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bongopukerat
2008-Nov-03, 02:51 AM
I liked listening to astronomy cast and the impossible sounding contraptions they explain to get a point across. Like the light year long scissors being closed at light speed; the cross section moves faster than light speed but doesn't contain any information.

Anyways onto my question...
If there were a treadmill with the belt spinning at 50mph, a ball set on the treadmill will be flung off at 50mph. If you were to stack a second treadmill on top of the first one, it too would be flung off at 50mph. However, anything set on top of the second treadmill would be flung off at 30mph plus the 50mph that the second treadmill is already traveling at.
Remember the belts on the 2 treadmills do NOT come into contact with each other, which is why they can both be spinning in the same direction and have their speeds stack.
Now if you super-size this contraption with one treadmill running at light speed and the other also running at light speed, what happens to the ball?

Remember that the first treadmill is more than long enough to propel the second treadmill to light speed. If the speeds are slower (30mph, 50mph, etc), then the speeds get added together for the ball; relative to the stationary positions of say the pivot points of the first treadmill.

I know nothing can go faster than light, but where exactly would it break down?

2008-Nov-03, 04:49 AM
why would the top treadmill- which is also going 50mph- only fling the ball at 30 mph?
and how is your treadmill contraption any different than a mechanical tennis ball thrower that uses two opposing wheels to fling tennis balls? i don't think the tennis balls get thrown any faster than either wheel is spinning, even tho their forces combine to throw the ball.

astromark
2008-Nov-03, 05:07 AM
' novaderrik ' ... You are not seeing this right. The tread mill velocity is added because one tread mill has been thrown by the first. So the velocity is staked.

No., 'bongopukerat' ... Turning this into a question regarding light speed does not do the same thing. Light has a measured velocity in a vacuum. It is a constant. It can not be added to. doubled. The rules of this universe do not change. C. is it.

Jeff Root
2008-Nov-03, 05:37 AM
I liked listening to astronomy cast and the impossible sounding
contraptions they explain to get a point across. Like the light year
long scissors being closed at light speed; the cross section moves
faster than light speed but doesn't contain any information.
Did they say that??? It isn't right. The point of intersection would
indeed convey information, but it would only move at the speed of

Real treadmills, levers, scissors, multistage rockets, whatever... are
all made of real materials. They have masses and finite strengths.

Imaginary treadmills or whatever that are massless and have infinite
strength have imaginary properties. They can be fed endless amounts
of energy at unlimited power rates without being torn apart. When
operated in imagination, their behavior doesn't tell you anything about
the behavior of real objects.

-- Jeff, in Minneapolis

BigDon
2008-Nov-03, 07:12 AM
I think Bongo is asking a legit question. But I'll phrase it differently for him

If I had a an engine rated for .6 C and another rated for .5 C, if mounted on the same vehicle why can't I go 1.1C?

I'll leave others to answer this.

You answerers shouldn't have been obsessing on the analogy used but on what he is asking.

astromark
2008-Nov-03, 08:01 AM
And the answer to that question is... False. A mass-less particle known as a photon is known to travel at C. Not faster. Not slower. While any object that has mass can not ever reach that velocity. There is not sufficient energy available in the universe to do that.
Your engine might if it were extremely efficient and very very powerful after a awful lot of accelerating attain a velocity of 9.999999 of C. Never C., and most importantly never over C.

grant hutchison
2008-Nov-03, 12:16 PM
Several points apply for the OP:

1) It's best not to frame these puzzles using material objects moving at the speed of light. Material objects can't do that, and when you apply the necessary maths, you get a mess of zeroes and infinities that aren't very explanatory. The puzzle is just as puzzling if the two objects are moving at 0.6 of light speed.
2) People make a fuss when you use material objects in these puzzles. Sometimes the fuss is justified, as in Jeff's objection to the closing scissors above: if the crossing point moves faster than light, then you have infinite rigidity, which is impossible under relativity, and which can let you set up all sorts of relativity-violating scenarios if you sneak it in. (You can fix the scissors scenario simply by separating the scissor blades, setting them at a slight angle to each other, and firing them towards each other at close to light speed. Their point of intersection will indeed move faster than light, and will indeed carry no information.)

In general, you can set up any situation you like, with any addition of velocities you like, and the sum will always be less than lightspeed for any observer, in any state of motion.

Grant Hutchison

grant hutchison
2008-Nov-03, 12:35 PM
If I had a an engine rated for .6 C and another rated for .5 C, if mounted on the same vehicle why can't I go 1.1C?

I'll leave others to answer this.Don, I think your version is much more complicated than the OP!
Are the engines rated to these speeds because they're working against resistance (friction of the interstellar medium, for instance)? Then increased friction determines that the second engine won't add its rated performance to the first.
Or are they fuel-limited to that velocity change, at which point they cut out? Then we need to worry about the mass of the fuel, because each engine's performance will be degraded by the presence of the other engine's fuel.
If they're magic fuelless velocity-addition engines, then the scenarios begin to get unreasonable, because we have to fret about how the magic works in detail.

Grant Hutchison

bongopukerat
2008-Nov-04, 08:49 PM
Grant: I like that you pointed out the puzzle being just as puzzling even if both objects were traveling at .6 the speed of light. That was going to be my next question in response to Astromark saying no object can travel quite at the speed of light; what if both tread mills were traveling at just over half the speed of light, thereby causing my head to hurt.
But I guess it comes down to the fact that the speed is all relative on where you are standing.

One other question though. If you had stacked treadmills the same length as one normal treadmill side by side and all were going at light speed or near light speed, which of the two balls dropped at the beginning reach the end first? They should reach the end at the same time right?

I may not be remember the scissor thing exactly right. Maybe they were using it as an analogy to the two crossbeams. In the end though, no information will carry.

grant hutchison
2008-Nov-04, 08:57 PM
One other question though. If you had stacked treadmills the same length as one normal treadmill side by side and all were going at light speed or near light speed, which of the two balls dropped at the beginning reach the end first? They should reach the end at the same time right?Sorry, I don't understand the scenario you're describing.

Grant Hutchison

bongopukerat
2008-Nov-04, 09:12 PM
Basically two treadmills side by side or parallel. They're both the same length and let's say start at Star A and end at Star B.
Both are going at light speed so if two balls were placed on each treadmill they would reach Star B at the same time.
Now stack another treadmill on one of the two parallel treadmills. Basically it's my original scenario, except I'm trying to compare the time it takes to transport the ball from Star A to Star B versus just ONE treadmill going at light speed.

Two treadmills traveling at light speed, stacked on each other, shouldn't get a ball faster to "Star B" versus one treadmill traveling at light speed, right?

grant hutchison
2008-Nov-04, 09:16 PM
Basically two treadmills side by side or parallel. They're both the same length and let's say start at Star A and end at Star B.
Both are going at light speed so if two balls were placed on each treadmill they would reach Star B at the same time.
Now stack another treadmill on one of the two parallel treadmills. Basically it's my original scenario, except I'm trying to compare the time it takes to transport the ball from Star A to Star B versus just ONE treadmill going at light speed.

Two treadmills traveling at light speed, stacked on each other, shouldn't get a ball faster to "Star B" versus one treadmill traveling at light speed, right?OK, so you're doing that thing it's better not to do: setting up a thought experiment with stuff moving at lightspeed.
Under those circumstances, for an observer stationary relative to the stars, the second treadmill would add exactly zero velocity to the first treadmill, no matter how fast (or slowly) it ran.

