Can one of the more learned members please help me get my head around the idea of special relativity please?

Einstein's relativity states that the speed of light is the same for all observers, no matter what their relative speeds.

I can't understand how this can be true for an observer travelling toward a light source.
If the observer is travelling at .5 the speed of light, why wouldn't the approaching light appear to be travelling at 1.5 times light speed?

If that same observer, while travelling at .5 light speed were to shine a light forward, wouldn't a second observer in a stationary position directly ahead not see light approaching at 1.5 light speed?

Is the light emitted from a moving object not subject to the addition momentum of it's source?
Would light emitted from something already moving at light speed simply stand still or would it be projected at light speed relative to it's source?

Wouldn't this lead to an outside observer seeing light travelling at twice the speed of light?

Jeez, I hope this is clear enough to answer.

Mods...I meant to place this in the non-SETI science forum...you can move it if you want to.

Is the light emitted from a moving object not subject to the addition momentum of it's source?
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No is the answer!

If you were talking about throwing a ball forward from a moving object, like a car, then yes. Because you are talking about an object with mass.

But in the case of light, its speed is definitive. If you are traveling at the speed of light, and you shine a light forward, the light emitted would travel forward with you at the same speed.

John.

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It would take too much time and space to explain how Einstein arrived at the concept of light speed as a constant in all possible coordinate transformations. He passed from the Galileo transforms to the Lorentz transforms. I would advice you to read a good book, such as "The riddle of gravitation" by Peter G.Bergmann (a coworker of Einstein), or "Albert Einstein creator and rebel" by Banesh Hoffman and Helen Dukas (Einstein's secretary).
When you have read one of those try reading "Einstein - Subtle is the Lord" by Abraham Pais. Those are my favored books on Einstein, but there is plenty of them.
Tullio

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If you are traveling at the speed of light, and you shine a light forward, the light emitted would travel forward with you at the same speed.

Then the speed of light isn't the same for all observers, no matter what their relative speed.
As an observer travelling at the speed of light, the light I shine forward would appear to be standing still.

At the same time, were I able to shine a light backward, it would appear from my vantage point that the light moved away at light speed but to a stationary observer the light would appear to be standing still.

RRRRR
I'd better do the recommended reading before my head pops.

But there is a difference in speed. Why then is there the red shift and a blue shift? If light is coming at you it is shifted into the blue. and if it is going away it is shifted red. photons do have mass albeit small but still mass. that why a black hole will not let light escape.

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Old James

@James Sotherden:

I'm no physican (but have had 1 1/2 good german beers this evening) but i think you are talking about two different things: speed and frequency. Speed of light for example ~ 300.000km/s. Freqency (red shift/blue shift) 1/s. hope you understand what I mean, if not go buy and drink some good german beer and the all things will become clear ;-)

@James Sotherden:

I'm no physican (but have had 1 1/2 good german beers this evening) but i think you are talking about two different things: speed and frequency. Speed of light for example ~ 300.000km/s. Freqency (red shift/blue shift) 1/s. hope you understand what I mean, if not go buy and drink some good german beer and the all things will become clear ;-)

LOL your right i had the two confused. Please excuse me when i posted i had had only one beer. I read your reply after my second. I can see so much clearer now:)

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Old James

Einstein's relativity states that the speed of light is the same for all observers, no matter what their relative speeds.

This is the statement that throws me off so hard.

I can understand doppler effects and speed relative to the source, but I can't fathom how light speed can be said to be the same no matter the relative speed of all involved in observing the same beam.

Where's Spock when you need him?

Where's Spock when you need him?

Probably had his 5th beer this evening and is involved with a klingon waitress in some kind of interspecies mating ritual!

Sorry but that is all I'm able to contribute to this discussion at the moment. My second beer is not finished.

But if you like send me your email adress to: b-a-n-d-i-t@hotmail.de

and i will send you something back to read on this subject!

@James Sotherden:

I'm no physican (but have had 1 1/2 good german beers this evening) but i think you are talking about two different things: speed and frequency. Speed of light for example ~ 300.000km/s. Freqency (red shift/blue shift) 1/s. hope you understand what I mean, if not go buy and drink some good german beer and the all things will become clear ;-)

LOL your right i had the two confused. Please excuse me when i posted i had had only one beer. I read your reply after my second. I can see so much clearer now:)

Really, that was very funny....LOL


{Robert Waite:wrote}

Einstein's relativity states that the speed of light is the same for all observers, no matter what their relative speeds.

This is the statement that throws me off so hard.

I know why you are puzzled by the statement, its because you are reading it wrong! Let me clarify it for you!

What he means is that it does not matter what speed the observer is traveling, light will still be traveling at the same speed regardless! Thats what it really means!

Think about it like this - Light has 2 speeds, zero and 3.0x10^8 M/s. So light is either traveling at the speed of light or its stopped. If light is traveling, and you travel along beside it, light appears to stop. But really the light is just traveling along at the same speed as you.

A guy called Bill stands stationary on the planet earth and shines a light out into space. You then travel away from Bill in the other direction. Bill see's you traveling away from him at the speed of light, and Bill also sees the light traveling away in the other direction at the speed of light. Yes, the speed between you and the light going on the opposite direction is twice the speed of light, but that does not mean the light is going any faster. The light is still going at the speed of light.

Robert the statement by Einstein is complex and confusing because there are so many different theories of "What if this" and "What if that". Its not complex, it just means that light only travels at one speed - 3.0x10^8 M/s

John.

