time travel

time travel

today,time travel has become a household name.it's been a part of many sci-fiction films,but the question lies can the humans finally crack the nut now or in the near future?"paradox" or "self-contradicting events which may be true" has puzzled the scientists all over.it would require a herculean effort to crack the nut given the problems that one has to encounter.so what do you think of it?

Time travel can NEVER become a reality since it is near impossible to travel back in time... forward time time travel may be possible, but not backward in time.

We all travel in time, at a rate of one second per second.

Time travel is not contrary to basic Physics. Everyone is familiar with the term "wormhole", theoretically the other end of a wormhole could be anywhere in space and time - even in the past or future. Trying to control where and when you emerge is at present an unknown though, since we can't even create wormholes yet.

yep you are right.stephen hawking said that we can only slow the time of a particular place by travelling at a faster speed due to which the motion of the objects present in that particular place will slow down in order to obey the laws of physics.therefore,time travel in future is the only logical conclusion.but that too would come at a huge cost.

but how would "wormholes" explain the problem of "paradox"?

This depends on what exactly can be considered as "time travel".

For example, when we look at stars - we see them as they were some time in the past (because light speed is finite). So, it is very easy to travel back in time - just look at stars.

Another known effect is correlation of time and speed: time passes slower for objects that move fast. So, if you want to travel to the future: launch rocket, achieve speed comparable to speed of light, then return to Earth. Clocks on rocket will show time in the past, it means that rocket traveled to the future. Moreover, we already have people who travel in time this way - astronauts on ISS. But speed of ISS is low, so, those astronauts travel only few seconds to the future.

So, yes, there are effects that can be considered as "time travel". But modern science does not know any method to repeat events shown in "Back to the future" movie.

Originally posted by sgunhouse:

We all travel in time, at a rate of one second per second.

Time travel is not contrary to basic Physics. Everyone is familiar with the term "wormhole", theoretically the other end of a wormhole could be anywhere in space and time - even in the past or future. Trying to control where and when you emerge is at present an unknown though, since we can't even create wormholes yet.

Wormwholes are not that stable and can easily collapse on you while you are travelling through it. In addition, it requires tons of energy to create the wormholes in the first place.

Also, if a space traveler is traveling at the speed of light (not currently possible) time would "slow down" for him/her so s/he can essentially travel into the future using the "fast lane."

There's no theoretical problem with travelling through wormholes. What's less likely is being able to change events while you're there.

Things are the way they are because of the way they are.

Originally posted by johnnysaucepn:

There's no theoretical problem with travelling through wormholes. What's less likely is being able to change events while you're there.

Things are the way they are because of the way they are.

Yes, there is no problem except one: there is no experimental data that confirms existence of wormhole.

Originally posted by sgunhouse:

Time travel is not contrary to basic Physics.

Traveling back in time IS contrary to basic physics: law of thermodynamics says that entropy increases in time.

Originally posted by sergey-pypyrev:

Originally posted by sgunhouse:

Time travel is not contrary to basic Physics.

Traveling back in time IS contrary to basic physics: law of thermodynamics says that entropy increases in time.

Yes but according to Stephen Hawkings, the universe is like a balloon and there could be holes that loop back in time. But it may not be the same universe as us (alternate universe). Also, it could be possible to bend space and time for a shortcut into the future.

Originally posted by wikipedian:

Originally posted by sergey-pypyrev:

Originally posted by sgunhouse:

Time travel is not contrary to basic Physics.

Traveling back in time IS contrary to basic physics: law of thermodynamics says that entropy increases in time.

Yes but according to Stephen Hawkings, the universe is like a balloon and there could be holes that loop back in time. But it may not be the same universe as us (alternate universe). Also, it could be possible to bend space and time for a shortcut into the future.

There is no experimental or even theoretical evidence for existence of alternate universe(s).

Bending space/time: you probably talk about the fact that gravity and high speed changes how time passes. Yes, this fact is known and as I wrote earlier - our astronauts already travel to the future using this method. But I have no idea how this can be used practically.

We can develop topic about alternate universes even more. Idea for multiple universes was said to explain why physical constants have their values. Explanation was that there are many universes with different values of those constants. What exactly will happen if universes with different physical constants will be connected? Look what happens when ice and fire are connected: they are so different that they can not co-exist. I think that universes with different constants will be much more different than ice and fire.

I've traveled here from the past to tell you... Oh noes, I've said too much!

