[Guest]

Someone told me this paradox at University once to explain to me why he believed time travel could not be possible Although despite this I think he was wrong, I just think if you did try this you'd create such a big hole in space/time continuum you would be in major trouble...

A Grandma gives her son who is interested in time travel a coin on his 18th birthday and tells him she was given it by someone special and to keep hold of it until he invents a time machine

A few years later her son manages to invent a time machine and takes the coin with him back in time to his grandmother. He tells his grandmother about inventing the time machine and that she must keep hold of the coin till his 18th birthday and give it back to him and tell him about the time machine.

The coin has no markings on it to signify any date when the coin was made or anything.

Where did the coin come from?

And you can't create something out of nothing

[Guest]

time machine is a matter of movies or science fiction.

Thus, yours is not a paradox.

A paradox is a contradiction with known facts. (I suppose)

[Guest]

Time travel could be true. And even if no-one knew how to make one the Grandma could instead give her son the instructions of how to make the time machine and the son would make it when he was 18 and go back and tell his grandmother how to make it so she could tell his younger form. You also made a mistake, you said "a grandmother told her son" and then you said "he told his grandmother". Your making it suggest two different people.

[Guest]

Someone told me this paradox at University once to explain to me why he believed time travel could not be possible Although despite this I think he was wrong, I just think if you did try this you'd create such a big hole in space/time continuum you would be in major trouble...

A Grandma gives her son who is interested in time travel a coin on his 18th birthday and tells him she was given it by someone special and to keep hold of it until he invents a time machine

A few years later her son manages to invent a time machine and takes the coin with him back in time to his grandmother. He tells his grandmother about inventing the time machine and that she must keep hold of the coin till his 18th birthday and give it back to him and tell him about the time machine.

The coin has no markings on it to signify any date when the coin was made or anything.

Where did the coin come from?

And you can't create something out of nothing

Yup. Sounds paradoxical, but if you do the maths then it turns out not to be.... the coin appears in the universe the moment the son appears in the past, causing a local breakdown in conservation of energy/mass/whatever. But, 20-odd years in the future, that same coin then dissapears from the universe, balancing all the books. As for where it came from... it essentially pops out of no-where.

This sort of thing happens at the quantum level all the time (although not neccessarily with the time travel aspect).

Alternatively, if you follow the many-worlds interpretation of reality, one possibility is that it came from a different universe.

There are many apparent paradoxes in relation to time travel... many of which turn out to be plausible theoretically. But until we build a time machine, we can't really say if or how they will really work.

Incidently... one thing many sci-fi authors carefully ignore is that as soon as you can travel faster than light, then you've got a time machine.

[Shakeepuddn]

Time travel to the past is theoretically possible. However, it is impossible to arrive at a time prior to departure (time slows relative to all else), or to have a machine that can travel to a time pror to its invention.

[Guest]

Time travel to the past is theoretically possible. However, it is impossible to arrive at a time prior to departure (time slows relative to all else), or to have a machine that can travel to a time pror to its invention.

Yes.... but the universe may be set up to allow time travel, so you can travel to any time since the start of the universe (as in Godel's rotating universe which is physically plausible, but not the universe we are living in.) Or could make use of naturally occuring worm-holes, which already link to the past.

So... most (all?) plausible time machines can not travel to a point before they are created... but they can use things that were created some time ago, thus putting back the effective creation date.

[Guest]

Yes.... but the universe may be set up to allow time travel, so you can travel to any time since the start of the universe (as in Godel's rotating universe which is physically plausible, but not the universe we are living in.) Or could make use of naturally occuring worm-holes, which already link to the past.

So... most (all?) plausible time machines can not travel to a point before they are created... but they can use things that were created some time ago, thus putting back the effective creation date.

Or... time travel could just be impossible and we are wasting our time discussing the topic.

even though my thoughts on time travel is that it is impossible i may be persuaded otherwise if someone gives a good enough arguement FOR it.

sorry to be a buzzkill but hey what you gonna do.

