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Author Topic: could the light spot on an osciloscope move faster than c  (Read 6270 times)

Online syhprum

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I think it could but I would like other opinions


 

Offline DoctorBeaver

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could the light spot on an osciloscope move faster than c
« Reply #1 on: 04/01/2009 15:11:37 »
No. The trace you see on the scope is produced by electronics. Many steps need to be taken between the scope's input and showing the trace on the screen. The transfer of signals between the electronic components to make those steps happen cannot be faster than c. In fact, it is considerably less than c. I'm not sure what the fastest speed attainable in a scope is, but maybe someone here knows.
 

Offline lightarrow

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could the light spot on an osciloscope move faster than c
« Reply #2 on: 04/01/2009 15:29:33 »
Sincerely I don't know details of an oscilloscope's functioning, but DoctorBeaver should be correct.
 

Online syhprum

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could the light spot on an osciloscope move faster than c
« Reply #3 on: 04/01/2009 15:43:22 »
The moving light spot does not need to be produced by electronic means it could we be a laser beam reflected from a rotating mirror (oscilloscope's and TV displays working on this principal were in use well into the 1940,s)I refer here only to the speed of the horizontal deflection. 

 

Offline Pumblechook

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could the light spot on an osciloscope move faster than c
« Reply #4 on: 04/01/2009 15:44:26 »
Scopes traditionally used electrostatic plates in a CRT to deflect an electron beam.  Modern way is TV display, more than likley LCD, which produces a digitally produced 'artists impression'.  

The engineering restrants would severly limit the deflection circuits and plates in the CRT.  The frequency respone wouldn't allow anything like the frequency required but thinking about the problem abstractly it may be possible for the trace to move faster than light because it is an illusion.   Nothing is moving in the direction of the trace.  The electron beam is just hitting a different spot on the screen because the beam is being deflected at varying angles. 

If you think of a theoretical CRT which is many km long you wouldn't need to bend the electron much or very fast for the trace to more a long way and very fast on the screen.  

Think of the sea reaching a wall which is not quite parallel to the sea.. The sea would hit the wall at one end before the other and the speed of the point where the sea hits could be faster than light????
« Last Edit: 04/01/2009 15:48:36 by Pumblechook »
 

Offline DoctorBeaver

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could the light spot on an osciloscope move faster than c
« Reply #5 on: 04/01/2009 16:05:33 »
The moving light spot does not need to be produced by electronic means it could we be a laser beam reflected from a rotating mirror (oscilloscope's and TV displays working on this principal were in use well into the 1940,s)I refer here only to the speed of the horizontal deflection. 


In which case it would not be an oscilloscope; it would just be a reflected beam of laser light. A scope takes an input signal, processes it, and displays a trace on the screen that represents the input. The input cannot be processed fast enough for the trace to move faster than light.
 

Online syhprum

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could the light spot on an osciloscope move faster than c
« Reply #6 on: 04/01/2009 17:08:39 »
I checked the writing speed of my oscilloscope and found it to be .0001 c, this is a modest instrument and no doubt a high class machine would be at least ten times as fast.
As Pumblechook suggests if the half meter tube could be extended to 500m a writing speed of c could be obtained although focusing might be a problem.
« Last Edit: 04/01/2009 17:10:30 by syhprum »
 

Offline DoctorBeaver

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could the light spot on an osciloscope move faster than c
« Reply #7 on: 04/01/2009 17:13:43 »
As Pumblechook suggests if the half meter tube could be extended to 500m a writing speed of c could be obtained although focusing might be a problem.

I'm dubious about that. But even so, it could not exceed c.
 

Offline Pumblechook

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could the light spot on an osciloscope move faster than c
« Reply #8 on: 04/01/2009 17:54:25 »
I am thinking about a gun which fire bullets at 3 targets (A, B and C) which are at the same distance but spaced at 5 deg.   The gun fires at A rotates 5 deg and fires at B and then rotates 5 more deg and then fires at C.    Say it does this very rapidly within one sec.  If the targets are at a distance where the spacing between A and C is more than 300,000 km then the ripple effect or writing speed has travelled at more than C. 
 

Offline DoctorBeaver

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could the light spot on an osciloscope move faster than c
« Reply #9 on: 04/01/2009 17:57:28 »
If what you're saying is true, then we could communicate faster than light. I'm sure that is forbidden by the laws of physics.
 