Originally posted by sergey-pypyrev:

Originally posted by wikipedian:

Originally posted by sergey-pypyrev:

Originally posted by sgunhouse:

Time travel is not contrary to basic Physics.

Traveling back in time IS contrary to basic physics: law of thermodynamics says that entropy increases in time.

Yes but according to Stephen Hawkings, the universe is like a balloon and there could be holes that loop back in time. But it may not be the same universe as us (alternate universe). Also, it could be possible to bend space and time for a shortcut into the future.

There is no experimental or even theoretical evidence for existence of alternate universe(s).

If you read A Brief History Of Time: From Big Bang To Black Holes, Steven Hawking mention about a theoritical alternate universe.

Originally posted by sergey-pypyrev:

Look what happens when ice and fire are connected: they are so different that they can not co-exist.

When fire and ice are introduced to each other, you get a lot of water. There's nothing wrong with a universe made up of nothing but water.

To step away from the over-extended analogy for a moment: there is no reason that every combination of physical constant should produce a working, viable universe. Perhaps the majority of them are just foamy balls of sludgy proto-matter that never turned into anything interesting. We can only sit here and think about it because this universe had the right combination to produce something complex.

Originally posted by sergey-pypyrev:

Traveling back in time IS contrary to basic physics: law of thermodynamics says that entropy increases in time.

Yes - entropy increases with time. But we're talking about changing our position in time, not reversing the effects of it.

http://en.wikipedia.org/wiki/Closed_timelike_curve
http://en.wikipedia.org/wiki/Novikov_self-consistency_principle

And no, there is no experimental evidence. But the first step is determining the hypothesis.

It's a lot of fancy jargon by large foreheads this one. Sci-Fi films and comics have an awful lot to answer for. Personally I gave them up when I grew out of Primary School.

For instance, imagine going back in time and killing your own mother. Then she'd never give birth to you, and just how would you have been able to travel back in time to commit matricide in the first place?

Just as bad, what about traveling forward in time only to discover that you were dead.

A riddle, wrapped in a mystery, inside an enigma.
Winston Churchill, October 1939

.....

Originally posted by rjhowie:

It's a lot of fancy jargon by large foreheads this one. Sci-Fi films and comics have an awful lot to answer for. Personally I gave them up when I grew out of Primary School.

For anybody who's paying attention, he never "grew out of Primary School."

Originally posted by rjhowie:

It's a lot of fancy jargon by large foreheads this one. Sci-Fi films and comics have an awful lot to answer for. Personally I gave them up when I grew out of Primary School.

For anybody who's paying attention, he never "grew out of Primary School."

Originally posted by johnnysaucepn:

Originally posted by sergey-pypyrev:

Look what happens when ice and fire are connected: they are so different that they can not co-exist.

When fire and ice are introduced to each other, you get a lot of water. There's nothing wrong with a universe made up of nothing but water.

Yes, both original worlds will be destroyed and new world will be created. And ...

To step away from the over-extended analogy for a moment: there is no reason that every combination of physical constant should produce a working, viable universe. Perhaps the majority of them are just foamy balls of sludgy proto-matter that never turned into anything interesting. We can only sit here and think about it because this universe had the right combination to produce something complex.

...as you write - most probably, this new world will not be able to produce anything interesting. Conclusion: it's bad idea to travel in time using alt universes.

Originally posted by wikipedian:

Originally posted by sergey-pypyrev:

Originally posted by wikipedian:

Originally posted by sergey-pypyrev:

Originally posted by sgunhouse:

Time travel is not contrary to basic Physics.

Traveling back in time IS contrary to basic physics: law of thermodynamics says that entropy increases in time.

Yes but according to Stephen Hawkings, the universe is like a balloon and there could be holes that loop back in time. But it may not be the same universe as us (alternate universe). Also, it could be possible to bend space and time for a shortcut into the future.

There is no experimental or even theoretical evidence for existence of alternate universe(s).

If you read A Brief History Of Time: From Big Bang To Black Holes, Steven Hawking mention about a theoritical alternate universe.

Yes, alt universes is Hawking's idea. The problem is that this is pure math and pure math may be not related to real life. For example, did you ever hear about white holes? They are possible mathematically, but practically they were not found. We can have the same situation with alt universes.

Originally posted by sergey-pypyrev:

Traveling back in time IS contrary to basic physics: law of thermodynamics says that entropy increases in time.

