# Possbility of Time travel?

#### bayan

do you people believe in time travel?

I do not mean to be able to slow the time down, but to actually go backwards in time!

Please give reason too (if possible

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#### Sirus

Well, in theory you can travel into the future when you travel faster than light. But that is also impossible in theory, so it's a bit of a contradiction. For now, all this stuff is just theoretical.

#### Gonzolo

Sirus said:
Well, in theory you can travel into the future when you travel faster than light. But that is also impossible in theory, so it's a bit of a contradiction. For now, all this stuff is just theoretical.
Isn't regular time dilatation "travel into the future"? Slightly less than the speed of light brings you there. Consider the twin "paradox". The one who has left the planet and come back is "in the future" just about as much as you would want him to be.

It is travel into the past which is more delicate. Nothing in conventionnal theories really allows it as far as I know.

#### primal schemer

Sirus said:
Well, in theory you can travel into the future when you travel faster than light. But that is also impossible in theory, so it's a bit of a contradiction. For now, all this stuff is just theoretical.
You dont need to go faster than the speed of light to go into the future.

I think there is a slight difference in terminology here. According to relativity it is possible for person A to travel into the future of person B. For example, if you remain on earth, and I get in a really fast space ship and travel around at speeds close to the speed of light for a while, then it may take an hour or so of me flying around to travel a couple of years into your future.
(I dont have the formulae for working out how fast/how long handy, but the speed would need to be very close to the speed of light.)
Getting back in time is a whole different story though......

Perhaps somebody could clear something up, but according to relativity, the closer you travel to the speed of light the faster (in time) you travel according to people standing still. So, if my reasoning is correct, if you are capable of travelling at a miniscule speed below the speed of light, then in theory you can travel as far in the future (according to other people) as you want.
So, (and Im asking someone to clear this up rather than saying it is definitely true), would it be possible to travel, say, thousands of years into person Bs future in a fraction of a second if person A was going *really* close to the speed of light.

Anyway, the difference in terminology I mentioned at the start refers to the difference between MY future and YOUR future. It is possible to me to go into your future (and vice versa), but me going into my own future is an entirely different matter that brings up a whole load of other 'back to the future' style questions.

Although I have never heard of anything that will allow us to go back in time to, say, the stone age, I remember reading somewhere that it could be theoretically possible to set up some sort of portal at time X that will allow anybody to travel forwards and backwards between time X and X + Y. (where Y is a positive time). Although as far as I remember, this relied on *alot* of if's https://www.physicsforums.com/images/icons/icon9.gif [Broken]
Unhappy

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#### Gonzolo

primal schemer said:
So, (and Im asking someone to clear this up rather than saying it is definitely true), would it be possible to travel, say, thousands of years into person Bs future in a fraction of a second if person A was going *really* close to the speed of light.
The answer "tends towards a yes", with serious limitations. What prevents us to apply this is the unimaginable energy that would be needed to accelerate someone to a sufficient speed. A Saturn rocket goes only a small fraction of c and most of its power is only to compensate the Earth's G-field. Now you need enough power to accelerate to near c, 4 times : away from the earth + decceleration, and back. The faster you want to do this trip, and the further in the future you want to go, the more energy you need, and it is not a linear relation.

Then there is the fact that a human can only support a few g's comfortably. Time travel into the future in a fraction of a second would mean accelerating so fast... well I won't write what I imagine the person would look like. :yuck:

#### pervect

Staff Emeritus
bayan said:
do you people believe in time travel?

I do not mean to be able to slow the time down, but to actually go backwards in time!

Please give reason too (if possible
Time travel may be possible through wormholes. But do wormholes exist? It's hard to say at this point. Even if wormholes do exist, there are some mechanisms that may prevent them from becoming time machines (chronology protection postulates, due to Hawking).

There's a pretty good popular article on wormholes and time machines by John Cramer, a physcisist, reprinted from his "Alternate Views" column in analog.

http://www.npl.washington.edu/AV/altvw33.html

If you browse around this webstie, he's written a few other columns about the same topic, as more information became available.

