Lifespan of Light

jungle11 (Greg) · #1 17-11-2008, 08:30 PM

There's proberly a couple of questions here, but my main one:

When you travel at lightspeed, time stops right? So why do lightwaves weaken over distance? Shouldn't the light be as bright as when it was emitted - regardless of distance travelled - because no time has passed for the light to be degraded during its travel (or whatever the technical term is)

take it easy on me guys!

bojan · #2 17-11-2008, 08:37 PM

photons do not weaken...
You are confusing flux (number of photons/square, intensity of source as we see it) and photon energy.

Light source emits a certain number of photons per unit of time.
This flow of photons produces a flux at some distance, and this will be lower as distance increases (inverse square law) .. but the energy of each individual photon will remain the same.
I am not taking into account the expansion of universe here :-)
I believe you haven't thought about it either...

jungle11 (Greg) · #3 17-11-2008, 08:49 PM

Quote:

Originally Posted by bojan

I am not taking into account the expansion of universe here :-)
I believe you haven't thought about it either...

You'd be right there.

I should have realized that light scattered, so totally obvious!

Thanks Bojan

[1ponders] (Paul) · #4 17-11-2008, 10:28 PM

Re the initial thread heading. Someone might like to check, but from memory doesn't a photon have a half life of something like 6.5 billion years.

bojan · #5 18-11-2008, 06:57 AM

and it decays in photon :-)

jungle11 (Greg) · #6 18-11-2008, 10:07 AM

Is that measured by the distance over which we can resolve visible light?
Also, with my relative viewpoint from earlier, wouldn't such a process of decay occur instantaneously to the photon?
How can any change occur to the wavelength of the photon instantaneously?

bojan · #7 18-11-2008, 10:43 AM

Quote:

Originally Posted by jungle11

Is that measured by the distance over which we can resolve visible light?
Also, with my relative viewpoint from earlier, wouldn't such a process of decay occur instantaneously to the photon?
How can any change occur to the wavelength of the photon instantaneously?

Sorry mate I was only joking :-)
Photon is stable and does not decay. We can detect it even if it is coming from near the edge of the visible Universe.
What is happening, thought, wavelength is changed (it becomes longer) because of expansion of the universe (similar to Doppler effect), so the photons from the edge of the visible Universe would have wavelengths infinitely long (frequency would have been zero, energy also zero).

jungle11 (Greg) · #8 18-11-2008, 11:29 AM

Thanks, that seems to make more sense.

I was reading awhile ago about experiments in quantum comunication using photons. Aparently these comms could be 'undecipherable' to unwanted eavesdroppers. I think they've only managed it over small distances because they are sending and receiving through an atmosphere, but considering what you said about the photon's stability, you could communicate all over the galaxy with this method. Pretty cool stuff.

Someone should tell SETI!

bojan · #9 18-11-2008, 01:17 PM

You are talking about communication using so called "entangled particle pairs".
Yes this is interesting, but, mind you we are using photons for communication already.
Laser light are photons.. and so are radio-waves (only those latter have very low energies compared to their visible siblings)

xelasnave · #10 20-11-2008, 01:27 PM

I am not trying to be cute or disrespectful but I have been thinking about all of this and must ask

.....When one throws a pebble in a pond the waves seem to legthen the further from the impact point ...I will go to the dam and check that my recollection is correct... but the pond is not expanding and yet the lenghting of the wave is apparent...the further the wave travells the more the wave lenghtens so why would something similar not apply to a light wave thru space...I mean if we took away the notion that the Universe is expanding does not the pond example point to a different explaination as to why the wave changes over time... as I said the pond does not expand but the wave lenghtens...
Could it be that similar happens with light in the Universe

...
alex

bojan · #11 20-11-2008, 03:03 PM

Alex,
surface of water and space are completely different media, so you can not compare them to such extent.
The main differences are:

1) water surface is 2D medium, space is 3D (but this is not so important)

2) water surface is lossy media - that means that energy of waves is dissipated to heat (because water molecules are moving against each other as wave propagates).

Much better analogy may be a surface wave on solids. Very good example is application of effect called SAW filter: Surface Acoustic Wave Filter, widely used in communication industry.
Here, the losses due to heat dissipation are almost null, as the quartz (used as a substrate) is almost ideally elastic (means you can apply a pressure on it and it will deform, but when the pressure is removed it will bounce back to the original shape).
SAW filter would not work as it does if wavelength would be changing as the wave travels across its surface

Now, I am not sure if your observation (waves on water are changing wavelength) is correct, but it may be.. I will check this at some stage :-)
The space behaves much more like quartz (in terms of losses and wavelength). It behaves like it is ideally elastic (Maxwell, energy preservation etc). Where would that energy loss go? If photons are loosing energy, it must go somewhere..
And NOTHING like this was EVER observed.
So, no Alex, photons do not get tired... And the space-time expands since the BB, and it will go on expanding... until... (?)

xelasnave · #12 20-11-2008, 04:06 PM

but at the scale of the filter would one notice any loss of energy? and I will bet that if one measured the "efficiency" of the filter one may find a loss...may... I say... but most things seem to show a loss of "efficiency" I thought ...like a car motor..which is lost I think in mechanical form...

But certainly if an eather and all the premises were accepted (relationship in some way to light) I could see how that would change everything because then light would be in the "lossy" media or the eather... and so it could lose energy that way.... I am not saying I support the eather either but you know I like the idea but what I do like is simply learning and considering alternatives... but I can see why the eather would present a problem on the first look for universe expansion...

I read all of Maxwells lectures ages ago and it is so interesting how some things seem to fit (when I am in the Big Bang Universe) I mean coming together for me.

