[morgsboi]

IM Egdall, on 2 February 2012 - 09:12 PM, said:

I like Hawking's point that there is empirical evidence time machines do not exist. He asks "where are all the tourists from the future visiting the present?"

Well, they could be on an alternate timeline.

[Airbrush]

IM Egdall, on 2 February 2012 - 09:12 PM, said:

I like Hawking's point that there is empirical evidence time machines do not exist. He asks "where are all the tourists from the future visiting the present?"

They would use stealth, out of grave necessity. They would have "all the time in the world" to go back and remove evidence of their time travel. If ETs can travel great distances, they may also be able to time travel.

[MigL]

Did not know that about Mitchell, DrR. Then again, when I was in my teens I read all of Andre Norton's work ( some about time travel including my first science ficrion story 'Galactic Derelict' ), without realising that he was actually a she. An older woman with a thing for cats ( she had many ), not your typical science fiction writer.

[IM Egdall]

Airbrush, on 3 February 2012 - 03:55 PM, said:

They would use stealth, out of grave necessity. They would have "all the time in the world" to go back and remove evidence of their time travel. If ETs can travel great distances, they may also be able to time travel.

Ya, good point. I was thinking about this too. Maybe if future scientists have the technology to make "time machines", they also may have the ability to cover up their visits to our time in some way. But, then again, it may not be so easy to do this. So perhaps Hawking's idea might be a good one. Fun to think about, but no way to know for sure (currently).

morgsboi, on 3 February 2012 - 03:35 PM, said:

Well, they could be on an alternate timeline.

If such a thing as alternate timelines really exist.

[morgsboi]

IM Egdall, on 3 February 2012 - 10:23 PM, said:

If such a thing as alternate timelines really exist.

I see them as "ifs". As in "if" this happened, then the universe would be in some way different.

[zapatos]

IM Egdall, on 3 February 2012 - 10:23 PM, said:

Ya, good point. I was thinking about this too. Maybe if future scientists have the technology to make "time machines", they also may have the ability to cover up their visits to our time in some way. But, then again, it may not be so easy to do this. So perhaps Hawking's idea might be a good one. Fun to think about, but no way to know for sure (currently).

Maybe there is nothing to cover up because they haven't been here. If you could pick any time in history to visit, would it be 2012?

[*puffy* japanisthebest]

Chap, on 22 January 2012 - 01:58 AM, said:

There is this video clip on Youtube saying that its possible to make a "time machine", which it claims allows us to travel into the 'past' (not only back in time). I have given the link below:

http://www.youtube.c...h?v=oRWwI61so5Q

I see several flaws with this video, so please check whether my arguments are correct:

1)The video has been posted before it was confirmed that particles can indeed travel faster than the speed of light, so obviously they didn't have that information at the time. Viewing the video in that context: The argument of the coffee bean doesn't make much sense to me. I mean; when the coffee bean (or whatever) approaches the speed of light, its weight would approach infinity. Therefore, even all the energy in the stirring motion will not be sufficient to make the coffee bean move faster (since any practical engine will fail somewhere before infinity, no matter how close to infinity it can get).

2)I also can't quite understand how Blackholes allows us to go back into the past (not 'time'). Its reasonable to assume if a particle can go faster than light, then "time" will reverse itself for that particle, but how can that particle go back into the "past"? For example, let's say I step into a machine which can travel faster than light and I start travelling. Depending on how much faster than light I travel, "I" will get younger, while everything around me will get older, right? When I step out (say 2 seconds later), I would have grown younger than when I initially stepped into the machine, while everything else around me would have aged naturally. So I would have gone to the future (kind of), not into the past?

Please convey your ideas on this matter.
P.S physics is not my area of expertise, therefore forgive me for missing obvious details.

i think that you can't travel to the past...but you can to the future... because if you go fast enough... time will slow down for you... so on earth it could be 100 years...but 20 years for you! <--- just a theory

[Tres Juicy]

*puffy* japanisthebest, on 6 February 2012 - 03:17 PM, said:

i think that you can't travel to the past...but you can to the future... because if you go fast enough... time will slow down for you... so on earth it could be 100 years...but 20 years for you! <--- just a theory

http://en.wikipedia....y_of_relativity

[IM Egdall]

*puffy* japanisthebest, on 6 February 2012 - 03:17 PM, said:

i think that you can't travel to the past...but you can to the future... because if you go fast enough... time will slow down for you... so on earth it could be 100 years...but 20 years for you! <--- just a theory

You are correct. But this is actually an every day effect. Say you leave your home and drive to work or school in the morning and return home that evening. Per special relativity, because of your motion in your car, time has run a tiny bit slower for you than time in your home. So you return home a tiny bit into the future. We don't notice this effect because it is so miniscule, but it is real.


