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3rd Gear Chirper
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Okay, I have wondered this forever now, maybe somebody can answer it.

If a car was driving at the speed of light and the driver turned the headlights on, would you be able to see them ahead of the car, or even at all?

I searched online, and did find a statement from someone saying that the photons would actually be displaced to the rear of the car, so the light would shine on them after the car had passed. But you would have to exceed the speed of light to get them behind the car, what if you drive at exactly the speed of light?
 

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If everything is traveling at the speed of light, then the light would travel along with the other matter at the place it was being emitted.

If a jet is traveling the speed of sound, then you would not hear it until the exact moment it got to you, since the jet is traveling at the same speed as the sound that it is making.
 

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Watch this 3 hour series. I don't remember which episode but one of them addresses your question.
In short, light travels at a constant speed. You would see the light move at the speed of light away from the car. A person on the side of the road will see the light moving towards him at... the speed of light.
 

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Speed is relative, so the question is moot. :tongue: :D

Relative to the driver , (assuming he isn’t pure energy…) the photons would appear to be traveling away at the speed of light.
Relative to a stationary observer within the universe (assuming he had real quick vision), the photons would appear to be traveling at 2x the speed of light and assuming the driver is approaching him. (Don't forget about Doppler shifts).
Relative to a stationary observer outside the universe the photons would appear to be traveling at multiple times the speed of light (assuming the universe is expanding at a c+ rate and the flashlight is headed towards the observer.)

"Speed of light" is nothing more than a set constant of the measurement of speed of all electromagnetic radiation, including visible light, in free space. And it has been proven that going faster than the speed of light is possible.

And since that is just a constant measurement, it does not exclude speeds greater than that constant. It isn’t a “speed limit”. The “limit” is from Einstein’s Theory of Relativity stating (basically) that anything with mass requires infinite energy to accelerate to the speed of light. So if the object has no mass, then there is no speed limit.

http://en.wikipedia.org/wiki/Speed_of_light

And the speed limit has been broken: “One recent experiment made the group velocity of laser beams travel for extremely short distances through cesium atoms at 300 times c (or about Warp 3.8…. http://en.wikipedia.org/wiki/Image:Warptable.gif :rofl: ).”

But this subject is still being debated/discussed/argued my physicists around the world and I don’t think anyone here will have a definitive answer. :D
 

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Voice/Data Guru
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Watch this 3 hour series. I don't remember which episode but one of them addresses your question.
In short, light travels at a constant speed. You would see the light move at the speed of light away from the car. A person on the side of the road will see the light moving towards him at... the speed of light.

Tinman Thank you for the link...Very interesting
 

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If you are already moving at the speed of light, and turn the lights on, you will still see the light ahead of you as normal, because the source of the light is already moving. Think of it like throwing a ball from the back seat to the front seat of the car. From within the car, the ball appears to be moving slowly, however to an observer outside the car, the ball would appear to be moving slightly faster than the car itself. The same thing happens with the light. What would be really cool though, would be if you were travelling at the speed of light, and someone turned on a set of headlights right at the moment you passed them, then you would be going along next to leading edge of the light, and you would see a point with the light shining right next to a point with no light.
 

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And it has been proven that going faster than the speed of light is possible.


http://en.wikipedia.org/wiki/Speed_of_light

And the speed limit has been broken: “One recent experiment made the group velocity of laser beams travel for extremely short distances through cesium atoms at 300 times c (or about Warp 3.8…. http://en.wikipedia.org/wiki/Image:Warptable.gif :rofl: ).”

But this subject is still being debated/discussed/argued my physicists around the world and I don’t think anyone here will have a definitive answer. :D
ah, but there is a huge difference between group velocity, and wave velocity, young Skywalker :D

while you may be able to exceed the group velocity, the wavefront velocity will decrease appropriately so that the speed of light in that medium will remain constant....and the speed of light in a medium is always less than c, the speed of light in a vacuum

since nothing with mass can reach the speed of light, say it is going very very close to the speed of light........yes, you will actually be able to see ahead of the car......from your perspective, light still moves forward and at the same exact speed as if you were stopped with respect to everything else.....if you're thinking that this would make the speed of light faster than "itself", then that is incorrect.....Lorentzian spacetime contractions would have a huge effect....this is what preserves the speed of light as the ultimate speed limit for anything that has mass in this universe
 

