Speed of light relative to what?

[Theo1]

Can someone help me with speed relative to different objects!

Nearly everything in everything in everyday life is relative to the earth, however if i was traveling a little be slower than the speed of light, relative to the earth, why am i not traveling faster than the speed of light relative to the sun or the centre of the galaxy or another galaxy (when the Earth adds to your speed relative to these other objects).

Another example is with a bulb and photons of light:
Photon 1<-----|Bulb|----->Photon 2...(photons traveling at speed of light)
If each photon is traveling at the speed of light relative to the bulb, at what speed is photon 1 traveling at relative to photon 2...twice the speed of light?

I have read that you travel relative to the ether or something, however i do not fully understand that theory!
Could you please help because however much i read i still can't really understand it

 

[mathman]

In all inertial systems the speed of light (in a vacuum) relative to any object in the system is the same (c). When you travel at near relativistic speeds compared to something else, the speeds don't simply add.
See http://en.wikipedia.org/wiki/Special_relativity to get appropriate formulas for velocity composition as well as a general picture of the subject.

 

[Meir Achuz]

The speed of light relative to anything is thw same c.

 

[physeven]

c. c will always be c.

 

[TheTechNoir]

I have read that you travel relative to the ether or something, however i do not fully understand that theory!
Could you please help because however much i read i still can't really understand it

LOL
Sorry I'm not laughing at YOU, just the misunderstanding is pretty funny because of how dated it is.

As others have said the speed of light is always the same for every observer. So no matter your speed the speed of light doesn't change (which is what causes and requires the strange features of special relativity like length contraction and time dilation). Anyways, you were probably reading about the ether in the context of history and what WAS believed to be the case (this is why I found it funny, it's a very very very old theory that was debunked a hundred years ago).

Before Einstein it was believed that there was an absolute reference frame called the ether in which everything moved relative to. The we could never catch up to a beam of light like we should be able to do if the ether was true was what made Einstein first conceive of special relativity I believe.

 

[physeven]

LOL
Sorry I'm not laughing at YOU, just the misunderstanding is pretty funny because of how dated it is.

As others have said the speed of light is always the same for every observer. So no matter your speed the speed of light doesn't change (which is what causes and requires the strange features of special relativity like length contraction and time dilation). Anyways, you were probably reading about the ether in the context of history and what WAS believed to be the case (this is why I found it funny, it's a very very very old theory that was debunked a hundred years ago).

Before Einstein it was believed that there was an absolute reference frame called the ether in which everything moved relative to. The we could never catch up to a beam of light like we should be able to do if the ether was true was what made Einstein first conceive of special relativity I believe.

Well i guess its funny now since we know better. but i can imagine myself laughing at the theory back then. I MYSELF would be the laughing stock... but fortunately i was born in a period where they are the laughing stock and not i. good point there.

 

[TheTechNoir]

Yeah it's also even just the misunderstanding it's self that I find funny, not so much the once held belief of the ether (that seems very intuitive/reasonable). But the fact that he read about a -really- old theory (pre-1900s) and thought it was the current state of things. Now if it was a theory from, say, 1999 maybe it was thought to be true when written/uploaded but now is known to be debunked. But obviously anything online talking about the ether is strictly a historical account as it predates the internet.

Very easy mistake to make though if you're new to special relativity and read something unaware it is in a historical context, just amusing. :)

 

[ghwellsjr]

Before Einstein it was believed that there was an absolute reference frame called the ether in which everything moved relative to.

...but now is known to be debunked.

All kidding aside, do believe the theory of Special Relativity debunks the idea of an absolute reference frame?

 

[mathman]

All kidding aside, do believe the theory of Special Relativity debunks the idea of an absolute reference frame?

That is one of the fundamental postulates of special relativity.

 

[Theo1]

LoL...i knew it was old and i thought it had been disregarded however i don't understand like 70% of special relativity so i just put it in there as a small possibility. Thanks for not TOTALY ripping into me tho! I think i understand it a lot more now...