Grant Hutchison

bongopukerat
2008-Nov-04, 09:27 PM
OK, so you're doing that thing it's better not to do: setting up a thought experiment with stuff moving at lightspeed.
Under those circumstances, for an observer stationary relative to the stars, the second treadmill would add exactly zero velocity to the first treadmill, no matter how fast (or slowly) it ran.

Grant Hutchison

Awesome i think you answered my question. I knew the balls couldn't go faster than light but was confused one exactly where things started to break down to prevent the ball from doubling its speed. And I guess it's time itself will pretty much compensate since space/time are interlocked.

Jeff Root
2008-Nov-05, 03:34 AM
I may not be remember the scissor thing exactly right. Maybe
they were using it as an analogy to the two crossbeams. In the
end though, no information will carry.
Most likely they discussed both. The point of intersection in the
scissor carries information but is limited to the speed of sound in
the scissor material, while the point of intersection of two angled
beams moving past each other can move at any speed but carries
no new information.

-- Jeff, in Minneapolis

Jeff Root
2008-Nov-05, 03:59 AM
Awesome i think you answered my question. I knew the balls couldn't
go faster than light but was confused on exactly where things started
to break down to prevent the ball from doubling its speed.
"Things" start to "break down" as soon as there is any speed at all.
However, at low speeds, the "breakdown" is so miniscule as to be
completely undetectible and ignorable. At the speed of satellites
orbiting the Earth, the difference between Newtonian speed and
relativistic speed is still very tiny, but can be important in precision
measurements. It is only when the speed is a large fraction of the
speed of light that the differences start to become large.

-- Jeff, in Minneapolis

hhEb09'1
2008-Nov-05, 02:29 PM
Did they say that??? It isn't right. The point of intersection would
indeed convey information, but it would only move at the speed of
sound in the material the scissor blade was made of.No, that is not true.

With a pair of blades whose intersection is R from the end of one blade, and the pivot is a distance r from that end, and θ is the angle between the blades, I get the following relationship:

dR = (r + R2/r) dθ

So, given a constant speed of dθ (which we can assume has been established in some manner), the rate of change of R (the intersection point "speed") can be made to be as large as we want just by making r small, no matter how fast the blades are rotating.

If r is zero, then the speed of the intersection point is infinite. That would be the closing of an (ideal) book, where the pivot is the binding, and the intersection is at the binding, and the intersection of the pages then goes from the binding to the edge instantaneously.

No need for lightyear long scissors :)

2) People make a fuss when you use material objects in these puzzles. Sometimes the fuss is justified, as in Jeff's objection to the closing scissors above: if the crossing point moves faster than light, then you have infinite rigidity, which is impossible under relativity, and which can let you set up all sorts of relativity-violating scenarios if you sneak it in. (You can fix the scissors scenario simply by separating the scissor blades, setting them at a slight angle to each other, and firing them towards each other at close to light speed. Their point of intersection will indeed move faster than light, and will indeed carry no information.)To tell you the truth, I don't really understand the distinction you are making there, between the two examples.

PS: For two blades at a constant angle θ, where one is moving at a speed S perpendicular to the other, the "speed" of the intersection is S times the cotangent of θ. The "speed" of the intersection can be made as great as we want, just by making the angle small.

Jeff Root
2008-Nov-05, 03:34 PM
Establishing the speed dθ is what proceeds at the speed of sound along
the length of the scissor blade. Even if the blade is only an inch long, the
far end of the blade will not begin to move until a time after the near end
has started to move, equal to the time for sound to travel that distance.

The clock starts ticking when you start to close the scissors. If you
assume that the entire blade is already in motion, then you are starting
with time already on the clock. That's essentially what happens with
the two beams moving linearly past each other.

-- Jeff, in Minneapolis

hhEb09'1
2008-Nov-05, 04:19 PM
The clock starts ticking when you start to close the scissors. If you
assume that the entire blade is already in motion, then you are starting
with time already on the clock. That's essentially what happens with
the two beams moving linearly past each other.I'm not sure it matters. With the very long scissors of the OP, the ratio of blade length to pivot size is very large. The instantaneous velocity of the intersection will be very large at the point in time that the ends of the scissors finally begin to close (no matter how long after the start).

grant hutchison
2008-Nov-05, 05:28 PM
To tell you the truth, I don't really understand the distinction you are making there, between the two examples.The second elimates all this discussion you and Jeff are having about the speed of sound. I think it's worth eliminating. :)

Grant Hutchison

hhEb09'1
2008-Nov-05, 05:35 PM
The second elimates all this discussion you and Jeff are having about the speed of sound. I think it's worth eliminating. I meant, as near as I can tell, they're the same example.

As I described above, the speed of sound doesn't affect it.

grant hutchison
2008-Nov-05, 06:05 PM
With the very long scissors of the OP, the ratio of blade length to pivot size is very large. The instantaneous velocity of the intersection will be very large at the point in time that the ends of the scissors finally begin to close (no matter how long after the start).I don't think this is so.
Imagine the unobtainable limit to the scissor scenario, in which the blades are perfectly parallel, separated by an infinitesimal gap. The gap is closed by moving the two blades together at one end. This disturbance (the sideways movement of each blade) propagates down the blades at the speed of sound, and so the "point of intersection" propagates down the blades at the speed of sound, too.

Edit: This counts as a response to your post immediately above, too.

Grant Hutchison

hhEb09'1
2008-Nov-05, 06:16 PM
I don't think this is so.
Imagine the unobtainable limit to the scissor scenario, in which the blades are perfectly parallel, separated by an infinitesimal gap. The gap is closed by moving the two blades together at one end. This disturbance (the sideways movement of each blade) propagates down the blades at the speed of sound, and so the "point of intersection" propagates down the blades at the speed of sound, too.Take it a slight bit farther out. If you close the blade slowly enough, the disturbance will have time to propagate to the end by the time the blade closes all along its length.

Edit: This counts as a response to your post immediately above, too.
What about the ones before that? :)

grant hutchison
2008-Nov-05, 06:18 PM
(Text originally edited into my previous post, but moved to make a separate response, since it really was too late for a second edit above.)

Some discussion here (http://www.math.ucr.edu/home/baez/physics/Relativity/SR/scissors.html) which makes the same points Jeff and I are making, and also describes a scenario in which the intersection point can go superluminal. This essentially converts the scissors into the scenario I described: the closure signal has time to travel the length of the scissor blades and they "go inertial" before the blades intersect. Its seems you're imagining the latter scenario, and Jeff and I are describing the former. Hence, I think, the usefulness of setting the scenario up with inertial blades from the outset, to avoid all this discussion. Which we've just had.

Grant Hutchison

astromark
2008-Nov-05, 06:25 PM
It really is hard to imagine how such a protracted discussion can be had about the action of a pair of scissarse... :) is there a point to this, cut it out... Oh please, now you have me doing it... where on this key bd is the logic key?... oh, I can not see it. Mark.

AndreasJ
2008-Nov-05, 06:28 PM
The speed of sound issue arises if the scissors are closed the usual way, by applying a torque on the pivot. One could imagine that one blade is made to move by the timed firing of small rockets or whatever, making it start moving simultaneously all along its length.

Edit: multiple xpost :o

John Mendenhall
2008-Nov-05, 06:31 PM
What about the ones before that? :)

Refer to Grant's 1 and 2 in post #7. Avoid physically impossible 'thought experiments' as if they were the plague. It takes endless discussion and usually someone at Grant's level or above (and there aren't many of them here) to find the error.

