# Thread: Speed of Light, should be named different?

1. We all know the speed of light is a constant.

299792458 m / s

But I was watching the movie Clockstoppers (worth only watching for the fun).... and at some point I noticed some flaws in it (it has a lot of flaws in it)... which got me thinking about something.

The above speed,... we call the Speed of Light. But should it not be named The Speed At Which Light Travels. Because there is a slight difference here.

This speed is not exclusive to light alone. If our Sun would explode, it would take us on earth 8 minutes to realise this. First because it would take the light approx 8 minutes to reach us, for us too see the Sun exploded,... second because we would suddenly notice the earth will go straight, shooting out of our solar system, as the gravity of the sun would suddenly not hold us anymore.

So in that case... it took the Gravity Distortion in the spacetime fabric also 8 minutes to reach us.

So my guess is, though this speed is named like that because of our scientific history,... this speed is not the Speed of Light (though light travels at that speed) but the speed of something else.

Am I making any sence here? I know it is just screwing around with words and what they mean,... but for accuracy in science... still important to determine.

2.

3. Originally Posted by Estheria Quintessimo
The above speed,... we call the Speed of Light. But should it not be named The Speed At Which Light Travels.
Given your subsequent reasoning it should be called "The speed at which light and gravity travel"

So in that case... it took the Gravity Distortion in the spacetime fabric also 8 minutes to reach us.
AFAIK it's not been conclusively established that gravity "moves" at c. The last time I looked the experimental error allowed for some doubt as to the final figure (not that I dispute c for gravity).

So my guess is, though this speed is named like that because of our scientific history,... this speed is not the Speed of Light (though light travels at that speed) but the speed of something else.
No, it's the speed of light.
Gravity's speed was unknown (Newton thought it was instantaneous, Einstein said different, but we didn't have any data) so, since light was the only thing we knew that travelled at that speed, that's what it was called. Or "c".

Am I making any sence here? I know it is just screwing around with words and what they mean,... but for accuracy in science... still important to determine.

4. Well ofcourse. I am not saying gravity moves. I said it takes the gravity distortion in the fabric of spacetime... approx 8 minutes to reach the earth,... afterwhich the earth starts going in a straight line due to the Suns gravity being gone.

Let me clarify,... Yes it is the determined maximum speed in the universe. But calling it The Speed Of Light,... may confuse people that Light its self has something to do with it perhaps. The only reason we call it that, is because we measured it for light,... only to determine later other things are bound to this maximum speed too, in fact everything basically.

So something is causing this to be the maximum speed. Seeing the research done at the LHC concerning gravity... perhaps we may know that in some years to come in the future.

Yes it wont change anything for real,.... but changing the nature of the meaning of the words has happened in the past too. Before Hubble,... the universe was the same as the Milkyway. We know better now when we talk about the universe.

Yes it is the Speed of Light,... but more accurant it is the Speed at which Light Travels. Determining why that speed is like it is,... will change the meaning The Speed Of Light. And that speed should be given another name.

... I am not sure how I can say it any different.

5. Originally Posted by Estheria Quintessimo
Before Hubble,... the universe was the same as the Milkyway.
What?

Yes it is the Speed of Light,... but more accurant it is the Speed at which Light Travels.
Er, what was the speed of that car?
I don't know, I can only state what speed that car was travelling at.

There's no sematic difference.

Determining why that speed is like it is,... will change the meaning The Speed Of Light.
No it won't
We didn't change, for example, "positive" and "negative" when we discovered the electron and its role in electric current.

6. Originally Posted by Dywyddyr
Originally Posted by Estheria Quintessimo
Before Hubble,... the universe was the same as the Milkyway.
What?

Yes it is the Speed of Light,... but more accurant it is the Speed at which Light Travels.
Er, what was the speed of that car?
I don't know, I can only state what speed that car was travelling at.

There's no sematic difference.

Determining why that speed is like it is,... will change the meaning The Speed Of Light.
No it won't
We didn't change, for example, "positive" and "negative" when we discovered the electron and its role in electric current.
Yes but surely EQ's point is not that absurd: is it not the case that INFORMATION cannot propagate faster than light?

7. Originally Posted by exchemist
Yes but surely EQ's point is not that absurd: is it not the case that INFORMATION cannot propagate faster than light?
Which discovery also came AFTER we established a value for c and that c is a limiting speed.
We have to call it something.
If we change the name then we'd have to alter all the text books, all the documentaries, heck, all the science fiction...

My point, illustrated with the positive/ negative example, is that so long as everyone knows what we're talking about what it's called doesn't really matter.

8. Yes, that's fair enough. A lot of labels are to some extent arbitrary or historically determined. And at least light is more tangible - easier to visualise - than "information".

9. I actually agree that the term "speed of light" to describe c is a misnomer, though I am inclined to think that "the speed at which light travels" is even worse. Calling c the "speed of light" gives the mistaken impression that relativity is about light... it isn't. c is geometric property of spacetime that provides the conversion factor between units of time and units of length... nothing more. Indeed, one can in principle measure the value of c without measuring the speed at which light travels (highlighting why I think this phrase is even worse). One need only measure the speed of the same object in two different inertial frames of reference and apply the relativistic velocity addition formula (of course, measuring the speed at which light travels is more accurate, but that is not the point).

10. Originally Posted by Dywyddyr
Originally Posted by exchemist
Yes but surely EQ's point is not that absurd: is it not the case that INFORMATION cannot propagate faster than light?
Which discovery also came AFTER we established a value for c and that c is a limiting speed.
We have to call it something.
If we change the name then we'd have to alter all the text books, all the documentaries, heck, all the science fiction...

My point, illustrated with the positive/ negative example, is that so long as everyone knows what we're talking about what it's called doesn't really matter.
Well that is not that hard,... changing the books. It wont be the first time a specific description for something very important has been changed!!!

'What is the Universe?' (I am assuming they knew the word universe at that time,... it was a thought up example)

They would have told you:

'Well that is easy,... all the stars around you are like our Sun. These make up our Universe'

But the people at that time had not realised, that some of these stars, factually were not stars at all. These were milkyways in their own right. So after Hubble, the Universe got alot bigger.

The definition for the 'Universe' would change.

To not bother changing the definition of something this important, just because you would have to change all the books.... would be rather silly.

The Speed Of Light is not the speed of light, because light can only travel at that maximum speed,.... it has that speed because something else is causing the Speed Of Light to have that maximum.

It is why I mentioned the exploding Sun. It takes the Earth to 'notice' approx 8 minutes the Sun is gone. This has got nothing to do with The Speed of Light,.... but with something else.

But every documentary I see about the Universe,.. always mentions The Speed Of Light. That is wrong,... unless you are actually indeed talking about light. That speed happens also to be the speed of light... but it is not the speed of light. It is the speed of something totally different. It is not light that is dominant here... it is something else causing light to have that maximum speed.

Thus it should be named accordingly.

I realize it is screwing around with words and definitions. But in science these are important to know what you are actually talking about.

It wont change theories... Just look at Newtonian physics. We still use these in our daily lives, though we realise Newton was wrong. But his laws can still be used on the basic levels if we are talking about simple motions and do simple vector calculations on stuff that moves.

In The Netherlands we have a saying: 'Noem het beestje bij de naam,' wich would ruffly translate into english as 'Call the little animal by its name.'
We use this sentence to tell someone, who is talking about something, but is not willing to mention its real name, to tell him/her they should not be afraid to not mention it's real name.
Science -I would gather- would prefer accuracy before laziness.

11. But it's a meaningless change, and only in English.

12. Originally Posted by Estheria Quintessimo
Well that is not that hard,... changing the books.
You think?

It wont be the first time a specific description for something very important has been changed!!!
Such as?

'What is the Universe?' (I am assuming they knew the word universe at that time,... it was a thought up example)
They would have told you:
'Well that is easy,... all the stars around you are like our Sun. These make up our Universe'
But the people at that time had not realised, that some of these stars, factually were not stars at all. These were milkyways in their own right. So after Hubble, the Universe got alot bigger.
I'm not sure if you're referring to the person or the telescope.
Either way you're not exactly correct with your first assertion.

To not bother changing the definition of something this important, just because you would have to change all the books.... would be rather silly.
It's not a definition it's a convenient "description". That's all.

The Speed Of Light is not the speed of light, because light can only travel at that maximum speed,.... it has that speed because something else is causing the Speed Of Light to have that maximum.
So what?
It's speed light travels at. Ergo: speed of light.

It is why I mentioned the exploding Sun. It takes the Earth to 'notice' approx 8 minutes the Sun is gone. This has got nothing to do with The Speed of Light,.... but with something else.
Wrong again. The "speed of light" which is the term used is the speed at which the information propagates.
What actually propagates is irrelevant.

