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How does it work if nothing can go faster than the speed of light but black holes trap light through I’m assuming over powering force?

The first part of the sentence isn`t necessarily true.

How enlightening… :unamused:

Any mass creates a gravity well. This means that objects need to reach a certain velocity to escape (for example, a rocket needs to accelerate to incredible speed to leave the earth)

Black holes are so dense that the escape velocity is higher than the speed of light, therefore nothing can escape, not even light. The only force acting is gravity, it does not require that anything move faster than light to cause it.

cba1067950

(1) Matter and energy are the same thing. When you drive your car at 50 miles per hour, your car is bigger than when it is parked or when you drive it at a slower speed. That is because its energy as motion is added to its energy as mass. Matter and energy are the same thing.

When an object in space is accelerated, it gets bigger. Its energy as motion is added to its energy as mass. As the object gets larger, it requires more energy to move it. As it approaches the speed of light (186,000 miles per second), the object would become so big that furthering its motion would take more energy than remains in the universe.

At the speed of light, energy as motion and energy as mass cancel each other out. Hence, nothing can travel faster than light.

I believe that sedm1000 is referring to tachyons. Tachyons are particles of “negative mass.” While tachyons travel faster than light (about 250,000 miles per second), they are nothing, i. e., contain no information. The rule stands: Nothing (as in no thing) can travel faster than light.

(2) I agree with HVD’s explanation that light cannot escape the gravitational pull of a black hole. However, according to Hawking, black holes emit radiation (see the energy and matter thread in this forum).

HVD stated:

In pursuit of finding how deep my ignorance goes I will subject my logic to what I understand in the above statement and what I think to be wrong with it. While my knowledge is thorough enough to know about escape velocities, speed of light, gravity, and somewhat in black holes…I have the following question in mind:

  1. If the escape velocity is higher than the speed of light, but nothing can move faster than the speed of light we have a contradiction in terms, or so I believe, in that to suggest a faster speed of light one must know of a speed faster than the speed of light, but we agreed that nothing is faster than the speed of light; hence to talk of things faster than the speed of light is absurd. But you do say that nothing is required to move faster than the speed of light in order for gravity to suck in light…please explain this.

Imagistar stated:
At the speed of light, energy as motion and energy as mass cancel each other out. Hence, nothing can travel faster than light.

Imagistar, if at the speed of light energy as motion and as mass cancel each other out, what do we have left? How do you derive the conclusion that nothing can travel faster than light, from the premise “At the speed of light, energy as motion and energy as mass cancel each other out.”? I don’t see how the jump is made.

What’s your take?

Magius

You write, “…if at the speed of light energy as motion and as mass cancel each other out, what do we have left?”

A shortstop with an accordion.

You continue, “How do you derive the conclusion that nothing can travel faster than light, from the premise ‘At the speed of light, energy as motion and energy as mass cancel each other out.’? I don’t see how the jump is made.”

Energy and matter are the same thing. A particle accelerated to the speed of light will become indefinitely large, having converted all the energy as motion of the universe into the energy as mass of the particle. No energy as motion remains to further its speed. Hence, nothing can travel faster than light.

For more, check out “special theory of relativity” on the 'Net.

What about gravity itself… (I think it is carried by a hypothetical particle called a graviton or a gravitron ). Gravity exerts an effect on an object ‘instantaneously’ does it not? So gravitational forces do travel faster than the speed of light.
But I guess this would not be true if gravity is a part of space time itself, nonetheless one could still ask; wouldn’t it still take Some time before the effect of gravity reaches an object?

My view is that gravity is ‘instantaneous’.

scientists don’t know what causes gravity. perhaps when they figure that out, they can figure out if light is a wave or a particle. and then everybody’s questions can be answered.

tRippq,
the question about whether light is a particle or a wave was answered over 50 years ago and is taught in first year university courses of astronomy, chemistry, and physics. Light is both a particle and a wave. It has both properties.

Not necessarily - one view of gravity is of polarization in the quantum vacuum (yeah, nice). If gravity warps space, then it can travel faster than light.

[quote=“sliver”]
What about gravity itself… (I think it is carried by a hypothetical particle called a graviton or a gravitron ). Gravity exerts an effect on an object ‘instantaneously’ does it not? So gravitational forces do travel faster than the speed of light.
But I guess this would not be true if gravity is a part of space time itself, nonetheless one could still ask; wouldn’t it still take Some time before the effect of gravity reaches an object? quote]

Not necessarily - one view of gravity is of polarization in the quantum vacuum (yeah, nice). If gravity warps space, then it can travel faster than light.

I need to log myself in before I post!! :blush:

I was responding first to sliver.

Most of what is being discussed here is rather elementary.

