speed of light question

[BH]

Hello,

I have read the speed of light is 168,000 miles a second. My question is this. I assume this is referring to visible light. Do other forms of light like infrared move at a different speed than the light we can see? Thanks.

 

[J842P]

It refers to all forms of light, not just the visible spectrum

 

[zorq]

It applies to everything on the electromagnetic spectrum. Radio waves, X-rays, Gamma rays, Infra-red light, Ultraviolet light, Etc.

Although perhaps according to this link, gamma rays can go faster than the speed of light sometimes???

 

[bilby]

'The speed of light' is a colloquial approximation of the more precise 'The speed of propagation of electromagnetic radiation in a vacuum'. It applies to all frequencies of electromagnetic radiation.

According to international standards, the speed of light is fixed; it is exactly 299,792,458 metres per second. The second is also given a fixed value; it is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom, at rest, and at absolute zero (or to look at it another way, the radiation emitted by that transition is defined as being exactly 9.19263177 GHz).

From these two fixed points, we can derive the length of the metre; this is, perhaps counter-intuitively, a variable whose value depends on how accurately we measure things experimentally. So an experiment to determine a more accurate value for the speed of light is actually determining a more accurate value for the length of the metre, as the speed of light is not changed regardless of the results of any experiments.

 

[Bomb#20]

Hello,

I have read the speed of light is 168,000 miles a second. My question is this. I assume this is referring to visible light. Do other forms of light like infrared move at a different speed than the light we can see? Thanks.

First off, it's 186,000, not 168,000. Second, what you read was referring to the speed of light in a vacuum. How fast light goes depends on what it's travelling through. It goes about 140,000 miles a second through water, about 120,000 mi/sec through glass, and about 77,000 mi/sec through diamond.

So to finally answer your main question, yes, if the light is going through a material then different wavelengths get slowed down by different amounts. Red light will typically go a couple thousand miles per second faster through glass than blue light will; infrared will go even faster. But it's the slow-down that's affected by wavelength, not the basic speed -- in a vacuum there's no slow-down so all wavelengths go at the same speed.

 

[DBT]

''Light can change speed, even in a vacuum, a new paper reports. The discovery could change the way we think about one of the constants of the universe.''

''For this reason, c is correctly referred to as “the speed of light in a vacuum.” However, in a paper on arXiv, Miles Padgett from the University of Glasgow has shown that even this needs a rethink. He manipulated the wave structure of some photons and sent them on a path of the same length as unaltered packets of light. The manipulated photons arrived later, indicating they were travelling more slowly.''

''The manipulation occurred by twisting a plane wave (one where the wave front is a parallel plane at right angles to the direction of travel) into a conical wave front, which is analogous to focusing a wave from a spread-out source onto a single point.''

''The slowing occurs at a rate of about one part in a hundred thousand. So in the time it takes unmodified light to travel a meter, the adjusted light makes it 0.01 millimeters less. With some understatement, Padgett and his co-authors note, “Measuring the arrival time of single photons with femtosecond precision is challenging.” The team achieved this by producing strongly correlated photon pairs and having them meet at the destination so that tiny variations in their arrival times would be revealed as phase differences. ''

 

[Playball40]

'The speed of light' is a colloquial approximation of the more precise 'The speed of propagation of electromagnetic radiation in a vacuum'. It applies to all frequencies of electromagnetic radiation.

According to international standards, the speed of light is fixed; it is exactly 299,792,458 metres per second. The second is also given a fixed value; it is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom, at rest, and at absolute zero (or to look at it another way, the radiation emitted by that transition is defined as being exactly 9.19263177 GHz).

From these two fixed points, we can derive the length of the metre; this is, perhaps counter-intuitively, a variable whose value depends on how accurately we measure things experimentally. So an experiment to determine a more accurate value for the speed of light is actually determining a more accurate value for the length of the metre, as the speed of light is not changed regardless of the results of any experiments.

That just made my brain explode.

 

[Peez]

Bomb#20:
So to finally answer your main question, yes, if the light is going through a material then different wavelengths get slowed down by different amounts. Red light will typically go a couple thousand miles per second faster through glass than blue light will; infrared will go even faster.

Note that this is related to the separation of colours that may occur when light passes through a prism.

Peez

 

[Loren Pechtel]

Hello,

I have read the speed of light is 168,000 miles a second. My question is this. I assume this is referring to visible light. Do other forms of light like infrared move at a different speed than the light we can see? Thanks.

First off, it's 186,000, not 168,000. Second, what you read was referring to the speed of light in a vacuum. How fast light goes depends on what it's travelling through. It goes about 140,000 miles a second through water, about 120,000 mi/sec through glass, and about 77,000 mi/sec through diamond.

And note that this is why lenses work--going from a medium with one speed to another with a different speed causes a wave to bend.

 

[barbos]

''Light can change speed, even in a vacuum, a new paper reports. The discovery could change the way we think about one of the constants of the universe.''

