Light Photons and how do they go on for billions of years at light speed??

johnwest288

Ok so Im trying to get my head around Light Photons...

Ok so the Hubble deep field picture shows stars that are about 13 Billion years old......

Ok so there was a star 13 billion years ago and a light photon was fired off from it and 13 billion years later it popped through the viewing hood on Hubble and Voila.

What is this thing called a photon, how can it have so much energy to travel at the speed of light and travel for billions of years???
Can these light photons just travel indefinitly at the speed of light?
When the photon hit the hubble scope it was destroyed i take it?

Bloody hell my heads wrecked trying to get my head around

srsly78

They ARE light. Can be thought of as a wave or a discrete particle - this is one of the freaky quantum things. http://en.wikipedia.org/wiki/Wave%E2%80%93particle_duality

Light = electromagnetic radiation.

When it hits the hubble it interacts with the sensor, transforming it's energy into electricity which gets processed by a computer.

This explains how the Hubble sensor works: http://en.wikipedia.org/wiki/Charge-coupled_device Your phone/camera also uses these, just not as fancy as what telescopes have.

Outkast_IRE

johnwest288 wrote: »

Ok so Im trying to get my head around Light Photons...

Ok so the Hubble deep field picture shows stars that are about 13 Billion years old......

Ok so there was a star 13 billion years ago and a light photon was fired off from it and 13 billion years later it popped through the viewing hood on Hubble and Voila.

What is this thing called a photon, how can it have so much energy to travel at the speed of light and travel for billions of years???
Can these light photons just travel indefinitly at the speed of light?
When the photon hit the hubble scope it was destroyed i take it?

Bloody hell my heads wrecked trying to get my head around

Im not big into physics/astrophysics
but id imagine the photon can travel that far for that long, because space puts up no resistance to it, so it can travel indefinetly as long as its not affected by an external force.

srsly78

Yep they can get blocked/absorbed by stuff in between. Note however that material interacts with light in different ways depending on the lights wavelength. So even tho there might be dust clouds in the way certain frequencies will get through and others won't.

The pictures we always see of space have nearly always been digitally manipulated to change the colours into something that looks good for us. In reality the Hubble does not "see" the image in those colours at all.

Gurgle

johnwest288 wrote: »

What is this thing called a photon

this photon thing is a little packet of energy

johnwest288 wrote: »

how can it have so much energy to travel at the speed of light and travel for billions of years???

Traveling doesn't require energy, acceleration of mass requires energy.
A photon has no mass, therefore no momentum, therefore no energy is expended in accelerating it to the speed of light.

Take a Newtonian look at it:
F = ma
m = 0 => F = 0

johnwest288 wrote: »

Can these light photons just travel indefinitly at the speed of light?

Short answer: Yes
Long answer: As it was created traveling at the speed of light, time didn't pass for the photon. It was emitted from the star 13 billion years ago and immediately hit a telescope.

johnwest288 wrote: »

When the photon hit the hubble scope it was destroyed i take it?

Short answer: Yes
Long answer: You can't destroy energy, but it was turned from a photon to a phonon when it was absorbed by Hubble's image sensor.

Neodymium

Photons are a massless particle and as a consequence they must travel at the speed of light due to Einsteins's theory of special relativity. In a way the speed of light could be called the speed of massless particles, but the photon was the first massless particle we discovered so the name stuck.

If you consider the mass-energy equivalence equation E=mc^2 which came as a product of special relativity, the 'm' in this equation refers the the mass of the particle at rest because due to relativity, the mass of a particle increases as its speed increases. The 'c' refers to the speed of light.

A more 'correct' way of representing the equation would be:



'v' is the velocity of the particle.
Now, you can rearrange this equation to give you:



We know that the mass of the photon is zero so we say m = 0 giving us:



The energy above and below cancel and we take the square root of both sides to give us:



This means that the velocity of this massless particle i.e a photon must be precisely at the speed of light at all times in a vacuum.

When we say the speed of light, people are usually referring to the speed of light in a vacuum which is around 300'000 kilometers per second.

Morbert

Gurgle wrote: »

this photon thing is a little packet of energy

No it isn't. A photon is a quantised excitation of the electromagnetic field. It has energy, sure, but it is not energy itself.

Traveling doesn't require energy, acceleration of mass requires energy.
A photon has no mass, therefore no momentum, therefore no energy is expended in accelerating it to the speed of light.

Take a Newtonian look at it:
F = ma
m = 0 => F = 0

Photons do have momentum, and they cannot be accelerated or decelerated, as massless particles must travel at the speed of light.

krd

Morbert wrote: »

Photons do have momentum, and they cannot be accelerated or decelerated, as massless particles must travel at the speed of light.

Yes, they cannot be accelerated or decelerated, but their momentum can change.

And this is why the sky is blue.

Actually, I'm not 100% sure if these photons with altered momentum should be classed as the same photons or as new photons created in the collisions. But it is the reason the sky is blue.

Morbert

krd wrote: »

Yes, they cannot be accelerated or decelerated, but their momentum can change.

And this is why the sky is blue.

Actually, I'm not 100% sure if these photons with altered momentum should be classed as the same photons or as new photons created in the collisions. But it is the reason the sky is blue.

That is not why the sky is blue. The Tyndall effect is why the sky is blue.

http://en.wikipedia.org/wiki/Tyndall_effect

krd

Morbert wrote: »

That is not why the sky is blue. The Tyndall effect is why the sky is blue.

http://en.wikipedia.org/wiki/Tyndall_effect

You're just being a smart arse. ....And by Wikipedia, the Tyndall effect is scattering in colloid or particles in a fine suspension. Particles above 40 nanometres.


