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Aschig
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| | Friday, August 01, 2003 - 12:08 am: | 
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Shrini, I can not claim to be a time travel expert, but we can perhaps discuss our understandings here.
Though the granny paradox is easy to understand, a few years ago it was decided to make it much simpler by removing the psychological and unnecessary aspects of it. I quote here the classic and the modern versions of the paradox which tries to put time travel in a spot.
Classic: You go to the past and kill your granny when she was a baby so you were never born so you could not go back so your granny grew up and you were born and you went back ....
Modern: You direct a billiard ball through a wormhole so that it hits its old self so that the old self goes off and you are not able to direct the newer ball into the wormhole to hit the old self so it is not deflected so you can direct it to hit itself and so on.
One of the way outs is that a "sum over histories" approach can be taken avoiding the paradox altogether i.e. consider all possible paths that the ball can take with probabilities attached and then you obtain a mean and that is what happens. SO most likely what happens is that as the ball approaches the wormhole in order to aim itself at the older self, suddenly a ball appears out of the other end of the wormhole deflecting this ball away and itself entering the wormhole. Thus there are in fact two balls involved rather than one.
As far as entropy is concerned, it is not being said that all actions get reversed for everybody but just that an individual goes to an earlier time period when the entropy was indeed less.
It may happen that we may not be able to go back in time to a point when the time machine did not exist. That is often quoted to be a reason why we have not had any visits from the future so far.
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Shrini
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| | Friday, August 01, 2003 - 4:37 pm: |
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Aschig, as I said in my earlier posting, can we define 'time' the way we can (and do) define 'space', or any other physical concepts like speed/velocity/work/power ?
Do we understand the concept of time formally or in an intuitive manner ? If only the latter, can we even begin to think of manipulating it ?
What are the physical changes occuring to
a) the person and b) her environment, when she is time travelling ? If she goes in past (where her 'older' self exists), she will occupy multiple 'spaces' at the same 'time'. Is that not a paradox ? One way to go around it is to say that there's some law that forbids you going back to all such times where you already exist... not very appealing...
In my opinion, the closest we can get to travelling back in time, is by capturing the light rays that actually originated in past, and trying to see the objects that they were reflected from. By this, theoretically, we would even be able to watch 'true' replays of Ramayan and Mahabharat. Ofcourse, we won't be able to alter any past events, only view them as they occur.
As far as travelling to future is concerned, I can't even imagine any reasonable way to approach it. 
It's always possible to eliminate paradoxes by making rules that won't allow them to occur in the first place. But again, that's not much fun, isn't it ?
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Aschig
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| | Friday, August 01, 2003 - 8:45 pm: |
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Let me try to talk about a few points that you have raised:
Do we understand time: To the best of current understanding, it is on the same footage as space. Just like mass and energy are really two aspects of the same entity, so also space-time are one entity. Mathematically (and hence physically if we do not bring in our restricted expereinces) time can be treated in the same
way as space. In fact, rather than distances, there are intervals given by:
sqrt(-c*c*t*t+x*x+y*y+z*z)
that separate events. So we do understand it. BTW, we can not necessarily manipulate things if we understand what they are. For example, space - we can traverse it, but I will not call that manipulation.
What are the physical changes occuring? Nothing. It will all have to be consistent. If you go to the past, just like in good sci-fi stuff, it will have to turn
out that you had been there anyway i.e. your old self may not believe you, you may get treated as an unusual but normal person.
Also, being able to do time travel on the scale of humans is obviously a far cry. It will be great even if we could transport some devices which could record conditions and get back.
Photons do not carry the memory of where they have been. You will have to have a sequence of photons and create 2D patterns from them. But thats nowhere near being in another time.
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Shrini
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| | Friday, August 01, 2003 - 11:23 pm: |
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By 'manipulation' I meant being able to travel in time w.r.t. the current context. My apologies for its incorrect use. The point is, do we understand 'time' clearly enough to start thinking about travelling through it ? If I have to go from point 'a' to point 'b' in space, I know exactly what I need to do to get there. Can we say the same thing about going from point 'a' to point 'b' in time ? From a physical point of view, what would such a process involve ?
