does time travel?

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Originally Posted by hypatia_

I don't think there's very much scientific evidence supporting the claim that the universe will continue expanding forever.

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I believe you are mistaken. As I said, the picture of the universe expanding forever is the picture that the great majority of evidence suggests. This picture is the current scientific consensus. You may not like this picture of the fate of the universe, but that has exactly zipp to do with whether it's actually true or not. The evidence is the important part.

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And the term "dark energy" irritates me. It is mostly a vague term used to fill in holes in relativity. Don't get me wrong, it definitely is labeling something that exists, but I'm not sure we are anywhere close to understanding it or it's relationship with pulling apart the universe.

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Of course it's a vague term. We have no idea what dark energy is. All we know is that something is making all the space in the universe expand. So when a I, or a physicist, is talking about dark energy, it's just a shorthand way of saying "The whatever that seems to be pulling the universe apart." The fact that something is pulling the universe apart (and not at a steady rate but actually an accelerating rate) has been very conclusively shown to be a reality. Even if there wasn't dark energy, in a flat universe we would expect the universe to expand outward but slowly lose it's speed and eventually become stagnant and die of heat death all the same. So even without dark energy, the fate of our universe wouldn't be much different. The fact that something is pulling the universe apart makes it even more of a sure thing that the universe is going out with a whimper.

But none of this should depress you. The human race will almost certainly be long gone before any of this would worry us. We have plenty of time to enjoy our life. We have much greater things to worry us, like the human race killing itself off due to nuclear warfare, biological catastrophe, or environmental hazards.

Also, as I think Yesno alluded to, we have no idea if our observable universe is all there is anyway. This could just be one bubble in an infinitely large foam of universes. If this is the case, our universe growing cold and dying is no more important than a puddle full of little bacteria drying up. There is still a whole world of puddles and bacteria out there, so the one drying up and dying isn't such a big deal--unless you're those bacteria of course. Multiple universes is a fun thing to think about, but ultimately it's all speculation. Actual beliefs should be based on evidence, regardless how bleak the evidence suggests the outcome is.

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Originally Posted by Frostball

I believe you are mistaken. As I said, the picture of the universe expanding forever is the picture that the great majority of evidence suggests. This picture is the current scientific consensus. You may not like this picture of the fate of the universe, but that has exactly zipp to do with whether it's actually true or not. The evidence is the important part.



Of course it's a vague term. We have no idea what dark energy is. All we know is that something is making all the space in the universe expand. So when a I, or a physicist, is talking about dark energy, it's just a shorthand way of saying "The whatever that seems to be pulling the universe apart." The fact that something is pulling the universe apart (and not at a steady rate but actually an accelerating rate) has been very conclusively shown to be a reality. Even if there wasn't dark energy, in a flat universe we would expect the universe to expand outward but slowly lose it's speed and eventually become stagnant and die of heat death all the same. So even without dark energy, the fate of our universe wouldn't be much different. The fact that something is pulling the universe apart makes it even more of a sure thing that the universe is going out with a whimper.

But none of this should depress you. The human race will almost certainly be long gone before any of this would worry us. We have plenty of time to enjoy our life. We have much greater things to worry us, like the human race killing itself off due to nuclear warfare, biological catastrophe, or environmental hazards.

Also, as I think Yesno alluded to, we have no idea if our observable universe is all there is anyway. This could just be one bubble in an infinitely large foam of universes. If this is the case, our universe growing cold and dying is no more important than a puddle full of little bacteria drying up. There is still a whole world of puddles and bacteria out there, so the one drying up and dying isn't such a big deal--unless you're those bacteria of course. Multiple universes is a fun thing to think about, but ultimately it's all speculation. Actual beliefs should be based on evidence, regardless how bleak the evidence suggests the outcome is.

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It does not depress me. The universe has been doing this cosmic dance for 13.6 billion years, and will continue to do so for a very long time.

Please contribute evidence that the universe will expand forever. P.S. "The universe's expansion is accelerating" is not enough. That idea is based on measured speeds at which other celestial objects are moving away from us, but unfortunately this is as egocentric attitude, similar to when we used to think the Earth were the center of the solar system.

