Rotating a laser beam faster than light?

[matheinste]

Hello robbie3370

Quote:-

---thats what i said, not only does nothing material move from one point on orbit to other faster than light, not even the light beam does-----

Sorry i must have misinterpreted what you said.

Matheinste

 

[robbie7730]

That's not how the paradox is normally done, you wave a laser beam at say 100deg/s.
The first photon hits the left side of moon after 2.5sec and the second photon emitted a 1/200 second later hits the other side of the moon 2.52sec after it leaves the laser.
So the 'spot' takes 0.02 seconds to cross the moon but no photon does.

You have not calculated correctly here.

first photon hits left of moon after 2.5 sec. after laser lit. And other emmited 1/200th sec later hits moon 2.52 sec after it leaves laser. Very carefully think now.

first is 2.5 sec. hits left of moon
1/200th sec later second beam leaves laser and takes 2.5 sec`s to reach right of moon. Thats 5.2 seconds total

in reality it takes 1.5 sec`s for light to reach moon but that's neither here nor there but means laser fired at left of moon takes 1.5 sec`s. Then laser in 1/ 200th second swings to point at right of moon, light now leaving at this instant takes 1.5 sec`s also so 3 seconds to hit left edge then right edge, no faster than light could cross the surface distance direct

 

[robbie7730]

i suppose another way of viewing it is that if you hold the laser perfectly still pointing at the moon, then instantly move it to another position, the new position of laser will emit light at that new angle, and that new light will form a spot 1.5 seconds later on the new position on the moon, now if that new position was 10 full moon widths away from moon and the laser still only takes 1.5 seconds to reach that position, it still sould be no faster than light would cross the same expanse itself.

Example...
pretend there`s 2 moons in orbit
one is opposite side of Earth to other
shine laser on one. takes 1.5 seconds to reach it. instantly turn laser to shine at other, still takes 1.5 seconds for light leaving laser to reach other moon. light leaving one moon would take 3 seconds to reach other moon. And so laser can not reach both moons any quicker than light leaving one moon can reach other moon.
simple

 

[Vanadium 50]

I read that the shuttle's clocks went slower; not faster?

That is correct. I was wrong the first time. I just did the calculation and GPS clocks run (45-7) = 38 microseconds per day fast, but space shuttle clocks run (11-25) = 14 microseconds per day slow. In one case the GR effect is larger and the other the SR effect is larger.

Vanadium, how do you get a stationary clock at the same altitude?

What difference does it make? You can do the calculation regardless. That's a little like asking, "but where do they get those frictionless planes and stretchless ropes?"

 

[Doc Al]

You have not calculated correctly here.

mgb_phys is correct.

first photon hits left of moon after 2.5 sec. after laser lit. And other emmited 1/200th sec later hits moon 2.52 sec after it leaves laser. Very carefully think now.

first is 2.5 sec. hits left of moon
1/200th sec later second beam leaves laser and takes 2.5 sec`s to reach right of moon. Thats 5.2 seconds total

You are thinking:
(1) Beam is fired to left of moon at t = 0.0 sec
(2) Beam hits left of moon at t = 2.5 sec
(3) Beam is swung to right at t = 2.52 sec
(4) Swung beam hits right of moon at t = 5.02 sec

Correct way of thinking:
Beam is swung from left to right in 0.02 sec
(1) Beam is fired to left of moon at t = 0.0 sec
(2) Beam is fired to right of moon at t = 0.02 sec
(3) Beam hits left of moon at t = 2.5 sec
(4) Beam hits right of moon at t = 2.52 sec

The "spot" (not a physical thing, remember) sweeps across the moon in 0.02 sec.

 

[gonegahgah]

Russ, I believe you.
What is the latency time between the back of an object changing speed and the front responding? Is it the speed of sound through that material / length?
Also interestingly from that, during acceleration the front of an object will always be traveling slower than the accelerated back as the front takes time to respond to the change of the back. Because the front is always moving slower that will mean that during acceleration an object will continue to compress throughout its length getting shorter and shorter for the duration of the acceleration; doesn't it?

Thanks Vanadium.
What are the relative speeds of the GPS satellites, the shuttle and the ground?
Just want to understand the -25 due to SR for the shuttle & -7 due to SR for the GPS.

