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why are photons affected by gravity


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If the photon truly has no mass then why are they bent by gravity fields ? having no mass should mean that gravity has nothing to attract or pull against and yet quite clearly it does. To my mind either the photon does have some physical mass after all, or it is not the particles themselves that are bent around large masses, but the energy waves like a fluid is directed around a boulder in a stream. If the later is true then does this not mean that light itself is just energy waves of varying frequency not particles. There is a third option which might explain this which is that the photon particles don't actually move very much but rather collide together transferring their energy as observed in a newton cradle or as the ocean waves behave what do you think?

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Indeed, a photon at rest is massless. However, photons usually travel at the speed of light so they have a momentum which is proportional to its wavelenth. It is this momentum which acts as a pseudo-mass source which is affected by gravity.

Steve..

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guys can I throw another idea into the arena which is its not the photon we have the problem with it is in fact gravity itself in that it is not really a force at all but instead just the gradient of the curvature of space and that what we observe to be gravitational effect are in fact the intere-action of differences of density of mass or to put it more crudely the effects of buoyancy

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The role of gravity has given the great minds of our time massive headaches for many years (i.e. Stephen Hawking). Gravity is indeed a force, it is the weakest of the four fundamental forces know to physicists today, the others being two atomic (forces holding neutrons and protons together and radioactive decay) and electromagnetic forces. It is exactly how gravity works that is the question and its relative weakness compared to the other three forces has been the ultimate question in physics recently. Current thinking shows multiple dimensions which 'weaken' or dissipate the effect of gravity. There are currently experiments being developed at one of the worlds largest particle accelerators to prove the existence of a multidimensional universe.

Steve..

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Physics, and especially astrophysics is a great science if you love open questions with no answer, or mind boggling if you dont lol. Definatly not for those who like clear cut answers. They are theorising that there are tons of tiny extra dimensions all spewing out bits of gravity which the SERN LHC maybe able to spot http://public.web.cern.ch/Public/en/LHC/LHC-en.html as Paxo was saying, and if its turns out to be true it will lead to endless new questions and possibilities. A very exciting time.

Until then however, I tend to use this method:

post-14143-133877338962_thumb.jpg

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If the later is true then does this not mean that light itself is just energy waves of varying frequency not particles.

Indeed, light is a wave of electromagnetic energy. The particle mental picture results from the observed fact that this wave is always emitted and absorbed in "packets" (called quanta or photons) whose energy is only dependent on the frequency.

Gravity, in Einstein's model, is expected to act on all energy content (and mass is one of those) so there is no conceptual problem with light being bent by gravity.

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Einstein's equation tells us that energy equals mass (times the square of the speed of light). So energy and mass are really the same thing, apart from a conversion factor (like dollars and pounds or miles and kilometres). Light has energy, so light has mass: photons have zero "rest mass" but never actually stand still. Blue photons have higher energy than red photons, so they have more mass.

If you throw a ball in the air, gravity reduces its energy and makes it slow down. If you throw photons in the air (from a torch, say), gravity can't slow the photons down, but it drains some of their energy by lengthening their wavelength, in other words "redshifting". For a torchbeam leaving Earth the effect is too minute to notice, but for photons leaving very massive stars it's significant - gravitational redshift was first measured in the 30s, I believe, from studying white-dwarf spectra.

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If you throw a ball in the air, gravity reduces its energy and makes it slow down. If you throw photons in the air (from a torch, say), gravity can't slow the photons down, but it drains some of their energy by lengthening their wavelength, in other words "redshifting". For a torchbeam leaving Earth the effect is too minute to notice, but for photons leaving very massive stars it's significant - gravitational redshift was first measured in the 30s, I believe, from studying white-dwarf spectra.

A very strange side effect here with black holes - if you are a stationary observer "at infinity", and you watch an object fall into a black hole, instead of just disappearing over the event horizon, the object instead slows down as it approaches the event horizon, and fades away becoming more and more redshifted over time. Most odd. This is a combination of gravitational redshifting, and time dilation in a gravitational field.

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Light has energy, so light has mass

I don't think any physicist would be comfortable with that way of saying things. Light does not have mass although its energy can be turned into mass, as in "pair-production" of an electron and its anti-particle.

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You're right, Themos, physicists generally aren't happy with it - but that's what the equation says. All a matter of interpretation, really.

(BTW I PM'd you about another topic).

Cheers, Andrew

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You're right, Themos, physicists generally aren't happy with it - but that's what the equation says. All a matter of interpretation, really.

