefore will not propogate in front of you. However, notice the subtle difference between this and the speed of light - the speed of light is relative to space, whereas the speed of sound is relative to the air. Thus, with a strong tail-wind, an aircraft at Mach 1, and its sound-wave, will have a higher ground-speed than in still air (even if the temperature, and therefore the local speed of sound, were the same).
The bullet is different yet again, because the speed of a bullet is relative to the gun. So if you move your gun at the speed of a bullet, then fire it, I believe the bullet would be moving at twice the speed of a bullet. (Although, as Ludwig points out, it would slow down fairly quickly.)
I have no idea what this has to do with Private Flying, and I'm sure BRL will move it to somewhere where more educated minds can tell me I'm wrong! (Not that us private pilots aren't an educated bunch, of course!)
FFF
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Yay, a distraction from babysitting my lab 
FFF, if you turned on the lights (forgetting of course that at that moment, having a non-zero rest mass, you and your car would have to have an infinite amount of kinetic energy) they would light the way ahead of you because the speed of light is a constant everywhere - that's the headwrecker. You, in the car, would measure the speed of light emitted from your headlamps at c - and so would an observer sitting on a planet as you pass by, even though you're travelling at c yourself....
Of course, while the headlights would light the way, they'd be doing so with gamma rays since as the light source approaches relativistic velocities, the light increases in frequency (the "red shift" and "blue shift" you hear so much about in astronomy), sort of like a doppler shift for light waves.
Ah, that was fun
FFF, if you turned on the lights (forgetting of course that at that moment, having a non-zero rest mass, you and your car would have to have an infinite amount of kinetic energy) they would light the way ahead of you because the speed of light is a constant everywhere - that's the headwrecker. You, in the car, would measure the speed of light emitted from your headlamps at c - and so would an observer sitting on a planet as you pass by, even though you're travelling at c yourself....
Of course, while the headlights would light the way, they'd be doing so with gamma rays since as the light source approaches relativistic velocities, the light increases in frequency (the "red shift" and "blue shift" you hear so much about in astronomy), sort of like a doppler shift for light waves.
Ah, that was fun
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I could be completely wrong cos it's a long time since I studied this but....
First of all I should mention that it's illegal to drive a car at the speed of light, even on the autobahns in Germany.
Secondly it's impossible to travel at the speed of light because your mass approaches infinity as you approach the speed of light so your own gravity makes you collapse in on yourself, or something like that.
However ponder this....
A man is travelling on a speeding train. He switches a torch on and shines it in front of him. As far as he is concerned the light leaves the torch at the speed of light. The train passes through a station and another man is standing on the platform. As far as the man on the platform is concerned is the light from the torch travelling at the speed of light plus the speed of the train? No, apparently time is going slower for the man of the platform so the light from the torch actually travels at the speed of light (and no more). Something to do with Relativity which I can't remember much about.
BRL, you might like to try this the next time you go to work.
Secondly it's impossible to travel at the speed of light because your mass approaches infinity as you approach the speed of light so your own gravity makes you collapse in on yourself, or something like that.
However ponder this....
A man is travelling on a speeding train. He switches a torch on and shines it in front of him. As far as he is concerned the light leaves the torch at the speed of light. The train passes through a station and another man is standing on the platform. As far as the man on the platform is concerned is the light from the torch travelling at the speed of light plus the speed of the train? No, apparently time is going slower for the man of the platform so the light from the torch actually travels at the speed of light (and no more). Something to do with Relativity which I can't remember much about.
BRL, you might like to try this the next time you go to work.
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Are yes the old relativity thing. I recall seeing James Burke explain this late one night on BBC2. If I recall the faster you go the thinner things get so, to get your car through a narrow gap, drive fast, but presumably, don't stop half way through otherwise the car wil expand again!
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Refering to the case where a bullet is fired from a car moving at the muzzle velocity of the bullet, it will leave the barrel at the same speed it usually does, but it's velocity relative to the ground the car is driving on is two times what it would be were it at rest. However, the car will not catch up to the bullet to have it crash through the window. Gravity is acting on it and once it leaves the barrel, it will begin falling. So by the time the car catches up with it, the bullet will have fallen to the ground. (Unless there is an incredible headwind to slow the bullet down)
Once comment about Rote 8's, first post in this thread. If I recall my physics classes correctly, the speed of sound increases with a decrease in temperature, not with an increase. That makes sense too because when it's colder, the molecules are closer together. When something disturbs them (ie sound) they don't have to move very far to crash into other molecules to propagate the sound wave. In warmer air, they're further apart so they have to travel a bit before they can pass on the wave.
I may be wrong, but that is how I believe it works. I await correction
Once comment about Rote 8's, first post in this thread. If I recall my physics classes correctly, the speed of sound increases with a decrease in temperature, not with an increase. That makes sense too because when it's colder, the molecules are closer together. When something disturbs them (ie sound) they don't have to move very far to crash into other molecules to propagate the sound wave. In warmer air, they're further apart so they have to travel a bit before they can pass on the wave.
I may be wrong, but that is how I believe it works. I await correction
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A good thread. I see its already on the tech forum so I will close it here and let it continue on that other forum. The usual e-mail address for your moans and groans about moving it etc......