Objects moving away from Earth at a faster rate does not have any relationship with the rate of expansion/contraction of the Universe.

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Originally Posted by hypatia_

It does not depress me. The universe has been doing this cosmic dance for 13.6 billion years, and will continue to do so for a very long time.

Please contribute evidence that the universe will expand forever. P.S. "The universe's expansion is accelerating" is not enough. That idea is based on measured speeds at which other celestial objects are moving away from us, but unfortunately this is as egocentric attitude, similar to when we used to think the Earth were the center of the solar system.

Objects moving away from Earth at a faster rate does not have any relationship with the rate of expansion/contraction of the Universe.

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You're going to have to research it a bit on your own if you really want to understand it. Wikipedia's article on dark energy is a good start, as it summarizes several methods by which evidence of dark energy has been obtained.

I'm not a physicist, I don't think I would really do it justice. But I do know that dark energy, and the idea that the universe's expansion is not slowing but speeding up, is the consensus of physicists today. The great majority of people who have spent their lives learning about and studying this stuff agree on this point. So I trust them. It's not a blind trust, it's a trust built from some knowledge of how the scientific method works, how the scientific community works, and the fact that any time I have actually done the legwork of finding out how something works in detail, it inevitably lines up with what I've heard from scientists already. So by all means, if you think this is incorrect do your own research and prove those physicists wrong.

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Originally Posted by hypatia_

Please contribute evidence that the universe will expand forever. P.S. "The universe's expansion is accelerating" is not enough. That idea is based on measured speeds at which other celestial objects are moving away from us, but unfortunately this is as egocentric attitude, similar to when we used to think the Earth were the center of the solar system.

Objects moving away from Earth at a faster rate does not have any relationship with the rate of expansion/contraction of the Universe.

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On the other hand, after reading your post again, I think I might be able, even with my layman's knowledge of cosmology, to assist you on some misunderstandings I think you have.

To the extent that the fact of the universe's accelerating expansion is evidenced by measurements of objects from the perspective of earth, it is done with a great amount of prior knowledge about our place in the universe. You seem to think we're just seeing everything move away from us from our perspective, and so we're just assuming from that that everything is expanding faster just because that's how it looks from earth. I may not have explained it well, but I think I know what you're saying. To that, I say that astronomers and physicists are a lot smarter than you give them credit for, because that's just entirely too obvious a thing not to think of. What I mean, is that to the extent that we're using plain old telescopes and observations of objects as evidence for dark energy, it isn't just earth we're watching but the whole universe and the relations of all objects with each other.

There is one misconception that I think is really common about this. People have this idea of the big bang as an explosion that blew all the bits of matter into space. This is incorrect. There was no space for matter to blow up into. During the big bang, space itself and all the matter were all compressed to a tiny speck, and during the time before the planck time (an extremely short amount of time) the universe expanded extremely fast, space, matter, and all. To this day, space is STILL expanding. So when we're talking about the universe expanding, we're not talking about objects moving away from each other according to classical physics. What is happening is that the actual space itself is expanding apart. The actual space between galaxies is growing larger. This kind of concept is mind boggling, but the idea of space as a thing itself is also rather incontrovertible nowadays, as it goes back to Einstein's theories of relativity. If we didn't account for the theory of general relativity, GPS system's wouldn't work.

In no way is any of this based on an egocentric perspective of earth as somehow special. I also want to state again that I am not a physicist but a geeky layman who is interested in science and space.

It is a good habit to be skeptical of what people claim to be facts, especially scientific facts. Here is the evidence that I would use to support a belief that the universe is flat: http://map.gsfc.nasa.gov/universe/uni_shape.html

That evidence is just an overview of other evidence. In order to accept that summary report, I have to trust it in some way. I don't really trust that NASA actually sent humans to the moon round 1970, but I don't have any reason to doubt the above evidence.

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Originally Posted by Frostball

On the other hand, after reading your post again, I think I might be able, even with my layman's knowledge of cosmology, to assist you on some misunderstandings I think you have.