Yes - entropy increases with time. But we're talking about changing our position in time, not reversing the effects of it.

http://en.wikipedia.org/wiki/Closed_timelike_curve
http://en.wikipedia.org/wiki/Novikov_self-consistency_principle

And no, there is no experimental evidence. But the first step is determining the hypothesis.

It looks like adding time to coordinates system confused many people. It is true that time is coordinate in theory of relativity. But this was done only to simplify math. Theory of relativity does not change physical nature of time. Physical nature of time stay the same - it is how we intuitively feel time. So, theory of relativity does not state that you can change time coordinate the same way as you can change other coordinates.

So, you can not say "I will change my position in time" the same way as "I will change my location". Physical sense of theory of relativity does not allow you to do this.

Originally posted by sergey-pypyrev:

For example, did you ever hear about white holes? They are possible mathematically, but practically they were not found.

Blackholes or objects scientists think are blackholes are found everyday.

Originally posted by wikipedian:

Originally posted by sergey-pypyrev:

For example, did you ever hear about white holes? They are possible mathematically, but practically they were not found.

Blackholes or objects scientists think are blackholes are found everyday.

Of course, I know about existence of black holes and that their existence is explained mathematically and confirmed by practical observations. But I talk about white holes: http://en.wikipedia.org/wiki/Whiteholes. They are possible mathematically, but their existence is not confirmed by observations (although they should be much easier to detect than black holes).

So, my main point here is that not everything possible in math exists in real world.

Why, yes.


You can "easily" do something resembling a warp. This is called space-time angular deficit. When you pass through an area of angular deficit, transit time beomes zero. Pic related.

Here's picture of Kerr black hole which in theory could be created:

So you don't enter singularity instead you warp by using it as accelerator.

Originally posted by sergey-pypyrev:

Yes, alt universes is Hawking's idea. The problem is that this is pure math and pure math may be not related to real life. For example, did you ever hear about white holes? They are possible mathematically, but practically they were not found. We can have the same situation with alt universes.

We have the same situation with practically all of this universe. White holes, black holes, dark mater, gravity in general, anit-anything, gamma bursts, time dilation, spooky action a distance, dimensions, universal brains, electricity, quarks, photon teleportation and the big one.... life.

Mathematics and/or observations have been made on pretty much all them, still that don't prove we know much about any of it. It's hard to sit in one spot for a short time and understand the full scope of anything. Our observational perspective comes from what we think we can see from our tiny little planet and our understanding of that comes from how we twist our math around to try and explain it. It's like going to a baseball game having never seen a game played and watching one inning from the top deck then trying to explain all the possibilities of what will/can happen in the rest of the game from the calculations you make based on what you just observed.

Originally posted by beiren:

So you don't enter singularity instead you warp by using it as accelerator.

Yeah Spock did that with an old Klingon bird of prey... easy peazy

Originally posted by ensbb3:

spooky action a distance

Gravity?

Originally posted by beiren:

You can "easily" do something resembling a warp. This is called space-time angular deficit. When you pass through an area of angular deficit, transit time beomes zero. Pic related.

"space-time angular deficit" - funny great words that sound smart and cool for those who do not have enough knowledge! Unfortunately, these words are not known in science.

It is easy to warp, easy to create black hole, easy to enter black hole and stay alive, easy to connect to another universe, etc. etc. etc. But easiest thing here is to forget that most important components in proposed time machine may not even exist:

1. Alternate universes is pure theory without single evidence of existence.

2. Wormhole is pure theory without single evidence of existence.

3. Everybody saw demonstration of gravity that uses piece of material. This demonstration is easy to understand, so, people who never saw even a single formula of Einstein's theory begin to think that they understand how gravity and space work. So, they begin to think they can do with space anything they can do with piece of material. For example, they can warp piece of material into something that resembles worm hole and travel this way. The problem is that piece of material has nothing common with real complex physical processes that happen in universe. So, warping space like you can warp piece of material and expecting the same effects is a complete BS.

Originally posted by wikipedian:

Originally posted by ensbb3:

spooky action a distance

Gravity?

uhmmm. what?

Originally posted by sergey-pypyrev:

Originally posted by wikipedian:

Originally posted by sergey-pypyrev:

For example, did you ever hear about white holes? They are possible mathematically, but practically they were not found.

Blackholes or objects scientists think are blackholes are found everyday.