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If you are already moving at the speed of light, and turn the lights on, you will still see the light ahead of you as normal, because the source of the light is already moving. Think of it like throwing a ball from the back seat to the front seat of the car. From within the car, the ball appears to be moving slowly, however to an observer outside the car, the ball would appear to be moving slightly faster than the car itself. The same thing happens with the light. What would be really cool though, would be if you were travelling at the speed of light, and someone turned on a set of headlights right at the moment you passed them, then you would be going along next to leading edge of the light, and you would see a point with the light shining right next to a point with no light.
without taking into account Lorentzian spacetime transformations, no this is incorrect...see my above post.....gallilean velocity transformations do not apply here, since this is far from newtonian physics
 

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ah, but there is a huge difference between group velocity, and wave velocity, young Skywalker :D

While you may be able to exceed the group velocity, the wavefront velocity will decrease appropriately so that the speed of light in that medium will remain constant....and the speed of light in a medium is always less than c, the speed of light in a vacuum

since nothing with mass can reach the speed of light, say it is going very very close to the speed of light........yes, you will actually be able to see ahead of the car......from your perspective, light still moves forward and at the same exact speed as if you were stopped with respect to everything else.....if you're thinking that this would make the speed of light faster than "itself", then that is incorrect.....Lorentzian spacetime contractions would have a huge effect....this is what preserves the speed of light as the ultimate speed limit for anything that has mass in this universe
But "speed" is relative. The only way to have speed or velocity it to compare two "things" to each other (i.e. movement) over a specific time.
For example: I'm sitting at my desk (at the equator). How fast am I moving? :2huh:
Relative to my office = 0 mph
Relative to the N/S poles = 1000 mph
Relative to the sun = 67,000 mph (+ or – 1000 mph depending on heading away or towards the sun on the surface of the earth)
Relative to the Milky Way = 560,000 mph (+ or – 67,000 mph depending…)
Relative to the other Galaxies = 670,000 mph

Yes, the speed of light in a vacuum when referenced to said vacuum is a constant. Nowhere does it address the issue if the vacuum is moving.

But I’m just applying common sense, considering I’m not a physicist. “Theory” is all cool and everything, but until it can be proven, it is just someone’s “guess”, even if it is a “very, very well educated” guess. (Many very educated (for their time) theorists said the Earth was flat until proven wrong):D

Can God make an unmovable bolder travel faster than the speed of light? :tongue:
 

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well I am a physicist, having taken pretty much all undergrad courses available in physics :D

and yes you're right...speed is relative, for objects with mass at subluminal speeds, but not when you're talking about a hypothetical light source moving at c....the speed of the wavefront of photons is still c, even if the source itself is moving at c...so no, the question is not moot, since c is still the ultimate speed limit in this universe for objects with mass

so in other words, the only reason the group velocity of the electromagnetic wave pulse exceeded the speed of light, is because the medium reduced the wave velocity of light....it is the only permissible way for the group velocity to exceed c, so that the product of the group velocity and the wave velocity equal to c² in that medium
 

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well I am a physicist, having taken pretty much all undergrad courses available in physics :D

and yes you're right...speed is relative, for objects with mass at subluminal speeds, but not when you're talking about a hypothetical light source moving at c....the speed of the wavefront of photons is still c, even if the source itself is moving at c...so no, the question is not moot, since c is still the currently theorized ultimate speed limit in this universe for objects with mass.
cough cough Show me a physics degree... cough cough :tongue:

Just messin' with ya!! And I modified/corrected your last sentence. :D :thumbsup:

But that’s the thing, c = 299,792,458 meters per second. To have a measurement of a distance of 299,792,458 meters traveled, you have to have something to compare “it” to, a reference. It is only to that reference that the speed of light is a constant.

Also, since a photon doesn’t have any mass, does Einstein’s “speed limit” for mass even apply? :2huh:
 

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haha well my degree will show physics as a minor, but the classes are there in my transcript hehe :)

oh i'd say you're incorrect about the referenced speed of light....it is equivalent to 299,792,458 meters per second, but nature knows no meter, nor does it know any second....those numbers are based on our units of time and space, now a days defined by us from radiation from cesium-133 atoms.....the speed of light by any other measurement would cause the same results on our spacetime......it is a true constant regardless of what reference is chosen to measure it....in any referece frame, by any number system conceivable, c (not as defined by us numerically, but simply read -the speed of light-, in its purest form) is constant.......this is the very reason absolute time and absolute space were abolished when relativity "came to light" in 1905 :)

Einstein's speed limit still applies alright, in the sense that nothing with mass can ever reach the speed of light, which is not a "speed limit" in the sense that it can be reached and not exceed it...the photon itself is always moving at c in a vacuum since it is the carrier of electromagnetism


