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[P]
Time travel a possibility?

By Tatarigami in News
Tue May 22, 2001 at 04:40:29 AM EST
Tags: Science (all tags)
Science

Leading on from recent experiments which have demonstrated light particles can be slowed from their usual impressive speed to a rate of mere meters per second in a suitable medium, a scientist at Connecticut University in the US has proposed that he can build a genuine time machine.


This New Scientist article details upcoming efforts by a University of Connecticut scientist to turn back time, if only briefly.

(Big thanks to the editors, who know lots of neat stuff and work fast, too. I would order pizza from you guys any day.)

Building a time machine relies on creating a device which would generate a 'closed time-like loop'. This device would take the form of a circle of material with the odd physical property of having negative energy density, rotating at high speed. A material which possesses a negative amount of energy sounds unlikely, but nothing in our current knowledge of physics actually forbids it. On the other hand, the only time we know 'exotic matter' like this has existed is very briefly after the Big Bang, before the universe had decided what it was going to be when it grew up.

Ronald Mallett, the Connecticut scientist has proposed using a rotating circle of not physical matter, but instead, light. It's a little-known and extremely odd property of light that while it is usually massless, if you slow photons down by passing them through a 'Bose-Einstein condensate' at temperatures close to absolute zero, they gain mass, which makes light an ideal material to form the closed loop in the experiment.

Fans of science-fiction writer Stephen Baxter are probably clicking their fingers and remarking "Ah-hah! That's how the alien Xeelee escaped a dying universe in the excellent novel 'Ring'!" Well, right you are.

What are the researchers expecting from the experiment? Well, considering they're proposing to build a genuine time machine, they're being understandably cagey. The first thing they've assured the public is that no-one will be going back in time to murder our grandparents. (Okay, I admit I'm paraphrasing a little.) In fact, they've admitted that they're not holding out any real hopes that the experiment will work. Although the theory seems sound, there may be unknown factors at work in the universe which act to preserve causality by preventing time travel from being possible. When the machine is turned on, they're hoping it will be possible for matter inside the ring to travel backwards or forwards in time to any other point during which the machine was on. In other words, leave it on for an hour, and just before you pull the plug, you'll be able to send something back in time an hour. The researchers have also been quick to point out that since the entire experiment will be conducted at temperatures a few fractions of a degree above absolute zero, we don't currently have the technology to send anything through safely, let alone teams of temporal assassins.

If the experiment works, the researchers hope to see the number of particles inside the circle increase as multiple instances of the same particle at different times fill the space within the ring. Arnold Schwarzenegger is not expected to put in an appearance.

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Display: Sort:
Time travel a possibility? | 86 comments (74 topical, 12 editorial, 0 hidden)
Frankly it sounds completely ridiculous (4.44 / 9) (#4)
by SIGFPE on Mon May 21, 2001 at 07:17:49 PM EST

I know my general relativity well and nothing going on in Bose-Einstein condensates has much to do with GR. What you can do with Bose-Einstein condensates is set up situations where the passage of light through a medium is described by the same equations as light travelling through curved spacetime.

Play with this stuff enough and you can start setting up things like the Bose-Einstein equivalent of black holes - regions of condensate from which no light can leave. But it's not a real black hole in that no real distortion of space or time takes place (except for the tiny tiny amounts associated with any matter/energy). Sounds to me like this guy has taken the analogy too far rather than calculate when and how the breakdown in the analogy will occur (and it will).
SIGFPE

Completely Ridiculous (3.66 / 3) (#12)
by dram on Mon May 21, 2001 at 08:32:09 PM EST

In modern physics there are many things that sound 'completely ridiculous'. I do not know all the physics or math behind it (if I did I would think about getting my PhD) but I have read about some of the insane things. 11D Super Gravity and theorys of everything...time travel and slow light. All ridiculous things. If you want a good book about it all read The Elegant Universe by Brian Greene. Can't wait until they have an improbability drive and a way for me to avoid hitting the ground.

-dram
[grant.henninger.name]

[ Parent ]
Actually... (3.66 / 3) (#17)
by SIGFPE on Mon May 21, 2001 at 09:00:25 PM EST

11D Super Gravity and theorys of everything
Been there. Done that. I studied strings for many years and did a PhD on string related stuff. Even given all that I still think this result sounds ridiculous! I think it's a publicity stunt.
SIGFPE
[ Parent ]
Wow (3.00 / 1) (#18)
by dram on Mon May 21, 2001 at 09:06:56 PM EST

I bow befor you, I wish I was that smart. I tried to my BS in physics and it was just to much math.

-dram
[grant.henninger.name]

[ Parent ]
Well even mathematicians get scared... (3.50 / 2) (#25)
by SIGFPE on Mon May 21, 2001 at 09:58:24 PM EST

...by the mathematics physicists use. I know - I was one of them!
SIGFPE
[ Parent ]
Hmmm.... (3.00 / 1) (#29)
by pete on Mon May 21, 2001 at 10:27:24 PM EST

So, given that I also just finished reading The Elegant Universe, and you're the first person I've ever encountered who's actually worked with this stuff: do you think string/M-theory is true? To my uninitiated mind, 7 extra Calabi-Yau shaped spatial dimensions is a pretty difficult thing to swallow.

--pete


[ Parent ]
I have never believed in String Theory (4.66 / 6) (#36)
by SIGFPE on Tue May 22, 2001 at 01:23:16 AM EST

But I'm more of a mathematician than physicist. But I think String Theory needs to be studied for a number of reasons:

  • I think we need to be able to understand String Theory to be able to do harder stuff. The mathematics underlying Strings is hard and it really isn't self-consistent yet. It's worrying because you quickly get into a mathematical mess starting with simple postulates. As some of those postulates are basic to much of modern physics (some of it quite practical) they're likely to stick around whatever new theories appear next (just as we still use Newton's Laws) so we ought to understand where the problems come from. (I'm thinking of issues like renormalisation, anomalies, rigorous definitions of path integrals etc.)
  • There's something deep underlying Strings. Weird and partly understood mathematics is tied up with Strings and it'd be cool to dig it up. About 20 years ago String Theory fragmented into a number of different theories and then somehow, almost by magic, M-theory reuinted them again. There were hidden layers of order waiting to be discovered and it'd be sad to leave the remaining ones untouched. These subtleties may have nothing to do with physics, just mathematics.
  • Who knows? Maybe it is correct. It certainly has a certain simplicity that the horrible Standard Model lacks.

IMHO The extra dimension thing isn't a big deal and I think people can be unnecessarily be scared of by them. Dimensions are just numbers. Extra dimensions are just like extra numbers associated to particles. That's all. You can think of the quantum mechanics of a particle, say, as 1-dimensional, 3-dimensional, 4-dimensional or infinite dimensional depending on your point of view. They're all equivalent. It's not a big deal.
SIGFPE
[ Parent ]

Why I like string theory (none / 0) (#60)
by a humble lich on Tue May 22, 2001 at 07:25:43 PM EST

While I am not a string theorist by any means (but I know some), I must say I like sting theory. As you seemed to imply, I don't think it is really physics, and I don't think the question as to whether it is "right" will be answered in a long time.

However, what is promising about it is that it shows that it is at least possible to write a self consistent theory of quantum gravity (although you say it isn't really self consistent that isn't something I've heard before). Also the fact that there seems to be some deep underlying unifying ideas in string theory give it credibility.