Of course, I know about existence of black holes and that their existence is explained mathematically and confirmed by practical observations. But I talk about white holes: http://en.wikipedia.org/wiki/Whiteholes. They are possible mathematically, but their existence is not confirmed by observations (although they should be much easier to detect than black holes).

So, my main point here is that not everything possible in math exists in real world.

Maybe we have... the Big Bang could be a white hole...

Originally posted by sergey-pypyrev:

Yes, both original worlds will be destroyed and new world will be created. And ...

...as you write - most probably, this new world will not be able to produce anything interesting. Conclusion: it's bad idea to travel in time using alt universes.

If you'll remember, your comment was on the particular subject of the physical properties of universes, not time-travel.

Originally posted by sergey-pypyrev:

1. Alternate universes is pure theory hypothesis without single evidence of existence.

2. Wormhole is pure theory hypothesis without single evidence of existence.

3. Everybody saw demonstration of gravity that uses piece of material. This demonstration is easy to understand, so, people who never saw even a single formula of Einstein's theory begin to think that they understand how gravity and space work. So, they begin to think they can do with space anything they can do with piece of material. For example, they can warp piece of material into something that resembles worm hole and travel this way. The problem is that piece of material has nothing common with real complex physical processes that happen in universe. So, warping space like you can warp piece of material and expecting the same effects is a complete BS.

Gravity is a theory to begin with. String theory which is qurrently the best attempt at theory of everything requires multiple dimensions to work, minimum of 11 if I recall correctly. In effect the same math which tells that something like gravity must exist requires multiple dimensions. String theory is indeed a theroy, as it is something that can be observed for evidence, as it incorporates quantum theory (microprocessors & satellites rely on that) & attempts to explain how/what particles are. Quantum theory then can be derived to hypotheses like alternate universes & wormholes. I don't pretend to understand math behind that, but at least I know how science works.

You are mixing hypothesis with theory. Stop doing that, it's number uno reason why bydlos can't understand how science works. Theory has evidence, hypothesis has none.

Originally posted by sergey-pypyrev:

So, my main point here is that not everything possible in math exists in real world.

There are a lot of things in real world that weren't thought to exist until someone did the maths.
Einstein's theory of special relativity was purely mathematical until someone devised the experiment necessary to observe it.
Until you have a mathematical model, a prediction of what may be, you can't begin to confirm the existence of anything experimentally.

The mathematics show that, based on what we do know to be fact, that time travel is not impossible. That wormholes are not impossible. Of course it doesn't tell us that we can do it, or that they occur naturally, or that humans may be able to create them artificially. It also doesn't rule out the discovery of new laws that do preclude it.

Originally posted by tsgtanvir:

can time travel become a reality?

Well, I will defend that matter doesn't exists and everything, including all of you, this forum and the entire universe is nothing but the product of my prodigious conscience's imagination.
Since traveling in time is something that I can perfectly imagine, sure is possible and so "real" as anything else.

Originally posted by beiren:

String theory which is qurrently the best attempt at theory of everything requires multiple dimensions to work, minimum of 11 if I recall correctly. In effect the same math which tells that something like gravity must exist requires multiple dimensions. String theory is indeed a theroy, as it is something that can be observed for evidence, as it incorporates quantum theory (microprocessors & satellites rely on that) & attempts to explain how/what particles are.

See the below on string theory, a book that I'm now reading.
http://www.math.columbia.edu/~woit/wordpress/?cat=2

Originally posted by beiren:

You are mixing hypothesis with theory. Stop doing that, it's number uno reason why bydlos can't understand how science works. Theory has evidence, hypothesis has none.

Agreed. I used word "theory" only because it is faster to type. Of course, those things are not more than hypothesis.

Originally posted by Belfrager:

Originally posted by tsgtanvir:

can time travel become a reality?

Well, I will defend that matter doesn't exists and everything, including all of you, this forum and the entire universe is nothing but the product of my prodigious conscience's imagination.
Since traveling in time is something that I can perfectly imagine, sure is possible and so "real" as anything else.

This guy need beer option .

Originally posted by johnnysaucepn:

Originally posted by sergey-pypyrev:

So, my main point here is that not everything possible in math exists in real world.

There are a lot of things in real world that weren't thought to exist until someone did the maths.

I understand you main point. But some things that you write are completely wrong.

Einstein's theory of special relativity was purely mathematical until someone devised the experiment necessary to observe it.

