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Hey all!

I have a couple of beginner questions after reading some basic astrophysics. I would really appreciate it if somebody could give me some answers.

1. Does Newtons 3rd law apply in space (to every action there is an equal and opposing reaction)? I dont mean on other planets, moons or entities, just space (the gaps) between the objects.

2. Assuming that this law does apply, wouldnt that mean, that at the end of a black hole is not really destruction but creation? I read that some people hypothesized that at the end of black holes would be so called white holes and that they could've been quasars?

3. Assuming that this law does not apply in space, wouldnt that mean constant acceleration (with necessary fuel) is not impossible and an object traveling through space would eventually reach or exceed the speed of light, should it be able to survive at those speeds?

Ok the last one is kind of stupid, I was just wondering, so bear with me please:

Could it be that the black matter in space between galaxies is just expanded black holes that have reached a, well 'critical mass' or something?

Would be great if anyone could help me out, I understand that everything is not so simple as I have put it but I'd like to learn :eek:

Thanks ;)

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Newton's laws are laws of classical mechanics, which is only an approximate description of nature. Relativity and quantum mechanics give a better approximation. So Newton's laws don't hold absolutely anywhere: classical concepts like force, action/reaction etc aren't applicable in quantum theory, except as approximations applied to bulk matter.

With that proviso, Newton's 3rd law applies just as well in space as on Earth: it's how rockets work. But discussion of black holes involves general relativity, so a Newtonian approach is no longer helpful there.

Your own example shows the limitation of Newton's laws: they predict that constant force could accelerate something beyond the speed of light. Einstein proved otherwise.

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Thanks for your replies. It clears a little bit up now that I know that Newtons theory cant really be applied to this scenario.

themos, I know there is no specific theory that can satisfy that question. I was just wondering, if all that mass when into something and that something kept growing from the beginning of time, it doesn't make much sense to me how everything would still exist. Which is why I think that although destruction appears at the beginning of the black hole, a reverse should be happening on the other end of it. I also read in a book that black holes could lead to another universe but that its also unlikely.

I guess the answer to that will not exist in our lifetime...

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wouldnt that mean constant acceleration (with necessary fuel) is not impossible

In theory, this is possible

and an object traveling through space would eventually reach or exceed the speed of light, should it be able to survive at those speeds?

but this isn't. As acey has said, relativity come into play, and, In relativity, time is relative. Acceleration is rate of change of velocity with respect to time, and velocity is rate of change of position with respect to time, and since Newton and Einstein treat time differently, it is to be expected that Newton and Einstein give different results for acceleration and velocity. The relativity of time prevents speeds faster than the speed of light, even for the case of constant acceleration. Consequently, a ship can accelerate forever with a ship-measured constant acceleration.

As a concrete example, suppose a ship starts from rest in the Earth's frame and, for the traveler's comfort, accelerates at a constant acceleration of a = 1 g, so the traveler experiences artificial Earth-like gravity. If the Newtonian expression v_n = a*t were true, then v_n (with respect to the Earth) would soon be greater than the speed of light. If I've done the calculations correctly, the actual speed v with respect to the Earth is given by

[FONT=Verdana]
t v v_n

0.1 0.1026 0.1032

0.5 0.46 0.52

1.0 0.72 1.03

2.0 0.90 2.06

3.0 0.95 3.10

10.0 0.995 10.32
[/FONT]

t is Earth time in years, speeds are such that the speed of light is 1, the speeds predicted by constant acceleration in relativity are in the middle column, and the speeds predicted by constant acceleration in Newton's theory are in the right column.

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I was just wondering, if all that mass when into something and that something kept growing from the beginning of time, it doesn't make much sense to me how everything would still exist.

I don't see the problem with that, personally.

What scientists are up against is that their equations are telling them "something" but they don't yet know how to interpret that something. This has happened many times before, of course. In maths, there were "complex numbers" and people really didn't know if they should be arguing mathematically about them and how. This was all worked out eventually and now, with quantum mechanics, complex numbers are essential to our understanding of what the equations are telling us.

