PDA

View Full Version : Speed of Light Moving Relative to What?



Wicked Lad
2007-Aug-30, 02:58 PM
(Amateur and n00b here. All relevant apologies-in-advance and disclaimers apply.)

When we speak of the speed of light being (pretty much) constant, we mean the speed of light as it moves relative to what? Isn't everything in the universe moving? Is there a "center" of the universe that can serve as a reference point for the speed of light?

BTW, I came here wondering how the most distant object we can observe is 13.6 billion light years away, dating to when the universe was .9 billion years old, thinking about what that implies about the universe's (accelerating!) rate of expansion and mass of objects. Searching the site I found some explanations to the effect that space is expanding faster than the speed of light, but objects themselves are not moving that fast. I'm still trying to get my wimpy 3-D mind to picture how the distance between two objects can be increasing at greater than the speed of light, and yet they may not be moving, relative to one another.

01101001
2007-Aug-30, 03:09 PM
When we speak of the speed of light being (pretty much) constant, we mean the speed of light as it moves relative to what?


(In a vacuum) everything.

Maybe better: it is constant in each and every reference frame.

Wikipedia: Special relativity (http://en.wikipedia.org/wiki/Special_relativity)


First postulate - Special principle of relativity - The laws of physics are the same in all inertial frames of reference. In other words, there are no privileged inertial frames of reference.
Second postulate - Invariance of c - The speed of light in a vacuum is a universal constant, c, which is independent of the motion of the light source.

01101001
2007-Aug-30, 03:37 PM
I'm still trying to get my wimpy 3-D mind to picture how the distance between two objects can be increasing at greater than the speed of light, and yet they may not be moving, relative to one another.

Cosmology Primer FAQ (http://preposterousuniverse.com/writings/cosmologyprimer/faq.html#ftl):


Are distant galaxies moving faster than the speed of light? Wouldn't that violate relativity?
A profound feature of relativity is that two objects passing by each other cannot have a relative velocity greater than the speed of light. An even more profound feature, one which has received much less publicity, is that the concept of "relative velocity" does not even make sense unless the objects are very close to each other. In Einstein's general theory of relativity (which describes gravity as the curvature of spacetime), there is no way to define the velocity between two widely-separated objects in any strictly correct sense. The "velocity" that cosmologists speak of between distant galaxies is really just a shorthand for the expansion of the universe; it's not that the galaxies are moving, it's that the space between them is expanding. [...]

Sock puppet
2007-Aug-30, 03:52 PM
When we speak of the speed of light being (pretty much) constant, we mean the speed of light as it moves relative to what? Isn't everything in the universe moving? Is there a "center" of the universe that can serve as a reference point for the speed of light?

Relative to everything.
If i stand still, and measure the speed of light in all directions, i will measure it to be c.
If I jump on a bus going at 60 km/h and perform the same measurements, I will measure it as c.
If I get in a rocket and accelerate (relative to the earth) to .9c, and repeat my measurements I will still find that light travels at c relative to me!

This is the really counter-intuitive thing about special relativity. We're all used to using newtonian mechanics, and adding velocities with galilean transformations (that just means: if a train is going at speed x, and I run after it at speed y, I see it as moving at x-y). But they just don't apply at relativistic velocities.

John Mendenhall
2007-Aug-30, 04:30 PM
Look at it this way: no matter where you are or how fast you are going relative to things around you, you will always measure the speed of light as c, both from light emitters traveling with you and from ones traveling past you.

And try the links previously posted above, they're all good.

Wicked Lad
2007-Aug-30, 05:12 PM
Thank you, folks. I'll read those links. From your explanations, I can see how we get strange phenomena like differences in elapsed time for someone who's standing still versus someone who's traveling fast. That is, it helps me understand some of this stuff in the abstract.

My Newtonian brain, however, rebels. How do we know which observer is traveling fast and which is standing still, other than by the relative elapsed time? (That's a rhetorical question, at least for now. I'll ponder as I peruse.)

You've served up a lot of good food for thought, and I'll chew through it. Again, many thanks!

