Go Back   AnandTech Forums > Hardware and Technology > Highly Technical

Forums
· Hardware and Technology
· CPUs and Overclocking
· Motherboards
· Video Cards and Graphics
· Memory and Storage
· Power Supplies
· Cases & Cooling
· SFF, Notebooks, Pre-Built/Barebones PCs
· Networking
· Peripherals
· General Hardware
· Highly Technical
· Computer Help
· Home Theater PCs
· Consumer Electronics
· Digital and Video Cameras
· Mobile Devices & Gadgets
· Audio/Video & Home Theater
· Software
· Software for Windows
· All Things Apple
· *nix Software
· Operating Systems
· Programming
· PC Gaming
· Console Gaming
· Distributed Computing
· Security
· Social
· Off Topic
· Politics and News
· Discussion Club
· Love and Relationships
· The Garage
· Health and Fitness
· Home and Garden
· Merchandise and Shopping
· For Sale/Trade
· Hot Deals with Free Stuff/Contests
· Black Friday 2014
· Forum Issues
· Technical Forum Issues
· Personal Forum Issues
· Suggestion Box
· Moderator Resources
· Moderator Discussions
   

Reply
 
Thread Tools
Old 10-29-2012, 02:20 PM   #1
Mark R
Diamond Member
 
Mark R's Avatar
 
Join Date: Oct 1999
Posts: 8,308
Default Shape of a photon's gravity "field"?

So what is the shape of a photon's gravity field?

OK, these are massless particles, but they have momentum and energy. I'm guess it's not "spherical" as a photon's energy-momentum tensor is somewhat anisotropic.
Mark R is offline   Reply With Quote
Old 10-29-2012, 03:53 PM   #2
SMOGZINN
Diamond Member
 
SMOGZINN's Avatar
 
Join Date: Jun 2005
Location: Texas
Posts: 7,252
Default

Gravity fields don't have a shape, they extend out forever getting less and less powerful as you get further away from the center of mass of the object.
__________________
"The open society, the unrestricted access to knowledge, the unplanned and uninhibited association of men for its furtherance? These are what may make a vast, complex, ever-growing, ever-changing, ever more specialized and expert technological world nevertheless a world of human community." - J. Robert Oppenheimer
SMOGZINN is offline   Reply With Quote
Old 10-29-2012, 08:49 PM   #3
videogames101
Diamond Member
 
videogames101's Avatar
 
Join Date: Aug 2005
Location: 52375
Posts: 6,241
Default

Photons don't produce a gravitational field, they're "affected" by gravity only because spacetime is warped by gravity.
__________________
3570K
HD7870 (Tahiti LE)
videogames101 is offline   Reply With Quote
Old 10-30-2012, 12:31 PM   #4
Jeff7
Lifer
 
Jeff7's Avatar
 
Join Date: Jan 2001
Posts: 39,230
Default

Quote:
Originally Posted by videogames101 View Post
Photons don't produce a gravitational field, they're "affected" by gravity only because spacetime is warped by gravity.
What would happen if you converted a certain amount of mass into a bunch of energetic photons, maybe in a matter/antimatter reaction, or something along those lines.
Does the gravity "disappear" then? Example: Convert 1kg into a tightly-concentrated burst of energy. Does that burst generate any kind of warping in spacetime?
__________________
.
"Homeopathy is what happened when snake oil salesmen discovered that water is cheaper than snake oil."
Jeff7 is offline   Reply With Quote
Old 10-30-2012, 02:24 PM   #5
Paul98
Diamond Member
 
Join Date: Jan 2010
Posts: 3,158
Default

Quote:
Originally Posted by videogames101 View Post
Photons don't produce a gravitational field, they're "affected" by gravity only because spacetime is warped by gravity.
Sure they do just like any mass/energy.
Paul98 is online now   Reply With Quote
Old 10-30-2012, 02:55 PM   #6
Mark R
Diamond Member
 
Mark R's Avatar
 
Join Date: Oct 1999
Posts: 8,308
Default

Hmm. I've just had a bit of a revelation, and my question is not one that is presently answerable (at least, as it is phrased) as there is no established quantum theory of gravity. GR is a classical theory, and a photon is a quantum construct. You therefore cannot examine one's properties in the context of the other.

So, while you cannot ask the question do photons (meaning particles) produce gravity, you can ask the question does light (meaning wave-like light) produce gravity. And the answer to this latter question, is yes.

So, to take Jeff7's question; if you convert 1kg of matter into a burst of energy that is emitted in a symmetrical spherical pattern, then at the instant of conversion the energy has the same gravitational effect as the original mass did.

