Silly supersonic question

[tank slapper]

Ginetta G15 Girl said:

As I understand it PD RADARS rely on the fact that a moving target, that has a coming towards you or moving away from you vector, causes a Doppler shift in the wavelength of the returned target signal compared to the other returns, and it is this that allows them to break a target out from the ground clutter (because the ground isn't moving). Thus giving you the option of 'Look Down/Shoot Down'.

A counter to a PD RADAR is to turn on a 90 degree track to the interceptor - there is now no Doppler shift and the target a/c disappears.

Obviously it's a wee bit more complex in real life.

I recall reading an account of an air combat training exercise, where the pilot of one aircraft attempted to avoid detection on the opponents radar by diving at the appropriate angle to maintain a constant range. He thought he had been successful, but in fact the opposing pilot had anticipated the move and switched off his radar when he saw them make the dive to trick them into thinking he had fallen for it. I can't remember what that was in - it may have been Sharkey Ward's book, but I just had a look and couldn't find it.

[Mr E]

Simpo Two said:

That would imply that if you put enough diamonds together, the photons will eventually stop and can be swept up with a brush and dustpan...

Not really, it'll just move through all the diamond at about .41C

The slowest photons have been reduced to is something like 60kph through sodium as a bose-einstein condensate near absolute zero.

I can ride a bike faster than that (briefly)



Physics. Everything you ever leant at school was lies to children. Modern research physics is probably slight more accurate and complex lies to adults. It's all bloody weird.



Edit to add;

Simpo Two said:

If you have a stationary photon is it still light? Can you see it?

You don't see photons. You see the result of photons hitting things.

Edited by Mr E on Friday 23 July 09:55

[Eric Mc]

I bet the OP wished he never asked the question now.

[Simpo Two]

eharding said:

In much the same way that the little '30' repeater speed limit signs are placed at intervals after the big '30' speed limit sign, it means you can keep going at 30, until you see the NSL speed limit sign, when you can accelerate to the speed of light again.

I see, so the photons, when emerging form the diamond, suddenly speed back up to 'c'? Where do they get the extra energy from?

[Eric Mc]

That is why Einstein was a genius and we aren't.

Edited by Eric Mc on Friday 23 July 10:24

[tank slapper]

Simpo Two said:

I see, so the photons, when emerging form the diamond, suddenly speed back up to 'c'? Where do they get the extra energy from?

Unsurprisingly, it isn't as simple as that. You have to remember that photons have no mass, and due to quantum electrodynamics weirdness light sometimes displays the properties of a wave and sometimes a particle. The change in speed of light (and I am probably going to get this wrong, it's been a while since I studied physics) is due to the interaction of the wave with the electromagnetic properties of the medium it is travelling through causing a slight change in wavelength. As the frequency remains the same, the velocity must alter. How much by, depends on the refractive index of the material, with different frequencies being altered by different amounts (eg a prism).

[JuniorD]

OK, here's one can a radio wave be slowed down significantly (>10000mph) without loss of signal stength?

[tank slapper]

JuniorD said:

OK, here's one can a radio wave be slowed down significantly (>10000mph) without loss of signal stength?

I don't know if anyone has slowed down a radio wave to that extent at all regardless of signal strength. I think most of that type of experiment have been done with lasers, which are of a much higher frequency.

I would have thought that the answer is, that whenever you pass a wave through a medium there would be attenuation to some degree, because there will be scatter. The extent of it will depend on lots of different things - some materials are opaque and transparent at different frequencies for example.

Edit - I misread your question: In comparison to c, 10,000mph isn't that much at all, so it would depend on the refractive index of the material it passes through so to slow it down by that much may not be too significant. I initially read it as slowed down to 10,000mph.

Edited by tank slapper on Friday 23 July 12:26

[JuniorD]

tank slapper said:

JuniorD said:

OK, here's one can a radio wave be slowed down significantly (>10000mph) without loss of signal stength?

I don't know if anyone has slowed down a radio wave to that extent at all regardless of signal strength. I think most of that type of experiment have been done with lasers, which are of a much higher frequency.

I would have thought that the answer is, that whenever you pass a wave through a medium there would be attenuation to some degree, because there will be scatter. The extent of it will depend on lots of different things - some materials are opaque and transparent at different frequencies for example.

Edit - I misread your question: In comparison to c, 10,000mph isn't that much at all, so it would depend on the refractive index of the material it passes through so to slow it down by that much may not be too significant. I initially read it as slowed down to 10,000mph.

Edited by tank slapper on Friday 23 July 12:26

My mistake, I meant slowed to less than than say 10000mph or even 500,000mph for that matter. But over a good distance in air. It's a silly question anyway, I can't see any purpose for a radio wave that could travel continents at rocket speeds.

[dilbert]

JuniorD said:

tank slapper said:

JuniorD said:

OK, here's one can a radio wave be slowed down significantly (>10000mph) without loss of signal stength?

I don't know if anyone has slowed down a radio wave to that extent at all regardless of signal strength. I think most of that type of experiment have been done with lasers, which are of a much higher frequency.