But every documentary I see about the Universe,.. always mentions The Speed Of Light. That is wrong,... unless you are actually indeed talking about light. That speed happens also to be the speed of light... but it is not the speed of light. It is the speed of something totally different. It is not light that is dominant here... it is something else causing light to have that maximum speed.
Right.
So by this logic we should also change "speed of sound"?

I realize it is screwing around with words and definitions. But in science these are important to know what you are actually talking about.
As I have stated before: REGARDLESS of what the term is everyone knows what it means.

In The Netherlands we have a saying: 'Noem het beestje bij de naam,' wich would ruffly translate into english as 'Call the little animal by its name.'
We use this sentence to tell someone, who is talking about something, but is not willing to mention its real name, to tell him/her they should not be afraid to not mention it's real name.
Science -I would gather- would prefer accuracy before laziness.
So we can also get rid of barn, colour (for quarks), spin, up, down...?

13. Originally Posted by KJW
I actually agree that the term "speed of light" to describe c is a misnomer, though I am inclined to think that "the speed at which light travels" is even worse. Calling c the "speed of light" gives the mistaken impression that relativity is about light... it isn't. c is geometric property of spacetime that provides the conversion factor between units of time and units of length... nothing more. Indeed, one can in principle measure the value of c without measuring the speed at which light travels (highlighting why I think this phrase is even worse). One need only measure the speed of the same object in two different inertial frames of reference and apply the relativistic velocity addition formula (of course, measuring the speed at which light travels is more accurate, but that is not the point).
Thank you.

But I was not proposing to redefine the term Speed Of Light to the Speed At Which Lights Travels. The speed of light is... The speed of light afterall, which is 299792458 m/s, in this Universe.
You would only need to do so, if the maximum speed at which light travels suddenly -in some new theory- is random, and unless the laws of physics suddenly change, or our perception and understanding of these (now fixed in sci theory) laws... I do not think that would happen, but who am I?

I think I said something like this in my first post at the end,.... asking if there was already another definition in use by the sci community. If not....

They should come up with a new definition for the maximum speed allowed by the physics in this Universe.

Basically,... in my limited sci understanding,.... the speed of light is caused by something. So naming it Speed of Light is the wrong name for something very important.

If the physics laws in our Universe had been slighty different,... this maximum speed allowed in that universe may be different. .... If these physics would allow such a Universe to remain in existance, this maximum speed allowed in this universe may have totally different effects on ALL its components,... inlcuding light and spacetime... perhaps.

That is what is bugging me and why I think we should not confuse the speed at which light travels (in this Universe) with the maximum speed in a universe.

For the definition, as I said in previous posts,... this 299792458m/s is not the Speed Of Light. It is the maximum speed allowed in our Universe. Perhaps the properties of light are such in all theoretical universes it always 'wants' to travel at that speed. Fact is we do not know that. But fact it is that this speed is not exclusive to light.

KJW said it indeed much better then I could ever had, not being a scientist... thank you for this...
...Calling c the "speed of light" gives the mistaken impression that relativity is about light... it isn't. c is geometric property of spacetime that provides the conversion factor between units of time and units of length... nothing more. Indeed, one can in principle measure the value of c without measuring the speed at which light travels. One need only measure the speed of the same object in two different inertial frames of reference and apply the relativistic velocity addition formula (of course, measuring the speed at which light travels is more accurate, but that is not the point).
KJW lost me on the last sentence ('One need only...')... me being a noob who likes to watch documentaries and think about stuff.

14. this 299792458m/s is not the Speed Of Light. It is the maximum speed allowed in our Universe. Perhaps the properties of light are such in all theoretical universes it always 'wants' to travel at that speed. Fact is we do not know that. But fact it is that this speed is not exclusive to light.
It is the speed light travels. Light travels at no other speed. That's the speed of light. It's not the maximum speed allowed. It's the maximum speed possible, and light travels at that speed, and no other speed.

I think your whole point is pretty pointless. Maybe it makes better sense in Dutch.

15. I think I understand that the OP is getting at. c is the maximum speed of cause and effect - the maximum speed of causality. Light (and any other particles with zero rest mass) travel at that speed.

16. Originally Posted by Estheria Quintessimo
That is what is bugging me and why I think we should not confuse the speed at which light travels (in this Universe) with the maximum speed in a universe.
Well call me parochial but I, all my friends, every scientist I know, and all the non-scientists too, happen to live in this universe.
And then there's the fact that A) we don't know of any other universes, B) we probably wouldn't be able to access them any way and/ or C) if we could then, by definition, they'd become part of this universe.
Given all of the above I think it's pretty pointless to change a well-established convention (I note you haven't given any prior examples of altering all the text books due to a name change) just because you can't grasp the idea involved.

17. Originally Posted by Estheria Quintessimo
This speed is not exclusive to light alone. If our Sun would explode, it would take us on earth 8 minutes to realise this. First because it would take the light approx 8 minutes to reach us, for us too see the Sun exploded,... second because we would suddenly notice the earth will go straight, shooting out of our solar system, as the gravity of the sun would suddenly not hold us anymore.

So in that case... it took the Gravity Distortion in the spacetime fabric also 8 minutes to reach us.
I realize you were using this as an example. But it's not correct. The light from the sun exploding would reach us in eight minutes. However the gravity is not going to disappear in 8 minutes. As the exploding mass expands it is still mass and will contribute to the center of gravity the Earth is orbiting around. For the Earth that gravity won't change much until the expanding mass passes the orbit of the Earth. When that happens the Earth will have a lot more to worry about than a bit less gravity to orbit around.

18. Originally Posted by Dywyddyr
As I have stated before: REGARDLESS of what the term is everyone knows what it means.
Well that is your problem! You think the science community should not change and progress,... just because youy feel comfy with the currect definitions?

So what?
It's speed light travels at. Ergo: speed of light.
It is why I mentioned the exploding Sun. It takes the Earth to 'notice' approx 8 minutes the Sun is gone. This has got nothing to do with The Speed of Light,.... but with something else.
Wrong again. The "speed of light" which is the term used is the speed at which the information propagates.
What actually propagates is irrelevant.
But every documentary I see about the Universe,.. always mentions The Speed Of Light. That is wrong,... unless you are actually indeed talking about light. That speed happens also to be the speed of light... but it is not the speed of light. It is the speed of something totally different. It is not light that is dominant here... it is something else causing light to have that maximum speed.
Right.
So by this logic we should also change "speed of sound"?
What logic you think I am using here? Speed of Sound... seriously? Light in this Universe is constant. Sound is not.

Im not sure what you think of me... You seem to have a cerain high... Seriously... comparing speed of sound,.. to the speed of light....

I am well aware of both. But what has the speed of sound got to do with the maximum speed in this Universe?

19. You think the science community should not change and progress,.
Really don't see any 'progress' in your proposal. Just seems like a meaningless semantic change for the sake of semantics.

20. Originally Posted by Dywyddyr
Originally Posted by Estheria Quintessimo
That is what is bugging me and why I think we should not confuse the speed at which light travels (in this Universe) with the maximum speed in a universe.
Well call me parochial but I, all my friends, every scientist I know, and all the non-scientists too, happen to live in this universe.
And then there's the fact that A) we don't know of any other universes, B) we probably wouldn't be able to access them any way and/ or C) if we could then, by definition, they'd become part of this universe.
Given all of the above I think it's pretty pointless to change a well-established convention (I note you haven't given any prior examples of altering all the text books due to a name change) just because you can't grasp the idea involved.
Really...?

I'll give you a word which's definition has changed significantly over the centuries,...

Sun

....

Now back to you.

I'll give you a heads up.... It actually is not a god or goddess,... atleast not these days, and the Moon is not it's brother or sister. And I do not think the Moon and the Sun had sex with eachother either.

It is also not a large pile of wood set on fire....

Guess what it is... Can you describe this word? ... 'SUN'...

....

But... I am not a scientist.... thus I must be a moron.

21. Originally Posted by Estheria Quintessimo
Well that is your problem! You think the science community should not change and progress,... just because youy feel comfy with the currect definitions?
On the contrary, it's your problem.
Particularly because it's not just me.
There are valid reasons why c is called the speed of light, and no valid reasons to change that practise.

What logic you think I am using here? Speed of Sound... seriously? Light in this Universe is constant. Sound is not.
But, and here's something you'll not like, the speed of sound is generally regarded as fixed (although at at least two different values) for most purposes.

Seriously... comparing speed of sound,.. to the speed of light....
Wrong again.
I'm comparing your argument for changing the name of speed of light to a similar one for changing the name of the speed of sound.

I am well aware of both. But what has the speed of sound got to do with the maximum speed in this Universe?
It's quite simple:
You - but it is not the speed of light. It is the speed of something totally different (which is incorrect, of course).
Me - when we use the term "speed of sound" or, more specifically, Mach we're mostly talking about something OTHER than sound.
I.e. a flimsy argument predicated on... not much at all.

I note you're carefully ignoring my question about barn, colour, etc.