Right.......I really shouldn't rub it in, but you asked for it.

From the same Wikipedia page.

On a day when the sky is overcast, the sunlight passes through the turbid layer of the clouds, resulting in scattered, diffuse light on the ground. This does not exhibit Tyndall scattering because the cloud droplets are larger than the wavelength of light and scatter all colors approximately equally.

Okay, that doesn't make you look like an ass. But this bit does.

On a day when the sky is cloud-free, the sky's color is blue in consequence of light scattering, but this is not termed Tyndall scattering because the scattering particles are the molecules of the air, which are much smaller than the wavelength of the light.

Colloids.........Though I have noticed the way milk can look blue in a glass. And I suppose the bluishness of wallpaper paste would be the Tyndall too.

But nothing to do with the blueness of the sky.

johnwest288

Now I am going to have to digest this information this evening over a glass i mean a liter of vodka,,,



"It was emitted from the star 13 billion years ago and immediately hit a telescope":eek: i find this mind bloody bending crikey



JEBUZ.... Right ill need to re-read this thread a few more times to get the jist but thanks for all this information im a fair bit clearer what light is now

Gurgle

Morbert wrote: »

No it isn't. A photon is a quantised excitation of the electromagnetic field. It has energy, sure, but it is not energy itself.

Excitation = energy
No?

Photons do have momentum, and they cannot be accelerated or decelerated, as massless particles must travel at the speed of light.

I thought it was an equivalence, not 'actual' momentum. Can you link somewhere that explains?

krd

Gurgle wrote: »

I thought it was an equivalence, not 'actual' momentum. Can you link somewhere that explains?

No, it's not a equivalence. It's another freaky thing about light. It does have momentum. Though it always travels at the speed of light.

When photons collide with atoms, they can cause them to move. They lose some of their momentum - but they're still going at the same speed. Their frequency changes - lowers. They lose a little energy. The energy of a photon is given by E = hfrequency.

Morbert

krd wrote: »

You're just being a smart arse. ....And by Wikipedia, the Tyndall effect is scattering in colloid or particles in a fine suspension. Particles above 40 nanometres.


Right.......I really shouldn't rub it in, but you asked for it.

From the same Wikipedia page.

On a day when the sky is overcast, the sunlight passes through the turbid layer of the clouds, resulting in scattered, diffuse light on the ground. This does not exhibit Tyndall scattering because the cloud droplets are larger than the wavelength of light and scatter all colors approximately equally.

Okay, that doesn't make you look like an ass. But this bit does.

On a day when the sky is cloud-free, the sky's color is blue in consequence of light scattering, but this is not termed Tyndall scattering because the scattering particles are the molecules of the air, which are much smaller than the wavelength of the light.

Colloids.........Though I have noticed the way milk can look blue in a glass. And I suppose the bluishness of wallpaper paste would be the Tyndall too.

But nothing to do with the blueness of the sky.

It has everything to do with the blueness of the sky.

http://pubs.acs.org/doi/abs/10.1021/ed048p389.1

Tyndall effect is the more encompassing term, and it was John Tyndall who first suggested that, if the particles are sufficiently smaller than the wavelength of light, the scattered radiation would be primarily blue. Rayleigh scattering is a specific instance of the Tyndall effect.

Also, I have no idea why you have started calling names, but I would ask you to stop such childishness.

Morbert

Gurgle wrote: »

Excitation = energy
No?

An excitation is a state of the system. Energy is a property of things, it is not a thing that exists by itself. Just as a "fast" does not exist by itself. A photon is "made of" the electromagnetic field insofar as it is a quantised excitation of the field.

I thought it was an equivalence, not 'actual' momentum. Can you link somewhere that explains?

It is an equivalence. But it is also actual momentum. In Newtonian mechanics, there is the usual relation between momentum and mass:

p = mv

In relativity, it turns out the deeper relation is the energy-momentum relation

E^2 = m^2 + p^2

or

p^2 = E^2 - m^2

So you can see that, even if something has no mass, it still has momentum. The only time it has 0 momentum is when it is at rest (and a photon is never at rest)

krd

Morbert wrote: »

Tyndall effect is the more encompassing term, and it was John Tyndall who first suggested that, if the particles are sufficiently smaller than the wavelength of light, the scattered radiation would be primarily blue. Rayleigh scattering is a specific instance of the Tyndall effect.

Yeah, I know it's the same scattering effect. The different names given for specific instances all refer to the same effect but it might be treated different with different bands and energies.

Also, I have no idea why you have started calling names, but I would ask you to stop such childishness.

I was just being silly. Wikipedia is not a good source. Both the Tyndall and Rayleigh entries are in conflict as both are credited with discovering why the sky is blue. But on the page for the Tyndall effect, it's explicit in saying that it's not the reason for the sky being blue (I think on the basis of the aerosols in the atmosphere generally not being that big. Most dust is very near the ground.)

I don't know who should be credited. Generally, it's Baron Rayleigh.

krd

Morbert wrote: »

So you can see that, even if something has no mass, it still has momentum. The only time it has 0 momentum is when it is at rest (and a photon is never at rest)

It's kind of mind boggling. If you consider something like laser cooling of gas. The laser is tuned to just a little bit below the frequency of an absorption line. The atom flies into the incoming beam - the velocity of the atom Doppler shifts the light up to its absorption frequency. It's absorbed, then re-emitted at the higher frequency/energy. The extra energy in the new photon comes from the atom's momentum. The atom slows down.

When I'm trying to get to sleep I try to visualise laser cooling. It's actually really interesting to think about.