Light, What is it relative to.

[-Demosthenes-]

Okay, a man is in a train that is going 25 mph. If he runs at 5 mph in the train relative to the train he's going 5 mph. But relative to the earth he's going 30 mph (if he's running the same direction).
So if light always goes the same speed, what is it relative to? If it's going the speed of light relative to the earth, then how fast is it going relative to the sun? or the other planets? How can we even classify speed at all? I'm confued, will someone talk some sence into me?

[JaKiri]

-Demosthenes- said in post # (http://www.scienceforums.net/forums/showthread.php?s=&postid=35609#post35609):

So if light always goes the same speed, what is it relative to?

ALL observers.

You shine a torch at me, you'll see the light going at c (if it's a vacuum).

If I'm running away, I'll see the light going past me at c.

And you're actually incorrect about the man on the train. He's actually going ever so slightly less than 30MPH.

The equation for adding velocities isn't a + b = velocity, it's (a+b)/(1+ab/c^2)

[-Demosthenes-]

Why is that(about the train and the man)

And about the light, it's relative to all observers, you mean everyone who can see it? Does it have to be a person? Or is it relative to all things?

Thanks!

[JaKiri]

-Demosthenes- said in post # (http://www.scienceforums.net/forums/showthread.php?s=&postid=35624#post35624):
Why is that(about the train and the man)

And about the light, it's relative to all observers, you mean everyone who can see it? Does it have to be a person? Or is it relative to all things?

Thanks!

Relative to anything possible ever.

Even if you 'were' a light beam, light going the other way would still be going off at the speed of light.

And as to, the train and the man it's called a relativistic effect. It's a consquence of the thing about the speed of light.

[-Demosthenes-]

Relative to everything at once?

[JaKiri]

-Demosthenes- said in post # (http://www.scienceforums.net/forums/showthread.php?s=&postid=35632#post35632):
Relative to everything at once?

Whereever you are, whatever speed you're going at, the speed of light in a vacuum will still be 299792458ms^-1

[Cap'n Refsmmat]

If you are going the speed of light, and shine a flashlight, the light will not move (relative to you!) because it is going the speed of light, and so are you, so it is going the same speed as you, and relative to you, is not moving.

[JaKiri]

Cap'n Refsmmat said in post # (http://www.scienceforums.net/forums/showthread.php?s=&postid=35640#post35640):
If you are going the speed of light, and shine a flashlight, the light will not move (relative to you!) because it is going the speed of light, and so are you, so it is going the same speed as you, and relative to you, is not moving.

Nope. It'll go away from you at lightspeed.

[Cap'n Refsmmat]

But to someone going at 0, it will go at lightspeed too, relative to them? But then... it would be going twice as fast as you, yet to them, as fast as you? Elaborate.

[JaKiri]

Cap'n Refsmmat said in post # (http://www.scienceforums.net/forums/showthread.php?s=&postid=35650#post35650):
But to someone going at 0, it will go at lightspeed too, relative to them? But then... it would be going twice as fast as you, yet to them, as fast as you? Elaborate.

Lightspeed is the same to all observers.

All rest frames are equally valid.

Look at it this way; when you're going at 30mph, what are you doing it relative to?

The planet.

However, it's equally valid to say that the planet is going 30mph with you at rest (assuming no acceleration occurs).

Another way of looking at it is...

You're in space. You think you're stationary, yes?

Then some other guy drifts past you.

To HIM, it feels like he is stationary, and you are drifting past HIM.

[Cap'n Refsmmat]

That still doesn't make sense. Your argument is true, it just doesn't explain it. The light from your flashlight will go the speed of light past you, so from the observers point of view, the light is going TWICE the speed of light. That's just not possible.

[NavajoEverclear]

Light Squared? I'm also interested to the explanation for this.

