Send Yourself an Encouraging Message: High School Edition

[Pagan Harpoon]

On the afternoon of Friday 9th of October 2009, you will be walking beside the Cricket pitch when your analysis teacher will pass you and wave. It is possible that you won't wave back and will only realize a few seconds later that it was him and that he was waving at you. You must be prepared for this encounter so that you can return the wave.

 

[BobG]

One of my favorite thought experiments from physics is described by Igor Novikov, in The Future of Spacetime. He describe a game of billiards in which a ball entering a certain pocket will enter a time machine, and then emerge in such a way that it interferes with its own path so as to prevent the ball from entering the pocket first place. Were the ball's path changed so that it misses the pocket, we would have a paradox. But it is found that while the ball may be deflected by its future self, it cannot be deflected enough to miss the pocket. THAT is an amazing result, even for a thought experiment.

Why? Just saying that is one way out of a paradox, but it would be more interesting if he'd at least suggest some reason the paradox is avoided. Conservation of momentum across temporal dimensions would be kind of an interesting avenue to explore as a thought experiment - especially since the entire Earth, the Solar System, and the Galaxy are screaming through space at incredibly high speeds, so real time travel would require a substantially large change in position as well as time.

Novikov refers to rigorous calculations first discovered by Kip Thorne. [p.78]

I checked just to be sure.

Wrong book. Yes, Novikov refers to Kip Thorne's ideas, but those are discussed in Kip Thorne's "Black Holes and Time Warps" along with Hawking's views on the subject (even though they are briefly mentioned in the book you mentioned) ...

Relative to what?

It's not just a spurious question. One can deduce that, regardless of how the time travel might or might not work, we cannot escape the conservation of momentum issue and have to deal with it somehow.

... and, yes, Dave's question is not just a spurious question. It makes a difference.

A time machine definitely can't be envisioned using classical physics. You would wind up with a runaway effect that would violate conservation of energy and momentum (Robert Geroch and Robert Wald). I don't think that saying any radiation or energy would be defocused, resulting in only a small portion of energy reflecting back through a worm hole really resolves the issue. In fact, it's even worse. Instead of the wormhole being destroyed by a runaway feedback loop, you have a stable wormhole creating an imbalance in the entire universe.

You still have a billiard ball from the future appearing in a present with a particular kinetic energy and momentum that duplicates already existing kinetic energy and momentum. In fact, if the time machine were capable of spitting the billiard ball out at different times (instead of just a single time and location), then you could wind up with a billiard ball colliding with itself billions of times, with all of the collisions creating an equilibrium that works for the billiard ball, but not for the rest of the universe that has one billiard ball turning into a sum of masses billions of times greater than the initial billiard ball.

In other words, what happens to the future billiard ball after it has finished deflecting the current billiard ball into the time machine? (This is what I meant about both Novikov and Thorne not going quite far enough.)

That's inductive logic and somewhat inconclusive, but the fact that we do have conservation of energy and momentum certainly indicates that time travel must be incredibly rare, if it exists at all, since there's no reason all of those billiard balls wouldn't be suddenly appearing both in our present and in our past (or some other type of matter/energy besides billiard balls if you don't like that analogy).

Or, perhaps it is inductive logic, but I'm going the wrong the direction in having energy and mass from the future appear in the present. Considering our world's energy problems (even if they are really energy conversion problems at this point), if a civilization were capable of inventing a time machine, it would probably make more sense to pull unused energy and mass from the past into the future. That would wreak havoc with our calculations about the universe's future.

Unfortunately, I don't have enough familiarity with the laws of quantum gravity to go that direction - the direction that Thorne went and the direction that Hawking says would probably lead to the immediate destruction of a time machine as soon as it came into existence.

 

[BobG]

OK, I've got a corollary question now:

If you were to receive a message right now from your future, what would it probably be?
.
.

And, in light of the previous post, my message to myself from the future would be to buy a good helmet to protect myself from all those damn billiard balls! (I'm suddenly filled with doubt about my tin foil helmet being quite sturdy enough.)

 

[Alfi]

Instead of the wormhole being destroyed by a runaway feedback loop, you have a stable wormhole creating an imbalance in the entire universe.

Just curious.
An imbalance?
What 'in the entire universe' is it that gets imbalanced by a stable wormhole?

 

[songoku]

It would be either "Do it" or "Don't do it." And I would know exactly what I'm talking about.

It's the same for me

Go to https://www.physicsforums.com/showthread.php?t=341033 after September 28, 2009. You can send a message back to yourself.

:rofl::rofl:

And message for me :

Join PF now and stop asking physics and math problems to that fat moron...

 

[fluidistic]

My message would be : "You won't die in the next 5 years, happy? You can test the veracity of the sentence if you like."
It makes no sense, sending a message would alter my life. In what way would I like it to be modified? Maybe I should have tried some things with some girls when I had the opportunity.

 

[BobG]

Maybe I should have tried some things with some girls when I had the opportunity.

No, don't do it!

One of them will get knocked up and your son will come back from the future to kill you before you get the chance to knock her up. Except, since that's an unallowable paradox, he'll actually just cripple you for life, dooming you, your knocked-up girlfriend, and your son to poverty for life, which is why he came back to kill you in the first place!

If your name is Oedipus, definitely don't do it! Only bad things come of having the future revealed to you!

 

[DarrenM]

Darren,

1) When Mom gets sick (Las Vegas), tell her to go to see an oncologist. If they don't find anything, tell her to go to another one.
2) Dance with your mother at your wedding; you know, I know, and everyone that has ever met you knows you can't dance. Just do it this once.
3) Read William James on free will.
4) Read Bertrand Russell. Anything.
5) Get a 5-year plan.
6) Get a 10-year plan.
7) It's not the end of the world, until it is.

Darren

 

[BobG]

2) Dance with your mother at your wedding; you know, I know, and everyone that has ever met you knows you can't dance. Just do it this once.

Number 2 is a good one. You can rarely change a person's destination, but you can at least enjoy the time you have with them.

A neat tradition of three hugs (especially with holidays coming):

First hug: Take a slow breath in and out while thinking of what life will be like when the person you're hugging is gone.

Second hug: Take a slow breath in and out while thinking about how you might be the one that's missing next time around. (And, yes, "don't let me go through that alone" is an acceptable emotion)

Third hug: Take a slow breath in and out while thinking about how the two of you are both here, at this particular time and that's something you darn well should enjoy.

 

[Ian_Brooks]

1. Ask her out
2. Ask her out
3. Ask her out

 

[Nephyte]

Hrm, high school... What grade makes such a difference. I'll assume when I started high school.

1) Befriend Martin, Eddie and Mat earlier, these guys will be friends for life.
2) Get a job you lazy bastard and then save the money to buy RIM stock. (You said no Microsoft... nothing against RIM*)
3) Apply yourself to academia, you'll thank me (yourself) later.*The job part would go without saying anyways regardless of buying stock. However, considering I graduated in 1997 RIM seems like the best stock to go with. Buying for under a dollar pre-2000 and then selling in 2007 early 2008 for 150$ is some pretty good profit. 6,666 dollars would roughly make you a millionaire plus quarterly dividends.

 

[physics girl phd]

...A neat tradition of three hugs...

This is a good one, but be careful. I likely got hitched because of the awkward hug a friend (now husband) and I shared... it was awkward for him because he thought it was rather long, and me because I thought it was rather short... and odd enough so that we both knew something was different.