Regards, John M.

cjameshuff
2008-Nov-05, 06:44 PM
I don't think this is so.
Imagine the unobtainable limit to the scissor scenario, in which the blades are perfectly parallel, separated by an infinitesimal gap. The gap is closed by moving the two blades together at one end. This disturbance (the sideways movement of each blade) propagates down the blades at the speed of sound, and so the "point of intersection" propagates down the blades at the speed of sound, too.

Changes in the closing/opening state, the acceleration of the blades, propagate at the speed of sound in the blade materials. Once the blades are moving, the "motion" of the cutting point where the blades start to overlap easily exceeds that speed, but attempts to stop it would be delayed by the speed of sound.

I don't think your idealization is very useful at all. You start with the scissors closed, and with the slightest motion opening them...in which case, yes, the intersection point will propagate down the blades at the speed of sound. If they are separated by a finite amount, however, the blades will bow and cross at some point away from the pivot point, forming two intersection points that travel in both directions, initially much faster than the speed of sound.

grant hutchison
2008-Nov-05, 07:18 PM
I don't think your idealization is very useful at all ...I don't think it's very useful either. :)
My only point in all this furore is that the "superluminal scissors" scenario always creates all this discussion, which is best avoided if all you're trying to do is say something simple about superluminal motion. It's ironic that I've actually perpetuated the discussion by saying:
The second elimates all this discussion you and Jeff are having about the speed of sound. I think it's worth eliminating. :)and I can only apologize to the outraged astromark.

If we want an example in which a lack of rigidity prevents superluminal signalling, then we can wave a single long pole.
If we want an example in which superluminal motion appears but transmits no information, then we can set up a pair of slightly inclined rods moving past each other inertially.

The scissors are, in comparison, a complete mess, producing aspects of both the simpler scenarios.

Grant Hutchison

hhEb09'1
2008-Nov-05, 07:25 PM
Refer to Grant's 1 and 2 in post #7. Avoid physically impossible 'thought experiments' as if they were the plague. It takes endless discussion and usually someone at Grant's level or above (and there aren't many of them here) to find the error.Grant's 1 referred to the treadmills, not the scissors, and I referred to his 2 in post #7 in my first post. :)

The whole discussion started with the OP's reference to the astronomy cast thought experiment, so that's kinda out of my hands. I just disagreed with Jeff's response, which said that it wouldn't work (besides the you know finding a long pair of scissors :) but as I've shown, you do not need a long pair of scissors, just a pair where the ratio of length to pivot is large )

mugaliens
2008-Nov-05, 08:10 PM
Establishing the speed dθ is what proceeds at the speed of sound along
the length of the scissor blade. Even if the blade is only an inch long, the
far end of the blade will not begin to move until a time after the near end
has started to move, equal to the time for sound to travel that distance.

Let's try a new thought experiment that will, hopefully, rectify some of these cause-effect problems...

Two circular wires of nearly identical radii are attached to the same axis. One is stationary, the other is rotating about the axis at a constant rotational velocity.

Now you have your intersection, which begins a v=0 at the axis when the wires are perpendicular to one another and which reaches infinity as the dual intersections race from the poles to the meridian as the wires become parallel.

What's the issue, again?

tommac
2008-Nov-05, 08:28 PM
The one on top would be part of the refernce system of the one on the bottom one and would be time dialated ( and elongated ? ) to adjust. The ball would be spit out at the speed of light.

I liked listening to astronomy cast and the impossible sounding contraptions they explain to get a point across. Like the light year long scissors being closed at light speed; the cross section moves faster than light speed but doesn't contain any information.

Anyways onto my question...
If there were a treadmill with the belt spinning at 50mph, a ball set on the treadmill will be flung off at 50mph. If you were to stack a second treadmill on top of the first one, it too would be flung off at 50mph. However, anything set on top of the second treadmill would be flung off at 30mph plus the 50mph that the second treadmill is already traveling at.
Remember the belts on the 2 treadmills do NOT come into contact with each other, which is why they can both be spinning in the same direction and have their speeds stack.
Now if you super-size this contraption with one treadmill running at light speed and the other also running at light speed, what happens to the ball?

Remember that the first treadmill is more than long enough to propel the second treadmill to light speed. If the speeds are slower (30mph, 50mph, etc), then the speeds get added together for the ball; relative to the stationary positions of say the pivot points of the first treadmill.

I know nothing can go faster than light, but where exactly would it break down?

John Mendenhall
2008-Nov-05, 09:00 PM
Let's try a new thought experiment that will, hopefully, rectify some of these cause-effect problems...

Two circular wires of nearly identical radii are attached to the same axis. One is stationary, the other is rotating about the axis at a constant rotational velocity.

Now you have your intersection, which begins a v=0 at the axis when the wires are perpendicular to one another and which reaches infinity as the dual intersections race from the poles to the meridian as the wires become parallel.

What's the issue, again?

Neat idea, Mugsy. It is:

1. Physically possible, right here. There's a display the local shopping center that does exactly this. And is it ever maddening to watch.

2. Repeatable, verifiable, predictable, etc.

Quibble: the apparent motion is from the poles to the equator, right? Not the meridian.

Regards, John M.

tommac
2008-Nov-05, 09:25 PM
Quibble: the apparent motion is from the poles to the equator, right? Not the meridian.
.

Hey the czecs like to go to warmer climates also.

Jeff Root
2008-Nov-06, 06:07 AM
AndreasJ,

The scenario you describe is equivalent to the "parallel beams" scenario
we have several times referred to immediately after discussing the scissor
scenario. Two parallel or nearly parallel straight edges, or beams, of any
length you want, are set in motion so that they pass each other, the angle
between them remaining constant. That scenario requires that one of the
beams be rigged ahead of time to receive an even push along its length,
so that it doesn't bend. That means there has to be a delay between the
time the signal is sent to push the beam and the time the beam actually
gets pushed. That delay prevents any signal from being transmitted at
high speed by the intersection point. The speed is limited to the speed
of the signal reaching the ends of the beam, and the pushing mechanism's
response time.

Your scenario just puts the same mechanism on a rotating scissor blade.

-- Jeff, in Minneapolis

hhEb09'1
2008-Nov-06, 06:25 AM
The scenario you describe is equivalent to the "parallel beams" scenario
we have several times referred to immediately after discussing the scissor
scenario. Two parallel or nearly parallel straight edges, or beams, of any
length you want, are set in motion so that they pass each other, the angle
between them remaining constant. That scenario requires that one of the
beams be rigged ahead of time to receive an even push along its length,
so that it doesn't bend. That means there has to be a delay between the
time the signal is sent to push the beam and the time the beam actually
gets pushed. That delay prevents any signal from being transmitted at
high speed by the intersection point. The speed is limited to the speed
of the signal reaching the ends of the beam, and the pushing mechanism's
response time.Does that "speed" I've highlighted in red refer back to the "high speed"?

Jeff Root
2008-Nov-06, 06:36 AM
If you close the blade slowly enough, the disturbance will have time to
propagate to the end by the time the blade closes all along its length.
First, the only point of this exercise is to try to transmit a signal
faster than light.

Second, the location of the pivot point is irrelevant. You aren't going
to believe that, so I'll repeat it: THE LOCATION OF THE PIVOT POINT
IS COMPLETELY, TOTALLY, UTTERLY IRRELEVANT! I hope that will
motivate you to prove it to yourself, because I don't want to bother.

Third, the speed at which you close the blades is irrelevant, as shown
by Grant's scenario in which the blades are only infinitesimally far apart.
In that situation, the blades will be fully closed at any given point along
the blades the instant that the blades begin to move at that point.

