Questions?

[Guest liliangrn]

Hi everybody,

 

My first question is:

 

Imagine I place a torch perpendicular to the ground and shine it up wards it red shifts (i.e. it's kinetic energy is turned into potential energy) as it leaves the earths gravity.

 

Say this light were to meet up with a planet of equal mass and size, in a direct line to it's centre of gravity, and hit the planets surface. Would it equally blueshift and be reverted to it's original state?

 

My second question is:

 

Does a black hole have an inertial mass?

 

Thanks

 

Josephine

[C1ay]

Imagine I place a torch perpendicular to the ground and shine it up wards it red shifts (i.e. it's kinetic energy is turned into potential energy) as it leaves the earths gravity.

 

Say this light were to meet up with a planet of equal mass and size, in a direct line to it's centre of gravity, and hit the planets surface. Would it equally blueshift and be reverted to it's original state?

 

Think of it like this. Imagine a photon as a baseball and toss it into the air. As it travels upward, gravity will slow it down, i.e. redshift. It will eventually stop and fall toward the ground. As it falls, gravity will accelerate it to the same velocity at which it left your hand to begin with, i.e. blueshift. BTW, it can be useful to share this with those that shoot guns into the air for celebration. Most do not realize that their bullet returns to the ground at the same speed it left the muzzle at and is just as deadly to people on the ground as if you had shot the bullet directly at them

 

Does a black hole have an inertial mass?

 

Yes.

[Qfwfq]

Hi Josephine.

 

The answer to the second question is yes.

 

The answer to the first is yes, if the other planet's radius is also equal. In this case the potential energy will be the same at each surface. More in general you may find the condition of equal potential energies from:

 

M/R = M'/R'

 

in the Newton approximation which is fine if R is large enough compared to the Schwarzschild radius for M, as in the case of most planets.

[Qfwfq]

Most do not realize that their bullet returns to the ground at the same speed it left the muzzle at and is just as deadly to people on the ground as if you had shot the bullet directly at them

Not exactly the same, loss to friction in air can be very high in the initail path, but all the same the bullet can hurt badly when it gets back down.

[Guest liliangrn]

Thanks heaps guys!

[C1ay]

Not exactly the same, loss to friction in air can be very high in the initail path, but all the same the bullet can hurt badly when it gets back down.

Yeah, there's also that terminal velocity thing. The point is that there are many more people killed by falling bullets than many people realize, it is not a safe way to celebrate.

[Guest liliangrn]

That's O.k. Clay,

 

I understood the meaning of your analogy, due to the fact that I forgot about the 'terminal velocity thing' aswell. So we cancelled out the problem together.

 

How many people get killed or injured by celebration shots, fired in the air, anyway?

Is there an actual number?

 

Josie

[C1ay]

How many people get killed or injured by celebration shots, fired in the air, anyway?

Is there an actual number?

 

Josie

Don't know. It seems I read somewhere that 30+ people died in the L.A. area alone from falling bullets one year. I also seem to recall something about a bunch of Kuwatis getting killed as well when they were liberated. I imagine there could be some indirect, related deaths as well in populated areas where a falling bullet hits a moving car for instance and the driver has an accident that kills somebody.

[Qfwfq]

They can certainly hurt, in many ways.

 

I didn't mention terminal velocity but it's high for a dense object like a bullet, large hailstones can cause damage and a bullet is denser and harder.

[Guest liliangrn]

The force if the bullet falling to the ground is F=ma

 

But the bullet leaving the gun, in classical terms, is:

 

F = mv/a

 

In relative terms, it would be:

 

1/2 (Dv)^2 = GMm/DR

 

Is that right?

 

Jos

[cindy 2005]

Think of it like this. Imagine a photon as a baseball and toss it into the air. As it travels upward, gravity will slow it down, i.e. redshift. It will eventually stop and fall toward the ground. As it falls, gravity will accelerate it to the same velocity at which it left your hand to begin with, i.e. blueshift. BTW, it can be useful to share this with those that shoot guns into the air for celebration. Most do not realize that their bullet returns to the ground at the same speed it left the muzzle at and is just as deadly to people on the ground as if you had shot the bullet directly at them

 

 

 

Yes.

