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**KrazyHorse** · August 12, 2005, 00:46

Re: Questions about astrophysics...

Originally posted by Albert Speer
First, I've heard it said repeatedly that if one were to travel at speeds exceeding the speed of light, one would be able to go back in time. I've even read 'explanations' of this phenomenum but I still didn't understand it. Why would you go back in time if you travelled faster than light?

You can't go faster than the speed of light. Period. Any speculation on what would happen if you could is just so much silliness.

Secondly, perhaps somewhat related... the images from the Wilkinson Microwave Anistoropy probe from two years ago were described as showing the universe as it was billions of years ago when it was only 400,000 years old. The thing I don't understand is how images from the edge of the universe could be so old (by their very nature they must be new as they are at the EDGE of an EXPANDING universe that was not 14 billion light years across 14 billion years ago). How could the radiation 14 billion light years away have been at the edge of the universe 14 billion years ago? How could a quasar lying 12 billion light years away have been there 12 billion years ago? I'm sure there's something real obvious I'm not getting but it doesn't make sense to me unless the universe and its constituents expanded faster than the speed of light.

a)

This is funny, because I currently work for the dude who's in charge of WMAP (Chuck Bennett, the most cited physicist of 2004). In fact, I was in the control room at Goddard 2 weeks ago when we did a station-keeping orbit change on the WMAP satellite (it's at L2, which is a saddle point, so while it's stable in the angular degrees of freedom it's unstable in the radial direction, so it has a tendency to slide inwards)

b) You're messing around with some sophisticated ideas here, and I have the feeling that you don't quite understand the fundamentals of GR. Concepts like "distance" and "time" need to be precisely defined or else you run into the conceptual problems you're facing now. Suffice it to say that there's no fundamental necessity in what you've described to have the Universe expand faster than the speed of light, HOWEVER

c) Other data (namely the large-scale uniformity of the Universe) does indeed suggest that for a period the Universe underwent rapid expansion. This theory is known as "inflation" and is the dominant theoretical construct in cosmology today.

Thirdly, how is gravity a force? I thought general relativity postulates that gravity isn't a force per se but actually the effect that matter has in warping the surface of space-time, causing objects following newton's laws of motion to APPEAR to be attracted to each other. Yet I hear that gravity is one of the four fundamental forces in physics.

Good question. When physicists know whether or not gravity is a force in the sense that EM or the nuclear forces are we'll get back to you.

Not astrophysics specifically, but what in the hell does it mean if an object has Spin 1/2? how the hell is it possible for an object to not return to it's same 'face' after a complete 360 rotation? How could anything take 720 degrees worth of rotation before returning to its earlier 'face'?

Welcome to the weird world of quantum mechanics. Here's the fundamental thing: ELECTRONS ARE NOT ACTUALLY LITTLE SPINNING BALLS. Any "common sense" gained by interacting with matter on the everyday scale does not apply. You can't actually take an electron and "rotate" it, because it doesn't have a physical form in the sense you think of. It has an extended waveform with certain intrinsic parity properties. All of its spin behaviour can be easily derived from writing out the 2-d irreducible representation of SU(2), and this behaviour has been painstakingly verified by experiment.

thank you

You're welcome.

**Nostromo** · August 12, 2005, 02:24

Good questions, Al. A friend recently told me: "Physicists frequently claim that the Universe is infinite. But they can't mean that. Either that or they don't know what infinity means. The universe can't possibly be infinitely large." I didn't know what to answer. At first glance, he seems to have a point. The Universe started about 15 billion years ago, and even if everything moved at the speed of light, it couldn't possibly be infinitely large. Granted, its ****ing huge, but not infinitely large.

**Winston** · August 12, 2005, 02:32

I don't think physicists claim that the Universe is infinite. They say it is without boundary, but finite in size.

**Urban Ranger** · August 12, 2005, 02:34

Re: Re: Questions about astrophysics...

Originally posted by KrazyHorse
You can't actually take an electron and "rotate" it, because it doesn't have a physical form in the sense you think of. It has an extended waveform with certain intrinsic parity properties.

That's according to the Standard Model, right?

I read something about David Bohm's Wave Guide Hypothesis, and it's even spookier than the Standard Model. Actions at a distance...

**Saras** · August 12, 2005, 02:42

Re: Re: Questions about astrophysics...

Originally posted by KrazyHorse

You can't go faster than the speed of light. Period. Any speculation on what would happen if you could is just so much silliness.

Secondly, perhaps somewhat related... the images from the Wilkinson Microwave Anistoropy probe from two years ago were described as showing the universe as it was billions of years ago when it was only 400,000 years old. The thing I don't understand is how images from the edge of the universe could be so old (by their very nature they must be new as they are at the EDGE of an EXPANDING universe that was not 14 billion light years across 14 billion years ago). How could the radiation 14 billion light years away have been at the edge of the universe 14 billion years ago? How could a quasar lying 12 billion light years away have been there 12 billion years ago? I'm sure there's something real obvious I'm not getting but it doesn't make sense to me unless the universe and its constituents expanded faster than the speed of light.

a)

This is funny, because I currently work for the dude who's in charge of WMAP (Chuck Bennett, the most cited physicist of 2004). In fact, I was in the control room at Goddard 2 weeks ago when we did a station-keeping orbit change on the WMAP satellite (it's at L2, which is a saddle point, so while it's stable in the angular degrees of freedom it's unstable in the radial direction, so it has a tendency to slide inwards)

b) You're messing around with some sophisticated ideas here, and I have the feeling that you don't quite understand the fundamentals of GR. Concepts like "distance" and "time" need to be precisely defined or else you run into the conceptual problems you're facing now. Suffice it to say that there's no fundamental necessity in what you've described to have the Universe expand faster than the speed of light, HOWEVER

c) Other data (namely the large-scale uniformity of the Universe) does indeed suggest that for a period the Universe underwent rapid expansion. This theory is known as "inflation" and is the dominant theoretical construct in cosmology today.