 

[justPAB]

Since we're talking about light.. can you guys clarify this confusion of mine:

Once a star blows up 50 light years away, does the light emitting from the star continue to shine for 50 years before it stops shinning; even though the star is no longer there?

 

[mariush]

Since we're talking about light.. can you guys clarify this confusion of mine:

Once a star blows up 50 light years away, does the light emitting from the star continue to shine for 50 years before it stops shinning; even though the star is no longer there?

Well, when you look up on the sun, what you really see is an 8.3 minute old version of it. The same is true for yours. But the light doesn't actually stop shining, it only reaches you.

 

[Theo1]

Correct me if I am wrong...
But all the light is emitted in a short amount of time, it then takes 50 years to reach us and we can see it as it look 50 years ago, as it blew up...that why its a bit like looking into the past - when u look at a normal star, say 2 light years away you see it as it looked 2 years ago, for all we know it may have gone by now

My little graphic didn't work...nvm

 

[justPAB]

Correct me if I am wrong...
But all the light is emitted in a short amount of time, it then takes 50 years to reach us and we can see it as it look 50 years ago, as it blew up...that why its a bit like looking into the past - when u look at a normal star, say 2 light years away you see it as it looked 2 years ago, for all we know it may have gone by now

My little graphic didn't work...nvm

Yeah that's exactly what I mean, are we right or wrong? lol

 

[Aemilius]

Can someone help me with speed relative to different objects!

Nearly everything in everything in everyday life is relative to the earth, however if i was traveling a little be slower than the speed of light, relative to the earth, why am i not traveling faster than the speed of light relative to the sun or the centre of the galaxy or another galaxy (when the Earth adds to your speed relative to these other objects).

Another example is with a bulb and photons of light:
Photon 1<-----|Bulb|----->Photon 2...(photons traveling at speed of light)
If each photon is traveling at the speed of light relative to the bulb, at what speed is photon 1 traveling at relative to photon 2...twice the speed of light?

I have read that you travel relative to the ether or something, however i do not fully understand that theory!
Could you please help because however much i read i still can't really understand it

Because you are trying to add values that belong to different coordinate system!

BTW. Light does not travel, light propagates.

 

[mariush]

Yeah that's exactly what I mean, are we right or wrong? lol

Yep, it's right. Everithing you see is with some delay, all depending on how far the light has to travel to reach you.

 

[ghwellsjr]

All kidding aside, do believe the theory of Special Relativity debunks the idea of an absolute reference frame?

That is one of the fundamental postulates of special relativity.

There are two postulates of SR, which one debunks the idea of an absolute reference frame?

 

[Grep]

All kidding aside, do believe the theory of Special Relativity debunks the idea of an absolute reference frame?

That is one of the fundamental postulates of special relativity.

The postulate doesn't really debunk the idea of absolute reference frame. It's just a postulate. Experiments debunk the idea of an absolute reference frame. Experiments like the Michelson-Morley experiment, and a host of others that seem to be listed here:

http://www.edu-observatory.org/physics-faq/Relativity/SR/experiments.html

An important distinction, I think.

 

[SamRoss]

Can someone help me with speed relative to different objects!

Nearly everything in everything in everyday life is relative to the earth, however if i was traveling a little be slower than the speed of light, relative to the earth, why am i not traveling faster than the speed of light relative to the sun or the centre of the galaxy or another galaxy (when the Earth adds to your speed relative to these other objects).

What you are talking about is a well known problem in relativity known as composition of velocities. Here's the pre-relativity way of thinking about it. Suppose object B, let's say the Earth, is traveling away from object A, say the sun, at a velocity "v". Let's also suppose there's an object C, say a spaceship, traveling away from B at a velocity "w" relative to B. We're also assuming all of this is in a straight line. Now the pre-relativity way of determining how fast C (the spaceship) is moving away from A (the sun) is just to add the velocities. That is, v+w. But thanks to Einstein, we now know that this intuitive, obvious, no-way-it-can-be-wrong formula... is wrong. The correct formula is (v+w)/(1+vw/c^2). Notice that this is the intuitive formula, v+w, with a correction factor of (1+vw/c^2)^-1. So for objects traveling at a speed much smaller than the speed of light, vw/c^2 is approximately zero so we get our good old v+w back. Also notice that this solves the problem you brought up in your post, for suppose v=c and w=c (so B is moving away from A at the speed of light relative to A and C is moving away from B at the speed of light relative to B). Then C is moving away from A at a speed of (c+c)/(1+c^2/c^2)=2c/2=c. This is a far cry from the "common sense" answer of twice the speed of light that we would get by thinking about it in the old way.