Oh, and you can and do travel into the past as well as the future simply by changing altitude. Per general relativity, time runs slower as it gets closer to the Earth (gravitational time dilation). So say you are at home and you go upstairs to bed. All night long, time upstairs runs a tiny bit faster for you than time downstairs. So when you come down to breakfast in the morning, you are travelling into the past. Again, this is a miniscule amount so you don't notice it. But it is real.


We are all time travellers.

This post has been edited by IM Egdall: 7 February 2012 - 12:25 AM

[*puffy* japanisthebest]

IM Egdall, on 7 February 2012 - 12:15 AM, said:

You are correct. But this is actually an every day effect. Say you leave your home and drive to work or school in the morning and return home that evening. Per special relativity, because of your motion in your car, time has run a tiny bit slower for you than time in your home. So you return home a tiny bit into the future. We don't notice this effect because it is so miniscule, but it is real.


Oh, and you can and do travel into the past as well as the future simply by changing altitude. Per general relativity, time runs slower as it gets closer to the Earth (gravitational time dilation). So say you are at home and you go upstairs to bed. All night long, time upstairs runs a tiny bit faster for you than time downstairs. So when you come down to breakfast in the morning, you are travelling into the past. Again, this is a miniscule amount so you don't notice it. But it is real.


We are all time travellers.

WOW i never thought travelling to the past was possible... but the first paragraph i knew...but thanks

[Moontanman]

You guys have obviously never heard of Chronoton particles.... oh well back to watching Star Trek Voyager....

[*puffy* japanisthebest]

???

[Moontanman]

I think all the real UFO's, not the fake and misidentified ones, are the result of one time traveler, who has been bouncing back and forth through time for possibly thousands of years from our perspective but only a few years at most for him....

Poor guy, he only wants to go home.... TT go home....

*puffy* japanisthebest, on 7 February 2012 - 03:38 PM, said:

???

I'm in a silly mood, it's just technobabble from Star Trek...

[*puffy* japanisthebest]

ooohhhhhhhhhhhhhhhhhhhh

[Airbrush]

Moontanman, on 7 February 2012 - 03:43 PM, said:

I think all the real UFO's, not the fake and misidentified ones, are the result of one time traveler, who has been bouncing back and forth through time for possibly thousands of years from our perspective but only a few years at most for him....

Poor guy, he only wants to go home.... TT go home....

If any of the UFOs are actually ETI controlled, then probably they would have some ability to time travel, because how else can they travel so far from their planet, which is probably over 100 or 1,000 light years away? The distances are too vast for conventional space travel. Or they travel so close to light speed, that time dialation for them is significant, and maybe they travel with their bodies in a state of suspended animation.

This post has been edited by Airbrush: 7 February 2012 - 04:08 PM

[Moontanman]

Airbrush, on 7 February 2012 - 04:07 PM, said:

If any of the UFOs are actually ETI controlled, then probably they would have some ability to time travel, because how else can they travel so far from their planet, which is probably over 100 or 1,000 light years away? The distances are too vast for conventional space travel. Or they travel so close to light speed, that time dialation for them is significant, and maybe they travel with their bodies in a state of suspended animation.

Hmmmm.... not ETI, TTI... From our own future... how else could they look so human...iod....

On the other hand, wouldn't a time traveler have to cover many light years of space as well as travel in time? If I were to go back in time to say 70 million years ago and I stayed in the same place i am now, the Earth would be many thousands of light years away ... wouldn't it?

[DrRocket]

IM Egdall, on 7 February 2012 - 12:15 AM, said:

You are correct. But this is actually an every day effect. Say you leave your home and drive to work or school in the morning and return home that evening. Per special relativity, because of your motion in your car, time has run a tiny bit slower for you than time in your home. So you return home a tiny bit into the future. We don't notice this effect because it is so miniscule, but it is real.


Oh, and you can and do travel into the past as well as the future simply by changing altitude. Per general relativity, time runs slower as it gets closer to the Earth (gravitational time dilation). So say you are at home and you go upstairs to bed. All night long, time upstairs runs a tiny bit faster for you than time downstairs. So when you come down to breakfast in the morning, you are travelling into the past. Again, this is a miniscule amount so you don't notice it. But it is real.