Of course until experiments can be done we will never know if it is right (my experimentalist friends have this crazy notion that physics is an experimental science). And I agree that the extra dimensions are not that big a deal. Again we have to go with the experimental science question, and if no experiments have been done that disprove there are unseen dimensions we have no grounds to say there are none. Of course we also have to admit that we can't say there are additional dimensions until experiment confirming that have been done.

[ Parent ]

String theory (none / 0) (#67)
by jadams2484 on Tue May 22, 2001 at 10:49:38 PM EST

To the mathematician, yeah, its troubling how calculus doesn't exactly work in the curly dimensions... but think about this for one second, even if you're not convinced by string theory: You know how string theory has the minimum distance, the breadth of the string, and nothing can be smaller, the planck distance? Could there be like a minimum time too, like the amount of time it takes light to travel that distance... Life could be seen as happening in frames, if you accept the assumption stuff has to 'move' to 'happen.'

[ Parent ]
Planck time (none / 0) (#72)
by spiralx on Wed May 23, 2001 at 05:54:54 AM EST

Could there be like a minimum time too, like the amount of time it takes light to travel that distance...

It's called the Planck time and is about 10^-43 seconds.

You're doomed, I'm doomed, we're all doomed for ice cream. - Bob Aboey
[ Parent ]

Discrete universe and smallest time steps. (5.00 / 1) (#76)
by SIGFPE on Wed May 23, 2001 at 01:10:39 PM EST

Could there be like a minimum time too
Remember, that there might be a 'minimum time' doesn't necessarily imply discrete 'frames'. These minimum times could be fuzzy. Or space-time could be tesselated in 4D so that time steps in different places are all slightly different slices. There's certainly no reason to expect 'frames' stretching coherently one after another stacked like sheets of paper.

Although physicists have been trying hard to figure out how to do quantum gravity in the 4 dimensions we live in 3d quantum gravity was figured out many years ago. This obviously doesn't describe the universe we see but it's a useful warmup exercise! The weird thing about it was that there were completely different looking alternative formulations of it that turn out to be equivalent deep down. Some of these descriptions are discrete, almost digital, and some are continuous. So it may turn out that whether there is a smallest time step simply depends on your point of view! (A link for this is here. John Baez is a well known physicist who also writes an (approx.) weekly column in sci.physics.research. Skip down to the word "sloppy" and read those paragraphs. Sorry - his articles are not easy to read for non-specialists but I can find no other articles on this subject.)
SIGFPE
[ Parent ]

Ridiculous (5.00 / 2) (#53)
by retinaburn on Tue May 22, 2001 at 12:46:43 PM EST

Frankly I think its ridiculous that light could ever be slowed down.

Frankly I think its ridiculous that man will ever get to the moon.

Frankly I think its ridiculous that man will ever fly.

Frankly I think its ridiculous to try and break the sound barrier.

Frankly I think its ridiculous to try to capture fire.

Famous quotes through the ages.... :)

I think that we are a young species that often fucks with things we don't know how to unfuck. -- Tycho


[ Parent ]
Wild guess at the effects .. (2.00 / 3) (#5)
by Highlander on Mon May 21, 2001 at 07:20:15 PM EST

From the article, I get the impression that what is affected by the time freeze is the beam itself.

This doesn't seem to be as strange as a "big" time machine. It could possibly be used to make the beam do some calculations immediately, in no time at all.

You could simulate an entire small universe in that ring.

Still, this assumes that in order to get the effect, you don't have to cool the light beam too much, since you still need some movement for quantum effects to happen.

Well, if it does something at all, it would give good data about elementary forces, I think :-)

Moderation in moderation is a good thing.

Effective Mass (5.00 / 6) (#6)
by sigwinch on Mon May 21, 2001 at 07:40:18 PM EST

(Reposted. It seems like I submitted just as the story was being edited and this comment got detached.)

I am not a physicist, but this seems either implausible or misinterpreted.

It's a little-known and extremely odd property of light that while it is usually massless, if you slow photons down by passing them through a 'Bose-Einstein condensate' at temperatures close to absolute zero, they gain mass, which makes light an ideal material to form the closed loop in the experiment.
You have to be very, very, very careful when talking about mass. Physicists frequently use "effective mass" when discussing systems of interacting entities. For example, imagine dragging a huge sponge through water. Since water is viscous, the entrained water becomes almost a part of the sponge. Water near the surface of the sponge is also dragged along and behaves much like it is a part of the sponge. Of course, the hydrodynamic equations to describe a sponge are horrendously complicated, so the usual approximation is to treat the sponge as if it had a larger effective mass while it is wet. A simple series of experiments can give you fairly accurate numbers for the effective mass, which you can plug right into Newtons equations of dynamics. However, the mass of the sponge hasn't changed a bit: it's just a notational approximation for analyzing the system.

Similar effects take place with the mobile electrons of many conductors. Since the effective mass of electrons determines the conduction properties (e.g., semiconductor bandgaps and superconductivity), there is much interest. Do a web search for "heavy electron" to be drowned in incomprehensible condensed-matter gobbly-gook. Google finds Dynamics of Heavy Electrons, which gives a taste of why people are interested.

In a similar vein, I would expect that the so-called "heavy photons" are merely photons that are spending a great deal of their lifetime as excitations of the Bose-Einstein condensate, and thus the properties of the ensemble depend not just on the photon wavelength, but also on the excitation duty cycle, and the mass and coherence of the condensate.

Any real physics care to enlighten us?

--
I don't want the world, I just want your half.

So... (4.85 / 7) (#7)
by baberg on Mon May 21, 2001 at 07:40:51 PM EST

This is gonna be a self-fulfilling prophecy. In other words, let's say that I turn on this time machine and "let it sit" for the hour that you're talking. Just before I pull the plug, I send something through (say, a tennis ball). Now, that tennis ball gets sent back through time to the moment I turned the machine on. However, if it truly crossed the time boundary, then it would have appeared immediately after I turned the machine on.

Thus, this is what would happen (if the machine works as described). I turn the machine on. It spits out a tennis ball. An hour later, I toss a tennis ball into the machine.

Here's my question: Let's say I get the tennis ball and then decide to NOT throw the ball into the machine. I just created a tennis ball out of nothing. But would I really have a choice in the matter?

While this would finally lay to rest the debate between free will and fate, I think it would cause a terrible paradox anyways.

Thoughts?

As I see it... (4.00 / 2) (#8)
by grahamsz on Mon May 21, 2001 at 07:54:11 PM EST

Maybe it works the other way round. How about looking at it like:

a) if a ball comes out of the machine then at that point it is determined that you will later throw the ball in

b) if no ball comes out then obviously you wont throw it in

Now if we have parallel universes then at the point you turn the machine on there will be a fork() that causes both realities to exist simultaneously.

However before any of this happens you have to have a tennis ball. What if you have ball x to start with, you switch the machine on and ball x' comes out of it (where x' is x + 1hr). What if you then throw x' back into the machine instead of x?

Maybe if you do that you'll end up with a cyclical relationship where the ball that comes out is infinitely old... therefore decayed to the point that you cant throw it, so maybe it wont come out after all.

but in the words of larry ellison - What will really get your noddle going is if I hadn't told you, would you still have done it? :)


--
Sell your digital photos - I've made enough to buy a taco today
[ Parent ]
I Don't Need No Stinking Ball (3.00 / 1) (#22)
by SEWilco on Mon May 21, 2001 at 09:40:51 PM EST

What if you turn the machine on and a tennis ball comes out? You didn't have a tennis ball. Maybe after you first turned on the machine you decided to go to the store and get a tennis ball, then tossed it into the machine. But now you have a ball and don't decide to do all that. Instead you update your research report. Maybe the peer reviewers say "you didn't prove time travel, you proved that you built a replicator which you can't yet control."