Also no-one answered my question. Do weightless clocks experience the same time dilation due to GR as non-weightless clocks?
ie if you stick a clock on a giant tower at 350km up (has weight) and another clock in a shuttle in orbit at 350km up (has no weight) will they both experience the same time dilation due to GR (ignoring the SR component)?

 

[mgb_phys]

What is the latency time between the back of an object changing speed and the front responding? Is it the speed of sound through that material / length?

Yes

Also interestingly from that, during acceleration the front of an object will always be traveling slower than the accelerated back as the front takes time to respond to the change of the back.

Yes

What are the relative speeds of the GPS satellites, the shuttle and the ground?
Just want to understand the -25 due to SR for the shuttle & -7 due to SR for the GPS.

Shuttle is in orbit at 300km with a period of 90mins gives a speed around 7.7Km/s
GPS satelites orbit at 20,000km but with a period of 12hours so around 4km/s

Also no-one answered my question. Do weightless clocks experience the same time dilation due to GR as non-weightless clocks?

Yes, GR is a function of the distortion of space time. It doesn't care how heavy your clock is.

A clock in orbit isn't weightless, it's simply falling just the same as if it was in a falling elevator. It's mass is the same.

 

[Savant13]

No. Think of the laser like a hose. The photons coming out go in the direction the laser was pointed at the time of emission. If you rotate the laser afterwards, the photons it already emitted are going in the same direction.

 

[simondupnik]

It is certainly difficult to practically verify what the imaginary dot from the rotating laser beam would or would not do on the Moon. So please consider the following fact - reality check regarding feasibility of the transverse beam: in 1970 Tectronix produced an oscilloscope whose horizontal deflection (timebase) was progressing across the screen faster than light. If you are nor gasping at this marvel of practical engineering, I propose another fact for consideration: at the time when Einstein was producing his theory which included an arbitrary (and probably unfounded) limiting of the Nature's velocities to the value of 'c', the existence of the superluminal speeds (ie greater than 'c') was known and well established. Not only in the electromagnetic transmissions, but also in the normally pedestrian, by comparison, propagation speeds of the sound waves in the dispersive liquids (ultrasonic pulse in the water mixed with powder, to you and me).

 

[robbie7730]

mgb_phys is correct.


You are thinking:
(1) Beam is fired to left of moon at t = 0.0 sec
(2) Beam hits left of moon at t = 2.5 sec
(3) Beam is swung to right at t = 2.52 sec
(4) Swung beam hits right of moon at t = 5.02 sec

Correct way of thinking:
Beam is swung from left to right in 0.02 sec
(1) Beam is fired to left of moon at t = 0.0 sec
(2) Beam is fired to right of moon at t = 0.02 sec
(3) Beam hits left of moon at t = 2.5 sec
(4) Beam hits right of moon at t = 2.52 sec


The "spot" (not a physical thing, remember) sweeps across the moon in 0.02 sec.

yes your right, that's what i was thinking, for 2 reasons

1. a few drinks while reading it
2. i was thinking that because the original question was can a laser spot which
has reached its target be moved along faster than light,

And the spot has not really swept across the moon, its a series of separate spots reaching it from a single point.

0.02 seconds is double the time light would take to cross the moon diameter. Although that's only a matter of swinging the laser faster.

But the original question was can a solid spot at a target billions of miles away move faster than light by moving laser. Well if you pick a distance of a light year and rotate laser a full circle in one minute, the light leaving that laser will reach the 1 light year distance in a year and the spot will in theory cover the 6.3 light year orbit in 1 minute. This is the same as a star exploding and all places a light year away will see it at the sime time even though opposite sides are 2 light years apart

So who knows the real answer, If someone speaks on a radio, 2 planets a light year away in opposite directions (and so 2 LY`s from each other) will get the information at same time, in a year, so has information passed faster than light?

 

[Doc Al]

So who knows the real answer, If someone speaks on a radio, 2 planets a light year away in opposite directions (and so 2 LY`s from each other) will get the information at same time, in a year, so has information passed faster than light?

No, why would you think that information passed faster than light?

And why do you say "Who knows the real answer?", as if this is some controversial issue? Lots of people know the real answer!

 

[robbie7730]

No, why would you think that information passed faster than light?

And why do you say "Who knows the real answer?", as if this is some controversial issue? Lots of people know the real answer!

a now look at my reply, why would i put 3 question marks.