Also, isn't it a physical paradox that the same object can coexist at different points in space at the same time ? Doesn't it violate physical laws ?
I know photons don't carry memories. We won't have to create a sequence of photons at all. If we can somehow capture those light rays that originated from a past event, we can see what happened. Of course this is not time travel in the physical sense, but I think that's the closest we might get to going back in time.
I recommend the story, 'the dead past' by Asimov. Will make an excellent reading, esp. w.r.t. this context.
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Prafull
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| | Friday, August 01, 2003 - 11:40 pm: |
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Can someone define basic 'funda' behind concept of time travel in brief and 'assumptions' required to prove it at least theoritically?
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Aschig
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| | Saturday, August 02, 2003 - 12:33 am: |
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The current theories which suggest time travel are in their infancy and can not tell you how to go from point A (in time) to point B. But if we could go from a point A to any other point B (i.e. some point B besides where the "flow" of time is taking us) that would be a big step too.
No, no physical law forbids "copies" from existing in different places. When you visit your old self, you are NOT one and the same person.
Is "dead past" part of some collection?
What do you mean by capturing photons? One photon at 4400 Ang. is no different from any other at that wavelength.
Prafull. we live life 1 second per second. If, by some quirk we could manage to find a vehicle to get to another point (and hopefully back) that is time travel.
A most trivial (but one way) technique is to go somewhere at a great pace. Time slows down for you. When you come back, others will have aged a lot compared to you. This is time travel in some sense. But that is not what we really mean.
Some theories suggest that using wormholes one could get to other parts of the universe, or even to other universes very much like in star trek, or contact, or star gate. These theories are based on the solid foundations of general relativity and quantum mechanics and time travel has not been seen to violate any known physical laws.
Of course actual time travel is no where near. The only currently known examples of possible wormholes are massive blackholes and may be very fast rotating neutron stars. But all these are so far away, that to get there we will need a time machine.
More exotic possibilities are the existence of tiny wormholes everywhere. If we learn how to use them, and prod them to carry objects like spaceships that will be the day.
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Aschig
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| | Sunday, August 03, 2003 - 4:15 am: |
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I read "the dead past" today. Its indeed a very nice story and the points it raises about time viewing are indeed very interesting.
However, (1) no particles can be used to record history since all particles of a kind are identical and do not carry any information that differs from another particle with the same properties, and
(2) time travel is different from something like time viewing bacause it will in some sense actually bring alive people from the past in a consistent way.
But I suppose you were using it only as an analogy anyway.
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Shrini
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| | Sunday, August 03, 2003 - 5:15 pm: |
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Aschig, about 'viewing' past (although it's not directly related to this discussion):
we 'see' things when light rays reflected from objects enter our pupils and create an image on our retina.
Now imagine that there are some light rays that were reflected from an 'event' say 5 minutes ago; and after that they just continued to travel straight, without coming across *any* object. SO if now someone 'sees' (or 'captures') these light rays, he will 'see' the past. Exactly equivalent to light reaching us from stars thousands of light years away... what we see now is actually the past of that star! Hope it's clear now!
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Shrini
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| | Sunday, August 03, 2003 - 5:21 pm: |
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>>No, no physical law forbids "copies" from existing in different places.
permit me to elaborate with an example.
Imagine that our Universe contains a time machine, an operator to operate that machine, and one free electron, at time t = 0.
Now, at time t = 1, the operator captures that electron and sends it back to time t = 0. So now, at time t = 0, we have *two* free electrons.
But 'initially' there was only one electron. This second electron appeared out of nowhere, which is against the law of conservation of energy. This is what I meant when I said about time travel violating physical laws. The same argument could be made for travelling to future too.
Is there a way out of it ? (except, ofcourse, modifying the law of c. o. e.)