Thirdly, how is gravity a force? I thought general relativity postulates that gravity isn't a force per se but actually the effect that matter has in warping the surface of space-time, causing objects following newton's laws of motion to APPEAR to be attracted to each other. Yet I hear that gravity is one of the four fundamental forces in physics.

Good question. When physicists know whether or not gravity is a force in the sense that EM or the nuclear forces are we'll get back to you.

Not astrophysics specifically, but what in the hell does it mean if an object has Spin 1/2? how the hell is it possible for an object to not return to it's same 'face' after a complete 360 rotation? How could anything take 720 degrees worth of rotation before returning to its earlier 'face'?

Welcome to the weird world of quantum mechanics. Here's the fundamental thing: ELECTRONS ARE NOT ACTUALLY LITTLE SPINNING BALLS. Any "common sense" gained by interacting with matter on the everyday scale does not apply. You can't actually take an electron and "rotate" it, because it doesn't have a physical form in the sense you think of. It has an extended waveform with certain intrinsic parity properties. All of its spin behaviour can be easily derived from writing out the 2-d irreducible representation of SU(2), and this behaviour has been painstakingly verified by experiment.

You're welcome.

Physics

I only spent one year at Vilnius Uni Physics department, later changed my mind and transferred to the next building, business school. Real science to BS, really. I should go back some day.

**KrazyHorse** · August 12, 2005, 02:57

Re: Re: Re: Questions about astrophysics...

Originally posted by Urban Ranger

That's according to the Standard Model, right?

No. That's according to basic quantum mechanics.

Az · August 12, 2005, 04:59

KH, could you elaborate on (b), please?

**TheStinger** · August 12, 2005, 05:12

I'm out my depth here but:

Is it the case that General relativitity is likely to be only an aproxiamtion of the force of gravity in the same way that Newtons laws whilst shown not to be correct are also a rough approximation (at our every day level)

Now I struggle with basic calculus so don't laugh

**KrazyHorse** · August 12, 2005, 09:57

Originally posted by TheStinger
I'm out my depth here but:

Is it the case that General relativitity is likely to be only an aproxiamtion of the force of gravity in the same way that Newtons laws whilst shown not to be correct are also a rough approximation (at our every day level)

Possibly. There is no real evidence one way or the other.

**dannubis** · August 12, 2005, 11:19

Originally posted by TheStinger
I'm out my depth here but:

Is it the case that General relativitity is likely to be only an aproxiamtion of the force of gravity in the same way that Newtons laws whilst shown not to be correct are also a rough approximation (at our every day level)

the stinger just described the essence of scientific evolution

**reds4ever** · August 12, 2005, 11:33

I enrolled on an Astronomy course last winter as a 'free' pick as part of my degree. There were 4 modules, 3 were Astronomy based (all well and good) and the fourth (which I'm doing now) is an introduction to Cosmology. It's just stopped me dead in my tracks, talk about complicated, I can't make head nor tail of it and it's only an INTRODUCTION for Gods sake!!

**KrazyHorse** · August 12, 2005, 12:42

It's okay; they often call graduate classes "Introduction to..." also

**Zkribbler** · August 12, 2005, 12:45

Re: Re: Questions about astrophysics...

Originally posted by KrazyHorse
You can't go faster than the speed of light. Period. Any speculation on what would happen if you could is just so much silliness.

No so much "silliness" as "science fiction -- that is, the projection of science."

When you're motionless, time runs normally, i.e. T=1. As you approach the speed of light, time slows down; supposedly at the speed of light, T=0. Thus IF the speed of light could be exceeded (which it can't), then time would move into negative numbers, (T<0) i.e., it would run backwards.

The universe is infinite...and also expanding.

Those who say it is not infinite, do not fully understand the concept of curved space. That is, if you throw a ball hard enough, it will eventually hit you in the back of the head.

Just like if you were an ant on the surface of a ball...and you started running, you'd eventually get back to where you started.

**KrazyHorse** · August 12, 2005, 15:08

Re: Re: Re: Questions about astrophysics...

Originally posted by Zkribbler

No so much "silliness" as "science fiction -- that is, the projection of science."

When you're motionless, time runs normally, i.e. T=1. As you approach the speed of light, time slows down; supposedly at the speed of light, T=0. Thus IF the speed of light could be exceeded (which it can't), then time would move into negative numbers, (T<0) i.e., it would run backwards

Actually, what would happen from the pure mathematics of it is that timeflow becomes imaginary, not that it goes negative.

gamma = 1/sqrt[1 - v^2/c^2]

if v>c then sqrt is of a negative number.

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