Maybe a bit more in depth reading of the other posts,

I am not asking if information, I am saying of course it does`t by putting 3 question marks.

And as for asking who knows the real answer,,, do you? You know for a fact the speed of light can't be passed? Who knows.

If one object is launched from one point and reaches 60 percent light speed, and the other goes in opposite direction at 60% also, they are in effect moving apart faster than light.
And how do we know when something is moving in space, its all relative. When something moves in space, what says its moving, relative to what?

 

[HallsofIvy]

a now look at my reply, why would i put 3 question marks.

Maybe a bit more in depth reading of the other posts,

I am not asking if information, I am saying of course it does`t by putting 3 question marks.

And as for asking who knows the real answer,,, do you? You know for a fact the speed of light can't be passed? Who knows.

People who have actually studied the subject, that's who.

If one object is launched from one point and reaches 60 percent light speed, and the other goes in opposite direction at 60% also, they are in effect moving apart faster than light.

No, that's clearly not true. if object A moves in one direction at 60% c (relative to the starting point) and object B moves in the opposite direction at 60%c (again relative to the starting point) then the speed of each, relative to the other, is about 88% the speed of light:
$$\frac{0.6c+ 0.6c}{1+ \frac{(0.6c)(0.6c)}{c^2}}= .88 c$$
nowhere near "faster than light". If you do not understand that calculation, then I recommend that you actually learn about physics before you talk about it.

And how do we know when something is moving in space, its all relative. When something moves in space, what says its moving, relative to what?

Because, people have done experiments with things moving in space. And "relative to what" is the whole point! A single object will have many different speeds relative to different frames of reference. The basic concept, based on experiment, is that the "laws of physics" must be true in any frame of reference and that leads to relativity.

Now my question is, if you really believe that no one can "know" anything, and that science is meaningless, as you imply, why in the world are you looking at a board like this one?

 

[robbie7730]

People who have actually studied the subject, that's who.


No, that's clearly not true. if object A moves in one direction at 60% c (relative to the starting point) and object B moves in the opposite direction at 60%c (again relative to the starting point) then the speed of each, relative to the other, is about 88% the speed of light:
$$\frac{0.6c+ 0.6c}{1+ \frac{(0.6c)(0.6c)}{c^2}}= .88 c$$
nowhere near "faster than light". If you do not understand that calculation, then I recommend that you actually learn about physics before you talk about it.


Because, people have done experiments with things moving in space. And "relative to what" is the whole point! A single object will have many different speeds relative to different frames of reference. The basic concept, based on experiment, is that the "laws of physics" must be true in any frame of reference and that leads to relativity.

Now my question is, if you really believe that no one can "know" anything, and that science is meaningless, as you imply, why in the world are you looking at a board like this one?

Where did i imply science is meaningless? I said only a couple of questions need to be asked before no one has the answer.

And you suggest i learn more about it before i comment, do you only talk about stuff your and expert in?

And yes i do understand your calculation as one object passing another both at c still only see the other passing at c.

But to show that formula as if that gives you more right to post here compared to someone that may not understand it, well ...

And why does light travel at the speed it does, o maybe i should`t be on this because i don't know that answer

 

[Doc Al]

a now look at my reply, why would i put 3 question marks.

Maybe a bit more in depth reading of the other posts,

I am not asking if information, im saying of course it does`t by putting 3 question marks.

Again I ask why do you think that information has passed faster than light? Saying "who knows?" is not an answer.

And as for asking who knows the real answer,,, do you? You know for a fact the speed of light can't be passed? Who knows.

If one object is launched from one point and reaches 60 percent light speed, and the other goes in opposite direction at 60% also, they are in effect moving apart faster than light.

According to the frame that measures their speeds as being 0.60c, the two objects separate at a rate of 1.2c. So? Still nothing is moving faster than light and certainly no information has flowed faster than light.

 

[robbie7730]

Again I ask why do you think that information has passed faster than light? Saying "who knows?" is not an answer.


According to the frame that measures their speeds as being 0.60c, the two objects separate at a rate of 1.2c. So? Still nothing is moving faster than light and certainly no information has flowed faster than light.

well they would be moving away from each other faster than light, say they were launced toward each other, they`d expect to pass at 1.2c

but that would`t happen and they`d only pass each other at 1c when looking at each other

and also look where you highlighted my comment in red,,,, i have ``does`t`` but you obviously did`t see the T

 

[robbie7730]

People who have actually studied the subject, that's who.


