- #1
redtree
- 321
- 13
Does a photon curve space-time, i.e., produce a gravitational field? Is the degree of curvature a function of its energy?
jnorman said:no. a photon exhibits no locality.
Yo jnorman! How's it going?jnorman said:no. a photon exhibits no locality.
jnorman said:hi pmb - i think we have had this discussion before, eh? once a single photon is emitted, it is essentially everywhere in the universe - ie, it displays no locality (ie, its position cannot be defined with ANY precision). since it has no defined position, it, ergo, cannot exhibit any local effect on gravitational field. as i recall, a light beam can, however, generate observable field effects. as always, feel free to correct me...
redtree said:Does a photon curve space-time, i.e., produce a gravitational field? Is the degree of curvature a function of its energy?
LURCH said:I am pretty sure the photon does not have a gravitational field. I say this because a photon has no mass other than relativistic mass.
LURCH said:I am pretty sure the photon does not have a gravitational field. I say this because a photon has no mass other than relativistic mass. If relativistic mass is a source of gravity, apparent paradoxes arise. If these paradoxes cannot be resolved, they serve as proof that relativistic mass cannot be a source of gravity, and gravity only proceeds from rest mass. Since a photon has no rest mass, it has no gravity. (That's my speculation.)
This quote was not by Einstein but by John Wheeler, and he said "matter", as a generic term, not "mass" (see C. W. Misner, K. Thorne, J. Wheeler, "Gravitation", W. H. Freeman (1973), page 5). Also it is just a pedagogical way to summarize the meaning of general relativity, one must always refer to the actual formulas to determine what the theory precisely says.mitesh9 said:As Einstein Said "The mass tells the space how to curve, and the space(-time curvature) tells the mass how to move", which may translate to "every thing that has mass create a curvature in spacetime and everything that follows the curved spacetime (i.e. responds to gravity) has mass".
Phrak said:Place equal amounts of matter and anitmatter in a box on a scale. It's a very good box; it's very reflective, and light doesn't get in or out. Allow all the stuff to annihilate to photons. Does the box change weight?
To be precise there is no quantum theory of gravity (or relativistic quantum mechanics) so we're really all taking an educated guess. However I see no reason to assert that a photon is everywhere in the universe. Quantum mechanics certainly makes no such assertion. All that can be said is that for each quantum state of any particle there is an associated wave function. The physical interpretation of that wave function is that the magnitude squared of the function represents the probability density of finding the particle in a particular region. Only when the exact value of the momentum is determined will the probability density be uniform and thus the chances of finding it anywhere in the universe will be zero. However that comes from non-relativistic quantum mechanics. Relativity restricts the speed of a particle to less than the speed of light and therefore the probability density can never be uniform. And even this interpretation of pronability refers only to essembles of identical experiments, not to individual experiments. There is no restriction on the limits of accuracy placed on each single measurement.jnorman said:hi pmb - i think we have had this discussion before, eh? once a single photon is emitted, it is essentially everywhere in the universe - ie, it displays no locality (ie, its position cannot be defined with ANY precision).
Wheeler made such statements in various places and using different terms each time. In Exploring Black Holes he phrased it using the term mass rather than matter. Due to the way the authors definined mass in that book I protested but Wheeler was adament about it.xantox said:This quote was not by Einstein but by John Wheeler, and he said "matter", as a generic term, not "mass" (see C. W. Misner, K. Thorne, J. Wheeler, "Gravitation", W. H. Freeman (1973), page 5). Also it is just a pedagogical way to summarize the meaning of general relativity, one must always refer to the actual formulas to determine what the theory precisely says.
Phrak said:Place equal amounts of matter and anitmatter in a box on a scale. It's a very good box; it's very reflective, and light doesn't get in or out. Allow all the stuff to annihilate to photons. Does the box change weight?
You're assuming the the mass-energy of the photon in question is so small as to be neglegible. I see no reason to make that assertion. I see no reason that a photon with large enough mass-energy can't generate a very strong and measureable gravitational field. Especially since the magnitude of the gravitational field is frame dependant and thus one can always transform to a new frame where the mass-energy is as large as one would like.LURCH said:I think we can all agree that it is not possible (at present) for us to measure the gravitational field of a photon. So, all of this discussion must be somewhat speculative. The correct answer to this question is not known with any high degree of certainty, is it?
That's the problem with using the term relativistic mass since it seems to imply that it is different than mass. Its well known that mass is the source of gravity. simply turn to page 404 in Gravitation by Misner, Thorne and Wheeler (MTW) to see that. I.e. the authors state on that page in the second paragraph thatI am pretty sure the photon does not have a gravitational field. I say this because a photon has no mass other than relativistic mass. If relativistic mass is a source of gravity, apparent paradoxes arise.
