# Physics question

#### HomeBrewerDude

##### Lifer

Speed of light is constant.
Light is bent by gravity.
Changing direction requires force, i.e. acceleration.
So how is light supposed to undergo acceleration if its speed is constant?

i am sure there is a simple explanation.... i just can seem to come up with it.

#### StageLeft

##### No Lifer
It isn't constant if gravity can affect it! If you're by a black hole it's going to slow down going away from the hole.

#### silverpig

##### Lifer
Originally posted by: HomeBrewerDude

Speed of light is constant.
Light is bent by gravity.
Changing direction requires force, i.e. acceleration.
So how is light supposed to undergo acceleration if its speed is constant?

i am sure there is a simple explanation.... i just can seem to come up with it.

Acceleration changes velocity, but not necessarily speed. If you run in a circle at a constant speed, you are constantly changing your velocity.

#### silverpig

##### Lifer
Originally posted by: Skoorb
It isn't constant if gravity can affect it! If you're by a black hole it's going to slow down going away from the hole.

Physics wouldn't be so hard if you weren't so stupid.

j/k

But seriously, this isn't quite right

#### woowoo

##### Platinum Member
The speed of light is not a constant.

#### StageLeft

##### No Lifer
Originally posted by: silverpig
Originally posted by: HomeBrewerDude

Speed of light is constant.
Light is bent by gravity.
Changing direction requires force, i.e. acceleration.
So how is light supposed to undergo acceleration if its speed is constant?

i am sure there is a simple explanation.... i just can seem to come up with it.

Acceleration changes velocity, but not necessarily speed. If you run in a circle at a constant speed, you are constantly changing your velocity.
That's fine. Now, how does light maintain a constant speed if it's being pulled by a black hole?

#### silverpig

##### Lifer
Originally posted by: woowoo
The speed of light is not a constant.

Got proof? If you do, I'll help you publish it and we can split the \$1 million Nobel Prize.

#### StageLeft

##### No Lifer
Originally posted by: silverpig
Originally posted by: woowoo
The speed of light is not a constant.

Got proof? If you do, I'll help you publish it and we can split the \$1 million Nobel Prize.
First of all a quick search on google will illustrate how it isn't constant. Secondly I'm still waiting on the answer to my black hole question.

#### silverpig

##### Lifer
Originally posted by: Skoorb
Originally posted by: silverpig
Originally posted by: HomeBrewerDude

Speed of light is constant.
Light is bent by gravity.
Changing direction requires force, i.e. acceleration.
So how is light supposed to undergo acceleration if its speed is constant?

i am sure there is a simple explanation.... i just can seem to come up with it.

Acceleration changes velocity, but not necessarily speed. If you run in a circle at a constant speed, you are constantly changing your velocity.
That's fine. Now, how does light maintain a constant speed if it's being pulled by a black hole?

The black hole just curves space as does any other massive object. Light must travel along this curvature.

Picture a car that can only go 60 mph. You put it on flat land, it goes 60, you put it on a curvy road, it goes 60. Now, a black hole (or star, planet etc) would be similar to putting a huge depression in the earth, like a funnel shape. The car must travel along this curve. It still goes 60 with respect to the road it's travelling on.

#### silverpig

##### Lifer
Originally posted by: Skoorb
Originally posted by: silverpig
Originally posted by: woowoo
The speed of light is not a constant.

Got proof? If you do, I'll help you publish it and we can split the \$1 million Nobel Prize.
First of all a quick search on google will illustrate how it isn't constant. Secondly I'm still waiting on the answer to my black hole question.

No it won't. Light has a constant speed.

#### StageLeft

##### No Lifer
Originally posted by: silverpig
Originally posted by: Skoorb
Originally posted by: silverpig
Originally posted by: HomeBrewerDude

Speed of light is constant.
Light is bent by gravity.
Changing direction requires force, i.e. acceleration.
So how is light supposed to undergo acceleration if its speed is constant?

i am sure there is a simple explanation.... i just can seem to come up with it.

Acceleration changes velocity, but not necessarily speed. If you run in a circle at a constant speed, you are constantly changing your velocity.
That's fine. Now, how does light maintain a constant speed if it's being pulled by a black hole?

The black hole just curves space as does any other massive object. Light must travel along this curvature.

Picture a car that can only go 60 mph. You put it on flat land, it goes 60, you put it on a curvy road, it goes 60. Now, a black hole (or star, planet etc) would be similar to putting a huge depression in the earth, like a funnel shape. The car must travel along this curve. It still goes 60 with respect to the road it's travelling on.
Well then how can gravity affect light?
No it won't. Light has a constant speed.
As jzero said - in a vacuum. In different mediums it most definitely changes its speed.

#### silverpig

##### Lifer
Originally posted by: Jzero
Speed of light in a vacuum is the constant c.

A photon can be slowed down by whatever medium it is passing through, that is what causes refraction and it can be slowed by gravity.

Recently scientist brought a photon to a near stand-still.

No, they didn't. I have to head to class now, but basically they just set up an elaborate energy level population in a (sodium?) atom whereby the effect was if they passed light through the sodium, the light waves would interfere with each other in such a way that everything was cancelled out except a bit of the wave which travelled at a very slow speed. I can explain more when I get back this afternoon, but in the mean time you can look up the ideas of group and phase velocity for waves and what happens when there is a difference in the transmission coefficients of light over a small wavelength window.

#### silverpig

##### Lifer
Originally posted by: Jzero
Still sceptical?

Media distorted reality

The technical explanation offers a different picture. 50 guys running at 10 m/s in a line. The first guy has a baton. As he crosses the start line he passes the baton backwards to the next guy, then to the next one etc. The first each guy takes just 10 seconds to cover 100 m, but the baton takes, oh... 15 seconds. The "group velocity" is still 10 m/s, but the "phase velocity" (god I hope I got those right ) is less than that.

