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View Full Version : Newton's Law of Gravity can predict gravitational lensing



william chan
09-08-2011, 11:27 AM
Some Food for Thought.

Everybody thought that it was Einstein's General Theory of Relativity about mass distorting space-time that first predicted that light can be bent by gravity.

However, if we consider light as balls of photon which has a mass, then by applying
F = G(M1M2/R*R) it becomes common sense that gravity would surely attract and bend light.

General Theory of Relativity is only one of many models that can predict gravitational lensing.
Why should we be so lucky that the first model that we can think of becomes the truth?

Tor
09-08-2011, 11:52 AM
Photons are massless.. that is, they don't have rest mass. (Not that they can ever be at rest though).

wjsteele
09-08-2011, 12:02 PM
"However, if we consider light as balls of photon which has a mass..."

Hence the fatal flaw. Photons do NOT have mass.

A lot of people assume that E=MC^2 is the correct formula to apply here, which would mean that because the photon has energy, then it must have mass. But it doesn't apply because E=MC^2 isn't the full equation... it only applies when the mass has zero momentum (Rest Mass is the term, like Tor said.) The full equation is E^2 = M^2C^4 + P^2C^2. If the photon was at rest, then P=0 and our formula becomes E=MC^2. Instead, however, for a photon with momentum but no mass, M=0... and our formula is actually P=E/C.

Bill

william chan
09-08-2011, 12:15 PM
Mass and Energy are interchangeable and they could even be the same thing taking different forms at low energies.
For example, 2 protons crashing into each other at high speed can produce anti-electrons.

Newton's equation can be expanded to be used for non-rest mass.

Humanoido
09-08-2011, 01:08 PM
It's exciting to actually verify the equations and see the effects of gravitational lensing through a large telescope that can visibly detect (and verify the absence of matter and through inference dark matter) at what we call the edge of our known Universe. Modern day telescopes are not only massive in terms of light gathering power, they act like much larger telescopes with modern Charge Coupled Devices. A ten inch is like a hundred, and a hundred is like a thousand. This gives amateurs working with modest equipment the ability to do professional research grade studies and programs. The first night with my 12.5-inch reflector and a CCD, acting like a 120-inch telescope, a new star was discovered in the M27 region of space. So it's not uncommon to go deep on the Abell lensing cluster of galaxies.

Spiral_72
09-09-2011, 07:55 PM
The first night with my 12.5-inch reflector and a CCD,

<sigh> You are my hero. I want one!

Duane Degn
09-09-2011, 08:21 PM
However, if we consider light as balls of photon which has a mass, then by applying
F = G(M1M2/R*R) it becomes common sense that gravity would surely attract and bend light.


I only minored in physics but I distinctly remember someone asking a professor about this exact thing. Using the attraction from gravity does not give the correct answer (to the about light is bent).

All of our general relativity questions were qualitative. I never learned the math necessary to calculate quantitative results for general relativity (I got pretty good at special relativity equations though).

Duane

Humanoido
09-10-2011, 02:51 AM
I only minored in physics ... All of our general relativity questions were qualitative. Duane

Duane:

Physics was one of my majors and the teacher went far beyond the MIT Physics book.. making every day a quantitative dream come true. As I recall, the chalk board running across the full width of the big room was not enough to contain all the equations.

At first, I turned 8 1/2 x 11-inch paper sideways to print long equations using a hard lead pencil and very tiny numbers. By the time I was senior, I was much smarter and used the back side of recycled very wide computer printer paper from a big IBM 360.

Spiral_72:

I think you should have one. Do you have any smaller telescopes so far? I began with a 30mm scope in grade school, then years later a view of Saturn's rings through an Edmunds 3-inch reflector telescope got me started! It was like an addiction and a need for increasingly larger telescopes to see more. The next one was a 4.25-inch, some refractors, and then I made more, several 8-inch Newtonians, 8-inch Schmidt, 12.5-inch reflector, 40-inch reflector, and the massive 50-inch.

BTW, Celestron is offering a 3-inch reflector telescope which can be purchased from Amazon.com for $35. Do a search for "Celestron FirstScope."

http://www.amazon.com/Celestron-21024-FirstScope-Telescope/dp/B001UQ6E4Y

Spiral_72
09-12-2011, 02:30 PM
Spiral_72:

I think you should have one. Do you have any smaller telescopes so far? I began with a 30mm scope in grade school, then years later a view of Saturn's rings through an Edmunds 3-inch reflector telescope got me started! It was like an addiction and a need for increasingly larger telescopes to see more. The next one was a 4.25-inch, some refractors, and then I made more, several 8-inch Newtonians, 8-inch Schmidt, 12.5-inch reflector, 40-inch reflector, and the massive 50-inch.

BTW, Celestron is offering a 3-inch reflector telescope which can be purchased from Amazon.com for $35. Do a search for "Celestron FirstScope."

http://www.amazon.com/Celestron-21024-FirstScope-Telescope/dp/B001UQ6E4Y

Wow, it sounds like you have some neat toys at your disposal :)
I used to use my uncle's "Telescope" when I were about 8yrs old to see Jupiter and Saturn's rings..... and of course the moon. This "Telescope" was actually an old 60mm rifle scope on a camera tripod.
Now I have a 8" reflector on an equatorial mount. I use a Canon Rebel 8Mp, with a single axis clock drive. It does alright.I have some images here: http://s250.photobucket.com/albums/gg256/spiral_72/

Humanoido
09-13-2011, 02:21 AM
Wow, it sounds like you have some neat toys at your disposal :)
I used to use my uncle's "Telescope" when I were about 8yrs old to see Jupiter and Saturn's rings..... and of course the moon. This "Telescope" was actually an old 60mm rifle scope on a camera tripod. Now I have a 8" reflector on an equatorial mount. I use a Canon Rebel 8Mp, with a single axis clock drive. It does alright.I have some images here: http://s250.photobucket.com/albums/gg256/spiral_72/

Spiral_72: That's a really great page of your astro photos. Thanks for sharing! I did telescope testing with the Celestron FirstScope last night and will post the results later today on this page: http://forums.parallax.com/showthread.php?124495-Fill-the-Big-Brain/page71

Ale
09-13-2011, 08:36 AM
Speaking of gravity, I was watching yesterday a movie called "Impact" from 2008. Besides the bad acting, and too many mentions to the us president as the only head of state interested in the mateer (!)... at least two mayor things were definitely wrong. A fragment of a "brown dwarf" collides with the moon and makes the moon two times heavier than the earth and modifies its orbit. Now, with such a pulling I'd expect the orbit of the earth to be also altered.... no mention of it, they just go on how only the moon moves towards the earth,,, The second is the supermassive meteorite they find, I mean if the matter is so compressed in the core of the star I'd expect than under normal conditions of temperature and pressure this matter could not possible exist, first it is just nuclei of elements compressed together (no electrons!) and not whole atoms.... Sorry for the movie bashing :).

Humanoido
09-13-2011, 12:15 PM
Ale, I think you could rewrite the movie for greater accuracy and plausibility.

Ale
09-14-2011, 08:06 PM
Hahahaha... but who will watch it ?... I mean, there are almost no sci-fi movies that get physics right... I wonder why....