[ Parent ]
The Grandfather Paradox. (3.60 / 5) (#9)
by Jacques Chester on Mon May 21, 2001 at 08:04:39 PM EST

There's a better example which is usually grasped more quickly.

Given that there are time machines, I travel back in time to kill my grandfather when he was a baby. This means that I should scrub myself out of existence. Except that, having scrubbed myself out, I cannot be around to kill my grandfather, right?

And so on and so forth. One of those awful paradoxes in physics.

--
Well now. We seem to be temporarily out of sigs here at the sig factory. We apologise for any inconvenience this may cause.
[ Parent ]

easily solved by parallel universes (3.00 / 1) (#54)
by coffee17 on Tue May 22, 2001 at 12:51:04 PM EST

in universe A, you go back in time (now in universe B) and convince your parents to were a condom. End result is you still existed in A, grew up, and then went to a time machine (maybe never to return again). However, in B, while you were not born, you stepped into existence in adult form with previous memories, and continue to exist there until you do something about it (either suicide, or travel to a new universe.

Now there's only the minor point of whether parallel universes exist or not. ;)


-coffee


[ Parent ]

That is a terrible paradox. (2.50 / 4) (#10)
by Crashnbur on Mon May 21, 2001 at 08:14:28 PM EST

Perhaps some ungodly (or maybe even godly) force would FORCE you to toss in the tennis ball. Or blow something up. Or end all existence, as you would have forced untruth of reality. YOU BASTARD.

Did that make any sense?

crash.neotope.com


[ Parent ]
The most twisted thing to do: (5.00 / 3) (#15)
by Burrito Supreme Dictator on Mon May 21, 2001 at 08:55:33 PM EST

1. Announce loudly that you will toss a tennis ball into the machine 1 hour hence.

2. Turn on the machine

5. Retrieve the future tennis ball from the machine, keeping it separate from the present tennis ball.

3. Wait one hour.

4. Toss the future tennis ball into the machine, keeping the original one from ever having gone into the machine.

Of course, in order for this to work, you have to ensure that if you do not get the tennis ball out of the machine in step 5, you will toss the present tennis ball into the machine at the end of the hour, to set up the initial iteration.

Of course, performing this experiment may be the only way to toss the ball in after 1 hour without having received any ball at the beginning. Effect -> Cause.

And now my head hurts.

-- This space devoted to wasting your bandwidth. (A token gesture, to be sure, in these days of high-speed connections. But it's the thought that counts, right?) --
[ Parent ]

I'll laugh when you pick up that tennis ball... (5.00 / 1) (#79)
by DavidTC on Wed May 23, 2001 at 02:15:25 PM EST

And it crumbles into dust cause it's 1423 years old. You aged it an hour each time, remember? ;)

-David T. C.
Yes, my email address is real.
[ Parent ]
LMAO! (none / 0) (#81)
by Burrito Supreme Dictator on Wed May 23, 2001 at 09:57:22 PM EST

So much for tossing both tennis balls in and getting an infinite tennis ball generator!

-- This space devoted to wasting your bandwidth. (A token gesture, to be sure, in these days of high-speed connections. But it's the thought that counts, right?) --
[ Parent ]
Better yet (4.62 / 8) (#20)
by Tatarigami on Mon May 21, 2001 at 09:20:06 PM EST

Here's my question: Let's say I get the tennis ball and then decide to NOT throw the ball into the machine. I just created a tennis ball out of nothing.

My god, do you realise what you're suggesting?

:o)

We could flood the international tennis ball market with cheap, paradoxical tennis balls and be rich!


[ Parent ]
Elementary Economics (3.00 / 1) (#26)
by baberg on Mon May 21, 2001 at 10:05:25 PM EST

We could flood the international tennis ball market with cheap, paradoxical tennis balls and be rich!

Yes, but you forget... If we were to flood the market with thousands of tennis balls, the value of a single ball would plummet. Therefore, we would have a warehouse full of tennis balls that, at one point, had incredible value (up to $1.00 per ball!) whereas they are now only worth a penny a piece.

The key is in moderation, my friend... We should only partially flood the market, while keeping our source of balls a secret... We could rule the world!!!!

[ Parent ]

Sneaky economics (3.50 / 2) (#28)
by Tatarigami on Mon May 21, 2001 at 10:18:41 PM EST

Now you're talking. Hey, once we have a generous supply of tennis-balls-from-nowhere stockpiled, we could generate an artificial tennis ball shortage by using our time machine to dispose of existing tennis balls in the past!

It's a foolproof... wait a minute. Can it be I've just figured out where those balls are coming from?

:o)


[ Parent ]
Thrice Upon A Time (4.00 / 3) (#21)
by SEWilco on Mon May 21, 2001 at 09:28:40 PM EST

The James P. Hogan book "Thrice Upon A Time" involves a time machine where they conduct a number of such experiments. Contradictory transmissions, overlapping, random actions. Of course fiction is no guarantee of how the real world works.

[ Parent ]
I liked that book. (none / 0) (#78)
by DavidTC on Wed May 23, 2001 at 02:10:31 PM EST

Especially when they realized, though random chance and dumb luck, they were in the one universe where they never tried to alter the past. ;)

And they also realized what did happen if you alter the past. Tada: New timeline instantly.

I've ofter thought, that if time travel was possible, time travellers would change the past instantly, causing a new time line. We'd go though an insane amount of timelines, until we happened to hit the one that didn't invent time travel, or somehow invent it but never cause a paradox, and we'd stick there. ;) So, my theory is, that some point in a past, one time traveller appeared, from a universe that was created though time travel, and so on, but this time he created a universe where no one created time travel. Either that, or we will create time travel, and be erased, but, logically, some day, though the infinite variations of the universe, eventually some universe won't invent it, and it will stop. I can't decide if that's a good thing or a bad thing.

-David T. C.
Yes, my email address is real.
[ Parent ]

My thoughts... (4.00 / 3) (#27)
by ism on Mon May 21, 2001 at 10:06:45 PM EST

There's an alternate universe in which a team of scientists are tossing tennis balls into an inter-universe wormhole. Ths alternate universe exists in 14-dimensional space where the laws of physics do not allow for time travel, but inter-universe travel. Our universe exists in 9-dimensional space, where inter-universe travel appears to be time travel.

So it's all an illusion, like if you lived in Flatland (two-dimensional) and tried to comprehend what ThreeDeeLand was like. We get the tennis balls and don't have to throw anything in. The universe(s) balances itself yet again.

[ Parent ]

Three universes (4.00 / 1) (#35)
by zerth on Tue May 22, 2001 at 12:25:41 AM EST

There are three possibilities, which would "occur" in this "order":

1. You throw a tennis ball, get nothing
2. You throw a tennis ball, get a tennis ball
3. You throw nothing, get a tennis ball.

You would probably get three universes, with a chance for an infinite amount of possibility 2s, depending on if one's mind can change between loops and if the universe knows a do loop(true) when it sees one:}

However, Stuff would probably be conserved because presumably you somehow wouldn't/couldn't throw the timetraveled tennis ball because it would eventually rot/disintegrate, which would definitely end the cycle, and while one universe would have two tennis balls, the initiator would have none.

Do I need more sleep?

