# Thread: "true" speed of light 187,000 mps?

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## "true" speed of light 187,000 mps?

I was reading a book called "Terrestrial Energy" (book is about energy sources, notably nuclear and how it is better than people give it credit for) and the author cited the speed of light at 187,000mps. Having read countless times that the speed of light is supposed to be 186,000mps, I at first thought it was an error. Then I thought "Awfully big error, must be something to it" so I looked into it more.

I found this website

http://www.is.wayne.edu/mnissani/a&s/light.htm

and saw the same 187,000 mps number. However, the article closes out with

"Today, we have excellent reasons to believe that nothing in the universe can possibly travel faster than 186,000 miles in one second."

That seems conflicting to what information was stated in the paragraph just above. Anyone able to clarify this perplexing issue? textbooks have been stating 186,000mps for decades now. Is this in error?

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It's just sloppy use of numbers. In fact, neither figure is accurate as they're both approximations to 186,282 mi/s (or more correctly in SI units 299 792 458 m/s).

3. Order of Kilopi
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The first number is incorrectly rounded up to three significant digits;
the other is correctly rounded down to three significant digits.

-- Jeff, in Minneapolis

Last edited by Jeff Root; 2009-Aug-17 at 05:23 AM. Reason: Read first, then write

4. Google says that the speed of light now is defined as exactly 299,792,458 meters per second which would be 186,232.4 miles per second. In other words, the speed of light now is used to exactly define the length of the meter.

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Originally Posted by Tuff
I found this website

http://www.is.wayne.edu/mnissani/a&s/light.htm

and saw the same 187,000 mps number. However, the article closes out with

"Today, we have excellent reasons to believe that nothing in the universe can possibly travel faster than 186,000 miles in one second."

That seems conflicting to what information was stated in the paragraph just above. Anyone able to clarify this perplexing issue? textbooks have been stating 186,000mps for decades now. Is this in error?
In the link you gave...You apparently read it wrong. It was comparing the Roemer's figures with two later measurements (in 1890 and in1874) and the later ones obtained 187,000 mps.

G^2

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Originally Posted by Tuff
I was reading a book called "Terrestrial Energy" (book is about energy sources, notably nuclear and how it is better than people give it credit for) and the author cited the speed of light at 187,000mps.
By definition, the speed of light is exactly 299,792,458 m/s. In fact, that's how the meter is now defined.

According to Google, that's 186,282.397 miles per second. So, even allowing for rounding error, those textbooks are wrong.

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Originally Posted by Gsquare
In the link you gave...You apparently read it wrong. It was comparing the Roemer's figures with two later measurements (in 1890 and in1874) and the later ones obtained 187,000 mps.

G^2
The last sentence where 186,000 is re-stated seemed contradictory. Plus 187,000 is apparently wrong from what I am reading here.

8. Yep. 187,000 is a wee bit too fast. Mugaliens gives the correct values.

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Originally Posted by mugaliens
By definition, the speed of light is exactly 299,792,458 m/s. In fact, that's how the meter is now defined.
That's correct now, at least.

But doesn't gravity affect the speed of light? No doubt, the definition of the speed of light will be refined even further.

10. Originally Posted by Gandalf223
That's correct now, at least.

But doesn't gravity affect the speed of light? No doubt, the definition of the speed of light will be refined even further.
No. The speed of light is constant no matter how deep the gravitational well that you are in, since a gravitational well simply is distorted space-time. Astronomers and physicists talk of light being "bent" by strong gravitational wells such as the sun or an intervening galaxy (gravitational lens). Yet in all cases the light traveled a perfectly straight path (from the light's point of view) through any distorted space-time caused by gravitational wells.

11. Order of Kilopi
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Measurement of the speed of light to nine significant digits was so consistent
that it was made the standard for defining the SI unit of length and all units
derived from it. So any further refinement of the speed to higher precision
could leave the least significant digits zeros. Whether it is done that way or
not, further refinement is extremely unlikely to change the ninth digit.

I don't know why the book which prompted the original post says "187,000",
but the same number in the web page linked in the original post is correct
for historical reasons. That page uses a value for the speed of light rounded
to three significant digits. There's nothing at all wrong with that.

-- Jeff, in Minneapolis

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