Welcome to
Science a GoGo's
Discussion Forums
Please keep your postings on-topic or they will be moved to a galaxy far, far away.
Your use of this forum indicates your agreement to our terms of use.
So that we remain spam-free, please note that all posts by new users are moderated.


The Forums
General Science Talk        Not-Quite-Science        Climate Change Discussion        Physics Forum        Science Fiction

Who's Online Now
0 members (), 181 guests, and 2 robots.
Key: Admin, Global Mod, Mod
Latest Posts
Top Posters(30 Days)
Previous Thread
Next Thread
Print Thread
Page 1 of 2 1 2
Joined: Aug 2010
Posts: 3,570
B
Bill S. Offline OP
Megastar
OP Offline
Megastar
B
Joined: Aug 2010
Posts: 3,570

Gravitational lensing causes light from a single object in distant space to split so that it passes on either side of the lensing object. Any sufficiently massive object, for example, a distant galaxy, can cause this phenomenon

If one of the routes around the lensing object is more direct than the other, then the light that takes that route will arrive sooner than the light that has taken the longer route. If the source of the light is a pulsar, then pulses of light will arrive at the detector. It has been established experimentally that a pulse of light that has taken the shorter route to the detector will arrive sooner than the same pulse that has taken the longer route. As would be expected, the speed of light has remained constant for both routes; however, the pulse that took the shorter route, and arrived ahead of the other appears to have travelled faster than light. There was no time difference when the pulse was generated, as it was then a single pulse; but there is a time difference at the detector. Not only is the perception of simultaneity different within the frames of reference of the source and the detector, but, it is argued, the first arriving pulse has apparently travelled faster than light, and has, therefore, travelled into the past. Of course, all this is relative, so the argument can be reversed. We can, with equal validity, maintain that the second pulse has travelled more slowly than light, and has therefore, within the terms of this argument, travelled into the future. However, there seems to be something of a paradox here, because, relativity says that the faster one travels the slower time passes, thus the further one travels into the future, yet in the case of the two light beams, it is the slower one that moves into the future relative to the faster one. So, can both of the above assertions be true?

We have to consider the fact that the arrival of the pulses at different times is due to the fact that one has taken a longer route than the other, so neither has actually travelled faster than light. Nor does the fact that the two pulses leave the source as a single pulse actually alter the situation. After all, if we split a single beam of light from a source into two beams, send one directly to a detector and the other, via a mirror placed a light year away, to the same detector, the first beam will arrive two years before the other, but we would not argue that it had travelled faster than light, or that it had time-travelled – or are there those who would?


There never was nothing.
.
Joined: Dec 2010
Posts: 1,858
B
Megastar
Offline
Megastar
B
Joined: Dec 2010
Posts: 1,858
Originally Posted By: Bill S.

Gravitational lensing causes light from a single object in distant space to split so that it passes on either side of the lensing object. Any sufficiently massive object, for example, a distant galaxy, can cause this phenomenon

Now I am about to do some nit-picking. You have the right idea about this, but the way you said it might be a little misleading. The light from the distant single object isn't really split. Rather 2 light beams which are diverging at a very small angle will be bent so that they appear to come from different sources. Analysis however shows that they came from the same source.

Originally Posted By: Bill S.
If one of the routes around the lensing object is more direct than the other, then the light that takes that route will arrive sooner than the light that has taken the longer route. If the source of the light is a pulsar, then pulses of light will arrive at the detector. It has been established experimentally that a pulse of light that has taken the shorter route to the detector will arrive sooner than the same pulse that has taken the longer route. As would be expected, the speed of light has remained constant for both routes; however, the pulse that took the shorter route, and arrived ahead of the other appears to have travelled faster than light. There was no time difference when the pulse was generated, as it was then a single pulse; but there is a time difference at the detector. Not only is the perception of simultaneity different within the frames of reference of the source and the detector, but, it is argued, the first arriving pulse has apparently travelled faster than light, and has, therefore, travelled into the past. Of course, all this is relative, so the argument can be reversed. We can, with equal validity, maintain that the second pulse has travelled more slowly than light, and has therefore, within the terms of this argument, travelled into the future. However, there seems to be something of a paradox here, because, relativity says that the faster one travels the slower time passes, thus the further one travels into the future, yet in the case of the two light beams, it is the slower one that moves into the future relative to the faster one. So, can both of the above assertions be true?

