time reversal

[rathfon]

I'd like to see people reply more to this, as I was quite interested.

 

[alpha88]

It's conceivable to go back in time, but not prior to the invention of a time machine.

 

[mordantmonkey]

Originally posted by: kotss

Originally posted by: 91TTZ
You know what always got me wondering? What if there were things that could go faster than the speed of light, but since we have no technology that's able to measure it, you couldn't tell. We only have light to use as a reference, so the fastest thing we can measure using light is light itself.

Also, at what speed does gravity propagate?

Gravity propogates at the speed of light. If the sun were suddenly to disappear, it would
take ~8 minutes for the earth to know that.

do you have a resource?

how the hell would they measure that? you can't just make mass disappear. you can turn a light source on and off, but gravity? how do we know gravity doesn't travel faster than light?

edit: heh incoherent light disapates exponetially like gravity...we have lasers now, i wonder if well ever be able to make a coherent graviton wave. GRAVITY GUN, oh yeah

 

[kotss]

Well in doing more "research" into the subject I have found conflicting results from
various sources. Some say the speed of light others say substantially faster.

My original statement came from having read and watched the Elegant Universe.

In doing a search on google this seems to be debated subject among some physicists.

Sources for :
1) Elegant Universe (Brian Greene)
2) Wikipedia

Source against: Wikipeida - Tom Van Flandern

Now, I would side with the explanations that also support general and special relativity.
Does this mean that I am right? Not necessarily.
People who are not physicists (such as myself) must do "research" and come to grips
with what makes the most sense to them. I side with the speed of light.

 

[mordantmonkey]

I think it makes more sense that gravity would travel faster than the speed of light, there are other things that can also by the way.
hell i don't even think it travels at a certain "speed" like a light wave, i think its a different kind of energy propogation. something like a field generation. but this is just the way i feel about what i know

 

[f95toli]

Again, nature does not care about wheter or not we think it makes sense or not. Gravity DOES travel at the speed of light, there is a lot of experiemental evidence for that.

Right now researchers are trying to measure gravitaional waves, there are two experiments running; LIGO has a rather good site where you can find more information link.

 

[mordantmonkey]

they are trying to prove that. the experimental evidence so far isn't solid enough to prove such. it's still debated, when enough experimental data is collected you may turn out be correct.

when developing theories we are going beyond the scope of hard facts. we must use our sense to expand on facts, and form hypothesis to test these theories.

 

[f95toli]

AFAIK all experiments that have been performed so far can be explained within the framework of GR.

And as I have already stated: Common sense is more or less useless when dealing with modern physics so we can not use it to "expand on facts", we can of course extrapolate and try to predict what will happen in a given situation but then we need to use math; trying to guess based on what "makes sense" will inevitable give us the wrong answer.

 

[bluslice]

i just read the first page of replies to this topic and i have something to say to them. You guys have absolutely no idea what the hell you are talking about and I have no friggin idea what you guys are talking about. You talk about time and space like you were in a philosophy class. There's a reason why this forum is called 'Highly Technical'. Please only post what makes sense. I'm not saying that I know it all, but please don't post jibber jabber. I think i felt my IQ go down a few points.

 

[Alex]

of course its possible... didnt u see the movies?

 

[cyclohexane]

Theoretically, my thoughts could transcend time...

 

[bfonnes]

Originally posted by: sao123

This would seem to be a parallel to something i read about the event horizon of a black hole? I dont remember everything, but it has something do with a pairing of a particle with a virtual particle on each side of the horizon, one falls into the black hole, and the other escapes from it. Also the particle and virtual particle have opposite spin.

Ummm... not correct... That explanation is to quantum teleportation.
BFonnes

 

[kotss]

Originally posted by: mordantmonkey
I think it makes more sense that gravity would travel faster than the speed of light, there are other things that can also by the way.
hell i don't even think it travels at a certain "speed" like a light wave, i think its a different kind of energy propogation. something like a field generation. but this is just the way i feel about what i know

What are the other things that can go FTL?
I would be interested to see the resources for this.

 

[mordantmonkey]

Originally posted by: kotss

Originally posted by: mordantmonkey
I think it makes more sense that gravity would travel faster than the speed of light, there are other things that can also by the way.
hell i don't even think it travels at a certain "speed" like a light wave, i think its a different kind of energy propogation. something like a field generation. but this is just the way i feel about what i know

What are the other things that can go FTL?
I would be interested to see the resources for this.