#### omin

bayan said:
do you people believe in time travel?

I do not mean to be able to slow the time down, but to actually go backwards in time!

Please give reason too (if possible
I'm just begining physics study, so take that in consideration when you read my answer.

Basic common sense says time travel is impossible. I think Newtons laws imply the impossibility also.

Reason: Each object that exists in all of the universe is dictated by the laws of physics. Time travel implies breaking the laws of physics.

Time travel into the future implies: I can slow the physics laws in my frame of reference, while the laws of physics operate at a faster rate outside my reference. But, that's not how physics works.

The clock inside a space ship that leaves the earth at near the speed of light will tick according to it's physical make-up, not it's relative speed to any thing else.

If a clock makes one tick every second, and it's going 1.0 meter per second, that means over one meter, one tick will sound. So, when it's going this velocity away from the Earth, one tick will have sounded and it will be one meter away from the Earth.

If it's going 1 million meters per second, one tick will occur every million meters. 3 million meters per second, will result in one tick every three million meters.

The clock will not slow down based upon speed. The rest of the universe will not speed up because of the clocks relative speed. Time is based upon the movement of objects in the universe, therefore time can't speed up or slow down either.

But, if other forces act upon things at high speed, causing what appears to be time travel, then it's a specific force that acts upon the clock, not mere speed.

Many claim, very fast speed causes time travel. Bull! Speed merely causes distance travel.

#### Gerinski

Omin said:
Reason: Each object that exists in all of the universe is dictated by the laws of physics. Time travel implies breaking the laws of physics.
Several interpretations of the laws of physics (as described by current theories) allow for time travel into the past theoretically, without creating any conflict with the laws themselves.
Wormholes, Frank Tipler's rotating cylinder, or cosmic strings are just some examples.

The problem is that the solutions allowing for time travel into the past require very extreme conditions, so we could not check them out so far (and we may never be able to do so). Time travel into the past is just possible theoretically (as a mathematical solution to the equations).

Even if the theoretical solution yields a time travel into the past, it's not clear at all which past it would be, the same that was experienced by the traveling entity (a time loop), or a different one (perhaps a parallel universe in the multiverse)

#### mijoon

omin said:
.................... I think Newtons laws imply the impossibility also.................
This is the age of Einstein, not Newton. Even though Einstein himself was not happy about it, he had to admit that time travel is possible. Look up Godels solution, for example.

The effects predicted by relativity theory have been experimentally verified to many, many decimal places. And thats no bull.

#### Gonzolo

omin said:
I'm just begining physics study, so take that in consideration when you read my answer.
When you start learning about relativity, you will learn that Newton never asked himself whether the light coming from the headlamps of a moving car was travelling faster than that from a standing lamp-post. If you try to solve this problem, you come up with the conclusion that clocks can depend on nothing but speed. As any junior physicist and upwards, starting with Einstein, I can, with elementary mathematics, in less than a page of demonstration show that intervals of time between the ticks of a clock depends on its speed. Your reasoning is only o.k. if you're moving slowly, it doesn't work if you're going half the speed of light.

Elementary relativity says you can go into the future. (Particles do it all the time in accelerators.)

Advanced relativity wonders whether you can go into the past. (Only Doctor Who, Micheal J. Fox and the like have done this yet.)

#### Ba

And as I understand it that if you travel close to the speed of light then to the people in a different referance frame you will travel faster in time according to those people, but at the same time from your referance frame those people in the other referance frame will be travelling faster in time than you. So I think that if you go out and back close to the speed of light that your time phase to the people will be the same as their time phase to you. Maybe?

