# Speed of Light frame of reference

#### SanderBuruma

##### New member
Hi all,

I've been wondering over the last couple days over the speed of light and what reference frame it is based on. Officially (ie. on Google/Wiki) the speed of light is 299,792,458 m/s. The problem I haven't found an answer for yet is what that is in reference to?

Is in reference to the earth, the sun, the galactic core or something else? If it is in reference to the sun then the value may be off by up to ~400m/s due to the earths rotation. If it is in reference to the Great Galactic Core (of our galaxy) then it'll vary by up to 30km/s depending on which way our orbit is inclined compared to the suns path of travel and our position along our orbit at the time of measurement.

How accurately do we know for sure that our measure of the speed of light is?

#### Urwumpe

##### Not funny anymore
Donator
It is its own reference. The nice feature of the universe is, that the vacuum speed of light is always the vacuum speed of light - regardless if you add or subtract relative velocities to it.

Regardless how you are travelling relative to a light-beam, it will always be moving at the speed of light to you - unless you travel at the speed of light yourself. It is one of the few real universal constants in our universe.

The fun question is : What would you see, if you are moving at the speed of light?

http://en.wikipedia.org/wiki/Michelson–Morley_experiment

[ame="http://en.wikipedia.org/wiki/Modern_searches_for_Lorentz_violation"]Modern searches for Lorentz violation - Wikipedia, the free encyclopedia[/ame]

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#### rainiere

##### New member
Exactly, this fact is the very core of relativity.

#### HarvesteR

##### Member
or subtract relative velocities

Really? C is constant, but it is not infinite. Is it really impossible to decelerate from 1C in practice? (or well, whatever it is that we can call practice in theoretical physics).

Not doubting, merely curious.

Cheers

#### Urwumpe

##### Not funny anymore
Donator

54000 m/s is not c. Not even near it. Cherenkow radiation is caused by particles travelling faster than the "speed of light inside the medium". Which does not mean photons move at less than c. They just get absorbed and reemitted by the molecules along the way.

---------- Post added at 11:29 PM ---------- Previous post was at 11:06 PM ----------

Really? C is constant, but it is not infinite. Is it really impossible to decelerate from 1C in practice? (or well, whatever it is that we can call practice in theoretical physics).

Practically yes.

But then, you can cheat. For example you can slow light down to pedestrian speed - but not the photons that constitute the light. These still travel at c. you just make the light of many photons behave like in slow motion.

#### HarvesteR

##### Member
Ah I see. So this applies to photons and massless things in general.

It makes sense. In the same way a non massless object cannot reach C without becoming impossibly massive and blowing up physics, it's similarly impossible for a massless object like a photon to travel at any less than C.

Is my reasoning right?

Cheers

#### Urwumpe

##### Not funny anymore
Donator
Ah I see. So this applies to photons and massless things in general.

It makes sense. In the same way a non massless object cannot reach C without becoming impossibly massive and blowing up physics, it's similarly impossible for a massless object like a photon to travel at any less than C.

Is my reasoning right?

Cheers

Pretty much yes. As far as we know today.

And we of course wish to be wrong there. :lol:

One important question around the Higgs boson is the question, how you can turn such energy that can move only at c, into energy with mass that cannot.

#### HarvesteR

##### Member
Hehe, we certainly do... Hey, FTL travel only violates relativistic causality... There may be hope still.

Cheers

#### SanderBuruma

##### New member
My question still remains then, how accurately do we really know the speed of light? I've already explained that I think that the observers point along earths orbits contributes a potential inaccuracy up to 30km/s and ~0.4km/s from earth's rotation.

#### HarvesteR

##### Member
There is no such inaccuracy, because the core postulate of special relativity is precisely that regardless of reference frames (be it solar-centric or earth-centric), C (and all other laws of physics) will be the same. From this, we must conclude that time must be variable.

Cheers

#### boogabooga

##### Bug Crusher
There is no such inaccuracy, because the core postulate of special relativity is precisely that regardless of reference frames (be it solar-centric or earth-centric), C (and all other laws of physics) will be the same. From this, we must conclude that time must be variable.

Cheers

Correction, INERTIAL reference frames, which earth is not, due to rotation and revolution. But I would assume this has been corrected for by now?