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Aschig
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| | Monday, August 04, 2003 - 6:16 pm: |
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We do see the sun as it is 500 seconds ago. But that amount is always fixed by the distance and the time light takes at the fixed velocity c to traverse it.
rarely we do see lensed QSOs where the light is taking different paths to reach us, and so we see a few different instances of the past of that object. But this is always fixed by the spacetime geometry. Even if you choose, you can not see the way the sun was just before the first human was born. The best you could do will be to
(1)determine the exact time when that happened,
(2) draw a sphere of that radius and get to its surface to catch those photons
Even then, since the intensity goes down as the surface area, you will get very few photons there and you will be able tomake out a very dim, faint object. So imagine how it will be if you want to view an entire scene. Then there is the question of coherence. I do not have to state here how difficult it is for even specialised apparatus like LASERs which use light amplification, and energy level inversions to create specialised populations of excited electrons.
I will answer your second question with the help of a diagram.
If you have encountered Feynman diagrams you know that electrons travelling back in time are nothing but anti-electrons i.e. positrons.
So before t=0 we have only one electron, and after t=1 also we have only one electron.
The electron that starts travellingback in time at t=1 (as a positron of course) would in fact have been created in pair with another electron at t=0 which you will see going away at an angle. So, contrary to your knowledge, there were in fact two electrons (not tomention the positron) between t=0 and t=1, exactly during the time when you had sent the electron back. Your original electron collided with the positron at t=1 and vanished whereas the other electron is what continued on beyond t=1 thus ensuring that the law of conservation of mass+energy is upheld.
(that also answers the question as to what happens to the second electron once it has reached t=0)
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Vijay
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| | Thursday, June 10, 2004 - 9:04 am: |
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Hello friends
I am new to this group. Though I am not a stuedent of science (which, I think you all are, from all the discussion above)I am keenly interested in this topic of "time travel" as you guys seem to be.
It would look a basic question to you guys, but I do not understand "HOW" we will be able to travel in time?
To the extent I understand, the term "time" is defined in the context of the speed of light. (I might be wrong. If so, please correct me).
If this is the case, then we would need to move faster than light in order to achieve either a backward or a forward movement in "time". This is not permitted by the equation E= m * (c square).
Again, if blackhole or wormhole is the "media" of time travel, then what would happen to the object travelling thru it at "Event Horizon" (Ref: Brief History of Time by Stephen Hawkings).
So, if at all we (general term - mankind) are thinking of time travel, how would we achieve it? Has there any progress / research made in this regard?
Also, I want some help from you people. Can anyone suggest me the books I can read on this topic or the specific web sites giving information related to this?
Thanks
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Agnihotry
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| | Sunday, January 23, 2005 - 8:11 am: |
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Aschiq : Have u read the book "Autobiography of a YOGI" by Shri Paramahansa Yogananda? In one of the chapters Yogananda says that his Guru-Shri Yukteshwara was expected from Puri to Calcutta (I suppose) and he appeared (gave darshan) to him in his hostel room in Calcutta(?) saying that he will be delayed and should not hurry for the station as he will be coming in the next train and not the first one futher he also mentioned that he will be escorted by a small boy with a silver/brass kamandalu/water bearing vessel. It seems the same message/darshan was given to his roommate too, but believing his Guru, Yogananda didnt go to station to receive him at the appointed time of first train while his roommate went, and as expected the Guru didnt come and the roommated came in a huff to tell Yogananda. Yogananda told his friend that the Guru is coming in the next train and that they should hurry. As they reached the station, Yogananda was indeed surprised to see his Guru being escorted by the small boy with kamandalu and all the details that his Guru had told him.
See if u can get some help/link or something clicks reading about it.
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Manjiri_g
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| | Monday, February 21, 2005 - 7:07 pm: |
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Yes, I have read that book. The book is amazing !
People were capable of time travel and had some special powers in old days. It is not frequent now. There are 4 Yugas: Sat, Treta, Dwapar and Kali. and these Yugas come one after the other and are linked back to back. Now, we, humans keep taking birth in different Yugas. In Kali Yuga, intellectual level of humans and values are at a very low level. Whereas in SatYuga, it is the highest. So, accordingly people in our mythological stories had special powers, which now we do not have !
Even now, some saints are known to do time travel. But, they are enlightened. Science may not have an answer for everything.
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Vishal_b
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| | Monday, May 23, 2005 - 6:46 pm: |
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It is sad that I missed out on such an interesting discussion!