22. Originally Posted by Estheria Quintessimo
I'll give you a word which's definition has changed significantly over the centuries,...
Sun
(And the argument doesn't apply anyway. IF the definition had "changed significantly" over the centuries how does that compare to taltering all of the textbooks now because you want to introduce a fiat change? A gradual shift in usage is one thing: a ridiculous and unwaranted change, at a particular instance, is another).

23. The description of what the sun is has changed significantly over the ages. But the name hasn't. Similarly, there's no reason to change the phrase 'speed of light'. It accomplishes nothing, makes nothing any clearer, and does nothing for any accuracy issues.

In fact, this whole argument is looking very like argument for arguments sake.

24. Originally Posted by Dywyddyr
Originally Posted by Estheria Quintessimo
I'll give you a word which's definition has changed significantly over the centuries,...
Sun
(And the argument doesn't apply anyway. IF the definition had "changed significantly" over the centuries how does that compare to taltering all of the textbooks now because you want to introduce a fiat change? A gradual shift in usage is one thing: a ridiculous and unwaranted change, at a particular instance, is another).
Geez man... get over your own arrogance!

You gave me a challenge... You asked me a question. I gave you my answer.....

What the hell are you doing?

You deny the fact that the definition of 'SUN' our bright yellow star,... has changed over the centuries?

If you deny this... then we are done talking.

If your goal is to only come out on top, all the time, in every discussion... I aplaud you. It is a noble goal, to be perfect. I would want that too. But you just lost this one.

You asked me to provide a word which definition has changed over time. I gave you my answer. it was:
SUN

I realize you have a lot of knowledge, I can see this from your respondses, you are a smart person. Please keep your respect.

25. FFS!!
Just call it Martin and leave it at that.
And the speed of sound should be called Doris.

26. Originally Posted by RedPanda
FFS!!
Just call it Martin and leave it at that.
And the speed of sound should be called Doris.
I mean no disrespect. This was my first post. Please do not ban me for speaking out, my mind.

27. Originally Posted by Estheria Quintessimo
You gave me a challenge...
I did.
Specifically: If we change the name
And your claim: It wont be the first time a specific description for something very important has been changed!!!
I'm still waiting for that.

You deny the fact that the definition of 'SUN' our bright yellow star,... has changed over the centuries?
You mean "that big bright thing in the sky"?
Yes. I deny that's changed as far as any relevance to this topic is concerned.
Unless, of course, you have access to numerous textbooks (which I mentioned and you claimed had been altered), documentaries (same), etc.
Any books that referred to the Sun as anything other than what we know it as today were A) extremely limited in number and B) equally limited in distribution.

If you deny this... then we are done talking.
In other words you can't can't support your claim.

You asked me to provide a word which definition has changed over time.
No I didn't.
And that's not what you claimed either.

Sun
While its supposed attributes may have changed over centuries (which, again, comes back to "not happened as a result of a single decision" - which was also your claim) it was A) still described as the Sun and B) acknowledged as the source of light (e.g. being a "god" or whatever else was a secondary aspect).

28. Originally Posted by RedPanda
FFS!!
Just call it Martin and leave it at that.
And the speed of sound should be called Doris.
No. Wanna call it Eric!

29. Originally Posted by Estheria Quintessimo
Originally Posted by RedPanda
FFS!!
Just call it Martin and leave it at that.
And the speed of sound should be called Doris.
I mean no disrespect. This was my first post. Please do not ban me for speaking out, my mind.
Erm...it was just meant to be a joke.

30. Originally Posted by Dywyddyr
No. Wanna call it Eric!
If we called it Eric then people would get confused with the other Eric.

31. Originally Posted by Dywyddyr
Originally Posted by Estheria Quintessimo
You gave me a challenge...
I did.
Specifically: If we change the name
And your claim: It wont be the first time a specific description for something very important has been changed!!!
I'm still waiting for that.

You deny the fact that the definition of 'SUN' our bright yellow star,... has changed over the centuries?
You mean "that big bright thing in the sky"?
Yes. I deny that's changed as far as any relevance to this topic is concerned.
Unless, of course, you have access to numerous textbooks (which I mentioned and you claimed had been altered), documentaries (same), etc.
Any books that referred to the Sun as anything other than what we know it as today were A) extremely limited in number and B) equally limited in distribution.
Pathetic and disgusting! So you claim the definition of The Sun has not been changed over hunderds of centuries... if I can not come up with textbooks? ROFL. Okay then... lets make this indeed scientific, as you wish.

(forgetting the fact I actually just already DID provide you with the information you wanted)

What is your definition of our Sun?... So I can have a reference based on you definition. And on what knowledge is your specific knowledge of our Sun based on?

So you think the Sun is what? ...... Heavy need of your specific knowledge now (!). .... Our sun is What.... (?)... ... ...

32. Originally Posted by Estheria Quintessimo
Pathetic and disgusting! So you claim the definition of The Sun has not been changed over hunderds of centuries...
Oh dear. Perhaps you should learn to read.
1) The definition changing is not what is under dispute, as I have pointed out.
2) I also pointed out that, for the purposes of this dicussion (i.e. the scientific designation) any "change" made of the Sun's definition is inavlid. Because science (which is what what we're talking about) didn't use any prior definitions.
3) I note that we've gone from a large and significant change engendered by single decision (yours), to a cghange over centuries to, now, a change over hundtrreds of centuries.

if I can not come up with textbooks? ROFL. Okay then... lets make this indeed scientific, as you wish.
(forgetting the fact I actually just already DID provide you with the information you wanted)
No. You have not provided a single reference.
You've made claims, but not one reference.

What is your definition of our Sun?... So I can have a reference based on you definition. And on what knowledge is your specific knowledge of our Sun based on?
So you think the Sun is what? ...... Heavy need of your specific knowledge now (!). .... Our sun is What.... (?)... ... ...
The topic is a change of name being efffected in all existing textbooks due to a single decision ( which you claimed has occurred before).
It's nothing to do with my definition (but you're welcome to Google for one).

PS I quite agree with the "pathetic and disgusting" comment.
So far you've:
made assertions while failing to support them despite repeated requests.
moved the goalposts at least three times (name change - definition change/ single decision change - over centuries/ over centuries - over hundreds of centuries)
switched from the scientific usage to my personal definition.

Since you have failed to be honest I'm out.
Welcome to my ignore list.

33. Originally Posted by Estheria Quintessimo
KJW lost me on the last sentence ('One need only...')... me being a noob who likes to watch documentaries and think about stuff.
Suppose a spaceship is moving at velocity relative to some observer, and a bullet is fired from the spaceship at velocity relative to the spaceship, then for collinear motions, the velocity of the bullet relative to the observer is given by the relativistic velocity addition formula:

However, this formula can be rearranged to express in terms of the three velocities, which would be the measured quantities:

Thus, we have determined the value of as a quantity that has nothing to do with the speed at which light travels.

34. This is funny. People arguing about the name of "Speed of Light"?

I think the name is just fine.

Fits the description.

Isn't it, after all...."THE SPEED OF LIGHT"?

Then again, I'm not a scientist.

35. Originally Posted by babe
Isn't it, after all...."THE SPEED OF LIGHT"?
Correct - but it is also the speed of other things (like gravity waves).
So, it has been proposed that it should be called "The Speed At Which Light And Other Things (e.g. Gravity Waves) Travel In A Vacuum Measured In Metres Per Second".
Or TSATWLAOTEGGWTIAVMIMPS.......for short.

36. Originally Posted by RedPanda
Or TSATWLAOTEGGWTIAVMIMPS.......for short.
By choosing the right units, we can shorten that to 1.

37. Originally Posted by RedPanda
Originally Posted by babe
Isn't it, after all...."THE SPEED OF LIGHT"?
Correct - but it is also the speed of other things (like gravity waves).
So, it has been proposed that it should be called "The Speed At Which Light And Other Things (e.g. Gravity Waves) Travel In A Vacuum Measured In Metres Per Second".
Or TSATWLAOTEGGWTIAVMIMPS.......for short.
Jesus Christ...let's just simplify it, shall we?

Howling!! Oh my ..

I have a question

Are you stating your problem with sex or quoting the bible? *L*

38. Originally Posted by Strange
Originally Posted by RedPanda
Or TSATWLAOTEGGWTIAVMIMPS.......for short.
By choosing the right units, we can shorten that to 1.
Thank you! Thank you!! Thank you!

39. Originally Posted by RedPanda
Originally Posted by babe
Isn't it, after all...."THE SPEED OF LIGHT"?
Correct - but it is also the speed of other things (like gravity waves).
So, it has been proposed that it should be called "The Speed At Which Light And Other Things (e.g. Gravity Waves) Travel In A Vacuum Measured In Metres Per Second".
Or TSATWLAOTEGGWTIAVMIMPS.......for short.
Actually, shouldn't be called the speed of anything. It is a conversion factor between units of length and units of time. Clearly, metres/second is also a unit of speed, but that is somewhat coincidental to being a conversion factor. I sometimes call the "special relativity constant".