No, that's clearly not true. if object A moves in one direction at 60% c (relative to the starting point) and object B moves in the opposite direction at 60%c (again relative to the starting point) then the speed of each, relative to the other, is about 88% the speed of light:
$$\frac{0.6c+ 0.6c}{1+ \frac{(0.6c)(0.6c)}{c^2}}= .88 c$$
nowhere near "faster than light". If you do not understand that calculation, then I recommend that you actually learn about physics before you talk about it.


Because, people have done experiments with things moving in space. And "relative to what" is the whole point! A single object will have many different speeds relative to different frames of reference. The basic concept, based on experiment, is that the "laws of physics" must be true in any frame of reference and that leads to relativity.

Now my question is, if you really believe that no one can "know" anything, and that science is meaningless, as you imply, why in the world are you looking at a board like this one?

Again i have to say about ``maybe i should learn about physics before i talk about it``
How much about physics should someone learn before they are qualified to talk about it?

Lucky people down through history did talk about stuff they did`d understand.


If one object does actually head in one direction at 0.6c, how long will it take to reach a light year distance, and will it be a shorter time for the occupants than the observer, and if so why, and can we be certain of the answer based on vessels traveling at 0.00005c with an atomic clock on it.

I think I am just as entitled to ask as a person who see`s himself as more qualified and see`s others as lesser entitled until they ``learn about physics`` . It did`t even say learn more about physics,,,

Anyway, it was interesting

 

[Doc Al]

well they would be moving away from each other faster than light, say they were launced toward each other, they`d expect to pass at 1.2c

Again, according to the frame that sees them traveling at 0.6c towards each other, they will close at a rate of 1.2c. They do not travel at 1.2c with respect to each other.

but that would`t happen and they`d only pass each other at 1c when looking at each other

Their relative speed is 0.88c, as explained by Halls, meaning that they each see the other passing by at 0.88c.

and also look where you highlighted my comment in red,,,, i have ``does`t`` but you obviously did`t see the T

Ah...You meant doesn't. (You might want to pay more attention to punctuation and spelling if you want to be understood.)

 

[robbie7730]

yes 0.6 pass each other at 0.88

i meant if they both at 1c they pass each other at 1c as observed from each other.

the spelling, well maybe your right there

im surprised at bein told i should learn about physics before commenting though, just because he thought i might not understand that formula, anyone has the right to
comment i would of thought. Is the title not physics help and maths help.

Maybe it should be changed to forum for physics experts and mathematicians only

 

[gonegahgah]

Thanks mgb.

Shuttle is in orbit at 300km with a period of 90mins gives a speed around 7.7Km/s
GPS satelites orbit at 20,000km but with a period of 12hours so around 4km/s

Cool. The following is true isn't it?
To get to a higher orbit when in orbit a ship or satellite accelerates in the direction it is orbiting. To get to a lower orbit a ship or satellite decelerates against the direction it is orbiting. So I would have thought that the higher up you were the higher your velocity.
Obviously not by the numbers you've worked out, but how does it work?
Could I also ask your forbearance and get you to kindly insert the math for me, if you would please?

A clock in orbit isn't weightless, it's simply falling just the same as if it was in a falling elevator. It's mass is the same.

If you weigh the clock on the shuttle it will show as zero weight therefore it is weightless, I'm pretty sure of that but correct me if not; but I certainly agree that it still has the same amount of mass no matter how much it weighs. Please assure me or correct me on the difference between weight and mass.

Because the front is always moving slower that will mean that during acceleration an object will continue to compress throughout its length getting shorter and shorter for the duration of the acceleration; doesn't it?

mgb, Is this correct also?

 

[mgb_phys]

The period of an orbit is given by Keplers third law (sorry can't get the tex to work out right - see wiki for nicer formatting)

(Period/2pi)^2 = radius^3 / G(M+m)
M is the mass of the planet and m is the mass of the orbiting body - which you can ignore for a tiny spaceship orbiting the Earth. Remember that radius is from the centre of the Earth - so you need to add 6400km to the altitude.
The speed is then simply the distance = 2 pi r, divided by the period.