This is a pretty good effort at linking and explaining the debate pretty simply.

these experiments may not ultimately prove FTL movement, but i'm just saying why limit our thinking of possibilities? It may not make "sense" according to our current thinking, but lots of things don't. I just don't think we should rule it out just because of a theory that doesn't necessarily accept it. Theories are not laws for a reason.

 

[f95toli]

I don't know anyone who believes that is it is possible to transfer information FTL using tunneling (which is what is described in the link), and I know quaite a few people who should know since quantum mechanical tunneling in solids (superconductorsin my case) is what I work with.
What it comes down to is the interpretation, you could of course say that "something" is traveling FTL but that this "something" can not carry information (or energy), however this is just a classical intepretation of what is a quantum mechanical phenomena.

From a quantum mechanical poit of view this "speed" is just the time it takes for the wavefunction to collapse when you measure it on the other side of the barrier divided by the thickness of the barries (more or less), this has absolutely nothing to do with FTL movement.

 

[mordantmonkey]

well that "phenomena" may not carry information...yet.

edit: and there are people that believe it can be done, and even claim to have done it. time will tell i suppose.

 

[f95toli]

And it never can, it is simply impossible according to all known facts about tunneling.

It is important to understand that tunneling is actually a simple phenomena and we can describe what is happening with extrememe precsion, but theequations that describes tunneling so well also tells us that you can not use this phenomena to transfer information FTL. I think there is even a mathematical proof of that.

Of course I can not rule out that we someday might discover a phenoemena that allows FTL information transfer, but what I am sure of is that it will NOT be tunneling as we know it AND we will not be able to describe it using any known rules of quantum mechanics.

 

[mordantmonkey]

I am waiting for nimtz to publish.

 

[f95toli]

Nimtz has already published (you should read you own links), his experiments were done several years ago; the outcome was that they do NOT involve any FTL transfer of information.

 

[mordantmonkey]

i'm sorry, i'm sure he has. i just haven't found anything technical on the supposed transmission of mozart's 40th as to medium of transmission. i doubt it was on the wave front but still i would like to read more. i agree that no information has yet been observed to travel ftl, at least not on a wave initially travelling ftl. but the fact that these anomalies exist, makes me raise an eyebrow or two. It's not as if the field of quantum mechanics has been completely exhausted.

 

[f95toli]

Again, tunneling is not an anomalie. Tunneling is just a consequence of the wave-nature of particles, you only need to know some basic quantum mechanics in order to understand it (you can ususally find examples of tunneling through a barrier in the first chapter in any text on basic quantum mechanics).

The so-called "transmission" of Mozart's 40th was just a demonstration, no new physics. Basically, Nimtz did not know what he was doing and when people startet pointing out that he was wrong he used this demonstration to attact attention; he did NOT transder information FTL in that experiment/demonstration.

The problem here is that in order to give a technical answer to why tunneling does not permit FTL transmission of signals you need to use relativistic quantum mechanics which is VERY complicated (but it has been known for a long time, I think Dirac added the relativitic corrections around 1930 or so), it is quite straightforward to calculate things like escape rate, transmission probablities etc but if you start taking relativistic effect into consideration it becomes very messy; fortunately the end-result is the same so you do not need to use the full equations; it is enough to solve the Schroedinger equation.

Quantum mechanics has not been exhaused, tunneling just happens to bea problem that i s very well understood,

 

[ahurtt]

No, you can't go back in time. The closest you are gonna come is loading up an old 8mm movie in a film projector and watching your grandparent's wedding. You can only go forward in time. Not back. In fact, I am travelling forward in time right now!

 

[Gilby]

when developing theories we are going beyond the scope of hard facts. we must use our sense to expand on facts, and form hypothesis to test these theories.

Quite wrong. Physics on the edge often starts with pure thought experiments, in many cases well away from common sense or basic human sensory data.

 

[w00t]

Originally posted by: ggjb
first off you would need a flux capacitor, a delorean, and would need to reach 88mph to travel back in time.

 

[rdegler]

Here are two problems with time travel

1) Where do you go? Looking at relativity you can not determine an absolute to make a measurement from. in other words not only do you have to travel in time but you must travel in space. Just going back one second without "correctly" calculating where to end up might place you inside the earth or out in space. Instant death.

2) Time is part of the phenominal world not the nominal world. It is only a creation of our senses and brain. This is a very long winded one. Read this link explaning transendental idealism by I. Kant. http://www.rep.routledge.com/article/DB047SECT5