I would have thought that the answer is, that whenever you pass a wave through a medium there would be attenuation to some degree, because there will be scatter. The extent of it will depend on lots of different things - some materials are opaque and transparent at different frequencies for example.

Edit - I misread your question: In comparison to c, 10,000mph isn't that much at all, so it would depend on the refractive index of the material it passes through so to slow it down by that much may not be too significant. I initially read it as slowed down to 10,000mph.

Edited by tank slapper on Friday 23 July 12:26

My mistake, I meant slowed to less than than say 10000mph or even 500,000mph for that matter. But over a good distance in air. It's a silly question anyway, I can't see any purpose for a radio wave that could travel continents at rocket speeds.

That's no mistake.

There are a wealth of uses for radio waves that can travel both quickly and slowly. The idea that the signal can be slowed, is critical to things like filters. For example, the capacity to slow radio waves can be used to select between radio one and two. As it happens it's not quite done that way, but it doesn't matter because the way they do do it, can't be used to filter light, which is almost as important.

The answer is yes.

Edited by dilbert on Friday 23 July 16:00

[navier_stokes]

Eric Mc said:

Did you fall asleep in Physics?

Sound is progated through a medium - such as air or water. It's speed varies depending on the density of that medium but rarely exceeds 700 mph or so.

Not strictly true, for a gas, speed of sound is related directly to temperature, not density.

[Eric Mc]

There will always be finer points to make.

But the important thing is that the speed of light can vary.

The OP obviously lacked even the most basic understanding of the difference between the speed of sound and the speed of light.

Some of the posts on this thread would bamboozle Stephen Hawking.

[BMWChris]

Eric Mc said:

There will always be finer points to make.

But the important thing is that the speed of light can vary.

The OP obviously lacked even the most basic understanding of the difference between the speed of sound and the speed of light.

Don't bring me into this. My question was answered in the third post. However, as the OP I object to the tone of this reply. I would have thought that the most basic difference between the speed of sound and the speed of light is that one is fast and one is very fast.

Seem to have understood that ok.

Edited by BMWChris on Friday 23 July 19:07

[dilbert]

Eric Mc said:

There will always be finer points to make.

But the important thing is that the speed of light can vary.

The OP obviously lacked even the most basic understanding of the difference between the speed of sound and the speed of light.

Some of the posts on this thread would bamboozle Stephen Hawking.

The important thing, is that to the casual observer, the speed of light can appear to vary.
The more important thing, is that to the critical observer, the speed of light does not change.

Edited by dilbert on Friday 23 July 19:58

[Semi hemi]

BMWChris said:

Eric Mc said:

There will always be finer points to make.

But the important thing is that the speed of light can vary.

The OP obviously lacked even the most basic understanding of the difference between the speed of sound and the speed of light.

Don't bring me into this. My question was answered in the third post. However, as the OP I object to the tone of this reply. I would have thought that the most basic difference between the speed of sound and the speed of light is that one is fast and one is very fast.

Seem to have understood that ok.

Edited by BMWChris on Friday 23 July 19:07

You live & learn.
Google might be harder to track down specific info on, and Wiki may possibly be inaccurate... but unlike PH neither will be condecending or downright insulting.

[Simpo Two]

BMWChris said:

Do radio signals travel at the speed of sound?

Eric Mc said:

The OP obviously lacked even the most basic understanding of the difference between the speed of sound and the speed of light

Eric was,as ever, simply being factual. It's a fundamental mistake, but people just can't bear to be told they're ignorant.

760 miles an hour
vs
186,000 miles per second

Somewhere between general knowledge and 'O' level physics. And so now you know Chris

Edited by Simpo Two on Friday 23 July 20:08

[Eric Mc]

One of the first things I ever learned in science.

[Mr E]

Eric Mc said:

One of the first things I ever learned in science.

I learnt to check the bunsun was off before reaching over it.

[dilbert]

Simpo Two said:

BMWChris said:

Do radio signals travel at the speed of sound?

Eric Mc said:

The OP obviously lacked even the most basic understanding of the difference between the speed of sound and the speed of light

Eric was,as ever, simply being factual. It's a fundamental mistake, but people just can't bear to be told they're ignorant.

760 miles an hour
vs
186,000 miles per second

Somewhere between general knowledge and 'O' level physics. And so now you know Chris

Edited by Simpo Two on Friday 23 July 20:08

Factoidial?

Edited by dilbert on Friday 23 July 21:02

[ATG]

dilbert said:

Eric Mc said:

There will always be finer points to make.

But the important thing is that the speed of light can vary.

The OP obviously lacked even the most basic understanding of the difference between the speed of sound and the speed of light.

Some of the posts on this thread would bamboozle Stephen Hawking.

The important thing, is that to the casual observer, the speed of light can appear to vary.
The more important thing, is that to the critical observer, the speed of light does not change.

Edited by dilbert on Friday 23 July 19:58

The speed of the propogation of electromagnetic waves does change ... em wave prop being a perfectly reasonable definition of the "speed of light". "c", the constant that turns up in special relativity, is known as the "speed of light" ... but is primarily a property of space time rather than light. Thus we need to be careful about what we mean when we see "speed of light".