[ Parent ]
Quantum Mechanics (4.00 / 1) (#57)
by baberg on Tue May 22, 2001 at 02:42:50 PM EST

I'm not sure how familiar you are with the precepts of Quantum Physics, but the thought of the 3 universes led me to think of superposition of wave states.

In short, whenever the universe has a decision to make (whether a coin will be heads or tails, for instance) then the universe splits into 2 identical universes, alike in every way except for the outcome of that coin flip. When the coin flip is observed (i.e., when we see what happens) then the other universe collapses into nothingness and we're left with the one universe.

I'm a little worried about the situation where you get a tennis ball without throwing one in, though. The precepts of "Conservation of Energy" comes to mind (yes, energy; mass is just concentrated energy, remember). If I just created a tennis ball, then I'm violating some precept that was developed well before we invented tennis, and what would happen then would be a Very Bad Thing(tm). I don't know what, but something would not work right after that paradox.

But what do I know? I'm just a godless heathen :-)

[ Parent ]

QM (none / 0) (#61)
by kubalaa on Tue May 22, 2001 at 07:42:29 PM EST

I'm curious, does QM actually obey conservation of energy? I thought it only had to in a probalistic sense. Or maybe it's conservation of information that it has to obey. Or maybe it obeys conservation of energy across all possible universes, so the universe where you lost the tennis ball balances the one where you gained one.

[ Parent ]
Sort of. (5.00 / 1) (#69)
by physicsgod on Wed May 23, 2001 at 12:07:13 AM EST

In QM you can "borrow" a quantity of energy deltaE for an amount of time deltaT such that deltaExdeltaT=
--- "Those not wearing body armor are hereby advised to keep their arguments on-topic" Schlock Mercenary
[ Parent ]
oops (5.00 / 1) (#70)
by physicsgod on Wed May 23, 2001 at 12:13:17 AM EST

Guess something didn't like my math, here's the stuff that got chopped off:

"<hbar. In other words you can violate conservation of energy, but only for short times, and the more you violate it the shorter the time.">

So that equation should read "deltaE times deltaT is less than or equal to hbar."

--- "Those not wearing body armor are hereby advised to keep their arguments on-topic" Schlock Mercenary
[ Parent ]
Annoying time paradox (3.00 / 1) (#39)
by swr on Tue May 22, 2001 at 02:10:38 AM EST

Here's my question: Let's say I get the tennis ball and then decide to NOT throw the ball into the machine. I just created a tennis ball out of nothing. But would I really have a choice in the matter?

Similarly, what happens if you have no tennis balls until one pops out of the machine, and then you toss the same tennis ball back in?

It comes out earlier in time because you threw it in later in time. But where did the tennis ball originally come from?

Moreover, the tennis ball that came out can not possibly be the same one that you later throw in! When you go to throw the tennis ball back in, an hour has passed, and so the tennis ball is an hour older than the one you received. If you throw in the tennis ball which is now an hour older, the tennis ball you will have received will have been an hour older than the tennis ball which you did previously receive (if you follow).

So either you can't throw the tennis ball back in, or the tennis ball will be sent back into a different universe from the one you had experienced.

Now, assuming there are an infinite number of universes, what are the odds of receiving one of those tennis balls?



[ Parent ]
Hmm...Grandfather paradox with parallel universes? (4.00 / 1) (#41)
by swr on Tue May 22, 2001 at 06:28:30 AM EST

Here I am replying to my own post... Isn't that a sign of insanity?

Now, assuming there are an infinite number of universes, what are the odds of receiving one of those tennis balls?

Okay, if there are an infinite number of universes from which to send a tennis ball, and an infinite number of universes to send the tennis ball to, then the probability of receiving a tennis ball depends on how sparse or dense the distribution of tennis-ball-sending universes is.

Assumption: If 1/n universes will send a tennis ball, then 1/n universes will receive a tennis ball. (If someone more familiar with math than I cares to correct me on that, please do (infinity is a weird concept)).

That seems simple enough. But what would happen if, instead of just sending a plain old tennis ball, those 1/n universes sent back something that would affect the value of n? For example, a note that reads "Please do not send notes such as this one back in time", instead of a tennis ball. For the sake of example let's use something that can't be ignored as easily as a note... Say, a doomsday weapon that would destroy all civilization, preventing them from ever sending a doomsday weapon back in time.

So we have these 1/n universes each sending on average 1 doomsday bomb back in time, which will be distributed among 1/n other universes. If the distribution is totally random, then every universe has 1/n probability of being destroyed, including the universes that are going to be sending bombs. So basicly:

1/n = (1-1/n)/n (the "- 1/n" part is the 1/n that were destroyed)

...which makes no sense. And because n has "suddenly gotten lower" (which is obviously a meaningless statement if n exists outside of time), the process repeats, which means n "becomes" ever lower:

1/n = (1-1/n)/n = (1-(1-1/n)/n)/n = (1-(1-(1-1/n)/n)/n)/n = ...

No universe is causing it's own destruction in a kill-your-own-grandfather kind of way - they are all destroying other past universes. Being destroyed or not is all pure chance. But collectively, they are affecting the probability of affecting the probability.

Any ideas to sort this mess out?



[ Parent ]
Not quite, but... (4.00 / 1) (#43)
by spiralx on Tue May 22, 2001 at 07:05:40 AM EST

The author mentioned in the article, Stephen Baxter, has already considered this in his excellent follow-up to HG Wells' The Time Machine, called The Time Ships. It specifically deals with both the situations you're talking about (start with 0/1 balls and a time loop) as well as interactions between the ball and itself. I can't remember off the top of my head the conclusions he drew though...

But it's a damn good read though, and well worth buying...

You're doomed, I'm doomed, we're all doomed for ice cream. - Bob Aboey
[ Parent ]

As put forth in some hard sf I read... (none / 0) (#82)
by mindstrm on Fri May 25, 2001 at 12:50:04 PM EST

If you turned it on, and a tennis ball flew out, then you WOULD, in the future, throw a tennis ball into the machine.
This would seem rather wierd, but you would do it. You would not feel 'compelled' to do it or anything... that's just how time works. Everything has already happened.

It would all be perfectly natural.


[ Parent ]
A resolution to the paradox, splitting universes (2.80 / 5) (#19)
by sanity on Mon May 21, 2001 at 09:11:53 PM EST

One possibility to avoid the "grandfather paradox" is that when you send something back in time, the universe splits at that point, creating two divergent universes. This means that if you killed your own grandfather, that would be find, since he would still exist in the parallel universe in which you existed.

Nice defense tactic... (3.00 / 1) (#31)
by brion on Mon May 21, 2001 at 11:57:13 PM EST

Well, if I'm ever up on murder charges (not that I expect such a thing to happen... no, not at all, no reason, just hypothetical) I'll try that defense. "Yes, I killed that man, but it's okay! He's still alive in a parallel universe and doing just fine!"

However, I suspect this would be countered by the fact that in another universe, I won't be jailed and/or executed, so I shouldn't complain...

(Yes, I know that's missing the point. It was a joke.)



Chu vi parolas Vikipedion?
[ Parent ]
WTFM! (3.00 / 2) (#23)
by SEWilco on Mon May 21, 2001 at 09:48:35 PM EST

He should inscribe on each of his machines the space-time coordinates of when the machines are operational. He already says he's trying to create a device which can measure a time-caused change. So he's building something which will detect a signal.