The reason I made the point about weightless is that many people get confused about a body in orbit and believe that it is weightless because there is no gravity in space. If the shuttle was a large distance away from the solar system in inter-stellar space there would be no gravity and object would indeed be weightless. Objects that are in orbit (like the shuttle) are weightless because they are falling but they still receive (almost) the same force of gravity on them as they would on earth.

The accelrating rod getting shorter would depend on the speed of the accelrating force. If one end is being pushed faster than the speed of sound in the material then as long as it keeps up this speed then the rod will get shorter. You can see this in explosively deformed materials and things like shaped charges.

 

[russ_watters]

well they would be moving away from each other faster than light, say they were launced toward each other, they`d expect to pass at 1.2c

They might, but only if they don't know anything about Relativity. If they do know about Relativity, they would not expect to pass each other at 1.2C.

I know that some say the object in orbit is falling, causing it to be weightless, but is it not more true to say the outward force away from the planet being orbited due to centrifugal force from traveling in a circular path is equal to the objects gravitational mass and so appears weightless.

No. The "centrifugal force" is often considered to be "ficticious" because it is a secondary reaction force and isn't always felt. If you are driving in a car, you feel the car pushing you around a corner. In the case of an orbit, you don't feel the space station pushing your toward earh. The only force is the centripetal force of Earth pulling you toward it.

It's a chicken or egg thing: You are moving in a circular path because the Earth is pulling at you, not the other way around.

You also mixed the concepts of force and mass there.

 

[russ_watters]

The accelrating rod getting shorter would depend on the speed of the accelrating force. If one end is being pushed faster than the speed of sound in the material then as long as it keeps up this speed then the rod will get shorter. You can see this in explosively deformed materials and things like shaped charges.

Expansion: Again, think of the rod as a spring. If you accelerate the rod, it will shorten the way a spring does until the force of the coiled spring equals the force of the acceleration and the pressure wave of the start of the acceleration propagates through it. Then it will continue to accelerate as a shortened rod.

If you try to accelerate a rod by hitting it with something going faster than the speed of sound in the rod, it will push through its own shock wave and permanently deform/be destroyed and the rules change a bit. In other words, there is nothing preventing a rod from moving faster than its own speed of sound, but one part of a rod cannot be moving at the speed of sound while an adjacent piece is stationary, without damaging it.

 

[pallidin]

If you try to accelerate a rod by hitting it with something going faster than the speed of sound in the rod, it will push through its own shock wave and permanently deform/be destroyed and the rules change a bit. In other words, there is nothing preventing a rod from moving faster than its own speed of sound, but one part of a rod cannot be moving at the speed of sound while an adjacent piece is stationary, without damaging it.

Russ, that reminds me of what I read on some legitimate military web-site. Sorry, I don't recall the link.
Anyway, it involves the use of a flat, circular tungsten disk placed in some type of barrel that contains a high explosive.
When the explosive goes-off, the force is so great that the tungsten disk is deformed(conveniently into a conical shape) and expelled through the barrel at over 7,000 mph.

Not sure if this was a production or experimental weapon at the time of that writing but I think it was experimental.

 

[mgb_phys]

It may have just been a high pressure experiment to create an unusual state of the metal.
You can make industrial parts by explosive forming, it's quite a good technique for small production runs where a high power press would be too expensive.
Basically you lay a piece of metal on top of a concrete or metal former shape, put a bit of C4 on top and surround the whole thing with water - then retire to a safe distance!

 

[madmike159]

I havn't read all of this post because its too long, so I'm sorry if some one has already asked this.
If you shon a laser on to the inside wall of a circle 1 light second away, it would have a circumference of about 60 light years. If your laser emmiter ratated at 1 rpm then would the laser light start to bend into a sprial because the point at the end would not be able to move as fast?

 

[Fredrik]

I havn't read all of this post because its too long, so I'm sorry if some one has already asked this.
If you shon a laser on to the inside wall of a circle 1 light year away, it would have a circumference of about 60 light years. If your laser emmiter ratated at 1 rpm then would the laser light start to bend into a sprial because the point at the end would not be able to move as fast?

