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Originally Posted By: K
An object is decellerating such that its position asymptotically approaches x=2m. How long does it take to reach 2m? Isn't that infinite time?


Now you're trying to confuse me with technical stuff, that's "candy from a baby"...too easy to be any fun. However, learning trumps point scoring in the serious threads, in my book, so here goes.

Would I not be right to say that an asymptote is just a mathematical device which bears no more relevance to the real world than did Zeno's paradox. An asymptotic relationship involving deceleration breaks down at Planck's length and Planck's time, if not before. Mathematical infinity, yes; physical infinity, no.


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Originally Posted By: Bill S.
Now you're trying to confuse me with technical
No

Quote:

involving deceleration breaks down at Planck's length and Planck's time, if not before. Mathematical infinity, yes; physical infinity, no.


Hmm well yea I suppose it wouldn't continue moving as the classical mathematical description says. But that means this whole infinity philosophy depends on QM and would have been impossible to consider 100 years ago. Seems strange for philosophical possibilites to change so fast.

Anyway, here's another way. Have a wave, say EM. Reduce its frequency all the way to zero. What happens to its wavelength or period?


By the way, we can cancel this whole discussion if you reckon an object moving 'backwards' at 3m/s doesn't have a physical velocity of -3m/s. Pretty much any measurable quantity can be measured in different ways to give different numbers representing the same thing. We could make up new units to give all sorts of numbers - even complex numbers to represent physical things.


Back to way back. If numbers aren't physical things then what did you mean about all that infinity business? It should all be invalid because there's no physical infinity, just as there's no physical -3.

I have a feeling you just invented a new concept which nothing is known to comply with, gave it some properties, called it 'infinity', then made some deductions based on all this made-up stuff, and using some assumptions, made a conclusion about the possibility of multiple universes.

Reminds me of the guy who builds his castle higher by taking bricks out of the bottom and putting them on the top wink

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Originally Posted By: K
depends on QM and would have been impossible to consider 100 years ago


Same can be said for wave/particle duality etc. etc. What's your point here?

Quote:
Have a wave, say EM. Reduce its frequency all the way to zero. What happens to its wavelength or period?


Come back Zeno...all is forgiven. Have you un-quantised your EM radiation for the sake of argument?

Quote:
It should all be invalid because there's no physical infinity, just as there's no physical -3.


If there is no physical infinity, then presumably, when we talk of an infinite universe (cosmos, or whatever), all we are saying is that it is unbounded. So why not say that?


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Originally Posted By: gan
we can breakthrough the constant of time ( that means we can travel past or future ) as well


It is sometimes interesting to look back to see where discussions like this started. An important point here seems to be the possibility (or otherwise) of time travel; with past-directed TT being particularly relevant in view of recent experimental results with the speed of light.

In the course of thinking about TT, I found myself going round in circles, so I took my wife's advice and wrote everything down. The following is something I wrote a couple of years ago, which seems relevant here.

Think about other people! This is not an exhortation to indulge in charitable thoughts, laudable as such a practice might be, it is, rather, an invitation to think about the people who would be left behind by someone who travelled through time. Suppose you were to travel back a hundred years, what would become of your family and friends? Indeed, what would become of all the people who were living on Earth at the time from which you departed? The simple answer is that they have not yet been born. This is true in your frame of reference, but what of theirs? Suppose for a moment that it was not you who frisked off into the past; suppose it was your friend, or your neighbour, or even someone you did not know. What would happen to you? Nothing, I suspect! Life for you would not change. If you go back in time, your family and friends have to get on with their lives without you. They do not cease to be – in their frame of reference – only in yours.

What does this tell us about the nature of time? Intuitively, we think of only the present as being present. We reason, like St Augustine, that the past no longer exists and the future has not yet happened, so only the present is real; but if you travel to the past, then the past becomes your present, while the present which you left behind still exists for those who did not travel with you. This must raise questions about how you could have gone to the past, if the past were not real, and how your loved ones could still be living in the future, when the future has not yet happened. It begins to look as though time, past, present and future, are all relative: as though time might be a static entity, of which we are able to experience only an “infinitesimal point” as we progress through it. If time really is static, and we are moving through it, then surely we have overcome the first hurdle in our quest for time travel. We are already travelling through time, all we have to do is find some way to change the direction or speed at which we travel. If that sounds easy – think again!


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really confuse.... is that possible to change the direction or speed at which we travel???

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I think that we’re overlooking something here. One of the most fundamental and revered laws of physics is the conservation of mass. If one could travel back to Jan. 1, 1812; they would be injecting additional mass into the universe of 1812. This isn’t going to happen.

