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Albert Speer
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Puerile Polemic
Mar 1999
time: 00:37
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There are four aspects of physics, chiefly astrophysics, which I can not seem to get my head around. I know people here are knowledgeable in these matters so could someone please enlighten me?
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?
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.
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.
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'?
thank you
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KrazyHorse
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Macedonia
May 2001
time: 00:37
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quote:
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.
quote:
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.
quote:
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.
quote:
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.
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Saras
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Vilnius, Lithuania
Apr 1999
time: 06:37
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quote:
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.
quote:
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.
quote:
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.
quote:
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.
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Zkribbler
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Los Angeles, CA, USA
Feb 1999
time: 21:37
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quote:
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.
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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.
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KrazyHorse
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Macedonia
May 2001
time: 00:37
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quote:
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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Albert Speer
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Puerile Polemic
Mar 1999
time: 00:37
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KH:
but how can we be witnessing quasars at the edge of the universe as they were 14 billion years ago unless the matter in the universe expanded faster than light? I understand that this isn't the case of expansion like an object expands if it has more and more stuff entering it and filling it. I understand it's more like the inflation of a balloon. If one were to mark points on an un-inflated balloon then fill it with air, the points and the distance between them would stretch so the relative size of the points and the length of the distance between them would remain the same though the balloon (or the universe) would be quite bigger... so in that case, I understand that quasars 14 billion light years away can be 14 billion years old. but wouldn't the light that they reflected 14 billion years ago (when the absolute size of the universe was smaller, though the relative size remained the same) have reached us much sooner? either the universe inflated faster than light or light travelled more slowly in the past.
am i wrong?
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Albert Speer
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Puerile Polemic
Mar 1999
time: 00:37
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Saras:
quote:
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. |
no kidding. I'm a business student myself though I don't know what I want to do specifically (finance? marketing? accounting? management? economics? business law?)... sometimes I think i would make a better choice if i just chose some major in the social sciences, became a professor and try to be some sort of polymath... but there's not as much potential money in that as in business...
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Albert Speer
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Puerile Polemic
Mar 1999
time: 00:37
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reds:
quote:
or the light waves are stretched as the 'balloon' expands, and so take longer to reach us? Then we have red-shifted quasars. |
why would it take longer now? if it took less time in the past than the light would have reached us much sooner than today. light must have increased in speed.
i just don't understand how we could look at the edge of the universe and the light that reaches us reflected off the radiation 14 billion years ago at the dawn of the universe. If the universe expanded at the speed of light then it should take 28 billion years for us to see this radiation at the edge of the universe. 14 billion years to get out that far and 14 for the light to come back to us. there's a couple ways of looking at this but all seem to involve the speed of light changing or the universe expanding faster than light.
Last edited by Albert Speer on 13-08-2005 at 06:59
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Joseph
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Ca. USA
Feb 2000
time: 21:37
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quote:
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.
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Then what about the partial that was shot at L. Livermore Lab a few years ago that did go faster then light speed?
So you are saying that Star Trek will never happen?
Last edited by Joseph on 13-08-2005 at 10:33
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12-08-2005 08:36
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