Completely wrong. Einstein theory was never pure math. In fact, no physics theory can be pure math. All physics theories are based on experimental data. Theory that does not explain existing experimental data has no use.

Until you have a mathematical model, a prediction of what may be, you can't begin to confirm the existence of anything experimentally.

Completely wrong. Science first gathers experimental data and then invents theory that explains this data. Do you know when length of year was measured? 5000 years ago by Sumerians. Sumerians were able to predict movement of Sun, planets, and stars without even close understanding what's going on. So, human kind started to gather experimental data for Newton theory thousands years before Newton was born!

The mathematics show that, based on what we do know to be fact, that time travel is not impossible. That wormholes are not impossible. Of course it doesn't tell us that we can do it, or that they occur naturally, or that humans may be able to create them artificially. It also doesn't rule out the discovery of new laws that do preclude it.

Yes, there is difference between "may" and "must. Now, in these terms let's look at some known scientific facts.

--- From Newton theory
Object with speed less than 8km/s MUST fall to Earth

This speed was consequence from math, and it was confirmed by experiments. It is important to note that math implies "MUST" here. So, results of mentioned experiments (aka sputniks) were known long time before experiments.

--- From Einstein theory
Black holes MAY exist
White holes MAY exist

Math implies "MAY" here. So, really those objects may exist or may not exist. Multiple black holes were found after their existence was predicted (BTW, Einstein himself thought that they do not exist). But no white holes were found although they should be much easier to detect than black holes. This still does not allow us to be sure that white holes do not exist, but at least we can say that their existence is questionable.

--- From <not sure what branch this is>
Alt universes MAY exist
Wormholes MAY exist

Math implies "MAY" here. Before saying "if it's possible in theory - we will find a way to do it" we should remember that every theory has it borders where it can be applied. For example, Einstein theory works well on star level, but does not work well on galaxy level. You probably heard about dark matter and dark energy - this is just an early attempt to explain why Einstein theory does not work on galaxy level. So, if we will launch rocket to center of galaxy and calculate it movement according to Einstein theory - most probably, mission of rocket will fail simply because Einstein theory will be applied outside of it current borders. So, those alt universes and wormholes are on extreme borders of current theories and it is very likely that those theories will not work in those extreme conditions.

--- From thermodynamics
entropy MUST increase in time

Math implies "MUST" here. So, this does not allow traveling back in time. But please don't say that "I will change my position in time". Time as coordinate is pure math, and nothing more. No modern physics theory changes sense of time. Intuitive understanding of time that all humans have is very scientific. And our intuition says that time goes in one direction only.

Originally posted by sergey-pypyrev:

This guy need beer option .

A Brain in a Vat only drinks wine or wiskie.

As it happens Jaybro, Primary Schools have a high record here so nah, nah to you. If I could go back in time I would re-do the War of Independence and then many of you wouldn't possibly be here. I am sure the world would thank me for it being a man with a generous streak by nature. Judging by those who get to the White House over there and thinking of intelligence and education you don't seem to be doing too well yourselves in those stakes are you??

sergey-pypyrev, you have the cart before the horse.

What you're describing is an ongoing cycle.
The Sumerians may have known the length of last, but that's all you can get from a single observation. What you then need to devise is a mathematical model based on the assumption that all years are the same length, and then devise an experiment to confirm this. Which would involve waiting a year, and seeing if your prediction is correct.

That's about as trivial an example as you can get. Einstein, and all other physicists you might care to mention, took what was already known, devised a mathematical model that fitted all that data, and that mathematical formulation demonstrated several properties that must also be true for the rule to hold. It was many years until the experiments were devised to prove the rule.

The work on wormholes, or other methods of jumping through time and space, are based on known science, confirmed by experiment. That data, and the laws known, predict results. We just have to work out how to devise an experiment to test them.

Originally posted by ensbb3:

Originally posted by sergey-pypyrev:

Yes, alt universes is Hawking's idea. The problem is that this is pure math and pure math may be not related to real life. For example, did you ever hear about white holes? They are possible mathematically, but practically they were not found. We can have the same situation with alt universes.

We have the same situation with practically all of this universe. White holes, black holes, dark mater, gravity in general, anit-anything, gamma bursts, time dilation, spooky action a distance, dimensions, universal brains, electricity, quarks, photon teleportation and the big one.... life.