[Guest]

Or... time travel could just be impossible and we are wasting our time discussing the topic.

even though my thoughts on time travel is that it is impossible i may be persuaded otherwise if someone gives a good enough arguement FOR it.

sorry to be a buzzkill but hey what you gonna do.

According to what I've read and seen (by scientists), time travel is possible, as soon as you can biuld a machine that can travel at the speed of light (or faster), like armcie said.

Let's ask the physicist to explain how this is possible theoretically.

[Guest]

is that when you time travel, you don't go to your own past, but to a different equal one, so the coin came from an "original" time and now its traveling through diferent times in a sort of loop, thus the coin came from one time, and it still exists in all the other times, but not in the same place as that time traveling one.

Edited January 6, 2009 by Yaru

[Guest]

Or... time travel could just be impossible and we are wasting our time discussing the topic.

even though my thoughts on time travel is that it is impossible i may be persuaded otherwise if someone gives a good enough arguement FOR it.

sorry to be a buzzkill but hey what you gonna do.

Meh. OK, I'll bite.

First of all... the interesting time machine is one that sends people into the past. We can easily send Alex into the future by using relativity - accelerate someone fast enough and time slows down for him. So, he can move 1000 years into the future, while for him only a few seconds have passed. The problem lies in getting him Back to the Present.

One of the ways to do this to use a wormhole*... a tunnel through space, which we can turn into a tunnel through time. Sit one end of the wormhole at home. Give Alex the other end of the wormhole and accelerate him into the future. Now, the two ends of the wormhole are seperated in time as well as space (because one end is in the future) and once Alex reaches the future he could pop back down the wormhole to the present.

The problem with this lies in wormholes... we don't know if they exist. And if they do they are likely to be really really small, and we'd need some negative energy (probably in the form of 'exotic matter') to keep it open wide enough to send something down it. We also don't know if such exotic matter exists**.

* wormhole: imagine the universe is two dimensional... like a piece of paper. Fold the paper in half and punch a hole through both sheets. Conect the holes with a piece of straw. Now, you can instantly travel from one side of the paper to the other by popping down the wormhole rather than taking the long route round the hole page. Now add a dimension.

** although the Casimir effect is a genuine, demonstrateable example of negative energy.

The other plausible time machines are... a little harder to picture. But they tend to involve big, long, spinny things. Basically a very heavy object spinning very fast will distort space and time enough so that you could go out in a little space ship, plot a suitable course twisting and spiralling round the spinny thing, such that when you came out you've moved through time.

Mathematicians have proven* that suitable long heavy spinny things include:

- the entire universe

- an infinitely long, extremely dense cylinder

- a sufficiently distorted atomic nucleus

We don't live in a rotating universe. And things of infinite length are hard to find, and hard to spin. And a nucleus isn't big enough to allow anything other than... something like an electron or smaller to rotate about it. But those are the objects for which the maths is relatively easy. It may be that other set-ups would also work.

* assuming our current understanding of physics is about right

Paradoxes? What paradoxes? There's something called the Novikov self consistency conjecture which hypothesises that paradoxes* are impossible. Its been demonstrated on fairly simple models (involving billiard balls and worm-holes in pool table pockets) that for every trajectory where there is a chance that a ball may roll into one pocket, pop out of another back in time and hit itself to stop it going into the wormhole, then there are infinitely more self consistent solutions where the ball pops out of the wormhole at an (apparently unexpected) angle at which it will hit itself so that it enters the hole at that (previously unexpected) angle.

* such as the 'kill your own grandfather' one

In summary... time travel may or may not work. But even if it doesn't, its certainly worth finding out why it doesn't. So not a waste of time. Certainly no more a waste of time than discussing what we would do if stood in a line with an unhinged executioner who's just put some coloured hats on our head...

[Guest]

The coin originally came from the son's mother.

