Thought Experiment

[Daffy]

I am sitting at my desk, watching my celing fan spin (OK, I'm bored). But I just wondered: the outside tip of the fan blades are turning much faster than the inside hub (or at least more space is covered in the same amount of time). What would happen if you had some sort of unbreakable material (I can't imagine what), and a strong enough motor so you could make a "fan" in space long enough that the outside tips were moving at (or faster) than the speed of light?

How would the increased mass at the edges appear, as part of the same "blades?"

I hope this doesn't seem like a stupid question...

[Grey]

What would happen if you had some sort of unbreakable material (I can't imagine what), and a strong enough motor so you could make a "fan" in space long enough that the outside tips were moving at (or faster) than the speed of light?

You can't. One of the consequences of relativity is that there is no such thing as an infinitely rigid material, even in theory. At best, the fan blades would bend or break.

How would the increased mass at the edges appear, as part of the same "blades?"

You would notice all the normal relativistic effects on the outer edges of the fan blades as they approached the speed of light. It might seem weird that things like Lorentz contraction would vary along the length of the blade, but that's the way it would work. It would be a fairly messy problem to work out all the details for an extended object like this.

I hope this doesn't seem like a stupid question...

Not at all! It's a pretty good question, actually.

[jfribrg]

The easy way to look at this is to show that you would need an infinite centripetal force to keep you rotating, but you could always assume that you had that much torque along with a material that can handle it.

So, assume you are on the edge of one blade and looking at the center of rotation. I would assume that the straight fan blade would appear to be more massive and warped near the center. Since the fan blade near you would not be moving much with respect to you, it would appear straight, but the farther away the blade is from you, the closer it is to the speed of light, so the far away portion of blade would appear more massive and it's clock would move slower, resulting in the velocity being less than light speed.

[skrap1r0n]

On a similar vein, I have often wondered why there was a fear of the sound barrier when simple observation would reveal that the tips of propeller blades regularly exceeded the speed of sound.

as far as your question goes, theoretically at least, it may be possible to approach C. Think of it this way, In order to come close to approaching this, you would have to have the "fan" spinning at 60rpm with each blade a little less that 30k miles long. That would put the circumference around 186k miles.

[publiusr]

There is a book FASTER THAN LIGHT: SUPERLUMINAL LOOPHOLES IN PHYSICS IIRC.

The only 'things' capable of exceeding light speed would be patterns. The ring of lightning called "Elves" expands at 3 times lightspeed--though the light itself is limited to c of course.

If you were over jupiter--and its surface as solid--you could hold a laser pointer--snap your wrist--and the spot would travel over its surface faster than light--though the actual beam itself goes no faster.

Phase waves seem to be superluminal--but none of these concepts seem to be able to carry useful info.

[Daffy]

Thanks all! Weird stuff...especially that last Jupiter bit.

[Wally]

On a similar vein, I have often wondered why there was a fear of the sound barrier when simple observation would reveal that the tips of propeller blades regularly exceeded the speed of sound.

Is this true??? I was under the impression that the forward speed of helicopters was limited to speed of sound minus the rotational speed of the blade tips. This, so that on their forward trip in the rotation, the blade tips would not break the sound barrier and create shock waves.

If this is in fact true, I would think the much shorter blades of a plane prop would also need to remain below the barrier for the same reason. Or, is it because they're so much shorter that their able to withstand the shockwave(s) created?

[jfribrg]

I didn't think that it was possible under normal circumstances to exceed Mach 1 with propellers. As I understood it, as a plane approached Mach 1, the shock wave accumulates, causing problems. In addition,the drag from the propellers increases to the point where the drag exceeds the thrust. Since the thrust of a prop plane is dependent on the medium (air), it takes extraordinary circumstances to exceed it. The speed of the propeller tips is irrelevant since that is not in the direction of the thrust. Jet and rocket engines can exceed Mach 1 because they don't rely on the medium for thrust. Yes, jet engines rely on air for the O2 but the thrust is a consequence of the law of conservation of momentum.

[Wally]

jfrebrg. We're strictly talking about the speed of the blade tips here, not the speed of the aircraft itself. Do you know if the tips of regular plane props exceed the sound barrier??? Wouldnt' this create some insermountable problems with stability/vibration/etc.

[Daffy]

There is a book FASTER THAN LIGHT: SUPERLUMINAL LOOPHOLES IN PHYSICS IIRC.

The only 'things' capable of exceeding light speed would be patterns. The ring of lightning called "Elves" expands at 3 times lightspeed--though the light itself is limited to c of course.

If you were over jupiter--and its surface as solid--you could hold a laser pointer--snap your wrist--and the spot would travel over its surface faster than light--though the actual beam itself goes no faster.

Phase waves seem to be superluminal--but none of these concepts seem to be able to carry useful info.

I have been thinking more about this example. Wouldn't this mean the photons are moving faster than light across the surface? And couldn't they contain information?

[Grey]

If you were over jupiter--and its surface as solid--you could hold a laser pointer--snap your wrist--and the spot would travel over its surface faster than light--though the actual beam itself goes no faster.