We have to consider the fact that the arrival of the pulses at different times is due to the fact that one has taken a longer route than the other, so neither has actually travelled faster than light. Nor does the fact that the two pulses leave the source as a single pulse actually alter the situation. After all, if we split a single beam of light from a source into two beams, send one directly to a detector and the other, via a mirror placed a light year away, to the same detector, the first beam will arrive two years before the other, but we would not argue that it had travelled faster than light, or that it had time-travelled – or are there those who would?

I don't think that anybody who is a reputable scientist would make any claim for time travel in this case. Well, maybe you could say that the beams traveled into the future. But then everything is traveling into the future. But since Relativity destroyed the idea of simultaneity we observe them at different times and this is perfectly normal.

Bill Gill


C is not the speed of light in a vacuum.
C is the universal speed limit.
Joined: Feb 2007
Posts: 1,840
R
Megastar
Offline
Megastar
R
Joined: Feb 2007
Posts: 1,840
As you say in the 1st sentence of the 2nd paragraph, Bill S:

"...the arrival of the pulses at different times is due to the fact that one has taken a longer route than the other, so neither has actually travelled faster than light..."

You might have spared yourself some mental contortions if you'd made that the 2nd sentence of your 1st paragraph, and stood by it. smile


"Time is what prevents everything from happening at once" - John Wheeler
Joined: Aug 2010
Posts: 3,570
B
Bill S. Offline OP
Megastar
OP Offline
Megastar
B
Joined: Aug 2010
Posts: 3,570
Thanks for the comments, folks.

Bill, I don’t think you were nit picking, I agree with your comment. I have grave doubts that I shall ever find the source of the “split beam” idea, but I will try.

Quote:
I don't think that anybody who is a reputable scientist would make any claim for time travel in this case.


Now, that really is throwing down the gauntlet. smile The search for quotes is on! At least, it will be next time I go up into the loft to look for some books.


There never was nothing.
Joined: Aug 2010
Posts: 3,570
B
Bill S. Offline OP
Megastar
OP Offline
Megastar
B
Joined: Aug 2010
Posts: 3,570
Originally Posted By: Rede
You might have spared yourself some mental contortions if you'd made that the 2nd sentence of your 1st paragraph, and stood by it.


Where's the fun in avoiding mental contortions? Anyway, they help to ward off Alzheimer's, if my memory serves me rightly! smile


There never was nothing.
Joined: Dec 2010
Posts: 1,858
B
Megastar
Offline
Megastar
B
Joined: Dec 2010
Posts: 1,858
Originally Posted By: Bill S.

Where's the fun in avoiding mental contortions? Anyway, they help to ward off Alzheimer's, if my memory serves me rightly! smile

I think I remember something about that. But I forgot what it was.

Bill Gill


C is not the speed of light in a vacuum.
C is the universal speed limit.
Joined: Aug 2010
Posts: 3,570
B
Bill S. Offline OP
Megastar
OP Offline
Megastar
B
Joined: Aug 2010
Posts: 3,570
The first quote is from Jenny Randles, “Breaking the Time Barrier”

Talking of work by J Richard Gott’s team, she says: “The effect they were looking for is quite real and is known as gravitational lensing because the gravity well of such a massive object acts like a lens to focus light lays down split paths converging on the Earth.” They were dealing with pulses of light, one of which arrived a year before the other. She says: “That first light ray traveled backwards in time by approximately a year”.


There never was nothing.
Joined: Feb 2007
Posts: 1,840
R
Megastar
Offline
Megastar
R
Joined: Feb 2007
Posts: 1,840
"Randles specializes in writing books on UFOs and paranormal phenomena"
http://en.wikipedia.org/wiki/Jenny_Randles

It would probably be worth reading the actual words of Professor Gott.


