[Physics]Light (it's speed and fun things to do with it...)

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[Physics]Light (it's speed and fun things to do with it...)

Postby devinhoo » Thu Dec 04, 2008 2:54 am UTC

This is a theoretical experiment.

Car traveling forward at the speed of light. Turns on its headlights. What happens?
No light is emitted because the photons are moving at the same speed as the car.
Car traveling backwards at the speed of light. Turns on its headlights. What happens?
A static trail of protons is left behind.

(Just my high school thoughts, please correct me if I'm wrong, this stuff is fun.)
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Re: [Physics]Light (it's speed and fun things to do with it...)

Postby BlackSails » Thu Dec 04, 2008 3:57 am UTC

Both are wrong.

1) Cars, having mass, cannot travel at the speed of light.

But lets say the car is traveling 99.9999% the speed of light, and turns its lights on, what happens?
The people in the car observe photons traveling at the speed of light away from them. An observer at rest observes the photons to be going at the speed of light.


Car moving backwards at 99.9999% the speed of light, and turns its lights on, what do the people inside see?
The people in the car observe photons traveling at the speed of light away from them. An observer at rest observes the photons to be going at the speed of light.

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

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Re: [Physics]Light (it's speed and fun things to do with it...)

Postby Cryopyre » Thu Dec 04, 2008 4:14 am UTC

Lol, spoiler tags.

Spoiler:
Well relativity is always important to keep in mind when talking about light. Light is a constant, so wherever you are you will always observe it travelling the spped of light, which is why things like time slowing down and things contracting are side-effects of high "relativistic" speeds
Last edited by Cryopyre on Thu Dec 04, 2008 4:20 am UTC, edited 1 time in total.
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Re: [Physics]Light (it's speed and fun things to do with it...)

Postby Cryopyre » Thu Dec 04, 2008 4:21 am UTC

How does one derive relativity? I thought the key components of it could only be observed

I mean
Spoiler:
The one that light is a constant
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Re: [Physics]Light (it's speed and fun things to do with it...)

Postby Carnildo » Thu Dec 04, 2008 4:49 am UTC

devinhoo wrote:This is a theoretical experiment.

Car traveling forward at the speed of light. Turns on its headlights. What happens?
No light is emitted because the photons are moving at the same speed as the car.


Right for the wrong reason. Ignoring the fact that a car, as a massive object, can't travel at the speed of light, no light will be emitted because, from the point of view of the headlights, no time is passing.

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Re: [Physics]Light (it's speed and fun things to do with it...)

Postby telcontar42 » Thu Dec 04, 2008 4:55 am UTC

Well, you can sort of reason out why the speed of light has to be constant. Picture two cars colliding at a 90 degree angle. Assuming the velocity of the light from a moving source was c+v, where c is the speed of light emitted from a stationary source and v is the velocity of the source in the direction of the emitted light, an observer directly in front of one of the cars would see that car reach the point of collision first, crashing into nothing, and then later would see the second car reach the point of collision. It doesn't make sense. If you take the speed of light to be c always, this problem disappears.

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Re: [Physics]Light (it's speed and fun things to do with it...)

Postby Matterwave1 » Thu Dec 04, 2008 9:35 am UTC

Assumed:

1) Light travels at the speed of light (c) in a vacuum in all reference frames.


2) The laws of physics are the same in all inertial reference frames.

Go ahead and derive special relativity now. :)

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Diadem
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Re: [Physics]Light (it's speed and fun things to do with it...)

Postby Diadem » Thu Dec 04, 2008 12:45 pm UTC

That's actually not that hard. It's done in any 1st year physics course on relativity.
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Re: [Physics]Light (it's speed and fun things to do with it...)

Postby danpilon54 » Thu Dec 04, 2008 1:26 pm UTC

Diadem wrote:That's actually not that hard to see it done. It's done in any 1st year physics course on relativity.


fix'd unless you're a really really smart freshman, or copying notes.
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Re: [Physics]Light (it's speed and fun things to do with it...)

Postby rho » Thu Dec 04, 2008 1:29 pm UTC

Cryopyre wrote:How does one derive relativity? I thought the key components of it could only be observed

I mean
Spoiler:
The one that light is a constant

As far as I know, this is not peer reviewed yet but: http://arxiv.org/abs/0806.1234

Mitchell Feigenbaum isn't exactly a crackpot.
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Re: [Physics]Light (it's speed and fun things to do with it...)

Postby Tass » Thu Dec 04, 2008 2:01 pm UTC

rho wrote:
Cryopyre wrote:How does one derive relativity? I thought the key components of it could only be observed

I mean
Spoiler:
The one that light is a constant

As far as I know, this is not peer reviewed yet but: http://arxiv.org/abs/0806.1234

Mitchell Feigenbaum isn't exactly a crackpot.


