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Q & A: origins

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Most recent answer: 10/22/2007
Q:
dear sir,I was just reading "A BRIEF HISTORY OF TIME" that suddenly a question arose in my mind,hope you will be kind enough to clarify my doubt... IT IS A COMMON BELIEF THAT OUR UNIVERSE IS CREATED BY "BIG BANG"..AND ANOTHER THEORY SUGGESTS THAT ANY MASS IN OUR UNIVERSE THAT EXCEEDS "CHANDRASEKHER LIMIT" WILL EVENTUALLY BE CONVERTED INTO BLACK HOLE or WHITE DWARF or NUTRON STAR..THEN HOW COME THE MASS OF WHOLE UNIVERSE WAS CONCENTRATED AT ONE PLACE BEFORE BIG BANG..WHY DOESN’T IT TRANSFORMED INTO A BLACK HOLE n WHAT MAKE IT EXPLODE TO FORM THE UNIVERSE???WAS IT POWERFUL ENOUGH TO MAKE MASS TRAVEL FASTER THAN SPEED OF LIGHT?? AND WAS THERE ONLY ONE MASS OF THAT KIND OR NUMBER OF IT WAS PRESENT WHICH HAS FORMED ANY SIMILAR UNIVERSE ELSE WHERE??
- ajit (age 20)
national institute of technology, trichurappalli,tamilnadu,I
A:
Nice questions.

Although there is strong reason to believe that any star more massive than the Chandrashekar limit will turn into a black hole, that does not mean that any collection of things whose total mass is that big will do so. Consider two stars, each with 2/3 the Chandrashekar mass, very far apart and departing rapidly from each other (faster than the classical escape velocity). They will never get together and form a black hole.

So whether objects collapse depends not only on their total mass but also on the initial distribution of relative velocities.

There is no way within the known laws of physics that any object can travel past any other object with relative speed greater than the speed of light. However, in general coordinate systems designed to include all objects in our universe, the rate of growth of the spatial coordinate of some object, as a function of its time coordinate, can exceed c. That object would be outside our horizon.

As for the question of what set the initial conditions for the Big Bang, we just don't know. People are trying to work out a consistent combination of quantum mechanics and gravity, which may lead to pictures of how the spacetime of our universe is included in some larger manifold. If that works, we may have some more definite reasons for thinking of other universes.

Mike W.

Some theorists speculate that we live in one of a large multitude of possible universes, each with its own unique physical laws. This is a fun idea to entertain, although I have yet to see anyone make an acutal prediction using this idea which can be tested in the laboratory or in anywhere else in the accessible universe. Maybe someone will come up with something good (but given that signals travel only as fast as light, I'm afraid we're stuck in the corner of just this one universe we can see. Fortunatley this includes a vast amount of past history of things far away, but still not enough for this idea).

Tom

(published on 10/22/2007)

Follow-Up #1: speed limit

Q:
As part of your answer to this question you state that "There is no way within the known laws of physics that any object can travel past any other object with relative speed greater than the speed of light. However, in general coordinate systems designed to include all objects in our universe, the rate of growth of the spatial coordinate of some object, as a function of its time coordinate, can exceed c. That object would be outside our horizon."

It seems to me that the original question postulates that relative velocities of matter at the big bang must have exceeded the speed of light in order for our universe not to collapse back into a black hole. Is this a true statement?

If no, then there must have been some sub-light speed that matter must exceed, and clearly since the Universe has continued to expand, matter exceeded this limit in the beginning. (T/F)?

But, if not, i.e. if matter must have exceeded the speed of light in order for the universe not to collapse into a black hole, then matter clearly broke the laws of physics! But you say something about spatial coordinates of objects with reference to their time coordinates might have been the get out of jail free card. I guess I just donít understand; is it, or is it not possible for matter to exceed the speed of light? If not, and if matter would have needed to exceed C at the Big Bang in order for the universe to expand beyond being a black hole, then...well...isnít there a problem?

Thanks.
- Les Clark (age 45)
New Boston, New Hampshire
A:
Hereís the speed limit: take any two objects that are near each other. Let an observer on either object pick any coordinate system (remember, nature doesnít just hand us fixed coordinates) within which the laws of physics holds and he is at rest. Then the other objectís speed in that coordinate system will not be greater than c. (One can pick any other local coordinate systems in which both objects are moving, in which their relative speed can range up to 2c.)

What about objects that are not near each other? In that case, the greater variety of coordinate systems allowed by General Relativity  becomes relevant. (In small patches of space, these all look like the Special Relativity coordinate systems described above, with a speed limit.) So there is no longer a unique speed assigned to object B by an observer fixed on object A. Thatís why our language describing this problem becomes more complicated. There is no logical way to extend our local speed limit to a speed limit for distant objects. In any of the standard modern pictures of the universe there are perfectly good coordinate systems within which some objectís distance from a ífixedí point grows faster than c. Without such 'speeds', the universe would have collapsed, as you say.

The key point which I think is hard to grasp is that objects don't have speeds stamped on them. Speed is a function of our choice of coordinate system. the laws of physics allow many equally good choices, and speeds are not the same in different systems. This is not a claim that 'everything is relative', just the opposite. It's a claim that the things which aren't relative (e.g. whether two objects bump into each other) are not quite the the same as our intuition suggests. Speed is a relative quantity.

Mike W.

(published on 10/22/2007)

Follow-up on this answer.