>>SO if now someone 'sees' (or 'captures') these light rays, he will 'see' the past. Exactly equivalent to light reaching us from stars thousands of light years away... what we see now is actually the past of that star!
- Or may be if we use a really good, high-end telescope to look deep into space, then we might think of it as observing an event which is happening in the past.
There is a very nice book by George Gamov 'Mr. Tompkins in wonderland'. In one of the chapters the Professor explains Mr. Tompkins about the peculiar focusing effects of the light rays occuring due the curvature of the space. I would like to refer to one of his statements in that chapter: "our Universe is so big that it takes the light milliards of years to go round. You could have seen the hair cut on the back of your head without any mirror, but after milliards of years after you had been to the barber."
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Maj
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| | Tuesday, May 24, 2005 - 11:07 am: |
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theres another such problem which says that if you keep two mirrors facing each other, light rays will continue to travel back and forth (in ideal world, forever). so at any point in time, theoritically u shud be able to see in the past, since light takes time (howsoever little) to travel between the mirrors.
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Aschig
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| | Tuesday, May 24, 2005 - 11:21 pm: |
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Vishal, you have quoted the comment jocularly, right? Because that scenario assumes a universe with positive curvature (as against one with negative curvature, or one that is flat). Current theories suggest that it is likely to be flat.
Maj, you won't be able to "see" the past unless you stick your eye (or a detector) between the mirrors and then the back and forth travel will be broken (besides not being able to determine how long the photons had been travelling that way). An unbelievably flat mirror will of course have to be assumed for such reflections.
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Maj
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| | Wednesday, May 25, 2005 - 2:26 am: |
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Aschiq: Can the problem you mentioned above be overcome by using a "transparent" mirror (can see thru one side, but is a mirror on the other side)?? so that we could keep on watching the past from outside without actually breaking the travel?
And yes of course, there is assumption of the mirrors being absolute flat.
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Aschig
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| | Wednesday, May 25, 2005 - 6:53 am: |
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> (can see thru one side, but is a mirror on the other side)??
Unfortunately not. When you "see" through one side, photons are entering your eyes (and hence not being refelected). All that it means is that photons on your side can not cross through to the other side.
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Vishal_b
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| | Wednesday, May 25, 2005 - 10:46 pm: |
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Aschig, in this chapter the Prof tries to explain Mr. Tompkins about curvature of space and that 'if' our Universe has a positive curvature, then in that case it would have these peculiar focusing effects.
Can you please cite the reference where they propose that space is flat and not curved?
In his book, Gamov mentions that a gravitational field is just a physical manifestation of the curvature of space. And using a fundamental equation of Einstein he suggests that if we consider a space uniformly filled with masses, as, for example, our space is filled wth stars and stellar systems, we shall come to conclusion that, apart from ocassionally large curvatures near seperate stars, the space should possess a regular tendency to curve uniformly on large distances.
I am just quoting the lines from his book here, but the explanation does make sense and makes you think that it is indeed curved and not flat. Now whether it closes onto itself (+ve curvature) or it opens up infintely like a saddle, is hard to answer.
Maj, when you say transparent mirror, that means the mirror transmits some of the photons and reflects some of them. So as time progresses the mirror will transmit a lot of photons and most of the information about the past will leak out. And the rate with which the information leaks out would depend on the percentage reflectance of that mirror.
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Maj
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| | Thursday, May 26, 2005 - 2:07 am: |
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Vishal_b: Agreed. I guess Aschiq also meant the same thing. But now one more assumption. What if I have a mirror which can be transformed in a "transparent" one? In that case I could see in the past at some time in the future.
I agree that we cannot see in the past for ever. But the point is that there is a way by which this could be done (atleast for some time).
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Hi Ashish, Vishal And Mahesh
A very interesting discussion indeed!
Here is what I think.
First of all I can't call it time travel. May be we can call it time flashback. I don't think we can interfere in the things, which have already happened. It is inconsistent. The examples are given above by you i.e. Killing ur granny etc.