 

I am confused that the gravity can slow down a photon that is moving? Light is defined as a stream of photons which are massless. If a photon is massless, gravity can slow down? I think it may make sense when we consider the mass of photon is approximated to 0 (but not equal to 0.)

[Guest liliangrn]

I am confused that the gravity can slow down a photon that is moving? Light is defined as a stream of photons which are massless. If a photon is massless, gravity can slow down? I think it may make sense when we consider the mass of photon is approximated to 0 (but not equal to 0.)

 

Hi Cindy,

 

The way this was explained to me is that firstly gravity doesn't move. Gravity is not energy it's a force. Gravity is the curvature of spactime and a mass caught in a gravity field follows the curvature. Say you placed a dip in the middle of a flat surface and rolled a ball across the surface. At the dip the ball would speed up and slow down as it exited the dip on the other side. Now this represents spacetime and because the curvature is radial (i.e. curved in all directions from the centre of gravity) we thus then see an object move in a straight line.

 

Light however does have a mass. Gravitational mass = Inertial mass according to einstien but since light cannot be slowed down so it has no inertial mass. It does have a gravitational mass. So it can be bent if a light ray passes on a tangent or parallel to a gravitational field. It will also bend if it hit's water for some reason (perhaps someone can explain this to me).

 

When a light ray leaves a gravity field the shorter frequency waves begin to move toward the longer frequency waves and it's kinetic energy is converted into potential. Within a black hole the red shift is infinite and all the kinetic energy is converted into potential energy. The light ray no longer has any energy therefore doesn't exist. The potential energy is absorbed by the black and converted to mas so the black hole is now a teensy bit bigger. Oh yeh light doesn't have a rest mass either.

 

Hope this helps.

 

Josephine

[C1ay]

I am confused that the gravity can slow down a photon that is moving? Light is defined as a stream of photons which are massless. If a photon is massless, gravity can slow down? I think it may make sense when we consider the mass of photon is approximated to 0 (but not equal to 0.)

If that confuses you then you'll really like this article about an optical computer using frozen light ;)

[Turtle]

___Speaking of terminal velocity, I find the phrase is often misrepresented. Particularly in relation to skydivers, wherein it is often claimed they have a terminal velocity of under 200mph. In point of fact, the USAF in the fiftys hauled a guy up to 100,000 feet on a ballon & then he jumped off & before opening his chute he broke the sound barrier!

___Terminal velocity is relative to many things & just keep in mind the acceleration of gravity.

[Guest liliangrn]

There you go Clay,

 

I guess my explanation was overly simplified. Light must have inertial mass; like passing through atoms at extremely low temperatures or increases in density. I've just read that light slows down due to an increase in density (e.g. passing through water or glass). It also speeds up with a decrease in density. Light must have a rest mass aswell. Michelson/Morley found that light travels at a slower constant speed through water aswell regardless of the movement of the water (About 75% the speed of light in vacuo).

 

Professor Lene Hau

http://www.physicscentral.com/people/people-02-2.html

 

Refractions and Rainbows

http://www.rebeccapaton.net/rainbows/refrctn.htm

 

This site has an experiment you can try at home.

http://www.ology.amnh.org/einstein/stufftodo/see_refraction.html

 

There you go.

 

Josephine

[C1ay]

___Speaking of terminal velocity, I find the phrase is often misrepresented. Particularly in relation to skydivers, wherein it is often claimed they have a terminal velocity of under 200mph. In point of fact, the USAF in the fiftys hauled a guy up to 100,000 feet on a ballon & then he jumped off & before opening his chute he broke the sound barrier!

___Terminal velocity is relative to many things & just keep in mind the acceleration of gravity.

I once performed an analysis of the terminal velocity of skydivers who's chutes had not opened. I found it was the same for all of them, i.e. TV = splat ;)

[Guest liliangrn]

Will light bend if it is travelling perpendicular to the centre of gravity?

Also will light bend if hits glass at an angle perpendicular to it's surface?

 

I'm guessing no but it still slows down? This would be why light still travels in straight lines through water.