1. Jun 28, 2008

### JKFlyguy

"OK, since this is physics and not philosophy, can you please show me an experiment that measured the speed of light in our false vacuum and showed that it is below the published value for "c"? Can you also open, say, the standard value of c as published in CODATA and tell me how they were able to obtain that value?

Zz."

Yes, I can show you an experiment where we slowed light down below light speed with matter. In fact, this is called refraction. Materials have a refractive index that shows how much light is slowed by the medium. Light is never as fast as light in a vacuum, or c, and since there is no true vacuum, that speed does not exist.

Here's how people slowed down light.

http://www.hno.harvard.edu/gazette/2001/01.24/01-stoplight.html [Broken]

Last edited by a moderator: May 3, 2017
2. Jun 28, 2008

### JesseM

That is not what ZapperZ asked. He was responding to your claim that even in what we call a "vacuum" the speed of light is not at its maximum, and asking for evidence of this. If it were true, then the speed of light as measured in a vacuum would be expected to be different than the "c" that appears in relativistic equations describing the behavior of things other than light, like the equation for time dilation as a function of velocity $$t*\sqrt{1 - v^2/c^2}$$ (note that time dilation for an object in a medium does not depend on the speed of light in that medium)

3. Jun 28, 2008

### malawi_glenn

It would be more honest if you linked to the original thread JKFlyguy ...

And again, he wonders what the difference of the thing you call 'false vacuum' and the value of c in that vacuum, and then compare that value with $$c_0$$, i.e the constant that physicsits use as the velocity of light in 'real' vacuum.

4. Jun 28, 2008

### JKFlyguy

I'm not saying it does

I'm saying that it is impossible for light not to run into matter. c will never be the speed of light. c will only be the speed of light in a (very very hypothetical) vacuum.

In the deepest regions of space, molecules still exist 10 meters apart. Matter is still there, it is just not very dense.

I'm saying that you can't measure a speed of light in a vacuum if there is no vacuum. Sure, the speed works for practical purposes (like what could be considered, for the most part, a vacuum compared to pressure at sea level), but if it is supposed to record light without the interaction of matter, you will never have a precise value. In fact, without the presence of matter to base the distance traveled by light off of, then you wouldn't have a vacuum for it to travel through in a first place.

Here's what I'm saying - if you can't have a true vacuum then you can't have the speed of light in a TRUE vacuum because the definition of "the speed of light in the TRUE vacuum" is false because there is no true vacuum.

The vacuum is hypothetical, just like the speed, and so is therefore philosophy, and not science.

Also, somebody locked the other link so i couldn't post to it.

Last edited: Jun 28, 2008
5. Jun 28, 2008

### JesseM

OK, but given that our experimental techniques don't have infinite precision anyway, the factor by which such small number of particles actually slow light down would presumably be too small to make a difference in the number of decimal places we are capable of measuring its speed anyway. I think one should be able to use quantum electrodynamics to quantify theoretically about how much a given density of hydrogen molecules would slow light waves from their ideal speed of c (without making any assumptions about what the value of c actually is).
It's not philosophy because as I said, in principle you can measure the value of the constant c with experiments that have nothing to do with light, like measuring time dilation as a function of velocity for massive particles.

6. Jun 28, 2008

### ZapperZ

Staff Emeritus
I'd be happy if you can make a measurement that showed the difference between the speed of light in air versus in a vacuum of 10-5 torr. If you can show that, then this isn't philosophy. If you can't, you are making a whole lot of hot air with no empirical evidence, and this thread is in violation of our Guidelines.

You might also want to figure out the mean free path of collision when you get to 10^-10 torr, for example. I can easily get that level of vacuum in the beamline that I work at. Want to bet what is the speed of light that I measure?

Zz

7. Jun 28, 2008

### JKFlyguy

"the factor by which such small number of particles actually slow light down would presumably be too small to make a difference in the number of decimal places we are capable of measuring its speed anyway."