To the extent that the fact of the universe's accelerating expansion is evidenced by measurements of objects from the perspective of earth, it is done with a great amount of prior knowledge about our place in the universe. You seem to think we're just seeing everything move away from us from our perspective, and so we're just assuming from that that everything is expanding faster just because that's how it looks from earth. I may not have explained it well, but I think I know what you're saying. To that, I say that astronomers and physicists are a lot smarter than you give them credit for, because that's just entirely too obvious a thing not to think of. What I mean, is that to the extent that we're using plain old telescopes and observations of objects as evidence for dark energy, it isn't just earth we're watching but the whole universe and the relations of all objects with each other.

There is one misconception that I think is really common about this. People have this idea of the big bang as an explosion that blew all the bits of matter into space. This is incorrect. There was no space for matter to blow up into. During the big bang, space itself and all the matter were all compressed to a tiny speck, and during the time before the planck time (an extremely short amount of time) the universe expanded extremely fast, space, matter, and all. To this day, space is STILL expanding. So when we're talking about the universe expanding, we're not talking about objects moving away from each other according to classical physics. What is happening is that the actual space itself is expanding apart. The actual space between galaxies is growing larger. This kind of concept is mind boggling, but the idea of space as a thing itself is also rather incontrovertible nowadays, as it goes back to Einstein's theories of relativity. If we didn't account for the theory of general relativity, GPS system's wouldn't work.

In no way is any of this based on an egocentric perspective of earth as somehow special. I also want to state again that I am not a physicist but a geeky layman who is interested in science and space.

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I get the idea the space between galaxies is growing larger, and that is an indicator of expansion. I just don't understand how you can assume based on that fact that it will continue forever. I think we lack the perspective to make such a conclusion.

I find the concept of a lack of "space" fascinating. The idea that the universe is expanding reminds me of a concentration gradient.

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Originally Posted by hypatia_

I get the idea the space between galaxies is growing larger, and that is an indicator of expansion. I just don't understand how you can assume based on that fact that it will continue forever. I think we lack the perspective to make such a conclusion.

I find the concept of a lack of "space" fascinating. The idea that the universe is expanding reminds me of a concentration gradient.

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If it is expanding into nothing then there is no friction or resistance acting upon it, so why WOULDN'T it keep expanding forever?

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Originally Posted by hypatia_

I get the idea the space between galaxies is growing larger, and that is an indicator of expansion. I just don't understand how you can assume based on that fact that it will continue forever. I think we lack the perspective to make such a conclusion.

I find the concept of a lack of "space" fascinating. The idea that the universe is expanding reminds me of a concentration gradient.

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Another thing to remember is the fact that when we look out into space, we're looking into the past because of how long it takes the light to reach us. So we don't just have our single timeframe to look at, in a way, we actually have a great many time periods of space that can be observed. The farther away the galaxy, or object is, the longer the light took to reach us, the further into the past the observations are. So if we see this kind of thing in effect on galaxies that are 7 billion light years away, then that's evidence for the fact that it was happening 7 billion years ago as well.

The most important thing to remember is the fact of a convergence of evidence. Similar to evolution, how no single piece of evidence can prove it to be true, it's the fact that fossil evidence, DNA evidence, geological evidence (and surely more I can't think of) all converge to the same exact conclusion. They all make sense if you account for evolution, and none of it makes sense without it.

Dark Energy is similar, though perhaps not as incontrovertible as evolution surely is. Our measurement of the total matter in the universe from cosmic background radiation suggest there is a missing 70%. Also, the large scale picture of the universe, with galaxies and clusters, also suggests that the density of matter in the universe is about 30% of the critical density, which also suggests a missing 70%. It's things like this that end up confirming a hypothesis and turning it into a real theory. It's the fact that all the current models of the universe make a lot more sense when accounting for dark energy, and they stop making sense without it.

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Originally Posted by cacian

well for me the word 'travel' means forwards. winds travel they go forward but not the opposite in one forward direction. clouds travel .they move from one spot to the next then aspire. they could not if they travelled backward.



what do you think you know? :)


time travel means in constant move. big bang means something stopped. there could not be an explosion if something was moving.
a bit like watching a bullet. it travels fast but it only explode because it had hit a target. it has stopped moving.




if and when something stops means something has caused it to. friction is usually one of them. an explosion is usually because that something has stopped moving and it has complied from lack of movement to the point of an explosion.