He should always record his detector's signal, and he can watch for someone from the future sending messages back to his detector. If he's lucky the future will tell him how to make his machine better. At least he shouldn't have the "Search For Future Intelligence" problem, as any sender should begin with simple Morse or ASCII signals which will explain the technology for future messages. And the sender will know how to code the signal in a way which will be detectable by his equipment.

parallel universes and conspiracies (3.00 / 2) (#24)
by dnuoforp on Mon May 21, 2001 at 09:51:52 PM EST

i wonder how long until some government agent is tasked with spying on this project. and an interesting thought: if time travel as it is popularly imagined will actually just be transportation between alternate universes, as the article suggests in the end, how will these universes interact? since anything you do in another universe won't affect this universe, then you can perform any kind of experiment you like. including blowing up a whole country to see the ramifications, or testing out some synthetic virus on millions of people. but then, what if these other universes get the ability to come to our universe, and we end up with a Multiverse War?

No need for spies! (3.00 / 1) (#30)
by brion on Mon May 21, 2001 at 11:50:28 PM EST

i wonder how long until some government agent is tasked with spying on this project.

Don't be silly, you know the answer: the project will be successful, so nobody needs a spy - they just have to download the information off the net when it's declassified in 2051 and send it back to the present. So they already have anything they need to know!

And of course, if the project isn't successful, then they'll already know not to bother, because they haven't gotten any data from the future.



Chu vi parolas Vikipedion?
[ Parent ]
Time Travel? (4.25 / 4) (#37)
by Prophet themusicgod1 on Tue May 22, 2001 at 01:46:55 AM EST

sure. why not. i mean, if i can move through the x axis why not through a previouslly undetected one?
but seriously, if you were going to travel back in time, and land in the *exact* same place, only even one hour earlier....you'd probably encounter some difficulty.
  1. the earth is rotating. you would likely end up a few meters at best, a few kilometres at worst from your starting location. that's no big deal...
  2. the earth is orbiting the sun. you can sort of see where i'm getting at this.
  3. the solar system is orbiting some central point in the galaxy, and quickly too...
and so on and so fourth...who knows how fast this quadreant of the universe/plane/whatever you want to call it is moving. we could be all moving faster than light at an unknown and unpredicted direction and we would never know it...so if we actually *did* find a way to go back in time...and retain physical standing....we could likely end up millions of miles away.
on the same principle however you could also base a time machine on...but...tha'ts another story.
and this story did mention unforseen details...one of which will be this one.
"I suspect the best way to deal with procrastination is to put off the procrastination itself until later. I've been meaning to try this, but haven't gotten around to it yet."swr
Relative positioning and velocity (3.33 / 3) (#40)
by jesterzog on Tue May 22, 2001 at 03:02:08 AM EST

I won't pretend to understand how this would work since I'm not very qualified. If someone who's more informed wants to correct me, I welcome it.

Isn't movement relative? The Earth only moves at 29km/s (or is it 29miles/s?) relative to the Sun. The Sun is moving relative to the centre of the galaxy (and who knows how many other stars), and the galaxy is moving relative to the local group (cluster) as well as other galaxies..

So to make a measurement like this, you have to choose a central point to base the relative velocity on. But with the possible exception of the centre of the Universe that everything's moving away from, there aren't any obvious candidates.

I guess my argument is that if you're only moving relative to something else, everything else may as well all be moving relative to you. Congratulations - you're completely still in the Universe.

This whole thing makes me think of an Infinite Improbability Drive.


jesterzog Fight the light


[ Parent ]
relativity (4.00 / 2) (#42)
by fvw on Tue May 22, 2001 at 06:36:14 AM EST

actually, if you move in time, there is no 'the same place', just as there is no absolute 'the same time' for people at different locations... I'd try to explain it but I'd just confuse everybody... googling found this link which seems to explain it reasonably though

[ Parent ]
Stays With The Machine (4.00 / 1) (#85)
by Shalom on Wed May 30, 2001 at 06:37:11 PM EST

If I understand this correctly, the machine is what makes the circle of energy, and that circle stays with the machine at all times no matter how much things around it move. So you would be guaranteed of going back to wherever the machine was when it was turned on.

[ Parent ]
impossible... (1.83 / 6) (#44)
by gibichung on Tue May 22, 2001 at 07:15:59 AM EST

I don't care what "scientists" guessing on top of who knows how many previous guesses think is possible...time travel (at least, backwards, anyway) isn't. How many time travelers have been encountered so far in history? None. Since time keeps going on into infinity, anything that can happen will eventually happen. Well, it didn't. Now, what I'd be interested is slowing down the effects of the passage of time enough so that it seems like you're jumping foward in time.. I wanna live forever, and in say 5000 years, who knows? I just hope I end up as lucky as Phillip J. Fry did and survive two sucessive alien annihilations and a midevil period before I wake up.

impossible? (4.50 / 6) (#46)
by F8alist on Tue May 22, 2001 at 08:10:04 AM EST

"When the machine is turned on, they're hoping it will be possible for matter inside the ring to travel backwards or forwards in time to any other point during which the machine was on."

The machine has to actually be on for it to receive something from the future. Since we don't currently have time travel devices, it wouldn't be possible for someone using this method to send something back to us.

"Since time keeps going on into infinity..."

Says who?

Libertarianism: The absurd notion that an individual is capable of running his own life, and that the government has anything but his best interests at heart
[ Parent ]

yeah, impossible.. (1.33 / 3) (#47)
by gibichung on Tue May 22, 2001 at 08:57:54 AM EST

Cause and effect. Everything happens for a reason. For every action there's an equal and opposite reaction. Et cetera. If the reaction happens before the action it changes the action so the action can't happen exactly like it did to produce the reaction, so the universe kerplodes or gets stuck in an infinite loop or something. It doesn't matter since it'll never happen.

I wasn't talking about this particular time machine anyway, but it's still not possible.

>> Since time keeps going on into infinity...
> Says who?

Every day I've been alive yesterday was the day before and tomorrow was the next day, so I guess I'll live in my fantasy world until someone proves me wrong, heh.

[ Parent ]

If time travel existed, (3.00 / 3) (#49)
by pallex on Tue May 22, 2001 at 09:54:27 AM EST

surely it would be owned by the Americans or Russians, right? If either of those `powers` DID own it, we`d not have the humiliating, yet amusing, examples of Vietnam, crap Russian pilots/submariners etc.

Therefore, no superpower owns time travel.

Perhaps Manchester United own one though?

[ Parent ]
Holes in this theory? (3.00 / 1) (#50)
by vinnyr on Tue May 22, 2001 at 11:59:09 AM EST

I can't even begin to explore the holes in that argument? It seems like your first mistake is to assume the US and Russia (which you claim is still a superpower) will continue to exist as superpowers through infinite time. If the invention of a viable time machine happens in 12,000 years do you still think the US will own it? Do you still think they'll want to go back and erase the Vietnam War? What does a time travveller do? Sneak into Nixon's office using his invisibilty device and give him the secret handshake that the US developed in 1945 for the purposes of recognizing a <Insert spooky music> real traveler from the future? How do you know no one from the future has ever been here? And why do you think they'd be interested in the late 20th century and it's follies? There are plenty of stories from the distant past that make no sense....and the god myths and stories of people from the sky.... Vinny

[ Parent ]
Nurse! He`s coming round! (1.50 / 2) (#51)
by pallex on Tue May 22, 2001 at 12:10:04 PM EST

Seriously though, i was joking.

And you`re right. America no longer has the power, money and respect it once had - I shall no longer refer to it as a super-power.