I have been thinking more about this example. Wouldn't this mean the photons are moving faster than light across the surface? And couldn't they contain information?

Nope. The photons are never moving across the surface at all, they're always moving from the laser pointer toward Jupiter. So if you're one light second away from Jupiter, it takes one second after you flick your wrist before the spot moves. When you move a laser pointer, you see the spot move, but there's not any physical object or even any information that actually travels from where the old spot was to where the new spot is.

[Daffy]

If you were over jupiter--and its surface as solid--you could hold a laser pointer--snap your wrist--and the spot would travel over its surface faster than light--though the actual beam itself goes no faster.

I have been thinking more about this example. Wouldn't this mean the photons are moving faster than light across the surface? And couldn't they contain information?

Nope. The photons are never moving across the surface at all, they're always moving from the laser pointer toward Jupiter. So if you're one light second away from Jupiter, it takes one second after you flick your wrist before the spot moves. When you move a laser pointer, you see the spot move, but there's not any physical object or even any information that actually travels from where the old spot was to where the new spot is.

But...what if instead of a simple spot, the pointer were generating a pattern, or even words? Wouldn't that mean the information travelled faster than light?

[skrap1r0n]

Is this true??? I was under the impression that the forward speed of helicopters was limited to speed of sound minus the rotational speed of the blade tips. This, so that on their forward trip in the rotation, the blade tips would not break the sound barrier and create shock waves.

Strictly the speed of the prop tips. The max speed of the P-51H Mustang was 490 knots (565 MPH, Mach .80) at an altitude of 25k. The speed of sound at that altitude is about 693. I believe the operating RPM at military power was around 3000 RPM. I cannot find exact specs on the propellor, but in images it looks to be a bit taller than a man so I'll go with a 3.5ft radius, giving it close to a 22ft circumference.

I do not know how the prop was geared from the drive shaft. I also have no idea how to figure the math on this but if the plane has a forward motion of 565 mph, and the prop is rotating at 3000 rpm with about a 22ft circumcerence, then I'd be willing to bet the tips exceed the speed of sound.

[Grey]

But...what if instead of a simple spot, the pointer were generating a pattern, or even words? Wouldn't that mean the information travelled faster than light?

Again, remember that the information in the pattern travels from the laser pointer to the spot at the speed of light. It never travels from the location of the old spot to the new spot. If you were sitting at the location of the old spot, and you wanted to send information to the location of the new spot, can you think of a way that you could use my flick of the laser pointer to do that? Unless, of course, you sent me message asking me to move the pointer, but in that case that request travels from you to me (at the speed of light) and then the information travels from me to the new location (again at the speed of light). Does that make more sense?

[Daffy]

But...what if instead of a simple spot, the pointer were generating a pattern, or even words? Wouldn't that mean the information travelled faster than light?

Again, remember that the information in the pattern travels from the laser pointer to the spot at the speed of light. It never travels from the location of the old spot to the new spot. If you were sitting at the location of the old spot, and you wanted to send information to the location of the new spot, can you think of a way that you could use my flick of the laser pointer to do that? Unless, of course, you sent me message asking me to move the pointer, but in that case that request travels from you to me (at the speed of light) and then the information travels from me to the new location (again at the speed of light). Does that make more sense?

It does. Thanks!

[Suths]

On a similar vein, I have often wondered why there was a fear of the sound barrier when simple observation would reveal that the tips of propeller blades regularly exceeded the speed of sound.

Is this true??? I was under the impression that the forward speed of helicopters was limited to speed of sound minus the rotational speed of the blade tips. This, so that on their forward trip in the rotation, the blade tips would not break the sound barrier and create shock waves.

If this is in fact true, I would think the much shorter blades of a plane prop would also need to remain below the barrier for the same reason. Or, is it because they're so much shorter that their able to withstand the shockwave(s) created?

/delurk

This really needs an aeronautical engineer to give a concise answer but the layman's answer is this...

The real problem with the sound barrier, and the reason for the fear of it, was that the aircraft suffered severe trim changes approaching the speed of sound. And the aircraft's control surfaces suffered all sorts of weird effects including flutter, and the real killer... control reversal. This effectively made the aircraft uncontrolable. Bell solved this by giving the X-1 an all moving tailplane (no seperate elevator) after getting test flight data from Yaeger and the other test pilots.

Most of the WW2 Prop planes had large enough props that the tips exceeded mach 1 at max revs.

The problem with helicopter's rotors is that you have to add the a/c's speed to the advancing blade and subtract it from the retreating blade ( whereas with a conventional a/c prop all the blade tips are moving at the same speed). This would mean that while the advancing blade is exceeding mach 1 the retreating blade is only moving at a few hundred mph. I would imagine that the resulting imbalance between the advancing and retreating blades coupled with having a supersonic shockwave on only one side of the aircraft would have undesired consequences but i'm not qualified to give a definitive answer... much as I enjoy dabbling in amatuer aeronautics I don't think that building and playing with R/C models qualifies me as an Engineer =P

/lurk

[TheGalaxyTrio]

If you were over jupiter--and its surface as solid--you could hold a laser pointer--snap your wrist--and the spot would travel over its surface faster than light--though the actual beam itself goes no faster.

