Where do photons come from?

[RiaX]

You travel at the speed of light (apparently) on the EH, almost every documentary ive watched on black holes have "stated" this.

Im aware you cant travel FTL but if you cant travel FTL how is that the pull of gravity of a black hole exceed that of c?

 

[Sinbad]

How can a pull have a speed?

 

[Moosedrool]

You travel at the speed of light (apparently) on the EH, almost every documentary ive watched on black holes have "stated" this.

Actually no. To stay on it you need to travel light speed relative to space-time away from the black hole.

Im aware you cant travel FTL but if you cant travel FTL how is that the pull of gravity of a black hole exceed that of c?

This is the tricky part of relativity. People got this idea on what they were told of FTL travel being impossible but first you have to understand the concept of General relativity.

Relative to what is light moving? If there were just you and a flashlight in the universe and you are actually moving at 200m/s. You assume you are stationary, Shining the flashlight in the direction of motion will actually cause the light to travel 200m/s slower, while shining it backwards it will seem to travel 200m/s faster. This is because of this fabric Einstein referred too called space time. Only relative to that fabric the speed of light is governed.

So what happens if you distort the fabric? Then space time distorts and you get gravity. A beam shining down to earth will move faster relative to observers than one shined straight into the sky, gravity drags on time causing this bend in time itself. Relative to space time the light travels at a constant speed.

A gravity well (Black Hole)'s event horizon is extremely interesting because that is the exact point where escape velocity is = to the speed of light meaning its impossible to remain stationary right there. Inside the escape v increases even more but nothing actually travels faster than light relative to space-time.

On earth you feel an accelerating force pushing you from the floor away from its centre but you remain stationary, Relative to the space fabric of space-time you are indeed moving across its like climbing up the down escalator.

 

[Swa]

No. C is defined as the speed of light in a vacuum.

And how do you know that c is correctly measured and that it should be the speed of light?

Actually no. To stay on it you need to travel light speed relative to space-time away from the black hole.

But you're not actually moving.

 

[Sinbad]

And how do you know that c is correctly measured and that it should be the speed of light?

It's DEFINED... Also, if light travels at less than the speed of light, then surely the speed of light is less than the speed of light and we just divided by zero?

 

[Humberto]

The mass m' of a body B is transformed according to the equation

m' = m[SUB]0[/SUB] / (1 - v[SUP]2[/SUP]/c[SUP]2[/SUP])[SUP]1/2[/SUP]

where m[SUB]0[/SUB] is the resting mass of the object, v is the speed of the object and c is the speed of light.

1. In what reference frame is m[SUB]0[/SUB] measured? I presume m[SUB]0[/SUB] is measured in the reference frame F which has B as a stationary body.
2. Then I take it the subsequent speed v of B is taken relative to the reference frame F.

If photons travel at the speed of light, how would one measure or describe their resting mass?

According to http://en.wikipedia.org/wiki/Photon#Experimental_checks_on_photon_mass:

The photon is currently understood to be strictly massless, but this is an experimental question. If the photon is not a strictly massless particle, it would not move at the exact speed of light in vacuum, c. Its speed would be lower and depend on its frequency. Relativity would be unaffected by this; the so-called speed of light, c, would then not be the actual speed at which light moves, but a constant of nature which is the maximum speed that any object could theoretically attain in space-time. Thus, it would still be the speed of space-time ripples (gravitational waves and gravitons), but it would not be the speed of photons.

 

[Swa]

It's DEFINED... Also, if light travels at less than the speed of light, then surely the speed of light is less than the speed of light and we just divided by zero?

And how do you know it's correctly defined? Division by zero is exactly what you get if photons have no mass so you end up needing infinite energy. But infinity multiplied by zero is still zero so photons can't travel. You are looking only at the formula (that gives its own problems) in order to justify that they are massless but it's not a settled matter.

 

[Humberto]

As I understand it, mass in the context of relativity theory refers not to the amount of matter in a body, but to the inertia of the body. The mass m of a body is given by the formula

F = ma

where F is the force applied to the body to achieve acceleration a.

Since photons have constant speed and cannot be accelerated (sped up or slowed down), how would their mass then be defined?

 

[Humberto]

Let F be a reference frame and let B be a body that is stationary relative to F. Let m[SUB]0[/SUB] be the mass of B as measured in F.

Let G be a reference frame which is in motion relative to F with a speed v. Let m' be the mass of B as measured in G.

Then

m' = m[SUB]0[/SUB] / (1 - v[SUP]2[/SUP]/c[SUP]2[/SUP])[SUP]1/2[/SUP].

As v increases, the mass (in the sense of inertia) of B, relative to G, increases.