[iglak]

actually, what would the speed of light be relative to you if you were going the speed of light relative to a stationery observer?

the speed of light is only able too look like the speed of light to all observers because all observers are going slower than the speed of light, so the light can look that speed if certain visual spacial and temporal (i think that's the word for time-ial) restrictions are put in place. but if you are going the speed of light, the exact speed of light relative to a stationery observer, the light could not look like it was going the speed of light, because it isn't going faster than you.

i think you wouldn't be able to move. i think that as one gets infinitely close to the speed of light that length contraction has such a big effect that it would look to you as if you crossed the galaxy in an instant (it would just look like a very short galaxy).

so, if you were going the exact speed of light, then it would appear to you as if the universe was nothing. you wouldn't be able to percieve it since it would be going by you all at once (i thought of a mathematical term for this before: "everything"). thus, you would never be able to stop. since you can't even move...

in graphical terms, easier to understand:
think of a line that stretches to infinity, and put markers on it at specific intervals. the faster you go, the closer the markers get together, but since the line is infinitely long, you just keep seeing it forever. as you approach the speed of light, the markers get infinitely closer, so that the line looks like 2d versions of the markers stacked on top of eachother with absolutely no space inbetween, but the line is still infinitely long. at the point you reach the speed of light, the infinite line becomes nothing. this is because you are passing by all points on the infinite line at the same time. you are passing by "everything" ("everything" can also be used to identify things divided by zero :D). it is not infinitely close to zero distance, for it is infinite distance, and can never look finite. thus, it is zero, or "everything". thus, you can never stop going that speed once you start since time no longer applies to you, you pass everything all at once in zero time.

<edit>
P.S. i just thought of an addition to this.
also, relative to a stationery observer, it would appear that you are infinitly thin as you approach the speed of light, and at the speed of light you would appear to be gone, because others would see your "everything" pass in zero time too, because of relativity. so then the question is now... do you simply cease to exist? or does each atom/sub atomic particle become a photon?

P.P.S. in case you haven't noticed, "everything" only gives meaning to "undefined". without my "everything", "undefined" is misenterpereted as "impossible", and only remains undefined.

[JaKiri]

iglak said in post # (http://www.scienceforums.net/forums/showthread.php?s=&postid=35683#post35683):
the speed of light is only able too look like the speed of light to all observers because all observers are going slower than the speed of light

No.

[JaKiri]

Cap'n Refsmmat said in post # (http://www.scienceforums.net/forums/showthread.php?s=&postid=35658#post35658):
The light from your flashlight will go the speed of light past you, so from the observers point of view, the light is going TWICE the speed of light. That's just not possible.

You only observe velocities from your own rest frame.

[YT2095]

ok, 2 flash lights back to back, switch them on and both beams travel in opposite directions, how fast is the front of the 1`st beam moving away from the front of the other beam?

[JaKiri]

YT2095 said in post # (http://www.scienceforums.net/forums/showthread.php?s=&postid=35730#post35730):
ok, 2 flash lights back to back, switch them on and both beams travel in opposite directions, how fast is the front of the 1`st beam moving away from the front of the other beam?

Assuming it's a vacuum, c.

[YT2095]

so beam A is moving away from the torch at half C?

[JaKiri]

YT2095 said in post # (http://www.scienceforums.net/forums/showthread.php?s=&postid=35732#post35732):
so beam A is moving away from the torch at half C?

Nope. c.

[edit]

If it WAS c/2, the combined speed would only be 3/4 c.

[YT2095]

how do you work that out? or arrive at 3/4 (.75).

I see it as, there`s 2 cars back to back and strating point X car A and car B take off in opposite directions from point X at 50 mph.
car A is moving from car B at 100 mph, rellative to each other, is that right so far?

[JaKiri]

YT2095 said in post # (http://www.scienceforums.net/forums/showthread.php?s=&postid=35734#post35734):
how do you work that out? or arrive at 3/4 (.75).

I see it as, there`s 2 cars back to back and strating point X car A and car B take off in opposite directions from point X at 50 mph.
car A is moving from car B at 100 mph, rellative to each other, is that right so far?

No.

They're actually seperating at 99.999999999998887mph.

[edit]

And read my FIRST POST IN THE THREAD, I went over this.

[YT2095]

I did and it didn`t make any sense
why 99.99988 blah blah, it`s 100 mph surely, it`s just basic maths, 50+50 is 100.