Suppose I have a pair of scissors designed to work as Grant described:
The blades are parallel and infinitesimally far apart (that means "real
close together" for anyone who wasn't sure). And suppose that the
speed of sound in the steel scissor blades is 5000 m/s. I squeeze the
handles of the scissor together. At my hand, the blades come together
simultaneous with my squeezing them. One metre away from my hand,
the blades come together 1/5000 second after I squeeze. One kilometre
away from my hand, they come together 1/5 second after I squeeze.
One thousand kilometres away from my hand, the blades come together
200 seconds after I squeeze. The steel bends along the length of the
blades in a pulse traveling at the speed of sound in that material.

-- Jeff, in Minneapolis

Jeff Root
2008-Nov-06, 06:44 AM
Does that "speed" I've highlighted in red refer back to the "high speed"?
Yes, it refers to the signal speed that you achieve with the apparatus.
You are trying to achieve a superluminal speed, but the actual speed
is limited by the communication lag and response time of the mechanism
which pushes evenly on the whole length of the beam.

-- Jeff, in Minneapolis

hhEb09'1
2008-Nov-06, 08:02 AM
First, the only point of this exercise is to try to transmit a signal faster than light.No, that's baloney. The OP mentioned the scissors described by astronomy cast. Astronomy Cast is aware that signals cannot be transmitted faster than light. Surely their use of the scissors is to demonstrate that certain mental constructs can move faster than the speed of light. Astronomy Cast is not trying to show that we can transmit a signal faster than light.

Second, the location of the pivot point is irrelevant. You aren't going
to believe that, so I'll repeat it: THE LOCATION OF THE PIVOT POINT
IS COMPLETELY, TOTALLY, UTTERLY IRRELEVANT! I hope that will
motivate you to prove it to yourself, because I don't want to bother.No need to shout :)

Third, the speed at which you close the blades is irrelevant, as shown
by Grant's scenario in which the blades are only infinitesimally far apart.
In that situation, the blades will be fully closed at any given point along
the blades the instant that the blades begin to move at that point.

Suppose I have a pair of scissors designed to work as Grant described:
The blades are parallel and infinitesimally far apart (that means "real
close together" for anyone who wasn't sure). And suppose that the
speed of sound in the steel scissor blades is 5000 m/s. I squeeze the
handles of the scissor together. At my hand, the blades come together
simultaneous with my squeezing them. One metre away from my hand,
the blades come together 1/5000 second after I squeeze. One kilometre
away from my hand, they come together 1/5 second after I squeeze.
One thousand kilometres away from my hand, the blades come together
200 seconds after I squeeze. The steel bends along the length of the
blades in a pulse traveling at the speed of sound in that material.
If I take a pair of scissors .5 meters long, with a pivot point 10 angstroms off, and the far ends 2mm open and I start to close them at a speed such that it would make the far end points converge at a rate of 1.0m/s, and using your figure of 5000m/s, the signal reaches the end of the scissors in .1msec when the ends of the scissors are still much more than 1mm apart. Using the equation (http://www.bautforum.com/questions-answers/80795-treadmill-top-treadmill-stacked-speed.html#post1358312) I derived before, when the scissors do close, the point of intersection is moving faster than (.5m/10angstroms)(1.0m/s), which is greater than the speed of light.

Scissors of .5m length are large, but I've seen larger. 10 angstroms is not far, but it's a lot farther than infinitesimally far. It's physically manageable--it's possible to create scissors where the pivot has zero offset. If, instead of 10 angstroms, I'd used 100 angstroms, the resulting speed of the intersection point is less than the speed of light. So, I think it is relevant.

Jeff Root
2008-Nov-06, 02:15 PM
First, the only point of this exercise is to try to transmit a signal
faster than light.
No, that's baloney. The OP mentioned the scissors described by
astronomy cast. Astronomy Cast is aware that signals cannot be
transmitted faster than light.
And you are aware that computers can be used to send messages.

OF COURSE Fraser and Pamela or whoever did the Astronomy Cast
are aware that signals cannot be transmitted faster than light!
I didn't hear the program, but I know that the scissors problem is
to try to describe a method of transmitting a signal faster than the
speed of light.

Surely their use of the scissors is to demonstrate that certain
mental constructs can move faster than the speed of light.
Astronomy Cast is not trying to show that we can transmit a
signal faster than light.
And I'm not trying to prove that I own light-year long scissors.

Enough with the silly strawmen, already!

I know what you are doing. You are testing to see if you will be
treated differently from the kooks. Well, you will. You are not a
kook. Deal with it.

From the original post and a followup by the OPer, I deduced that
the Astronomy Cast talked about both the scissors problem and some
version of the linearly-moving beam problem. The scissors fail to
transmit a signal FTL because of the limited stiffness of material, which
is in turn due to the speed-of-light communication between electrons
in the material which hold the material together. The linearly-moving
beams fail to transmit a signal FTL because a light-speed-limited
signal first has reach all parts of the beam before any part of the
beam starts to move. The signal to start moving the beam is what
is being timed.

If I take a pair of scissors .5 meters long, with a pivot point
10 angstroms off, and the far ends 2mm open and I start to close
them at a speed such that it would make the far end points converge
at a rate of 1.0m/s, and using your figure of 5000m/s, the signal
reaches the end of the scissors in .1msec when the ends of the
scissors are still much more than 1mm apart. Using the equation I
derived before, when the scissors do close, the point of intersection
is moving faster than (.5m/10angstroms)(1.0m/s), which is greater
than the speed of light.
No, the point of intersection is moving at 5000 m/s. Or thereabouts.
The fact that the scissor has ends complicates the motion.

The equation you derived has to do with geometry, not scissors.
Yes, you have to get the geometry right, but then you have to get
the physics right, too. I assume that the Astronomy Cast was more

Scissors of .5m length are large, but I've seen larger. 10 angstroms
is not far, but it's a lot farther than infinitesimally far. It's physically
manageable--it's possible to create scissors where the pivot has zero
offset.
That's the point. The offset is irrelevant. You can have the scissor
blades come together at any angle you desire. It makes not the
slightest difference. The speed of the signal is limited by the speed
of sound in the scissor material.

The scissor can be made so that the blades are exactly parallel. Your
geometric analysis would have them come together along their entire
lengths at the same instant. But that ignores the physics. It doesn't
matter whether the blades are a centimetre long or a light-year long,
the speed at which the point of intersection moves is limited by the
speed of sound, which is in turn determined by the speed of light.

-- Jeff, in Minneapolis

cjameshuff
2008-Nov-06, 05:05 PM
No, the point of intersection is moving at 5000 m/s. Or thereabouts.

With an offset, the intersection can be moving at any arbitrary speed.

That's the point. The offset is irrelevant. You can have the scissor
blades come together at any angle you desire. It makes not the
slightest difference. The speed of the signal is limited by the speed
of sound in the scissor material.

The offset is not irrelevant.
The whole point of the exercise is to demonstrate how an apparent superluminal motion fails to transmit a superluminal signal. With an infinitesimal offset, you do limit the motion of the intersection point to the speed of sound in the material...making for a completely useless illustration.

The signal is the change in velocity of the blades. With a non-zero offset, the signal can start propagating toward the ends of the blades before the intersection point even exists. When the blades do cross, the intersection point can move along the blades at any arbitrary velocity...but it is not the signal, the signal has already been transmitted and is on its way to the other end of the blades. The arrival of the crossing point at the end of the blades is limited by the speed of sound, but not its propagation speed along the blades.

John Mendenhall
2008-Nov-06, 05:33 PM
The arrival of the crossing point at the end of the blades is limited by the speed of sound, but not its propagation speed along the blades.

Jeff, do you have that swimming through glue feeling?