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I'd like the way you explain things here. But my thing is if time travels means constant move or continually moving then surely it will create expansion at all phases. Not sure though?! :)

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Originally Posted by Volya

If it is expanding into nothing then there is no friction or resistance acting upon it, so why WOULDN'T it keep expanding forever?

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How do you know there is no resistance acting upon it? In fact, what evidence do you have that we are expanding into "nothing?"

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Originally Posted by Frostball

Another thing to remember is the fact that when we look out into space, we're looking into the past because of how long it takes the light to reach us. So we don't just have our single timeframe to look at, in a way, we actually have a great many time periods of space that can be observed. The farther away the galaxy, or object is, the longer the light took to reach us, the further into the past the observations are. So if we see this kind of thing in effect on galaxies that are 7 billion light years away, then that's evidence for the fact that it was happening 7 billion years ago as well.

The most important thing to remember is the fact of a convergence of evidence. Similar to evolution, how no single piece of evidence can prove it to be true, it's the fact that fossil evidence, DNA evidence, geological evidence (and surely more I can't think of) all converge to the same exact conclusion. They all make sense if you account for evolution, and none of it makes sense without it.

Dark Energy is similar, though perhaps not as incontrovertible as evolution surely is. Our measurement of the total matter in the universe from cosmic background radiation suggest there is a missing 70%. Also, the large scale picture of the universe, with galaxies and clusters, also suggests that the density of matter in the universe is about 30% of the critical density, which also suggests a missing 70%. It's things like this that end up confirming a hypothesis and turning it into a real theory. It's the fact that all the current models of the universe make a lot more sense when accounting for dark energy, and they stop making sense without it.

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Yes yes, the first two paragraphs I learned in middle school, and the third is something I can find on Wikipedia in 5 seconds. But none of it is indicative of an indefinitely expanding universe.

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Originally Posted by hypatia_

How do you know there is no resistance acting upon it?

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Because from what I am aware (theoretically) it is expanding into nothing. I'm not really an expert on physics though, this is just what I've deduced from GCSE level Physics at school.

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Originally Posted by Frostball

Multiple universes is a fun thing to think about, but ultimately it's all speculation. Actual beliefs should be based on evidence, regardless how bleak the evidence suggests the outcome is.

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There is actual evidence supporting the multi-verse in the form of observations of the CMBR. Some (like myself) would argue that the best current interpretation of quantum mechanics tells us there's a multiverse as well, though that's a whole other issue.

Whether one can travel in time depends on what time is. Time is the duration of events and between events. It is analogous to physical dimensions. Between the Battle of Hastings and me writing this there is a span of 948 years and three thousand and some odd miles. The question isn't whether it is possible to travel through time but whether one can do so a rate that is other than the traditional rate and whether one can control such travel. Then there is the question of whether one can get into the branches of the multiverse that are near this one. And if you were to travel into the past of the future, what would you see? What is there?

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Originally Posted by free

I've read that time doesn't exist and that it is an illusion of our senses.

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You were misinformed. Time is the separation between events. If there were no time,then everything would happen at the same time, and that would be inconvenient.

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Originally Posted by PeterL

Whether one can travel in time depends on what time is. Time is the duration of events and between events. It is analogous to physical dimensions. Between the Battle of Hastings and me writing this there is a span of 948 years and three thousand and some odd miles. The question isn't whether it is possible to travel through time but whether one can do so a rate that is other than the traditional rate and whether one can control such travel. Then there is the question of whether one can get into the branches of the multiverse that are near this one. And if you were to travel into the past of the future, what would you see? What is there?

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I agree that time is the duration of events within a frame of reference.

One should be able to reach a future event in some frame of reference A faster than those remaining in frame of reference A by accelerating oneself closer to the speed of light in a different frame of reference B moving away from frame of reference A and then turning around and coming back to that frame of reference A and syncing up one's speed with it. One would reach a future point in A using less time as measured by both frames of reference. I think that is just special relativity.

