Time travel: Misconception?

[Karik]

Hi
I'm new in this forum, but I was reading about what is time, and its concept is not very well defined.

UPDATE: I found a text from wired.com with an interview with Sean Carroll that do explain my point. Under there is a part that may help to understand what I'm saying!

Wired.com: So what happens to the arrow in places like a black hole or at high speeds where our perception of it changes?
Carroll: This goes back to relativity and Einstein. For anyone moving through spacetime, them and the clocks they bring along with them – including their biological clocks like their heart and their mental perceptions – no one ever feels time to be passing more quickly or more slowly. Or, at least, if you have accurate clocks with you, your clock always ticks one second per second. That’s true if you’re inside a black hole, here on Earth, in the middle of nowhere, it doesn’t matter. But what Einstein tells us is that path you take through space and time can dramatically affect the time that you feel elapsing.
The arrow of time is about a direction, but it’s not about a speed. The important thing is that there’s a consistent direction. That everywhere through space and time, this is the past and this is the future.
Wired.com: So you would tell Michael J. Fox that it’s impossible for him to go back to the past and save his family?
Carroll: The simplest way out of the puzzle of time travel is to say that it can’t be done. That’s very likely the right answer. However, we don’t know for sure. We’re not absolutely proving that it can’t be done.
Wired.com: At the very least, you can’t go back.
Carroll: Yeah, no. You can easily go to the future, that’s not a problem.

In physics it is said that time is a dimension or a process, depending on what you think is right.

For example, if time is a dimension, it means you cannot change its properties, as you cannot distort the third, second or first dimension (it will always be first second or third). But it is observable that time does change its properties, going slower or faster (speed can change) depending on gravitional distortions (near blackholes time goes slower, for example).
If time is a process, than it does have a direction (arrow) that may or may not be changeable.

Anyway, what came to my mind is: What does a "time machine" (that makes time travel) do?
Obviously, it goes back in time. But how? How can particles "travel" through time?
I ask this because the "how" changes a lot depending on the mechanisms time travel works

What i'm trying to say is that if the "time machine" (and everything inside it) goes back in time, within a linear time line, we can imagine the classic time travelling issues and histories, as you just invert the arrow of time to get back at some past point. But I thought that, as blackholes slow times near it, what if the time machines does not go back in time? What if the "time machine" makes the universe around go back in time? That would not change the direction of the arrow of time as relative to the "time machine" you are still going throught one direction in time (isolated system?). All the issues and paradoxes of time travel would go away!

Imagine time as a linear dimension or process: If the universe goes back, than it means that each time it goes back, all the particles that make you are in the "time machine", so in the instant it stops going back all the particles that makes you will be in you: this means that there is no killing your father or going back where the time machine does not exist problem! Another you will not exist: it will simple not exist as you. Killing him does not mean killing you, because he is made of diffent particles. This keeps the linearity of time, doesnt it?

Also this leads to another problem: If the universe goes "back", or if you go back in time in a universe, it goes to a state of less entropy, which does not make a lot of sense (every moment the entropy in a isolated system increases). So you would just hop into a diferrent universe to this to work, or something that I have not thought yet....

I don't know if I explained it well, but what do you think?

 

[Einstein]

What do I think? First eliminate all your assumptions. Second remove existing theory. What you are left with is facts about time.

The facts show time to be variable in magnitude with direction. No documented time reversal has been observed to my knowledge.

As for time being linear? Why not spiral? Or toroidal?

We don't even know if time is a basic force. No one has been able to filter it out, or separate it from existing phenomena.

What you are proposing is beyond what we know about time.

 

[RainmanTime]

As for time being linear? Why not spiral? Or toroidal?

I don't think you are understanding what Karik meant when he used the word "linear." It is clear to me he did not mean linear in a geometric sense, which is what your reply implies you inferred. Rather, I believe he meant mathematically linear. They are related, but not the same.

RMT

 

[Darby]

Hi
I'm new in this forum, but I was reading about what is time, and its concept is not very well defined.
In physics it is said that time is a dimension or a process, depending on what you think is right.

For example, if time is a dimension, it means you cannot change its properties, as you cannot distort the third, second or first dimension (it will always be first second or third). But it is observable that time does change its properties, going slower or faster depending on gravitional distortions (near blackholes time goes slower, for example).
If time is a process, than it does have a direction that may or may not be changeable.