[ Parent ]
Not Impossible (none / 0) (#84)
by Shalom on Wed May 30, 2001 at 06:32:43 PM EST

I don't think you read the story's method of time travel. It is saying that you can only go back to a time when the machine was in existence in the first place. Thus, these myriad time travelers will only start to appear when the machines have already been built. And they will only be as numerous as the machines themselves, so never fear a veritable flood of future people entering society--unless we decide to turn on a veritable flood of machines.

[ Parent ]
Why matter, why not just information (3.50 / 2) (#45)
by andyclap on Tue May 22, 2001 at 07:25:42 AM EST

After reading the article in NS, I though to myself: "Why try and send matter back - wouldn't it be more useful initially to send information back?"

Consider for a moment what would happen were you to put a feedback loop on the device, so that any information coming out of the device is automatically sent back in an hour later. This allows us to bypass the grandfather paradox - the information is always put into the loop, but creates a whole new paradox: You don't actually need to work out what to put in, as you already have it. Just by thinking about what the information from the future you are about to extract should represent, you get the information you requested.

I've gone cross-eyed...

Sending information back (none / 0) (#58)
by ceruleanlobster on Tue May 22, 2001 at 03:16:41 PM EST

I think that's a great idea; after all, you can send back so much information in just a tiny amount of space. it would probably travel easier than anything with alot of mass. Wish someone would send me back a directive, sometime back in November, saying "Get Norton Anti-Virus now!"

[ Parent ]
Timescape (none / 0) (#59)
by SlydeRule on Tue May 22, 2001 at 05:22:02 PM EST

This allows us to bypass the grandfather paradox
Not really. This is the premise of Gregory Benford's classic "hard Sci-Fi" novel Timescape.

Recommended reading for the speculative science, although the story is not really a page-turner. Also, one must make allowances for the fact that when Timescape was written (1980), the year 1998 was well in the future.

[ Parent ]

The information I'd like to send back... (none / 0) (#65)
by Tatarigami on Tue May 22, 2001 at 10:36:19 PM EST

...is to tell the me of a week ago to avoid the teriyaki beef. Being able to do that would justify the entire project, from my point of view.

(moan)


[ Parent ]
How do you encode "just information" (none / 0) (#75)
by krlynch on Wed May 23, 2001 at 12:16:02 PM EST

"Why try and send matter back - wouldn't it be more useful initially to send information back?"

Yes, but HOW do you send "just information" back through the machine? You have to ENCODE that information somehow. There is no way to encode information that wouldn't require you to send some actual physical entity through the machine. You could use photons, or spin states of electrons, or a piece of paper with written words on it, but you have to actually send SOMETHING back in order to send information.

[ Parent ]

Research Funding... (3.60 / 5) (#48)
by Wojina on Tue May 22, 2001 at 09:15:15 AM EST

At least the research for this could be self-funding once they get the prototype working...just throw tomorrow's paper in it and do some day trading. :)

I already have a time machine (4.50 / 2) (#52)
by mauftarkie on Tue May 22, 2001 at 12:11:33 PM EST

Most others do, too, only they don't realize it. Where do you think your socks go in the washing machine? Think about it: they only disappear when you put them in and turn the machine on (which happens to spin... in a circle!).

I just feel sorry for the past (or future) person who is getting all my socks...


--
Without you I'm one step closer to happiness without violence.
Without you I'm one step closer to innocence without consequence.


What's new? (3.25 / 4) (#55)
by jd on Tue May 22, 2001 at 01:37:22 PM EST

Sorry, but this is OLD news. So old, even Slashdot covered it. You'll find references to this sort of work all over the Internet, scientific papers, etc.

Essentialy, this entire domain of research started when Carl Sagan phoned Professor Kip Thorne on how to travel interstellar distances in a reasonable time, in a scientifically-correct way.

Professor Thorne discovered that there was a solution to the equations for Black Holes which was meta-stable. It required matter of a negative mass to be present. Within that constraint, though, essentially you could use this device to connect any two points of space/time together that you liked.

There was a brief concern that the whole thing would explode, because you could get positive feedback through the system. However, this was quickly dismissed, when it was shown that the "wormhole" - as it was called - repelled matter and energy, rather than attracting it. This would mean that the system would be self-regulating, and positive feedback would not pose a significant problem.

The next challange came from those who argued that exotic matter does not exist. This same claim is made in the title argument. However, that is not entirely correct. It does indeed exist, it's just very difficult to obtain. The references you want to look up are for the "Casmir Effect", where two plates are placed close together and heavily charged. The effect of this is to produce a system between the plates in which no positive matter or energy is valid, under the laws of quantum mechanics. In that region, exotic matter does exist. Unfortunately, there is no aparatus yet built that is capable of reaching this state. The negative state causes the aparatus to implode.

There are alternative ways to stabilise a wormhole, of course. A Kerr Ring Singularity would permit such a thing. However, the gravitational tidal forces - never mind the effort involved in flying perfectly through the hole, without smashing into the ring - would be an interesting challange.

However, what any of this has to do with the speed of light in different medium is anyone's guess. That is NOT time-travel, nor is it any function of time-travel. That's simply a property of waves under different conditions. You might as well argue that your house time-travels, as the speed of light through a house brick is zero.

Casimir effect (none / 0) (#62)
by kubalaa on Tue May 22, 2001 at 07:50:09 PM EST

I have to correct you (I think) on the point that nobody has ever created negative energy using the Casimir effect. I understand experiments have been done which actually measure the negative energy, and there's no reason the plates should "implode"; you just use force to keep them apart. The issue is that this technique is only good for creating very tiny amounts of negative energy, not nearly enough for any real uses.

[ Parent ]
Casimir effect (none / 0) (#73)
by spiralx on Wed May 23, 2001 at 05:59:49 AM EST

The references you want to look up are for the "Casmir Effect", where two plates are placed close together and heavily charged. The effect of this is to produce a system between the plates in which no positive matter or energy is valid, under the laws of quantum mechanics. In that region, exotic matter does exist. Unfortunately, there is no aparatus yet built that is capable of reaching this state. The negative state causes the aparatus to implode.

Sort of. What happens is that since virtual particles are being created and destroyed all the time in a vacuum through the vacuum can be considered to have a load of waves of different wavelengths in it constantly.

However between the two plates only certain wavelengths can occur - those for whom the distance between the plates is an integer number of wavelengths, which means that only particle/anti-particle pairs with certain energies can be created and others are suppressed. This leads to a lower energy density between the plates than in the empty space around it, and thus a radiation pressure pushing the two plates together.

It's been done in the lab though, and there's no implosion. The force is rediculously small...

You're doomed, I'm doomed, we're all doomed for ice cream. - Bob Aboey
[ Parent ]

I don't believe this stuff (1.00 / 1) (#63)
by newellm on Tue May 22, 2001 at 08:09:33 PM EST

This is all a bunch of crap. All these scientist do is base their theories on other scientist crazy theories. None of this has any real evidence.

On the other hand, the only time we know 'exotic matter' like this has existed is very briefly after the Big Bang, before the universe had decided what it was going to be when it grew up.

We don't know anything about the big bang. It is just another theory that hasn't gained enough opposing evidence to disprove it. That is what all this shit is based on, not having enough knowledge to disprove the shit that the shit is based on.

I don't even believe in special relativity as it was taught to me. It is based on the assumption that one thing is moving faster than something else. How can this be possible when we don't have any absolute position to measure motion against.