40. Originally Posted by Estheria Quintessimo
We all know the speed of light is a constant.

299792458 m / s

But I was watching the movie Clockstoppers (worth only watching for the fun).... and at some point I noticed some flaws in it (it has a lot of flaws in it)... which got me thinking about something.

The above speed,... we call the Speed of Light. But should it not be named The Speed At Which Light Travels. Because there is a slight difference here.

This speed is not exclusive to light alone. If our Sun would explode, it would take us on earth 8 minutes to realise this. First because it would take the light approx 8 minutes to reach us, for us too see the Sun exploded,... second because we would suddenly notice the earth will go straight, shooting out of our solar system, as the gravity of the sun would suddenly not hold us anymore.

So in that case... it took the Gravity Distortion in the spacetime fabric also 8 minutes to reach us.

So my guess is, though this speed is named like that because of our scientific history,... this speed is not the Speed of Light (though light travels at that speed) but the speed of something else.

Am I making any sence here? I know it is just screwing around with words and what they mean,... but for accuracy in science... still important to determine.

I suppose you could just call it the "Universal Speed Limit" or something.

41. Originally Posted by KJW
Actually, shouldn't be called the speed of anything. It is a conversion factor between units of length and units of time.
Exactly. And, as a consequence, it appears in the conversion factors for many other things (e.g. for mass <-> energy) based on the same system of arbitrary units.

42. Originally Posted by babe
I have a question

Are you stating your problem with sex or quoting the bible? *L*
Was the "Revelation 22:12" not enough of a clue?

Or do you think Revelations is all about me?
"Even so, come, Lord Jesus." Revelation 22:20

43. Originally Posted by Estheria Quintessimo
You would only need to do so, if the maximum speed at which light travels suddenly -in some new theory- is random, and unless the laws of physics suddenly change, or our perception and understanding of these (now fixed in sci theory) laws... I do not think that would happen, but who am I?
In that case, we appear to disagree on why should be renamed. In the "old" days before was defined to be exactly 299792458 m/s, the metre and the second were independently defined respectively in terms of the length properties and time properties of specified physical objects. Thus, the numerical value of c depended on the way we chose to define the metre and the second, and this is independent of any considerations about the properties of light. To say that c could be different in some alternative universe, or even at different locations in space and time in this universe, is tantamount to saying that the laws of physics that govern the time properties of physical objects are not the same laws of physics that govern the length properties of physical objects. In other words, the frequency of the emitted light from a given type of atom is connected to the size of a different type of atom by the one set of physical laws, and part of those physical laws is the speed at which light travels. Putting it another way, to suggest that c can have different values is attributing to physical reality degrees of freedom it does not possess.

44. Originally Posted by AlexG
this 299792458m/s is not the Speed Of Light. It is the maximum speed allowed in our Universe. Perhaps the properties of light are such in all theoretical universes it always 'wants' to travel at that speed. Fact is we do not know that. But fact it is that this speed is not exclusive to light.
It is the speed light travels. Light travels at no other speed. That's the speed of light. It's not the maximum speed allowed. It's the maximum speed possible, and light travels at that speed, and no other speed.

I think your whole point is pretty pointless. Maybe it makes better sense in Dutch.
This highlights one of the main problems with calling it "the speed of light" because this isn't true.

Light travels at all kinds of speeds, depending on what medium it is propagating through. C is only the speed it travels when it's in a vacuum.

Certain topics, such as Cherenkov Radiation become incredibly hard to explain to someone if they don't know the difference between "The Speed of light (in a medium)" and C.

45. Incorrect. Light always travels at c. The increased time of propagation through a medium is due to absorbtion and re-emission of the photons. But the speed of the photons is always c.

46. Originally Posted by kojax
Originally Posted by AlexG
this 299792458m/s is not the Speed Of Light. It is the maximum speed allowed in our Universe. Perhaps the properties of light are such in all theoretical universes it always 'wants' to travel at that speed. Fact is we do not know that. But fact it is that this speed is not exclusive to light.
It is the speed light travels. Light travels at no other speed. That's the speed of light. It's not the maximum speed allowed. It's the maximum speed possible, and light travels at that speed, and no other speed.

I think your whole point is pretty pointless. Maybe it makes better sense in Dutch.
This highlights one of the main problems with calling it "the speed of light" because this isn't true.

Light travels at all kinds of speeds, depending on what medium it is propagating through. C is only the speed it travels when it's in a vacuum.

Certain topics, such as Cherenkov Radiation become incredibly hard to explain to someone if they don't know the difference between "The Speed of light (in a medium)" and C.
Sorry but that's not right either. The only reason light seems to move at less than C through a medium is that it is absorbed and re-admitted by the electrons that compose the elements of the medium. However, when it's traveling between electrons it is moving at C. All those little stops and re-admissions are what causes light to appear to take more time traveling through a medium. However you want to describe a photon, particle or wave it always moves at C when it's moving freely.

47. Originally Posted by RedPanda
Originally Posted by babe
I have a question

Are you stating your problem with sex or quoting the bible? *L*
Was the "Revelation 22:12" not enough of a clue?

Or do you think Revelations is all about me?
"Even so, come, Lord Jesus." Revelation 22:20
I see.

48. Originally Posted by Bad Robot
Originally Posted by kojax
Originally Posted by AlexG
this 299792458m/s is not the Speed Of Light. It is the maximum speed allowed in our Universe. Perhaps the properties of light are such in all theoretical universes it always 'wants' to travel at that speed. Fact is we do not know that. But fact it is that this speed is not exclusive to light.
It is the speed light travels. Light travels at no other speed. That's the speed of light. It's not the maximum speed allowed. It's the maximum speed possible, and light travels at that speed, and no other speed.

I think your whole point is pretty pointless. Maybe it makes better sense in Dutch.
This highlights one of the main problems with calling it "the speed of light" because this isn't true.

Light travels at all kinds of speeds, depending on what medium it is propagating through. C is only the speed it travels when it's in a vacuum.

Certain topics, such as Cherenkov Radiation become incredibly hard to explain to someone if they don't know the difference between "The Speed of light (in a medium)" and C.
Sorry but that's not right either. The only reason light seems to move at less than C through a medium is that it is absorbed and re-admitted by the electrons that compose the elements of the medium. However, when it's traveling between electrons it is moving at C. All those little stops and re-admissions are what causes light to appear to take more time traveling through a medium. However you want to describe a photon, particle or wave it always moves at C when it's moving freely.
If a lay person was not already confused by the idea of light traveling slower through a medium, I'm sure explaining about the stops, and re-admissions will confuse them still worse.

It's true, though, that that explanation is more accurate.

49. Originally Posted by kojax
If a lay person was not already confused by the idea of light traveling slower through a medium, I'm sure explaining about the stops, and re-admissions will confuse them still worse.

It's true, though, that that explanation is more accurate.
If members are confused they can ask more questions to become less confused. If they are not members, maybe they will become a member so they can ask more questions. Or they can do a little research on their own. Either way I have little sympathy for confused people to lazy to become less confused.

50. OK....help me out here.

The Speed of Light has a maximum speed it can possibly go.

Correct?

So if that is correct, when people refer to the speed of light is not like an infinite speed?

51. Originally Posted by babe
OK....help me out here.

The Speed of Light has a maximum speed it can possibly go.

Correct?

So if that is correct, when people refer to the speed of light is not like an infinite speed?
No, light only has one speed and that speed is the fastest possible speed this universe allows. It's called a universal constent for a reason. Nothing ever goes faster and light never goes slower. When it travels for any amount of distance it always travels at the speed of light which happens to be C.

52. Originally Posted by Bad Robot
Originally Posted by babe
OK....help me out here.

The Speed of Light has a maximum speed it can possibly go.

Correct?

So if that is correct, when people refer to the speed of light is not like an infinite speed?
No, light only has one speed and that speed is the fastest possible speed this universe allows. It's called a universal constent for a reason. Nothing ever goes faster and light never goes slower. When it travels for any amount of distance it always travels at the speed of light which happens to be C.
Mahalo Brat ROBOT!

THat was very clear and concise.

53. Originally Posted by babe
OK....help me out here.

The Speed of Light has a maximum speed it can possibly go.

Correct?

So if that is correct, when people refer to the speed of light is not like an infinite speed?
Light goes always at the same speed, the numerical value of which is the same everywhere and in all frames of reference. That means that light cannot be accelerated, and thus its speed never changes. The value of the speed of light is well-defined and quite finite.

54. Originally Posted by Markus Hanke
Originally Posted by babe
OK....help me out here.

The Speed of Light has a maximum speed it can possibly go.

Correct?

So if that is correct, when people refer to the speed of light is not like an infinite speed?
Light goes always at the same speed, the numerical value of which is the same everywhere and in all frames of reference. That means that light cannot be accelerated, and thus its speed never changes. The value of the speed of light is well-defined and quite finite.
Thank you kind sir.