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Thanks George - So basically with Newtons theory the object in 10 years would be traveling at almost 10 times the speed of light, with a constant 1g acceleration whereas Einstein says it could only reach to 0.995 of light speed. Theoretically Einstein describes time travel better with more accuracy than Newtons theory? Okay I know you guys have tried to explain but I'm still asking why? I mean in theory if an light can travel that fast, isnt it also possible that perhaps something out there that we cannot see, can travel twice as fast or more? I mean if something is traveling 10x the speed of light, chances are we're not going to see it right or our brain might not even be able to perceive what we saw right?

themos - I mean if a blackhole existed at around the time the universe was created and grew bigger and bigger and never stopped but just kept sucking up everything it came into contact with, then how could the universe still exist? Especially with more than just 1 blackhole. Of course theres the argument that the universe is just too big, I suppose.. :eek:

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I mean if a blackhole existed at around the time the universe was created and grew bigger and bigger and never stopped but just kept sucking up everything it came into contact with, then how could the universe still exist?

The black hole has infinity gravity only at the singularity. Far away from it, its gravity field is just like any other star's. We don't see stars capturing too much matter into their stellar systems so black holes have the same problem finding enough stuff to feed on.

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You seem to be forgetting a rate of absorbtion ino a black hole. If the black hole kind of bumped into an atom a year since the start then not a lot of atom's and no significant change to the universe's count of atoms.

Agreed it will be more, a lot more. It isn't just a case of how long it has existed.

If you want a firework display wait until our galaxy and andromeda combine. When those 2 black holes merge it should be spectacular.

Why infinite gravity, and I presume that singularity = zero size, why, if that is the case, zero size?

Have noticed he term coming more and more into use but never read anything proving it.

Just wondering if someone used the term "singularity" it caught on then it became "zero size", simply because it sounds "good".

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Black holes actually shrink over time, when not actively being 'fed', via a process known as hawking radiation.

Heres a nice diagram from stephen hawkings universe in a nutshell:

Hawking.jpg

small black holes evaporate by this process much faster than larger ones, which is why microblackholes, if they exist, would likely not last very long.

It has been proposed that future starships of an advanced race could be powered extremely efficiently by hawking radiation, i'll dig up the article if anyones interested.

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It has been proposed that future starships of an advanced race could be powered extremely efficiently by hawking radiation, i'll dig up the article if anyones interested.

What an interesting idea! Yes, I'm very interested in the article, but before you post a reference, I would first like to have a go at doing some calculations myself.

I have a little experience playing around with equations for (special) relativistic variable-mass rockets, and for the power of emitted Hawking radiation, and I would like to see if I can put these things together.

I'll work on it this weekend, but I have some other interesting stuff on which I should work also.

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Thanks for that diagram. I didnt know that they evaporate when not actively being fed. Yes I would like to read that article about hawking radiation. Every piece of new information is a learning experience. So if you could find it, I would really appreciate it. Thanks :eek:

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  • 2 weeks later...

I recall reading some article by an astrophysicist who stated...and I paraphrased... "that a Black Hole the size of our universe would have about the same mass as we observe in our universe", when you include all that stuff we cannot find. Which I found curious.

And is not the the effect of Hawking Radiation so minute that a stellar size black hole would take millions or billions times the alleged age of the universe to evaporate? So its effect on SMBH's is not even worth discussion. Except perhaps in relation to information collection of entangled pairs.

Perhaps here on this forum someone might put me right on some of my own muddled thinking. As I understand it Black Holes are the coldest environment we can imagine due to matter being packed so tightly atomic movement, and thus heat generating friction, is stilled. Just like a Bose-Einstein condensate in the lab it would start to take on 'weird' quantum properties. In this state matter is no longer really matter but quantum potential where any point within the overall mass has the potential to take on the properties of any and all potentials. Since our universe is governed at a fundamental level by quantum physics could it indeed be that it is a black hole? Anybody have a simple rebuttal to such a musing?

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