#### jangofett287

##### Heat shield 'tester'
The value of C is exactly 299,792,458 m/s. The Meter is defined as the distance light travels in 1/299,792,458ths of a second.

#### boogabooga

##### Bug Crusher
The value of C is exactly 299,792,458 m/s. The Meter is defined as the distance light travels in 1/299,792,458ths of a second.

Doesn't eliminate the problem. The question just becomes "are all of our tools that measure a meter correct?"

My question still remains then, how accurately do we really know the speed of light? I've already explained that I think that the observers point along earths orbits contributes a potential inaccuracy up to 30km/s and ~0.4km/s from earth's rotation.

Wait,
If you mean you are treating the revolution of the earth as if it were an inertial frame moving at 30km/s, then no. It wouldn't directly add like that.

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#### Linguofreak

##### Well-known member
Hi all,

I've been wondering over the last couple days over the speed of light and what reference frame it is based on. Officially (ie. on Google/Wiki) the speed of light is 299,792,458 m/s. The problem I haven't found an answer for yet is what that is in reference to?

In the mid 1800's, scientists thought that the speed of light had to be in reference to something, and started devising experiments to try an determine what the reference frame for the speed of light was. And they got a very weird result: The speed of light is exactly the same in every reference frame. The theory of relativity was developed to explain this phenomenon.

#### jedidia

##### shoemaker without legs
My question still remains then, how accurately do we really know the speed of light?

Not quite as accurately as we know Pi, but as with Pi, the inaccuracy is pretty much negligible at this time.

I've already explained that I think that the observers point along earths orbits contributes a potential inaccuracy up to 30km/s and ~0.4km/s from earth's rotation.

You still didn't understand the central thing here. LinguoFreak already described it, I'll try to expound on that a bit.
Once upon a time A guy (I forgott his name) made a thought experiment: If he was on a ship, below decks in an isolated room on an absolutely calm sea, could he tell if the ship was moving or not? He came to the conclusion that he could, IF the ship was accelerating. If it wasn't, there was no way he could tell how fast it was moving or if it was moving at all. Any measurement that could settle the question needed a reference from outside the room.
He thus postulated the the laws of nature are exactly the same in any inertial frame of reference (that is, a frame of reference moving at a constant velocity).

So later, as Linguofreak stated, they measured the speed of light in the hope of finding the Universes Rest-frame, only to be grieviously disappointed and finding that the speed of light in a vacuum was constant no matter how you turn it. It was thus postulated that the speed of light is in fact a law of nature, and thus the same in any given inertial frame of reference.

Along comes Einstein, a man with a keen ability to spot the odd detail nobody thought about, and went: "Wait a minute, velocity is distance by time. If it is constant in any reference frame, then somebody is screwing with my watch, or my ruler, or both!" And thus a whole new understanding of the universe was born...

#### sorindafabico

##### New member
You can measure speed of light itself (the first of this kind was the measure of the "delay" of an Io transit of Jupiter caused by the limited speed of light) or calculate it by measuring vacuum permeability and vacuum permittivity, since c=1/sqrt(u0*E0). But I don't know what kind of measurement is more precise.

#### n122vu

Donator
SanderBurma, let me ask you this. Picture cars travelling on a US Interstate highway, as shown in the attached. Some cars continue travelling on the Interstate at a constant speed, some decide to decelerate and exit.

Does the speed or position of the cars on the exit ramps in any way affect the speed of the cars that continue straight along the highway?

#### Attachments

• Highway Clover Leaf.jpg
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#### SpaceNut

##### New member
I don't see how something itself can be it's own reference. I always understood it as the point of observation. For example, if I'm in space and turn on a flashlight, the light is moving away from me at the speed of light, hence, I'm the point of reference. Now let's say you're behind me, and moving towards me (relative to me), and I turn on a flashlight. Even though I'm not moving relative to the flashlight, and you are, we'll both measure the speed of light as the speed of light. Which is where the "problem" is. How can something viewed from two different frames of reference ( you moving relative to the flashlight, and me not), measure the light moving at the same rate? The answer is of course, time, as you are moving relative to me, you pass through time slower than I do, and hence observe light moving at the same rate as me. No where is "light it's own reference".

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