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# NEW SPEED OF LIGHT TOPIC

EanofAthenasPrime
UM...Hello. I hate that other speed of light topic. It started with a stupid question and got super intelligent and confusing responses. This new topic has a question thats not that stupid and hopefully will get responses some who hasn't graduated from college can understand.

It is well known that the speed of light in a vacuum is a constant 3*10^8 mps.
However, I had to research to find out that like can travel considerably slower! So...Question 1. Since light normally goes considerable slower depending on its medium that it travels, would its speed still be seemingly constant no matter what frame of reference? Ie. Light going the speed of light, no matter if you are going half the speed of light and you look at the sun, the sunlight would appear to go the same speed as it went when you weren't moving. However, if light went less than c, would it still exhibit this property? I think no. I think light exhibits this property not because photons have some magic powers, but that that is what happens when you go c. Usings Einsteins time dilation equation, no mathematical errors occur when you are traveling less than 1 c. But at 1 c, a math error occurs. Past c, Division by 0 errors occur. I think the first error, is represented by light not being relative, as I mentioned earlier. So all math, even errors, have things they represent! Anyways, I don't know what division by zero would represent in real life, scientists assume backwards time travel. But here is the problem with proving me wrong. I think scientists can only test if light is constant when it is in a vacuum. This makes it hard to prove my theory that light switches to Galilean/Einstein relativity when it is subluminal.
Indi
It's a little misleading to say that light slows down in a medium. It's not false, it's just not completely true.

Photons always travel at c (BUT THERE'S A CATCH HERE! i'll get to it later). The reason that light appears to move slower in a medium is because the photons are interacting with the particles of the medium. There are two ways that they can interact. The first is easy - they get jostled around slightly by the tiny gravitational fields of the molecules. So instead of travelling like this:
 Code: →→→→→→→→→→
They travel like this:
 Code: ↗↘↗↘↗↘↗↘↗↘
Obviously, that means they take a little longer.

The second thing that is happening is that photons travelling through a medium sometimes hit the particles of that medium. When they do, the particle absorbs the photon and gets boosted to a higher energy state. Then, usually, it immediately drops back down to the lower energy state and re-emits the photon. However, this absorption and re-emission takes a small but finite amout of time, which delays the photon slightly.

At no time is the actual "speed" of the photons changing, they're just taking a more complex path instead of flying straight.

Now for the catch.

Remember that photons are not tiny little "balls of light". Photons are wavepackets - superpositions of waves that form a packet. There are several "speeds" invovled in wave packets - phase velocity, group velocity and signal velocity. Generally speaking, when we talk about the speed of light (or the speed of any wavepacket particle, including things like electrons), we are talking about the group velocity... but not always. Sometimes the wavepacket can be distorted in a way that makes the group velocity much larger or smaller than c... but the signal velocity will always be c. Sounds weird, i know... you kinda have to go through the math of wavepackets step-by-step to get it.
EanofAthenasPrime
Thanks for clarifing that stuff it makes a lot more sense now!

Except for wavepackets.
Indi
 EanofAthenasPrime wrote: Thanks for clarifing that stuff it makes a lot more sense now! Except for wavepackets.

Yes, well. ^_^; Wavepackets are really hard to explain without a whiteboard to sketch on... and a solid understanding of sine functions, and maybe statistics (particularly, the Bell curve).
EanofAthenasPrime
Indi wrote:
 EanofAthenasPrime wrote: Thanks for clarifing that stuff it makes a lot more sense now! Except for wavepackets.

Yes, well. ^_^; Wavepackets are really hard to explain without a whiteboard to sketch on... and a solid understanding of sine functions, and maybe statistics (particularly, the Bell curve).

ah...I under stand sine, now for the chalkboard...
newolder
Indi wrote:
 EanofAthenasPrime wrote: Thanks for clarifing that stuff it makes a lot more sense now! Except for wavepackets.

Yes, well. ^_^; Wavepackets are really hard to explain without a whiteboard to sketch on... and a solid understanding of sine functions, and maybe statistics (particularly, the Bell curve).

Didn't Bikerman post a graphic or a link thereto (that showed wave-packet behaviour very well), in a recent topic that i can't bring to surface memory?

Perhaps a Moderator could help, please?

ed.
Indi
newolder wrote:
Indi wrote:
 EanofAthenasPrime wrote: Thanks for clarifing that stuff it makes a lot more sense now! Except for wavepackets.

Yes, well. ^_^; Wavepackets are really hard to explain without a whiteboard to sketch on... and a solid understanding of sine functions, and maybe statistics (particularly, the Bell curve).

Didn't Bikerman post a graphic or a link thereto (that showed wave-packet behaviour very well), in a recent topic that i can't bring to surface memory?

Perhaps a Moderator could help, please?

ed.

Yes, but that applet was more to show the difference between group velocity and phase velocity. If you don't understand what's going on with superposition to begin with, it will probably only confuse the hell out of you.
newolder
Okay Indi.

So, armed with what's written so far, the reader could visit this site too:: http://pr.erau.edu/~vanda029/phy40e/40e_ani_packet.html

and there are probably many others too...
EanofAthenasPrime
 newolder wrote: Okay Indi. So, armed with what's written so far, the reader could visit this site too:: http://pr.erau.edu/~vanda029/phy40e/40e_ani_packet.html and there are probably many others too...

thanks man...ill try and read it during class, that way I'll actually learn something.
gong_05sk
Thanks for clarifing that stuff it makes a lot more sense now! it í very usefuly
thanks a lot.
ReubenWilliams
Indi wrote:
It's a little misleading to say that light slows down in a medium. It's not false, it's just not completely true.

Photons always travel at c (BUT THERE'S A CATCH HERE! i'll get to it later). The reason that light appears to move slower in a medium is because the photons are interacting with the particles of the medium. There are two ways that they can interact. The first is easy - they get jostled around slightly by the tiny gravitational fields of the molecules. So instead of travelling like this:
 Code: →→→→→→→→→→
They travel like this:
 Code: ↗↘↗↘↗↘↗↘↗↘
Obviously, that means they take a little longer.

The second thing that is happening is that photons travelling through a medium sometimes hit the particles of that medium. When they do, the particle absorbs the photon and gets boosted to a higher energy state. Then, usually, it immediately drops back down to the lower energy state and re-emits the photon. However, this absorption and re-emission takes a small but finite amout of time, which delays the photon slightly.

At no time is the actual "speed" of the photons changing, they're just taking a more complex path instead of flying straight.

Now for the catch.

Remember that photons are not tiny little "balls of light". Photons are wavepackets - superpositions of waves that form a packet. There are several "speeds" invovled in wave packets - phase velocity, group velocity and signal velocity. Generally speaking, when we talk about the speed of light (or the speed of any wavepacket particle, including things like electrons), we are talking about the group velocity... but not always. Sometimes the wavepacket can be distorted in a way that makes the group velocity much larger or smaller than c... but the signal velocity will always be c. Sounds weird, i know... you kinda have to go through the math of wavepackets step-by-step to get it.

very very clear. Thanks