Relativity simply states that to accelare an object you must give it energy, and by giving it energy you also give it mass (it doesn’t not actually get bigger, in fact the faster it’s going relative to you, the more “squeezed” it appears in the direction of motion.) It becomes increasingly more difficult to accelerate it the faster it is moving (relative to you). The limiting speed, c, turns out to be the speed of light in vacuum, supporting the hypothesis that light is composed of particles with zero rest-mass. (i.e. it takes no energy at all to accelerate them to the maximum speed possible, so they are always travelling at this speed, at least in vacuum. [Recent experiments have actually managed to stop light completely, in a gas cloud: http://www.nature.com/nsu/010125/010125-3.html.])

It’s easy to imagine speeds greater than c where no objects or information actually travel at this speed. Take a lighthouse for example, if we consider the speed at which the beam sweeps across an arc far out to sea, then it’s clear that once we get far enough away the beam will shoot past us at a speed far greater than c (if it were a bright enough and focused enough beam that we could still see it).

An easier example to conduct by yourself is to take two straight rulers, (or edges of paper), and to intersect them at angle theta. Then move one towards the other, maintaining the angle. Now consider the velocy the point of intersection moves parallel to either of the rulers. The smaller you make theta, the faster this point moves in proportion to the speed you move the rulers. When theta=0 the point moves at infinite velocity, and it exist simultaneously all along the rulers and is moving both forwards and backwards at the same time!

What’s important about these two examples is that no physical objects (or for that matter anything carrying information) is travelling faster than c. But as you can see velocities greater than c can be considered, and have useful meaning.

Tachyons could hypothetically exists if they were initially created travelling at a speed >c, and it is proposed that this would mean they would propergate backwards in time. In fact particles travelling backwards in time play an important part in the theory of Quantum Electro-Dynamics (QED), and without considering their effects QED would not be able to account as accurately as it does for so many physical quantities.

Gee, I made the same mistake :laughing:
That was my post above, this post is just so I can watch this thread! :astonished:

Albert stated:

Im not sure if you realized that you quoted me, but your response appears to also be in response to me, but I never stated that the thing travelling gets bigger. Someone else stated that in another post. Helpful hint: It usually helps if you actually add the name of the person you are quoting. If you split ideas, ie. Answer mine and then answer someone elses, you should make it clear. This way people reading your quote know exactly whats going on and are more likely to read your posts in the future; especially any long ones. Anyway, back to your post, your comment about it taking no energy to accelerate light to the maximum speed possible - can you explain this further. As far as my physics goes that is impossible. To say that no energy is needed to accelerate light, is to say no energy is needed to create light. The energy creating light is the energy accelerating it. C is light traveling at 300,000km/s in a vacuum, so its not traveling that fast here on earth. So different speed are possible for light. I was wondering what you thought of a funny theory I once heard, that if you could set up a mechanism for measuring the speed of light, lets say on a boat. And the light travelled at 300,000km/s, but the boat itself the mechanism was on travelled at 50km/h, it would raise the light overall speed and hence breaking the light travelling barrier. Believe it or not I learned this theory in an introductory course in Astronomy, I dont remember what it was called; but I wanted to know what you thought about it.

What’s your take?

The question of why light can’t escape from a black hole has been explained rather neatly by the “escape velocity” argument above. But it’s not really the whole story, since it fails to address how gravity can have influence over light in the first place (if gravity is just a force mediated by gravitons, presumably traveling at the speed of light, how can they influence light travelling at the same speed?)

It turns out that we must consider gravity differently from other forces (at least in this case), and it’s better to consider what we observe as gravity as actually being a warping of space-time itself.

To explain this, consider throwing a ball on the surface of the Earth. Ignoring air-resistance it moves in a parabolic arc (relative to us, on the surface). If we throw the ball harder, it will land further away from us, if we throw it hard enough (at the escape velocity), it will still fall towards the earth, but the curvature of it’s path would be such that as it descends the Earth’s own curvature would bend away from it by a larger amount, and so it would be trapped in a perpetual elliptical orbit. Throw it harder still and it will veer off away from the earth on a hyperbolic path (initially distorted by the Earth’s gravity, but straightening out asymptotically to a straight line as it gets further away from us).

Now consider a small intelligent ant trapped inside a bubble of air at the exact center of the ball we’re throwing. Once the ball has left it’s ejection apparatus, and is no longer under the influence of any force other than gravity, the ant will feel weightless. In fact, he wouldn’t be able to tell the difference whether he was falling back to Earth, orbiting around it, or leaving on a hyperbolic trajectory (he doesn’t have any windows to look out of). For all intents and purposes he might as well be in deep space, traveling in a straight line far away from any gravitating objects.

