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Superluminal motion

johnfosteruk

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johnfosteruk

johnfosteruk

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3 hours ago, ollypenrice said:

Once you 'get it' you get it, but it can tae a while to click!

I think Professor Merrifield's analogy with the laser pen helps with that very much.

 

11 hours ago, robin_astro said:

I remember following this phenomenon with  modified video and webcams  back in 2003-2005.

http://www.threehillsobservatory.co.uk/astro/astro_image_37.htm

I knew about it being a light echo but didn't realise the geometry made the echo appear to be moving outwards faster than light. Thanks for posting !

Robin

v838Mon_anim.gif

Great to see yours along with Hubble's Robin, nice one.

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ollypenrice

ollypenrice

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The explanation is indeed excellent. In fact the idea that you can cause a dot of light to trace across the surface of the moon at more than C, but not the end of a long metal bar, is a nice way to think about relativity.

Olly

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AKB

AKB

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Ah, so this seems to be exactly the difference between phase and group velocity in standard wave propagation theory.  You can see the effect also on the wave breaking on a beach: the wave is actually travelling in quite slowly, but hitting the beach at an oblique angle, so the breaking wave travels across the beach much, much faster.  Indeed, if the wave comes straight in to the beach, the 'phase velocity' across the beach is infinite. 

 

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ollypenrice

ollypenrice

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4 hours ago, AKB said:

Ah, so this seems to be exactly the difference between phase and group velocity in standard wave propagation theory.  You can see the effect also on the wave breaking on a beach: the wave is actually travelling in quite slowly, but hitting the beach at an oblique angle, so the breaking wave travels across the beach much, much faster.  Indeed, if the wave comes straight in to the beach, the 'phase velocity' across the beach is infinite. 

 

Choice: read up on 'Phase Velocity' or go back out and cut down more brushwood?

Phase velocity wins...

Thank you AKB!

Olly

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andrew s

andrew s

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As with Olly this is more interesting than, in my case, painting the conservatory.

I think the video is very good but the diagram of the laser pointer hitting the moon is misleading. in reality if you plot the trajectory of the light from the laser it will not be a straight line but a curve the shape of which will depend on the scanning speed.

 To see this consider that the  moon is about 1.3 light seconds away and the moon subtends about 1/2 degree so it would be quite simple to scan the laser across the moon several times a second. To take a simple case say you scan across from one side to the other in 1.3 seconds so by the time first light to leave the laser is hitting the moon you are pointing at the other side. The "illuminated" track off the laser (i.e. the red path shown in the video) must be a curve between these points. 

NB I have assumed a flat space time between the earth and the moon for simplicity and this assumption was good enough for the moon landings! Besides I could not work it out otherwise.

Regards Andrew

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ollypenrice

ollypenrice

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8 minutes ago, andrew s said:

As with Olly this is more interesting than, in my case, painting the conservatory.

I think the video is very good but the diagram of the laser pointer hitting the moon is misleading. in reality if you plot the trajectory of the light from the laser it will not be a straight line but a curve the shape of which will depend on the scanning speed.

 To see this consider that the  moon is about 1.3 light seconds away and the moon subtends about 1/2 degree so it would be quite simple to scan the laser across the moon several times a second. To take a simple case say you scan across from one side to the other in 1.3 seconds so by the time first light to leave the laser is hitting the moon you are pointing at the other side. The "illuminated" track off the laser (i.e. the red path shown in the video) must be a curve between these points. 

NB I have assumed a flat space time between the earth and the moon for simplicity and this assumption was good enough for the moon landings! Besides I could not work it out otherwise.

Regards Andrew

Agreed, but I thought that the point of the 'laser on the moon' experiment was simply to show that no physical object was moving. Maybe I'm mising a wider point? The key for me point was the variety in light paths from the same source and consequent light arrival time.

Olly

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andrew s

andrew s

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13 minutes ago, ollypenrice said:

Agreed, but I thought that the point of the 'laser on the moon' experiment was simply to show that no physical object was moving. Maybe I'm mising a wider point? The key for me point was the variety in light paths from the same source and consequent light arrival time.

Olly

and

13 minutes ago, Stub Mandrel said:

Of course the fun thing is that is a light spot races towards you over the moon's surface faster than the speed of light, you see it moving away from you instead...

I agree I was just being picky. However, having though about it what if you replaced the laser with a very rapid fire, 1000s of rounds per second, Gatling gun then (neglecting issues of escape velocity etc. etc.) the impact of the bullets would race across the moon just as the laser did at superluminal speed would they not. Just as the bullets would be spaced out so would the photon detection for a finite laser power.   What I think this shows is that a spatially separated sequence of events can appear as superluminal motion however it is triggered. Maybe I am mistaken. 

I do agree it can't be done by a solid rod.

Regards Andrew

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ollypenrice

ollypenrice

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48 minutes ago, andrew s said:

and

I agree I was just being picky. However, having though about it what if you replaced the laser with a very rapid fire, 1000s of rounds per second, Gatling gun then (neglecting issues of escape velocity etc. etc.) the impact of the bullets would race across the moon just as the laser did at superluminal speed would they not. Just as the bullets would be spaced out so would the photon detection for a finite laser power.   What I think this shows is that a spatially separated sequence of events can appear as superluminal motion however it is triggered. Maybe I am mistaken. 

I do agree it can't be done by a solid rod.

Regards Andrew

...but if we fired the Gatling gun through two slits...

:icon_mrgreen:lly

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andrew s

andrew s

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9 minutes ago, ollypenrice said:

...but if we fired the Gatling gun through two slits...

:icon_mrgreen:lly

...we would have some very badly messed up slits. Care to try this on your double Takahashi rig?

Regards Andrew

PS You can tell when a thread has come to the end of it's useful life it diffracts off into levity.

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SilverAstro

SilverAstro

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7 hours ago, ollypenrice said:

...but if we fired the Gatling gun through two slits...

:icon_mrgreen:lly

Point of order,

I think the Gatling gun should remain fixed, relatively,

it's the bullets that we need to know the deBroglie wavelength of ?

 

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