Nice article. Thanks. No that is deffinitely not the work of a crackpot.

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Re: [Physics]Light (it's speed and fun things to do with it...)

Postby thoughtfully » Thu Dec 04, 2008 2:12 pm UTC

The constant speed of light comes out of Maxwell's equations. This was one of Einstein's main motivations. Starting from this, you can derive the Lorentz transformations and mass-energy equivalence from a light clock in motion and basic geometry. http://galileoandeinstein.physics.virgi ... lwhat.html

EDIT: Ok, probably not mass-energy equivalence. It still boggles me how far a light clock can get you, though :)
And everything you ever wanted to know about Special Relativity is at http://www.learner.org/resources/series42.html, episodes 41-44.
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Re: [Physics]Light (it's speed and fun things to do with it...)

Postby dagothar » Thu Dec 04, 2008 7:42 pm UTC

This has always bugged me: why is it always stated that someone _sees_ light travelling at speed - isn't it impossible? How do you see light? Isn't that true that you can see it only if it hits you? You certainly wouldn't be able to see light speeding away from you or just approaching.

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Re: [Physics]Light (it's speed and fun things to do with it...)

Postby danpilon54 » Thu Dec 04, 2008 7:52 pm UTC

it means within your reference frame, light takes x/c seconds to reach an object, where x is the distance and c is the speed of light.
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mdyrud
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Re: [Physics]Light (it's speed and fun things to do with it...)

Postby mdyrud » Fri Dec 05, 2008 12:02 am UTC

Didn't special relativity also rely heavily on the Lorentz transformations?

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Re: [Physics]Light (it's speed and fun things to do with it...)

Postby thoughtfully » Fri Dec 05, 2008 12:22 am UTC

mdyrud wrote:Didn't special relativity also rely heavily on the Lorentz transformations?

Special Relativity derived the Lorentz transformations with a more convincing motivation than originally used by Lorentz.
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Re: [Physics]Light (it's speed and fun things to do with it...)

Postby Diadem » Fri Dec 05, 2008 2:36 am UTC

danpilon54 wrote:
Diadem wrote:That's actually not that hard to see it done. It's done in any 1st year physics course on relativity.


fix'd unless you're a really really smart freshman, or copying notes.


Well I said it's done in 1st year courses. I didn't say it was done by the freshmen themselves. But it's usually demonstrated on the blackboard. And they understand it too. Once you have your two postulates, the math really isn't that hard.

In fact most of the math had been done before Einstein even came along. Lorentz transformation were already known. The really hard part was putting it all together, and drawing the right conclusions from it: Ie, comming up with these two postulates. That's what was Einstein's big contribution.
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Re: [Physics]Light (it's speed and fun things to do with it...)

Postby thoughtfully » Fri Dec 05, 2008 2:48 am UTC

Yeah. General Relativity is another ball of worms entirely, though. One has to be a pretty advanced undergraduate to grok the basics, and more typically, it waits until graduate coursework. Dirac wrote a pretty slim volume on it, though, so it isn't like you have to absorb Misner/Thorne/Wheeler.

Topology, differential geometry, and really fun notation conventions, oh my!
and who doesn't love nonlinear PDEs?
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Re: [Physics]Light (it's speed and fun things to do with it...)

Postby ConMan » Fri Dec 05, 2008 3:12 am UTC

thoughtfully wrote:Yeah. General Relativity is another ball of worms entirely, though. One has to be a pretty advanced undergraduate to grok the basics, and more typically, it waits until graduate coursework. Dirac wrote a pretty slim volume on it, though, so it isn't like you have to absorb Misner/Thorne/Wheeler.

Topology, differential geometry, and really fun notation conventions, oh my!
and who doesn't love nonlinear PDEs?


Surely everybody loves solving [imath]\nabla_{\dot{x}}\dot{x}=0[/imath] and [imath]G=\kappa T[/imath] for an unspecified T, don't they?
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Re: [Physics]Light (it's speed and fun things to do with it...)

Postby devinhoo » Fri Dec 05, 2008 4:08 am UTC

Wow, cool!

This was really fun to mess with my friends and math teacher. I'm going to bother my physics teacher about it next simester... back to my Electromagnetic Spectrum paper for now though... (And yes, I'm using xkcd =D )
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Oh, and if you feel the need, check out my blog and my webcomic at devinhoo.com

Thanks! :)

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Re: [Physics]Light (it's speed and fun things to do with it...)

Postby meat.paste » Mon Dec 08, 2008 2:53 pm UTC

Cryopyre wrote:How does one derive relativity? I thought the key components of it could only be observed

I mean
Spoiler:
The one that light is a constant


Once the assumption that c is a constant and independent of the reference frame is made, I thought that the special relativity effects (time dilation and length contraction) become a trig problem
Spoiler:
gamma=1/sqrt([v2/c2]-1)
Huh? What?