Going backward in time requires that one avoids the "grandfather paradox" which basically says you shouldn't be able to kill your own grandfather if you try such a stunt. I understand that many worlds thinks it can accommodate this behavior by just splitting out a new parallel universe, but that begs the question whether there are such parallel universes at all. I find them highly unlikely, with probability 0, that they exist.

I think one could get hints of what happened in the past if there existed some dimensions that one could access that would allow one to read a history of the past more easily than physical or cultural clues surviving in our present. That is not usually considered time travel as such, but I suspect it is the only alternative for past events. To some extent one can predict future events that appear to be deterministic. Intuitives or psychics can sometimes predict future events that are not as deterministic. Although I wouldn't mind knowing what the stock market will be a year from now, I suspect I will just have to wait and see.

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Originally Posted by YesNo

I agree that time is the duration of events within a frame of reference.

One should be able to reach a future event in some frame of reference A faster than those remaining in frame of reference A by accelerating oneself closer to the speed of light in a different frame of reference B moving away from frame of reference A and then turning around and coming back to that frame of reference A and syncing up one's speed with it. One would reach a future point in A using less time as measured by both frames of reference. I think that is just special relativity.

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I have more problems with travel to the future than with travel into the past. There is no future; there are just possibilities. But Mr Everett has pointed out that all of the inifinty of possibilities has to come into existence. That makes the problem o travel to the future going home.

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Going backward in time requires that one avoids the "grandfather paradox" which basically says you shouldn't be able to kill your own grandfather if you try such a stunt. I understand that many worlds thinks it can accommodate this behavior by just splitting out a new parallel universe, but that begs the question whether there are such parallel universes at all. I find them highly unlikely, with probability 0, that they exist.

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I was about to abandon hope for parallel universes a few months ago when I wrote some blog posts on time travel and the Many World Interpretation, but I discovered that there appears to have been laboratory evidence for the Many Worlds Interpretation being correct. If MWI is true, then the grandfather paradox is just something or ignorant writers of fiction.

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I think one could get hints of what happened in the past if there existed some dimensions that one could access that would allow one to read a history of the past more easily than physical or cultural clues surviving in our present. That is not usually considered time travel as such, but I suspect it is the only alternative for past events. To some extent one can predict future events that appear to be deterministic. Intuitives or psychics can sometimes predict future events that are not as deterministic. Although I wouldn't mind knowing what the stock market will be a year from now, I suspect I will just have to wait and see.

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According to the theory it is impossible to access other branches of the multiverse, but I think that there is a minor adjustment that could be made to the theroy that would allow for movement from branch to branch, and it would also reduce the number of worlds from infinite to just many. But the multiverse as a whole is deterministic; although there is free will within each universe, because each decision creates a new branch.

Going a year into the future to buy a copy of the stock tables for a day would create a new branch, and investing on that table would be a crap shoot, but you could travel into the past with the daily price moves from then to now and invest on that basis, and it would create new branches, if your invesments were digfferent from the original, but the tables probably would remain accurate. Now I just have to get my time machine running.

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Originally Posted by PeterL

...But Mr Everett has pointed out that all of the inifinty of possibilities has to come into existence. I was about to abandon hope for parallel universes a few months ago when I wrote some blog posts on time travel and the Many World Interpretation, but I discovered that there appears to have been laboratory evidence for the Many Worlds Interpretation being correct. If MWI is true, then the grandfather paradox is just something or ignorant writers of fiction.

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I don't think you've comprehended what you've started. There have been about 3 or 4 epic threads by now with people (mostly me) trying to explain MWI to YesNo. You are in for a lot of this: :mad2: if you continue on this course.

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Originally Posted by PeterL

But the multiverse as a whole is deterministic; although there is free will within each universe, because each decision creates a new branch.

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Not really sure what you mean here. MWI says that each option of the "choice" happens in different worlds, so I'm not sure what you mean you'd have "free will within each universe, because each decision creates a new branch." Besides, that's not entirely accurate of what happens. It's more accurate that each choice diverges an intersects with an existing branch. It's not really clear if any genuinely "new" worlds are created as opposed to all worlds co-existing, diverging, and intersecting at each decoherence point.