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# Random question about light and gravity. watch

1. Light, clearly, has mass and is affected by gravity. Consider a hypothetical scenario, kind of representative of the universe, in which you have a single point of large mass in space from which a photon is fired.

Clearly initially all of the energy is kinetic, but as it moves away from the gravity some must be converted to potential, surely? Hence the light must slow down.

However, (and this is where i'm a bit out of my depth) applying general relativity you could argue that whilst the light may slow down from the light's perspective, time will also slow down as it exits the gravitational field so from our perspective it will remain at light-speed (i think that works...).

If this is true, how could the universe ever implode? surely from our perspective it will continue to expand indefinitely at the speed of light, although from the edge of the universe, the rate of expansion will tend towards 0? How can that initial light ever go backwards, basically?

(of course the physics governing this would be immense but if there is a simple explanation it is appreciated).
2. (Original post by nexttime)
Light, clearly, has mass and is affected by gravity. Consider a hypothetical scenario, kind of representative of the universe, in which you have a single point of large mass in space from which a photon is fired.

Clearly initially all of the energy is kinetic, but as it moves away from the gravity some must be converted to potential, surely? Hence the light must slow down.

However, (and this is where i'm a bit out of my depth) applying general relativity you could argue that whilst the light may slow down from the light's perspective, time will also slow down as it exits the gravitational field so from our perspective it will remain at light-speed (i think that works...).

If this is true, how could the universe ever implode? surely from our perspective it will continue to expand indefinitely at the speed of light, although from the edge of the universe, the rate of expansion will tend towards 0? How can that initial light ever go backwards, basically?

(of course the physics governing this would be immense but if there is a simple explanation it is appreciated).
Does it?
3. I thought photons were massless...

http://en.wikipedia.org/wiki/Photon#cite_note-8
4. (Original post by strangej)
I thought photons were massless...

http://en.wikipedia.org/wiki/Photon#cite_note-8
they are.

but, try and prove it...
5. Photons cannot have rest mass as such as other wise they wouldn't be able to travel at the speed of light due to relativity? but relativity also makes it need mass....
It amazing how much you forget in a month or two, but I'm pretty sure this was one of the off topic questions my physics teacher answered.

EDIT: In terms of an implosion or the universe collapsing in on itself, it is dark matter and sub-atomic particles whose mass has not been found which are of concern.
6. If a photon had (rest) mass it would not be able to move at the speed of light - the nasty effects of Special Relativity. So, light has zero rest mass but is sometimes said to have relativistic mass - the mass equivalent of the energy it has.

As light doesn't have any rest mass it is not effected by gravitation. Space, however, is - the realm of GR now. Light appears to move towards a black hole as the colossal amount of mass causes the space through which the light travels to be distorted and thus the path of the photon appears bent to the observer.

Accepting that the universe will achieve critical density and collapse and what-not, the collapsing mass of universe will become so great and dense that the surrounding space and therefore all the massless particles contained will be dragged back in.

No sources, just intuition and something a physics teacher might have said al long time ago! See what everyone thinks.
7. Yes, as said before, light is massless. You may have been confused due to the effects of 'bending' the light due to General Relativity, but this does not mean the light has mass.
8. But it can have momentum. Try to sort that one out in your head
9. (Original post by nexttime)
Light, clearly, has mass and is affected by gravity. Consider a hypothetical scenario, kind of representative of the universe, in which you have a single point of large mass in space from which a photon is fired.

Clearly initially all of the energy is kinetic, but as it moves away from the gravity some must be converted to potential, surely? Hence the light must slow down.

However, (and this is where i'm a bit out of my depth) applying general relativity you could argue that whilst the light may slow down from the light's perspective, time will also slow down as it exits the gravitational field so from our perspective it will remain at light-speed (i think that works...).

If this is true, how could the universe ever implode? surely from our perspective it will continue to expand indefinitely at the speed of light, although from the edge of the universe, the rate of expansion will tend towards 0? How can that initial light ever go backwards, basically?

(of course the physics governing this would be immense but if there is a simple explanation it is appreciated).
[Amateur information follows]

Light has no mass. It is affected by gravity because light takes a straight path in space-time. By G.R., mass bends space-time, and light thus follows what we perceive to be a curved path that is the shortest in the curved space-time between two points- the geodesic.

