Is Light Truly Constant Despite Its Properties?

[f95toli]

The current speed of light was estimated at least 150 years ago by a guy with mirrors measuring minute angles, and that popluar opinion has remained the same ever since.
Don't mean to be a doubting Thomas, but I do like to play the deils advocate occaisionally.
Mainly because I fear that accepting what everyone else has already taken for granted may cloud my ability to identify hidden truths.

Before the re-definition of the meter in the SI 1983(?) several NMIs put a LOT of effort into acuratelly measuring the speed of light. This means that extremely accurate indepdendent measurements were done in labs all over the world over a long period of time; needless to say they all got the same value (with some the error bars). Some labs even built dedicated facilities to do this; meaning a lot of money and many man-hours went into this project.
It might perhaps be worth pointing out that it is not at all "obvious" that they would get the same value; measurements at this level are extremely complicated and small errors can cause a lot of problem; but a change in the SI is only done if ALL labs agree to within some agreed accuracy.

(right now the two best measurments of Planck's constant do NOT agree which is why the SI won't be modified to take these values into account this time round)

The point is that we can be VERY sure that the speed of light is constant and that the current value is extremely good (although we can of course never be 100% sure about anything in science).

 

[statdad]

"But just as was mentioned, many people once believed the Earth to be flat, because that's was the popular opinion. "

this is actually not true: the notion of the Earth being essentially spherical (decidedly not flat) occurred long ago: Aristotle reasoned the Earth wasn't flat by considering how the constellations appeared to rise earlier when sailors were headed south than when they were headed north. that was about 330 BC.

Pliny the Elder asserted, in approximately 1AD, that people should be in agreement that the Earth was spherical.

Indian astronomers reached the same conclusion later, and Jesuits had a hand in spreading the idea of a round Earth in parts of china in the 17th century.

Why do I mention this? To indicate that it doesn't make sense to make a scientific assertion that runs counter to theory and evidence, and fall back on "well yes, but for many years scientists thought the Earth was flat, and look how that turned out" as your defense.

I'll now return to "statistician with an interest in history and physics who lurks around reading these posts" mode.

 

[DukeofDuke]

Hello DukeofDuke.

Light transmission was known to be not instantaneous long before special relativity was required to solve other problems. The postulate of the same constant speed of light for all inertial observers is really the cornerstone of special relativity. What that speed happened to be is immaterial.

It is of course true that relativistic effects are only significant at very high velocities, but they apply at any subluminal velocity.

Matheinste.

yeah I'm agreeing with all of that =)

All I'm saying is that at very slow relative speeds, light seems to be practically instantaneous. Whereas, when you take speeds a tenth or more of light you think you'd observe light slowing or speeding up as you go with it classically, yet the valid assumption is made that c is the greatest possible speed (at least, that's the assumption for the Lorentz), and from that we get space and time dilation to fix the apparent contradiction. And then we have experiment verify it for us.

I was just mentioning noticeably "lag time" in information flow as one of the reasons special relativity makes some intuitive sense. Merely noting that before you even need to think about the Lorentz transformations, you can sort of make sense of some correction needed to classical ways of thinking about speed, because if you go fast enough you'll lose the "instantaneous" apparent affect. And you'd think that'd mess with your perceptions of space and time, because our assumption that event A happened at this time t is based on the assumption that we as an observer can locally measure some light from event A at around this time t as well. Once you get to relativistic speeds, you need to redefine your notion of space and time to account for the fact that you don't really know what's going on where, because its impossible for signals to reach you at anything considered near instantaneous. =)

 

[Pippo]

I understand that it makes the relativity theory work nicely, but...


Whether it's a particle or wave, light must have mupltiple speeds.
Light particles can be charged or excited which would increase the speed of the particle.
Light also has different wavelengths, each with different oscillations and travel length.


All this seem to support that light is NOT a constant.

I would say:
Maximum speed is just a characteristic of the environment called space-time and speed of light is a show of it, so it is independent by the intensity or frequency. Light conforms with, and so well describes, the behaviour of our space-time.