Perhaps I could rephrase my original question, in the light of the above as: what happens next (as the energy starts to travel outward)?
Mark R is offline   Reply With Quote
Old 10-31-2012, 02:15 AM   #7
PowerEngineer
Platinum Member
 
PowerEngineer's Avatar
 
Join Date: Oct 2001
Posts: 2,493
Default

Quote:
Originally Posted by Mark R View Post
Perhaps I could rephrase my original question, in the light of the above as: what happens next (as the energy starts to travel outward)?
pun intended?

Why wouldn't it be conceptually the same as what happens when a 1 Kg ball of mass is exploded outward in a symmetrical spherical pattern (subtracting out the energy of the explosion)?
PowerEngineer is offline   Reply With Quote
Old 11-01-2012, 07:03 PM   #8
MrDudeMan
Lifer
 
MrDudeMan's Avatar
 
Join Date: Jan 2001
Location: Dallas, TX
Posts: 14,483
Default

Quote:
Originally Posted by Mark R View Post
Hmm. I've just had a bit of a revelation, and my question is not one that is presently answerable (at least, as it is phrased) as there is no established quantum theory of gravity. GR is a classical theory, and a photon is a quantum construct. You therefore cannot examine one's properties in the context of the other.

So, while you cannot ask the question do photons (meaning particles) produce gravity, you can ask the question does light (meaning wave-like light) produce gravity. And the answer to this latter question, is yes.

So, to take Jeff7's question; if you convert 1kg of matter into a burst of energy that is emitted in a symmetrical spherical pattern, then at the instant of conversion the energy has the same gravitational effect as the original mass did.

Perhaps I could rephrase my original question, in the light of the above as: what happens next (as the energy starts to travel outward)?
How do you know? I don't remember that from the one quantum class I took, so I'd like to read more if you have any information.
__________________
So banned it hurts.
Anandtech Moderator

105-1-0
MrDudeMan is offline   Reply With Quote
Old 11-02-2012, 03:00 PM   #9
Mark R
Diamond Member
 
Mark R's Avatar
 
Join Date: Oct 1999
Posts: 8,308
Default

Quote:
Originally Posted by MrDudeMan View Post
How do you know? I don't remember that from the one quantum class I took, so I'd like to read more if you have any information.
This is a GR concept, rather than a quantum concept - and as the two are mutually incompatible....

Anyway, I believe that this is a result of Birkhoff's theorem. Although, I'm not able to even begin to solve the EFE for myself.
Mark R is offline   Reply With Quote
Old 11-03-2012, 12:54 AM   #10
videogames101
Diamond Member
 
videogames101's Avatar
 
Join Date: Aug 2005
Location: 52375
Posts: 6,241
Default

Quote:
Originally Posted by Paul98 View Post
Sure they do just like any mass/energy.
Upon further investigation, this is correct and I was wrong.
__________________
3570K
HD7870 (Tahiti LE)
videogames101 is offline   Reply With Quote
Old 11-03-2012, 03:37 AM   #11
PlasmaBomb
Lifer
 
PlasmaBomb's Avatar
 
Join Date: Nov 2004
Location: In a pub... in Cumbria
Posts: 11,781
Default

Some quantum electrodynamics may help answer - or at least provide further reading Mark...

Quote:
The energy of a system that emits a photon is decreased by the energy of the photon as measured in the rest frame of the emitting system, which may result in a reduction in mass in the amount . Similarly, the mass of a system that absorbs a photon is increased by a corresponding amount. As an application, the energy balance of nuclear reactions involving photons is commonly written in terms of the masses of the nuclei involved, and terms of the form for the gamma photons (and for other relevant energies, such as the recoil energy of nuclei).[86]
This concept is applied in key predictions of quantum electrodynamics (QED, see above). In that theory, the mass of electrons (or, more generally, leptons) is modified by including the mass contributions of virtual photons, in a technique known as renormalization. Such "radiative corrections" contribute to a number of predictions of QED, such as the magnetic dipole moment of leptons, the Lamb shift, and the hyperfine structure of bound lepton pairs, such as muonium and positronium.[87]
Since photons contribute to the stress-energy tensor, they exert a gravitational attraction on other objects, according to the theory of general relativity. Conversely, photons are themselves affected by gravity; their normally straight trajectories may be bent by warped spacetime, as in gravitational lensing, and their frequencies may be lowered by moving to a higher gravitational potential, as in the Pound-Rebka experiment. However, these effects are not specific to photons; exactly the same effects would be predicted for classical electromagnetic waves.[88]
http://en.wikipedia.org/wiki/Photon#...ss_of_a_system
__________________
PlasmaBomb is offline   Reply With Quote
Old 11-26-2012, 11:15 PM   #12
disappoint
Diamond Member
 
disappoint's Avatar
 
Join Date: Dec 2009
Posts: 6,263
Default

Enter Nasa.gov:

http://imagine.gsfc.nasa.gov/docs/as...rs/961102.html

Quote:
The Question

(Submitted November 02, 1996) This questions has been bugging me and my chemistry class. Does light have mass? Most people would think not but here's why I argue against it. Even though light does not effect anything it its path like a solid object, it is affected by gravity. Anything that has mass is affected by gravity. Why do I say that light has mass? Well, If a black holes gravity field is so strong that light cannot escape itself, light must have mass? Am I right? Everyone argues against it.

