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Posted: 02-08-2007 00:35 Post subject:

Yes, I understand Einstein's assertion that spacial locations can never be defined with certainty. But how does that assure that the hypothetical time machine would vanish and reappear at the "same" place (relative to the Earth)?
Now, allowing here for the term Universe to be defined as a physical place currently expanding in otherwise empty space, what if we "enclosed" the Universe, as it is at this very instant, with eight imaginary balls that were stationary, such that the Universe is enclosed within a cube; beyond these 8 points is empty space into which matter has not yet spread. We don't, of course, but imagine that we did.
Now and, admittedly, only now, could we define "where" the Earth is at any particular moment. It is in this hypothetical scenario that my original question takes place.

Posted: 02-08-2007 01:07 Post subject:

Posted: 03-08-2007 21:29 Post subject:

So the universe (or space??) does not expand into but just keeps expanding??

Posted: 05-08-2007 04:19 Post subject:

That's pretty much it, yeah.

Posted: 05-08-2007 13:25 Post subject:

Hey, is there a 'but' after that 'yeah' bit?

Posted: 06-08-2007 10:11 Post subject:

There's always a 'but'... It goes without saying!! Wink

Posted: 29-06-2008 17:17 Post subject:

New model 'permits time travel'
By Julianna Kettlewell
BBC News science reporter

If you went back in time and met your teenage parents, you could not split them up and prevent your birth - even if you wanted to, a new quantum model has stated.

Researchers speculate that time travel can occur within a kind of feedback loop where backwards movement is possible, but only in a way that is "complementary" to the present.

In other words, you can pop back in time and have a look around, but you cannot do anything that will alter the present you left behind.

The new model, which uses the laws of quantum mechanics, gets rid of the famous paradox surrounding time travel.

Paradox explained

Although the laws of physics seem to permit temporal gymnastics, the concept is laden with uncomfortable contradictions.

The main headache stems from the idea that if you went back in time you could, theoretically, do something to change the present; and that possibility messes up the whole theory of time travel.

Clearly, the present never is changed by mischievous time-travellers: people don't suddenly fade into the ether because a rerun of events has prevented their births - that much is obvious.

So either time travel is not possible, or something is actually acting to prevent any backward movement from changing the present.

For most of us, the former option might seem most likely, but Einstein's general theory of relativity leads some physicists to suspect the latter.

According to Einstein, space-time can curve back on itself, theoretically allowing travellers to double back and meet younger versions of themselves.

And now a team of physicists from the US and Austria says this situation can only be the case if there are physical constraints acting to protect the present from changes in the past.

Weird laws

The researchers say these constraints exist because of the weird laws of quantum mechanics even though, traditionally, they don't account for a backwards movement in time.

Quantum behaviour is governed by probabilities. Before something has actually been observed, there are a number of possibilities regarding its state. But once its state has been measured those possibilities shrink to one - uncertainty is eliminated.

So, if you know the present, you cannot change it. If, for example, you know your father is alive today, the laws of the quantum universe state that there is no possibility of him being killed in the past.

It is as if, in some strange way, the present takes account of all the possible routes back into the past and, because your father is certainly alive, none of the routes back can possibly lead to his death.

"Quantum mechanics distinguishes between something that might happen and something that did happen," Professor Dan Greenberger, of the City University of New York, US, told the BBC News website.

"If we don't know your father is alive right now - if there is only a 90% chance that he is alive right now, then there is a chance that you can go back and kill him.

"But if you know he is alive, there is no chance you can kill him."

In other words, even if you take a trip back in time with the specific intention of killing your father, so long as you know he is happily sitting in his chair when you leave him in the present, you can be sure that something will prevent you from murdering him in the past. It is as if it has already happened.

"You go back to kill your father, but you'd arrive after he'd left the room, you wouldn't find him, or you'd change your mind," said Professor Greenberger.

"You wouldn't be able to kill him because the very fact that he is alive today is going to conspire against you so that you'll never end up taking that path leads you to killing him."

Greenberger and colleague Karl Svozil introduce their quantum mechanical model of time travel on the ArXiv e-print service.

http://news.bbc.co.uk/1/hi/sci/tech/4097258.stm

Background on Sum over Histories here:
http://www.einstein-online.info/en/spotlights/path_integrals/index.html

Posted: 29-06-2008 19:56 Post subject:

imho its more likley that the only form of timetravel possible is being able to remove an object from time,the same way you can remove weight from an object(in space).the said object will not travel back in time but stand still like removing it from a flowing river.once it rejoins time it appear exactly at the time it left ,the same way an object wont weigh any less after being weightless.

the obvious benefits are to not being subject to time are vast but imo wont lead to any form of "back to the future"type scenarios

why would it be possible to go back?its more likly to be that you simply stand still in normal time untill you decide to rejoin it.very handy if you want to travel very long distances though.

Posted: 29-06-2008 19:56 Post subject:

.....

Posted: 30-06-2008 00:03 Post subject:

I suspect that if time travel is possible, then forwards, or backwards, won't be the problem, it will be travelling sideways through possible pasts and futures that will be the problem. Getting back to one's own time-line might be very tricky.

That way, changing the past might well be possible, in one time-line, or other, well within the realms of quantum possibility and no unlikely quantum time-loops necessary.

Posted: 30-06-2008 16:34 Post subject:

Posted: 30-06-2008 18:25 Post subject:

Posted: 30-06-2008 23:11 Post subject:

Posted: 22-07-2010 12:48 Post subject:

Lots of comments at link.

Posted: 22-07-2010 12:55 Post subject:

I started (back when Virgin had the range) writing a Doctor Who novel, in which turned out that the TARDIS had a quantum computer at its heart that enabled it to navigate the time lines...