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

In article ,
Mark McIntyre wrote:

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.

The scissors can be of normal size with the blades moving at, say,
1 cm / second. If the angle is small enough (which of course it never
is with typical scissors, because of the way they are hinged) then
the crossing point can be made to move at arbitrarily high speeds.

Consider two blades, one moving up the y axis at 1 m/s, the other with
its edge at a very small angle to the x axis, say along the line
y = x / 10^10. Suppose at t=0 the first blade's edge is at y=0, so that
at time t its edge is along the line y = t.

Nothing is accelerating, no forces are acting, nothing physical is
moving at more than 1 m/s. No relativistic mechanics are involved.

The locus of the intersection of the blades is (10^10t, 0).

-- Richard

On 29 Mar 2006 13:47:49 GMT, in uk.sci.astronomy ,
(Richard Tobin) wrote:

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.

Consider how small such a blade would have to be.

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

Undoubtedly. Again, consider how small such a blade would have to be,
for the force to *instantaneously* be applied to the entire blade.

Mark McIntyre
--

On 29 Mar 2006 14:11:40 GMT, in uk.sci.astronomy ,
(Richard Tobin) wrote:

Consider two blades, one moving up the y axis at 1 m/s, the other with
its edge at a very small angle to the x axis, say along the line
y = x / 10^10. Suppose at t=0 the first blade's edge is at y=0, so that
at time t its edge is along the line y = t.

Nothing is accelerating, no forces are acting, nothing physical is
moving at more than 1 m/s. No relativistic mechanics are involved.

You've merely picked a bad frame of reference, and are applying
inadmissible maths to it. You can do the same parlour trick with
pretty much anything if you choose to ignore or sidestep the hard
maths. I've seen proofs that circles have smaller circumferences than
the inscribed square, that -1==1 and so forth, done similarly.

The locus of the intersection of the blades is (10^10t, 0).

In a bad frame of reference, and using inapplicable maths.
Mark McIntyre
--

In article ,
Mark McIntyre wrote:

Undoubtedly. Again, consider how small such a blade would have to be,
for the force to *instantaneously* be applied to the entire blade.

Why does the force have to be applied instantaneously? The edge just
has to have reached a uniform (low) velocity before the angle becomes
very small.

-- Richard

In article ,
Mark McIntyre wrote:

You've merely picked a bad frame of reference,

What's bad about it?

and are applying inadmissible maths to it.

What's inadmissible about it?

I've seen proofs that circles have smaller circumferences than
the inscribed square, that -1==1 and so forth, done similarly.

Yes, and I can see the mathematical errors in them. What's the
error in this case?

-- Richard

On 29 Mar 2006 14:43:04 GMT, in uk.sci.astronomy ,
(Richard Tobin) wrote:

Yes, and I can see the mathematical errors in them. What's the
error in this case?

If you have about four years, someone could explain it all I'm sure.
Otherwise, why not read up on relativity and see if you can work it
out.
Mark McIntyre
--

In article ,
Mark McIntyre wrote:

Yes, and I can see the mathematical errors in them. What's the
error in this case?

If you have about four years, someone could explain it all I'm sure.
Otherwise, why not read up on relativity and see if you can work it
out.

You know it doesn't work like that. No-one (well hardly anyone) is
going to devote a lot of time to learning a subject just to refute
someone on Usenet. Especially in this case, when the more readily
available sources (such as the page I found at your suggestion on
Google) seem to match what I understand already. So I'll just go on
believing that I'm right and you're confused.

-- Richard

On 29 Mar 2006 20:37:45 GMT, in uk.sci.astronomy ,
(Richard Tobin) wrote:

In article ,
Mark McIntyre wrote:

If you have about four years, someone could explain it all I'm sure.
Otherwise, why not read up on relativity and see if you can work it
out.

No-one (well hardly anyone) is
going to devote a lot of time to learning a subject just to refute
someone on Usenet.

So in other words, you concede this is too hard to understand
trivially.

Especially in this case, when the more readily
available sources (such as the page I found at your suggestion on
Google) seem to match what I understand already.

There are also pages about alien abductions, JFK assassination
theories, the moon landings and the face on mars. I'm guessing that
you discard any of those that don't agree with your preconceptions
too.

So I'll just go on believing that I'm right and you're confused.

IOW, you can't prove I'm wrong, it just "feels wrong", so therefore I
must be.

For what its worth, I suspect that I can find the flaw using a
combination of SR, quantum mechanics and plain old fashioned maths.
One thing you might want to consider is that no matter whether or not
the junction moved at c, it would be impossible to observe any
movement faster than c. Another to consider is how long the finite
rod would take to cross the axis.



-- Richard
Mark McIntyre
--

In article ,
Mark McIntyre wrote:
[...]

Well, let's have one more try, this time with the spot-of-light
version. Consider a light source and a screen 2 light seconds away (a
bit further than the distance from the earth to the moon), the screen
being 2 light seconds wide. The screen is curved so all points on it
ar two light seconds away. The source and screen are at rest relative
to each other and all measurements are performed in that frame. Where
does the following sequence go wrong? You may not have time to teach
me relativity, but you can surely tell me which is the first false or
impossible statement:

Experiment 1:

1 I turn on the light pointing at one end of the screen at time t=0.
At time t=2, the light reaches the screen.

2 At time t=4, I will see the reflected light illuminating a spot
at that end of the screen.

Experiment 2:

1 I turn on the light pointing at the other end of the screen at time t=0.
At time t=2, the light reaches that end of the screen.

2 At time t=4, I will see the reflected light illuminating a spot
at that end of the screen.

Experiment 3:

1 I point the light at one end, and turn it on at time t=0.
At time t=2, the light reaches that end of the screen.

2 At time t=0 I start turning the light to point at the
other end, finishing the turn at t=1. This is just a small
movement of my light source at low speed.
Light starts travelling to the far end at time t=1.

3 At time t=3, the light transmitted at t=1 reaches the other
end of the screen (the speed of light is not affected by
the motion of the source).

4 At times between t=2 and t=3, the light will have arrived at
intermediate positions on the screen.

5 The illuminated spot on the screen will have moved 2 light seconds
in one second.

6 At time t=4 I will see the illuminated spot at the first end.

7 At time t=5 I will see the illuminated spot at the other end.

8 Between times t=4 and t=5 I will see the spot moving across a
screen 2 light seconds wide in 1 second.

-- Richard

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.

They have expressed the standard SR description. Check any reasonable
physics text that deals with the superluminal scissors paradox. The
words in the FAQ are the result of stepwise refinement by several
authors from an original draft. Everyone is out of step but you... I
think that it could be clearer (it spends too much time describing
configurations that would not work and not enough time explaining the
one that does and why there is no conflict with SR). Also I reckon the
French guillotine with a gently sloping blade is a much simpler
geometry to analyse.

Mark it is you who does not understand special relativity. There is no
conflict at all with SR in the superluminal scissors "paradox". Nothing
physical is moving faster than the speed of light. You might be able to
grasp this if you imagine what observers on a pair of exactly parallel
blades would observe when they crossed.

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.

There is no need to get around SR. It is your misunderstanding of SR
that is the problem here.

Regards,
Martin Brown