Wicked Lad
2007-Aug-30, 05:19 PM
(Psst! John! I grew up near where your profile places you. I have a question about your name. Your inbox is full, so I can't PM you. If you wouldn't mind such a question, would you please send me a PM or e-mail about how to reach you?)

mugaliens
2007-Aug-30, 05:30 PM
(Amateur and n00b here. All relevant apologies-in-advance and disclaimers apply.)

When we speak of the speed of light being (pretty much) constant, we mean the speed of light as it moves relative to what? Isn't everything in the universe moving? Is there a "center" of the universe that can serve as a reference point for the speed of light?

I actually asked the same question here, once. At least I've learned something:

The speed of light will always appear to be moving at the speed of light relative to the observer, no matter where the observer is or what his velocity might be.

The only indication of relative motions that are available is the red-shifting of known frequencies, including spectra of known elements, or a frequency such as the Pioneer sending unit.

alainprice
2007-Aug-30, 10:25 PM
We are all accustomed to relativity without knowing it.

If I drive at normal speed down the highway, I am going 65 mph. This is only true if we all understand that I am travelling at 65 mph RELATIVE to the ground beneath the tires. Relative to a river current running beside the road, I may be travelling faster or slower.

If I get into a head on collision with another car, you automatically assume the speed of impact was 130 mph. This is because you rightly assumed my speed is relative to the other car.

Everytime we say a speed(or velocity), we are speaking relatively.

mugaliens
2007-Aug-31, 08:16 AM
Everytime we say a speed(or velocity), we are speaking relatively.

Yes - you're speaking relatively.

No - You're not speaking of relativity.

Car A and Car B travelling towards one another at .6 c do not see it as a looming 1.2 c crash. It's close to a .92 c crash. And despite the fact that, to an outside observer, their rate of closure appears to exceed the speed of light, to both cars, it does not, and they can actually see one another.

That's relativity vs relatively.

grant hutchison
2007-Aug-31, 11:32 AM
It looks as if alainprice is using the term with reference to Galilean relativity rather than special relativity. That's a kind of simple relativity that we encounter every day, as he describes.

Grant Hutchison

mugaliens
2007-Aug-31, 11:48 AM
But that's not the relativity that wicked lad was talked about when he asked, "Speed of Light Moving Relative to What?"

Or, more specifically, "When we speak of the speed of light being (pretty much) constant, we mean the speed of light as it moves relative to what? Isn't everything in the universe moving? Is there a "center" of the universe that can serve as a reference point for the speed of light?"

That question cannot be addressed, as you know, with Galilean relative motion, as the next question becomes, "But what about two people, each travelling .6 c, moving away from one another?" which has already, and quite recently, been well addressed, here.

grant hutchison
2007-Aug-31, 12:37 PM
I was just remarking on what alainprice seemed to be up to, and how it can quite correctly be called relativity. Presumably alainprice will come back and argue relevance if he feels strongly about the matter. :)

Grant Hutchison

alainprice
2007-Aug-31, 06:08 PM
Why not?

If we add a postulate that there is no absolute center and the speed of light (in vacuo) is constant, we can derive special relativity.

My main point is that we speak of velocities, we speak of them relatively. In other words, no one should need to ask "relative to what?" . You tell me.

mugaliens
2007-Aug-31, 06:12 PM
Ok. Also: :)

John Mendenhall
2007-Aug-31, 09:27 PM
(Psst! John! I grew up near where your profile places you. I have a question about your name. Your inbox is full, so I can't PM you. If you wouldn't mind such a question, would you please send me a PM or e-mail about how to reach you?)

Mail fixed, sorry about that.

EvilEye
2007-Sep-02, 01:26 PM
Carl Sagan explained it pretty simply.

If light was not constant in reletivity, then when approaching a crossroad, I would veer to avoid someone coming the other way before they were ever even there. You CANNOT add YOUR speed to the speed of light.

It is just nature's way of keeping things simple. Imagine if you had to adjust a camera because your nose is reflecting light before your forehead!