I am just as enamored by Sci-Fi lore as anyone else. The notion of time travel to the past is romantic, mysterious, paradoxical… Fortunately, we don’t have to worry about the safety of our grandfathers. The past is not changeable because the laws of physics prohibit it… lucky us.


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Originally Posted By: gan
is that possible to change the direction or speed at which we travel???


Let’s start from the idea that we are all travelling through spacetime at the speed of light. Obviously, not everyone will agree with that, but I thing you will find it is supported by S R. This will mean that any change in our speed through space will influence our speed through time.

We cannot claim that we are stationary in any absolute sense, so we usually regard the Earth as being stationary, and measure our movement relative to that.

When stationary, relative to the Earth, I am travelling through spacetime at “c”.
If I hop into an aeroplane and travel at “v”, relative to the Earth, I am still travelling through spacetime at “c”, so I am travelling through time at “c” – “v”.
I am moving into the future, relative to people on Earth, albeit at a very tiny rate.

When stationary on Earth, my speed through space, relative to Earth, is “v” = 0.
There is no way I can give “v” a minus value, so I can neither increase my speed through time, nor travel back in time.

Having said all that, there are those who believe that it is possible to exploit closed time-like loops to travel back in time, but that’s another story.


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Originally Posted By: Bill S.
Originally Posted By: gan
is that possible to change the direction or speed at which we travel???


Let’s start from the idea that we are all travelling through spacetime at the speed of light. Obviously, not everyone will agree with that, but I thing you will find it is supported by S R. This will mean that any change in our speed through space will influence our speed through time.

We cannot claim that we are stationary in any absolute sense, so we usually regard the Earth as being stationary, and measure our movement relative to that.

When stationary, relative to the Earth, I am travelling through spacetime at “c”.
If I hop into an aeroplane and travel at “v”, relative to the Earth, I am still travelling through spacetime at “c”, so I am travelling through time at “c” – “v”.
I am moving into the future, relative to people on Earth, albeit at a very tiny rate.

When stationary on Earth, my speed through space, relative to Earth, is “v” = 0.
There is no way I can give “v” a minus value, so I can neither increase my speed through time, nor travel back in time.

Having said all that, there are those who believe that it is possible to exploit closed time-like loops to travel back in time, but that’s another story.



Interesting viewpoint (v0=c). In my mind, the only aspect that can belie that possibility is length contraction. Why would travel at a slower speed result in more length compaction? Now, if the “fabric” of space were traveling at c, I would expect that result. For reasons that I don’t want to get into, I have my doubts about either scenario (Earth at c or space at c). However, I have no doubt that Earth is at some fraction of c (to say the least).

None-the-less… an interesting viewpoint. I don’t think that this scenario can be ruled out 100% and therefore, we should remain mindful of the possibility.

I also believe that exchanging the roles of the “background” and “figure” is a valuable exercise that can result in enlightenment.

Do you subscribe to this as a possibility or as a probability? If the “singularity” was traveling through space at the speed of light; how would this be convenient?

Please keep in mind that I am not busting chops here...I already know that you have a large abstract capacity.


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Originally Posted By: KG
The past is not changeable because the laws of physics prohibit it… lucky us.


Enter the multiverse! Personally, I think it doesn't provide an answer to the problems of past-directed time travel, but it certainly has a good try.


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Albert Ainstein predicted that if you travel faster than light, you will travel backwards in time, and shrink. But is that so shure? If you trawel in the speed of light you woud reash you destination (b) at the exact time the light from your destination woud reash you start possision (a) at the exact time. So if you were going faster than light, you woud come to b before the light from b woud come to a. So from a it woud lock that you (c) have traweled back in time, but from you c and b time woud be normal, cuse light is yout a picture of whats happened. So a million lightyears we have the time to trawel from a to b with c. Then, c and light (d) Woud be each fast, and be at a and b at the exact same time, if c traweled each constant as d. So then c woud be this fast -> and d woud be this fast <- . Each fast. But when c came to b, a woud need another 1 000 000 lightyears to see the light from your ship at b. But if c traweled twice as fast as d you woud be traweling --> also twice the speed of light. Then, you woud be at b 500 000 years before d vame to a. Then they woudent see you traweling, cuse you are traweling faster than light. So if you sent a signal when you reached b with the speed -> same 2 000 000 years, but with the speed --> it woud be 1 500 000 years, cuse light still woud be traweling each fast. So if e is speed, the formular woud be: c= -> d=<- e= c=--> d= <- e 1/4 ore somthing. I am not good at formulars, and this is youst my thought.


