The speed of light....

[b0redom]

So as I understand it, the speed of light is the limiting factor in the Universe. Speed is measured relative to something though as it's a vector.

Relative to us light is travelling towards us from the sun at the speed of light, but on the other side of the sun presumably light is travelling away at the speed of light. That means that relative to the light coming towards us, light on the other side of the sun is receding at 2 times the speed of light?

Am I missing something?

[Saracen]

Yes, you're missing something ... the headache that results when trying to grasp how somewhat unintuitive theoretical principles apply to the real world, on a scale that we have very great trouble observing empirically so have to infer.

Your problem lies with something physicists have struggled with for hundreds of years, and which are sort-of explained by Einstein's special theory of relativity, and about exactly what the fixed frame of reference is, and about space-contraction and time-dilation at or near the speed of light.

For instance, to us, space is fixed and time runs at a constant rate, but grasping relativity (to me, anyway) is a bit like trying to explain how shades of green differ to someone that has been blind since birth and for whom colour itself is an abstract concept. That is, it'll give you a headache and annoy the blind person.

So, as I already have a headache, I'm bowing out of this one, and I recommend a healthy supply of aspirin, or better yet Tramadol, to anyone who goes for it.

[wazzickle]

Relative is the key word. The point that's difficult to grasp, because it runs counter to our experience, is that speed is not additive when you get to the bigger numbers. Think diminishing returns. In an absolute world, the sum of two speeds in one frame of reference would be the same as their relative speeds, but because speed behaves relatively, and it's limited, it doesn't.

As to why.... well, that's even less easy to understand or explain, but I'd say that it's to do with the properties of the medium through which light and matter (i.e. energy) changing when you speed up.

[BobF64]

I always went on the principle that "everything in theoretical physics exists to make the maths work", and just left it at that.

[flearider]

Relative to us light is travelling towards us from the sun at the speed of light ....yes
but on the other side of the sun presumably light is travelling away at the speed of light...yes
That means that relative to the light coming towards us, light on the other side of the sun is receding at 2 times the speed of light... no

it's still only going the same speed .. unless it hit's a blackhole or another massive gravitational force then it could speed up or slow down.

[aramil]

SpaceTime is complex, and sometimes very very odd, but this is not to complex.

Ie two cars travelling in opposite directions @ 30Mph = The gap between them grows at 60 Mph, both cars are still doing 30Mph - Relative to a static observer who can see both cars, in normal spacetime.

[Saracen]

SpaceTime is complex, and sometimes very very odd, but this is not to complex.

Ie two cars travelling in opposite directions @ 30Mph = The gap between them grows at 60 Mph, both cars are still doing 30Mph - Relative to a static observer who can see both cars, in normal spacetime.

But the whole point of special relativity is that simple vector maths that apply elsewhere don't apply at or near light speed, so such analogies are meaningless in that very special stuation.

[aramil]

But according to the maths, two items travelling in opposing directions @/near to "C" without a fixed point of reference will not interact "in normal space" with each other, and so both are independantly travelling @ or around "C". So from their own perspective, and the "effect of spacetime distrotion" both would not see each other (as the red shift would mean the information from one would never reach the other). So you would need a 3rd party fixed location to measure them.

[Lanky123]

This plot taken from stackexchange shows the point at which you need to start taking account of relativistic effects in velocity addition (and shows the equation to use in blue). The x axis shows the individual velocities (as a fraction of c) and the y axis the relative (added together) velocities with classical and relativistic calculations. So with the correct calculations the relative velocity still only tends towards c as an absolute upper limit.

As a point of reference the fastest man-made object is the Juno spacecraft on approach to Jupiter at 0.00023 c.

[ik9000]

my old A-level text book gives this equation: v=[u1+u2]/[1+(u1.u2)/c^2]

which when applied to a similar argument as the OP's with speeds u1=2c/3, u2=2c/3 in the opposite direction gives relative speed v=12c/13. Even with equal u1=u2=c the relative speed v is still only c

It contrasts this with Newtonian mechanics which would give v=u1+u2 and be wholly incorrect. The topics to read up on are special relativity and general relativity. I've spent this afternoon re-reading my A-levels. Interesting to see how much I'd forgotten.

Note that for slow speeds the denominator converges to 1 - so Newtonian mechanics is correct for the majority of cases we deal with in our day to day lives. It just doesn't scale well to space, small particles, very fast speeds and the like.

Edit - if you are interested in a bit more reading, not too high-brow then I recommend Muncaster's Relativity and Quantum Physics A-level book if you can find a copy. I always found his books approachable.

[wazzickle]

Carlo Rovelli has just released a book called '7 brief lessons on physics' which I found to be very interesting. The blurb describes it as aimed at those with a low level of science knowledge but I consider myself reasonably scientifically educated and the current state of theories still blew my mind and I found quite difficult to get to grips with. Makes a good late christmas present.