#### Gonzolo

Ba said:
And as I understand it that if you travel close to the speed of light then to the people in a different referance frame you will travel faster in time according to those people, but at the same time from your referance frame those people in the other referance frame will be travelling faster in time than you. So I think that if you go out and back close to the speed of light that your time phase to the people will be the same as their time phase to you. Maybe?
At some point, you have to turn around to meet each other again. If both reference frames turn around at exactly the same rate etc, you are right. If only one does and it comes back to join the other, the situation is asymetric and the "phase" as you call it is different, resulting in definitive time dilatation of the accelarated frame relative to the other.

#### omin

If an object must travel what we call forward through time, then relative to everything else, it must slow down while everything else speeds up.

The amount of energy it would expend, must then be drawn from it to the system outside itself.

Energy flows at a rate determined by the all things physical.

All energy is conserved, and all rates of flow of energy are consistent between objects that exist in the universe.

When the energy of the object that must slow down to travel through time flows backs to the system outside itself so it can move faster, there is a equal and opposite physical effect, which is the rate determined by physical laws, making it impossible for time to appear different to either reference frame.

I think the effects of time travel people are stating is just unawareness of some force acting upon the matter that appears to be traveling through time.

Fields of force of some sort seem like they could have an effect as things move at high speeds, where they don't have the same amount of an effect at low speeds. A frictive medium must create the rate of energy change, giving the illusion of time travel. With my limited knowledge, and I admit it is, time travel is only possible when things can't be accounted for. I say ignorance of some force makes time travel possible only.

#### Gonzolo

omin said:
If an object must travel what we call forward through time, then relative to everything else, it must slow down while everything else speeds up.
Yes, its time must "slow down", not its speed. That happens with decaying particles.

omin said:
The amount of energy it would expend, must then be drawn from it to the system outside itself.
Not necessarily.
omin said:
Energy flows at a rate determined by the all things physical.
If you mean that it flows according to the laws of physics, I agree. But the fact that c is invariant is practically a law of physics now.

omin said:
All energy is conserved, and all rates of flow of energy are consistent between objects that exist in the universe.
Rates of flow of energy... I agree that all must always be consistent.

omin said:
When the energy of the object that must slow down to travel through time flows backs to the system outside itself so it can move faster, there is a equal and opposite physical effect, which is the rate determined by physical laws, making it impossible for time to appear different to either reference frame.
Not quite sure what you mean. When a rocket slows down near a planet, the planet couldn't care less. The equal and opposite effect is between the rocket and its fuel.

omin said:
I think the effects of time travel people are stating is just unawareness of some force acting upon the matter that appears to be traveling through time.

What people? What force? There is no physical evidence of this "force" you mention.

omin said:
Fields of force of some sort seem like they could have an effect as things move at high speeds, where they don't have the same amount of an effect at low speeds. A frictive medium must create the rate of energy change, giving the illusion of time travel. With my limited knowledge, and I admit it is, time travel is only possible when things can't be accounted for. I say ignorance of some force makes time travel possible only.
Well, until you can demonstrate that such a "force" and "frictive medium" exist, time dilatation is true, and has, on the other hand, been verified experimentally with very fast particles and very precise atomic clocks. "some sort of force", "seems", "think", and unstated "physical laws" won't do much to convince serious physicists. If you are confortable with classical mechanics (end of highschool level), I invite you to grab any book on special relativity and look for a single mistake in the mathematical reasoning.

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#### bayan

Well, until you can demonstrate that such a "force" and "frictive medium" exist, time dilatation is true, and has, on the other hand, been verified experimentally with very fast particles and very precise atomic clocks. "some sort of force", "seems", "think", and unstated "physical laws" won't do much to convince serious physicists. If you are confortable with classical mechanics (end of highschool level), I invite you to grab any book on special relativity and look for a single mistake in the mathematical reasoning.
Those books are full of mistakes and i will someday prove it (Hopefully).

#### Gonzolo

Light momentum, increase of mass, a maximum speed that is constant, and time dilatation come from experimental observations themselves, not books.

What relativity books do is make sense of it all, binding these effects together in a coherent theory.