So it is finite!

Mahalo!

55. In fact, it is a fundamental law of nature that anything that has zero rest mass MUST move at the speed of light from the instant it is created, undergoing no acceleration in the process.

56. Originally Posted by Bad Robot
Originally Posted by babe
OK....help me out here.

The Speed of Light has a maximum speed it can possibly go.

Correct?

So if that is correct, when people refer to the speed of light is not like an infinite speed?
No, light only has one speed and that speed is the fastest possible speed this universe allows. It's called a universal constent for a reason. Nothing ever goes faster and light never goes slower. When it travels for any amount of distance it always travels at the speed of light which happens to be C.
That is just not true. It is the whole point of my argument actually,... You are actually making my point here!

I should have mentioned this earlier... As it is one of the pointers for renaming.

Explain a prism to your poster! ... Any knowledgable Cable Guy will tell you that whilst installing fiber at your home... that the more bends you'll have in your fiber-optics,.. the more you will loose bandwidth. So yes there you have light traveling... But it is NOT traveling at the scientic accepted Speed of Light. Why? Because that speed is NOT the Speed of Light!!!

Light is going through a medium... and is slowing down. In the case of fiber optics,... different frequencies will result into a red and blue shift, causing some of the information getting faster at the receiving point then other frequencies.
Yes,... if it had no medium it would be travelling at the meaximum speed which we tend to call now... The Speed Of Light.

But its not! ... the Speed Of Light....

That maximum Speed is only that ... because we measured it for Light,... and that speed is the maximum Speed allowed according to modern physics.

Yes the Maximum Speed of Light... is that Speed. But its not The Maximum Speed Of Light. If the Laws of Physics in our universe would be different,... that speed would differ too. To name the Max speed 'The maximum Speed Of Light' suggests that speed has got something to do with light.

THE MAXIMUM SPEED IN THIS UNIVERSE HAS GOT NOTHING TO DO WITH LIGHT. It has that speed, because of something totally different.

57. Originally Posted by KALSTER
In fact, it is a fundamental law of nature that anything that has zero rest mass MUST move at the speed of light from the instant it is created, undergoing no acceleration in the process.
Thanks Kalster!

58. Originally Posted by Estheria Quintessimo

No, light only has one speed and that speed is the fastest possible speed this universe allows. It's called a universal constent for a reason. Nothing ever goes faster and light never goes slower. When it travels for any amount of distance it always travels at the speed of light which happens to be C.
That is just not true. It is the whole point of my argument actually,... You are actually making my point here!

I should have mentioned this earlier... As it is one of the pointers for renaming.

Explain a prism to your poster! ... Any knowledgable Cable Guy will tell you that whilst installing fiber at your home... that the more bends you'll have in your fiber-optics,.. the more you will loose bandwidth. So yes there you have light traveling... But it is NOT traveling at the scientic accepted Speed of Light. Why? Because that speed is NOT the Speed of Light!!!

Light is going through a medium... and is slowing down. In the case of fiber optics,... different frequencies will result into a red and blue shift, causing some of the information getting faster at the receiving point then other frequencies.
Yes,... if it had no medium it would be travelling at the meaximum speed which we tend to call now... The Speed Of Light.

But its not! ... the Speed Of Light....

That maximum Speed is only that ... because we measured it for Light,... and that speed is the maximum Speed allowed according to modern physics.

Yes the Maximum Speed of Light... is that Speed. But its not The Maximum Speed Of Light. If the Laws of Physics in our universe would be different,... that speed would differ too. To name the Max speed 'The maximum Speed Of Light' suggests that speed has got something to do with light.

THE MAXIMUM SPEED IN THIS UNIVERSE HAS GOT NOTHING TO DO WITH LIGHT. It has that speed, because of something totally different.
The speed of light through any transparent medium has been explained. It simply takes more time do to the interaction with electrons. In between electrons light still travels at the speed of light. A comparison that might help you visualize what is actually happening is, imagine your driving your car on a freeway, that's light traveling through space. Now imagine your driving down a road with stoplights and stop signs. Each time you have to stop and start again takes more time to go from point A to B. That's light through a medium. Keep in mind that light never accelerates, each photon travels at C. No faster and no slower. That goes for all frequencies of electromagnetic radiation from gamma rays all the way down to radio waves.

Now as far as that being the maximum speed anything can travel at in our universe. We've never observed anything that has traveled faster than light. But we still have a lot to learn about the universe we live in. Nothing with a rest mass greater than 0 will ever travel at the speed of light. At the moment I can't think of anything but electromagnetic radiation that has a rest mass of 0 and it can only move at C.

59. THE MAXIMUM SPEED IN THIS UNIVERSE HAS GOT NOTHING TO DO WITH LIGHT. It has that speed, because of something totally different.]
Incorrect. This is simply arguing for the sake of losing.

60. Markus Hanke gave me the best explanation!

I appreciate!

61. Originally Posted by Bad Robot
At the moment I can't think of anything but electromagnetic radiation that has a rest mass of 0 and it can only move at C.
There are only two other particles which have vanishing rest mass - the gluon ( one of the mediators of the strong force ), and the hypothetical graviton.

62. challenge

what does this mean in lay terms

63. Originally Posted by babe
challenge

what does this mean in lay terms
It means: "Look, I'm a clever bastard, so don't mess with me."

64. Originally Posted by Estheria Quintessimo
But it is NOT traveling at the scientic accepted Speed of Light
Yes it is.

Light is going through a medium... and is slowing down.
This is only an apparent effect; light does not actually slow down. What happens inside the medium is that it gets absorbed and re-emitted by the electrons present there, i.e. it no longer travels in a straight line, and therefore has to traverse a longer distance than in vacuum. Hence the delay. Locally it still travels at exactly the speed of light though, only globally it takes longer to traverse prism. For ease of calculation, materials are thus given an index of refraction, to account for how strongly light interacts with the electrons inside the material.

That maximum Speed is only that ... because we measured it for Light,... and that speed is the maximum Speed allowed according to modern physics.
It is the "maximum speed" because it defines the boundary of a light cone; in other words, travelling at the speed of light is travelling along a null geodesic in space-time. The surface made up of all such null geodesics emanating from a given event forms the boundary between what is causally connected in space-time and what is not. What we termed the "speed of light" is ultimately the geometric ratio between space and time; for every 300,000km you traverse in space, you traverse 1 second in time. It is a fundamental geometric invariant.

The only reason why it was called the "speed of light" is that this fundamental constant was discovered and measured long before the model of 4-dimensional space-time came into common use; the reasons are thus purely historical. You could call it anything else you wanted, and it wouldn't change its fundamental meaning.

To name the Max speed 'The maximum Speed Of Light' suggests that speed has got something to do with light.
But it does. Light traces out null geodesics in space-time; at every point along such null geodesics you can define a unit 4-velocity vector which is tangent to the world line ( with respect to the affine parameter ). If you look at the magnitude of this vector you will find that it is constant and exactly c everywhere, and has dimensions of m/s due to the definition of 4-velocity. It is perfectly reasonable to speak of "speed of light".

THE MAXIMUM SPEED IN THIS UNIVERSE HAS GOT NOTHING TO DO WITH LIGHT.
"Maximum speed" would be the speed at which two events in space-time can exchange information, i.e. can be causally connected. The boundary surface that separates regions which are causally connected from those which are not is made up of null geodesics - which is precisely what light traces out in space-time.

65. Originally Posted by babe
challenge

what does this mean in lay terms
Consider two people standing somewhere along the equator on earth, maybe one in South America, the other one in Africa, but both on the equator. They now start walking exactly north, towards the north pole, on initially parallel trajectories. Now picture in your mind what happens - the further they walk north, the closer they get to each other ! At the north pole, they meet, even though they were initially separated by a large distance, and even though they both walked exactly north ( not towards each other ).

Why is that ? It is because the earth's surface is not flat, but curved - the shortest route between two points is a great circle segment. On a flat plain parallel lines remain parallel, i.e. two people walking in the same direction will never meet. On a curved surface that is not so - initially parallel lines will deviate, and two people walking in the same direction will either approach one another, or move away from one another over time. This concept is called geodesic deviation; it is a way to quantify the geometry of a manifold such as for example the surface of the Earth.

The above equation quantifies this notion of geodesic deviation, which is why it is unsurprisingly called the equation of geodesic deviation. It is of central importance in Einstein's theory of General Relativity - essentially it tells us how a test particle under the influence of gravity behaves compared to a completely flat and straight reference geodesic.

66. Originally Posted by exchemist
Originally Posted by babe
challenge

what does this mean in lay terms
It means: "Look, I'm a clever bastard, so don't mess with me."
does not....

it means I am cutting up a chicken

the D2 Du shit means I got the bird
and it is demised..
the plus R means I got the knife
the instrument of choice
the dbd....etc, means I done split that chicken into BBQ! PARTS and the =O , means I am one proud assed lady! *buffing nails*

67. Originally Posted by Bad Robot
The speed of light through any transparent medium has been explained. It simply takes more time do to the interaction with electrons. In between electrons light still travels at the speed of light.
I actually agree with Estheria Quintessimo on this one. The empirical speed of light in a medium is less than . The stop-start motion of photons in a medium is based on a simplified model that may not represent the actual physics.