[JaKiri]

YT2095 said in post # (http://www.scienceforums.net/forums/showthread.php?s=&postid=35738#post35738):
I did and it didn`t make any sense
why 99.99988 blah blah, it`s 100 mph surely, it`s just basic maths, 50+50 is 100.

If you want to add speeds a and b (well, velocities strictly speaking) it IS NOT a + b. It's (a+b)/(1+ab/c^2)

[YT2095]

so when they callibrate the speedo in a car, that`s how they do it?

[JaKiri]

YT2095 said in post # (http://www.scienceforums.net/forums/showthread.php?s=&postid=35742#post35742):
so when they callibrate the speedo in a car, that`s how they do it?

No.

a+b is a fine working approximate until you reach higher velocities.

[Sayonara³]

In any case, speedos don't measure velocity.

[JaKiri]

Sayonara³ said in post # (http://www.scienceforums.net/forums/showthread.php?s=&postid=35744#post35744):
In any case, speedos don't measure velocity.

Most of them approximate velocity from other measurements rather than speed, though.

[JaKiri]

Sayonara³ said in post # (http://www.scienceforums.net/forums/showthread.php?s=&postid=35744#post35744):
In any case, speedos don't measure velocity.

And AW and TSA still aren't working

[YT2095]

true, probably a bad example then.

it just seems to all run a little contrary to things you consider to be perfectly sensible.

[JaKiri]

YT2095 said in post # (http://www.scienceforums.net/forums/showthread.php?s=&postid=35748#post35748):
it just seems to all run a little contrary to things you consider to be perfectly sensible.

People didn't think the earth being a spheroid was sensible either.

[Sayonara³]

MrL_JaKiri said in post # (http://www.scienceforums.net/forums/showthread.php?s=&postid=35745#post35745):
Most of them approximate velocity from other measurements rather than speed, though.
Let's not overcomplicate it that far :-p

MrL_JaKiri said in post # (http://www.scienceforums.net/forums/showthread.php?s=&postid=35747#post35747):
And AW and TSA still aren't working
Gavin's on it.

[JaKiri]

Sayonara³ said in post # (http://www.scienceforums.net/forums/showthread.php?s=&postid=35750#post35750):

Gavin's on it.

Why not just buy him a chair?

[Sayonara³]

Servers are a lot warmer.

[JaKiri]

Sayonara³ said in post # (http://www.scienceforums.net/forums/showthread.php?s=&postid=35753#post35753):
Servers are a lot warmer.

My cat likes to sit on my underwear.

[Sayonara³]

Filthy filthy MrL.

/me calls RSPCA

[JaKiri]

Sayonara³ said in post # (http://www.scienceforums.net/forums/showthread.php?s=&postid=35757#post35757):
Filthy filthy MrL.

/me calls RSPCA

It's not a filthy cat, I sometimes find it sleeping in the bath.

[Sayonara³]

Is that where you keep your undies?

Sorry I thought you meant you were wearing them at the time.

[-Demosthenes-]

Does light have totally different properties from everything else in the universe? One of thoses things that I'll never inderstand, seems like.
Very interesting.

[-Demosthenes-]

So light beam A and B are heading in opposite directions. They are only going *c, then isn't it going half of c relative to the planet?

*(I think I've learned that c represents the speed of light from other's post, tell me if I'm wong)

[JaKiri]

-Demosthenes- said in post # (http://www.scienceforums.net/forums/showthread.php?s=&postid=35797#post35797):
So light beam A and B are heading in opposite directions. They are only going *c, then isn't it going half of c relative to the planet?

*(I think I've learned that c represents the speed of light from other's post, tell me if I'm wong)

c = 299792458ms^-1, the speed of light in a vacuum.

And NO THEY'RE NOT ONLY GOING HALF c

I gave you the sodding equation TWICE, USE IT.

[JaKiri]

-Demosthenes- said in post # (http://www.scienceforums.net/forums/showthread.php?s=&postid=35795#post35795):
Does light have totally different properties from everything else in the universe? One of thoses things that I'll never inderstand, seems like.
Very interesting.

