This is art.

[32BT]

Oscar I'm sorry, but do you know what any of these words mean?

Of course not, why do you think this keeps me from sound sleep?

[Rob C]

You should hear him on quantum mechanics!

;-)

No, Keith, for that one needs the combined efforts and collected reminiscences of David Bailey and the late, great Brian Duffy. Oh! What a Lovely War was where the mechanics really came together, with his, Bailey's, initial investment being sold off to another punter, making Bailey the only one to come out of it unscathed. I believe Duffy clung on, perhaps just to prove his mucked mistaken, but that takes more detective work than I could finance at short notice. One of the Attenboroughs was involved, not the one famed for life in the wild. Which might seem a bit ambiguous, now that I think about it. Perhaps both habitats were/are a bit wild...

For deeper clarification, dig this:

https://www.youtube.com/watch?v=dMWHi-SPEf0

:-)

Rob

[GrahamBy]

Ok, I'll attempt my two paragraph explanation of special relativity.

Suppose you have a stick, 1 metre long. You lay it on the ground pointing straight away from you. So you could also describe it as stick that goes from here, 1m forward. However if turn a little bit, it nows goes mostly forward, but a bit to the side. Our brains see this as quite ordinary, and if we wanted to do it with maths we'd say that in one reference frame the stick went to the point (1,0) and in the other to (cos(theta), sin(theta)). What is important is that 1²+0²=cos²(theta)+sin²(theta)=1... the length of the stick doesn't change, I'm just looking at it from a different angle.

The idea of space time is that different speeds are just moving through space-time at a different angle: if you position (x) doesn't change as t increases, we say you are stationary... tilt things over a bit and x increases in proportion to t. That means you are moving. In fact it turns out that the invariant is not x²+y² as it is in the spatial directions, but x²-t² (or x²+y²+z²-t² in full). If you observe something from different reference frames, you rotate a bit between t and x, but x²-t² stays constant. The corresponding rotation functions are cosh(v/c) and sinh(v/c) instead of cos(theta) and sin(theta), and the time dilation and length contraction effects come from applying them like a rotation.

And that's it.

[Rob C]

Shit, Graham, if you'd just stated that at the begining, there'd be no confusion: everyone would have understood right away.

Rob

[32BT]

Perhaps i should restate my confusion with red/blue shift.

I'm on earth looking into the night sky with a very expensive telescope and a fancy spectroscope. For simplicity's sake we will assume the earth fixed in space and a galaxy i'm observing is moving with considerable speed straight towards me.

I detect blue shift.

Somebody tells me, yes, that's a doppler effect, no different than the higher pitch of sound from a sound source that is travelling towards you.

Except, i realise, that can't be the case, because:
A: there is no "carrier" for light in a vacuum as there is for sound pressure waves in air.
B: no matter how fast the galaxy is travelling towards me, the light will still reach me with the  speed of light which we assume as fixed.

Because the speed of the photons reaching me is fixed, the speed of throughs and peaks in the wave, if that would be applicable, don't change for me as observer, unless time is somehow compressed in the photon itself.

So, the question remains: what causes the shift?

Does this make my confusion any clearer? 

[Eric Myrvaagnes]

Since I'm color-deficient, does that mean that blue-shift and red-shift don't apply to me?   

[Rob C]

Since I'm color-deficient, does that mean that blue-shift and red-shift don't apply to me?   

Thinking this question through, Eric, I'm inclined to conclude that just as long as your green credentials are strong enough, you can safely disregard the reds and the blues.

Just avoid fossil fuels and all will be well with the continuum.

[Telecaster]

Oscar, in your example you'll see lightwaves emitted by an object coming toward you at high speed as shorter (blueshifted) compared to what you'd see if the object were moving slower or not at all. It's a relative rather than absolute thing.

-Dave-

[32BT]

Oscar, in your example you'll see lightwaves emitted by an object coming toward you at high speed as shorter (blueshifted) compared to what you'd see if the object were moving slower or not at all. It's a relative rather than absolute thing.

-Dave-

I know, i already mentioned that i would detect a blue shift, but the question is: what's causing the shift? The speed of the photons (relative to me) doesn't change, regardless of how fast the source is moving (relative to me) when the photons are emitted.

[Telecaster]

Because the "speed of light" is an upper limit on velocity, you can't add the speed of the source emitting light (the galaxy in your example) to the speed of the light itself and get a faster speed. Instead what happens is that the light coming from the approaching galaxy appears compressed in wavelength (that is, blueshifted) compared to if the galaxy were standing still relative to us. Spectrographic studies of galaxies show this effect. In some cases the rotation of a galaxy around its own center will result in blueshifted light from the part rotating toward us and redshifted light from the part rotating away from us.

-Dave-

[32BT]

Because the "speed of light" is an upper limit on velocity, you can't add the speed of the source emitting light (the galaxy in your example) to the speed of the light itself and get a faster speed. Instead what happens is that the light coming from the approaching galaxy appears compressed in wavelength (that is, blueshifted) compared to if the galaxy were standing still relative to us. Spectrographic studies of galaxies show this effect. In some cases the rotation of a galaxy around its own center will result in blueshifted light from the part rotating toward us and redshifted light from the part rotating away from us.

-Dave-

Yep, and the fact that we can see the galaxy side-to-side, regardless of which direction it rotates, tells us something else: the photons emitted are always emitted at the speed of light. Their speed towards us is not influenced by the speedvector of the source. Even if the source is moving away from us, you don't have to subtract that speed. The photon still travels with maximum lightspeed towards us after emission. Which perhaps isn't as strange as it seems since they don't have mass and therefore the speed of the source isn't transferred to the photon by impuls or other conventional earth physics.

So, the questions becomes:
1. how is the photon created
2. what causes the photon to be compressed/expanded?

The problem of course is, this being relativistic, that neither the source, nor the observer is moving as far as the photon is concerned. 

Something happens during the exact moment of creation that influences the (perception of) the photon.

Some possibilities:
If the duration of creation of a photon is always fixed, then the movement of the galaxy relative to me changes that duration in my perception. So that could cause a shift. Though this doesn't quite explain why there is such a thing as gravitational shift.

If the creation of a photon is influenced by the size of an electron jump (classical model) then perhaps the electrons are moving in a non-circular cloud around their atom core. This would happen in a gravitation field, or when the atom has been under acceleration.

Whatever happens during creation, the same thing should happen in reverse during the detection of the photon. If both me and the galaxy are moving at considerable speed in the same direction, I would not see any shifts. In fact, I would be unable to tell we are both moving.

[GrahamBy]

I know, i already mentioned that i would detect a blue shift, but the question is: what's causing the shift? The speed of the photons (relative to me) doesn't change, regardless of how fast the source is moving (relative to me) when the photons are emitted.

What do you mean by "cause"? You are looking at them differently, so they look different. That's all. Same as looking at a stick from a different angle, that's how it is.

Another way of saying it is: "because it's a necessary consequence of a universe where everyone sees the speed of light as the same"

What you might find you mean by "cause" is: "please explain this to me in a way that fits with my intuition", ie in a way that doesn't require you to change how you think things work. Can't do that... your intuition is wrong, it needs to be re-trained. But it's a MUCH less violent mental re-configuration than quantum mechanics ("If you think you understand it, then you don't understand it" - Paul Dirac)