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Re: [Physics]Light (it's speed and fun things to do with it...)

Postby QuantumShinobi » Tue Dec 09, 2008 6:54 am UTC

thoughtfully wrote:The constant speed of light comes out of Maxwell's equations. This was one of Einstein's main motivations. Starting from this, you can derive the Lorentz transformations and mass-energy equivalence from a light clock in motion and basic geometry. http://galileoandeinstein.physics.virgi ... lwhat.html

EDIT: Ok, probably not mass-energy equivalence. It still boggles me how far a light clock can get you, though :)
And everything you ever wanted to know about Special Relativity is at http://www.learner.org/resources/series42.html, episodes 41-44.


Mass-energy isn't too bad mathematically
Once you have the lorentz transformations, you have proper velocity [math]u = \gamma * ( c, \vec{v})[/math]
then you can define the 4 momentum [math]p = m * u = \gamma (mc, m \vec{v}) = (E , \vec{p}c )[/math]

the invariant length of this 4-vector is [math]c*p^2 = E^2 - (\vec{p}c)^2 = m^2c^4[/math]
so that when the particle is at rest [math]E_0 = m c^2[/math]

coming to embrace this result conceptually is another thing though...

(I have no idea if I've used the math tags right... it doesn't seem to want to render for me...)

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Re: [Physics]Light (it's speed and fun things to do with it...)

Postby Tass » Wed Dec 10, 2008 9:36 am UTC

QuantumShinobi wrote:(I have no idea if I've used the math tags right... it doesn't seem to want to render for me...)


It is nicely rendered for me.

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Re: [Physics]Light (it's speed and fun things to do with it...)

Postby Chfan » Wed Dec 10, 2008 10:05 pm UTC

There was a question I was thinking of while reading my Brian Greene book.

Let's say you are on a ship off of Earth capable of moving at 99.99999...(to a huge amount of 9's)% of the speed of light and you are participating in an experiment with a super powered laser being shot from Earth by some scientists. If the laser is fired directly in your direction just as the ship begins moving at max speed (so that you are, for a short period of time, ahead of the laser), you won't see it, will you? You won't be able to tell if the laser has been fired yet?
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Re: [Physics]Light (it's speed and fun things to do with it...)

Postby ThinkerEmeritus » Wed Dec 10, 2008 11:46 pm UTC

There will be a time delay before you see that the laser has been fired so long as you are any nonzero distance from the laser at the time it is fired (time as measured in your frame), regardless of the speed of the ship. The length of the time delay will depend on the initial distance from the laser and in your frame is given by t = d / c . The light will be moving at v = c with respect to the ship, no matter how fast the ship is moving with respect to the earth.
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Re: [Physics]Light (it's speed and fun things to do with it...)

Postby Carnildo » Thu Dec 11, 2008 8:57 am UTC

ThinkerEmeritus wrote:The length of the time delay will depend on the initial distance from the laser and in your frame is given by t = d / c . The light will be moving at v = c with respect to the ship, no matter how fast the ship is moving with respect to the earth.


Keep in mind that an observer on Earth and an observer aboard the ship may disagree on how far apart they are.

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Re: [Physics]Light (it's speed and fun things to do with it...)

Postby Hydralisk » Thu Dec 11, 2008 2:50 pm UTC

meat.paste wrote:
Cryopyre wrote:How does one derive relativity? I thought the key components of it could only be observed

I mean
Spoiler:
The one that light is a constant


Once the assumption that c is a constant and independent of the reference frame is made, I thought that the special relativity effects (time dilation and length contraction) become a trig problem
Spoiler:
gamma=1/sqrt(1-[v2/c2])

Fix'd.

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Re: [Physics]Light (it's speed and fun things to do with it...)

Postby MotorToad » Thu Dec 11, 2008 3:59 pm UTC

dagothar wrote:This has always bugged me: why is it always stated that someone _sees_ light travelling at speed - isn't it impossible? How do you see light? Isn't that true that you can see it only if it hits you? You certainly wouldn't be able to see light speeding away from you or just approaching.

Heh, that struck me as rather funny, since that's what seeing is. :)

Anyway, what's meant by "seeing light" is that the speed of light is the same in every frame of reference. It's not that you see an individual photon speeding away from your headlights, but that thought experiment is what started Einstein on this whole relativistic venture. Since the apparent speed of the photon is the same to a person standing still (or as still as you can stand on a planet rushing around the sun at thousands of mph) as it is to a person traveling at 185,000 miles/second, to arrive at the same value either the miles or the seconds have to be different sizes for each person.
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