''For this reason, c is correctly referred to as “the speed of light in a vacuum.” However, in a paper on arXiv, Miles Padgett from the University of Glasgow has shown that even this needs a rethink. He manipulated the wave structure of some photons and sent them on a path of the same length as unaltered packets of light. The manipulated photons arrived later, indicating they were travelling more slowly.''

''The manipulation occurred by twisting a plane wave (one where the wave front is a parallel plane at right angles to the direction of travel) into a conical wave front, which is analogous to focusing a wave from a spread-out source onto a single point.''

''The slowing occurs at a rate of about one part in a hundred thousand. So in the time it takes unmodified light to travel a meter, the adjusted light makes it 0.01 millimeters less. With some understatement, Padgett and his co-authors note, “Measuring the arrival time of single photons with femtosecond precision is challenging.” The team achieved this by producing strongly correlated photon pairs and having them meet at the destination so that tiny variations in their arrival times would be revealed as phase differences. ''

Smells like a total bullshit, because path light takes is longer than a straight line, hence "slower" speed.
Also, reading further confirms these people have no clue what they are talking about (the part of the hollow waveguide).

 

[DBT]

''Light can change speed, even in a vacuum, a new paper reports. The discovery could change the way we think about one of the constants of the universe.''

''For this reason, c is correctly referred to as “the speed of light in a vacuum.” However, in a paper on arXiv, Miles Padgett from the University of Glasgow has shown that even this needs a rethink. He manipulated the wave structure of some photons and sent them on a path of the same length as unaltered packets of light. The manipulated photons arrived later, indicating they were travelling more slowly.''

''The manipulation occurred by twisting a plane wave (one where the wave front is a parallel plane at right angles to the direction of travel) into a conical wave front, which is analogous to focusing a wave from a spread-out source onto a single point.''

''The slowing occurs at a rate of about one part in a hundred thousand. So in the time it takes unmodified light to travel a meter, the adjusted light makes it 0.01 millimeters less. With some understatement, Padgett and his co-authors note, “Measuring the arrival time of single photons with femtosecond precision is challenging.” The team achieved this by producing strongly correlated photon pairs and having them meet at the destination so that tiny variations in their arrival times would be revealed as phase differences. ''

Smells like a total bullshit, because path light takes is longer than a straight line, hence "slower" speed.
Also, reading further confirms these people have no clue what they are talking about (the part of the hollow waveguide).

I was wondering about that. But it seemed odd that they would make such a silly error.

 

[barbos]

Smells like a total bullshit, because path light takes is longer than a straight line, hence "slower" speed.
Also, reading further confirms these people have no clue what they are talking about (the part of the hollow waveguide).

I was wondering about that. But it seemed odd that they would make such a silly error.

You will be surprised how much bullshit get shamelessly published.
Also this part

The team achieved this by producing strongly correlated photon pairs and having them meet at the destination so that tiny variations in their arrival times would be revealed as phase differences

Suggests that these people do not understand the difference between phase velocity and group velocity.
Basically they don't know physics.
and then this part

Professor Robert Boyd of the University of Rochester, New York, told Science News, “I’m not surprised the effect exists. But it’s surprising that the effect is so large and robust.”

what?! you are not surprised? really?

 

[beero1000]

Posted on arXiv != published.

That isn't to say that bullshit isn't actually published, but peer review catches at least some of it first...

 

[DBT]

Must be an embarassing error given his credentials:

Padgett is internationally recognised for his leadership in the field of optics and in particular of optical momentum. His best known contributions include an optical spanner for spinning micron-sized objects, use of orbital angular momentum to increase the data capacity of communication systems and an angular form of the Einstein-Podolsky-Rosen (EPR) quantum paradox.[5]''

Not to mention the publicity:

''A team of Scottish scientists has made light travel slower than the speed of light.

''They sent photons - individual particles of light - through a special mask. It changed the photons' shape - and slowed them to less than light speed.''

''The photons remained travelling at the lower speed even when they returned to free space.''

''The experiment is likely to alter how science looks at light.''

 

[barbos]

''The photons remained travelling at the lower speed even when they returned to free space.''

''The experiment is likely to alter how science looks at light.''

Professor Robert Boyd of the University of Rochester said "Meh, I knew it"
Preprint is 8 months old without peer reviewed publication. That should give you a clue.

 

[barbos]

And I vaguely remember a thread about that or something very similar at the time it was "pre-published".
Authors said that effect is the largest (or present) when distance is shorter. Which really should give you a pause because it clearly looks like nothing is being slowed down there, it's just idiots have some dT somewhere miscalculated.

 

[DBT]

''The photons remained travelling at the lower speed even when they returned to free space.''

''The experiment is likely to alter how science looks at light.''

Professor Robert Boyd of the University of Rochester said "Meh, I knew it"
Preprint is 8 months old without peer reviewed publication. That should give you a clue.

Fair enough. I'm not arguing for anything, just gathering thoughts on the article.

 

[barbos]

Must be an embarassing error given his credentials:

You would think that but I have seen people who were not embarrassed by worse things.
I read russian website which is about computer related news but they post science news like that too. And they have "british scientists invented" meme there. It means bullshit