I want to understand all the forces as best I can..I am trying to understand how a reactor works and and I am working on a question because of an impass in my reading...I have found out most things to keep me happy but I can not find how they actually start a chain reaction in a fission reactor...but my point it is wonderful the net we are so lucky to live at such a time in history.

alex

xelasnave · #13 20-11-2008, 04:09 PM

What does a photon look like anyways.
alex

Zuts · #14 20-11-2008, 04:30 PM

Quote:

Originally Posted by xelasnave

What does a photon look like anyways.
alex

Hi,

I dont think this is a meaningfull question. Light exhibits both particle and wavelike properties.

In any case, by example what does water look like, nothing like a H2O molecule yet this is water. How can human beings possibly know what a water molecule let alone a photon actually looks like. The question is absolutely meaningless.

However just because we cant know what it looks like does not mean we cant know how it behaves and create predictive theories like quantum mechanics which while based in mathematics at least allow us to describe many properties and interactions with the real world.

Cheers
Paul

jungle11 (Greg) · #15 20-11-2008, 04:42 PM

Quote:

Originally Posted by xelasnave

I want to understand all the forces as best I can..I am trying to understand how a reactor works and and I am working on a question because of an impass in my reading...I have found out most things to keep me happy but I can not find how they actually start a chain reaction in a fission reactor...but my point it is wonderful the net we are so lucky to live at such a time in history.

alex

I agree, the internet is the only thing that keeps me sane - until, like you, I wonder 'hmm...how does that work' - which then drives me insane

I think Bojan is right in that there is no real shortcut to fully understanding some of these concepts. University is the best way, but expensive and maybe doesn't fit in to your life to well.
But most things are probally floating out there on the net somewhere - the hard part is knowing what things you need to know (might not seem to be related to the question at all, first look) and how they fit together in practice and in theory.

I'm up to newton's laws of motion and gravitation in Astronomy 161, and i think im actually getting the hang of the expressions and equations now. I'm finding it real enjoyable too, thinking maybe i might even have a crack at astronomy at uni in the near future.

Here's hoping anyway

bojan · #16 20-11-2008, 04:47 PM

Yes.. meaningless question, totally.
Things "look like" because light (photons, light waves) bounce from them and enter our eyes where the image of them is formed by eye lens...
So for a photon to "look like", we have to illuminate it with light (but how??? photons do not bounce from other photons?!) to be able to see it...
A bit easier problem is to try to see other particles... but then uncertainty principle comes in a way.. and higher resolution we want, the shorter wavelengths we have to use, so the object we are trying to see will be more disrupted (because shorter the wavelength, higher the energy of collision, if I may use this "mechanistic" term here, and I know it is not appropriate...
Alex, your thinking is too much aligned with our everyday macro world. In order to understand micro-world, you have to abandon everyday common sense and start thinking in terms of quantum mechanics. Otherwise, you will always be confused.

xelasnave · #17 20-11-2008, 04:55 PM

Zuts and Bojan
Yes the duality thing

...but when it is a particle I wondered if it had a form.. as a particle I presumed it a physical reality... so if a physical thing I wondered about what it may look like.

Your H2O example does not help now that has me wondering also

It is wonderful we (folk other than me) can work on such things...

I would love to be able to visit that world and comprehend all I see... I mean imagine being able to actually see what any of the particles looked like ....I know it can never happen in such a way and anyway everything would be travelling so fast you could not see it...

I really posted that line because at the time I was thinking what I now explain and hoped others may comptemplate what it may look like

.

But I read that by observing it changes everything as far as particles go so it is an imagination trip

.

ANyways with the rain here I think I will be thinking about the H20 molecule more than photons

.

Thanks for your input hope the weather is kind at your place

.

alex

xelasnave · #18 20-11-2008, 05:00 PM

AND Bojan I am trying but I am old ..which I am proud to have made it this far..but that does place me at numerous disadvantages butyou would not believe how far I have come..at least I know what confusses me... and so I go on... I will get there, I am patient and I have no wild ideas of finding anything that will change the outside world but my world is constantly growing because I am becoming aware of more and more things... once all I could think about was girls...so I am ahead I reckon.

alex

xelasnave · #19 20-11-2008, 05:11 PM

Greg...

That is wonderful that you are developing yourself.

I read a fair bit and probably know I little more than I let on sometimes so I probably look like an idiot to most and that is reasonable what else could you think given my presentation....

I do like math to be honest but have not gone near it for over a decade but I used to use spread sheet for years and that sort of corrupted my learning because I know there is a real simple way if I need it as far as math goes...

I go to sleep by drawing house plans in my head or a boat ..whatever and then work out the material list, the number of nail, timber cement sand etc etc down to how much paint at a certain paint thickness, and when I play guitar it is all done by maths really in so far it is all number relation ships recurring pattens and the like.. I dont play a cord as CFG but as the numbers the notes are..major chord is 1 3 and 5 of the major scale and then there are heaps of numerical relationships to give you all the chords...I never know what note I play only the numbers.

But I dont need to go to University as it would take the place of someone more worthy and with life ahead of them to help humanity.
I like the net I can go to any University on the planet..I can go into the work shops aat NSSA and look around I can go into a reactor and have a look..so I am happy..

Thank you for your thoughts

alex

jungle11 (Greg) · #20 20-11-2008, 05:56 PM

Quote:

Originally Posted by xelasnave

Greg...
I read a fair bit and probably know I little more than I let on sometimes so I probably look like an idiot to most and that is reasonable what else could you think given my presentation....
alex

Alex, I'm sure nobody here - definately not me - thinks you're an idiot. You certainly know more about this stuff than i do. Anyway's, I flunked maths at school - that was why i was so happy to be picking up equations and (beginning to) understand them.

cheers

greg