#### Jzero

##### Lifer
No, they didn't.
I guess the link I posted was just a big lie by NASA like the moon landing?
I have to head to class now...
Is it physics? Maybe you should ask the professor?

#### amoeba

##### Diamond Member
Pretty sure speed of light is only C in a vaccuum.

#### Heisenberg

##### Lifer
Originally posted by: silverpig
Originally posted by: Jzero
Still sceptical?

Media distorted reality

The technical explanation offers a different picture. 50 guys running at 10 m/s in a line. The first guy has a baton. As he crosses the start line he passes the baton backwards to the next guy, then to the next one etc. The first each guy takes just 10 seconds to cover 100 m, but the baton takes, oh... 15 seconds. The "group velocity" is still 10 m/s, but the "phase velocity" (god I hope I got those right ) is less than that.
I think you have group and phase velocity backwards. Phase velocity is the individual wave, group velocity is the wave envelope. I don't think people here understand the difference and that's what is leading to the confusion. Although the phase velocity can exceed c, the group velocity never can. This is a pretty complex topic, and isn't something you can explain in a paragraph on a BBS IMO.

#### StageLeft

##### No Lifer
Originally posted by: Heisenberg
Originally posted by: silverpig
Originally posted by: Jzero
Still sceptical?

Media distorted reality

The technical explanation offers a different picture. 50 guys running at 10 m/s in a line. The first guy has a baton. As he crosses the start line he passes the baton backwards to the next guy, then to the next one etc. The first each guy takes just 10 seconds to cover 100 m, but the baton takes, oh... 15 seconds. The "group velocity" is still 10 m/s, but the "phase velocity" (god I hope I got those right ) is less than that.
I think you have group and phase velocity backwards. Phase velocity is the individual wave, group velocity is the wave envelope. I don't think people here understand the difference and that's what is leading to the confusion. Although the phase velocity can exceed c, the group velocity never can. This is a pretty complex topic, and isn't something you can explain in a paragraph on a BBS IMO.
And even still, it doesn't try to dispute the fact that light does indeed slow in various mediums.

#### Heisenberg

##### Lifer
Originally posted by: Skoorb
Originally posted by: Heisenberg
Originally posted by: silverpig
Originally posted by: Jzero
Still sceptical?

Media distorted reality

The technical explanation offers a different picture. 50 guys running at 10 m/s in a line. The first guy has a baton. As he crosses the start line he passes the baton backwards to the next guy, then to the next one etc. The first each guy takes just 10 seconds to cover 100 m, but the baton takes, oh... 15 seconds. The "group velocity" is still 10 m/s, but the "phase velocity" (god I hope I got those right ) is less than that.
I think you have group and phase velocity backwards. Phase velocity is the individual wave, group velocity is the wave envelope. I don't think people here understand the difference and that's what is leading to the confusion. Although the phase velocity can exceed c, the group velocity never can. This is a pretty complex topic, and isn't something you can explain in a paragraph on a BBS IMO.
And even still, it doesn't try to dispute the fact that light does indeed slow in various mediums.
Well, yes and no. It depends on what light you're considering. There are complex interference, absorption, and transmission effects at work. All those news articles are written for the public and therefore are oversimplified. I'm not trying to sound like an ass, but one really needs at least an undergraduate optics course to get a full view of what's happening.

#### dxkj

##### Lifer
Originally posted by: woowoo
The speed of light is not a constant.

Yup. You can slow it down depending on what its passing through, just like sound.

The question is, what can it pass through that will speed it up? (water vs air for sound?)

#### WhiteKnight

##### Platinum Member
Originally posted by: dxkj
Originally posted by: woowoo
The speed of light is not a constant.

Yup. You can slow it down depending on what its passing through, just like sound.

The question is, what can it pass through that will speed it up? (water vs air for sound?)

There's not necessarily a direct correlation with phenomena observed with sound, since sound is a wave and light is a particle/wave.

#### dxkj

##### Lifer
Originally posted by: WhiteKnight
Originally posted by: dxkj
Originally posted by: woowoo
The speed of light is not a constant.

Yup. You can slow it down depending on what its passing through, just like sound.

The question is, what can it pass through that will speed it up? (water vs air for sound?)

There's not necessarily a direct correlation with phenomena observed with sound, since sound is a wave and light is a particle/wave.

I know, hence the smiley

#### silverpig

##### Lifer
Originally posted by: Jzero
No, they didn't.
I guess the link I posted was just a big lie by NASA like the moon landing?
I have to head to class now...
Is it physics? Maybe you should ask the professor?

Yeah, it was actually... both of them, and then the seminar after

But yeah, as Heisenberg has backed me up on this, it is much more complicated than saying the car goes at 60 on the road and 40 in the mud. A car is, to an extremely good approximation, a particle. Light however isn't so. It travels in waves which can add and subtract from each other. If the conditions are just right they can do so in a way such that they form a secondary waveform which travels at a different speed. Again, depending on how it's all set up, you can have the phase velocity at any speed you want, including greater than c. It happens that in media other than a vacuum this phase velocity is less than c. This is the speed of light in the medium. It doesn't change how fast the group velocity of light is though, as that is always constant.

There is an interesting little consequence of this though. You know that blue-green glow that you see in nuclear reactors? Well that's a result of particles "going faster than light". The phase velocity of light in water is c/1.33 (approx), and some of the subatomic particles travel faster than this. The result is a photonic boom, and photonic cone similar to that of a sonic boom when a plane goes faster than sound. That blue-green colour is this radiation, called Cerenkov radiation. Google it for a better explanation.