[Guest]

Seeing the past is one thing, actually participating with it is another. You can not go back in time as you have no ability to retract light. One it has happened its moment has been sent into antiquity as the light energy travels away from the source, once gone it will not return hence the moment has passed. You could view it if you were able to outrace light and look back with a telecope and see it but you most definitly could not touch it. But to answer the question the son took it out of his pocket when he built the time machine and tested it.

Edited January 6, 2009 by matslund19

[Guest]

For starters, I will assume that time travel is somehow possible for the sake of this, through whatever combination of misshapen atoms, super-lightspeed travel, or exotic matter/wormholes is required. It is just more fun that way!

I will also assume that grandma gave it to her grandson, so that the relations all work out.

[spoiler='

Therefore...']

suppose the guy travels back and gives the coin to the grandma on his 1st birthday. At that point, granny has the coin, and there are briefly 2 versions of the guy, time traveling visitor and poopy diaper wearer.

Between that time and his 18th bday, granny keeps the coin.

On his 18th, he gets the coin, and keeps it until he invents the time machine.

At that point, he goes back in time and divests himself of the coin - or does he? Does he have it after returning to his own time? I will say no, the coin is on a kind of 'temporal mobius strip'.

However, this violates the 'pops into existence' clause. Hmmm.

But the no-markings is easy to explain. It came from Chuck E. Cheeses arcade!

Someone told me this paradox at University once to explain to me why he believed time travel could not be possible Although despite this I think he was wrong, I just think if you did try this you'd create such a big hole in space/time continuum you would be in major trouble...

A Grandma gives her son who is interested in time travel a coin on his 18th birthday and tells him she was given it by someone special and to keep hold of it until he invents a time machine

A few years later her son manages to invent a time machine and takes the coin with him back in time to his grandmother. He tells his grandmother about inventing the time machine and that she must keep hold of the coin till his 18th birthday and give it back to him and tell him about the time machine.

The coin has no markings on it to signify any date when the coin was made or anything.

Where did the coin come from?

And you can't create something out of nothing

[Guest]

The coin came from the future

and kept returning

[Guest]

Someone told me this paradox at University once to explain to me why he believed time travel could not be possible Although despite this I think he was wrong, I just think if you did try this you'd create such a big hole in space/time continuum you would be in major trouble...

A Grandma gives her son who is interested in time travel a coin on his 18th birthday and tells him she was given it by someone special and to keep hold of it until he invents a time machine

A few years later her son manages to invent a time machine and takes the coin with him back in time to his grandmother. He tells his grandmother about inventing the time machine and that she must keep hold of the coin till his 18th birthday and give it back to him and tell him about the time machine.

The coin has no markings on it to signify any date when the coin was made or anything.

Where did the coin come from?

And you can't create something out of nothing

It came from the boy in the first place, when he invented the time machine...i think.

[Jiminy Cricket]

Someone told me this paradox at University once to explain to me why he believed time travel could not be possible Although despite this I think he was wrong, I just think if you did try this you'd create such a big hole in space/time continuum you would be in major trouble...

A Grandma gives her son who is interested in time travel a coin on his 18th birthday and tells him she was given it by someone special and to keep hold of it until he invents a time machine

A few years later her son manages to invent a time machine and takes the coin with him back in time to his grandmother. He tells his grandmother about inventing the time machine and that she must keep hold of the coin till his 18th birthday and give it back to him and tell him about the time machine.

The coin has no markings on it to signify any date when the coin was made or anything.

Where did the coin come from?

And you can't create something out of nothing

The Grandma was given the coin by her Grandson while she was a little girl.

Many years passed and she gave the coin to her Grandson and told him to keep it.

More years pass and the Grandson invents a time machine.

(For the purpose of this answer I will assume a working time machine capable of going back prior to its own creation)

The Grandson goes back in time and gives the coin to his Grandmother and immediately returns to his present.

The coin, also known as an Artifact, came from an Ancient race that lives in the folds of time and are knows as the Illuminati - heard of them?