Mathematics and/or observations have been made on pretty much all them, still that don't prove we know much about any of it. It's hard to sit in one spot for a short time and understand the full scope of anything. Our observational perspective comes from what we think we can see from our tiny little planet and our understanding of that comes from how we twist our math around to try and explain it. It's like going to a baseball game having never seen a game played and watching one inning from the top deck then trying to explain all the possibilities of what will/can happen in the rest of the game from the calculations you make based on what you just observed.

I think the correctly analogy is that scientists observe only an inning of a baseball game and then try to predict what happened and what could possible happen. Then they develop a set of rules to predict how all baseball games will carry out.

Originally posted by johnnysaucepn:

What you're describing is an ongoing cycle.
The Sumerians may have known the length of last, but that's all you can get from a single observation. What you then need to devise is a mathematical model based on the assumption that all years are the same length, and then devise an experiment to confirm this. Which would involve waiting a year, and seeing if your prediction is correct.

Sumerians did know length of year and not only length of last year: http://en.wikipedia.org/wiki/Sumerian_calendar. Of course, it's not possible to create calendar from single observation. Do you know why we have so strange time units (24*60*60)? Because number 60 in Sumerian math was close to number 100 in our math. So, we did improve Sumerian calendar, but we still use Sumerian time without improvements (although it is terrible for our math). So, science in some form started much earlier than you can imagine.

That's about as trivial an example as you can get. Einstein, and all other physicists you might care to mention, took what was already known, devised a mathematical model that fitted all that data, and that mathematical formulation demonstrated several properties that must also be true for the rule to hold. It was many years until the experiments were devised to prove the rule.

I don't understand you. I say the following: any physics theory is based on existing experimental data and can not be pure math. So, before theory is used to predict events, it should be checked against existing old data.

Do you agree with this or not? It looks like you do agree, but then I don't understand why we continue to argue and why I have cart before horse.

For example, let's look at Einstein theory. It explained movement of Mercury (which was not explained properly by Newton theory). This was old experimental data, and Mercury was probably one of first tests for Einstein theory. Next - Einstein theory predicted possibility of black holes, and they were found later. At the same time another prediction (white holes) is questionable. And one more prediction (movement of stars in galaxy) is complete failure.

So, second important thing that I said is that not all things possible theoretically are possible in practice. And this is not because they can be difficult to achieve, but because theory can be wrong somewhere. See example above - stars in galaxy don't want to move according to Einstein theory.

The work on wormholes, or other methods of jumping through time and space, are based on known science, confirmed by experiment. That data, and the laws known, predict results. We just have to work out how to devise an experiment to test them.

Things that you mention are NOT confirmed by experiments.

Just as i thought lots of would-be intellectualism but no beef.

Originally posted by sergey-pypyrev:

I don't understand you. I say the following: any physics theory is based on existing experimental data and can not be pure math. So, before theory is used to predict events, it should be checked against existing old data.Do you agree with this or not? It looks like you do agree, but then I don't understand why we continue to argue and why I have cart before horse.

Yes, of course I agree. But proving that we have a mathematical model for what we already know is only part of the, if you'll pardon the pun, equation.

Once you have a mathematical model, you'll notice that it has implications. Things to test further. The mathematical model predicts that a structure such as a wormhole would fit the model nicely, and answer some fundamental questions.

Where you and everyone else in the thread differ is that you consider everything that has not been observed yet to be unsupported nonsense, and only consider that which has already been observed. If we relied on that, we would never discover everything new. We need to equally consider that which we have not observed yet.

Originally posted by rjhowie:

Just as i thought lots of would-be intellectualism but no beef.

Spoken in the manner of a man who has never seen a cow.

Originally posted by johnnysaucepn:

Where you and everyone else in the thread differ is that you consider everything that has not been observed yet to be unsupported nonsense, and only consider that which has already been observed. If we relied on that, we would never discover everything new. We need to equally consider that which we have not observed yet.

I don't deny ability of math models to predict things that humans didn't observe yet. I only listen to common sense and distinguish things that are likely to happen in the future and those that most probably will never happen.

Traveling back in time contradicts both common sense and modern scientific view. So, most probably, it will never happen.

Originally posted by sergey-pypyrev:

Traveling back in time contradicts both common sense and modern scientific view. So, most probably, it will never happen.