68. Originally Posted by KJW
The stop-start motion of photons in a medium is based on a simplified model that may not represent the actual physics.
So what's your take on what happens inside the medium, given that microscopically it is largely just vacuum ?

69. Originally Posted by Markus Hanke
Originally Posted by KJW
The stop-start motion of photons in a medium is based on a simplified model that may not represent the actual physics.
So what's your take on what happens inside the medium, given that microscopically it is largely just vacuum ?
I can only speak about the classical wave picture, but as the electromagnetic wave passes through the medium, the charges within the medium move in response to the electromagnetic field. The electromagnetic field of the moving charges combine with the electromagnetic field of the light to produce a total electromagnetic field whose phase is retarded relative to the original phase of the light. It is important to note that the refractive index of a material depends on the mobility of the charge distribution at the frequency of the light passing through it.

If one wants to consider the quantum electrodynamic picture, then don't forget that virtual particles, including virtual photons, can be "off shell", and therefore the speed of virtual photons need not be .

70. Originally Posted by KJW

I can only speak about the classical wave picture, but as the electromagnetic wave passes through the medium, the charges within the medium move in response to the electromagnetic field. The electromagnetic field of the moving charges combine with the electromagnetic field of the light to produce a total electromagnetic field whose phase is retarded relative to the original phase of the light. It is important to note that the refractive index of a material depends on the mobility of the charge distribution at the frequency of the light passing through it.

If one wants to consider the quantum electrodynamic picture, then don't forget that virtual particles, including virtual photons, can be "off shell", and therefore the speed of virtual photons need not be .
Yes, so the main point being that the incoming wave / photon interacts with particles present inside the atoms making up the material in some way; the overall refractive index is hence a result of these interactions ( whatever their exact nature ), which distinguishes light going through a medium from a reference light wave going through vacuum. That's really all I was trying to point out. I still don't think it is quite correct to speak of a reduction in the speed of light; if I was an observer somehow co-moving with an electron, at any given time ( QM effects aside for the moment ) I should see the photon coming towards me at exactly c in vacuum within a small neighbourhood around myself.

This actually raises an unrelated yet interesting question - what would it be like to ride on a quantum object like an electron, with all QM effects considered ? Is that even a meaningful question to ask ? What does the universe look like to, say, a QM electron ?

71. Originally Posted by Markus Hanke
Yes, so the main point being that the incoming wave / photon interacts with particles present inside the atoms making up the material in some way; the overall refractive index is hence a result of these interactions ( whatever their exact nature ), which distinguishes light going through a medium from a reference light wave going through vacuum. That's really all I was trying to point out. I still don't think it is quite correct to speak of a reduction in the speed of light; if I was an observer somehow co-moving with an electron, at any given time ( QM effects aside for the moment ) I should see the photon coming towards me at exactly c in vacuum within a small neighbourhood around myself.
I think the main point is that irrespective of the mechanism by which light propagates through a medium at the microscopic level, the empirical speed of light in a medium at the macroscopic level is less than . However, it is worth noting that between the charges of the medium isn't a "vacuum" but an electromagnetic field that is itself part of the propagating wave.

72. Originally Posted by KJW
the empirical speed of light in a medium at the macroscopic level is less than
Well yes, I can agree with this statement.

However, it is worth noting that between the charges of the medium isn't a "vacuum" but an electromagnetic field that is itself part of the propagating wave.
True enough. This entire scenario is really much less trivial than would initially appear to be the case.

But all these complications aside, if we go back to the OP at this point, I think it would be fair to say that "speed of light" is a term born mainly out of convention, and that the OP is obviously correct in saying that light is not the only thing that propagates at that speed. I don't see any reason to change the term though, any more than I see reason to call a banana "curved yellow fruit" from now on. It's just simply become a matter of convention, and anyone with any knowledge of physics understands that it is not exclusive to light alone.

73. Originally Posted by Markus Hanke
Originally Posted by KJW
the empirical speed of light in a medium at the macroscopic level is less than
Well yes, I can agree with this statement.

However, it is worth noting that between the charges of the medium isn't a "vacuum" but an electromagnetic field that is itself part of the propagating wave.
True enough. This entire scenario is really much less trivial than would initially appear to be the case.

But all these complications aside, if we go back to the OP at this point, I think it would be fair to say that "speed of light" is a term born mainly out of convention, and that the OP is obviously correct in saying that light is not the only thing that propagates at that speed. I don't see any reason to change the term though, any more than I see reason to call a banana "curved yellow fruit" from now on. It's just simply become a matter of convention, and anyone with any knowledge of physics understands that it is not exclusive to light alone.
"speed of light" is C, which means something specific, i.e. the speed of massless particles in a vacuum, like you say.

Originally Posted by KJW
However, it is worth noting that between the charges of the medium isn't a "vacuum" but an electromagnetic field that is itself part of the propagating wave.
But that is true for any region of space, not so? Any region of "vacuum".

74. Originally Posted by KJW
However, it is worth noting that between the charges of the medium isn't a "vacuum" but an electromagnetic field that is itself part of the propagating wave.
Furthermore, the electrons of a medium should not be regarded as point particles with empty space between them as they are not position eigenstates. Therefore, they should be regarded as a charge density for which the classical electrodynamic equation:

can be applied. Note that the source term is not zero and therefore is not describing propagation of at .

75. Originally Posted by Markus Hanke
But all these complications aside, if we go back to the OP at this point, I think it would be fair to say that "speed of light" is a term born mainly out of convention, and that the OP is obviously correct in saying that light is not the only thing that propagates at that speed. I don't see any reason to change the term though, any more than I see reason to call a banana "curved yellow fruit" from now on. It's just simply become a matter of convention, and anyone with any knowledge of physics understands that it is not exclusive to light alone.
The OP and I do not necessarily agree on why should be renamed. For me, it isn't really because there are other things that travel at , or because light doesn't travel at under all circumstances. It is because , as it applies to relativity, refers to a relationship between space and time, and that the speed at which light travels in a vacuum is merely coincidental (although the speed at which light travels in a vacuum was part of the development of special relativity).

Obviously, I'm not suggesting that the name be changed officially, or that all the books on relativity be rewritten. However, I do think that it should be recognised that calling the "speed of light in a vacuum" is somewhat of a misnomer that overstates the significance of light to the theory of relativity.

76. Originally Posted by KJW

Furthermore, the electrons of a medium should not be regarded as point particles with empty space between them as they are not position eigenstates. Therefore, they should be regarded as a charge density for which the classical electrodynamic equation:

can be applied. Note that the source term is not zero and therefore is not describing propagation of at .
Ok, I am probably missing something crucial here, but wouldn't the solutions to these inhomogeneous equations be retarded and advanced potentials, which would still propagate at exactly c into the future ( and backwards ) ? If not, could you please show me what propagation speed is extracted from these inhomogeneous equations ?
I should probably know this myself, but I need to firm up on my differentials equations a little...

77. Originally Posted by Markus Hanke
Ok, I am probably missing something crucial here, but wouldn't the solutions to these inhomogeneous equations be retarded and advanced potentials, which would still propagate at exactly c into the future ( and backwards ) ? If not, could you please show me what propagation speed is extracted from these inhomogeneous equations ?
I should probably know this myself, but I need to firm up on my differentials equations a little...
Just as the propagation speed for the Klein-Gordon equation:

is not well-defined, the speed of propagation of the inhomogeneous wave equation is not well-defined. However, one can think of it as follows: Consider the equation locally and for each individual component. Suppose we have:

If the term can be expressed as , then we have the homogeneous wave equation:

with as the propagation speed, though because we are considering this locally, will depend on the coordinates (making it ill-defined).

78. Originally Posted by KJW

Just as the propagation speed for the Klein-Gordon equation:

is not well-defined, the speed of propagation of the inhomogeneous wave equation is not well-defined. However, one can think of it as follows: Consider the equation locally and for each individual component. Suppose we have:

If the term can be expressed as , then we have the homogeneous wave equation:

with as the propagation speed, though because we are considering this locally, will depend on the coordinates (making it ill-defined).
Hm, ok. I get the maths, just need to get my head around why this is so physically, because it doesn't really make sense to me.
Leave it with me for a while.

79. Never mind, actually - it does make sense. The key is as you said that charges ( electrons ) are accelerated ( "shaken" ) by the incoming wave, but that acceleration must of course have a finite value, i.e. can't be instantaneous. The re-radiated outgoing wave due to the accelerated charge thus has the same frequency as the incoming one, but will be out of phase by some amount. In other words - there will be a difference between phase speed and propagation speed ( the overall wave is a superposition of original and re-radiated elements ), hence giving a refractive index that differs from unity.