And herein lies the clue to the nature of gravity. The motion of any object is in fact always in a ‘straight line’ in the absence of a force (other than gravity). Gravity is not really a force in this sense; rather it is curvature of the very fabric of space-time itself, in such a way, that to us, objects in the vicinity of other massive objects appear to move along curves. These curves are actually just geodesics in a non-Euclidian space – lines of shortest distance between points.

In the case of a black hole, space-time is distorted to such an extent that the geodesic ‘straight lines’ which light travels on, appear to us (at a great distance away) to be so curved that they completely wrap-in on themselves, and so even light itself cannot escape.

Magius, sorry if I confused the quotes above, it wasn’t meant to be directed at you, or anybody really, I just wanted to introduce some simple thought experiments into the arguments.

To respond to some of your (Magius) queries:-

Now we’re getting into the realms of quantum mechanics, and things start to get very weird indeed. As you probably know, physicists are still trying to marry their two great theories of the 20th century, quantum mechanics and general relativity. The creation of light occurs through quantum processes. This happens several ways: electrons falling to a lower energy orbital around atoms or molecules, particles decaying into something else, or matter/anti-matter annihilations.

What’s important about all these types of interaction is that the photons (whether they be visible light, or gamma ray burst or whatever), are considered to come into existence instantaneously as a result of the “quantum event”. Any attempt to try to consider them coming into being somehow at rest, and then being accelerated up to the speed of light is basically futile, since a) quantum physics doesn’t work that way, and b) since they have strictly zero rest mass anyway, then any tiny amount of energy they have, or can get from their immediate environment, would instantly accelerate them up to c anyway (that’s just my way of thinking about it by the way)

Of course the photons of light do carry energy, and therefore have mass, but this energy is from their frequency (E=hv), which in turn was determined by the wave equations that caused their creation. This energy of course came from somewhere.

Well this is really the whole point of relativity (back to relativity again, phew!). No mater whether you were on the boat measuring the speed of light, or on the shore measuring the same light, you would always get the same answer, 299,792,458 m/s. The point being that the velocity of light is a fundamental constant of nature. Since speeds are just ratios of distance over time, what gives is space and time in order to preserve this fundamental constant. So on your boat, your apparatus to measure the speed of light would be of length d, and you’d have a clock ticking to measure time t that the light takes to span that distance (c=d/t). But on the shore an observer would see your apparatus contracted a little bit, and would see your clock ticking a little bit more slowly, in just the right amount their ratio would still be c

Everything you said above correlates with what I have learned, hence I am still confused on the point, which I should have cut out more clearly as being…

If C is the speed of light in a vacuum, what is the speed of light not in a vacuum? I too have heard about experiments that have stopped light due to clouds as happens naturally in space, which explains Olbers Paradox (how come the sky is not all white from the light of all the stars…) - but if light is C in vacuum, and light can be stopped by cloud, doesn’t it follow that light also can be slowed down? You also have failed to explain how light particles become accelerated without any energy.

What’s your take?

Magius wrote:

The experiments which stopped light were actually created using very specialized lasers and gas, and exploited unsual quantum phenomena (creating standing waves I believe). In the universe at large light doesn’t usually ever stop in an interstellar gas cloud (not that we know of anyway).

Physicists use the following formula to calculate the speed of light through any medium: v=c/n, where n is the refractive index of the medium in question. For vacuum n=1, for air n=1.0003, for water n=1.33, for glass n=1.5, or thereabouts.

The transmission of light though a medium is itself another quantum interaction, there’s nothing fundamental about the speed of light through media other than vacuum, as is explained here…

[the following curtosy of http://www.physlink.com/Education/AskExperts/ae23.cfm]
In a medium other than a vacuum, light propagates by a cascade of interactions with the molecules or atoms that make up the medium. Photons, the ‘particles’ of light, are absorbed and re-radiated (by atoms or molecules), absorbed again and re-radiated again, and over-and-over as they pass through the medium.

Since each absorption and re-radiation interaction takes time, though it be very little, the cascade cannot travel as fast through a material medium as light can in a vacuum. Light travelling in a vacuum does not have to ‘waste time’ being repeatedly absorbed and re-radiated. For this reason, the speed of light in a material medium is generally less than c, and will exhibit a specific value that depends upon the characteristics of the medium.

Since a vacuum exists between the atoms or molecules of a material medium, the speed of light between absorption and re-radiation sites is still c. It is the interactions that cause the overall speed through a material medium to differ from the standard value c in vacuum.

As for Olber’s paradox, the dust cloud/interstellar gas arguments dont really work [as is explained here: http://math.ucr.edu/home/baez/physics/Relativity/GR/olbers.html]

Currently accepted reasons why the night sky isn’t infinately bright, or blazingly white are:

The Universe is expanding, so distant stars are red-shifted into obscurity.
The Universe is young. Distant light hasn’t even reached us yet.