Quote: Original post by irreversible
...
Allow me to refer you to Brian Greene's The Fabric Of The Cosmos

This seemingly simple tidbit implies more than it seems to: it implies determinism.

I used to think that the universe was deterministic. If you could accurately model the movement of every particle, you could predict with exact precision the future state of the universe. However, it's impossible to know the exact movement of some particles. Where will an electron be at a point in time? When a material experiences radioactive decay, which particle is the decay particle? It's impossible to know ... and not because our models are incomplete.

So, if the universe is not deterministic, then we can logically conclude that anything which implies that it is, must necessarily be false.

in response to the OP:
If you're standing on the beach in California and you throw a pebble into the ocean, will the pebble's ripple reach the shore in Japan? People who think a buttefly flapping its wings could spawn a hurricane would say "yes" but I disagree. The minor ripple caused by a pebble or butterfly are quickly lost in entropy as the ocean/atmosphere tries to maintain its equilibrium state.

As depressing as it is to consider, the sum of all of human achievement and action is a mere pebble thrown into the vast ocean of time and space. So cosmically insignificant...it's really not worth mentioning.

Quote: Original post by Sneftel
This question seems about as meaningful as asking "how far does it take for over there to get over here."

You can actually solve that problem with a range. It never gets over here or it travels at the speed of light since that's the fastest information can travel.

The OP's question has no good analogy I can think of that isn't a separate problem with a different solution.

IANAP, but I imagine it travels at the normal progression of time. Though I don't believe there is a past or a future. There's only a present and imagining a past that you could go to and test this is a bit silly.

Quote: Original post by slayemin

Quote: Original post by irreversible
...
Allow me to refer you to Brian Greene's The Fabric Of The Cosmos

This seemingly simple tidbit implies more than it seems to: it implies determinism.

I used to think that the universe was deterministic. If you could accurately model the movement of every particle, you could predict with exact precision the future state of the universe. However, it's impossible to know the exact movement of some particles. Where will an electron be at a point in time? When a material experiences radioactive decay, which particle is the decay particle? It's impossible to know ... and not because our models are incomplete.

So, if the universe is not deterministic, then we can logically conclude that anything which implies that it is, must necessarily be false.

I'm not talking about determinism as such - but rather implied determinism, which takes the past-to-future temporal vector and makes it symmetric with a (hypothetical) future-to-past vector.

Eg if you were to ignore the second law of thermodynamics (that is increasing entropy, which you'd probably have to flip) and were to simply "run the universe in reverse", the exact same things that have happened, would happen in reverse. Determinism is most likely not something to get hooked up on because we exist in time and as such do not have the outside vantage point needed to make the final assessment. Note, however, that even quantum physics hasn't definitively proven that there isn't a single solution to summed probabilities of a particle's probability waves. Feynman's idea takes the notion of "all probabilities" and averages them so that there's only one outcome that is the likeliest (the double slit experiment shows that this outcome needn't at all be a fixed path - as long as not only the experiment, but also the path isn't contaminated). Who's to say that that by itself couldn't be defined as determinism?

Determinism is a far more subtle topic than people generally tend to think. Truth is, to us, for all intents and purposes the Universe isn't deterministic because it's just too complex - which could just be a nice way of saying that we're to simple.

>> When a material experiences radioactive decay, which particle is the decay particle?

Indeed, the weak nuclear force is a bit of a wildcard. I'm not informed enough to follow up on this in more detail, though.

Quote: Original post by Sirisian

Quote: Original post by Sneftel
This question seems about as meaningful as asking "how far does it take for over there to get over here."

You can actually solve that problem with a range. It never gets over here or it travels at the speed of light since that's the fastest information can travel.

Er, OK. So how far, in meters, does it take for 5 meters away to get here? Remember, the speed of light in a vacuum is approximately 3x10^8 m/s.

Quote: Original post by Sneftel

Quote: Original post by Sirisian

Quote: Original post by Sneftel
This question seems about as meaningful as asking "how far does it take for over there to get over here."

You can actually solve that problem with a range. It never gets over here or it travels at the speed of light since that's the fastest information can travel.

Er, OK. So how far, in meters, does it take for 5 meters away to get here? Remember, the speed of light in a vacuum is approximately 3x10^8 m/s.

In Minkowskian spacetime the distance and time metrics are interchangeable.

Quote: Original post by irreversibleEg if you were to ignore the second law of thermodynamics (that is increasing entropy, which you'd probably have to flip) and were to simply "run the universe in reverse", the exact same things that have happened, would happen in reverse.

Sorry if I'm picking at details I can't prove, but to "run the universe in reverse" and then forward again would require a force to be exerted on the universe which by itself could be sufficient to change the outcome of events. You'd have to somehow pull yourself out of the universe and be an observer capable of watching events occur backwards and forwards like some sort of cosmic VCR.

if i would kill my dad in the past (before my mom getting pregnant), it would, according to all the knowledge i have, immediately kill me.

the paradox is, it would immediately make it impossible that i could travel back in time to kill my dad, so nothing would have been there to kill him.

this would be an instant infinite loop that could not resolve itself.

there would not be a form of ripple, it would be instant.


i remember how in back to the future, the reaction to time changes was not instant.

Quote: Original post by kseh

Quote: Original post by irreversibleEg if you were to ignore the second law of thermodynamics (that is increasing entropy, which you'd probably have to flip) and were to simply "run the universe in reverse", the exact same things that have happened, would happen in reverse.

Sorry if I'm picking at details I can't prove, but to "run the universe in reverse" and then forward again would require a force to be exerted on the universe which by itself could be sufficient to change the outcome of events. You'd have to somehow pull yourself out of the universe and be an observer capable of watching events occur backwards and forwards like some sort of cosmic VCR.

Indeed - assuming an observer were required :). What I'm saying is that by starting at a "particular moment in time" (whatever that means) and suddenly reversing all acting forces in the universe, the universe would run backwards, just like the VCR you mentioned. This by itself is not dependent on an observer being present (no one needn't be able to observe this) - it just demonstrates that the laws of physics are symmetrical with respect to the arrow of time. The truth of the matter is, that isn't the case and if nothing else, then entropy disqualifies this as a possible realistic outcome. It doesn't invalidate it, however.

Quote:
if i would kill my dad in the past (before my mom getting pregnant), it would, according to all the knowledge i have, immediately kill me.

I'm sure locality would have a few more qualms with that than just your individual existence :).

On a more philosophical note, however - assuming for a moment such a thing as omniverse exists, there are universes out there where you go back in time in every imaginable way and kill your dad in every imaginable way. And unimaginable, for that matter.

Maybe time doesn't exist and the universe is ever present simultaneously and your just some cosmic spark of consciousness which wills itself to manifest at this particular coordinate phase space ;) if that's the case it won't take any time at all for the changes to propagate but unfortunately you can't make any changes which haven't already been done.

Enjoy!

-ddn

Quote: Original post by davepermen
if i would kill my dad in the past (before my mom getting pregnant), it would, according to all the knowledge i have, immediately kill me.

the paradox is, it would immediately make it impossible that i could travel back in time to kill my dad, so nothing would have been there to kill him.

No, Sir. Remember, this is how Marty MacFly got into that trouble in the first place! :-)