Anyway, what came to my mind is: What does a time machine (that makes time travel) do?
Obviously, it goes back in time. But how?
I ask this because the "how" changes a lot depending on the mechanisms time travel works

What i'm trying to say is that if the time machine (and everything inside it) goes back in time, within a linear time line, we can imagine the classic time travelling issues and histories. But I thought that, as blackholes slow times near it, what if the time machines does not go back in time? What if the time machine makes the universe around go back in time? All the issues and paradoxes of time travel would go away!

Imagine time as a linear dimension or process: If the universe goes back, than it means that each time it goes back, all the particles that make you are in the time machine, so in the instant it stops going back all the particles that makes you will be in you: this means that there is no killing your father or going back where the time machine does not exist problem! Another you will not exist: it will simple not exist as you. Killing him does not mean killing you, because he is made of diffent particles. This keeps the linearity of time, doesnt it?

I don't know if I explained it well, but what do you think?

Karik,

Linear Time

Coordinate time is a concept in Special and General Relativity where time is viewed as one of four coordinates in a time-space continuum. In that concept time and space are not separate entities. They are inseparable parts of a whole.

In Special Relativity there are assumptions made about the scenario in order to make certain points about how the world works. The assumptions are movement in a straight line and absolutely no accelerations. No acceleration means no change in speed or direction. This implies that no gravitational force (or any other unbalanced force) is working on the system and the object cannot be spinning (inertial frames). You also have an observer who is stationary with respect to the moving object under observation.

In Special Relativity the rate of the passing of time as measured by the stationary observer's clock with respect to the moving object's clock varies is a function of the difference in velocity between them. Neither the observer nor the moving object will notice any change in their own frame. There are no accelerations. Either side can correctly state that they are at rest and it is the other that is moving.

Similar observations can be made about the mass and measuring rod of the moving object as measured by the stationary observer.

And finally it is stated that the laws of physics are the same in every inertial frame. This means that if the same experiment is run in both the stationary frame and the moving frame the results will be the same in both frames.

Is time travel possible and has it been accomplished? Yes as to both - at least to the future if you mean is it possible for me to arrive at January 1st, 2015 before you arrive there. We observe this every day. For example, we observe it in satellites orbiting the earth, in particle accelerators and in certain particles raining down from the sky after cosmic rays collide with nitrogen atoms in the air. All it takes for two previously synchronized clocks to get out of synchronization is for one to travel faster than the other.

Time travel to the future is no longer something for theoretical physicists to contemplate as a basic science. Applied physics and theoretical engineering hold the torch today.

But there is a limit, also stated directly by Special Relativity. The speed of light is the maximum speed and objects with rest mass cannot be accelerated to the speed of light.

The real math is complex. But you can get a feel for the implications by using the simplified form of the Lorentz Transformations used in Special Relativity. They are simplified in the sense that they are stated in straight high school algebra rather than university graduate level calculus and the parameters/coordinates/DOFs (time, speed, mass, length) are taken separately.

In General Relativity accelerations, specifically gravity, are added to the mix. Now you have curved space-time. General relativity models the real world where we do have accelerating forces everywhere. Special Relativity is a subset (limiting situation) of General Relativity.

 

[Dizzie]

Thanks for that overview, Darby. Do you have any thoughts in layman's terms as to why the speed of light is the limit of speed? If the limit of speed were theoretically higher, would light theoretically travel faster, or is it the properties of light itself that create the speed boundary? I never have really understood if there is a "why" behind the speed maximum or if that's "just the way it is", so to speak.

 

[Darby]

Thanks for that overview, Darby. Do you have any thoughts in layman's terms as to why the speed of light is the limit of speed? If the limit of speed were theoretically higher, would light theoretically travel faster, or is it the properties of light itself that create the speed boundary? I never have really understood if there is a "why" behind the speed maximum or if that's "just the way it is", so to speak.

First, the answer to "why is the speed of light the maximum" is we don't know. It's a postulate of Special Relativity that is not derived from any first principles of physics. It's observed to hold true experimentally, has never been found to have been violated and is stated such that it can be falsified. Not a very satisfying answer but it is the correct answer.