No, it doesn't. It's pretty normal for a boson.

[blike]

Like he said, they're always going c, no matter where you are, how fast your moving.

[YT2095]

if we forget the flashlights for a second, it`s the 100 mph parts that throws me?

"They're actually seperating at 99.999999999998887mph."

surely 100 mph is just that? as in if both cars travel for 1 hour, they will be 100 miles appart.

[Sayonara³]

If you are dealing with speeds approaching c, the difference is a lot more than 0.000000000001 miles.

I think that was the point.

[YT2095]

well ok, so these 2 cars would be somewhat less than 100miles apart? or they would be 100 miles apart but get there at a slower speed, or time would say one hour was up to the driver when infact it fell short of 1 hour? WHAT?

I`m at a loss here, and I know there`s others in the queue (sp?) eagerly awaiting a responce other than a formula, a plain english answer maybe applicable here (I could be wrong, but working exapmples seem to get the point across a triffle better than a+bXz^c/a million+3.14 etc...)

:)

[Cap'n Refsmmat]

It looks right! I just don't see what MRL is saying.

[JaKiri]

YT2095 said in post # (http://www.scienceforums.net/forums/showthread.php?s=&postid=35861#post35861):
well ok, so these 2 cars would be somewhat less than 100miles apart? or they would be 100 miles apart but get there at a slower speed, or time would say one hour was up to the driver when infact it fell short of 1 hour? WHAT?

I`m at a loss here, and I know there`s others in the queue (sp?) eagerly awaiting a responce other than a formula, a plain english answer maybe applicable here (I could be wrong, but working exapmples seem to get the point across a triffle better than a+bXz^c/a million+3.14 etc...)

:)

Why not do it with the you used? 50 mph is about 22.3ms^-1.

(22.3 + 22.3)/(1+22.3*22.3/299792458^2) is what you're looking for. Which is about 44.6/1.0000000000001, or thereabouts (0's aren't exact, just 'a line of zeros'), which will be just less than 44.6ms^-1.

And after one hour, they would be about 99.999999999999998 whatever miles apart.

[YT2095]

Phew, and I thought I was the only one there for a sec :)

[JaKiri]

Cap'n Refsmmat said in post # (http://www.scienceforums.net/forums/showthread.php?s=&postid=35866#post35866):
It looks right! I just don't see what MRL is saying.

All rest frames are equally valid, is what I am saying.

[YT2095]

then maybe you`de like to explain how a GPS works, are you saying they too are wrong when they give the current velocity of your vehicle?

[JaKiri]

YT2095 said in post # (http://www.scienceforums.net/forums/showthread.php?s=&postid=35868#post35868):
Phew, and I thought I was the only one there for a sec :)

If you want the example of the torch beams, then a and b would both be c.

(c+c)/(1+c*c/c^2) = 2c/(1+c^2/c^2) = 2c/2 = c.

So the recession speed of two bits of light travelling at c in opposite directions is c.

[JaKiri]

YT2095 said in post # (http://www.scienceforums.net/forums/showthread.php?s=&postid=35870#post35870):
then maybe you`de like to explain how a GPS works, are you saying they too are wrong when they give the current velocity of your vehicle?

I find that relativistic effects are the least of your worries when it comes to the accuracy of GPSs.

[iglak]

MrL_JaKiri said in post # (http://www.scienceforums.net/forums/showthread.php?s=&postid=35691#post35691):
No.
why? i must be misunderstanding something if this is true.

[JaKiri]

iglak said in post # (http://www.scienceforums.net/forums/showthread.php?s=&postid=35911#post35911):

why? i must be misunderstanding something if this is true.

The speed of light looks like the speed of light to someone going the speed of light. Look at my posts on the 2nd page, and one on this one.

[YT2095]

ok then, put in laymans terms, it seems like you`re trying to say that the faster you move, the more you slow down?
or at least something like it, and that if you hit C then you stop?

I expect that this is probaly quite wrong again or the best over simplified, but on behalf of myself and one or 2 others that have posted in this thread, is that anything like close to a working example?