So we can only view the past. That too not by our own wish as we all are three-dimensional and lack the fourth time dimension. Hence whenever a thing happens at the x y z and say t (for time) coordinates. And again by the creator's wish, these same coordinates match each other or simply same time coordinate collides at the same place the thing can be viewed there. Also on that place at that time there should not be a major activity going on. Otherwise even if the review will happen but no one will notice it.
This can be one reason why people see things at quieter places.
As we are three-dimensional it is difficult for us to imagine this.
However, if we find out some technique to imitate such a situation, we can create an image of the past.
The other aspect is light rays. They are continuously traveling. So we can think of capturing light rays of past and view those things. But again this is also a very rare possibility. Because of the speed its difficult to find the rays, which are gone beyond hardly 2 minutes.
There should be a known surface in universe on a known distance from earth without any other possible obstacles. So that we can catch rays reflected from that surface. This too is very limited. The rays can get distorted by umpteen factors. So its not as easy as we think.
These are scientific aspects based on limited knowledge to mankind and a very very small fraction known to me. Creator is great. Yesterday's fiction has become today's discovery and tomorrows daily need. So think, discuss and you will discover!
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hI cacaa- dovanaagarItuna marazIt hÜvau Xakt naahI kaÆ 
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Bspujari
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| | Thursday, May 26, 2005 - 6:08 am: |
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>>..but the explanation does make sense and makes you think that it is indeed curved and not flat
actully the "flatness" of univesre is now proven on experimental basis... the famous experiment of "Boomrang baloon" has given this indication. The Boomerang experiment is a microwave telescope taken to 38 km altitude by a balloon over Antarctica. In a 249-hour flight the telescope mapped a part of the sky that is free of thermal radiation from interstellar dust.
The resulting maps,(and derived power spectrum,) shows the strength of the fluctuations on different angular scales, (variations at the level of 1 part in 100,000 in the cosmic background temperature of 2.73 K.) which contains information on such characteristics of the Universe as its geometry and how much matter and energy it contains... these results when analyzed showed the evidence that the universe is rather "flat" than curved...
( see P. de Bernardis, et al., Nature, 404(6781):955-9, 27 April 2000)
... one can find more info here:
http://news.bbc.co.uk/1/hi/sci/tech/727073.stm
http://physicsweb.org/articles/world/13/6/3
http://old-www.nersc.gov/research/annrep00/02compsci_boomerang.html
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I read the discussion and this concept is cool.
Manjirig and Agnihotry,
is there any physical evidence to testify such 'darshans'?!
I have been through the book. Although a good read I can not digest some things, eg.Darshan, as fact. As you mentioned it remains a myth. Which could also be called as fantasy.
It is like that Pushpak Viman thing. If indians had these concepts developed thousands of years ago then why are we are not able to make one of our own yet.
There is no proof of such thing besides our religious books. At least am not aware of.
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Vishal_b
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| | Thursday, May 26, 2005 - 6:48 pm: |
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Thank you Bhalchandra. This is a great piece of information. It is amazing to see how they have used the information from the small anisotropy in the measured temperature of the CMB. Though it is sad on my part that I did not read this article before.
One question which comes to mind after reading this article is that, how do you relate the flatness of the Universe to the peak that is observed at 45 arcminutes on the angular scale? Can you throw some light on it?
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Aschig
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| | Thursday, May 26, 2005 - 7:25 pm: |
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prashant, there is a separate BB about
Time travel in Hindu mythology. You can find the non-scientific (oops, I meant mythological) discussions there.
Bhalchandra, good that you included refs. There are some other separate studies that too are hinting at the flatness of the universe.
Incidently, the first ever (and the only ever needed, since future time travellers could always return to that location and place) time travellers convention was recently held. Unfortunately, no confirmed future traveller showed up, theough there had been a few RSVPs. First time traveller convention
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hI cacaa- dovanaagarItuna marazIt hÜvau Xakt naahI kaÆ
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Bspujari
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| | Friday, May 27, 2005 - 5:14 am: |
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ivaXaalaÊ 45 arcminute ivaYayaI IoP cyaa vaoba saa[T var jao ilaihlao Aaho to qaÜDo Apuro Aaho. %yaa "l" (spherical harmonics) cao AaiNa %yaa angle =45 arcm cao ek relation Aaho :
theta ~ 180(deg) /l
Aat theta = 45 arcmin jar zovalao tr l = 240 Asao ]<ar imaLona..AaiNa maga %yaa AaÌtIcyaa peak caI Ka~I pTona.