"Time is what prevents everything from happening at once" - John Wheeler
Joined: Aug 2010
Posts: 3,570
B
Bill S. Offline OP
Megastar
OP Offline
Megastar
B
Joined: Aug 2010
Posts: 3,570
OK, I admit that Randles was an easy starter. smile

I havn't had time for a major hunt yet, but a brief extract from my notes of a coulpe of year ago might serve as a stop-gap.

Richard Gott (Time Travel in Einstein’s Universe). ...provides an excellent description of cosmic strings, and the geometry of spacetime around a string. He also explains how cosmic strings might be detected because their enormously compacted mass – some “…10 million billion tons per centimetre,” – would give rise to gravitational lensing,

Gott shows how the distortion actually gives rise to the lensing effect, and this is where the potential for time travel enters the picture.

Anyone who has the above book might beat me to the actual quote.


There never was nothing.
Joined: Aug 2010
Posts: 3,570
B
Bill S. Offline OP
Megastar
OP Offline
Megastar
B
Joined: Aug 2010
Posts: 3,570
This may be the quote from Gott (same book) to which Randles was referring.

“When we see two images of a distant quasar, the distances to the two images along the two paths can differ slightly………Since light always travels at 300,000 kilometers per second, if one path is shorter than the other, a light signal from the quasar coming along the shorter path arrives sooner.

Similar effects occur when light is bent while traveling around the opposite sides of a massive galaxy.”

Gott goes on to describe his observation of “quasar 0957”

“ We observed a sharp drop in the brightness of image A, and given the lensing geometry, we predicted that this should be followed by a similar drop in the brightness of image B……417 days later there was an identical brightness drop in image B.”

“This shows that you can beat a light beam in a race………A spaceship traveling at 99.9999999999 percent of the sped of light along path A would have done just a little worse at the race, still beating a photon traveling along path B by 414 days.”

“……….the door to time travel to the past begins to crack open.”


There never was nothing.
Joined: Dec 2010
Posts: 1,858
B
Megastar
Offline
Megastar
B
Joined: Dec 2010
Posts: 1,858
Originally Posted By: Bill S.

“ We observed a sharp drop in the brightness of image A, and given the lensing geometry, we predicted that this should be followed by a similar drop in the brightness of image B……417 days later there was an identical brightness drop in image B.”

“This shows that you can beat a light beam in a race………A spaceship traveling at 99.9999999999 percent of the sped of light along path A would have done just a little worse at the race, still beating a photon traveling along path B by 414 days.”

“……….the door to time travel to the past begins to crack open.”


That still doesn't sound like time travel. It is only natural that if you and a friend travel from Point A to Point B by different routes at the same speed, but by different routes, the one taking the shorter route will get there first. I cannot see how this could be considered in any way to be time travel.

Bill Gill


C is not the speed of light in a vacuum.
C is the universal speed limit.
Joined: Feb 2007
Posts: 1,840
R
Megastar
Offline
Megastar
R
Joined: Feb 2007
Posts: 1,840
Originally Posted By: Bill S.
Richard Gott (Time Travel in Einstein’s Universe). ...provides an excellent description of cosmic strings, and the geometry of spacetime around a string. He also explains how cosmic strings might be detected because their enormously compacted mass – some “…10 million billion tons per centimetre,” – would give rise to gravitational lensing

Btw, thanks for the info, this is the first I've heard of R. Gott's 1991 time machine model based on cosmic strings. I just read about it in 'A User's Guide to the Universe', p.158 (by Dave Golberg and Jeff Blomquist, 2010). It seems that what you'd have to do is:

(1) find an infinitely long cosmic string
(2) accelerate it to close to the speed of light

That would take forever, eh? Is there no getting away from infinity, Bill grin


"Time is what prevents everything from happening at once" - John Wheeler
Joined: Aug 2010
Posts: 3,570
B
Bill S. Offline OP
Megastar
OP Offline
Megastar
B
Joined: Aug 2010
Posts: 3,570
Originally Posted By: Bill
I cannot see how this could be considered in any way to be time travel.