 

[Swa]

As I understand it, mass in the context of relativity theory refers not to the amount of matter in a body, but to the inertia of the body. The mass m of a body is given by the formula

F = ma

where F is the force applied to the body to achieve acceleration a.

Since photons have constant speed and cannot be accelerated (sped up or slowed down), how would their mass then be defined?

Mass seems to be rather poorly understood. Inertia is related to the amount of matter so in essence mass can be equated to a constant measurement of matter (weight changes according to gravity). It's thus possible for energy (and photons) to have mass as matter and energy can be converted from one form to another. This is how they were equated in Einstein's hypothesis where mass is actually energy in a combined form to make up subatomic particles.

As photons and other forms of energy can be thought of as constantly moving the term rest mass becomes rather void of meaning as it can't be measured like matter that is standing still. Their rest mass would still be defined as if they were standing still. Photons can actually slow down from their speed in a vacuum.

 

[Jings]

Mass seems to be rather poorly understood. Inertia is related to the amount of matter so in essence mass can be equated to a constant measurement of matter (weight changes according to gravity). It's thus possible for energy (and photons) to have mass as matter and energy can be converted from one form to another. This is how they were equated in Einstein's hypothesis where mass is actually energy in a combined form to make up subatomic particles.

As photons and other forms of energy can be thought of as constantly moving the term rest mass becomes rather void of meaning as it can't be measured like matter that is standing still. Their rest mass would still be defined as if they were standing still. Photons can actually slow down from their speed in a vacuum.

That basically explains Einstein's theory of relativity, where mass and speed are combined to determine escape velocity. But the theory falls apart when introducing black holes.

c is the constant for speed of light, and no object can go faster than the speed of light. The escape velocity of a black hole is so great that it emits no radiation so the return value on the formula is infinity, which is unacceptable in physics.

Here's the catch - general relativity says black holes can exist. We now need someone at Einstein genius level or greater to fix this.

 

[Humberto]

Mass seems to be rather poorly understood. Inertia is related to the amount of matter so in essence mass can be equated to a constant measurement of matter (weight changes according to gravity). It's thus possible for energy (and photons) to have mass as matter and energy can be converted from one form to another. This is how they were equated in Einstein's hypothesis where mass is actually energy in a combined form to make up subatomic particles.

As photons and other forms of energy can be thought of as constantly moving the term rest mass becomes rather void of meaning as it can't be measured like matter that is standing still. Their rest mass would still be defined as if they were standing still. Photons can actually slow down from their speed in a vacuum.

To my knowledge, light appears to slow down in a medium because of diffraction. The photons move through the medium in a zigzag-like path by bouncing around against molecules so that they don't move through the medium in a straight line, hence why it takes light longer to pass through the medium. Technically the photons do still travel at the constant speed of light though.

 

[Jings]

To my knowledge, light appears to slow down in a medium because of diffraction. The photons move through the medium in a zigzag-like path by bouncing around against molecules so that they don't move through the medium in a straight line, hence why it takes light longer to pass through the medium. Technically the photons do still travel at the constant speed of light though.

I thought photons can only travel in a straight line. How do x-rays and gamma rays work?

 

[RiaX]

I thought photons can only travel in a straight line. How do x-rays and gamma rays work?

what do you mean work ? they are part of the light spectrum with a small wavelength and hence high energy.

Photons generally travel in a straight line but gravity can bend it. They also change direction when entering a different medium (refraction) and they bounce around (again in straight lines) when they collide with atoms or matter

 

[Jings]

what do you mean work ? they are part of the light spectrum with a small wavelength and hence high energy.

Photons generally travel in a straight line but gravity can bend it. They also change direction when entering a different medium (refraction) and they bounce around (again in straight lines) when they collide with atoms or matter

Thanks, I remember now. Gravity doesn't alter light's path, it alters the bend of space-time that the light passes through.

 

[Moosedrool]

But the theory falls apart when introducing black holes.

Yup right at the centre or singularity and not just the infinite radiation but infinitely, small, infinitely dense etc...

I Believe quantum starts to play a little role here regarding the singularity. IMO it will be one planck length in size giving it a specific density depending on what has been sucked in. Hawking radiation doesn't affect larger black holes as much as tiny ones, could this be effected due to it's density? An infinitely small point that has mass should have an infinite amount of mass because it's infinitely dense. So how do we get large black holes and small black holes, This is why infinite doesn't work in physics.

 

[RiaX]

Thanks, I remember now. Gravity doesn't alter light's path, it alters the bend of space-time that the light passes through.

yes and hence causes a curving effect of the light. If the photon travels in a straight line within a cone like shape it doesnt actually move in a straight line

.... it moves in a straight line within a curved fabric giving the illusion that its bending ....... i need panado