No, it doesn't contradict either. Besides, common sense is a terrible basis - common sense tells us that a particle's position cannot be indeterminate, or that light can't be both a wave and a particle.
http://en.wikipedia.org/wiki/Time_travel#General_relativity

However, the theory of general relativity does suggest a scientific basis for the possibility of backwards time travel in certain unusual scenarios, although arguments from semiclassical gravity suggest that when quantum effects are incorporated into general relativity, these loopholes may be closed. These semiclassical arguments led Hawking to formulate the chronology protection conjecture, suggesting that the fundamental laws of nature prevent time travel, but physicists cannot come to a definite judgment on the issue without a theory of quantum gravity to join quantum mechanics and general relativity into a completely unified theory.

The common sense position is to acknowledge that we don't currently know either way.

Originally posted by johnnysaucepn:

Originally posted by sergey-pypyrev:

Traveling back in time contradicts both common sense and modern scientific view. So, most probably, it will never happen.

No, it doesn't contradict either. Besides, common sense is a terrible basis - common sense tells us that a particle's position cannot be indeterminate, or that light can't be both a wave and a particle.
http://en.wikipedia.org/wiki/Time_travel#General_relativity

If you're really interested in this subject - I can recommend you to read more advanced things than wikipedia. For example, if you'll read something serious about general relativity (with all formulas, etc.) - you will understand that time as coordinate is pure math, and nothing more. Idea was only to simplify math: when you consider x,y,z,t as one coordinate, it is much easier to calculate x,y,z based on given t. Unfortunately, this lead to misbelief that t can be changed the same way as x,y,z. I can calm you: general relativity is much easier than special relativity, so, it is not so hard core as you may expect.

But I doubt that you will read something serious. So, here is an easy to read pop article that explains why traveling back in time is not possible:

http://en.wikipedia.org/wiki/Entropy_(arrow_of_time)

This is opinion of modern science, not my own opinion.

However, the theory of general relativity does suggest a scientific basis for the possibility of backwards time travel in certain unusual scenarios, although arguments from semiclassical gravity suggest that when quantum effects are incorporated into general relativity, these loopholes may be closed. These semiclassical arguments led Hawking to formulate the chronology protection conjecture, suggesting that the fundamental laws of nature prevent time travel, but physicists cannot come to a definite judgment on the issue without a theory of quantum gravity to join quantum mechanics and general relativity into a completely unified theory.

BTW, your own quote says that Hawking is skeptical about time traveling. Unfortunately, next article says that "physicists cannot come to a definite judgment". Reality is different: only few dumb asses "cannot come to a definite judgment". Most of scientific world is skeptical about time traveling just like Hawking.

Originally posted by sergey-pypyrev:

If you're really interested in this subject - I can recommend you to read more advanced things than wikipedia. For example, if you'll read something serious about general relativity (with all formulas, etc.) - you will understand that time as coordinate is pure math, and nothing more. Idea was only to simplify math: when you consider x,y,z,t as one coordinate, it is much easier to calculate x,y,z based on given t. Unfortunately, this lead to misbelief that t can be changed the same way as x,y,z. I can calm you: general relativity is much easier than special relativity, so, it is not so hard core as you may expect.

I have. If the best argument you have is to be patronising, you're wasting both our time.

Originally posted by sergey-pypyrev:

BTW, your own quote says that Hawking is skeptical about time traveling. Unfortunately, next article says that "physicists cannot come to a definite judgment". Reality is different: only few dumb asses "cannot come to a definite judgment". Most of scientific world is skeptical about time traveling just like Hawking.

Yep, obviously M-theory is only for "dumbasses". You misread Hawking's position, he's not saying that he doesn't think that time travel can't be possible, he's proposing that some other law must be in place to protect causality. That's not incompatible with other self-consistency propositions.

The arrow of time explains the direction of time, how one time position may be distinguished from another. It say nothing about whether travel is possible from one point in time to another. As long as entropy is conserved, this doesn't apply.

Originally posted by johnnysaucepn:

The arrow of time explains the direction of time, how one time position may be distinguished from another. It say nothing about whether travel is possible from one point in time to another. As long as entropy is conserved, this doesn't apply.

I think that in this context "direction" and "travel" are almost synonyms.



Let's look at picture above. Let's say that on axis Y I can move only in one direction - forward. It means that I can travel only from y=1 to y=5, but not otherwise.

Now back to entropy. We know direction of time: it is always forward. It means that I can travel only from t=1 to t=5, but not otherwise.

I really don't know how this can be interpreted other way.