One experiment they did was flying a plane around with a nuclear(?) clock that was perfectly accurate(or so they say). They left another clock on the ground that was set to exactly the same time. When the plane landed it was observed that the clocks were at different times. They claim that this proves special relativity to be correct because the faster moving plane had time elapse at a different rate(This is somehow recursive). This assumes that the plane is moving faster than the clock on the ground. The speed however is measured relative to the earth. So the earth is not moving and the rest of the universe is. This cannot be true(or can it?).

Matt Newell

PS. I am still in highschool and haven't taken any advanced physics classes so give me a break if everything I said is wrong. I just don't think that there is enough evidence to support all of these theories people are always coming up with.

Little Appreciation for Science (none / 0) (#66)
by jadams2484 on Tue May 22, 2001 at 10:36:51 PM EST

These theories may be improbable, but they are not "crazy." Its just scientists (in this case, reputable ones) exploring possibilities. And the basis of this theory is not "shit," it was derived in a logical rational process. We will replace it one day with a more complete view, but given our present knowledge the big bang is one of the most reasonable conclusions to draw. If you don't believe in special relativity than you don't have much appreciation for science, unless you want to offer a more complete view of the universe than Einstein did. You state the problem without what you think is the reason behind it, so we can analyze your reason scientifically. And you don't need absolute position to measure motion against, just relative position; How fast an object moves away from another. And we do have the speed of light in a vacuum as an absolute speed to judge others against. If a theory sounds outlandish, one should work to find out more about it and disprove it, not just forget about it and move on... I think a scientist making what you call "crazy" claims should have his work studied (and perhaps debunked) more than one who makes small suggestions. You say "there isn't enough evidence to support all of these theories people are always coming up with," well, how do you expect people to get that evidence? 1. Saying "well this is 'crazy' and the basis is 'shit' so lets not get into it. *OR* 2. Saying "lets analyze this and if its wrong, we will debunk it and it will give way for theories that hold water."

[ Parent ]
You just proved it to yourself (or should have) (4.00 / 2) (#68)
by Dink Meeker on Tue May 22, 2001 at 11:30:39 PM EST

This assumes that the plane is moving faster than the clock on the ground. The speed however is measured relative to the earth.

It dosen't assume that the plane is moving faster, what relativity really assumes is that motion is possible (Zeno was disproven long ago), that the speed of light never changes, and that the universe is expanding. It's called the theory of relativity because it is about relative motion. The experiment you're describing involves a plane moving very fast relative to the earth and then seeing if an atomic clock on board is affected (it was and ordinarily should not have varied significantly from its stationary twin).

I don't even believe in special relativity as it was taught to me. It is based on the assumption that one thing is moving faster than something else. How can this be possible when we don't have any absolute position to measure motion against.

That's exactly the problem that Einstein was addressing when he developed his theory of relativity. Without a way to measure absolute motion in the universe (no fixed points in an expanding universe), he went for the next best thing and measured relative motion. The problem with relative motion is that it does funny things to your perception. If we are driving in separate cars, and I am traveling 3mph faster than you, I don't fly past your car, I slowly inch ahead. But the pedestrians I mowed down may have a different opinion about how fast I was going. It dosen't feel like I was going very fast to you, because I wasn't going very fast relative to you. Actually, I was going very damn fast (that's the technical term) if you take into account speed of car+speed of Earth's spin+speed of Earth's rotation around sun+speed of sun's motion in Milky Way+speed of Milky Way's rotation+speed of Milky Way's motion through the universe. My stupid little car actually adds very little into the equation, but most of that dosen't matter because it affects all of us, so only the relative motion among you, me, and the pedestrians really matter. (Actually, pedestrians don't matter all that much either).

If you get in a plane and travel at supersonic speeds while I stay on the ground, time will have less of an effect on you relative to me (a trillionth of a second or so, which is why they needed an atomic clock in the experiment). It may seem unlikely at first, but this is relative time between us and not absolute time (which we can't measure--same problem as absolute space).

Hopefully that helps. If I screwed up anything in this explination, I'm sure someone will point it out.

"Everybody in this room is wearing a uniform, and don't kid yourself." --Frank Zappa
[ Parent ]
Oh boy (none / 0) (#71)
by spiralx on Wed May 23, 2001 at 05:52:31 AM EST

This is all a bunch of crap. All these scientist do is base their theories on other scientist crazy theories. None of this has any real evidence.

Yes, scientists base their theories upon other theories. And there's plenty of evidence for most of them apart from the latest ones, which are obviously waiting for experimental confirmation or to be disproven.

We don't know anything about the big bang. It is just another theory that hasn't gained enough opposing evidence to disprove it. That is what all this shit is based on, not having enough knowledge to disprove the shit that the shit is based on.

Well, considering so far most the evidence is pretty much in line with the Big Bang theory (distribution of elements from primordial nucleosynthesis, microwave background raditation fluctauations and so on) calling it "shit" is pretty ignorant. I don't know that the steady state theory really has much going for it any more, and I don't know of any other alternatives based on science.

I don't even believe in special relativity as it was taught to me. It is based on the assumption that one thing is moving faster than something else. How can this be possible when we don't have any absolute position to measure motion against.

Because it's (the clue is in the name) relative motion. If you are in a car and another car overtakes you, then relative to your speed the other car is certainly moving faster isn't it?

One experiment they did was flying a plane around with a nuclear(?) clock that was perfectly accurate(or so they say).

It was accurate enough so that any error was small enough not to affect the results anyway. These things are taken into consideration in any worthwhile experiment.

They left another clock on the ground that was set to exactly the same time. When the plane landed it was observed that the clocks were at different times. They claim that this proves special relativity to be correct because the faster moving plane had time elapse at a different rate(This is somehow recursive). This assumes that the plane is moving faster than the clock on the ground. The speed however is measured relative to the earth. So the earth is not moving and the rest of the universe is. This cannot be true(or can it?).

From the point of view of someone on the ground it is after all. Alternatively from the point of view of someone on the plane, the Earth was moving and they weren't.

Have you ever sat on a train at a station with another train next to you, and then one of them starts moving and you can't work out which one it is until you look out at another window?

Probably the best example of how these different views are equivalent is muon decay in the Earth's atmosphere. I'll explain if what I've said isn't clear :)

PS. I am still in highschool and haven't taken any advanced physics classes so give me a break if everything I said is wrong. I just don't think that there is enough evidence to support all of these theories people are always coming up with.

There's a lot of evidence, it just isn't really something you can appreciate without a fair bit of advanced stuff. Quantum mechanics seems like a bizarre load of nonsense, but it has a good experimental base and is consistent with all the evidence we have so far. As is general relativity.

You're doomed, I'm doomed, we're all doomed for ice cream. - Bob Aboey
[ Parent ]

Well, then maybe you should.... (3.00 / 1) (#74)
by krlynch on Wed May 23, 2001 at 11:33:22 AM EST

PS. I am still in highschool and haven't taken any advanced physics classes so give me a break if everything I said is wrong. I just don't think that there is enough evidence to support all of these theories people are always coming up with.

Maybe part of your problem is that you don't know what you are talking about? It seems a bit presumtuous of you to tell us that all of modern science is crap when you don't even know what modern science says, isn't it? If a fourth grader came up to you and told you that everything you said about high school was crap, but that he may be wrong because he'd never been to high school, how much weight would you give to his opinion, hmm?

Come back and complain about modern theories when you have some idea of what you are talking about. Until then, the scientists who are actually doing the research have enough questions and technical issues with our theories to keep us plenty busy designing experiments and doing calculations. Maybe you'll even learn enough to be able to help someday. But right now, you're making yourself look like an uneducated idiot.