Light in a vacuum always travels at the same speed in all reference frames. The effect of gravity is not to slow it down, but to reduce the energy by causing shift to longer wavelength. This effect must actually be compensated for in GPS systems (with radio waves).

[and here's where I'm just speculating, but further research should be a straight-forward matter].
The reason the above red-shift occurs is as follows: If you imagine a someone on a space ship moving very fast shining a beam of light ahead of them, they see the light move away at speed c, and so do you (speed of light is constant in all reference frames). Thus this says two things: firstly, they you will see a smaller wavelength than the person on the space ship, and to compensate for this difference from the point of view of the person on the spaceship, time in their reference frame is slower. G.R. sets an equivalence between acceleration and gravity, so a beam of light shone in the direction of a planet will undergo the same effects as a person shining a beam of light when accelerating, it it will be seen to blue-shift (and thus gin energy). Conversely, going in the other direction, it will red-shift (and thus lose energy).

If a physicist could tell me where the above is wrong, I would appreciate it.
10. (Original post by nexttime)
Light, clearly, has mass
my physics knowledge is limited to a level - but i dont think you can treat that as a given fact
11. (Original post by ashy)
But it can have momentum. Try to sort that one out in your head
If they havent got mass then how can they have momentum?
12. (Original post by wizz_kid)
If they havent got mass then how can they have momentum?
Magic. They can exert a force, too.
13. (Original post by ashy)
Magic. They can exert a force, too.
JOkes!
14. (Original post by wizz_kid)
JOkes!
15. 'Relativistic mass' or whatever you're meant to call it - the important thing in this scenario is that it is affected by gravity - does that mean it loses energy and 'slows down' when it moves away from our theoretical point-mass though? unsure.

And if it does not slow down, how can the universe still implode? Surely the light will just continue forever if it is not affected by gravity? Or will it just... disperse as it gets further away?
16. (Original post by Dirac Delta Function)
[Amateur information follows]

Light has no mass. It is affected by gravity because light takes a straight path in space-time. By G.R., mass bends space-time, and light thus follows what we perceive to be a curved path that is the shortest in the curved space-time between two points- the geodesic.

Light in a vacuum always travels at the same speed in all reference frames. The effect of gravity is not to slow it down, but to reduce the energy by causing shift to longer wavelength. This effect must actually be compensated for in GPS systems (with radio waves).

[and here's where I'm just speculating, but further research should be a straight-forward matter].
The reason the above red-shift occurs is as follows: If you imagine a someone on a space ship moving very fast shining a beam of light ahead of them, they see the light move away at speed c, and so do you (speed of light is constant in all reference frames). Thus this says two things: firstly, they you will see a smaller wavelength than the person on the space ship, and to compensate for this difference from the point of view of the person on the spaceship, time in their reference frame is slower. G.R. sets an equivalence between acceleration and gravity, so a beam of light shone in the direction of a planet will undergo the same effects as a person shining a beam of light when accelerating, it it will be seen to blue-shift (and thus gin energy). Conversely, going in the other direction, it will red-shift (and thus lose energy).

If a physicist could tell me where the above is wrong, I would appreciate it.
1st paragraph - this is also true of planets orbiting the sun - they follow a straight path in 4D space. However, i thought this was only true due to planets (and it follows, light's energy) having mass. A theoretical body that was not subject to gravity would move in a straight line in 3D space, not four as you have described. Therefore mass is important.

2nd and third - special relativity, no? Relating to relative movement not gravity and space-time?
17. What is with peoples damned assumption that speed (or even velocity) has some kind of relationship (and in fact LINEAR) relationship with momentum.
These are the same damned people who are told that Inertial mass and gravitational mass are proportional to several orders of magnitude and they think "big whoop". Seriously.
18. No one seems to be addressing the main issue here (though granted the op did need to be told that light has no mass!! Christ!). The universe is not expanding at the speed of light, it's rate of expansion is not determined by light rays running away as you seem to describe it, op. I don't know the specifics of what would happen in the 'big crunch', but light follows the shortest path in space-time right, so what happens at the edge of the universe we don't really know (or at least I don't know) but it is plausible that the light would move in a circular direction looping back and then heading back where it came from. This solves your problem, the light never slowed down - it's not going backwards, it's going forwards in another direction. (this is all very very wrong I'm sure but it's a plausible solution to your problem and I think that plausibility is what you're really after here)

