# The Naked Scientists Forum

### Author Topic: ? Time travel  (Read 7666 times)

#### Jon Francis

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##### ? Time travel
« on: 07/08/2010 07:11:49 »
Imagine two people. One leaves Earth on a spaceship to travel at high speed around the solar system. When he returns he would be younger than the person that he left behind? In traveling he has travelled into the future whilst the person he left behind has travelled back in time? If time is a dimension does that mean that space has expanded in his travels for the person he left behind but not for him?

#### syhprum

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##### ? Time travel
« Reply #1 on: 07/08/2010 08:05:35 »
As you say the traveller due to the accelerations to which he has been subjected is now younger than he would have been if he had remained on Earth, this does not mean he has moved back in time only that his rate of aging has been slowed.
His hair and fingernails etc would have grown less than they would have if he had not travelled.
« Last Edit: 07/08/2010 08:08:45 by syhprum »

#### flr

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##### ? Time travel
« Reply #2 on: 07/08/2010 18:48:13 »
The traveler moved in his own present time all the time.
We are all 'out of sync', as what we call time is something relative.

#### simplified

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##### ? Time travel
« Reply #3 on: 08/08/2010 06:33:24 »
Just my opinion.Travel relatively of dominant masses slows down time of the traveler.May be this is speed of moving in the time = L-1
L=1/sqrt(1-v^2/c^2)

#### Farsight

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##### ? Time travel
« Reply #4 on: 08/08/2010 14:22:36 »
Imagine two people. One leaves Earth on a spaceship to travel at high speed around the solar system. When he returns he would be younger than the person that he left behind?
Yes. He's subject to time dilation, he ages less than the guy who stayed on Earth.

In traveling he has travelled into the future whilst the person he left behind has travelled back in time? If time is a dimension does that mean that space has expanded in his travels for the person he left behind but not for him?
No. There's no actual "travel" into the future, that's just a figure of speech. Both the space-traveller and the person on Earth experienced the thing we call time, but the space-traveller experienced less of it because of his motion through space.

#### Jon Francis

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##### ? Time travel
« Reply #5 on: 08/08/2010 16:27:15 »
Relative to the person remaining on Earth hasn't the traveller seemingly experiencing time at a slower rate actually moved faster through time?

#### Murchie85

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##### ? Time travel
« Reply #6 on: 08/08/2010 17:12:34 »
Simplified, You are right. Travelling at or close to the speed of light which requires a lot of energy and at the speed of light mass becomes infinite (mathematically speaking ). Gravitational field increases with mass and therefore increases with speed, the greater the gravitational field the slower time passes for the traveler with respect to a still observer. In short we can slow our ageing and isolate ourselves so that when we return say to earth, we have thus jumped to the future in a manner of speaking.

Travelling back in time is a different question and thus far only has theories to describe and is far less realistic an option than jumping to the future via dilation. My personal opinion is that time is connected to the movement of space so moving forward is easy (by isolating yourself via time dialation) but moving backwards would require manipulating the entire space around you for which you want to travel back (example would be winding a video tape backwards in that you need control of the whole system or space) and thus far far harder.
« Last Edit: 08/08/2010 17:18:13 by Murchie85 »

#### Jon Francis

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##### ? Time travel
« Reply #7 on: 08/08/2010 18:38:22 »
Assuming time only moves on. We are trapped in a moment of time. The past can never be revisited, the future always remains while the present is fleeting. But for the two people the present moment has been reached in a different interval of time. Movement through time is therefore relative.
Imagine that centuries ago the Isle of Wight had been transported through space at extreme speed and had now been returned to its original position but had not moved on in time relative to the surrounding land.
If we travelled to the Island would we travel back in time?

#### Jon Francis

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##### ? Time travel
« Reply #8 on: 08/08/2010 20:16:18 »
What I was trying to determine in my first posting was the following.
Is the movement of matter through spacetime at differing speeds giving us the impression that spacetime is expanding?