[ Parent ]

The problem you're having... (none / 0) (#77)
by DavidTC on Wed May 23, 2001 at 01:52:16 PM EST

...and which no one seems to be able to answer, is that time dilation isn't due to speed, persay, it's due to acceleration.

If you take off in a spaceship, fly three light years away at .9C, and then come back, it will only be a few months or whatever (too lazy to figure it out) for you, and 6 years for me. If you 3 light years, and stop and wait, then I fly three light years and catch you, it was three years waiting and a few months, for both of us.

This also happens in a graviational field, too. There is no observable difference between a gravataional field and acceleration. This leads to the legit question of:

Which clock went faster?

A clock in outer space or in orbit should go slower then one on earth, not having to cope with gravity as much. A clock on an airplane should go faster. Of course, all this is assuming they sync the clocks after they get up there, otherwise you have to factor in 'leaving the earth' acceleration minus 'earth's gravity' at each point in the takeoff.

And, before the nitpickers get me, I know relative speed is also a factor in time dilation. But acceleration is reason the earth appears to be the center of the universe. ;)

If we threw a person into space, attached hugeass engines to the earth, and took the planet out for a spin around the galaxic arm, the planet would suffer 'time dilation' relative to the now planetless people floating in space. (I put quotes because everything suffers this dilation on one scale or another, as the universe is expanding, so it's a bit of a misnomer.)

Anyway, this whole thing is insanely confusing, and congrats on picking out a seeming paradox most people don't catch. The very simple version: Things like the 'twins paradox' only happen when someone has a force act on them to change their speed, relative to the other person. If that doesn't happen, the only 'time dilation' that's noticable is basically mimicable by the dopler effect, aka, wavelengths shifting as someone comes closer to you.

-David T. C.
Yes, my email address is real.
[ Parent ]

Really? (none / 0) (#80)
by spiralx on Wed May 23, 2001 at 03:53:20 PM EST

...and which no one seems to be able to answer, is that time dilation isn't due to speed, persay, it's due to acceleration.

Well whilst time dilation is surely affected by general relativistic effects, time dilation is a consequence of special relativity and is caused by high speeds alone. In the twins paradox we conveniently ignore the fact that acceleration is required to speed up and slow down at the start and end of the journey, but it's a thought experiment so we can get away with it.

The lifetime of a muon passing through the Earth's atmosphere has nothing to do with acceleration, but still requires time dilation to be explained.

P.S. It's per se, not persay :)

You're doomed, I'm doomed, we're all doomed for ice cream. - Bob Aboey
[ Parent ]

Well, okay. (none / 0) (#86)
by DavidTC on Thu May 31, 2001 at 01:07:58 PM EST

The time dilation is caused by speed alone, but the effect of time dilation that make it appear that with regard to objects on earth vs objects in space, that the ones in space are moving, is due to acceleration. ;)

-David T. C.
Yes, my email address is real.
[ Parent ]
Special Relativity (none / 0) (#83)
by Shalom on Wed May 30, 2001 at 06:14:15 PM EST

It is good that you question. I had the same problems with quantum mechanics. I was sure that scientists were just copping out and a classical explanation was too hard for them to come up with. But then I realized that the double slit experiment exhibited behavior that cannot be explained with classical mechanics. You must question, but you must be willing also to change your answers when the data shows you up.

Both of Einstein's theories are nothing more than him working out the consequences of certain bizarre facts about the universe. Special Relativity is the simplest to understand, so I'll explain that here.

Special Relativity: Light Is A Constant

The bizarre fact that makes Special Relativity possible is that no apparently, no matter how fast they are going or who is looking at it, light always looks to be going the same speed to everybody. (This speed varies depending on what it is traveling through, but it looks the same to everyone.) We guessed this first in unproven mathematical theories, and then finally found out it was true through experiment--particularly the Michelson-Moreley experiment.

An Analogy: The Biker-Ball Universe

The reason this is so bizarre can be best demonstrated by analogy. Let's pretend there's a universe in which you can throw a ball of light at someone. And let's further pretend that light in that universe goes 60 KPH.

Now, let's say you're sitting on the ground having a nice picnic (you're moving at 0 KPH), and a Hell's Angel biker comes tearing across the grass towards you at 30 KPH. In reflex, you throw your ball of light at him.

Now the light is going away from you at 60 MPH, because that's the speed of light. And if this were an ordinary universe, the Hell's Angel would be scared as heck because it would look to him like the light was coming at him at 90 KPH (30 KPH + 60 KPH). Kind of like when two cars going 60 KPH hit each other head on we say they crashed at 120 KPH, because that's how fast they were going relative to each other.

But this is no ordinary ball, and no ordinary universe. This universe has our weird rule that says light has to look the same to everyone.

So because of that rule, you should see the light going away from you at 60 KPH and he should see the light coming towards him at 60 KPH, even though he's moving toward you really fast. How bizarre!

Why Space Time Has To Bend

The problem is, now that light is all screwed up, the biker and you are going to perceive the place where the light hits you totally differently. He will see it hit him when you two are 30 feet apart, and you will see it hit him when you are 40 feet apart.

Thus something has to give. The only two things there are in velocity are space and time (distance divided by time). So something has to bend in order for everything to work out. It turns out that both of them bend.

The Math

If you're 60 feet from him, and he's going 30 KPH, then you'd expect the light to hit him at 40 feet from you (the light traveled 2/3 the distance and he traveled 1/3). But from his point of view, the light is coming at him at 60 KPH and you are essentially coming closer at 30 KPH (just flip around how you see things, there's no difference in terms of the math). When the light hits him it will have traveled 60 feet and you will be 30 feet closer. So to the biker, it looks like the light hit him at 30 feet from you.

That number has to match up somehow or the universe would be screwed up: you could never say "X happened at time and place Y" because that time and place would be totally different for everyone. Since the velocity of light can't bend, something else has to. It turns out that what bends is space and time around the observer.

Hope this helps some and I haven't confused you more :)



[ Parent ]
Time Travel Research (none / 0) (#64)
by Dink Meeker on Tue May 22, 2001 at 10:23:41 PM EST

Whether it works or not, research in this area is bound to lead to a great breakthrough.

Einstein's theory of relativity clearly leaves open the possibility of time travel, a fact that troubled him, but he could not work out any way to prove that it was an impossibility. And the theory of relativity has been born out by several experiments.

The reason why Einstein is so famous is because relativity completely redefined our understanding of space, time and gravity. If time travel is indeed an impossibility, proving it would be a breakthrough as big as or even bigger than Einstein's since it would be another dramatic shift in our understanding of the universe.

We may also get a better understanding of what time is. The fact that there are so many examples of time paradoxes shows that we don't really have much of a grasp on the subject. Right now our view of time is that it always moves forward, but we may not be looking at it in the right context. When our context was limited to just Earth, our view of gravity was that things always fell down. When viewed in the context of the universe, gravity is a force of attraction between all matter. Sure, the "things fall down" is still an accurate description of the "real world" we live in, but Newton's theory of gravity (and Einstein's revolutionary refinement thereof) have expanded our knowledge tremendously.

And, of course, if they can build a time machine, it might make a bit of a stirr.

"Everybody in this room is wearing a uniform, and don't kid yourself." --Frank Zappa
Time travel a possibility? | 86 comments (74 topical, 12 editorial, 0 hidden)
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