Apparently nobody gets the idea that it is bad practice to propose physically impossible thought experiments.

Guys, go back and read Albert"s early papers again. They're readily available online. Note how carefully he constructs his 'thought experiments'. You have to have the boundary conditions nailed down.

Regards, John M.

hhEb09'1
2008-Nov-06, 05:59 PM
I didn't hear the program, but I know that the scissors problem is
to try to describe a method of transmitting a signal faster than the
speed of light.I disagree with that.

I know what you are doing. You are testing to see if you will be
treated differently from the kooks. I definitely disagree with that! :)

No, the point of intersection is moving at 5000 m/s. Or thereabouts.
The fact that the scissor has ends complicates the motion.
That doesn't seem to be the case. I've done the math above, what's wrong with it? I've even taken the delay due to the speed of sound into consideration.

The equation you derived has to do with geometry, not scissors.
Yes, you have to get the geometry right, but then you have to get
the physics right, too. I assume that the Astronomy Cast was more
about the physics of the scissors problem than about the geometry.I thought I included the physics. Which part am I missing?

That's the point. The offset is irrelevant. You can have the scissor
blades come together at any angle you desire. It makes not the
slightest difference. The speed of the signal is limited by the speed
of sound in the scissor material.The intersection of the blades is not a signal.
Jeff, do you have that swimming through glue feeling?

Apparently nobody gets the idea that it is bad practice to propose physically impossible thought experiments.Mine above (http://www.bautforum.com/questions-answers/80795-treadmill-top-treadmill-stacked-speed-2.html#post1359054) seems to be physically possible. Why not?

cjameshuff
2008-Nov-06, 06:20 PM
Apparently nobody gets the idea that it is bad practice to propose physically impossible thought experiments.

The basic experiment is not physically impossible. It's not even really that hard. There's just not much point in actually doing it, because it's easy to show that the "motion" of the crossing point is not limited by the speed of sound in the material. If the blades are rotating at a constant rate, the speed of sound in the material is in fact completely irrelevant. A signal transmitted through the blades is limited by that speed, though, and that's the point...the existence of apparently "FTL" phenomena does not mean FTL communication is possible.

Making the blade offset infinitesimally small is physically impossible, but also destroys the usefulness of the illustration. The solution is simple...just don't do that.

hhEb09'1
2008-Nov-08, 09:59 AM
Mine above seems to be physically possible.When the ends of the blades are one angstrom apart, the intersection is .5m/11 (a little more than 4 cm) from the ends. Since the ends are converging at 1m/s, they will touch in 10-10 second. That means the intersection travels over 4cm in 10-10 seconds, or an average speed over those last 4 centimeters of over 400,000 km/sec. Those are small (http://hypertextbook.com/facts/MichaelPhillip.shtml) distances, but not infinitesimally small. I could make the setup larger, but I thought it was neat to have one I could actually hold in my backpack. :)

AndreasJ
2008-Nov-08, 12:17 PM
AndreasJ,

The scenario you describe is equivalent to the "parallel beams" scenario
we have several times referred to immediately after discussing the scissor
scenario. ... Your scenario just puts the same mechanism on a rotating scissor blade.
Since you evidently believe I need to be told this, what on earth do you think the point of my example was?

Jeff Root
2008-Nov-08, 04:37 PM
AndreasJ,

The scenario you describe is equivalent to the "parallel beams" scenario
we have several times referred to immediately after discussing the scissor
scenario. ... Your scenario just puts the same mechanism on a rotating
Since you evidently believe I need to be told this, what on earth do
you think the point of my example was?
beams. Since you didn't mention that what you were describing had
already been described, and since you didn't refer to the beams in
any way, neither I nor any other reader could know whether you
was to tell us that the mechanism previously described could also be
applied to the scissors scenario. That was a good point. It changes
the failure from a speed-of-sound delay in the scissor blades to a
communication and reaction delay in the mechanism propelling them.

That is also the failure in mugaliens' rotating wire assembly.

-- Jeff, in Minneapolis

bongopukerat
2008-Nov-29, 09:26 AM
UPDATE:
Just watched The Universe Season 3 Episode 3 "Light Speed" on History HD and they played out the scenario I described with the treadmills. They used a bike and tennis ball analogy though. A guy on a bike standing still throws a ball vs. riding his bike and throwing the ball; the speeds stack. But when he does the same test with a light beam on his bike, the light takes the same amount of time to travel the same distance; the speeds do not stack.

mugaliens
2008-Nov-29, 10:22 AM
Jeff, do you have that swimming through glue feeling?

Reading through all the scissor postulations, I certainly do!

Apparently nobody gets the idea that it is bad practice to propose physically impossible thought experiments.

Agreed, which is why I proposed a perfectly possible and easily constructed thought experiment. But wait! It's more than a thought experiment - it's easily built, right here, right now.

Guys, go back and read Albert"s early papers again. They're readily available online.

Errrrppp! Instead, how about we revisit my idea, instead, as it, too, is readily available online. Unlike Albert's "thought experiments," however, mine's doable. Real machine, here and now, real time.

Neat idea, Mugsy. It is:

1. Physically possible, right here. There's a display the local shopping center that does exactly this. And is it ever maddening to watch.

2. Repeatable, verifiable, predictable, etc.

Quibble: the apparent motion is from the poles to the equator, right? Not the meridian.

Regards, John M.

Thanks!

Details and drawings to follow, shortly.

mugaliens
2008-Nov-29, 11:02 AM
Given a thickness n, with a constant angular velocity ω, and a radius r for two coaxial rings, a central observer would observe the intersection moving towards a point equidistant and 90 degrees from the axis. At the last instant, the apparent movement would be infinate, but would instead be a finite velocity v, where v=f(n, ω, r). However, that apparent velocity would be greater than the edge velocity of the rotating ring.

http://i35.photobucket.com/albums/d185/mugaliens/RotatingRings.jpg

WayneFrancis
2008-Nov-29, 01:02 PM
I liked listening to astronomy cast and the impossible sounding contraptions they explain to get a point across. Like the light year long scissors being closed at light speed; the cross section moves faster than light speed but doesn't contain any information.

Anyways onto my question...
If there were a treadmill with the belt spinning at 50mph, a ball set on the treadmill will be flung off at 50mph. If you were to stack a second treadmill on top of the first one, it too would be flung off at 50mph. However, anything set on top of the second treadmill would be flung off at 30mph plus the 50mph that the second treadmill is already traveling at.
Remember the belts on the 2 treadmills do NOT come into contact with each other, which is why they can both be spinning in the same direction and have their speeds stack.
Now if you super-size this contraption with one treadmill running at light speed and the other also running at light speed, what happens to the ball?

Remember that the first treadmill is more than long enough to propel the second treadmill to light speed. If the speeds are slower (30mph, 50mph, etc), then the speeds get added together for the ball; relative to the stationary positions of say the pivot points of the first treadmill.

I know nothing can go faster than light, but where exactly would it break down?

First you can't get even the first treadmill to the speed of light. But lets say you get the first tread mill to the speed of .99 the speed of light. If you introduced the same amount of energy you could get the 2nd tread mill to .99the speed of light in its own frame. But does this mean the 2nd tread mill will be 1.98x the speed of light? No because even at slow speeds 30mph + 30mph != 60mph but a slightly lower speed.