The Grandson encountered this race while traveling through time and spent many centuries learning from them some of the more mundane "secrets" of the Universe, such as "42" is not really the answer to life, the Universe and everything.

http://www.google.com/search?hl=en&q=a...+and+everything

He also learned the origins of Stonehenge, the statues on Easter Island, the drawings on the Nazca planes and all pyramids throughout the universe.

Sorry, but we do not have exclusive rights to pyramids on Earth.

He also learned how the "Great 'Wall' of China" was actually supposed to be a tower that the Chinese felt compelled to build so they could out do the Tower of Babel, but fell due to cheap foreign labor not being as skilled as the Hebrews, but hey, what are you going to do?

The Grandson showed such promise that the Illuminati offered to send him back to Earth's infancy and make him a lesser God.

The Grandson declined stating that he really needed to go back to Earth a few years prior to his invention of the time machine and make some key sports bets in order to finance his invention of said time machine. After all, if he did not travel back in time to finance his invention, he could never invent it!

But that's another issue in and of itself.

Besides, he loves knowing in advance that the Red Sox are going to win it all!

The Illuminiati are disappointed but understanding.

As a token of their mutual good will towards each other and the universe they gave him a year's supply of Rice-A-Roni and a lovely parting gift that came to be knows as "The Coin"

[bonanova]

Time travel extraordinaire ...

Excerpt from All You Zombies by Robert Heinlein

"A baby girl is mysteriously dropped off at an orphanage in Cleveland in 1945. "Jane" grows up lonely and dejected, not knowing who her parents are, until one day in 1963 she is strangely attracted to a drifter. She falls in love with him. But just when things are finally looking up for Jane, a series of disasters strikes. First, she becomes pregnant by the drifter, who then disappears. Second, during the complicated delivery, doctors find that Jane has both sets of sex organs, and to save her life, they are forced to surgically convert "her" to a "him." Finally a mysterious stranger kidnaps her baby from the delivery room.

"Reeling from these disasters, rejected by society, scorned by fate, "he" becomes a drunkard and drifter. Not only has Jane lost her parents and her lover, but he has lost his only child as well. Years later, in 1970, he stumbles into a lonely bar, called, Pop's Place, and spills out his pathetic story to an elderly bartender. The sympathetic bartender offers the drifter the chance to avenge the stranger who left her pregnant and abandoned, on the condition that he join the "time travelers corps." Both of them enter a time machine, and the bartender drops off the drifter in 1963. The drifter is strangely attracted to a young orphan woman, who subsequently becomes pregnant.

"The bartender then goes forward 9 months, kidnaps the baby girl from the hospital, and drops off the baby in an orphanage back in 1945. Then the bartender drops off the thoroughly confused drifter in 1985, to enlist in the time travels corps. The drifter eventually gets his life together, becomes a respected and elderly member of the time travelers corps, and then disguises himself as a bartender and has his most difficult mission: a date with destiny, meeting a certain drifter at Pop's Place in 1970."

Question: Who is Jane's mother, father, grandfather, grandmother, son daughter, granddaughter, and grandson? The girl, the drifter and the bartender are all the same person. If we draw Jane's family tree, we find the branches curl inward on themselves, in a circle. We come to the astonishing conclusion that she is her own mother and father. She is an entire family tree!

What is the probability her other sibling is a girl? Wait. Never mind.

[Guest]

he just took it from his grandmother's pocketbook when he got to the past, told her to give it to his 18 year old self, then left back for the future. He leaves the coin he had already gotten when he was 18 in the future?

[Jiminy Cricket]

Time travel extraordinaire ...

Excerpt from All You Zombies by Robert Heinlein

"A baby girl is mysteriously dropped off at an orphanage in Cleveland in 1945. "Jane" grows up lonely and dejected, not knowing who her parents are, until one day in 1963 she is strangely attracted to a drifter. She falls in love with him. But just when things are finally looking up for Jane, a series of disasters strikes. First, she becomes pregnant by the drifter, who then disappears. Second, during the complicated delivery, doctors find that Jane has both sets of sex organs, and to save her life, they are forced to surgically convert "her" to a "him." Finally a mysterious stranger kidnaps her baby from the delivery room.