#### omin

Gonzolo,

A have a first clock tied to a string. At the other end of the string is the second clock. I swing the second clock around the first clock. I bring the second clock near the speed of light, while the first clock is at near rest compared to me.

The energy had to go through the first clock to get to the second clock.

With this in mind, did the second clock really move further than the first clock?

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#### Gonzolo

The energy did not have to go through the first clock at all. You could simply put this 1st clock on a table nearby, unstringed and swing the second clock. When you stop swinging and compare the clocks, the one that has swinged (the second) will have lagged.

#### omin

Gonzolo said:
The energy did not have to go through the first clock at all. You could simply put this 1st clock on a table nearby, unstringed and swing the second clock. When you stop swinging and compare the clocks, the one that has swinged (the second) will have lagged.
No, no. I'm implying a connection here that isn't as easy to see without a string. Let's add your third clock under the first clock.

But as far as the first two clocks, the energy must be via the first clock to get to the second clock, right? If not, how does the second clock move?

#### Gonzolo

Well I'm saying that string or not, connexion or not, there is a lag. The non-moving clock could be hanging from a blimp floating on Jupiter.

If you insist on having all the clocks on a string, let's stick with only two such that :

(pivot hand)-----(1stclock)-----(2ndclock)

then since the second clock has a greater speed, it lags relative to the first, it has more energy, more mass, and it is shorter.

Whether there is a first clock on the string or not is irrelevant to the 2nd. The second clock knows nothing of the first. No energy is transmitted via the first clock or anywhere. The first has its KE, and the second has its greater KE, and that's all.

The 2nd could have started moving by fixing a small rocket to it, but once it starts making revolutions, it doesn't stop (gravity might make it fall down), just like the Earth never stops spinning.

Now if you decide to pull on the string you can give it energy, but you don't have to do this anymore than a nail would. Any energy you would need is only to counter gravity and air friction.

#### omin

Gonzolo said:
If you insist on having all the clocks on a string, let's stick with only two such that :

(pivot hand)-----(1stclock)-----(2ndclock)

then since the second clock has a greater speed, it lags relative to the first, it has more energy, more mass, and it is shorter.
I can't go that far yet. I need to stop you there.

The second clock has greater speed?

The pivot hand exerts a force upon the first clock.
The first clock exerts a force upon the string.
The string exerts a force upon the second clock.

The force is exerted upon the first clock is constant, but the first clock doesn't appear to be moving as fast as the second clock that is recieving about half of the force and expressing it as kinetic energy?

I'll claim the force in the first clock as more of a static (internal force) than kinematically expressed force, making the collective speed of each clock equal, thus energy equal?

Or is there energy loss outward from the second clock or inward flow to the pivot hand making a density of energy settle toward one of the clocks?

#### NoTime

Homework Helper
bayan said:
do you people believe in time travel?

I do not mean to be able to slow the time down, but to actually go backwards in time!

Please give reason too (if possible
Sure.
Get on a fast enough plane and travel from say New York to California.
You can arrive before you left.
Even easier.
In some locations there is a local time warp. So you can simply walk across the street and end up a whole hour in the past.
Even easier still.
Live in the right location.
You don't have to travel at all. You jest end up in the past or future occasionally.

Did you mean travel to your past?
In that case - No.

#### Gonzolo

omin said:
I can't go that far yet. I need to stop you there.

The second clock has greater speed? ?
Ok, first of all, I want to make it clear that whatever we learn in school (pre-university) is called classical physics (and electricity). For most people, it is all that they will ever need to know about physics, and even for most engineers (and I dare say most physicists), they will never have to use relativity (any extra years of study an engineer does is always for systems at low speeds : no relativity needed). What relativity does is account for the upper limit of speed, but it contains all of what school says, so it is crucial that you understand classical mechanics before going to far in relativity. The effects we are talking about here are only for fun (unless you ever become a theoretical physicist). But I think you're not through with classical physics. (It's ok though, we've all been there.)