[JaKiri]

YT2095 said in post # (http://www.scienceforums.net/forums/showthread.php?s=&postid=36039#post36039):
ok then, put in laymans terms, it seems like you`re trying to say that the faster you move, the more you slow down?

Nope. You still go the same speed. It's just that, as I've explained before, the recession speed isn't calculated a+b; that's incorrect, and it's something you're assuming.

[YT2095]

CAN YOU then, explain it in Laymans terms as opposed to calcs and observations then?

[JaKiri]

YT2095 said in post # (http://www.scienceforums.net/forums/showthread.php?s=&postid=36044#post36044):
CAN YOU then, explain it in Laymans terms as opposed to calcs and observations then?

The maximum possible recession speed (or approach speed, of course) between two states of motion (as it were; 'rest frames' is more accurate) is a maximum of c, and the deviation between the actual and just adding them becomes greater as you move away from relative rest and towards lightspeed.

[YT2095]

so working on the idea that a speedo in car works on RPMs of the wheel turning, you`re saying the faster you actualy travel, the more your speedo will be telling lies to you?

[Cap'n Refsmmat]

That's what he's saying. Not that I get any of it. He still didn't answer my question about the flashlight, at least in a way I understand.
The light from the flashlight would go 2x the speed of light, from the observer's point of view. Is that even POSSIBLE?

[JaKiri]

YT2095 said in post # (http://www.scienceforums.net/forums/showthread.php?s=&postid=36046#post36046):
so working on the idea that a speedo in car works on RPMs of the wheel turning, you`re saying the faster you actualy travel, the more your speedo will be telling lies to you?

THE RELATIVE SPEED OF AN OBJECT WITH SOMETHING AT REST IS UNCHANGED. IT'S ONLY WHEN YOU'RE ADDING VELOCITIES.

[JaKiri]

Cap'n Refsmmat said in post # (http://www.scienceforums.net/forums/showthread.php?s=&postid=36049#post36049):
That's what he's saying. Not that I get any of it. He still didn't answer my question about the flashlight, at least in a way I understand.
The light from the flashlight would go 2x the speed of light, from the observer's point of view. Is that even POSSIBLE?

No matter your velocity relative to anything else, the speed of light relative to you (ie the speed it moves towards/away/whatever) will always be the speed of light.

This is because all rest frames are equally valid, and you might as well say that everything else is going at the speed of light.

[YT2095]

MrL_JaKiri said in post # (http://www.scienceforums.net/forums/showthread.php?s=&postid=36051#post36051):


THE RELATIVE SPEED OF AN OBJECT WITH SOMETHING AT REST IS UNCHANGED. IT'S ONLY WHEN YOU'RE ADDING VELOCITIES.

WELL YOU CAN SHOUT ALL YOU WANT TO, IT DOESN`T MAKE YOUR POINT ANY THE BETTER UNDERSTOOD! dUUUUGH!

so how the hell does car A know what Car B is doing then?
for cryin out loud man!

[JaKiri]

YT2095 said in post # (http://www.scienceforums.net/forums/showthread.php?s=&postid=36057#post36057):


WELL YOU CAN SHOUT ALL YOU WANT TO, IT DOESN`T MAKE YOUR POINT ANY THE BETTER UNDERSTOOD! dUUUUGH!

so how the hell does car A know what Car B is doing then?
for cryin out loud man!

I can now use the quote in its native form.

I am not able rightly to apprehend the kind of confusion of ideas that could provoke such a question.

'Car A' 'knowing' what 'Car B' is doing is utterly, utterly irrelevent, (ignore this bit if you don't know what I'm talking about already, it'll probably confuse matters: although always possible by the nature of information exchange) and you may as well say 'How does car a know what car b is doing??? when you're adding together the speeds under newtonian mechanics (ie a+b).

It's not a property of the car, it's not a property of the speed it's going at, it's a property of the universe, and exists with respect to rest frames.

