So, I was sat on the loo and thought...

"JamesB" james wrote in message
...
one of those pointless hypothetical what-if questions...
So, assuming the rule of "you can't go faster than light" (E=MC2 and all
that) what would happen if we could attach a really long rod to the earth?
Presumably, if we made the rod long enough, the tip of it would be moving
through its local space at the speed of light? I havent done the maths on
it, but if you plug the speed of light into a calculation and throw in
some trigonometry and the angular rotation speed of the earth, you can
work out how long the rod would be.
And what would happen if you made the rod even longer so it was going
faster than light?
I'll leave now...
James

Is this analogous to the space elevator thing I read about a few years ago,
whereby a long cable is tethered between the Earth and an orbiting
satellite? Can't quite remember the details though.

Rob

On 28 Mar 2006 00:10:41 -0800, in uk.sci.astronomy ,
wrote:

You can make the end of a light beam sweep along a screen at any
arbitrary speed though - but nothing physical is actually moving faster
than the speed of light. Same with a very narrow angle pair of scissors
the crossing point can be made to advance at c (at least in
principle).

Er, no it can't. The classic mistake is to apply non-relativistic
equations of motion.

Mark McIntyre
--

On Tue, 28 Mar 2006 11:53:28 +0100, in uk.sci.astronomy , John Irwin
wrote:

...I'm still looking for a solution; do you have one for rotating
frames?

Any degree-level high energy physics book will cover this. Mine does,
though I'm too rusty to work through the sums.

Mark McIntyre
--

In article ,
Mark McIntyre wrote:
Same with a very narrow angle pair of scissors
the crossing point can be made to advance at c (at least in
principle).

Er, no it can't. The classic mistake is to apply non-relativistic
equations of motion.

Yes it can. The scissor blades don't even have to be moving very fast
if the angle is narrow enough. Relativity doesn't come into it.

You could shine a beam of light onto the moon, and if you moved it
across the disk in 1/100 second (which you could do by hand if you had
a powerful enough hand-held laser) the spot would move faster than
light.

-- Richard

wrote:
You can make the end of a light beam sweep along a screen at any
arbitrary speed though - but nothing physical is actually moving faster
than the speed of light. Same with a very narrow angle pair of scissors
the crossing point can be made to advance at c (at least in
principle).

Er, no it can't. The classic mistake is to apply non-relativistic
equations of motion.

Yes. It can. Nothing physical has to move faster than the speed of
light to acheive it. The scissors do have some technical difficulties,
but they can be avoided by using a French guillotine style setup with a
suitably shallow angle of cut. A 1m wide blade with a 1/100 radian
slope moving down at c/10 will do quite well enough to make the point.

Physical geometry requires that the crossing point advances 100x faster
than the closing speed of the blades. There is no conflcit with
relativity here. Nothing physical moves faster than light and the
mechanism cannot be used to send a signal FTL. See the Relativity
Physics FAQ for details:

http://jcbmac.chem.brown.edu/scissor...lScissors.html
(see the caveat at the bottom for details of how superluminal scissors
can be made to work)

This URL also deals in part with some practicalities relating to the
question of the OP.

Regards,
Martin Brown

On 28 Mar 2006 22:10:52 GMT, in uk.sci.astronomy ,
(Richard Tobin) wrote:

In article ,
Mark McIntyre wrote:
Same with a very narrow angle pair of scissors
the crossing point can be made to advance at c (at least in
principle).

Er, no it can't. The classic mistake is to apply non-relativistic
equations of motion.

Yes it can. The scissor blades don't even have to be moving very fast
if the angle is narrow enough. Relativity doesn't come into it.

This is one of the classic paradoxes of SR. You're applying newtonian
mechanics, which are inapplicable. I suggest you do a quck websearch
for it, and consider that the scissors /bend/.

You could shine a beam of light onto the moon, and if you moved it
across the disk in 1/100 second (which you could do by hand if you had
a powerful enough hand-held laser) the spot would move faster than
light.

Er, no. Again you're applying newtonian mechanics. The light beam
would in fact /bend/ in such a way that the end moved at, at most, c.

If this puzzles you, please do study some SR texts.
Mark McIntyre
--

On 29 Mar 2006 00:06:06 -0800, in uk.sci.astronomy , "Martin Brown"
wrote:

wrote:
You can make the end of a light beam sweep along a screen at any
arbitrary speed though - but nothing physical is actually moving faster
than the speed of light. Same with a very narrow angle pair of scissors
the crossing point can be made to advance at c (at least in
principle).

Er, no it can't. The classic mistake is to apply non-relativistic
equations of motion.

Yes. It can.

No it can't. The contact point moves at less than c at all times.
You're not reading the explanation properly

See the Relativity
Physics FAQ for details:

indeed.

http://jcbmac.chem.brown.edu/scissor...lScissors.html

And note this part:
"We have mistakenly assumed that the scissors do in fact close when
you close the handle. "
....
".The point at which the blades bend propagates down the blade at some
speed less than the speed of light."
Mark McIntyre
--

See the Relativity Physics FAQ for details:

indeed.

http://jcbmac.chem.brown.edu/scissor...lativity/relSc...

And note this part:
"We have mistakenly assumed that the scissors do in fact close when
you close the handle. "
....
".The point at which the blades bend propagates down the blade at some
speed less than the speed of light."

Clearly you did not read the caveat underneath that explained how to
make a proper superluminal pair of scissors where the blade crossing
point does move at a speed greater than c. As I pointed out the
technical issues with pivotted scissors can be neatly circumvented by
using a drop blade guillotine.

Regards,
Martin Brown

On 29 Mar 2006 03:02:30 -0800, in uk.sci.astronomy , "Martin Brown"
wrote:

See the Relativity Physics FAQ for details:

indeed.

http://jcbmac.chem.brown.edu/scissor...lativity/relSc...

And note this part:
"We have mistakenly assumed that the scissors do in fact close when
you close the handle. "
...
".The point at which the blades bend propagates down the blade at some
speed less than the speed of light."

Clearly you did not read the caveat underneath that explained how to
make a proper superluminal pair of scissors where the blade crossing
point does move at a speed greater than c.

Clearly, you didn't understand it (and I suspect that whoever wrote it
didn't fully, either, or at least wasn't able to explain to
themselves). The 'length' of such scissors would have to be
infinitesimal. By that stage, other problems will appear.

As I pointed out the
technical issues with pivotted scissors can be neatly circumvented by
using a drop blade guillotine.

Nope. You can't get round SR.
Mark McIntyre
--

In article ,
Mark McIntyre wrote:

Same with a very narrow angle pair of scissors
the crossing point can be made to advance at c (at least in
principle).

[...]

This is one of the classic paradoxes of SR. You're applying newtonian
mechanics, which are inapplicable.

The question of the crossing point is not a mechanics problem. No
physical object is moving at relativistic speed.

I suggest you do a quck websearch
for it

I did that. The first page I found is

http://math.ucr.edu/home/baez/physic.../scissors.html

It considers the case of some scissors with blades a light year long,
and indeed if you are relying on the distant end of the scissors to
start moving then it will be delayed - the end can't start moving
until the force has been propagated down the blades.

But as that article points out in the last paragraphs, this does not
apply to scissors where the blades are small enough and the motion
slow enough that the whole blade is moving before you start measuring.

This - not the giant scissors which I had not heard of before - is the
case that I and (presumably) the original poster were considering.

-- Richard