Relative speed of light

[livefree75]

Hi, I have a couple of questions about light:

1. How do redshift and blueshift work? I always thought it was due to the Doppler effect, in which the speed of sound or some other wave changes relative to you, and you perceive a different frequency. The question is, how does this work if the speed of light is always 186,000 miles per second, no matter what speed you're traveling at?

2. If two light beams are approaching each other at 186,000 miles per second, wouldn't the distance be decreasing by 372,000 miles per second? That violates relativity...or does it?

Thanks!

EDIT: 186,000 miles per second =~ 299,338 km/s

Edited July 10, 2015 by livefree75
Changed 186000 mps to 299338 km/s

[andrewas]

1) Its not the relative speed of the wave that's important, its the apparent wavelength. Move towards a wave source and the wavelength you measure is reduced.

2) Nothing is moving faster than c in any reference frame. From your frame, the distance between the two waves is reducing at 2c, but that distance doesn't correspond to anything phyiscal, its just a measurement between two points. If we substitute the light beams for two relativistic spacecraft, then we can talk about the frame of reference of those spacecraft. Both spacecraft will measure you coming towards them at near-c and the other spacecraft approaching at a slightly higher velocity.

[lajoswinkler]

1) You don't have to move (relative to the source) at relativistic speeds to observe the Doppler shift of color.

2) The key is "relative". When you're expressing speed, it's always "relative to ____". There is special static place in existence, dignified with a nice cartoonish red and white pole used as the measuring mark. It's all about moving relative to something else. Always.

(Please, for the sake of science and logic, use metric system. Not horses, thumbs, yards and sticks. )

[YNM]

1. The same thing as sound wave - the compressed parts are crossed more faster when the object approach you, and slower when it's leaving. Forgot the derivation for light waves, which (should) account relativity.

2. The most important constant when wondering space, is that photons always move at a defined (yeah, I really mean defined) constant speed of approx. 300,000,000 m/s (true number goes 299,9xx,xxx.xx m/s or something). So if you ask "how fast is another photon moves wrt a photon" and the answer stays the same.

[Kaos]

The most important thing for (2) is that the flow of time is not constant. With such an high speed, time will flow very slow and an observer in this object will see that the other thing is also slower than c.

[Mr Shifty]

(Please, for the sake of science and logic, use metric system. Not horses, thumbs, yards and sticks. )

Indeed. I've always found it easier to remember that the speed of light is 5.7 x 10¹² acre^1/2/fortnight

[How2FoldSoup]

Indeed. I've always found it easier to remember that the speed of light is 5.7 x 10¹² acre^1/2/fortnight

I need to know if this is an actual conversion for.....reasons. I'm lazy and really don't want to double check. The units check out though which gives me confidence..

[YNM]

Indeed. I've always found it easier to remember that the speed of light is 5.7 x 10¹² acre^1/2/fortnight

I'm not sure the details but the orders seems to approach it. Still, doesn't prevent me to be even more lazy and just pull it out of a calculator.

[Hcube]

for (2) : as i understand it, yes the distance is decreasing at 2*c, but since it is not an object or a physical point it's perfectly possible. It's like a shadow : imagine having a very powerful light source that would light the whole moon from the earth. if you put your hand in front of it it would create a huge shadow on the Moon (assuming the sun is not there). now if you wave your hand very quickly in front of the light source, the shadow on the moon would travel" at several times the speed of your hand... i used the moon as an example but if you take some large object very far away the shadow will be moving on the surface at several times the speed of light... but a shadow is nothing so it's totally OK

[wasmic]

for (2) : as i understand it, yes the distance is decreasing at 2*c, but since it is not an object or a physical point it's perfectly possible. It's like a shadow : imagine having a very powerful light source that would light the whole moon from the earth. if you put your hand in front of it it would create a huge shadow on the Moon (assuming the sun is not there). now if you wave your hand very quickly in front of the light source, the shadow on the moon would travel" at several times the speed of your hand... i used the moon as an example but if you take some large object very far away the shadow will be moving on the surface at several times the speed of light... but a shadow is nothing so it's totally OK

This analogy is not entirely correct. From the observers point of view, the photons would be moving towards each other at a velocity of 2c, but from the photon's FoR (frame of reference), nothing happens at all since photons move at the speed of light and therefore do not experience time.

Let us instead use this example:

You are Observer O. From one direction, spacecraft A is approaching you at 0.75c. From the directly opposite direction, spacecraft B is approaching you at the same velocity. From the FoR of O, the spacecrafts approach each other at a speed of 1.5c. This is a correct measurement. However, from the FoR of spacecraft A, A will approach observer O at 0.75c and spacecraft B at ~0.96c. This conflicting measurement is also correct.

What is important to remember is that information cannot travel faster than light - and since the information in Hcube's example is moving from Earth (the projector) to the Moon, not across the Moon, no rules are broken.