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Old Sep 2nd 2014, 01:49 PM   #21
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In SR you could probably get away with the statement that space is flat and time is "expanding." (Though this is not a good way to look at it.) In GR, on the other hand, space-time is influenced by the distribution of masses in it. Space-time itself is "evolving" and curved in 3 + 1 dimensions.

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Old Sep 4th 2014, 03:45 AM   #22
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Originally Posted by topsquark View Post
Actually it does. The photon, being massless, always travels at c. On the other hand we have the relationship: E = h(nu). As the photon travels "higher" out of the potential well created by the black hole it loses energy the only way it can: the frequency shifts to the red end of the spectrum. That's where the phrase "red shift" comes from.
That's what they say Dan. But it isn't true. Flip it around and think of blue shift. You start with a 511keV photon, and you send it into a black hole. The black hole mass increases by 511keV/c. Not by some other amount. Conservation of energy applies. Gravity is not a force in the Newtonian sense, there is no magical mysterious mechanism by which a descending or ascending photon somehow gains or loses energy in a gravitational field. It appears to, but it doesn't. It's a bit like what happens when you accelerate towards a photon in gravity-free space. The photon appears blue shifted to you, but it didn't change a jot. Instead, you changed.
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Old Sep 4th 2014, 11:48 AM   #23
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Originally Posted by Farsight View Post
That's what they say Dan. But it isn't true. Flip it around and think of blue shift. You start with a 511keV photon, and you send it into a black hole. The black hole mass increases by 511keV/c. Not by some other amount. Conservation of energy applies. Gravity is not a force in the Newtonian sense, there is no magical mysterious mechanism by which a descending or ascending photon somehow gains or loses energy in a gravitational field. It appears to, but it doesn't. It's a bit like what happens when you accelerate towards a photon in gravity-free space. The photon appears blue shifted to you, but it didn't change a jot. Instead, you changed.
I have to disagree again. If you are standing on a (very) large mass, you can use a black hole if you like, throwing a rock upward lessens its initial kinetic energy and trades it for potential energy. GR says that energy will do the same. In fact if we are within the Schwartzschild radius the photon not only loses "kinetic energy" but actually turns around, which is about as weird as it gets to my mind anyway. Energy is still conserved even while talking about a massless particle, but as the photon is trading "kinetic energy" it is gaining in potential energy. The way the photon loses kinetic energy is by changing its frequency.

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Old Sep 4th 2014, 12:22 PM   #24
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Originally Posted by topsquark View Post
I have to disagree again. If you are standing on a (very) large mass, you can use a black hole if you like, throwing a rock upward lessens its initial kinetic energy and trades it for potential energy.
No problem with that. You did work on the rock. The kinetic energy is converted into potential energy. This is in the rock*. So the mass of the rock has increased**. We could do the same with you. Then because your mass-energy has increased, the energy of the photon appears to have decreased. I swear this is how it is. Cross my heart and hope to die. But for the life of me I cannot explain why this is not what's taught. I've been thinking of writing a letter to Nature about it. Seriously.

Originally Posted by topsquark View Post
GR says that energy will do the same. In fact if we are within the Schwartzschild radius the photon not only loses "kinetic energy" but actually turns around, which is about as weird as it gets to my mind anyway. Energy is still conserved even while talking about a massless particle, but as the photon is trading "kinetic energy" it is gaining in potential energy. The way the photon loses kinetic energy is by changing its frequency.
I'm sorry Dan, but I fancy I know a thing or two about GR, and this doesn't sound right. Where have you got this from? The photon "is" kinetic energy. When you fire a photon straight up, it doesn't slow down or fall back down. And when you direct it straight down, it doesn't speed up. As for what happens within the Schwartzschild radius, that's another can of worms. Maybe we need a new thread.


* the momentum of the Earth and the rock is equal and opposite, but the Earth doesn't move much, so the rock gets the lion's share of the kinetic energy.

** if in doubt check out what happens to a falling rock. After the kinetic energy is radiated away, there's a mass deficit.
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Old Oct 23rd 2014, 12:26 PM   #25
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It has recently occurred to me that in my model of an infinite and expanding universe, all things would expand into existence as we witness them! This phenomena is akin to time being an illusion. If anyone is interested I will elaborate.
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Old Nov 16th 2014, 05:05 PM   #26
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Originally Posted by Troll View Post
It has recently occurred to me that in my model of an infinite and expanding universe, all things would expand into existence as we witness them! This phenomena is akin to time being an illusion. If anyone is interested I will elaborate.
Einstein was a bit ahead of you there... "Reality is merely an illusion, albeit a very persistent one!"

Imagine what would happen if you could travel at c, time would stand still from your inertial reference frame. But what would an observer see watching you pass by at the speed of light? Nothing! The only way an observer would see you is if you smacked them right in the eyes... either directly or by collision with some other matter/energy. Energy/information conservation seems to imply that if you could propel matter to c it would necessarily stay a beam of energy and travel indefinitely without losing any energy. But the most interesting part is what happened to the energy way back when it started to coalesce into particles...
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