Do Photons Have Mass? - Debate & Questions

[robinpike]

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

Thanks for the picture.

If, in that picture, gravity is a consequence of following the curved spacelines, then what is the force is pulling the spacelines down in the picture?

 

[Nugatory]

If, in that picture, gravity is a consequence of following the curved spacelines, then what is the force is pulling the spacelines down in the picture?

There isn't one. That picture can be used to give you a mental model of what curved space does, but doesn't help with why the space curves.

 

[HallsofIvy]

Space around a mass is curved as a result of the General Theory of Relativity, there is no force necessary.

 

[harrylin]

Thanks for the picture.

If, in that picture, gravity is a consequence of following the curved spacelines, then what is the force is pulling the spacelines down in the picture?

It is the opposite: the properties of space have an influence on the motion of everything that results in a contact force if you counter it. Newton called such a motion changing influence a force, but not everyone uses the exact same definition.

 

[robinpike]

There isn't one. That picture can be used to give you a mental model of what curved space does, but doesn't help with why the space curves.

So curved spacetime lines are a means to perform the calculation of how things behave near objects with mass, but the spacetime line concept in itself is not the explanation for gravity?

 

[harrylin]

So curved spacetime lines are a means to perform the calculation of how things behave near objects with mass, but the spacetime line concept in itself is not the explanation for gravity?

A spacetime line is a graphical sketch of the mathematics. The underlying concept is that space has properties that are influenced by nearby matter - in other words, GR is a field theory.

 

[georgir]

posts are getting deleted by database errors?

 

[pervect]

Good news, bad news...

Good news: One can understand GR as drawing space-time diagrams on the surface of some curved sheet of paper.

Bad news: The surface one really has to draw on doesn't look like anything the 250 px bmp attached earlier in the thread.

Good news: There's a paper by Marolf, http://arxiv.org/abs/gr-qc/9806123, that describes the surface you DO need to draw your space-time diagram on (an embedding) to model the r-t plane of a Schwarzschld black hole

Bad news: This paper, http://arxiv.org/abs/gr-qc/9806123, isn't terribly accessible to the layperson, even if one optimistically assumes that the reader is already familiar with drawing space-time diagrams and doing Lorentz transforms.

The important points about Marolf's shape is that Lorentz transforms work correctly in the flat tangent space on said curved surface, and furthermore that the natural path of matter free-falling is a geodesic path (the closest thing you can draw to a straight line) on said surface.

Good news: The "250 px bitmap" earlier mention in the thread does look a little bit like the purely spatial part of the space-time curvature around a massive object. Furthermore, the purely spatial part of the curvature explains a few interesting things about gravity.

Bad news: The interesting things spatial curvature does explain are second order effects - interesting, but not striking to the heart of the topic. Things like light deflection and Mercury's perihelion advance, which were crucial to the acceptance of the theory.

Good news: If you viewed space-curvature as an "add-on" to Newtonian theory, it might actually convey a lot of what happens - use Newtonian theory for the main predictions of gravity, and the spatial curvatrue part of "unexpected extra effects" that are due to the full GR theory and not expected by Newtonian theory or any simple "mash-up" arising from it.

Summary: There doesn't seem to me to be a really good way of describing GR without a significant prior background, which seems to include as a minimum understanding special relativity, the Lorentz transform, and enough about geodesics on curved surfaces to feel comfortable talking about them. Furthermore, diagrams which are commonly shown and do not presuppose this sort of background seem to be rather misleading.

 

[jtbell]

posts are getting deleted by database errors?

A person who was previously banned from here, reappeared under another name. We deleted his posts and the responses to them (which would have looked strange all by themselves).

 

[HmmTheCat]

If photons make up light (electromagnetic radiation) and radiation can be measured as energy, and energy can be measured as a mass, then why couldn't light have, at least, an extremely small mass?

Say you have an x-watt lightbulb

watts = Joules/second (power/sec)

Power is the conversion of energy (also radiant energy).

Energy is defined
- mechanically by ΔE=work
-- Work=Force X Distance
--- Force=mass X acceleration
- as E=mass X (speed of light)^2

 

[pervect]

If photons make up light (electromagnetic radiation) and radiation can be measured as energy, and energy can be measured as a mass, then why couldn't light have, at least, an extremely small mass?

Say you have an x-watt lightbulb

watts = Joules/second (power/sec)

Power is the conversion of energy (also radiant energy).

Energy is defined
- mechanically by ΔE=work
-- Work=Force X Distance
--- Force=mass X acceleration
- as E=mass X (speed of light)^2

We have a FAQ on this. Hopefully it will answer your question (just click on the link). If it doesn't, please give us some feedback so that the FAQ can be improved.