#### simplified

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##### ? Time travel
« Reply #9 on: 10/08/2010 18:32:19 »

Simplified, You are right. Travelling at or close to the speed of light which requires a lot of energy and at the speed of light mass becomes infinite (mathematically speaking ). Gravitational field increases with mass and therefore increases with speed, the greater the gravitational field the slower time passes for the traveler with respect to a still observer. In short we can slow our ageing and isolate ourselves so that when we return say to earth, we have thus jumped to the future in a manner of speaking.

Travelling back in time is a different question and thus far only has theories to describe and is far less realistic an option than jumping to the future via dilation. My personal opinion is that time is connected to the movement of space so moving forward is easy (by isolating yourself via time dialation) but moving backwards would require manipulating the entire space around you for which you want to travel back (example would be winding a video tape backwards in that you need control of the whole system or space) and thus far far harder.
Hi.I don't trust in speed of moving in the time.However we can play with it.
E=(L-1)m*c^2
(L-1) - is speed of moving in the time
Energy of photon=mc^2
Then Lorenz coefficient for a photon=2
A photon must to travel with a speed=259627884 m/s.
But the especial nature of a photon ignores this limiting of speed. However does not ignore absolute limiting of speed.

#### Murchie85

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##### ? Time travel
« Reply #10 on: 10/08/2010 18:42:47 »
Jon,

Answer to your question is that if we traveled to the island we would not have traveled back in time as time is relative. The only real thing that we can know according to modern physics and is not theoretical is to slow time down relative to something else. Also about spacetime expanding, as far as I know, space is expanding due to the big bang and time is ever moving only at different rates for different objects.

#### Farsight

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##### ? Time travel
« Reply #11 on: 10/08/2010 19:58:34 »
What I was trying to determine in my first posting was the following.
Is the movement of matter through spacetime at differing speeds giving us the impression that spacetime is expanding?
Jon: no. We don't actually "move" through spacetime. We move through space, along with everything else. Things like the inner workings of clocks. What clocks actually "clock up" is the motion of their cogs and gears, and they show it on a display of some kind. A quartz clock doesn't have cogs of course, instead it employs a vibrating crystal, but the same principle applies. Ditto for atomic clocks and light clocks. And the more you move in a macroscopic fashion through space, the less local motion is available to "clock up" the thing we call time. Have a look at Simple inference of time dilation for more on this. Also see Hubble's Law re the expansion of space. Not spacetime, space.

#### Joe L. Ogan

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##### ? Time travel
« Reply #12 on: 10/08/2010 22:14:17 »
I have never had an opportunity to try this but I have heard that a clock will actually not show as much time consumed in travelling through space.  I know that time in an airplane shows no difference than time on earth.  So I must assume that travel in this reference must be much faster than an airplane.  At what rate of speed does time slow?  Thanks for comments.  Joe L. Ogan

#### JP

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##### ? Time travel
« Reply #13 on: 11/08/2010 03:43:41 »
Measurements of time are going to be different no matter how fast you're going.  Clocks on a very fast plane run at a different rate than those on the earth's surface, but you need a very sensitive clock to measure the tiny differences (there's also a change due to the lower strength of gravity when you're far above the earth.)

The ratio of the two time measurements goes like the Lorentz factor, Sqrt(1-v2/c2), where v is the relative speed between the two clocks and c is the speed of light.  Remember that c is a huge number, so for most everyday speeds, v is small enough that v/c is basically zero, and the time measurements appear the same, especially if your clocks aren't extremely good.  If you want a number, there's roughly a 1% difference in time measurements when you travel at 1/8th the speed of light, if I've done my math right.

