[Kryptid]

interesting that they attract each other. ... should I assume the effect has negligible impact on the inverse squire law?

It would have no impact on the inverse square law. Any given photon being emitted from a point source will have other photons on all sides of it, meaning there is no preferential gravitational pull in any given direction.

[Bill S]

The photons will slowly approach each other over time, since their energy produces a tiny gravitational field.

It would be interesting to know how that impacts on the argument that photons "experience" neither time nor distance. 

Two photons that had travelled together for 1k years, in our FR, isolated from other photons, would be closer together at the end of that time than at the start.  If the time and distance are relevant only in our RF, what does that say about any measurements of the separation might be taken.

I realise that taking measurements tends to have a destructive influence on photons, but just thinking.

[raf21]

could this attraction of photons be tested with lasers? fire one laser at a target,  then fire another laser across the top very close at a 90 angle?  then shutting one laser off and seeing if the light "bends " back? perhaps 90 degrees creates to short an interaction window,  perhaps almost parallel,  say cross the beams at an extremely acute angle,  like a couple moa or even seconds of degree. ...   

[Kryptid]

It would be interesting to know how that impacts on the argument that photons "experience" neither time nor distance. 

Two photons that had travelled together for 1k years, in our FR, isolated from other photons, would be closer together at the end of that time than at the start.  If the time and distance are relevant only in our RF, what does that say about any measurements of the separation might be taken.

I realise that taking measurements tends to have a destructive influence on photons, but just thinking.

I'm not sure how a curved path for a photon in our reference frame would look in the photon's frame. We know it has to work somehow, since gravitational lensing is a confirmed phenomenon.

could this attraction of photons be tested with lasers? fire one laser at a target,  then fire another laser across the top very close at a 90 angle?  then shutting one laser off and seeing if the light "bends " back? perhaps 90 degrees creates to short an interaction window,  perhaps almost parallel,  say cross the beams at an extremely acute angle,  like a couple moa or even seconds of degree. ...   

I don't know if we have the needed sensitivity to perform such an experiment on Earth. If it was up to me, I'd prefer to perform the experiment in space where two laser beams fired parallel to each other could travel for millions of miles before hitting the detector.

[raf21]

I had wondered if it would be able to be detected in the small space in earth available to play with,  and that led me to thinking the experiment would possibly work better with two different strength lasers,  themain laser having a high photon density, and measurements taken from a weaker laser fired "across the bow" so to speak. ....

[Kryptid]

I had wondered if it would be able to be detected in the small space in earth available to play with,  and that led me to thinking the experiment would possibly work better with two different strength lasers,  themain laser having a high photon density, and measurements taken from a weaker laser fired "across the bow" so to speak. ....

Interestingly enough, the rate of deflection of the weak laser you speak of wouldn't change even if you powered up the weak laser to arbitrarily high levels. All objects fall at the same rate in the same gravitational field (recall Galileo's experiment). Increasing the strength of the "strong" laser would increase the rate of deflection of the weak laser, however.

[raf21]

but the higher density of photons in the strong laser would deflect the weaker laser?more so then the stronger laser was deflected?

[raf21]

and to go back to photons having "head on collisions"...... how rare is this occurrence?  it must not be very prevalent as the universe appears black,  and if it were to be common,  the whole universe would glow somewhat. ....?

[Kryptid]

but the higher density of photons in the strong laser would deflect the weaker laser?more so then the stronger laser was deflected?

The rate of deflection of the weak laser would depend upon the strength of the strong laser. The rate of deflection of the strong laser would depend upon the strength of the weak laser. The rate at which the gap between the two laser beams closes depends upon the strength of both of them.

and to go back to photons having "head on collisions"...... how rare is this occurrence?  it must not be very prevalent as the universe appears black,  and if it were to be common,  the whole universe would glow somewhat. ....?

I don't know what the actual numbers are, but it's a very rare phenomenon under most circumstances. The more energy that the photons have, the more likely that this scattering will occur. It's important to recognize that it isn't actually the photons themselves that are scattering off of each other. Instead, photons are capable of transforming, temporarily, into other particles (such as electron-positron pairs). If both photons that are approaching each other transform into such a particle pair at the same time, those particles are capable of interacting with each other and then scattering before returning to their previous photon-selves.

[PmbPhy]

but the higher density of photons in the strong laser would deflect the weaker laser?more so then the stronger laser was deflected?

Let us not forget relativity. What's a strong laser beam in one frame of reference is a weak beam in  another. So if you had two laser beams headed head on symmetry states that they'd collide. However if they two beams were parallel and not head on then a strong deflection in frame is a weak deflection in another frame.

[yor_on]

As far as I understand light can't have a 'rest mass', it's 'propagating', and never 'at rest'. When it comes to a box containing 'excited photons' it will be the box that has a increased 'rest mass', not the photons per se.