"Reeling from these disasters, rejected by society, scorned by fate, "he" becomes a drunkard and drifter. Not only has Jane lost her parents and her lover, but he has lost his only child as well. Years later, in 1970, he stumbles into a lonely bar, called, Pop's Place, and spills out his pathetic story to an elderly bartender. The sympathetic bartender offers the drifter the chance to avenge the stranger who left her pregnant and abandoned, on the condition that he join the "time travelers corps." Both of them enter a time machine, and the bartender drops off the drifter in 1963. The drifter is strangely attracted to a young orphan woman, who subsequently becomes pregnant.

"The bartender then goes forward 9 months, kidnaps the baby girl from the hospital, and drops off the baby in an orphanage back in 1945. Then the bartender drops off the thoroughly confused drifter in 1985, to enlist in the time travels corps. The drifter eventually gets his life together, becomes a respected and elderly member of the time travelers corps, and then disguises himself as a bartender and has his most difficult mission: a date with destiny, meeting a certain drifter at Pop's Place in 1970."

Question: Who is Jane's mother, father, grandfather, grandmother, son daughter, granddaughter, and grandson? The girl, the drifter and the bartender are all the same person. If we draw Jane's family tree, we find the branches curl inward on themselves, in a circle. We come to the astonishing conclusion that she is her own mother and father. She is an entire family tree!

What is the probability her other sibling is a girl? Wait. Never mind.

EXCELLENT!

I was just having fun with mine, but that excerpt...Holy Smokes! I'ma gonna have to read that one!

Nice pull, BN! :-)

[Guest]

Time travel extraordinaire ...

Excerpt from All You Zombies by Robert Heinlein

"A baby girl is mysteriously dropped off at an orphanage in Cleveland in 1945. "Jane" grows up lonely and dejected, not knowing who her parents are, until one day in 1963 she is strangely attracted to a drifter. She falls in love with him. But just when things are finally looking up for Jane, a series of disasters strikes. First, she becomes pregnant by the drifter, who then disappears. Second, during the complicated delivery, doctors find that Jane has both sets of sex organs, and to save her life, they are forced to surgically convert "her" to a "him." Finally a mysterious stranger kidnaps her baby from the delivery room.

"Reeling from these disasters, rejected by society, scorned by fate, "he" becomes a drunkard and drifter. Not only has Jane lost her parents and her lover, but he has lost his only child as well. Years later, in 1970, he stumbles into a lonely bar, called, Pop's Place, and spills out his pathetic story to an elderly bartender. The sympathetic bartender offers the drifter the chance to avenge the stranger who left her pregnant and abandoned, on the condition that he join the "time travelers corps." Both of them enter a time machine, and the bartender drops off the drifter in 1963. The drifter is strangely attracted to a young orphan woman, who subsequently becomes pregnant.

"The bartender then goes forward 9 months, kidnaps the baby girl from the hospital, and drops off the baby in an orphanage back in 1945. Then the bartender drops off the thoroughly confused drifter in 1985, to enlist in the time travels corps. The drifter eventually gets his life together, becomes a respected and elderly member of the time travelers corps, and then disguises himself as a bartender and has his most difficult mission: a date with destiny, meeting a certain drifter at Pop's Place in 1970."

Question: Who is Jane's mother, father, grandfather, grandmother, son daughter, granddaughter, and grandson? The girl, the drifter and the bartender are all the same person. If we draw Jane's family tree, we find the branches curl inward on themselves, in a circle. We come to the astonishing conclusion that she is her own mother and father. She is an entire family tree!

What is the probability her other sibling is a girl? Wait. Never mind.

Awestome

[Guest]

First of all... the interesting time machine is one that sends people into the past. We can easily send Alex into the future by using relativity - accelerate someone fast enough and time slows down for him. So, he can move 1000 years into the future, while for him only a few seconds have passed. The problem lies in getting him Back to the Present.