This probably sounds like word salad, but is now much clearer in my mind. The inhomogeneous wave equation then simply yields a propagation speed that is not a constant, but a function of coordinates; it should still be well-defined, but only locally for small regions and short periods of time along the wave trajectory, in the sense that it can be reasonably well averaged out.

80. If anybody wants a non-technical version of what KJW and Markus Hanke are talking about I can suggest is a Sixty Symbols video that explains this topic for a lay audience without the maths and jargon.

81. Originally Posted by AlexG
THE MAXIMUM SPEED IN THIS UNIVERSE HAS GOT NOTHING TO DO WITH LIGHT. It has that speed, because of something totally different.
Incorrect. This is simply arguing for the sake of losing.
Estheria Quintessimo is saying that the speed of light is actually the speed of any massless particle, and the reason for this being the maximum speed in the universe has nothing to do with light itself, but is due to the laws of physics in this universe. In which case, I completely agree with him.

82. Originally Posted by SpeedFreek
Estheria Quintessimo is saying that the speed of light is actually the speed of any massless particle, and the reason for this being the maximum speed in the universe has nothing to do with light itself, but is due to the laws of physics in this universe. In which case, I completely agree with him.
Yes, I concur - see post #71.

83. This question is a little off topic, but still in line with understanding light.

With transparent substances the photons are re-admitted at very much the same frequency, but with opaque substances the re-admission must be at a frequency that's not visible or are not re-admitted at all. My guess is that opaque substances mostly re-admit inferred photons, because when the sun shines on an opaque object it gets warm or hot.

I would like to know if there is an easy to understand explanation for this behavior of light interacting with different substances?

84. One suggestion I have always had in physics was that velocity, v, was always in units in respect to the speed of light. For instance, the rate of time flow in special relativity is expressed as sqrt(1 - v^2/c^2), but if the velocity is expressed as a fraction of the speed of light, sqrt(1 - v^2) works better. If people don't like that, then we can use another letter or symbol specifically to express speed as a fraction of the speed of light.
In response to your complaint, it seems that the term "speed of light" is a bit misleading, but I do not agree with your argument. The phrase "speed of light" does not necessarily imply that light is the only thing that travels at c (please correct me if that is wrong, or inform me if my counter-argument is a straw man). What is true is that light only travels at that speed in a vacuum, but most textbooks and references say that c is the "speed of light in a vacuum".

85. [QUOTE=Markus Hanke;458634]
Originally Posted by babe
challenge

what does this mean in lay terms
Consider two people standing somewhere along the equator on earth, maybe one in South America, the other one in Africa, but both on the equator. They now start walking QUOTE]

What makes them start walking?

86. Originally Posted by Markus Hanke
Originally Posted by babe
challenge

what does this mean in lay terms
Consider two people standing somewhere along the equator on earth, maybe one in South America, the other one in Africa, but both on the equator. They now start walking exactly north, towards the north pole, on initially parallel trajectories. Now picture in your mind what happens - the further they walk north, the closer they get to each other ! At the north pole, they meet, even though they were initially separated by a large distance, and even though they both walked exactly north ( not towards each other ).

Why is that ? It is because the earth's surface is not flat, but curved - the shortest route between two points is a great circle segment. On a flat plain parallel lines remain parallel, i.e. two people walking in the same direction will never meet. On a curved surface that is not so - initially parallel lines will deviate, and two people walking in the same direction will either approach one another, or move away from one another over time. This concept is called geodesic deviation; it is a way to quantify the geometry of a manifold such as for example the surface of the Earth.

The above equation quantifies this notion of geodesic deviation, which is why it is unsurprisingly called the equation of geodesic deviation. It is of central importance in Einstein's theory of General Relativity - essentially it tells us how a test particle under the influence of gravity behaves compared to a completely flat and straight reference geodesic.
I owe you an apology

I think I tried to read this the evening after surgery....but was so sick and woozy that it just didn't make sense, nor did anything else so I just tried to go to sleep.

However, I did read it tonight.

Thank you. That makes sense. Although I would not recognize the equation, as it is something I would not use, I do very much appreciate your explanation it.

It was very concise, and very informative.

Regards!

87. Originally Posted by bill alsept
What makes them start walking?
They start walking because I told them to. It is an analogy I use simply to demonstrate a geometric principle, not to defend any theory in physics, which you seem to imply.

Remember that all the equation does is quantify the deviation of geodesics at any given point of a manifold. It is a mathematical result in differential geometry, and not reliant on GR or any other physics. I used the analogy of the "walkers" only to better demonstrate the principle; however, the great circles on the Earth's surface will still meet at the pole, regardless of if or how someone walks along them. It's about the geometry of the geodesics, nothing more. The point is that this can be generalised to any number of dimensions > 2.

I find it funny that babe, a member who is clearly not involved in anything mathematical or physical, seems to clearly grasp the concept of an "analogy", whereas you appear to be unable to do so. But regardless, to answer your question - geodesics in 4-dimensional space-time will still deviate according to that very same equation, in the presence of curvature. What "makes them walk" is the simple fact that one of those dimensions is time - each point along a geodesic has three spatial coordinates, and a time coordinate. The geodesic deviation equation tells us how the separation between two geodesics changes with respect to a flat and straight reference geodesic, at any point in time along that world-line. Like most other relativity deniers, you seem unable to consider the fact that we are dealing with space-time here, not just space. Your inital question "what makes them walk" is thus meaningless; the equation is about the geometry of geodetic world-lines in 4-dimensional space-time, it has nothing to do with classical notions of motion, speed etc etc. You can think of those like cracks ( fault lines ) in a sheet of ice - the geometry of their twists and turns depends on the structure of the ice sheet, just as the geometry of geodesics depends on the geometric structure of 4-dimensional space-time. It is all just pure geometry.

88. Originally Posted by babe
Although I would not recognize the equation, as it is something I would not use
You would only encounter this equation if you were to study maths or physics at university level, so I'd say most people would be unable to recognize it

89. What puzzles me on the subject here, is this :

(It's a genuine question, not a statement with an air of arrogance for those who might want to interpret it as such.)

It is accepted that items without a restmass travel at the speed of light ?
It is accepted that items with a restmass travel at speed lower than the speed of light ?

But if there is nothing there with a restmass, then how can it travel ?
It travels apparently , so there is something there ?

So why coudn't we assume that the item at the speed of light has an infinite, or yet unable to have been measured, restmass,
since that is the fundamental explanation for it's ability to move, no ?

Analogically for instance : At this point of technology, we cannot state beyond any doubt to have found the smallest particle existing.
There might be particles smaller to be found in the future, there might not be.
We have no real reason at this time, to state that it is impossible to detect even smaller particles in the future, no ?

What is the oppinion on this ?

90. Originally Posted by Noa Drake
It is accepted that items without a restmass travel at the speed of light ?
It is accepted that items with a restmass travel at speed lower than the speed of light ?
Yes, both correct.

But if there is nothing there with a restmass, then how can it travel ?
Because it still has momentum, and hence energy. Mass is quite simply a form of energy, but it is not the only one there is. Having rest mass is not a necessary condition for something to "exist" and interact physically.

So why coudn't we assume that the item at the speed of light has an infinite, or yet unable to have been measured, restmass,
since that is the fundamental explanation for it's ability to move, no ?
A non-vanishing rest mass is not the fundamental explanation of an object's ability to move; I am not certain why you would think that. Measurements of the mass of photons have indeed been performed, and it has been experimentally constraint to an accuracy of about 10^-14 eV/c^2.

There are also a number of theoretical reasons for the photon to be massless; to cite just one example, if the photon had a non-vanishing rest mass, electromagnetism ( specifically Coulomb's law ) would need to be working differently than it does. Obviously, that can be experimentally tested as well, and has thus been ruled out.

At this point of technology, we cannot state beyond any doubt to have found the smallest particle existing.
Correct.

What is the oppinion on this ?
The opinion and consensus is that there is no reason to suspect that the photon has mass; it is pretty much ruled out on theoretical grounds, and no experimental evidence of a photon mass has ever been detected, in any experiment.

91. Thank you for a clear explanation.

*
But if there is nothing there with a restmass, then how can it travel ?

"Because it still has momentum, and hence energy. Mass is quite simply a form of energy, but it is not the only one there is. Having rest mass is not a necessary condition for something to "exist" and interact physically."

>> ok, it still has momentum and hence energy. > Hence it is some form of mass, no ?

*

What is the oppinion on this ?

"The opinion and consensus is that there is no reason to suspect that the photon has mass; it is pretty much ruled out on theoretical grounds, and no experimental evidence of a photon mass has ever been detected, in any experiment."

>> So their is a consensus that a photon has no mass, on theoretical grounds, not experimental grounds.
>So who's to rule out that it couldn't be detected in the future ?