I agree absolutely, this is why I wonder why it could be considered that it opens the door to TT to the past, even a crack.


There never was nothing.
Joined: Aug 2010
Posts: 3,570
B
Bill S. Offline OP
Megastar
OP Offline
Megastar
B
Joined: Aug 2010
Posts: 3,570
Originally Posted By: Rede
(1) find an infinitely long cosmic string
(2) accelerate it to close to the speed of light.


If I remember correctly, Gott's theory developed into one involving two infinitely long strings approaching each other at close to light speed, and one had, somehow, to go round both. I'll look it up.

Infinity is always with us. smile


There never was nothing.
Joined: Aug 2010
Posts: 3,570
B
Bill S. Offline OP
Megastar
OP Offline
Megastar
B
Joined: Aug 2010
Posts: 3,570
There is another point about cosmic strings that puzzles me.

Gott points out that, in spite of the great mass of cosmic strings (mentioned above), two such strings could be placed close together without attracting each other, because their gravitational attraction would be exactly offset by the negative pressure generated by the tension on the string, which would have a repulsive effect. The strings, it seems would be gravitationally neutral. What he does not explain is why gravitational lensing, which is the result of the distortion of spacetime in the presence of a strong gravitational field, would occur when the gravitational force involved had been cancelled out in this way.

Any ideas?


There never was nothing.
Joined: May 2011
Posts: 2,819
O
Megastar
Offline
Megastar
O
Joined: May 2011
Posts: 2,819
That is because the idea is nonsensical.

If you had too long cosmic strings as he proposes near each other the force at the ends could not possibly be balanced, try it with two magnets which has the same effect.

Basic physics tells you whats going to happen an open long string would not be stable anywhere near any other open long string.

You would need dohnuts or at least a closed shape to make this rubbish fly.
.


I believe in "Evil, Bad, Ungodly fantasy science and maths", so I am undoubtedly wrong to you.
Joined: Aug 2010
Posts: 3,570
B
Bill S. Offline OP
Megastar
OP Offline
Megastar
B
Joined: Aug 2010
Posts: 3,570
I guess that knocks on the head the idea of achieving PDTT using two infinitely long cosmic strings. I'm beginning to like this discussion. smile

Wait, though! Does the requirement that they be infinitely long make any difference? Could a string be considered open if there were no possibility of identifying either end?


There never was nothing.
Joined: May 2011
Posts: 2,819
O
Megastar
Offline
Megastar
O
Joined: May 2011
Posts: 2,819
That would be correct in that it denies ends of the strings ... you just have to believe in physical infinity now :-)


I believe in "Evil, Bad, Ungodly fantasy science and maths", so I am undoubtedly wrong to you.
Joined: Aug 2010
Posts: 3,570
B
Bill S. Offline OP
Megastar
OP Offline
Megastar
B
Joined: Aug 2010
Posts: 3,570
It's not believing in physical infinity that is the problem, it's what people try to do with it. smile


There never was nothing.
Joined: Feb 2007
Posts: 1,840
R
Megastar
Offline
Megastar
R
Joined: Feb 2007
Posts: 1,840
I know. That occurred to me when I tried to get my piano upstairs grin


"Time is what prevents everything from happening at once" - John Wheeler
Page 1 of 2 1 2

Link Copied to Clipboard
Newest Members
debbieevans, bkhj, jackk, Johnmattison, RacerGT
865 Registered Users
Sponsor

Science a GoGo's Home Page | Terms of Use | Privacy Policy | Contact UsokĀž»­¾W
Features | News | Books | Physics | Space | Climate Change | Health | Technology | Natural World

Copyright © 1998 - 2016 Science a GoGo and its licensors. All rights reserved.

Powered by UBB.threads™ PHP Forum Software 7.7.5