How do you know that? Prove that light speed in a vacuum would not be an irrational number. If light speed is infinite compared to motion, then, no, a couple of decimal places really would not matter.

In fact, no numbers would matter at all then because light speed is infinity and you cannot measure infinity (300,000 m/s for whatever your internal frame of reference is) lightspeed is not just c, but X plus c, for whatever X rate you are traveling. Therefore light is both 0 m/s (if you are traveling opposite of our current motion at 300,000 m/s) and 600,000 m/s (if you are traveling 300,000 m/s faster than our motion)

Why is lightspeed not immeasurable?

(keep in mind, i am on a quest for knowledge. Being my devil's advocate helps me tremendously)

8. Jun 28, 2008

### JKFlyguy

No, I can't show you a huge difference. I bet that I can show you a very small one. I bet I can show you an even larger difference between that and the refraction index of water. What I'm getting at is that you can get closer and closer to it, but you can never find the exact speed of light.

9. Jun 28, 2008

### ZapperZ

Staff Emeritus
Then SHOW IT! In physics, one has to show that the idea is empirically valid.

Zz.

10. Jun 28, 2008

### JesseM

The phrase "infinite compared to motion" seems meaningless. And note that I said the difference would be too small to measure, not that there would be no difference at all--do you understand that even if the actual value of c was irrational, any measurement would only have a finite precision, and that all measurements only go to a certain number of decimal places and include a set of error bars?
Again you appear to be talking gibberish here. If light speed "is infinity" then it can't have a finite value like 300,000 km/s.
In relativity velocities don't add that way--if you see me moving at speed v in your frame, and I shoot an object ahead of me which moves at speed u in my frame, the speed of that object in your frame is not u+v as it would be in Newtonian physics, instead it's (u + v)/(1 + uv/c^2). You can see that if the object I shoot ahead of me is a photon which moves at c in my frame, then in your frame it will move at:
(c + v)/(1+cv/c^2) = (c+v)/(1 + v/c) = c*(1 + v/c)/(1 + v/c) = c.

So if something moves at c in one frame, this formula says it moves at c in every frame. In fact this is one of the basic postulates of relativity.

Also, in relativity there is no absolute truth about "whatever X rate you are traveling"--the "rate you are traveling" is different in different frames, and no inertial frame is preferred over any other.

Last edited: Jun 28, 2008
11. Jun 28, 2008

### RandallB

Just because you cannot attain and measure something moving at the speed “c” does not mean the value “c” is hypothetical as if it did not exist.
Your logic is like claiming that the tangent to a circle cannot exist as a ‘real’ value because it is a straight straight line and there are no two points on the curve of a circle that are actually straight from which to extend such a tangent. Leibniz and then Newton taught us how find a real value for such things by finding their limit mathematically.

The speed of light as defined by “c” is a real value that be determined by finding its limit mathematically as well.
The logic your using is a sophist technique not much different than the dichotomy paradox (Zeno's paradoxes) attempt to turn the theory of motion against itself to validate sophism.

12. Jun 28, 2008

### JKFlyguy

"Again you appear to be talking gibberish here. If light speed "is infinity" then it can't have a finite value like 300,000 km/s."

yes, that is true. 300,000 is NOT infinity. But geometric infinity is 1 + the square root of five over two. And that has a value - 1.6180339887... that value is undefined (irrational) because the decimal places go on forever. Fractals geometrically use the golden ratio to logically repeat forever. Lots of constants in our universe, including pi and the golden ratio, cannot ever be finitely measured. The thing is that nothing can ever get to the limit; that's why it's a limit.

13. Jun 28, 2008

### JesseM

OK, but no one calls that "infinity", it's just an irrational number (the decimals of 1/3 also go on forever in base 10, but they repeat). And did you read what I said about all experimental measurements of physical constants having error bars? No one claims we can find perfectly precise values for the constants of nature.

14. Jun 28, 2008