# Photon Propagation : Straightline or Helix ?

Discussion in 'Physics & Math' started by The God, May 12, 2016.

1. ### The GodValued Senior Member

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Is it straightline or helix ?

The prevalent notion is straightline.

When almost nothing moves in straightline (through space), why photon ? The guy has got the momentum.

3. ### exchemistValued Senior Member

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I am sceptical that this post is being made in good faith. I suspect it is trying to provoke another silly argument.

5. ### originTrump is the best argument against a democracy.Valued Senior Member

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Everything moves through space in a straight line unless acted upon by a force.

7. ### The GodValued Senior Member

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Don't kill the thread in the beginning itself.

Stayput you will learn something from this thread too, but only if you allow it to build up.

8. ### The GodValued Senior Member

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This one needs rethinking on your part.......dig a bit deeper, you will know that, it is not the case.

9. ### exchemistValued Senior Member

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But now he will try to f*** you up with something about geodesics or something...oops too late.....

You wait, this will be another stupid exercise in coat-trailing and then nitpicking and deliberate obtuseness, all in order to start an unproductive argument.

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10. ### originTrump is the best argument against a democracy.Valued Senior Member

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No it doesn't.

Don't be coy, do you have a thought on what I wrote?

11. ### The GodValued Senior Member

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If you do not have anything worthwhile to contribute, pl stay away. Why provoke ?

12. ### originTrump is the best argument against a democracy.Valued Senior Member

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I suspect you are right, I just thought I would weigh in for a few posts before Paddoboy and him start their dueling flamethrowing.

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13. ### The GodValued Senior Member

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Give an example of motion of a particle which is in straightline through space ? (as seen from infinity).

14. ### exchemistValued Senior Member

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Well good for you, but, as you can perhaps tell, I'm really tired of this p........erson.

15. ### originTrump is the best argument against a democracy.Valued Senior Member

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I don't want to play games. I have already stated that everything moves in a straight line through space unless it is acted upon by a force so why are you asking me for an example? You indicated that you had a different thought on this, so what is your thought on this?
If you want to play games I will leave you to have your fun with others that have more patience.

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16. ### rpennerFully WiredStaff Member

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In Special Relativity (SR), all free particles, including photons, propagate in straight lines governed by
$E^2 = (m c^2)^2 + (c \vec{p} )^2 , \; E \vec{v} = c^2 \vec{p}$
so conservation of $E$ and $\vec{p}$ for an isolated system (such as a free particle) guarantees constant $\vec{v}$ so the equation of motion is a straight line.

In General Relativity (GR), all free particles, including photons, propagate in straightest possible lines in the curved geometry of a space-time manifold governed by
$\mathbf{p}^2 = m, \; \mathbf{\nabla}_{\mathbf{p}} \mathbf{p} = 0$.

Correct, but the definition of straight line changes between the two physical theories. Your choice of physical theory needs to be clarified.

Your objection appears to be misinformed. The orbits of the planets are to good approximation geodesics of space-time, so they are the straightest possible lines in GR.
Correct in either SR or GR and is the definition of free particle. In GR, gravity is not a force. In SR, we cannot correctly describe gravity. Your choice of physical phenomena needs to be clarified.
Your definition of straight line needs to be clarified.

[Moderator: The God is asked to respond with lengthier and better supported explanations of his reasoning, theoretical and factual basis before continuing. The God is suspected of arguing unfairly not explaining his preconceptions and/or setting up a trap and moving the goal posts when someone responds to the bait. Either would be trolling, which is forbidden by forum rules, so it is to The God's benefit to explain his position better and more fairly to his readers.]

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17. ### originTrump is the best argument against a democracy.Valued Senior Member

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LOL. That was great.

18. ### exchemistValued Senior Member

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But your response: "I have already stated that everything moves in a straight line through space unless it is acted upon by a force so why are you asking me for an example?" was also spot-on, I thought.

Anyway, our excellent mod seems to have put a stop to it.

19. ### krash661[MK6] transitioning scifi to realityValued Senior Member

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i find it odd that you are such a science genius, and yet here is this question.

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Not much digging is needed in actual fact. As origin said, all things move in a straight line unless acted on by a force.
Let me elaborate:
Light follows geodesic paths in curved spacetime. That is well known.
We see that geodesical path as gravitational lensing.
The rotation of Earth and planets could also be said to be following geodesics in spacetime, explainable in simple Newtonian terms as a tussle between the pull of gravity by the parent star, and the straight line motion of the planet/Satellite.
The same could be said to apply to the motions of the stars around the galactic center: The stars try and maintain their straight line motion, while the pull of gravity from the SMBH at the center, keeps them in orbit.
Conversely they are simply following the same geodesic paths in curved spacetime as is light.
Hope that helps.

21. ### The GodValued Senior Member

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Say, an observer records the motion of earth (any planet of our solar system) from infinity. The motion will appear like helical. Is it not ?

On the same front, sun is orbiting the GC and the entire Milky Way is moving in some direction (say towards Great Attractor), so in all likelyhood if orientation fits in, the motion of Sun is also helical as observed from infinity.

So that covers all the planets and stars of Milky way, all are moving in helical path through space. If any Galaxy has translational motion, than this logic can be extended to almost all the constituent stars/planets/particles of any Galaxy in universe.

Where is the straightline motion ?

Of course I am considering everything other than null geodesics and Euclidean straightlines, as curved lines. and please get the idea, orbital motion may be considered as straightline in GR, but we have not gotten rid of something called circular orbit, elliptical orbits, angular speed, time period etc....signifying falling back on classical mechanics. so pl do not push that all motion under GR are straightlines as they are on some geodesic or the other.

PS : On your false accusation of trolling in red color.
Trolling, I am not, Trolling is what 'Exchemist' is doing, can't you see that ? As a Moderator you must arrest the situation before it flys off, all his posts are unwarranted nonsense, meant to provoke. If it is not in forum rule book, then pl update the rule book to cover this kind of behaviour.

22. ### The GodValued Senior Member

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Parrotised stuff !
And incorrect/misleading at couple of places. I am sure Rpenner can see through it.

23. ### DaeconKiwi fruitValued Senior Member

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Is claiming that something is "parrotised" supposed to be an implication that it's incorrect?