The issue is if you get the first tread mill to .99 the speed of light then that first tread mill is time dialated by ~7x and its lengths reduced by 7x. So now to the external observer anything travelling inside the first frame of reference is 49x slower then what it appears inside the reference frame. It gets a bit more complicated

The 2nd tread mills speed can be computed by the following equation

V1 = speed of 1st treadmill
V2 = speed of 2nd treadmill

V1 + V2
______________ = Apparent V

1 + ( V1V2/C^2)

So if we plug in the number .99c for V1 and V2 then we get

1.98
____

1 + (0.9801/1)

or
1.98
_______
1.9801

or
0.99994949750012625624968435937579C

if you then fire a bullet at 50mph from the 2nd treadmill and just use the previous number as V1 and
0.000000074558246601427475190150705314526 in for V2

The number above is 50mph over the speed of light in MPH (670616629mph)

mugaliens
2008-Nov-29, 06:29 PM
V1=0.99994949750012625624968435937579 c

V2=0.000000074558246601427475190150705314526 c

Thus, the bullet is travelling at V3=(V1+V2)/(1+(V1V2/C^2))

I get: 0.999949497507656 c, which is > V1 and > V2 but < c.

You may want to check your math...

WayneFrancis
2008-Nov-30, 03:03 AM
V1=0.99994949750012625624968435937579 c

V2=0.000000074558246601427475190150705314526 c

Thus, the bullet is travelling at V3=(V1+V2)/(1+(V1V2/C^2))

I get: 0.999949497507656 c, which is > V1 and > V2 but < c.

You may want to check your math...

I'm confused on what you think the problem is.

If V1 is the 2 combined treadmills reference frame
and
V2 is the bullets speed (50mph within the V1 reference frame)
Then V should be > V1 and V2 and V < C. That is the entire point.

I think I'm missing something here.

Just went through this my son. Take 2 cars that hit each other while both where driving at 50mph. Normal thinking is that they would impact at 100mph but in reality they would hit at
99.999999999999444106784832330957mph

If You have a train going at .5C and car on top of that train that is doing .5C in the train's reference frame then to an external observer to the train the car is not doing C but .8C

cosmocrazy
2008-Nov-30, 12:09 PM
The whole problem with some thought experiments is the fact that fantasy can take over from reality even if it may seem possible.

I'm interested in your experiment Mugs and will watch and see how it develops.

In the scissor scenario the speed which any intersection takes place will never exceed C regardless of the "apparent" speed propagation of the signal and velocity of the blades and or the size of the scissors. even if the handles were squeezed at C the intersection would be measured to be occurring at no more than C to any relative observer even if all other material limiting factors had been eliminated . The problem seems to be arising because the measurements are being added together for what essentially is 2 different reference frames, point A the point of applied force & point B the point of intersection. From either point as soon as acceleration is applied to one point relative to the other, space-time would be dilated or stretched so the sizes measured from either point or any other relative point would not agree at the moment of closing the scissors. C would remain constant.

Durakken
2008-Nov-30, 12:47 PM
#1) imaginary things are very useful... shown to be so by the very fact of todays sciences as most of it is built on stuff one cannot see in most cases or at least when originally proposed to speculated upon were purely imaginitive.

#2) The answer to the OPs question is screwy. Assuming you have infinite energy reserves as far as I have read it should be possible for you to achieve speed faster than light. There is no barrier there that says you can't move faster than this, but rather there is just an impracticality of doing it that way. So moving on from that...

Let's say you actually had this limitless energy that could send something to say 1.5x light speed and did it...Can something go faster than light? The answer really is an odd one because like i said before there is no actual barrier that says at this point you can not travel faster than this speed...so in that sense there is no break down. The break down comes in the form of measurement. To measure something we need for two position to be known and what is the interval between them. Since this is the case and light doesn't ever stop being emitted/reflected from this object you can only measure the distance by gathering it and the quickest medium of gathering it is light and because the fastest we can measure from an outside perspective is that of the light reflect back is light speed...there is no way to measure beyond that... There is also time dilation that happens so it would be hard to take the on board info at face value...

The only way we could even say that something traveled beyond the speed of light, like we have in some experiments, is to know the point of origin for the light and the other object traveling faster and having them detected at a destination.

Btw traveling at those speeds would really be something to see because when it would be traveling away from you it would be an after image and you could create some fantastic illusions while when it is traveling towards you it would create an infinite regression image of sorts because the light wouldn't be able to keep up but would be projecting from multiple points in space at the same time from an observers point of view.

cosmocrazy
2008-Nov-30, 01:19 PM
#1) Let's say you actually had this limitless energy that could send something to say 1.5x light speed and did it...Can something go faster than light? The answer really is an odd one because like i said before there is no actual barrier that says at this point you can not travel faster than this speed...so in that sense there is no break down. The break down comes in the form of measurement. To measure something we need for two position to be known and what is the interval between them. Since this is the case and light doesn't ever stop being emitted/reflected from this object you can only measure the distance by gathering it and the quickest medium of gathering it is light and because the fastest we can measure from an outside perspective is that of the light reflect back is light speed...there is no way to measure beyond that... There is also time dilation that happens so it would be hard to take the on board info at face value...

The only way we could even say that something traveled beyond the speed of light, like we have in some experiments, is to know the point of origin for the light and the other object traveling faster and having them detected at a destination.

A couple of things regarding this.

If you were able to accelerate an object up to C, in that objects reference frame space-time would have dilated so much between the start and end point that it would measure zero distance & time elapsed. So for any object to travel any distance at all measured in its reference frame, then subliminal speed is required. Even If the object was proposed to have traveled greater than C, from any relative reference point the object would have measured to have traveled the distance at C. Regardless of the proposed speed achieved. If you try to imply any different you start to get problematic causality effects.

The cosmological constant "C" is measured by us to be around 300,000 km/s in a vacuum. But if it measured to be 1 km/s to infer it could be exceeded has no relative meaning.
"C" means constant, everything else has to change relatively.

1.0000000000000000001 x C = infinity x C . both have no real value in reality.

Durakken
2008-Nov-30, 01:33 PM
A couple of things regarding this.

If you were able to accelerate an object up to C, in that objects reference frame space-time would have dilated so much between the start and end point that it would measure zero distance & time elapsed. So for any object to travel any distance at all measured in its reference frame, then subliminal speed is required. Even If the object was proposed to have traveled greater than C, from any relative reference point the object would have measured to have traveled the distance at C. Regardless of the proposed speed achieved. If you try to imply any different you start to get problematic causality effects.

The cosmological constant "C" is measured by us to be around 300,000 km/s in a vacuum. But if it measured to be 1 km/s to infer it could be exceeded has no relative meaning.
"C" means constant, everything else has to change relatively.

1.0000000000000000001 x C = infinity x C . both have no real value in reality.

I think you said what i said better...

Basically from all reference points it is impossible to travel beyond the speed of light, but if you could measure time/distance without light then it would show that one had traveled faster...

mugaliens
2008-Nov-30, 02:41 PM
I'm interested in your experiment Mugs and will watch and see how it develops.

Thanks. I think it has a fighting chance for two reasons:

1. Because it involves two circular hoops, all points of the apparent intersection are equidistant from the obersver.

2. One hoop is stationary while the other rotates about a common axis at a constant angular velocity.

As a result of the geometry, the angle of the intersection goes from 90 deg when the hoops are at right angles to one another to 0 degrees the instant that both intersections travelling from each side meet up in front of the observer.

At that instant, as the intersection angle --> 0, the intersection velocity --> ∞, due to the fact that the angular velocity, ω, is a constant.

So, yes - the location of an intersection can indeed travel faster than the speed of light to an observer. An intersection, itself, however, does not travel faster than light, as an intersection is defined by to coincident points on intersecting lines. The intersection remains fixed at that coincident point. It only appears to travel as an infinite succession of intersections occurs along the two lines as they pass one another.