 

[rbj]

Hmm, there are some different definitions of mass that people have here. i, personally, like having the concept of relativistic mass and rest mass. the former is now a deprecated term (but i like it) and the latter is now usually called simply "mass" or sometimes "invariant mass".

anyway using the now deprecated terminology, photons with energy $E=h\nu$ have a relativistic mass of $m=E/c^2$. but their rest mass must be zero if they move at the speed of $c$. any particle that moves at the speed of $c$ must have an infinite energy if it has any non-zero mass at rest.

 

[Drakkith]

If photons make up light (electromagnetic radiation) and radiation can be measured as energy, and energy can be measured as a mass, then why couldn't light have, at least, an extremely small mass?

It is important to understand that "mass" refers to "invariant mass". That is, mass that doesn't change when you switch between different frames of reference. Light has energy, and will add to the mass of the SYSTEM it is in, but it itself does not have mass as in invariant mass. That said, it is always possible light is actually massive but has an extremely small amount of mass. Current measurements have shown that if light has mass it must be below 3x10^-27 eV/c². This is a VERY small number. For example, the energy of a visible light photon is around 1.5-3.0 eV.

http://en.wikipedia.org/wiki/Photon#Experimental_checks_on_photon_mass

 

[ZapperZ]

If photons make up light (electromagnetic radiation) and radiation can be measured as energy, and energy can be measured as a mass, then why couldn't light have, at least, an extremely small mass?

Say you have an x-watt lightbulb

watts = Joules/second (power/sec)

Power is the conversion of energy (also radiant energy).

Energy is defined
- mechanically by ΔE=work
-- Work=Force X Distance
--- Force=mass X acceleration
- as E=mass X (speed of light)^2

Please read the FAQ subforum

https://www.physicsforums.com/forumdisplay.php?f=210

Zz.

 

[harve]

Do you mean that light can be accelerated by gravity? Curvating motion also means acceleration.

With regards to the black hole gravity, that would probably mean that the curvature in spacetime in that locale is so great that even the speed of light cannot escape it.

That is just my take.[/QUOTE]

 

[Nugatory]

Do you mean that light can be accelerated by gravity? Curvating motion also means acceleration.

With regards to the black hole gravity, that would probably mean that the curvature in spacetime in that locale is so great that even the speed of light cannot escape it.

There's no substitute for actually doing the math, but the above is a pretty decent summary of how gravity affects light. You might want to be careful with that word "accelerated" - are you thinking that gravity can change the speed at which light moves? It doesn't, it just changes the direction of travel; this effect has actually been observed.

 

[TomTelford]

Now if I remember what I read correctly then the photon should be increasing in energy as it falls into the gravity well, if not velocity, effictively blue-shifting it off the scale. Let me know if this is incorrect.

Tom.

 

[Nugatory]

Now if I remember what I read correctly then the photon should be increasing in energy as it falls into the gravity well, if not velocity, effictively blue-shifting it off the scale. Let me know if this is incorrect.

That is correct.

 

[harrylin]

There's no substitute for actually doing the math, but the above is a pretty decent summary of how gravity affects light. You might want to be careful with that word "accelerated" - are you thinking that gravity can change the speed at which light moves? It doesn't, it just changes the direction of travel; this effect has actually been observed.

In fact it does, as expressed in "non-local" coordinates; the change of direction was first predicted as due to the gradient in speed (Huygens construction).

 

[harrylin]

Now if I remember what I read correctly then the photon should be increasing in energy as it falls into the gravity well, if not velocity, effictively blue-shifting it off the scale. Let me know if this is incorrect.

Tom.

That is only true in "local" coordinates, which do not conserve energy. As a matter of fact, the observed blueshift is ascribed to gravitational time dilation of the clocks at lower gravitational potential. There have been several discussions with detailed clarifications on that topic in this forum.

 

[harve]

tl;dr
But the idea that something needs mass to be affected by gravity is obviously false - all things that have mass are affected absolutely identically by gravity, they receive exactly the same acceleration, regardless of their mass. So even if they had zero mass, it would be normal to assume they will still be affected in the same manner and get the same acceleration.

Do you mean that light can be accelerated overcoming its constant speed,or simply follows the space curvature? But curvating motion also means acceleration.

 

[Drakkith]

Do you mean that light can be accelerated overcoming its constant speed,or simply follows the space curvature? But curvating motion also means acceleration.

The velocity of light is always c, yet it is affected by gravity and will change its direction of propagation.

 

[pervect]

One thing should be clarified. The velocity of light is always "c" using local clocks and rulers, which means that in a coordinate independent sense, it's always "c".

The rate of change of the distance coordinate with respect to the time coordinate isn't always "c". So it's important to know how you are defining velocity before you talk about it. If you define it as being measured by local clocks and rulers, then it's always constant.

It's a separate argument about why that's the best way to define velocity - I find that it's mostly a waste of time. it may be worth mentioning - errr repeating - that the issue is one of coordinate independence.