[iglak]

MrL_JaKiri said in post # (http://www.scienceforums.net/forums/showthread.php?s=&postid=35963#post35963):


The speed of light looks like the speed of light to someone going the speed of light. Look at my posts on the 2nd page, and one on this one.
you mean the candle light flashlight stuff?
ahh... i see...
then all i don't get is special relativity (darn, and i thought i understood it!), i need to look it up.

[NSX]

MrL_JaKiri said in post # (http://www.scienceforums.net/forums/showthread.php?s=&postid=35743#post35743):


No.

a+b is a fine working approximate until you reach higher velocities.

Like MrL says.

Speeds on our Earth are pretty far from relativistic motions.

For simple Earth speeds like you say, YT, cars, classical mechanics is good enough.

iglak said in post # (http://www.scienceforums.net/forums/showthread.php?s=&postid=36073#post36073):

you mean the candle light flashlight stuff?
ahh... i see...
then all i don't get is special relativity (darn, and i thought i understood it!), i need to look it up.

It's a long lookup
:lol:

[-Demosthenes-]

I've been reading the posts, but I honestly don't completely understand it. Can we put it all in a nutshell, just for me? I think I know "what" but I don't think I know "why".

Signed--Confused

[Cap'n Refsmmat]

I'm confused too.

[JaKiri]

Why is because the speed of light is constant for all observers. Everything else is due to it, and the mathematics derives from it.

If you want something better, then I'm afraid the only other explanation is 'that's the way it is'

[Cap'n Refsmmat]

Ok, let me restate my question. I'm not satisfied with the way you answered.
You are going the speed of light. You turn on a flashlight and point it forward. The light would accelerate ahead of you at the speed of light. To another observer, that light is going twice the speed of light. Not possible, right? So what would the light do?

[JaKiri]

Cap'n Refsmmat said in post # (http://www.scienceforums.net/forums/showthread.php?s=&postid=36094#post36094):
Ok, let me restate my question. I'm not satisfied with the way you answered.
You are going the speed of light. You turn on a flashlight and point it forward. The light would accelerate ahead of you at the speed of light. To another observer, that light is going twice the speed of light. Not possible, right? So what would the light do?

For a start, it wouldn't accelerate. Light isn't subject to forces in the same way as ordinary matter.

Furthermore, the observer would see the light going at the speed of light too. Not twice, but once.

The speed of light is a bad speed to do things at in this kind of discussion, though, as time technically does not exist and therefore you can't have done anything.

[Cap'n Refsmmat]

"Furthermore, the observer would see the light going at the speed of light too."
So that means the light would appear to stay in the flashlight (relative to the observer), because it is going the speed of light and so are you.

[JaKiri]

Cap'n Refsmmat said in post # (http://www.scienceforums.net/forums/showthread.php?s=&postid=36097#post36097):
&quot;Furthermore, the observer would see the light going at the speed of light too.&quot;
So that means the light would appear to stay in the flashlight (relative to the observer), because it is going the speed of light and so are you.

The speed of light is a bad, bad example, you see, and not only because it's impossible.

It's much better to view things going just under the speed of light, it'll help you understand better.

[Cap'n Refsmmat]

So what happens if you are going 99.999999999% of the speed of light, and you do that. Then it is going ALMOST twice, which isn't possible.

[JaKiri]

Cap'n Refsmmat said in post # (http://www.scienceforums.net/forums/showthread.php?s=&postid=36100#post36100):
So what happens if you are going 99.999999999% of the speed of light, and you do that. Then it is going ALMOST twice, which isn't possible.

What will happen is that the light will be moving at c in the rest frame of the observer, and you will be going 99.9999999% of the speed of light, so the light moves away from you veeerrry sloooowly.

In your rest frame, however, the light moves away from you at c.

This is consistent, because time slows down the faster you go (relative to the rest frame).

Hold on, I'll do a worked example.

Lets say you're going 99.999999 whatever% of the speed of light.

According to the person that is watching, it'll take quite some time for the light to move one light second away from you, because you're only just going slower than it.

(1 light second being 299792458m)

According to you, of course, the light will only take ONE second to go one light second away from you.

However, your time is slowed because you're going so close to the speed of light, and the factor by which it's slowed is exactly the same multiple as between your one second and his few hours.