Offline Pumblechook

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could the light spot on an osciloscope move faster than c
« Reply #10 on: 04/01/2009 18:03:15 »
  It is just an illusion.  Nothing is actually moving from A to C.  Nothing phyical.  No laws of physics broken.  The bullets could travel very slowly but it would still be true.  There is no 'communication' between A and C.
« Last Edit: 04/01/2009 18:05:01 by Pumblechook »
 

Offline DoctorBeaver

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could the light spot on an osciloscope move faster than c
« Reply #11 on: 04/01/2009 18:09:16 »
But if you spell out a message like that then it could be reflected back to you (at c, obviously) and someone standing next to you could read it. He would get the message faster than it could have been sent to him direct had he been twice the distance away as the targets.
 

lyner

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could the light spot on an osciloscope move faster than c
« Reply #12 on: 04/01/2009 18:31:25 »
This light speed thing is getting out of hand. You and I  (separated) can receive a message from a distant point at the same time. BUT I couldn't tell you I had received it any quicker than light could get from me to you.
The spot on a scope screen is just a record of having received an electron. The 'moving' spot is only a set of images which are generated in sequence. Nothing violates anything here.
 

Offline Pumblechook

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could the light spot on an osciloscope move faster than c
« Reply #13 on: 04/01/2009 18:37:25 »
Didn't really follow that.  If a message was sent on a bullet it would travel slowly and if the bullet bounced back it would travel back slowly but the ripple form A to B to C is still faster than light.  

I am thinking now of a radar which scans the sky at a rate of about 1 rev per second.  You can see the Moon on radar and at that distance it would be scanning the sky faster than the speed of light..... A circle of circumference of 2.5 million km in one second.  It would be scanning the surface of the Moon faster than light.

« Last Edit: 04/01/2009 18:41:40 by Pumblechook »
 

Offline LeeE

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could the light spot on an osciloscope move faster than c
« Reply #14 on: 04/01/2009 19:07:27 »
This is a bit like the waves washing up upon a straight beach.  If you start with the waves coming in parallel to the beach each wavefront will travel along the beach at the speed of the wave.  As the wind direction shifts though, and the waves start to come in at an angle to the beach, the wave fronts now move along the beach at a higher speed than the waves are actually moving.  When the wind direction shifts even further, so that it is blowing directly towards the beach, each wave front now hits every point along the beach at the same time and the effective speed of the wave front along the beach is now infinite.

Perhaps another way of looking at it is to consider an omni directional radio transmitter antennae; all the receivers at the same radius from the antennae, regardless of their direction, will detect any signals from it simultaneously.
« Last Edit: 04/01/2009 19:11:37 by LeeE »
 

Online syhprum

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could the light spot on an osciloscope move faster than c
« Reply #15 on: 04/01/2009 19:29:37 »
The Picoscope 9200 sampling oscilloscope has a maximum timebase speed of 10pS per division which equates to approximately 2.5 c.
This is of course only an illusion like the waves breaking on the sea shore
 

lyner

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could the light spot on an osciloscope move faster than c
« Reply #16 on: 04/01/2009 19:51:03 »
We all seem to be agreeing.







NEXT
 

Offline Madidus_Scientia

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Offline DoctorBeaver

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could the light spot on an osciloscope move faster than c
« Reply #18 on: 05/01/2009 10:30:55 »
I'm confused again now
 

Online syhprum

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could the light spot on an osciloscope move faster than c
« Reply #19 on: 05/01/2009 11:18:59 »
Dear doc
if it oscilloscopes you are confused about let me explain about the two different types, first we have analog where the input signal is displayed in realtime where the writing speed is limited by how fast a waveform can be supplied to the defletion plates and on a single sweep how intense an electron beam can be used, these are limited in readily available models to about .1 c.
Then there are sampling oscilloscopes that can only display continually repeating signals, these use a very short sampling pulse at a slightly different frequency to the input signal the result can be displayed on a low performance analog oscilloscope and the apparent scanning speed can be well over c
I beg pardon if I have bored everone with these technicalities

Since I started to air my knowledge of oscilloscope design I find I am about 25 years out of date, see this URL for a more authoritative account

http://en.wikipedia.org/wiki/Oscilloscope
« Last Edit: 05/01/2009 20:35:43 by syhprum »
 

Offline Pumblechook

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could the light spot on an osciloscope move faster than c
« Reply #20 on: 05/01/2009 11:40:49 »
I would imagine CRT scopes will soon be museum pieces. 

 

lyner

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could the light spot on an osciloscope move faster than c
« Reply #21 on: 05/01/2009 12:28:15 »
I'm confused again now
That thread goes on and on about the same point; just because you assume someone is doing something there is not necessarily any information reaching you that they actually ARE doing it. The actual information speed is limited by c but for the assumed information, there is no limit.
All these though experiments are based on the same idea.
 

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could the light spot on an osciloscope move faster than c
« Reply #21 on: 05/01/2009 12:28:15 »

 

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