In the scissor scenario the speed which any intersection takes place will never exceed C regardless of the "apparent" speed propagation of the signal and velocity of the blades and or the size of the scissors. even if the handles were squeezed at C the intersection would be measured to be occurring at no more than C to any relative observer even if all other material limiting factors had been eliminated . The problem seems to be arising because the measurements are being added together for what essentially is 2 different reference frames, point A the point of applied force & point B the point of intersection. From either point as soon as acceleration is applied to one point relative to the other, space-time would be dilated or stretched so the sizes measured from either point or any other relative point would not agree at the moment of closing the scissors. C would remain constant.

I'm not so sure about saying: "In your scissor scenario, that is true - the apparent intersection would never propogate faster than c."

In my dual hoop example, the apparent intersection most certainly propogates faster than c.

The reason I can't say the first one has to do with the case where the two pieces of a scissor are squeezed together, and accelerate until they are moving at a constant angular velocity relative to one another. At that point in time, the apparent intersection may very well occur at FTL.

There is no violation of the transformation of information at FTL velocities, here! Consider the example whereby a man directs two helpers, each armed with a timing device and an explosive charge. One goes a mile east, the other a mile west. Because the timers aren't perfect, one goes off a tiny fraction of a second before the other. Given their two-mile separation, the second report occurs at 3.7 c after the first. Did this violate c? No! The initial conditions were all established long before, as velocities well under c.

Similarly, once a scissors' two pieces are rotating at a relative angular velocity ω, any apparent succession of intersections may very well "move" FTL. But as mentioned earlier, the intersections themselves are moving. They're merely a succession of intersections, and only the location of the intersections moves.

It's like the explosive reports. The reports didn't move at FTL. There were two reports. They simply occured so close together in time that if they had been the same report, it would have had to travel at FTL.

But it wasn't the same report.

Mug's Axiom: The change of location of a succession of intersections may move at FTL speeds. The intersections themselves remain put.

cosmocrazy
2008-Nov-30, 02:59 PM
Basically from all reference points it is impossible to travel beyond the speed of light, but if you could measure time/distance without light then it would show that one had traveled faster...

Yes because the measurement would be unbounded all the way to infinity.

But the whole beauty of relativity and C is that in a sense it gives us that infinity boundary that can be measured in real terms. What i mean by this is, with relativity one can possibly travel at very close to the speed of light reducing any time and distance down to almost zero within one's reference frame. In doing so appearing to reach near infinite speed over near infinite distance in near zero time. But without creating problematic causality effects in another relative reference frame.
As an example ; Say you plotted a journey to Alpha Centuri 4.5 light yrs away. you set off in your space ship and accelerated up to very close to the speed of light. when you reached your destination just a few minutes later on your clock you send a signal to earth telling of your safe arrival which is received just over 9 years from the day you left. Now the point is, even if you were able to achieve light-speed trying to go any faster would be totally fruitless. Simply because the moment you reach light-speed your journey would be over, time and space would be dilated to zero. There would be no point going any faster cause you don't need to in your reference frame (irrelevant of the journey's distance). But the people back home would still see it take you 4.5 yrs to get there and 9 yrs to receive a signal from you since your departure.

Now if you could exceed C then you would in a sense travel back wards in time but over what distance could you achieve this speed? Traveling at C you could say you are going no distance in no time or you could say you are going infinite distance in zero time. Both make no sense in reality.:)

cosmocrazy
2008-Nov-30, 03:08 PM
Thanks. I think it has a fighting chance for two reasons:

1. Because it involves two circular hoops, all points of the apparent intersection are equidistant from the observer.

2. One hoop is stationary while the other rotates about a common axis at a constant angular velocity.

As a result of the geometry, the angle of the intersection goes from 90 deg when the hoops are at right angles to one another to 0 degrees the instant that both intersections traveling from each side meet up in front of the observer.

At that instant, as the intersection angle --> 0, the intersection velocity --> ∞, due to the fact that the angular velocity, ω, is a constant.

So, yes - the location of an intersection can indeed travel faster than the speed of light to an observer. An intersection, itself, however, does not travel faster than light, as an intersection is defined by to coincident points on intersecting lines. The intersection remains fixed at that coincident point. It only appears to travel as an infinite succession of intersections occurs along the two lines as they pass one another.

I'm not so sure about saying: "In your scissor scenario, that is true - the apparent intersection would never propagate faster than c."

In my dual hoop example, the apparent intersection most certainly propagates faster than c.

The reason I can't say the first one has to do with the case where the two pieces of a scissor are squeezed together, and accelerate until they are moving at a constant angular velocity relative to one another. At that point in time, the apparent intersection may very well occur at FTL.

There is no violation of the transformation of information at FTL velocities, here! Consider the example whereby a man directs two helpers, each armed with a timing device and an explosive charge. One goes a mile east, the other a mile west. Because the timers aren't perfect, one goes off a tiny fraction of a second before the other. Given their two-mile separation, the second report occurs at 3.7 c after the first. Did this violate c? No! The initial conditions were all established long before, as velocities well under c.

Similarly, once a scissors' two pieces are rotating at a relative angular velocity ω, any apparent succession of intersections may very well "move" FTL. But as mentioned earlier, the intersections themselves are moving. They're merely a succession of intersections, and only the location of the intersections moves.

It's like the explosive reports. The reports didn't move at FTL. There were two reports. They simply occurred so close together in time that if they had been the same report, it would have had to travel at FTL.

But it wasn't the same report.

Mug's Axiom: The change of location of a succession of intersections may move at FTL speeds. The intersections themselves remain put.

I agree, the change of intersection may appear to be FTL, but would that not be due to assuming a stationary reference point relative to all measured intersections at any one time?

Jeff Root
2008-Nov-30, 03:51 PM
Mugs,

As I said in post #14 of this thread, before you introduced the rotating
hoops geometry, the point of intersection can move at any speed, but
carries no information, so there is nothing very interesting about it. The
rotating hoops are in principle identical to two straightedges, or beams,
moving relative to each other without rotating. If two straightedges
are aligned parallel to each other, and move past each other, the edges
cross along their entire lengths simultaneously. That has no implications
for faster-than-light anything. It is just geometry, not physics.

Your experiment is indeed very simple and easy to do. It just doesn't
do anything very interesting. The whole point of the scissor scenario
is to transmit a signal FTL by closing and opening the scissor blades.
That fails because the acceleration of the blades is limited to the

-- Jeff, in Minneapolis

Jeff Root
2008-Nov-30, 04:26 PM
So many people appear to be confused that I'm going to re-state this:

We have two different scenarios here. One involves accelerating the
two straightedge blades of a pair of scissors to try to make the point of
intersection move faster than light. The other involves two straightedges
which are already in motion. These straightedges could be arranged like
the blades of a pair of scissors, but they could instead be moving linearly
past each other. This latter option has the advantage that no rotation
is involved, so there is no strain along the length of the material -- no
strain of any kind, actually. In the scissors scenario, the strain along
the length of the blades is important for long blades and high rotation
speeds. But the strain accelerating the blades is important in all cases,
and it involves the transmission of force through the blade material at
the speed of sound in that material.

In the scenario where the scissor blades are accelerated, the speed of
the intersectin is limited to the speed of sound. In the scenario where
the straightedges are already in motion, such as Mugs and the guy with
the red spraypaint avatar discuss, the speed of the intersection is not
limited in any way, but it conveys no information FTL since the signal to
accelerate the straightedge had to have been sent at an earlier time.
Two parallel straightedges can come together along their entire lengths
simultaneously (for certain definitions of "simultaneously"), and it is not
a particularly remarkable occurrance.