%yaa l vaÉna AiQak maaihtI kXaI imaLvaayaica (aivaYayaI http://etd.caltech.edu/etd/available/etd-05282004-140350/unrestricted/main.pdf ha PhD thesis jaast p`kaXa TakU Xakona.
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Aschig
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| | Friday, May 27, 2005 - 7:00 am: |
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limbutimbu, can you also point out how I can see marathi on hitguj while using mac/linux? I am sure you could read english. But there are many like me who can not read marathi. English is the accepted language for science allover and it should be okay to discuss it in english. I would appreciate it if you translate the last post by bspujari into English for me.
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Bspujari
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| | Friday, May 27, 2005 - 9:23 am: |
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oops Ashish, i didn't know that you cannot read what i said, otherwise i would have posted it in english.... but as a matter of fact, i am a "proud" user of Linux (RH9) and i can read/write marathi font easily! ... you just have to download and put it in proper directory ... you can give a try ...
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Aschig
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| | Friday, May 27, 2005 - 1:22 pm: |
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I have tried a few fonts where non-root access works (I do not maintain my computer). But do tell me more about it. Let us discussion about it offline.
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Vishal_b
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| | Friday, May 27, 2005 - 2:43 pm: |
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Thanks Bhalchandra. I will definitely go through the thesis some time.
BTW, do you work with Prof. Kanhere by any chance?
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Bspujari
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| | Saturday, May 28, 2005 - 2:54 am: |
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Yeah! absolutely right!!...how do u know him???..(simply looking at "advanced comp lab" u guessed it ?)
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Vishal_b
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| | Saturday, May 28, 2005 - 6:08 am: |
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Perfect guess! Well, you must be knowing Sharan. We did our M.Sc. together. And I worked with Prof. Gadre in Chemistry for some time. And as you know Prof. Gadre and Prof. Kanhere are..., well, good friends I should say! When I was in the Univ I often used to visit your lab. And BTW, Prof. Pathak taught us DFT!
Cheers!
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Bspujari
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| | Saturday, May 28, 2005 - 6:13 am: |
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once againg getting back to our time travel...what Sanghmitra was saying::
>>So we can only view the past.
actully viewing the past is not a big deal..that's what astronomers do...when they are "looking" at some galaxy say at redshift z=.3 (what ashish did !) they are actully looking in the past at about 1 or 2 million years! ... (meaning the state of the galaxy millions of year ago in past)...but the only question is how to view our "own" past..
>>Hence whenever a thing happens at the x y z and say t (for time) coordinates. And again by the creator's wish, these same coordinates match each other or simply same time coordinate collides at the same place the thing can be viewed there
..but for this thing to happen u need a "loop" with respect to time coordinate ..(one can draw simple diagram ...position -say x cordinate - on horizontal axis and time on verticle axis) ...but thats where the difficulty comes...if ligth has to come back to same position then it has to make a loop...but for that the light has to travel faster than the light (see fig..note that at "every point" light has to make 45deg with horizontal).. such a loop is aginst the laws of relativity...nothing can travel faster than light not even light!
Apart from that as you rightly said we need something "reflecting"...or if space had been spherical we would have expected light to take a long (i mean really looooong!!) journey and would have come back to us....but now with BOOMERANG and MAXIMA kind of experiments that is also not possible..!!....
so in nutshell i believe, at present, the time travel or vieving our own past is not possible ...
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Bspujari
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| | Saturday, May 28, 2005 - 6:19 am: |
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Wow!....u know our entire group!!....
indeed nice to meet you!
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| मायबोली |
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| चोखंदळ ग्राहक |
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| पांढर्यावरचे काळे |
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| गावातल्या गावात |
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| तंत्रलेल्या मंत्रबनात |
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| आरोह अवरोह |
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| शुभंकरोती कल्याणम् |
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| विखुरलेले मोती |
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