We are all time travellers.

You have misconstrued relativity. Time runs at 1 second per second. Always. Everywhere. To talk about the "rate of tiime" is meaningless. Since rate of something = change in something/change in time. When applied to time this yields rate of time = change in time/change in time = 1.

What is true is that clocks measure proper time and proper time is dependent on the world line of the clock. That world line is reflects acceleration, hence gravitational effects for anything not in free fall. But to compare clocks meaningfully one must compare them at points of intersection of their respective world llines.

The effects of which you speak are coordinate effects and only approximate a true comparison of the proper times measured by clocks.

This not "traveling into the past" or "traveling into the future" in any meaningful sense. It does illustrate that time is a local concept and that there is no such thing as a true global notion of time.

[IM Egdall]

DrRocket, on 7 February 2012 - 04:51 PM, said:

You have misconstrued relativity. Time runs at 1 second per second. Always. Everywhere. To talk about the "rate of tiime" is meaningless. Since rate of something = change in something/change in time. When applied to time this yields rate of time = change in time/change in time = 1.

What is true is that clocks measure proper time and proper time is dependent on the world line of the clock. That world line is reflects acceleration, hence gravitational effects for anything not in free fall. But to compare clocks meaningfully one must compare them at points of intersection of their respective world llines.

The effects of which you speak are coordinate effects and only approximate a true comparison of the proper times measured by clocks.

This not "traveling into the past" or "traveling into the future" in any meaningful sense. It does illustrate that time is a local concept and that there is no such thing as a true global notion of time.

But if I were to travel in a rocket into space and back ar relativistic speed, the time interval for my trip is less than the time interval for the trip as observed on Earth. In other words, my clock on the spaceship and my calendar measure say a trip of 10 years. But the clocks and calendars on Earth measure a time interval for the round-trip of say 30 years. Say I leave in the year 2012. So when I return, my calendar would say it is the year 2022. But Earth calendars say it is the year 2042. I have only aged 10 years, but 30 years have gone by on Earth. In that sense, I have traveled into the future.

In principle, at a fast enough speed, I could travel in the rocket for say one year, and thousands of years would have passed by when I return to Earth. Ain't this a kind of time travel?

Do not the rocket clock and Earth clock intersect at two world lines, at the beginning of the trip and at the end of the trip? Comparing them we see they do not show the same elapsed time.

Please enlighten me as to how this is incorrect.

This post has been edited by IM Egdall: 7 February 2012 - 10:53 PM

[morgsboi]

Just wondering, what would happen if you went in an orbit around a planet of anti-matter (if it existed)? Would there be a type of gravity that pushes you away? And would time speed up relative to Earth?

[DrRocket]

IM Egdall, on 7 February 2012 - 10:52 PM, said:

In principle, at a fast enough speed, I could travel in the rocket for say one year, and thousands of years would have passed by when I return to Earth. Ain't this a kind of time travel?

I would not call it time travel, but what you say is correct in terms of what would be observed. What I would call it is a demonstration of the fact that time is only a local quantity and two clocks with two different world lines measure their own proper time intervals, and those intervals are different.

IM Egdall said:

Do not the rocket clock and Earth clock intersect at two world lines, at the beginning of the trip and at the end of the trip? Comparing them we see they do not show the same elapsed time.

Please enlighten me as to how this is incorrect.

You have the right basic idea, but there are some wrinkles worth considering.

The rocket clock and the Earth clock do not "intersect at two world line" but rather the two world lines of the two clocks intersect at two spacetime points -- the beginning and end points of the trip. Those two world lines are different and the clocks measure the proper time of each world line and they are indeed different.

The thing to keep in mind is that the proper time associated with a world line is just the arc length of that line (divided by c unless you use units in which c is equal to 1). So what time really is in general relativity is the length of the world line of the clock that is being used to measure time. To compare clocks one needs to compare their readings between two points of intersection of their world lines. [Note that arc length is measured using the Lorentz/Minkowski metric, and it a decidedly non-Euclidean version of arc length.]

When you see things like "gravitational time dilation" what is going on is a local approximation to this situation. Only by using local approximations can you simultaneously compare clocks at different spatial locations. This can be quite confusing until you get used to it. But it is the fundamental issue with making sense of comparison of clocks that are removed from one another (you can do this in special relativity) in general relativity that makes use of the term "time travel" rather dicey. There is no "time" through which to travel, except as a local variable.

This post has been edited by DrRocket: 7 February 2012 - 11:43 PM