Relative to the driver , (assuming he isn’t pure energy…) the photons would appear to be traveling away at the speed of light.
Relative to a stationary observer within the universe (assuming he had real quick vision), the photons would appear to be traveling at 2x the speed of light and assuming the driver is approaching him. (Don't forget about Doppler shifts).
Relative to a stationary observer outside the universe the photons would appear to be traveling at multiple times the speed of light (assuming the universe is expanding at a c+ rate and the flashlight is headed towards the observer.)
cant believe i missed these...lol......first sentence is correct....the next two are absolutely incorrect.....show me the relativistic Lorentzian tranformation calculations that permit this, and i'll eat all my words :)

two-words: spacetime contraction :)
 

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Ah, I see the light. Sorry, couldn’t resist… :D

So the “constant” is the movement of light, photons, whatever (something massless) through a vacuum to which we assign an empirical number. The same way pi is a constant that is the ratio of a circle’s circumference to its diameter that we assign a base 10 number of 3.1415 to.

Now here’s a related question since it applies to speed of light in a vacuum: Can a pure or absolute vacuum (free space) even exist (or is the vote still out on that)? And if not, how can you have a constant (c) be based on something that may or may not exist? :2huh:

Doesn't the Doppler effect sort of support the statements on the perceived light from someone external to the referenced light? Or better, what would someone see if they are standing say one lightyear in front of the car or one lightyear to the rear of the car (assuming the "flashlight" is actually an unblocked light source)

I always loved Physics and it was my major at VA Tech before I dropped out (after only one year, so obviously my physics knowledge is EXTREMELY limited) But it always frustrated me because I’m a kind of person that does not like pushing the “I believe button”. :D
 

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So wait, I'm confused. Guitar Maestro, it sounds like you are saying that from the perspective of the driver, the light would travel forward at the speed of light, and that from the perspective of an outside observer, the light would also travel forward at the speed of light. How is that possible? That would mean the photons would have to be in 2 places at once, right? Cause that would mean after one year, from the perspective of a still observer the light would have to be one light year away, but from the perspective of the driver, who is also one light-year away from the still observer, the light would have to be one light year ahead of him, or 2 light years away from the still observer. Could you explain how this works? Or did I misunderstand you?
 

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well the density of outer space (seemingly empty) is roughly a few hydrogen atoms/m³, and the tube in which protons will travel thru in the main accelerating ring of the Large Hadron Collider at CERN in France/Switz is colder and emptier than that of outer space (not including the protons/anti-proton beams of course hehe)....so we're pretty close......there is an ultimate lower limit of vacuum which i was reading about when i was taking thermal physics, but i dont quite remember what it is.....it all depends on whether quantum mechanics is correct, or even complete as a theory at this point

as far as what someone would "see", well here is where things get tricky...the closer an object gets to c, the more and more spacetime contracts, along the axis of travel, for that object.....so everything in that reference frame literally shrinks as an outside observer would see it, yet light miraculously stays unaffected....both the moving and the stationary observers could both simultaneously conduct experiments to determine the speed of light in some agreed measurement system, and when they finally end up comparing notes, both will share the same answer: c

doppler effects do come into play here as well, but there are relativistic doppler calculations, just for scenarios like this one.....in front of such a quick moving light source, it would definitely be blue-shifted, and behind it would be red-shifted
 

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So wait, I'm confused. Guitar Maestro, it sounds like you are saying that from the perspective of the driver, the light would travel forward at the speed of light, and that from the perspective of an outside observer, the light would also travel forward at the speed of light. How is that possible? That would mean the photons would have to be in 2 places at once, right? Cause that would mean after one year, from the perspective of a still observer the light would have to be one light year away, but from the perspective of the driver, who is also one light-year away from the still observer, the light would have to be one light year ahead of him, or 2 light years away from the still observer. Could you explain how this works? Or did I misunderstand you?
if we could see such a driver, he would be shrunk into nothing.....the contraction of length and the slowing down of time along the axis of travel preserves the speed of light for him and us simultaneously....so yes, space literally contracts more and more as you get closer to the speed of light....but this doesnt affect the light beam....there will always be only one light beam swiftly moving along at the speed of...well...light.......nature has very clever ways of hiding truths like this from us
 

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yet light miraculously stays unaffected....both the moving and the stationary observers could both simultaneously conduct experiments to determine the speed of light in some agreed measurement system, and when they finally end up comparing notes, both will share the same answer: c
:rofl: :rofl: Where's my I believe button when I need it. :D

I'm so glad I never completed my degree... these types of discussions give me headaches (especially when having them with a college professor…)!!! :thumbsup:

The "light" is traveling at a constant speed, but the "light frequency" is not as evidenced by the Doppler shift!!! :bawling:

Just shoot me now! :bowdown: :thumbsup:
 
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