#### imatfaal

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##### ? Time travel
« Reply #14 on: 11/08/2010 13:01:13 »
Joe - if you are in a normal plane you just need accurate clocks as JP said

Hafele and Keating Experiment

Quote
"During October, 1971, four cesium atomic beam clocks were flown on regularly scheduled commercial jet flights around the world twice, once eastward and once westward, to test Einstein's theory of relativity with macroscopic clocks. From the actual flight paths of each trip, the theory predicted that the flying clocks, compared with reference clocks at the U.S. Naval Observatory, should have lost 40+/-23 nanoseconds during the eastward trip and should have gained 275+/-21 nanoseconds during the westward trip ... Relative to the atomic time scale of the U.S. Naval Observatory, the flying clocks lost 59+/-10 nanoseconds during the eastward trip and gained 273+/-7 nanosecond during the westward trip, where the errors are the corresponding standard deviations. These results provide an unambiguous empirical resolution of the famous clock "paradox" with macroscopic clocks."

J.C. Hafele and R. E. Keating, Science 177, 166 (1972)

#### simplified

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##### ? Time travel
« Reply #15 on: 11/08/2010 14:50:24 »
What is relative time?
We see disintegration of  slow particle earlier than disintegration of  fast particle. However the fast particle  sees  own disintegration earlier than  disintegration of the slow(relatively of us) particle.Blah.

#### chrisdsn

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##### ? Time travel
« Reply #16 on: 16/08/2010 05:30:36 »
There are two factors which effect the passage of time for a fast moving plane (as
measured by you; assumed to be stationary on the surface of the earth):

1) The plane is moving fast. Special relativity tells you that this slows the rate of
time for the plane.

2) The plan is high. General Relativity tells you that time moves slower the higher
the gravitational field; this contributes a speed-up of time for the plane.

For a plane, both these effects are tiny (and the Special Relativity effect is the
largest, so time moves slower). They have been measured, however.

An example of a situation where an understanding of relativity is really needed
for something in everyday life to work is the GPS network. The GPS satellites travel
very fast and very high, and are basically just sending out a clock signal.  The onboard
computer correct for both special and general relativity effects (and if they didn't
GPS wouldn't work).

http://www.phys.lsu.edu/mog/mog9/node9.html [nofollow]

#### yor_on

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##### ? Time travel
« Reply #17 on: 22/09/2010 17:51:22 »
"What is relative time."

A good question, looking at it from my own perspective time have a constant non-changing flow.
Comparing it between different frames of reference like the 'twin-experiment' time suddenly becomes 'two-folded'. My 'personal time-flow' as compared to another frame of reference. When the 'traveling' twin arrives back at his origin the 'time-flow' once more becomes coordinated as they join a common reference but, if Einstein is right, now with one twin having aged more than the other.

So what is time?
It's a causality-chain enabling us to observe a 'history' and a order. It also seems to have properties repeating itself creating a pattern, as proofed by chaos mathematics. What you can't use 'times arrow' for seems to be to define your 'time' as being the objective one. So does there exist a 'universal time'? I don't think so, I think time is related to your frame of observation only, it can be seen as universal in that causality will exist in all frames you experience, and in that from a personal definition you never will experience it breaking down.

So causality might be the better description, but even that may be questioned when comparing different frames of reference against each other. Although that may be a moot question as time always will 'tick the same' for you as compared to your wristwatch. In a way it's like we all carry 'times arrow' with us, unchanging and very real, and only when comparing very extreme 'frames of reference', also expecting them to relate to each other as we do when we use the twin experiments common origin (Earth) as the definition of an 'objective time-reference', will we ever see the inconsistencies of an 'objective common time'.

Clearly time is 'frame dependent'.
« Last Edit: 22/09/2010 17:55:26 by yor_on »

#### CPT ArkAngel

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##### ? Time travel
« Reply #18 on: 23/09/2010 02:09:55 »
What Murchie wrote is true. You must look at it in a parallel way of duration of time. The astronaut will see that people on earth just move faster, their time is going faster than his time. But when the astronaut will go down to meet them, he will change is relative accelerated frame of reference to be in the same (or almost the same) spacetime as them. You cannot go backward but you can go forward in time...