Ok, back to the problem, now, but let's get the classical thing going first, so : masses instead if clocks, we don't need the needles.

Suppose you swing the masses at 1 revolution per second. Clearly, the 2nd one does a greater diameter : d2, than the 1st : d1. It therefore has a greater speed, d2/t > d1/t. This is totally classical, any engineer knows this. Same frequency of revolution, but different speed.

omin said:
The pivot hand exerts a force upon the first clock.
The first clock exerts a force upon the string.
The string exerts a force upon the second clock.
Application of a force doesn't mean using energy : remember W = fd. If d is 0, W is zero, and f and d must be in the same direction. Our swinging masses have d and f in perpendicular directions => no energy is used (f along string, d on circle diameter).

omin said:
The force is exerted upon the first clock is constant, but the first clock doesn't appear to be moving as fast as the second clock that is recieving about half of the force and expressing it as kinetic energy?
The force is split in two. There is no "appear", the speeds are completely (classically) different as I said above.

omin said:
I'll claim the force in the first clock as more of a static (internal force) than kinematically expressed force, making the collective speed of each clock equal, thus energy equal?
No, they are exactly the same types of force, both centripetal and static. There is no "collective speed", see what I said above. Only a unique angular speed. But all I have said about relativity applies to linear speed (length per second), not angular (angle per second). I'm always talking about linear speed (that is what c is.).

omin said:
Or is there energy loss outward from the second clock or inward flow to the pivot hand making a density of energy settle toward one of the clocks?
No energy loss at all. No flow of energy. Only balanced forces. Let's get this classical thing right before going back to relativity.

#### ArmoSkater87

NoTime said:
Sure.
Get on a fast enough plane and travel from say New York to California.
You can arrive before you left.
Even easier.
In some locations there is a local time warp. So you can simply walk across the street and end up a whole hour in the past.
Even easier still.
Live in the right location.
You don't have to travel at all. You jest end up in the past or future occasionally.

Did you mean travel to your past?
In that case - No.
By saying "fast enough" im guessing you mean faster than the speed of light, which we all know is impossible. So from your example it would be impossible to time travel into the past, but it would be possible to time travel into the future because of time dilation.

As for time travel to the past, there are ways to theoretically do it. One way is to use a cosmic string (i think thats what its called). A cosmic string is i believe left over from the big bang and has a very high density, and causes the space around it to warp dramatically. You can think of the warp as a pizza made into a cone by cutting one slice out. Since light goes straight, light would have to travel down and up the inside of the cone, taking a longer time, while you can take a short cut around the cone. This makes it seem like you traveled faster than lightspeed because you got to the other side before the light would. Since you went faster than light did, you traveled into the past.

Another way is by using a black hole. If black holes spin, then that means because of its enormous gravitational field, it bends the space around it so much that it spins it around while the black holes is spinning it self (sorry for the bad terminology, i dont know how to really explain it). Now predend like you went next to the black hole with a space ship and started traveling around the black hole with the space ship at very high speed, close to the speed of light. So someone outside the spinning space and huge warp, it would seem like the space ship is traveling faster than light because the space itself it moving with the space ship also, giving the impression that the ship is going faster than light to someone outside the space where the gravity has that kind of an effect.
There are time machines that are being built right now that are similar to the black hole situation, one of them will have a cilinder where there would be very high energy laser beams circling around, which bend the space a great deal and in a sense, also bend time in to a "loop" if u think of time as a straight line.

Anyways, to put it short, i dont know all that much about time travel, i just watch a lot of TV programs, but i do know that the laws of physics dont forbid the possibility of time travel to the past, there are ways of geting around other laws that do forbit it, like traveling faster than the speed of light.

#### Gonzolo

ArmoSkater87 said:
By saying "fast enough" im guessing you mean faster than the speed of light, which we all know is impossible. So from your example it would be impossible to time travel into the past, but it would be possible to time travel into the future because of time dilation.
No, it is called a joke, related to time zones.