A Thought About Time Travel

[swashthebuckler]

OK, so I was thinking about time travel the other day, and basically formulated a kind of logic puzzle:

 

The Primary Assumption: Time Travel is possible.

 

Now assuming that the above is true, here's my scenario:

 

Suppose someone (e.g. Adam) is a normal guy. He somehow travels back in time and meets his mother. Adam then marries his mother and has a child, himself. Now, here's my paradox. If he travels back in time, then he would always have been his own father. I know this "time loop" thing has been proposed before and you probably all already know what it is. Now, unless parallel universes exist, Adam is his own father. Let's say Adam Original's genes are XY. X from his father, Y from his mother. However, if he is his own father, won't Adam New have a different set of genes than Adam Original? And if this keeps on happening, every Adam will have a different set of genes. According to the "time loop" scenario, everything should always be the same. However, if Adam fathers himself, his wife/mother's genes will be more and more prevalent in the new Adam's genes. The way I see it, the only way this scenario can work out is if:

1) time travel is not possible, and my original assumption is wrong

 

or

 

2) every time something time travels, an alternate/parallel universe is created which is different than the old one.

 

Feel free to respond

[ecoli]

sounds like a different spin to the same puzzle.

[Cap'n Refsmmat]

Why should Adam New have a different set of genes from Adam Original? Adam New's Y chromosome and one half of his genes are taken from his father, Adam Original. So Adam gives himself his own genes.

 

The dilemma I'd see is "where'd the Y chromosome come from originally if Adam's merely giving it to himself?"

[swashthebuckler]

Yes, but Adam's genes don't segregate exactly the same way each time during meiosis. If we consider it with two genes, one time the fertilizing sperm might be AB. Another time it might be Ab. The thing is, Adam's father/mother chromosomes don't segregate the exact same way each time, unless they have a "destiny"

[iNow]

Since you're breaking the laws of physics, you can postulate just about anything you want. With the scenario you posed, Adams mother could be a unicorn and it would be roughly equivalent.

[Cap'n Refsmmat]

Yes, but Adam's genes don't segregate exactly the same way each time during meiosis. If we consider it with two genes, one time the fertilizing sperm might be AB. Another time it might be Ab. The thing is, Adam's father/mother chromosomes don't segregate the exact same way each time, unless they have a "destiny"

It only happens once. Time doesn't repeat, so far as we know.

[swaha]

if u travel time may be u dont become part of it just see it like u do in tv that may also serve the postulate.

[NZL]

Here's what will happen according to me:

 

Before any of the time travelling was going on, Adam had to have a different father than himself, or he would have never existed to do his first time travel trip.

Now, when original Adam grows up to be a very bright scientist and travels back in time 9 months before he was born to go and impregnate his mother, his original father would not have the chance and the original Adam would have never been born and would never have travelled back in time.

So now, we're at the point where Adam never travelled back in time, and he does gets born, and does gets the chance to travel back in time. But as soon as he decides to go back, he will never have existed.

 

Therefore, it is impossible to travel back in time to conceive yourself with your mother.

[ajb]

Since you're breaking the laws of physics, you can postulate just about anything you want. With the scenario you posed, Adams mother could be a unicorn and it would be roughly equivalent.

 

I don't know much about DNA etc, but what laws have been broken?

[iNow]

Since you're breaking the laws of physics, you can postulate just about anything you want. With the scenario you posed, Adams mother could be a unicorn and it would be roughly equivalent.

 

I don't know much about DNA etc, but what laws have been broken?

 

One would presume a breakage of the 2nd law of thermodynamics considering the statement I've quoted below:

 

 

 

Suppose someone (e.g. Adam) is a normal guy. He somehow travels back in time and meets his mother.

[ajb]

I do not think that the 2nd law and entropy prohibit time travel. The energy expended in time travelling could off-set the entropy, for example.

[iNow]

I do not think that the 2nd law and entropy prohibit time travel. The energy expended in time travelling could off-set the entropy, for example.

Interesting insight. Thanks. I was of the impression that travel back in time had pretty serious issues with causality and entropy.

[ajb]

Interesting insight. Thanks. I was of the impression that travel back in time had pretty serious issues with causality and entropy.

 

Causality maybe, but it looks like there might be the Novikov self-consistency principle.

 

Novikov and his collaborators found that one could a billiard ball to scatter off itself via a wormhole. Slightly more that this, they found it hard not to get the ball to self-scatter. This led them to suggest the consistency principle.

 

Now in its simplest form, the consistency principle says that "you cannot change the past, you were always part of it". But where does this leave self-will?

 

Another possibility is that time travel places you in another universe via the many worlds interpretation. That way you can change the past in another world and not your original world.

 

Or maybe causality is an illusion and not a fundamental part of physics?

 

Or maybe time travel is just not possible.

 

The 2nd law by itself does not ban time travel. What it can do is place constraints on how you time travel.

 

More fundamentally, quantum field theory on curved space-times (semi-classical gravity) suggests that time travel is not allowed. It seems that the expectation value of the energy momentum tensor always diverges (tends to infinity) as you approach a closed time-like curve (time machine). This led Hawking to his chronological protection conjecture. "Time machines are not allowed".

 

However, it is possible that quantum gravity may provide a loop hole here by regulating the energy momentum tensor and then the issue of time machines becomes even more unclear.

Edited August 21, 2009 by ajb

[iNow]

However, it is possible that quantum gravity may provide a loop hole here by regulating the energy momentum tensor and then the issue of time machines becomes even more unclear.

LCG is remarkably interesting, and I'm looking forward to learning more as we progress in that field. I laugh a bit at the statement about it making "the issue of time machines even more unclear," since they are already an enormous bastion of non-clarity, but I really appreciate you taking the time to walk through some of the key issues which my post above completely brushed over.

[ajb]

I laugh a bit at the statement about it making "the issue of time machines even more unclear," since they are already an enormous bastion of non-clarity, but I really appreciate you taking the time to walk through some of the key issues which my post above completely brushed over.

 

Time machines are fascinating. I don't think (apart from Mallett) than anyone is trying to build a usable time machine. The idea is really to push general relativity to its limits to point towards new physics. Classical general relativity by itself does not seem to rule out time machines, though they may require matter that violates the energy conditions. Though this may not be a serious problem in general relativity as we know quantum fields tend to violate these conditions.

 

It is similar to the study of black holes. We know general relativity fails here, but can we get a window into quantum gravity by carefully studying them? I would say yes. Semi-classical results, such as the black hole entropy and Hawking radiation all help towards building a quantum theory of gravity.

 

It seems that quantum gravity, whatever that is will have the final say on time travel.

[jablan]

Anyone remember that John Titor guy a few years back? Claimed to be a time traveller.

 

He says that when you travel back in time a new timeline is created. You can never affect your original timeline negatively, thus eliminating any paradoxes.

 

Linky