Strictly speaking, we can easily send Alex into the future by just waiting........................

LOOK! He's in the future now. And so are we

Relativity requires that there be no objective present, and thus everyone is in 'the future' according to the proper frame of reference. Similarly, 4-dimensional space-time would indicate that this supposed paradox really isn't a problem. There's nothing incoherent about a coin, or a person, which has a 'life span' composed of durations that are not all concurrent in 'objective time'.

When did the coin 'come into existence'? Well, it's first moment of existence--measured by year, month, date, and time--would have been the moment the grandson appeared 'back' in time. It's last moment of existence would presumably (without more to the story) be the last moment before the grandson travels to the past. It has no 'creation' in that it is never stamped out of a machine, but that's not a paradox of logic. There's nothing logically necessary about a coin being stamped from a machine. That's just how we usually see them made

[Guest]

Oops. Edit time limit ran out.

To add to the above:

4-D space-time means that there is no 'first time' when the grandmother gave the coin to the grandson, or when the grandson gave it to the grandmother. There was no 'first time' when he went back in time, and the loop then 'began'. These are all events that simply happen/have happened/are happening. They can be labeled as 'earlier than', 'later than', or 'simultaneous too', but that's it.

Events in space-time simply exist, all of them at their x-y-z-t coordinates. They don't happen and then 'stop happening', like movie frames.

[Guest]

Simple way to disprove time travel is we don't know how to time travel.

If we invent time travel in the future, over a potentially infinite amount of time somebody would have inevitably come back and shared the technology. At the very least, we'd be seeing time travelers.

Edited January 14, 2009 by Llam4

[Guest]

Simple way to disprove time travel is we don't know how to time travel.

If we invent time travel in the future, over a potentially infinite amount of time somebody would have inevitably come back and shared the technology. At the very least, we'd be seeing time travelers.

Well, that's a way to *inductively* argue against the possibility of time travel. It's certainly not proof.

1) Who says time is infinite? Even if time is infinite, who wants to argue that the life span of the human race is infinite?

2) If time (or the existence of the human race) is not infinite, then what you have left is a "surely we'd have seen it by now" argument, which is almost never a good argument. The most effective response to that argument is to point out that, before the first time anything we HAVE discovered was discovered, there was probably someone saying "If one of those existed, surely we'd have seen one by now"

3) How do you know you haven't seen whole bunches of time travelers? Are they supposed to look remarkably different from us? Are they supposed to tell us that they are time travelers? What if they are Quantum Leap time travelers--most of their bodies stay behind, but their 'mind' travels through time. I might be one right now, or your next door neighbor. The next time someone you know acts strangely, ask yourself...could that be Sam Beckett?

[Guest]

Time travel to the past is theoretically possible. However, it is impossible to arrive at a time prior to departure (time slows relative to all else), or to have a machine that can travel to a time pror to its invention.

This reminds me of the classic "Chicken or the Egg" paradox because one cannot have a beginning without the other. (of course, scientifically one can explore the evolution of the 'chicken' from its 'chicken-like' ancestors and find the paradox irrelevant since it did not simply come into being one day. well... who knows? haha)

I agree with what Shakeepuddin said in an earlier post about the problem with going into the past, but I suppose anything is possible. There's always the unexpected.

I think that if it were possible to travel in time before the time of departure, the coin would have to exist only between the time when the boy gives his grandmother the coin and when he invents the time machine. As far as where the coin came from (which I know is the question) I don't know because it seems the coin only exists in a window of time.

I like this paradox! Thanks for posting

[Guest]

This reminds me of the classic "Chicken or the Egg" paradox because one cannot have a beginning without the other. (of course, scientifically one can explore the evolution of the 'chicken' from its 'chicken-like' ancestors and find the paradox irrelevant since it did not simply come into being one day. well... who knows? haha)

Aristotle--actuality precedes potentiality. The chicken is an actual chicken. The egg is a potential chicken. Thus, the chicken comes first.