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Guinness cann't be so good - come on you are drinking whisky ;-)

Speed is distance/time.

While the speed of light is constant for all observers it is because perception of time changes with the velocity (or the strength of gravity) for the local observer.

The person travelling at 0.99999... of light has his perception of time dialated so much that his local clock and local means of measuring distance have changed so much that he too measures light as traveling at 300 000 k/s.

We, standing still, measure the speed of light the same as the guy traveling along at just shy of c because our local time is different.

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Pure mathematics is, in its way, the poetry of logical ideas.

Albert Einstein

I imagine it to be like two adjacent escalators both running at same speed to represent light, also, in the same direction so two people running up two up escalators would be a good way of illustrating how the two runners would appear to each other, for example if they were both doing 1.5 x lightspeed then both would lose the sense of motion in each others movements and appear to stand still to each other. That would make a great day out if we could find a nice big escalator! Don't fancy being the one in front looking over the shoulder and running up an escalator but if anyone finds out how to crack lightspeed then feel free to take me for a spin to the nearest pleasure planet and drop me off thanx.

B

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Prelude to Einstein's Theory II :The Findings of Henry Lorentz

1. The mechanical universe
   
2. The mechanical universe
   
3. The mechanical universe
   

Fundamentals of Physics--->Yale Courses (PHYS 200)

This lecture offers detailed analysis of the Lorentz transformations which relate the coordinates of an event in two frames in relative motion.
It is shown how length, time and simultaneity are relative.

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I THINK... and i say this because the most advanced training I have ever had was physics in high school.

But.. I THINK.. what that statement means is that the speed of light is constant and light is either on or off. Like a laser.

If you were to go faster than light and pass it and look back... you wouldn't see dim light as it's coming. You'd see nothing. If you stopped and once it reached you, it would "turn on" and you would see it.

In this example;

"If that same observer, while travelling at .5 light speed were to shine a light forward, wouldn't a second observer in a stationary position directly ahead not see light approaching at 1.5 light speed?"

No.. they would not see a light approaching as the light has not reached them yet. Again think On or Off. No streching or change of intensity (if the light is a constant source).. just on or off.

If you think of light as on or off and having a constant speed, you will understand how the speed of light will be the same for all observers.

If you move further and further from a light bulb... it will become dimmer and dimmer and eventually you won't be able to see it. This is not a factor of the light not reaching you or how fast it is traveling, it is a issue of intensity/power. If you could move 2x the speed of light and turn a powerful light source on and ran away from it and stopped, you would see nothing until it reached you.

You are confusing your eye's interpretation of the world with constants I think.

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The links provided by Dirk were the most clear explanations of relativity I've seen.
The visuals used really made the issue clearer to me than I hoped possible.
Even my old monkey brain seemed to grasp the concept finally. (this doesn't imply that I can now write a thesis on the subject, but I'm no longer stupified by the basics)

Thank you Dirk

I knew I would get great responses from this crowd.

The big Q' I have on the whole thing is exactly how the intrinsic speed of light was calculated. Exactly how was the value of this so called constant determined? Was it S.W.A.G. and if not what was used to measure it's speed?

Seems to me it would be a sin if the equation was accepted and the constant value tweaked UNTIL the equation WORKED.

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Classic WU= 7,237 Classic Hours= 42,079

There were so many attempts during the last centuries to measure the speed of light, that it is an interesting story on its own to learn the different values and methods used to determine the speed.

First records are made from Galileo itself, just confirming a delay in visualizing the light depending on the distance in between two objects.

In 1667 Ole Rømer confirmed a time delay in astronomical observations and calculated the first speed value. Which was near to our today´s known constant, however the limitation of measurement devices of course did not make it a 100% right.

So over the years you can probably add a lot of names and values to the list.

In the 20th century spectroscopic methods became so much advanced that light speed was measured and set as a constant of 299797458 m/s in 1983 by the CGPM (http://www.bipm.org/en/CGPM/db/17/1/).

However even the NASA installed a mirror on the moon and measured, using a laser beam, the travel time of the light sent from earth and reflected back by the moon. ;-)

But we still have a problem with light speed
What if space is not uniform in structure, if so then the speed will differ depending on the "density/permeability/local properties" of the space it travels through.

Which of course begs the question does space actually have a structure.. which leads us to the fact that it has a dielectric value so hence it does have "something" there that supports electric fields, as a total nothing would not.
The electric/magnetic fields that pervade the universe need a background "something" to be able to exist.

Light slows down in a denser medium.
We have a "bow shock" surrounding the local solar system... therefore space is not totally uniform ...everywhere... but probably close enough that it is a minor matter.

However it will impact deep space astronomy slightly, and also seti by the nature of the medium being examined.

But we still have a problem with light speed
What if space is not uniform in structure, if so then the speed will differ depending on the "density/permeability/local properties" of the space it travels through. ...

That's a very good point. Is our electromagnetic view of the universe distorted by the mass and the medium through which we are viewing?

All a sort of more subtle combination of distortions due to gravitational lensing, universe expansion and the material through which we are squinting?

Is there enough gravitation that we would be able to see ourselves if we had been there (or would be there) by the time the light path had looped around?

(Aside: Conditionals in time are a good source of a linguistic headache. Note the musings of Douglas Adams when faced with a similar conundrum!)

Keep searchin'!
Martin

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See:

Hitchhiker's Guide to the Galaxy, Restaurant at the End of the Universe (Chapter 15) - Douglas Adams

Time Travelling Grammar

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