Physics / Maths class - ask a professor... or someone who thinks he is one :p

[smartquin]

An interesting question for which I currently do not have an answer. I shall endeavour to find one.

I shall this evening, shortly after arriving home.

Cheers Three PhDs!

[helio9]

Not a wavicle?

So depending on how we want to look at the photon as to weather we use a wave or particle theory? Physics has to be more accurate than that.

No, not really. Theres an entire field of physics devoted to reasearch in this area, Einstein did some work on this as well. The wave particle duality of light just means that in some instances, light behaves like a series of waves, but in others, it behaves like a stream of particles (See einstein's work on the photoelectric effect).

[SmallTimePerson]

How can it accelerate back to c? Where is the energy coming from? Or is it just a fact of life for a massless particle of energy that looks like a wave or a particle depending on the experiment? (these photons are getting wierder the more I learn about them).

photons have zero mass right?
so if there is no mass it doesn't require much energy to accelerate does it?

[helio9]

I have a question regarding black holes and light. I realize my problem may simply be a huge lack of knowledge and concepts required to understand this situation, but here goes...


Why can't light escape from a black hole? Escape velocity is a fuction of mass and density of an object (Vesc = sqrt(2GM/R)). So at some point, as mass increases and R decreases, Vesc becomes greater than c (speed of light).

However, universal gravitation says that the magnitude of the gravitational force of one mass on another is Fg = Gm1m2/(r^2).

But for photons, m = 0, so...why does the black hole even affect them? I guess what im trying to say is, if you had a ship near a black hole, a gravitational force would be applied on the ship by the black hole, proportional to the mass of the black hole and the ship (right?). But if a photon has mass zero...there's no force applied...whats the deal?

[SmallTimePerson]

I have a question regarding black holes and light. I realize my problem may simply be a huge lack of knowledge and concepts required to understand this situation, but here goes...


Why can't light escape from a black hole? Escape velocity is a fuction of mass and density of an object (Vesc = sqrt(2GM/R)). So at some point, as mass increases and R decreases, Vesc becomes greater than c (speed of light).

However, universal gravitation says that the magnitude of the gravitational force of one mass on another is Fg = Gm1m2/(r^2).

But for photons, m = 0, so...why does the black hole even affect them? I guess what im trying to say is, if you had a ship near a black hole, a gravitational force would be applied on the ship by the black hole, proportional to the mass of the black hole and the ship (right?). But if a photon has mass zero...there's no force applied...whats the deal?

I have a lack of knowledge too
But light is affect by gravity (i read this somewhere).

[helio9]

I think the only reason light is affected by gravity is because of the relativistic bending of space time (caused by large masses, AKA gravity). Otherwise, according to pure newtonian mechanics, I don't think theres really any reason for light to be trapped within a black hole.

[SmallTimePerson]

I think the only reason light is affected by gravity is because of the relativistic bending of space time (caused by large masses, AKA gravity).

I think that was what i was thinking of

[smartquin]

I think that was what i was thinking of

Yep, space-time warping. Should be something like s(reletivistic)=s(normal)*sqrt(1-(2Gm/sc^2))

[Three PhDs]

photons have zero mass right?
so if there is no mass it doesn't require much energy to accelerate does it?

a=f/m = infinite acceleration with any force.

I have a question regarding black holes and light. I realize my problem may simply be a huge lack of knowledge and concepts required to understand this situation, but here goes...


Why can't light escape from a black hole? Escape velocity is a fuction of mass and density of an object (Vesc = sqrt(2GM/R)). So at some point, as mass increases and R decreases, Vesc becomes greater than c (speed of light).

However, universal gravitation says that the magnitude of the gravitational force of one mass on another is Fg = Gm1m2/(r^2).

But for photons, m = 0, so...why does the black hole even affect them? I guess what im trying to say is, if you had a ship near a black hole, a gravitational force would be applied on the ship by the black hole, proportional to the mass of the black hole and the ship (right?). But if a photon has mass zero...there's no force applied...whats the deal?

In short, photon still follow the path of spacetime, and blackholes warp spacetime. The photon thinks it's still travelling ahead at the speed of light, but it's actually not, and is falling into an orbit.

[Lord §okar]

Ok, in a vacuum its constant, but when it enters a piece of glass it slows down, and when it exists it speeds back up again... is that right, coz it seems a bit odd to me

It's mean velocity of propagation changes, its speed does not.

[Three PhDs]

It's mean velocity of propagation changes, its speed does not.

That's a concept I'm not 100% familiar with (never thought about the hypothetical accleration/deceleration you'd see in light moving from medium to medium before), can you elaborate a little?

[Lord §okar]

There is an acceleration since it's d(ds)/(dt)^2. Light's movement though matter is series of absorptions and subsequent re-emissions which occurr over finite timescales and result in percievable decrease in velocity (as ds/dt), the speed of the actual photon doesn't differ a jot from c, though.

[Three PhDs]

Gotcha.

[smartquin]

There is an acceleration since it's d(ds)/(dt)^2. Light's movement though matter is series of absorptions and subsequent re-emissions which occurr over finite timescales and result in percievable decrease in velocity (as ds/dt), the speed of the actual photon doesn't differ a jot from c, though.

Ok, but doesnt that violate the uncertainty principle? You would know where the photon is, and what its speed is. I've probably missed something, please help.

[Three PhDs]

Ok, but doesnt that violate the uncertainty principle? You would know where the photon is, and what its speed is.

No, you don't know, you just accept that the apparent speed is less than c because there's some "To me, to you" going on with the photon in the medium it's travelling through.