#### yor_on

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« Reply #19 on: 23/09/2010 13:21:54 »
Hmm :)

To me all frames have the same time. There is only one in fact, the frame you are in. You are looking at it from assuming that there can be an 'objective time'. But there isn't any 'objective time' as I see it. Only when assuming a 'objective time-rate', a 'gold-standard' true for the whole universe can you call what the traveler sees as 'speeded up'. It can't be 'speeded up' if there exist no such 'objective universal frame of time-rate' it seems to me? This might seem only semantics of course but to me there is a difference.
« Last Edit: 23/09/2010 13:26:16 by yor_on »

#### CPT ArkAngel

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« Reply #20 on: 23/09/2010 13:31:33 »
That is why i wrote "or almost the same"...

#### simplified

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« Reply #21 on: 23/09/2010 16:04:07 »
Twins are identical. Relative speeds are identical. Accelerations are identical. But in the end they have different ages.

#### yor_on

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##### ? Time travel
« Reply #22 on: 23/09/2010 18:53:26 »
Yes Sim :)

Relative speeds are indeed identical, as long as we're discussing the rockets later 'uniform motion'. But the acceleration wasn't. And the discussion we had in that other thread was also around how the universe can differ between different uniform speeds, to present us time-dilation and Space 'deforming'. If SpaceTime can do so, and I will assume that it can, depending on my uniform velocity, then there is something hiding there that I don't really can define. Also, the acceleration in the 'twin experiment' can't be seen as 'equivalent' (Earth versus the traveling twins rocket) as only one of them expends energy to reach that uniform motion, well, as I see it.

#### yor_on

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##### ? Time travel
« Reply #23 on: 23/09/2010 19:19:37 »
What one might ask though, is if that rocket accelerated at a perfect one G at all times? Would that then be equal to being on Earth? And also, would the Twins age differ after the traveling Twin returned?

You might use this as a proof that a uniform acceleration and Gravity isn't exactly equivalent as there would be a age difference observed? but as long as we are thinking of it in form of energy expended I expect my argument to hold water any which way :)

I've never really felt that to be the best definition though, to me it's uniform motion and 'free falling' so called 'inertial frames of reference' that seems the real 'equivalence' in SpaceTime. so thinking of it, the equivalence between a rocket moving at a constant one G and Earths gravity will only hold true as long as the 'frames of reference' don't join a common origin in the end, as we otherwise will see different effects. But taken in a black box, and measured only there, without any 'history' I expect it to be impossible to differ that rockets gravity from Earths. It's all about 'frames of reference'. And only when we introduce the 'history' of it will we be able to differ between the rocket uniform acceleration and Earths gravity, but, not while inside that rocket uniformly accelerating.

#### simplified

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##### ? Time travel
« Reply #24 on: 25/09/2010 07:56:33 »
[:-'(]
What one might ask though, is if that rocket accelerated at a perfect one G at all times? Would that then be equal to being on Earth? And also, would the Twins age differ after the traveling Twin returned?

You might use this as a proof that a uniform acceleration and Gravity isn't exactly equivalent as there would be a age difference observed? but as long as we are thinking of it in form of energy expended I expect my argument to hold water any which way :)

I've never really felt that to be the best definition though, to me it's uniform motion and 'free falling' so called 'inertial frames of reference' that seems the real 'equivalence' in SpaceTime. so thinking of it, the equivalence between a rocket moving at a constant one G and Earths gravity will only hold true as long as the 'frames of reference' don't join a common origin in the end, as we otherwise will see different effects. But taken in a black box, and measured only there, without any 'history' I expect it to be impossible to differ that rockets gravity from Earths. It's all about 'frames of reference'. And only when we introduce the 'history' of it will we be able to differ between the rocket uniform acceleration and Earths gravity, but, not while inside that rocket uniformly accelerating.

A rocket spends energy,  particle in collider does not spend energy... Main thing is that I do not see a law of  clever person, I see the paradox of  idiot. [:-'(]

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« Reply #24 on: 25/09/2010 07:56:33 »

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