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I am wery awere of Einsteins prediction of that nothing can go faster than light. But we have to test it. dosent we?
That is the point of science.

Another formulation woud be this. Speed=S Destionation=B Startoftrawle=C Acserelaiton=A Gravity=G You=Y Light=L Year=U Infinitve=I

L/S=1 L/A=I G=0 L/A-B-A=2 000 000 U Y/S=1 Y/A=I G=0 Y/A-B-A=2 000 000

Here lightspeed is defined as 1. Something you shoud notice.
Lets analyce. Lightspeed=1 lightacseleration=Infinitive Gravity=zero, that means there is no gravitaytion to slow down anything, Light from A to B and then back to A, takes 2 000 000 years, thats means from A to B takes 1 000 000 years with the speed of light. In rest you trawel in the speed of light, youst the same. That mean if you trawel with speed of 2, gravity is 0 and acseleration infinitive, you woud trawle twice as fast as light, thats mean you coundt be detected at right spot cuse when light reached detector, you woud be 2 lightyears away from detector if light traweled one year. So you cudent be detected at great accuratcy before you have stopped. Also traweling to a area where light cames from the past when it reaches earth, dosent mean you are traweling backwards in time.


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Magiimice, would I be right in thinking that your line of reasoning is that any apparent past-directed time travel would depend on the frame of reference of the observer? If so, that, surely, is what relativity is "all about".
KG is right, we cannot change the past, because every point is an immutable spacetime event, and even the act of going to a point in the past, unless we were there originally, constitutes a change.
There are those who argue that the multiverse gets us round that difficulty, but I suspect that even the multiverse would not really solve the problem.


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Hi guys. Ever heard of a stasis box?. Like time travel, it's science fiction, but conceptually it's just a glorified refrigerator, and it tells you something important about "time travel".

No motion occurs inside a stasis box, so when I put you inside one, electromagnetic phenomena don’t propagate. So you can’t see, you can’t hear, and you can’t even think. Hence when I open the door 5 years later, to you it’s like I opened the door just as soon as I closed it. As far as you're concerned, you “travelled” to the future. But you did this by not moving. It wasn't you moving, it was everything else.

Ergo if you travel to the future by not moving, you aren't really travelling forward through time, hence there's no way you can travel backwards in time. For that to happen everything else in the universe would have to be subject to negative motion. And there's no such thing as negative motion. Motion is motion. You can reduce a rate of motion to zero, but you can't keep on reducing it.

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Being shut in the refrigerator for 5 years does not sound like my idea of fun.
However, it has a slightly more realistic feel to it than Richard Gott's suggestion for constructing a time machine, which starts:

“First, disassemble the planet Jupiter and use its material to construct around yourself an incredibly dense spherical shell whose diameter is just a bit larger than the critical diameter needed for the mass to collapse to a black hole (for a Jupiter-mass shell, that is a bit larger than 5.64 meters, roomy enough for you to sit inside).”

Perhaps high speed time travel is the best option after all.


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Have a look at the simple inference of time dilation. What actually happens when you move fast on a round trip through space is that all your macroscopic motion reduces local motion. If you could travel at 99.99999999% of the speed of light there's hardly any local motion left, so it's a bit like being in the "no motion" stasis box. But there are practical problem,s like you hit stuff. And it takes a lot of energy to accelerate then decelerate. The best practical way to "travel to the future" would be via cryogenics. After all, we can freeze embryos now, so one day we'll maybe be able to freeze and defrost adults.

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I want to ask that why SR say that actual time = dt-dx??? The twin paradox tell us that the traveler will have a longer time... but if minus dt than it will be shorter!!!! can anybody teach me???

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If you were to go faster than the speed(approximately 300000 meters per second) you bend space and time(much like a worm hole) shirnk space infront of you and expand it out the otherend,
thus,taking you to a differnt part of the universe and a different point in time.

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Hi, Astrophysisict, good to see a new poster.

I would be interested to see the theoretical stuff to back your idea up. The maths would have to be fairly simple for me to cope. smile

My own thoughts would be that if you could actually reach the speed of light, you would have no time dimension in which to travel, so you would be everywhere at once. No need for wormholes to take you to other places.


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A multiverse woud let us travel in time. Well not exsactly. Travelling in time is inpossible. What we sees as time traveling, is simply creating a new universe.


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Originally Posted By: magiimice
Travelling in time is inpossible.


Do you mean that past-directed TT is impossible, or do you include forward-directed TT as well.


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