Do we really know the speed of light ?

Question

with our experiments limited to a very very small portion of this galaxy (which is a a very very small portion of ...). How can this be in the KNOWN FACT catagory ?

HEY ALBANNACH read your references such as the link to LIGHT SPEED REDUCTION IN AN ULTRACOLD ATOMIC GAS what do YOU know about the conditions 10 galaxies away thanks for the help !!!!!!!!!!!!

wait don't do this' COMMON SENSE' has declared that we need not research this anymore WE KNOW THE ANSWER !

Answer 1

There are really 2 questions here, and they have very different answers:

1) We know the speed of light is a constant in our part of the universe because we can, and have, measured it on earth. No matter what technique is used (laser interferometry, motion of planets/moons of near objects, etc.) we always obtain the same answer within experimental uncertainties. So for those purposes, it is a "known fact" that the speed of light is a constant that can be measured.

2) However, we do not know if the speed of light is a universal constant throughout the universe, throughout all of time (time and space invariant). We can only hypothesize that it is, design our grand theories on the workings of physics, and perform experiments that will either tend to confirm or disprove these hypotheses.

Two of the fundamental ideas of our system of physics are that the laws of physics are time and space invariant. That's only a hypothesis - and you are correct that in our tiny portion of the universe, at this insignificant moment of eternity, we do not have enough information to state whether the speed of light is time and space invariant.

Answer 2

Well there isn't really any such thing as a "known fact", just useful theories. As it happens there is evidence to suggest that c (the speed of light) is flexible. It may have been slightly slower immediately after the big bang. But again, this still doesn't count as a "known fact" and may later turn out to be incorrect.

Answer 3

On Earth, in a vacuum, the spead is always the same, about 186,000 miles per second. However if it's always that speed in other places we don't really know. It would sure confuse light years though! There is some belief in some Creationists that the speed of light, or c, is decaying. That would explain how light from distant stars has made it to earth in just about 6,000 years. That's just an idea though. For now, it appears constant in our part of the universe.

Answer 4

Yes. The speed of light you are talking about refers to the speed in a vacuum. However the speed can slow down in mediums such as glass.

To measure light all you have to do is have a set distance in a vacuum. Then have a light detector at one end and a light emitter at the other with a very accurate clock.

Answer 5

Your point is well taken. There may even be some evidence that the speed of light changes over the aeons.

The velocity of light in a vacuum has been demonstrated to be consistent enough locally to become the accepted standard for the meter, since the duration of the second is defined so precicely.

Answer 6

The speed of light and other theory's may well prove to be different as more knowledge is available to us. But as for now in a vacuum it is the same and the fact that it is the same is what led us to understand that time and space are really the variables in the equations.

Answer 7

They measure the speed of light here on earth. If turn on a flashlight the speed in which it takes to go across say a football field can be measured.
The interesting thing is how nothing can travel faster than light for the power needed to propel a craft faster than that would have to outweigh the mass of that object and if your moter wighs more than your craft than it cant be attached to it for its mass would than be included in the total mass of the craft.
I hope I explained that right.

Answer 8

The speed of light in a vacuum is a constant, that is universal. It is in the KNOWN FACT category because it has been measured.

http://en.wikipedia.org/wiki/Speed_of_Light#Measurement_of_the_speed_of_light

Answer 9

And God said, Let there be light: and there was light. And God saw the
light, that it was good: and God divided the light from the darkness.
* * *
Why the quantum of light has such magic,
constant speed of motion с = 1 ?
* * *
On the planet of Earth there is the law - that every newborn cries,
and each nestling in order to be born must break the shell of the egg.
His new life starts with this moment.
In Kingdom of eternal cold / Vacuum / there is its law :
each quantum of light for its birth should make internal impulse h = 1
and then it gains constant speed of motion с = 1.
His new life starts with this moment.
===================================
Why the quantum of light has not such “magic “
constant : mass of rest equal to zero m = 0 ?
* * *
According to quantum theory, the particle's mass increases at approaching
to the speed of light quantum.
And at achievement of this speed, the particle's mass becomes infinite.
But the quantum of light is real particle and its mass is particular and not infinite.
How to leave this contradiction?
And the scientists invented an artful way: the quantum
does not possess the mass of rest and it is always in motion.
All other material particles have the mass of rest and
only quantum of light has a privileged position.
Why?
May be it so differs from material particles, that it is not material, is it?
And if it is not material, what is it?
It is written in the Bible:
«And God said: let there be light, and there was light.
And God saw the light and that it was good».
However, the Scientist interfered with the God's business and added:
« But let the quantum of light not possess the mass of rest and let it move eternally.
And who will doubt, let him be betrayed to anathema and fire! ».
Yes! It is difficult to be quantum of light.
All particles have mass of rest, and it does not.
All particles can have a rest, lie down, have a sleep.
And it, poor, does not have a second of rest, all the time in motion.
For what sins, I ask.
And as though God is pleased with it and praises it.
Only evidently, the quantum of light did not please the Scientist
with something, and he is angry about it and without pity runs it in the world.
God, save and protect me from such mentality.
=======================
Quantum of light is a privileged particle.
Only the speed of a light quantum in Vacuum has
a maximal, constant, absolute quantity of c=1.
No other particle can travel with the speed c = 1.
If quantum of light flies always rectilinearly c=1, it is a mad one.
Is he really mad?
No.
In Vacuum, in a condition of rest its internal impulse is equal to zero h=0.
But Quantum of Light has two kinds of internal impulse.
1)Under one internal impulse (Planck,s spin h =1)
a quantum of light flies rectilinearly with speed (c = 1).
A quantum of light behaves as a particle.
2) Under other internal impulse (Goudsmit-Uhlenbeck's spin ħ = h / 2)
a quantum of light rotates around of his diameter and is known as electron.
A quantum of light behaves as a wave.
Very strange particle is quantum of light.
Quantum of light stays in Vacuum and on it nobody and nothing renders influence.
It is independent and makes a decision in which of three conditions it occurs.
1. In a condition of rest its internal impulse is equal to zero h=0.
2 .In a condition of uniform rectilinear movement its impulse h=1.
3. In a condition of rotation around of his diameter its impulse ћ =h/2π.
So it can work only with particle that has his own consciousness.
They are alive, spiritual particles.
His own consciousness is not static but can develop.
The development of conscious scale goes " from vague wishes up to a clear thought ".
This evolution proceeds during hundred millions (billion) years.
Socratus.
http://www.socratus.com

Answer 10

Are you saying you think the speed of light is different elsewhere in the universe? If so I dont think so. I think it is the same as it is here.