The Answer

These are interesting issues that you bring up. Whether or not light (or more accurately photons, the indivisible units in which light can be emitted or absorbed) has mass, and how it is affected by gravity, puzzled scientists for many, many years. Figuring it all out is what made Albert Einstein famous. Bear with me and I'll try to explain both the theory and the observation. Back in the 1700s, scientists were still struggling to understand which theory of light was correct: was it composed of particles or was it made of waves? Under the theory that light is waves, it was not clear how it would respond to gravity. But if light was composed of particles, it would be expected that they would be affected by gravity in the same way apples and planets are. This expectation grew when it was discovered that light did not travel infinitely fast, but with a finite measurable velocity.

Armed with these facts, a paper was published in 1783 by John Michell, in which he pointed out that a sufficiently massive compact star would possess a strong enough gravitational field that light could not escape --- any light emitted from the star's surface would be dragged back by the star's gravity before it could get very far. The French scientist Laplace came to a similar conclusion at roughly the same time.
Not much was done over the next hundred years or so with the ideas of Michell and Laplace. This was mostly true because during that time, the wave theory of light became the more accepted one. And no one understood how light, as a wave, could be affected by gravity.
Enter Albert Einstein. In 1915 he proposed the theory of general relativity. General relativity explained, in a consistent way, how gravity affects light. We now knew that while photons have no mass, they do possess momentum (so your statement about light not affecting matter is incorrect). We also knew that photons are affected by gravitational fields not because photons have mass, but because gravitational fields (in particular, strong gravitational fields) change the shape of space-time. The photons are responding to the curvature in space-time, not directly to the gravitational field. Space-time is the four-dimensional "space" we live in -- there are 3 spatial dimensions (think of X,Y, and Z) and one time dimension.

Let us relate this to light traveling near a star. The strong gravitational field of the star changes the paths of light rays in space-time from what they would have been had the star not been present. Specifically, the path of the light is bent slightly inward toward the surface of the star. We see this effect all the time when we observe distant stars in our Universe. As a star contracts, the gravitational field at its surface gets stronger, thus bending the light more. This makes it more and more difficult for light from the star to escape, thus it appears to us that the star is dimmer. Eventually, if the star shrinks to a certain critical radius, the gravitational field at the surface becomes so strong that the path of the light is bent so severely inward so that it returns to the star itself. The light can no longer escape. According to the theory of relativity, nothing can travel faster than light. Thus, if light cannot escape, neither can anything else. Everything is dragged back by the gravitational field. We call the region of space for which this condition is true a "black hole" (a term first coined by American scientist John Wheeler in 1969).

Now, being scientists, we do not just accept theories like general relativity or conclusions like photons have no mass. We constantly test them, trying to definitively prove or disprove. So far, general relativity has withstood every test. And try as we might, we can measure no mass for the photon. We can just put upper limits on what mass it can have. These upper limits are determined by the sensitivity of the experiment we are using to try to "weigh the photon". The last number I saw was that a photon, if it has any mass at all, must be less than 4 x 10-48 grams. For comparison, the electron has a mass of 9 x 10-28 grams.

Hope this answers the questions that you and your Chemistry class have.

Good luck,
Laura Whitlock.
__________________
A man can't keep people from having a bad opinion of him, but he can keep them from being right about it.
We give evil it's greatest power through our belief in it.
"Pay no attention to what the critics say. A statue has never been put up to a critic." -Jean Sibelius
Per aspera ad astra: Through hardship to the stars.
disappoint is online now   Reply With Quote
Old 11-28-2012, 03:26 PM   #13
DrPizza
Administrator
Elite Member
Goat Whisperer
 
DrPizza's Avatar
 
Join Date: Mar 2001
Location: Western NY
Posts: 44,948
Default

Back to the OP - in whose frame of reference? The photon's frame of reference? Or our frame of reference?
__________________
Fainting Goats
DrPizza is online now   Reply With Quote
Reply

Thread Tools

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT -5. The time now is 12:31 AM.


Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.