Or we could go down the route that when (or rather IF, and that is a big if) space-time collapses on itself, the light wont be going backwards in the context of space time. It will be moving at the same velocity relative to space-time itself (I refuse to go into the arguments against substantival space at this time of night...), but yes in this case it is no longer an issue and this seems pretty right to me. Then again it is half two in the morning... meh.
19. (Original post by Audrey Hepburn)
No one seems to be addressing the main issue here (though granted the op did need to be told that light has no mass!! Christ!). The universe is not expanding at the speed of light, it's rate of expansion is not determined by light rays running away as you seem to describe it, op. I don't know the specifics of what would happen in the 'big crunch', but light follows the shortest path in space-time right, so what happens at the edge of the universe we don't really know (or at least I don't know) but it is plausible that the light would move in a circular direction looping back and then heading back where it came from. This solves your problem, the light never slowed down - it's not going backwards, it's going forwards in another direction. (this is all very very wrong I'm sure but it's a plausible solution to your problem and I think that plausibility is what you're really after here)

Or we could go down the route that when (or rather IF, and that is a big if) space-time collapses on itself, the light wont be going backwards in the context of space time. It will be moving at the same velocity relative to space-time itself (I refuse to go into the arguments against substantival space at this time of night...), but yes in this case it is no longer an issue and this seems pretty right to me. Then again it is half two in the morning... meh.
Try explaining why it is bent by gravitational fields, cannot escape from large gravity concentrations like black holes, can exert pressure, can change the velocity of particles when observing them, that E=mc(2) and light must have energy, and that it is an observable uncharged particle, then. It does have mass, just not at rest. I don't know how you would even begin to test that it even has 0 rest mass, tbh.

How is the universe expanding, then? It isn't just 'space expanding' - space with nothing in it is unmeasurable and so undefined. If its not determined by the quickest thing leaving the site of the big bang then what is it? (I know you have the strange proposed theory of hyperinflation in the early years, in which expansion was quicker than light, but after that i was sure it was expanding at the speed of light, because the boundary was being pushed by light itself).

The 'looping around' theory could be possible, i guess, but only due to the mass in the universe being unevenly distributed, and tbh i doubt if you worked it out mathematically that it would be feasible for every photon to work that way but you never know.
20. (Original post by nexttime)
Try explaining why it is bent by gravitational fields, cannot escape from large gravity concentrations like black holes, can exert pressure, can change the velocity of particles when observing them, that E=mc(2) and light must have energy, and that it is an observable uncharged particle, then. It does have mass, just not at rest. I don't know how you would even begin to test that it even has 0 rest mass, tbh.

How is the universe expanding, then? It isn't just 'space expanding' - space with nothing in it is unmeasurable and so undefined. If its not determined by the quickest thing leaving the site of the big bang then what is it? (I know you have the strange proposed theory of hyperinflation in the early years, in which expansion was quicker than light, but after that i was sure it was expanding at the speed of light, because the boundary was being pushed by light itself).

The 'looping around' theory could be possible, i guess, but only due to the mass in the universe being unevenly distributed, and tbh i doubt if you worked it out mathematically that it would be feasible for every photon to work that way but you never know.
It's already been explained to you why gravitational fields affect protons, it's due to their effects warping space-time and not because they effect the actual protons themselves. Also, fyi, [E^2 = (p^2)(c^2) + (m^2)(c^4)] in special relativity, then because photons have no mass this reduces to E=pc - and bear in mind that its momentum is NOT based on its mass but rather its wavelength (I think even at A level you should have studied deBroglie - I'm assuming you're not at degree level because otherwise you would have covered all of this before).

As for the universe expanding stuff, let me just point out that all evidence points to the fact that the rate of expansion of the universe is ACCELERATING. How could this be possible if it was expanding at the speed of light? It couldn't. It is indeed 'space' expanding that defines the expansion of the universe (the space between objects created in it though, I'm not trying to argue that there is an absolute space... though tbh our current system of newtonian mechanics does rely on this).

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