The dot would move at about a light-year per second, but it wouldn't be at the location on the wall that the laser is aimed at. The shape of the beam would be curved and the dot would lag behind the laser. It would be a crazy spiral since the laser has turned over 500 thousand times when the dot starts moving.

 

[robbie7730]

i think the circumference would be 6.28 light seconds, or 1.8 million km`s roughly. I`d say the photons would form a spiral, they still travel in a straight line but i`d say they form a spiral as they radiate out from the spinning laser.

 

[robbie7730]

1 light year away would be just over 6 light years circumference

 

[mgb_phys]

I`d say the photons would form a spiral,

To be more precise - a line joining the position of all the photons at a point in time would form a spiral. It's important to remember that individual photons travel in a straight line (as robbie said)

 

[robbie7730]

To be more precise - a line joining the position of all the photons at a point in time would form a spiral. It's important to remember that individual photons travel in a straight line (as robbie said)

yes no doubt that's the more precise reality of it

 

[gonegahgah]

The reason I made the point about weightless is that many people get confused about a body in orbit and believe that it is weightless because there is no gravity in space. If the shuttle was a large distance away from the solar system in inter-stellar space there would be no gravity and object would indeed be weightless. Objects that are in orbit (like the shuttle) are weightless because they are falling but they still receive (almost) the same force of gravity on them as they would on earth.

Thanks mgb. I'm in a rush this morning so I'll have to study what you've written for me later. I just have a quick question.

I understand what you are saying that we can have weightless in a gravity field (falling) and weightless in the absense of gravity. That's ok. No problems with that.

Acceleration is supposed to be equivalent to gravity is it not? Does this apply in respect to time dilation as well? If it does then does the equivalent amount of acceleration produce the equivalent amount of time dilation as an equivalent amount of gravitation?

 

[Fredrik]

Acceleration is supposed to be equivalent to gravity is it not? Does this apply in respect to time dilation as well? If it does then does the equivalent amount of acceleration produce the equivalent amount of time dilation as an equivalent amount of gravitation?

Short answer: Yes.

Longer anwer: I think you should think of it the other way round. We can measure the acceleration by measuring the relative ticking rates of two clocks attached to opposite ends of a solid object. This would be a perfectly valid way to design an accelerometer.

Also, keep in mind that gravitational time dilation (which is equivalent to time dilation due to acceleration in flat spacetime) has an entirely different cause than the time dilation that's relevant when two clocks are moving with constant but different velocities in flat spacetime. In the latter case, which is the one that's easier to understand, both world lines are straight, but there's still time dilation due to the fact that different observers don't agree about which events are simultaneous. In the case of time dilation due to acceleration (either caused by gravity or by something else), the cause of the time dilation is that the world lines of the two clocks are curved by different amounts.

 

[gonegahgah]

Also, keep in mind that gravitational time dilation (which is equivalent to time dilation due to acceleration in flat spacetime) has an entirely different cause than the time dilation that's relevant when two clocks are moving with constant but different velocities in flat spacetime. In the latter case, which is the one that's easier to understand, both world lines are straight, but there's still time dilation due to the fact that different observers don't agree about which events are simultaneous. In the case of time dilation due to acceleration (either caused by gravity or by something else), the cause of the time dilation is that the world lines of the two clocks are curved by different amounts.

Thanks for jumping ahead on a question I was wondering Fredrik comparing relative speed time dilation (SR) and accelerated or gravitational time dilation (GR). So they are completely unrelated and can be treated separately?

SR time dilation produces a slowing of time as does GR time dilation. That is:
- if something is in greater gravity it will animate slower by our time.
- if something is under greater acceleration it will animate slower by our time.
- if something is going faster then it will animate slower by our time.
And this is accumulative isn't it?
ie. if you combine gravity with different relative velocity you sum their time dilation amounts.

 

[mgb_phys]

Thanks for jumping ahead on a question I was wondering Fredrik comparing relative speed time dilation (SR) and accelerated or gravitational time dilation (GR). So they are completely unrelated and can be treated separately?

Correct - see the above calcs for a shuttle and gps clock.

if something is in greater gravity it will animate slower by our time.
if something is under greater acceleration it will animate slower by our time.
if something is going faster then it will animate slower by our time.

Yes

ie. if you combine gravity with different relative velocity you sum their time dilation amounts.

Yes, that's why a shuttle clock runs slow and a gps clock runs fast overall.