 

[JesseM]

There are two postulates of SR, which one debunks the idea of an absolute reference frame?

Assuming the two postulates are correct (which as Grep says is a matter for experiment), the first one is incompatible with the idea of a physically measurable absolute reference frame (you could still postulate a metaphysical truth about which frame's definition of simultaneity is metaphysically correct, but the first postulate says no experiment could empirically distinguish this frame from any other...and any of the ether theories which the OP was thinking of would say there would be some experimental way to distinguish the ether frame from other frames, otherwise they wouldn't really be 'theories' at all).

 

[ghwellsjr]

So, are you saying that SR debunks the idea of an absolute reference frame (and by that I mean, states that it cannot exist) or just that it's pointless to search for such a frame because we have no way of identifying it if it did exist?

 

[JesseM]

So, are you saying that SR debunks the idea of an absolute reference frame (and by that I mean, states that it cannot exist) or just that it's pointless to search for such a frame because we have no way of identifying it if it did exist?

If some proposed entity is in principle undetectable by any possible empirical observations, then I would say it is outside the domain of physics altogether. Of course SR does not make statements about wholly metaphysical entities, for example it does not rule out the idea that one particular frame is "preferred" in the sense that God loves it above all other reference frames. But SR is certainly incompatible with a physically distinguishable preferred frame.

 

[Theo1]

What you are talking about is a well known problem in relativity known as composition of velocities. Here's the pre-relativity way of thinking about it. Suppose object B, let's say the Earth, is traveling away from object A, say the sun, at a velocity "v". Let's also suppose there's an object C, say a spaceship, traveling away from B at a velocity "w" relative to B. We're also assuming all of this is in a straight line. Now the pre-relativity way of determining how fast C (the spaceship) is moving away from A (the sun) is just to add the velocities. That is, v+w. But thanks to Einstein, we now know that this intuitive, obvious, no-way-it-can-be-wrong formula... is wrong. The correct formula is (v+w)/(1+vw/c^2). Notice that this is the intuitive formula, v+w, with a correction factor of (1+vw/c^2)^-1. So for objects traveling at a speed much smaller than the speed of light, vw/c^2 is approximately zero so we get our good old v+w back. Also notice that this solves the problem you brought up in your post, for suppose v=c and w=c (so B is moving away from A at the speed of light relative to A and C is moving away from B at the speed of light relative to B). Then C is moving away from A at a speed of (c+c)/(1+c^2/c^2)=2c/2=c. This is a far cry from the "common sense" answer of twice the speed of light that we would get by thinking about it in the old way.

Thankyou SO much...this has been bugging me for a while and i have never found a way around it (by myself). Understand it now!

 

[Theo1]

Btw any1 have a project i can do (to do with physics) of between 4000-6000 words
I have to answer a question!
Nothing too hard tho as I am not at uni yet!...And no unanswerable questions like why a neutron is heavier than a proton...
Thought sofar are Time Dilation, QCD or Space colonization - however i do not have a question to answer in these areas yet
Thanks again for answering my original question

 

[Passionflower]

And no unanswerable questions like why a neutron is heavier than a proton...

Actually is that question unanswerable, or at least incalculable?

I opened a topic in the QM section: https://www.physicsforums.com/showthread.php?t=448428

[ Edited ] Fyi: The topic now lives in High Energy, Nuclear, Particle Physics

 

[SamRoss]

Thankyou SO much...this has been bugging me for a while and i have never found a way around it (by myself). Understand it now!

My pleasure

 

[John232]

If you where traveling close to the speed of light relative to Earth you would still have to go about 300,000 km/sec faster to catch up to a beam of light.