The context of that statement, given Eq. (17.1), the authors are really referring to what you call relativistic mass (and which I and MTW simply call mass). MTW also use the term mass-energy to refer to the same thing. In actuallity MTW are rwefering to the mass density in that expression and are speaking about the T00 component of the stress-energy-momentum (SEM) tensor. And there are no paradoxes exist which can't be resolved under close scrutiny.Mass is the source of gravity.
The source of gravity is not rest mass. Why would you even think that? Its well known that the mathematical quantity which acts as the source of gravity is the SEM tensor which does not vanish for a quantum of light. Even Einstein said that mass is completely determined by the SEM tensor. Since the SEM tensor has energy, momentum and stress terms it follows that each is a source of gravity. A photon has both energy and momentum.If these paradoxes cannot be resolved, they serve as proof that relativistic mass cannot be a source of gravity, and gravity only proceeds from rest mass. Since a photon has no rest mass, it has no gravity. (That's my speculation.)
If you consider the box of matter to be a closed system then photons cannot escape from the box. The walls of the box will then either reflect the photons and thus transfering momentum (and thus weight) to the walls or the walls absorb the energy of the photon with a corresponding increase in the weight of the box. Even a box of photons will generate a gravitational field.Phrak said:Place equal amounts of matter and anitmatter in a box on a scale. It's a very good box; it's very reflective, and light doesn't get in or out. Allow all the stuff to annihilate to photons. Does the box change weight?
pmb_phy said:To be precise there is no quantum theory of gravity (or relativistic quantum mechanics) so we're really all taking an educated guess. However I see no reason to assert that a photon is everywhere in the universe. Quantum mechanics certainly makes no such assertion. All that can be said is that for each quantum state of any particle there is an associated wave function. The physical interpretation of that wave function is that the magnitude squared of the function represents the probability density of finding the particle in a particular region. Only when the exact value of the momentum is determined will the probability density be uniform and thus the chances of finding it anywhere in the universe will be zero. However that comes from non-relativistic quantum mechanics. Relativity restricts the speed of a particle to less than the speed of light and therefore the probability density can never be uniform. And even this interpretation of pronability refers only to essembles of identical experiments, not to individual experiments. There is no restriction on the limits of accuracy placed on each single measurement.
Also keep in mind that all this is asssumes that a measurement has been made and the interaction of a particle with the photon's gravitational field might be considered as such an measurement.
Best wishes
Pete
pmb_phy said:However I see no reason to assert that a photon is everywhere in the universe.
jnorman said:i begin with the notion that a photon moves at C. at C, distance has no meaning - there is no distance between things. ergo, a photon is essentially everywhere at once. and of course, our old general rule - you cannot say anything about a photon in between the time it is emitted and the time it is absorbed...
again, please feel free to knock that down.
"10c km"? What does that mean?mitesh9 said:a distance of 10c km
OK, let's forget that version, as it is of course confusing.pmb_phy said:In Exploring Black Holes he phrased it using the term mass rather than matter.
pmb_phy said:Wheeler made such statements in various places and using different terms each time. In Exploring Black Holes he phrased it using the term mass rather than matter. Due to the way the authors definined mass in that book I protested but Wheeler was adament about it.
Pete
It would have to weigh slightly less. 2 D batteries contain about 150,000 joules of energy. m = E/c^2, so the flashlight will weigh 1.68x10^-12 kg less when it's drained.Or as one of my physics professors said, weigh a flashlight, turn it on till the batteries die then weigh it again now that all the light is out of it =)
Phrak said:This is all every nice, but to which parts of the stress-energy tensor does light contribute?
jnorman said:a photon is essentially everywhere at once.
kev said:I have read on a number of occasions that two parallel light beams do not gravitationally attract each other while two anti-parallel light beams do gravitationally attract each other. Is that true? If it is true, why does it work one way but not the other?
I always assumed the references to "anti-parallel" in this context to mean parallel beams with the photons in each beam moving in opposite directions as opposed to to two parallel beams that have photons moving in the same direction. That is just my interpretation. I assume we mean the same thing?Antenna Guy said:If two parallel light beams pass through a gravitational field, do they remain parallel?
[Clarification: What do you mean by "anti-parallel"? I've seen reference to opposite vs. same direction, but not the way you put it.]
Regards,
Bill
kev said:I always assumed the references to "anti-parallel" in this context to mean parallel beams with the photons in each beam moving in opposite directions as opposed to to two parallel beams that have photons moving in the same direction. That is just my interpretation. I assume we mean the same thing?