(BTW I PM'd you about another topic).

Cheers, Andrew

I think you are misreading the equation. The photon equation does not contain a mass term. Of course, any amount of energy can be equivalent to some amount of mass but that's not to say that any energy IS mass. Heat and electricity can both be measured in energy units but one is not the same as the other.

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I think there's a philosophical issue here about the verbs "equivalent to" and "is" (and a question of which is meant by the symbol = ). In natural units (c=h=1) the Einstein and Planck equations become E=m and E=f.

If you heat up a saucepan of water its mass increases. This is not measureable for a saucepan of water, but makes a significant contribution to the gravitation of stars (their internal pressure is also important - we know from general relativity that not only mass but also pressure is a source of gravity, via the stress-energy tensor).

Now, if I heat up a saucepan of water and find that its gravitational field increases, is it more correct to say that its mass has been raised, or that its energy has been raised? It is, as I say, a philosophical issue. We feel happy with a word like "mass" because we all have a sense of what it feels like when something lands on our toe; we're lesss comfortable with "energy", which has exotic or even mystical overtones. But what you feel when something lands on your toe is energy as much as it is mass.

You say potatoes and I say...

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Now, if I heat up a saucepan of water and find that its gravitational field increases, is it more correct to say that its mass has been raised, or that its energy has been raised? It is, as I say, a philosophical issue.

I would say that the stress-energy tensor, the source of gravity, has been changed.

For light/photons, I think the http://en.wikipedia.org/wiki/Electromagnetic_stress-energy_tensor formulas are correct and give you the stress-energy tensor in terms of the electric and magnetic fields associated with light.

Ok, maybe I am escaping into jargon here :afro:

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John Baez gives a very nice summary of general relativity at:

http://math.ucr.edu/home/baez/einstein/node3.html

"Given a small ball of freely falling test particles initially at rest with respect to each other, the rate at which it begins to shrink is proportional to its volume times: the energy density at the center of the ball, plus the pressure in the x direction at that point, plus the pressure in the y direction, plus the pressure in the z direction."

I suppose all we're really quibbling about is words (especially "energy"), we obviously agree on the equations.

And if I ever get that 12-inch dob I'll make damn sure I don't drop its stress-energy on my toe! :afro:

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I think there's a philosophical issue here about the verbs "equivalent to" and "is" (and a question of which is meant by the symbol = ). In natural units (c=h=1) the Einstein and Planck equations become E=m and E=f.

Lately I've been wondering...

Photons supposedly have no mass, but they contribute to the stress-energy tensor in general relativity and so they apparently exert gravity on objects with mass. They have pseudo-mass, like Steve said. What's more, with natural units (as Acey said) their "mass" = energy = frequency. Therefore, the "mass" of a photon is directly equal to it's frequency.

I daren't even hazard a guess at how many photons exist in the universe, so even if the most energetic gamma rays have only an infinitessimal mass, surely together they much exert a considerable amount of gravity.

So I apologise if this is a naive question, but I was wondering if anyone could tell me -- do the calculations that call for dark matter consider this? If not, why not? I know most physicists don't like to think of photons as having mass...

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In human terms, yes every photon in the universe would probaly add up to a large number, however in cosmological terms it will still be tiny.

Alot of people I talk to think its just a small ammount of mass (or as you imply very reasonably, gravitaional influence) we are missing (10-20% is a general number I get from people) but its actually more like 90% of the mass which is dark matter. So although you cant really rule anything out at the momment, I wouldnt put my money on it :shocked:

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Is it enough for a layman to think of each particle/ object as having a set amount of mass/ energy and that the two are interchangeable? ie the photon has a lot of energy but no mass, if it had a little less energy it would have a little more mass?

:? :?

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In human terms, yes every photon in the universe would probaly add up to a large number, however in cosmological terms it will still be tiny.

Heh... Envelope calculations. Because I can. :shocked:

If the Sun's total energy hitting the Earth is 174x10^15W, assuming average wavelength of all photons to be 500nm and assuming the sun's output is constant over 12 billion years, gave me this -- A star like the Sun will output somewhere in the region of 30 googol photons in it's lifetime!

Quite impressive...

Is it enough for a layman to think of each particle/ object as having a set amount of mass/ energy and that the two are interchangeable? ie the photon has a lot of energy but no mass, if it had a little less energy it would have a little more mass?

:? :?

Other way around. More energy means more effective mass. :lol:

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I always thought that it was the spactime in which photons travel that is effected by gravity not the photons themselves and that the photons are just travelling through warped space.

kpax

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