If we were in another universe and if in that universe the speed of light was greater than here then, yes, light would travel faster there. It wouldn't reveal anything useful for us because the laws of physics in that universe would be different than ours. Even the slightest change in the strong force it ends up forbidding quark formation, No quarks means no matter. Slightly change the electric force and electrons either can't bind or can't unbind. No active chemistry thus no molecules. Slightly change the weak force and we have no radioactive decay or runaway radioactive decay. No nuclear chemistry thus we have no elements beyond hydrogen. Our laws of physics seem to live in a Goldilocks Zone. The laws are just right. And the just right includes the light speed limit.

Why can't an object that has rest mass be accelerated to the speed of light? You see the answer in the Lorentz Factor for mass (and this has been proven experimentally to be true):

Gamma = 1/sqrt(1 - (v^2/c^2))

Look at the denominator. It is going to be the square root of 1 minus the velocity/the speed of light squared term. Its a vanishing quantity. The closer that "v"gets to the speed of light "c" the smaller the denominator. It tends to zero (vanishes). One over an infinitesimally small denominator tends to the other direction - infinity.

That's what the equation is telling you about the mass of the object observed to be accelerating to near the speed of light. It's mass tends to grow to infinity the closer you accelerate to the speed of light.

Mass has a property called inertia. Inertia quantifies the object's resistance to a change in velocity. More mass means more resistance. Each step up the mass scale takes more energy than the previous step to accelerate the object than the previous step. It grows as a function of the square of the change in velocity. It is not a linear progression.

This was a problem in 1905 when Einstein published his original paper on Special Relativity. But the original paper was classical physics based. Quantum physics enters at the same time. Now the problem is compounded. Classical physics deals with waves, not particles. Quantum mechanics is particle based. Specifically, energy itself is delivered in discrete energy packets (particles). There are no half packets, quarter packets or any other fractional packets. There are only whole packets.

Each step in adding energy to accelerate the object requires a quanta (packet) of energy. Eventually, before you can reach the speed of light, the next quanta required for any acceleration at all is greater than the entire mass of the universe (Remember E = m...energy and mass are equivalent. Forget the c^2 because it is the constant of proportionality in the equation E = mc^2).

That's the answer as to why we can't accelerate an object with rest mass to the speed of light. Insufficient energy in the entire universe because the energy requirement ends up being infinite.

How have we proven this to be true? Many ways but here's one example. The CERN Large Hadron Collider (LHC) accelerates individual atoms to very near the speed of light. Not time machines, spaceships or even grains of sand. Individual atoms. To do this they had to build an electric generation facility of a size that it could otherwise provide electric power for a city of 35,000 people...just to accelerate a few lead atoms at a time. That's a 5 megawatt generation plant.

In the 1920's the original accelerators could be plugged into a wall outlet and achieve 10% the speed of light. That's about 100 watts. Today we've increased the velocity 10 fold but the energy required has increased by a factor of 50,000.

 

[Timetravel_00]

Can someone fill me in on what's happened in the past 13 years.

 

[Dizzie]

...

Bravo, Darby. An excellent explanation with great examples and references. You stretched my mind.

 

[vodkafan]

Can someone fill me in on what's happened in the past 13 years.

If you are a time traveller surely you can go back and have a look? And if you are the supposed traveler known as JT why can't you go back to before you (supposedly) came last time and change your posts on that forum that no longer exists? Do things differently? That should be easy

 

[J.T._9663]

TT_00,
In answer to your question, people have become increasingly angry. Riots, conflicts are only a physical manifestation of a very real future that was foretold 13-14 years ago. Instead of directing that anger towards goverments and incompetent leaders, we have been, for the most, given a very limited 'narrative/propoganda' through media which polarizes and confuses us. 'Everything' is ok' message means we bottle up and then direct our frustrations on others. Our fellow man and woman. 2008 saw a horrific, yet staged transfer or wealth to the bankers and greedy. In the last few months, The American Federal Empire has been attempting to poke the Bear into a war. I think we all know how this one ends...
Any other questions, regardless of from where will be answered. Id much rather find immense enjoyment in talking to others who share a similar enthusiasm than be 'entertained' and spoon-fed, carefully crafted viewpoints of the 'truth'.
Think I just contradicted myself. Doesnt matter. Nobody is perfect.
Cheers mate.
J

 

[Karik]

I have updated the post, because my explanation was poorly made. Now I've put a interview with Sean Carrol that do explain my point!

Thanks.