[yor_on]

A  more interesting point here, and one that follows naturally, is what the heck we mean by 'propagating'.
That's worth thinking of

[Zer0]

This is a Nice general terms and definitions conversational thread.
It's very clear and informative. 👌
Helps to wash out confusion n put things into perspective, relatively.

Please keep this OP going! ✌

P.S. - Thanks to raf21 and all other contributing members. 👍

[raf21]

I do not deserve the thanks,  I've done nothing,  I'm the monkey not understanding why I can't grab the hologram banana.  bogie,  pmb, Bill,  Colin and kryptid, among others,  deserve the thanks.  I'm tempted to ask a couple black hole questions,  I think they relate too this discussion.  anyone object?

[Bogie_smiles]

I'm tempted to ask a couple black hole questions,  I think they relate too this discussion.  anyone object?

I would say go for it,  but again you pick a topic is about science that is still on the drawing board, lol.

[raf21]

am I understanding things correctly when they say a black hole is black due to the event horizion? event horizon (being an effect,  not an object?) is caused by light being unable to escape the singularities gravitational field. .. correct? and I've heard that as an object approaches an event horizion it appears to approach the horizon at a slower and slower rate and you'll never observe it passing though the horizon,  it appears to "freeze" at the surface. 
I cannot make this work in my mind.  if object appears to freeze at the surface,  then black holes would not be black,  but  very red, a Doppler shifted image of everything the black hole has swallowed for it's entire existence. 
....
I cannot rationalize that scenario with my limited intelligence. how could an image still exist on the surface of the EH after said object has passed through? where does the energy come from to continue sending the light from said object back to us?
and more important,  if light cannot escape due to gravity,  does this violate the light speed law that light has to move at light speed? gravity can bend light,  does gravity slow light like density can? compress space itself?  it's space being compressed in a black hole? if space can expand,  surly it can compress. .....how does light,  traveling in space that is contacting or expanding appear to act to an observer in normal space?  I'm sorry,  that's a bunch of questions,  I've been busy lately ast work and my mind sorta just unloaded.

[PmbPhy]

John Wheeler coined the term black hole due to the fact that anything that goes into it can never come out of it, including light. The term "black hole" sounded cool and it was accurate so it stuck.

[Zer0]

As far as I know a Black Hole has never been seen from a telescope or naked eyes. (I might be wrong)

I suppose AstroPhysicists collect radio signals or em waves or x or gamma rays n put the bunch through a computer program which processes all the information n then projects a cgi image on screen for a printout to be made possible. (I might be wrong again)

I certainly don't understand why it's still named Black, cause maybe Hawkings radiation could make it one of, or the brightest objects in the universe. (possibly wrong again)

N why add the Hole to it, when it's a bit clear now they are balls of dense matter. Whatever goes inside it isn't falling of into another parallel multiverse but adds on to its existing mass making it slightly more dense n larger than before orelse they would never grow in size. (Yeah right, u get it)duuh!

Nothing personal against Mr John Wheeler but if Pluto can be laid off from planetary status, then why can't Black Holes be renamed to something more precisely defined. (Yup I already know, not gonna happen)pfft!

P.S. - Isn't light made out of Photons, and don't Photons have incredible velocity but Zero Mass?
Doesn't that make Photons untouchables, as Gravity cannot act on massless particles?
Why then do they generally say that Light cannot escape a Black Hole, isn't light just following a straight path like a spaghetti n isn't the incredible dense gravity well of the blackhole creating a meatball by twisting the mere fabric of spacetime?
How could a train leave station (A) and arrive to another station (B) when the rail track itself is twisted n looped around station (A) and why would anyone blame the engine/train driver for not being able to escape station (A)
❓

[PmbPhy]

I certainly don't understand why it's still named Black, cause maybe Hawkings radiation could make it one of, or the brightest objects in the universe. (possibly wrong again)

Names don't get changed when new theories come out. As I already explained, its called a black hole because whatever falls in can never fall out. What makes you think Hawking Radiation is so strong for all black holes? By the way, Hawking Radiation does not come from inside the event horizon but just outside of it. The name is based solely on analogy as so many things are. You don't object to the term "wormhole" do you? After all the person who coined the term didn't think that spaceworms are going through it.

N why add the Hole to it, when it's a bit clear now they are balls of dense matter.

Take a guess from what I already posted. They used to be called frozen stars. Note: No ice was thought to be used in their makeup.

If you want to know why an object has a name read what the man who coined the term says.

[Zer0]

Well now that u have brought it to my notice, I most certainly object to the term ' WormHole ' .
👊
Makes little sense as to how someone could dig the spacetime fabric n create a hole in it.
A Tunnel seems more appropriate.
🚞
N don't even get me started on that ' worm ' thingy. PFFT!
🐛

P.S. - OK Mr John Wheeler I accept n agree to the term BlackHole, now stop makin dat face n plz put d gun down & R.I.P.
✌