# Some confusing things

Tags: confusing, things
P: n/a
 Originally posted by chroot Arcon, I don't know what you're talking about. A typical neutron star is about the mass of the earth and is about 7 km in radius. The photonsphere for an object with the mass of the earth is 1.33 centimeters. I have no idea where you got 3 million kilometers from. - Warren
Neutron stars do not have a mass ever close to that of the Earth. A neutron star is what is left after a high mass star goes supernova and has a mass of about the same as our sun.

Note that I didn't say that the calculation was for a neutron star. I said assume that the maximum density was that of a neutron star and then set R = 2.5GM/c2. That means assume that you have a spherical object whose mass density is that of the center of the nucleus of an atom and assume that the radius allowed for a photonsphere. What is that radius? Again - assume a naive calculation by leaving out pressure - just to give a rouch idea of the size of such an object.

The density of the nucleus = density of a neutron star = 3x1017kg/m3.

$$M = \rho V = \rho (\frac {4}{3}\pi R^{3})$$

Substitute that into R = 2.5GM/c2 and solve for R. You'll get about 2.2x109 meters.

The whole point was to illustrate that such an object is unlikely to exist.
 Emeritus Sci Advisor PF Gold P: 10,427 If you're asking: What would be the radius of an object, with density 3 * 10^17 kg/m^3, whose photonsphere is right at its edge? The answer is 18,901 meters. I still have no idea where you're getting 2.2 * 10^9 meters, and I have no idea why you're using a factor 2.5 instead of 3. - Warren
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 Originally posted by chroot If you're asking: What would be the radius of an object, with density 3 * 10^17 kg/m^3, whose photonsphere is right at its edge? The answer is 18,901 meters. I still have no idea where you're getting 2.2 * 10^9 meters, and I have no idea why you're using a factor 2.5 instead of 3. - Warren
I see what happened. I did the arithmetic wrong. I used 2.5 to be somewhere betweeen a black hold radius and a photonsphere radius. Otherwise you'd have the photons riding on the surface.
P: 36
 Originally posted by wolram may i with respect to other posters point out that gravitational radiation has not been found to date, and is hypothetical new detector might find it in a few years, but what if they do not?
Wolf, that is no longer true. Check out the BBC's Web Site, and look in the Science Section.

Some scientists have recently created a "Gravitational Wave Detector",
and have (to their satisfaction) proved the existence of a gravity wave, and obtained additional information about it.
P: 36
 Originally posted by Link how can a photon sphere have a mass? Isnt light massless?
Link, despite what I read in every physics book, photons have been found to NOT be massless particles.

If I recall correcly, the mass of a photon is estimated at 10^-48th
of the mass of an electron.

This is published on a reputable Web Site, which I appologize for not being able to provide, BUT I can provide other links that
can tell you the estimated mass of a photon:

"What is the Mass of a Photon?" See:
http://math.ucr.edu/home/baez/physic...oton_mass.html

For a discussion of it, in another String Theory Forum, See:
http://superstringtheory.com/forum/p...ages10/23.html

If you want more, Google: photon mass
(without quotes, using both words)
 Emeritus PF Gold P: 8,147 Treat, please stop posting this, you have misinterpreted Baez' site (a good one), and the superstring site is by a crank, kx21. Baez says, as I replied to you on another thread, that those small numbers are limits. They are the unavoidable experimantal error limits of the measurements; they are NOT the mass of the photon.
P: 176
 Originally posted by treat2 Wolf, that is no longer true. Check out the BBC's Web Site, and look in the Science Section. Some scientists have recently created a "Gravitational Wave Detector", and have (to their satisfaction) proved the existence of a gravity wave, and obtained additional information about it.
No, they haven't. If gravity waves had been detected, then more than just the BBC would be reporting it (specifically, every physics-related publication in the world).

Also note that there are more than one "gravity wave detector". The most commonly known is LIGO, but there are many others (LISA, etc...).
 P: 273 It seems to me nowadays people just believe in the rest mass. I remember seeing a news in yahoo.com sometimes earlier and Arcon mentioned that. The experiment was performed in measuring possible gravity effect between two light waves. I remember it did say the experiment shows positive result suggesting attracting force between two light waves. What is Modern Physics's opinion on this experiment?
 Sci Advisor P: 674 Gravity waves have been "detected" indirectly for a long time now. Hulse and Taylor won a Nobel prize for finding that a system of binary pulsars was losing energy at exactly the rate predicted by GR due to gravitational radiation. Recent observations have narrowed down the error bars dramatically on these types of systems, and GR still passes. I agree though that seeing gravitational waves through LIGO et al would be huge news. Anyway Link, gravitational waves are "created" similarly to electromagnetic waves. You accelerate charges to get light, and you accelerate masses to get gravity waves (more or less).

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