(I understand of course that this is more a discussion on how the human mind interprets observation and logic, rather than an scientific test-case.)

92. Originally Posted by Noa Drake
Thank you for a clear explanation.
No problem. If you ask genuine questions, as you did this time, you will always receive genuine answers.

Hence it is some form of mass, no ?
In physics there are two distinct notions of "mass" - firstly, there is rest mass, which is the same for all observers in all frames of reference ( hence also called invariant mass, because it does not vary with observers ). Secondly, there is relativistic mass, which can be understood as the total energy within a system or body; relativistic mass is therefore observer dependent, because it generally includes a contribution from relative speed as well.

In the case of photons, the situation is that its rest mass is exactly zero, whereas its relativistic mass is E=pc ( p = momentum, c = speed of light ). So yes, you can attribute a notion of mass to a photon, but you need to be clear that this is not rest mass. Again, I should mention that in the famous formula E=mc^2, the "m" is rest mass, and this formula is valid only for particles at relative rest.

So their is a consensus that a photon has no mass, on theoretical grounds, not experimental grounds.
A massive photon is incompatible with modern understanding of particle physics. Experimentally, the current state of affairs is that a photon mass, if it exists, must be smaller than 10^-14 eV/c^2; this lower bound will continue to decrease as the accuracy of our experimental setups increases.

So who's to rule out that it couldn't be detected in the future ?
It can never be completely "ruled out" as such that photons have indeed a very small rest mass. The problem is that should we ever experimentally find such a mass, however small, then a complete re-working of the Standard Model of Particle Physics would be required, which is difficult to imagine since the model very accurately predicts the outcome of current experiments performed in particle accelerators. A massless photon on the other hand is in accordance to both theoretical understanding and experimental evidence, so at the moment there is simply no reason to suspect that it does in fact have mass. You are right though that it can never be 100% "ruled out".

There is in fact a principle in the philosophy of science which says that theories can never be proven no matter how much evidence in its favour, yet it requires only one single piece of counter-evidence to render them false.

93. Very clarifying explanation, and food for thought.

94. Originally Posted by Noa Drake
Very clarifying explanation, and food for thought.
Thank you

95. Originally Posted by Markus Hanke
Originally Posted by bill alsept
What makes them start walking?
They start walking because I told them to. It is an analogy I use simply to demonstrate a geometric principle, not to defend any theory in physics, which you seem to imply.Remember that all the equation does is quantify the deviation of geodesics at any given point of a manifold. It is a mathematical result in differential geometry, and not reliant on GR or any other physics. I used the analogy of the "walkers" only to better demonstrate the principle; however, the great circles on the Earth's surface will still meet at the pole, regardless of if or how someone walks along them. It's about the geometry of the geodesics, nothing more. The point is that this can be generalised to any number of dimensions > 2.I find it funny that babe, a member who is clearly not involved in anything mathematical or physical, seems to clearly grasp the concept of an "analogy", whereas you appear to be unable to do so. But regardless, to answer your question - geodesics in 4-dimensional space-time will still deviate according to that very same equation, in the presence of curvature. What "makes them walk" is the simple fact that one of those dimensions is time - each point along a geodesic has three spatial coordinates, and a time coordinate. The geodesic deviation equation tells us how the separation between two geodesics changes with respect to a flat and straight reference geodesic, at any point in time along that world-line. Like most other relativity deniers, you seem unable to consider the fact that we are dealing with space-time here, not just space. Your inital question "what makes them walk" is thus meaningless; the equation is about the geometry of geodetic world-lines in 4-dimensional space-time, it has nothing to do with classical notions of motion, speed etc etc. You can think of those like cracks ( fault lines ) in a sheet of ice - the geometry of their twists and turns depends on the structure of the ice sheet, just as the geometry of geodesics depends on the geometric structure of 4-dimensional space-time. It is all just pure geometry.
I understand the analogies and that most of them do not really apply to the real world. My question hit the mark as I intended it and I thank you for your responce. I am no relative denier I only question it and there is nothing wrong with that. I question the basic premises of GR that mass somehow curves space and somehow that causes gravity. There is no proof of curved space and the premis doesn't even explain why or how space would curve. Prooving curved space would be like Prooving there is a god. Even if you can get beyond all that and just except curved space on faith alone there is still the second question as to why and how curved space causes gravity. And that is why I asked the question what makes them start walking in the first place. in your analogy the guys started walking because you told them to but curved space cannot do that. And if it could how?

96. I am no relative denier I only question it and there is nothing wrong with that.
Questioning would be more effective if you knew anything about it, other than you don't believe it.

There is no proof of curved space and the premis doesn't even explain why or how space would curve.
Incorrect. Star displacement in the 1919 solar eclipse observed by Arthur Eddington exactly matched the predictions of GR.

97. Originally Posted by Markus Hanke
A non-vanishing rest mass is not the fundamental explanation of an object's ability to move; I am not certain why you would think that. Measurements of the mass of photons have indeed been performed, and it has been experimentally constraint to an accuracy of about 10^-14 eV/c^2.

There are also a number of theoretical reasons for the photon to be massless; to cite just one example, if the photon had a non-vanishing rest mass, electromagnetism ( specifically Coulomb's law ) would need to be working differently than it does. Obviously, that can be experimentally tested as well, and has thus been ruled out.
Yes, one of the experimental tests that the photon is massless is to measure the electric field inside a charged hollow metal sphere. For the inverse-square law, this electric field will be zero, and deviations from the inverse-square law indicate that the photon has non-zero mass.

98. Originally Posted by Markus Hanke
A non-vanishing rest mass is not the fundamental explanation of an object's ability to move; I am not certain why you would think that. Measurements of the mass of photons have indeed been performed, and it has been experimentally constraint to an accuracy of about 10^-14 eV/c^2.

There are also a number of theoretical reasons for the photon to be massless; to cite just one example, if the photon had a non-vanishing rest mass, electromagnetism ( specifically Coulomb's law ) would need to be working differently than it does. Obviously, that can be experimentally tested as well, and has thus been ruled out.
Yes, one of the experimental tests that the photon is massless is to measure the electric field inside a charged hollow metal sphere. For the inverse-square law, this electric field will be zero, and deviations from the inverse-square law indicate that the photon has non-zero mass.

99. Originally Posted by Noa Drake
It is accepted that items without a restmass travel at the speed of light ?
It is accepted that items with a restmass travel at speed lower than the speed of light ?

But if there is nothing there with a restmass, then how can it travel ?
It travels apparently , so there is something there ?

So why coudn't we assume that the item at the speed of light has an infinite, or yet unable to have been measured, restmass,
since that is the fundamental explanation for it's ability to move, no ?
The zero mass of the photon is about the peculiar geometry of four-dimensional spacetime. The mass of any object is given by the formula (in Minkowskian coordinates):

For , it is the statement that:

Thus, the mass of an object is about the energy and momentum of that object, with the mass fixing the relationship between the energy and the momentum of that object.

100. Originally Posted by Markus Hanke
Originally Posted by bill alsept
What makes them start walking?
They start walking because I told them to. It is an analogy I use simply to demonstrate a geometric principle, not to defend any theory in physics, which you seem to imply.

Remember that all the equation does is quantify the deviation of geodesics at any given point of a manifold. It is a mathematical result in differential geometry, and not reliant on GR or any other physics. I used the analogy of the "walkers" only to better demonstrate the principle; however, the great circles on the Earth's surface will still meet at the pole, regardless of if or how someone walks along them. It's about the geometry of the geodesics, nothing more. The point is that this can be generalised to any number of dimensions > 2.

I find it funny that babe, a member who is clearly not involved in anything mathematical or physical, seems to clearly grasp the concept of an "analogy", whereas you appear to be unable to do so. But regardless, to answer your question - geodesics in 4-dimensional space-time will still deviate according to that very same equation, in the presence of curvature. What "makes them walk" is the simple fact that one of those dimensions is time - each point along a geodesic has three spatial coordinates, and a time coordinate. The geodesic deviation equation tells us how the separation between two geodesics changes with respect to a flat and straight reference geodesic, at any point in time along that world-line. Like most other relativity deniers, you seem unable to consider the fact that we are dealing with space-time here, not just space. Your inital question "what makes them walk" is thus meaningless; the equation is about the geometry of geodetic world-lines in 4-dimensional space-time, it has nothing to do with classical notions of motion, speed etc etc. You can think of those like cracks ( fault lines ) in a sheet of ice - the geometry of their twists and turns depends on the structure of the ice sheet, just as the geometry of geodesics depends on the geometric structure of 4-dimensional space-time. It is all just pure geometry.
Thanks...I'm science dumb but theatre arts smart!

101. Originally Posted by Noa Drake
Very clarifying explanation, and food for thought.
Markus and MANY OTHERS in here, are often kind to people, such as myself, who don't understand things. I know I appreciate it when I receive an explanation which gives me clarity, even if not a total grasp.

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