There's also a factor of length contraction, but that makes it a bit more complicated to word (the light second you see isn't the same as the light second the person at rest sees, distance wise)

You see?

[Cap'n Refsmmat]

So the light goes light speed according to him, and to you, since time is just crawling, it is moving away fast.
I'll think about it in bed. Don't worry, I can stand losing sleep. ;)

[JaKiri]

Cap'n Refsmmat said in post # (http://www.scienceforums.net/forums/showthread.php?s=&postid=36102#post36102):
So the light goes light speed according to him, and to you, since time is just crawling, it is moving away fast.
I'll think about it in bed. Don't worry, I can stand losing sleep. ;)

Basically, yeah.

But the odd thing is that it works THE OTHER WAY AROUND too.

[Cap'n Refsmmat]

WHAT? Now I'm doomed to confusion for life.

[JaKiri]

Cap'n Refsmmat said in post # (http://www.scienceforums.net/forums/showthread.php?s=&postid=36109#post36109):
WHAT? Now I'm doomed to confusion for life.

SR basically says that all rest frames are equally valid.

So no matter what speed someone else thinks you're going at, it's perfectly fine to assume that you're stationary and HE'S moving.

The best example of this would be the two astronauts drifting past eachother.

I drift past you at 20ms^-1. You're stationary.

It's equally valid to say that you're drifting past me, and I'm stationary.

Or that someone on a ship travelling close to lightspeed (relative to us) is stationary, and we're both moving very close to lightspeed, just with a 20ms^-1 (or thereabouts) difference.

The mathematics all works.

[edit]

This only works for CONSTANT velocities. If you accelerate, then it all changes, and YOU'RE the one moving (basically). See the thread on the twins paradox (with no planet) for more on that.

[Cap'n Refsmmat]

Yes, I might be moving, and you not, and you think YOU are moving. However, if our reference frame is Earth, for example, we know who is moving relative to Earth.

[greg1917]

Can I just take the chance to applaud MrL's patience for being asked exactly the same question repeatedly yet still posting answers?

The problem here is people cant seem to comprehend mechanics outside the nice, closed newtonian world where everything is wonderful and predictable.

if you are travelling at 99%c and turn on a torch, the photons from the torch will move away from YOU at c.

To a person standing still as you shoot past them, the same photons will also appear to be travelling at c.

For anyone else who happens to be jogging past this absurd scenario at ANY speed, the photons will be travelling at c.

to accomodate this, crazy things like time dillation and length contraction occur. Well, crazy for a chemist anyway. we like to stick to things like deciding the best solvent sytems for a tricky TLC the next day :-p

[Cap'n Refsmmat]

See, these things always confuse me. If they are moving away from you at C, but relative to someone else, they are going at c, that is confusing. At least until you account the time changing and so on.

[greg1917]

Thats why physicists aren't normal people. They think outside the box then mutter to themselves all day and quaff real ale.

[JaKiri]

Cap'n Refsmmat said in post # (http://www.scienceforums.net/forums/showthread.php?s=&postid=36111#post36111):
Yes, I might be moving, and you not, and you think YOU are moving. However, if our reference frame is Earth, for example, we know who is moving relative to Earth.

It gets a bit tricky with accelerations, as I said. You ARE moving on the earth, because you're undergoing acceleration (except in freefall, when you're not, sort of)

[JaKiri]

greg1917 said in post # (http://www.scienceforums.net/forums/showthread.php?s=&postid=36114#post36114):
Thats why physicists aren't normal people. They think outside the box then mutter to themselves all day and quaff real ale.

I'm more of a guinness kind of chap.

[greg1917]

Couldnt drink guinness myself, id let down my fellow Rangers fans by being associated with the dark side. Not that im pathetic or anything.

[JaKiri]

greg1917 said in post # (http://www.scienceforums.net/forums/showthread.php?s=&postid=36112#post36112):
to accomodate this, crazy things like time dillation and length contraction occur. Well, crazy for a chemist anyway. we like to stick to things like deciding the best solvent sytems for a tricky TLC the next day :-p

I like to give my equations a bit of TLC too.