-- Jeff, in Minneapolis

cosmocrazy
2008-Nov-30, 04:41 PM
Thank you Jeff and you are quite right.

I think with thought experiments such as these people tend to dismiss the "real" structural and information transfer ability of matter, when forces are applied.

hhEb09'1
2008-Dec-01, 04:42 AM
So many people appear to be confused that I'm going to re-state this:

In the scenario where the straightedges are already in motion, such as Mugs and the guy with the red spraypaint avatar discuss, the speed of the intersection is not limited in any way, but ... That contradicts what you said before. In this other post, you say my example is limited to 5000m/s

If I take a pair of scissors .5 meters long, with a pivot point 10 angstroms off, and the far ends 2mm open and I start to close them at a speed such that it would make the far end points converge at a rate of 1.0m/s, and using your figure of 5000m/s, the signal reaches the end of the scissors in .1msec when the ends of the scissors are still much more than 1mm apart. Using the equation (http://www.bautforum.com/questions-answers/80795-treadmill-top-treadmill-stacked-speed.html#post1358312) I derived before, when the scissors do close, the point of intersection is moving faster than (.5m/10angstroms)(1.0m/s), which is greater than the speed of light.No, the point of intersection is moving at 5000 m/s. Or thereabouts.I went back through the posts to see if you explain that discrepancy between your posts, but I can't find anything to that effect.

ETA: BTW, I went to the Astronomy Cast (http://www.astronomycast.com/)pages, and actually listened to a couple of them trying to find the source of the comment in the OP, with no luck. Anybody have any idea where the AC discussion arose?

WayneFrancis
2008-Dec-01, 02:17 PM
...
#2) The answer to the OPs question is screwy. Assuming you have infinite energy reserves as far as I have read it should be possible for you to achieve speed faster than light. There is no barrier there that says you can't move faster than this, but rather there is just an impracticality of doing it that way. So moving on from that...

I disagree. Even with infinate energy you can't get something to the speed of light. You can just get something REALLY REALLY close to the speed of light. That is if you trust SR.

The only way you can get something fast the the speed of light is if it has negative rest mass. Then the more energy you pump into it the slower it actually goes but it never goes slower then C

...
Let's say you actually had this limitless energy that could send something to say 1.5x light speed and did it...Can something go faster than light? The answer really is an odd one because like i said before there is no actual barrier that says at this point you can not travel faster than this speed...so in that sense there is no break down. The break down comes in the form of measurement. To measure something we need for two position to be known and what is the interval between them. Since this is the case and light doesn't ever stop being emitted/reflected from this object you can only measure the distance by gathering it and the quickest medium of gathering it is light and because the fastest we can measure from an outside perspective is that of the light reflect back is light speed...there is no way to measure beyond that... There is also time dilation that happens so it would be hard to take the on board info at face value...

Even if you could get up to the speed of light there is a problem. You would experience no time. You wouldn't ever know when to slow down. To yourself your travel would be instant, if someone/something else actually slowed you down.

...
The only way we could even say that something traveled beyond the speed of light, like we have in some experiments, is to know the point of origin for the light and the other object traveling faster and having them detected at a destination.

Btw traveling at those speeds would really be something to see because when it would be traveling away from you it would be an after image and you could create some fantastic illusions while when it is traveling towards you it would create an infinite regression image of sorts because the light wouldn't be able to keep up but would be projecting from multiple points in space at the same time from an observers point of view.

The problem with saying you can get some thing with a positive non-zero rest mass to C is that it would take infinate energy at which point it also would have infinate mass which means it would have infinate gravity.

If you can show me an accepted equation where you can actually get a positive non-zero rest mass object to C then I'd be interested.

Jeff Root
2008-Dec-02, 06:25 PM
In the scenario where the straightedges are already in motion, such
as Mugs and the guy with the red spraypaint avatar discuss, the
speed of the intersection is not limited in any way, but ...
That contradicts what you said before. In this other post, you say
my example is limited to 5000m/s

If I take a pair of scissors .5 meters long, with a pivot point
10 angstroms off, and the far ends 2mm open and I start to close
them at a speed such that it would make the far end points converge
at a rate of 1.0m/s, and using your figure of 5000m/s, the signal
reaches the end of the scissors in .1msec when the ends of the
scissors are still much more than 1mm apart. Using the equation
I derived before, when the scissors do close, the point of intersection
is moving faster than (.5m/10angstroms)(1.0m/s), which is greater
than the speed of light.
No, the point of intersection is moving at 5000 m/s. Or thereabouts.
I went back through the posts to see if you explain that discrepancy
between your posts, but I can't find anything to that effect.
Sometimes you seem to be talking about the accelerating scenario
and sometimes you seem to be talking about the non-accelerating
scenario. I assumed at first that you were talking about the
accelerating one since that is the only one that has any interesting
physics. The post of yours which you quote above pretty clearly
referred to the accelerating scenario, so I said that the speed of
sound limit applies. However, your repeated insistance on analyzing
the geometry but ignoring the acceleration and the material led me
to think that you were talking about the non-accelerating scenario,
in which the physics and material can safely be ignored, and only
geometry is involved, making the scenario physically uninteresting,
but putting no limitation on the speed of the point of intersection,
including simultaneous intersection along the entire length. That is
what I assumed in my last post. I made no attempt to sort through
your statements to figure out whether you jumped back and forth
between scenarios or were consistent and I just didn't understand
which scenario you were talking about, or what. I'm confused as
to why you seem not to "get" it because I know you are as smart
as I am. I think one of us must be more confused than the other
at the moment, but I don't know which one.

-- Jeff, in Minneapolis

hhEb09'1
2008-Dec-03, 08:20 PM
The post of yours which you quote above pretty clearly referred to the accelerating scenario, so I said that the speed of
sound limit applies.That's why I devised the scenario there, it does seem pretty clear, but it's also true that the speed of sound limit does not apply. Go ahead, work through it.

Jeff Root
2008-Dec-03, 09:46 PM
The post of yours which you quote above pretty clearly referred to the
accelerating scenario, so I said that the speed of sound limit applies.
That's why I devised the scenario there, it does seem pretty clear,
but it's also true that the speed of sound limit does not apply. Go

If I take a pair of scissors .5 meters long, with a pivot point
10 angstroms off, and the far ends 2mm open and I start to close
them at a speed such that it would make the far end points converge
at a rate of 1.0m/s, and using your figure of 5000m/s, the signal
reaches the end of the scissors in .1msec when the ends of the
scissors are still much more than 1mm apart. Using the equation I
derived before, when the scissors do close, the point of intersection
is moving faster than (.5m/10angstroms)(1.0m/s), which is greater
than the speed of light.
The ends of the scissors are still 2 mm apart when the signal
reaches them. That is when the ends begin to move. Using my
figure of 5000 m/s for the speed of sound in steel, the ends begin
to move 0.1 ms after the blades begin to move at the pivot.
It will take some time for the ends to reach a speed of 1.0 m/s.

-- Jeff, in Minneapolis

hhEb09'1
2008-Dec-04, 10:31 AM
It will take some time for the ends to reach a speed of 1.0 m/s.
Which is why I opened the blades to 2mm instead of 1mm :)

1 meter per second is pretty slow, but whatever figure that you use for the "some time" can be allowed for by just opening the scissors another mm or few. The end result is that the intersection point moves at a speed faster than the speed of light, with a not-too-very-special pair of hand shears.

In other words, what appeared to be one scenario instead has just enough time to develop into the other scenario.