I don't think I buy this. The photons travelling to the surface are limited to light speed. Wouldn't the beam just bend? If you suddenly flicked the light source over, the spot on the surface would smear (afte some propagation delay) in that directions and then reform as a spot in the new location, but the change (the "information front" so to speak) would never exceed c.

I think I need to write a simulation or something. I can visulaize it both ways and I'm not sure which one is right.

[Grey]

I don't think I buy this. The photons travelling to the surface are limited to light speed. Wouldn't the beam just bend? If you suddenly flicked the light source over, the spot on the surface would smear (afte some propagation delay) in that directions and then reform as a spot in the new location, but the change (the "information front" so to speak) would never exceed c.

Yes, the beam would bend, and yes, there would be a propagation delay before the change was observed on the planet, and yes, the speed at which the information travelled would never exceed the speed of light. However, imagine that you have two observing stations set up. To make things easier, we're going to move off of Jupiter so that we can place the observing stations even further apart. Let's say that the source is at the center of a circle one light minute in radius, and the two main observing stations are likewise separated by a light minute.

Now, at some point, I turn the laser so that it now no longer points to the first observing station, and instead points at the second one, and I do that in one second. Neither observing station knows about it yet, because there's still 59 seconds worth of light heading toward the first observing station, a curved string of photons aimed at various points between the two stations, and a steady beam just starting out for the second observing station. Now, after those 59 seconds have passed, the beam heading to the first observing station runs out. The photons sent out during the second that I was turning the laser hit at intermediate points between the two stations, with the ones emitted first, closer to the first station, also striking first, and the beam races over to the other station in a single second. Exactly 60 seconds after the beam was aimed at the second station, the light arrives there, and the spot remains steady, only one second after it left the first observing station. So, no photons or information ever travelled faster than light. However, if I had sensors strung all the way between the two observing posts, and timed when the spot was at each point (or if the circle was mirrored, so that it would reflect the light back to me), I'd see the spot race across a light minute of distance in only a second, 60 times the speed of light.

[hammo1j]

skrap1r0n with your Mustang example if the circumference that the tip of the prop moves is 22 ft and it rotates 3000 times per minute then the speed of the prop is 66,000 ft per minute which is 60 * 66,000 /5280 mph = 750mph.

The velocity of the tip of the prop is at right angles to the forward motion unlike the helicopter so there is no component provided by the plane flighing forward.

[PatKelley]

The tip of a propellor exceeding the speed of sound was part of the demise of the propellor based fighter. Engines and radial speeds could probably have gone on to higher speeds, and a number of transsonic propeller blade designs were tried, but the result of the mini sonic booms was enough to make them very scary to fly, not to mention the leading edge erosion. A good summary of attempts and a primary test pilot here.

[Frog march]

What if a rod of metal the size of Earth's orbit existed and it was spinning so that the ends were traveling at around the speed of light(which is the limit) and you have a wire traveling to the end and back from the middle to make a circuit including a light bulb battery and switch. When you close the circuit does the time dilation(time slowing down) near the end of the rod affect the rate of propagation of the electronic signal through the wire?

[A Thousand Pardons]

I think I need to write a simulation or something. I can visulaize it both ways and I'm not sure which one is right.

Try this one: from space, twirl a laser pointer so that it traces a circle on the earth about the size of the continental USA. Speed up until the entire circle is taking only one hundredth of a second. Each point on the circle, on earth, will receive a dose of photons every hundredth of a second--and like a wave formed in the crowd at a sporting event, it will whip around the USA one hundred times a second. Much faster than the speed of light.

[Frog march]

think of a wave approaching a beach at a slight angle. The point on the shore where the wave breaks would move a lot faster than the wave. Does that help?

[skrap1r0n]

Try this one: from space, twirl a laser pointer so that it traces a circle on the earth about the size of the continental USA. Speed up until the entire circle is taking only one hundredth of a second. Each point on the circle, on earth, will receive a dose of photons every hundredth of a second--and like a wave formed in the crowd at a sporting event, it will whip around the USA one hundred times a second. Much faster than the speed of light.

I may be muddled here but isn't that FTL "Wave" the same principle behind the Alcubierre drive?

[cjl]

think of a wave approaching a beach at a slight angle. The point on the shore where the wave breaks would move a lot faster than the wave. Does that help?

That is a very good analogy for what is happening - just because the wave break races down the beach at high speed doesn't mean that the actual wave is moving at that speed.

[genebujold]

The tip of a propellor exceeding the speed of sound was part of the demise of the propellor based fighter. Engines and radial speeds could probably have gone on to higher speeds, and a number of transsonic propeller blade designs were tried, but the result of the mini sonic booms was enough to make them very scary to fly, not to mention the leading edge erosion. A good summary of attempts and a primary test pilot here.

Actually, the blade tips of the C-130E propellers currently in service today routinely exceed the speed of sound, without being "very scary to fly".