Although that's frowned upon in the wider community.



AND THEY CALLED ME A PERVERT!

ME!

A PERVERT!

After I finish having sex with this sheep I'm going to go round there and TEACH THEM A LESSON

[JaKiri]

greg1917 said in post # (http://www.scienceforums.net/forums/showthread.php?s=&postid=36117#post36117):
Couldnt drink guinness myself, id let down my fellow Rangers fans by being associated with the dark side. Not that im pathetic or anything.

Rangers eh?

My team is single handedly responsible for knocking Celtic out of the Champions League.

[greg1917]

MrL a pervert? Ill alert lord Hutton right away to make sure your cleared of every charge and the BBC takes the rap!

[greg1917]

Your a lyon fan?

[JaKiri]

greg1917 said in post # (http://www.scienceforums.net/forums/showthread.php?s=&postid=36121#post36121):
Your a lyon fan?

Sunderland.

We loaned em Mickey Gray.

[greg1917]

Lol

Mind you, sunderland fans are still probably gutting themselves laughing at Newcastle getting knocked out by FK Partizan :o

[JaKiri]

greg1917 said in post # (http://www.scienceforums.net/forums/showthread.php?s=&postid=36123#post36123):
Lol

Mind you, sunderland fans are still probably gutting themselves laughing at Newcastle getting knocked out by FK Partizan :o

Nah, we've moved on to getting further than them in the FA Cup, when it was really really clear that Shearer and Robson wanted to win it oh so much, and Mick think's it's a bit of a laugh.

[edit]

Worst rumour of the season was that Arca was moving to Celtic (there was a link with Chelsea at one stage, but that's natural for every player)

[greg1917]

Oh bloody hell its almost 4am, im off to bed. frank de boers debut for Rangers tomorrow so im just hoping he hits the ground running.

[-Demosthenes-]

Okay, I think I can barely understand, but maybe not. You must have some kind of degree in light travel or something, I 'm going to bed.

[Cap'n Refsmmat]

Good for you.
I must be amazing. I actually figured out how it worked! (not that I'm not confused)

[-Demosthenes-]

yeah

[YT2095]

Hmmm... your post #75 makes a little more sense of it all now (to me anyway).
but what happens when you shine a torch or a laser at one star (say our sun for example), and then move it across the sky to another, the end of the beam moves several thousand light years in less than a second, alot faster than light itself can.
if you could look from the top down at this beam originating from earth, what would the beam appear like when you moved it from the sun to another star a million light years away?

[Sayonara³]

It made sense when I started, then gradually... less so :-(

[YT2095]

I envisage someling like an arc, and taken to the ultimate degree, if the laser were to be spun in motor, I`de imagine a catherine wheel spiral effect that radiates outwards. having said that, that`s obeying the physical laws that I KNOW and understand.
I see the problem arrise when this laser is spun SO fast that the light radiating outwards would be traveling almost sideways, and that I can`t picture? ;)

[Sayonara³]

I suppose it depends what your t-axis is doing.

[YT2095]

come again?

I only know of 3; X, Y and Z axis`

[Sayonara³]

Time dude.

The window during which you observe the light will have an effect. You don't want to trace its path forever, so where you decide "that's enough of watching it zoom off in that direction" becomes an important consideration.

Or something.

[YT2095]

well the way I picture it if you moved the beam from point A to B. and you looked down on the beam it would look like a \ that moves forwards because from the beam ON point at A it would still take time to physicaly move the laser to point B, during that time, the light from point A has traveled further.

[-Demosthenes-]

wow, I need some more brains or something.

[Jugs Monthly]

I have always been impressed by the knowledge of people on here such as YT2095 and read their posts with great respect and envy.
So it is a perverse pleasure to see them struggle with the concept of c being the same to all observers regardless of their position or motion relative to the source of said light. Something I finally managed to get my head around and accept some years ago.
No insults intended, it just makes me feel a little less thick when compared to the doubtless superior brains that post here.
May you all continue to amaze and educate me for years to come.