Reporting my own post to bring this to the attention of a moderator.

[e-pie]

Does anybody have any references where Einstein speaks about Time Machines?

From collected sources I found, he strongly rejected the idea of Time Machine but through his theory implied that time travel may be possible if wormholes existed.

Something needs to be infinitely fast to do this? Have their own gravitational field? Answers to these also please.
If something goes at infinite velocity doesn't wave nature becomes superior than their particle nature? So that means even if time travel is possible only wave packets of existing particles will reach the future or past?
Any definite sources such as books/journals/research papers etc.?

 

[Implication]

Not that this really answers your question, but from what we know about physics and how its applied in our world time travel is 100% possible. Move fast enough and everything else slows down around you. Once you slow back down a great deal of time has passed for everyone/thing else, but not for you.

 

[Ibix]

Some solutions to Einstein's field equations include "loops" in spacetime which you can travel around and meet yourself in the past. As far as I'm aware they're all either not descriptions of our universe, require stuff we've never seen (like concentrations of negative energy), or are idealisations that we don't expect to really exist in nature (the interior of Kerr black holes, for example). So we can describe such spacetimes, but we don't think they can exist.

You can communicate backwards in time if you can find particles that can exceed light speed. Again we've never seen such things and they lead to paradoxes (google for the tachyonic anti-telephone, which is a less bloodthirsty version of the shoot-your-own-grandad paradox).

I would assume it's the apparent potential for paradoxes that Einstein disliked. I'm afraid I don't have references to support that assumption, however.

 

[Ibix]

Move fast enough and everything else slows down around you. Once you slow back down a great deal of time has passed for everyone/thing else, but not for you.

There are caveats around this statement. It's broadly true, but there are quite a lot of ways that taking it as strictly true could come back to bite. There are a billion threads on the twin paradox in the relativity forum if you wish to know more.

 

[e-pie]

I heard somewhere that if we move at speeds greater than light, then the effects would occur much before the cause(Law of causality).
I think it would be in a light like world. (Minkowski's cone and Space time geometry).

 

[Ibix]

I heard somewhere that if we move at speeds greater than light, then the effects would occur much before the cause(Law of causality).

That's not really correct (which is probably something to do with "I heard somewhere" not really meeting PF's referencing standards). Anything moving faster than light is moving backwards in time in some choice of reference frame, but is also moving forwards in time in other reference frames. So from some perspectives, effect would precede cause, while from others it would follow. The order of events wouldn't be clearly defined in relativity - which doesn't matter because we've never seen anything traveling faster than light.

I think it would be in a light like world. (Minkowski's cone and Space time geometry).

I've never heard the term "light like world" - I think you are confusing a number of things.

The light cone is the surface that separates all events that you can reach traveling at or below the speed of light, starting from some given event (such as where you are now). You can affect events inside your future light cone, and can have been affected by events in your past light cone. Events outside the light cone can't affect you (yet) and can't be affected by you, so whether they happen before or after now isn't important (within limits). Those events are space-like separated from you. Events actually lying on the light cone are light-like separated from you.

 

[e-pie]

Events outside the light cone can't affect you (yet) and can't be affected by you, so whether they happen before or after now isn't important (within limits).

How is the order of precedence/succession of events decided?

Light like separated? Meaning...?

You are right. I used the line "I heard somewhere" because it is mostly from a combination of popular science books and videos+little memory that would not serve as a reliable source.

 

[Ibix]

How is the order of precedence/succession of events decided?

If you can get from one event to the other without exceeding the speed of light, the order of the events is fixed (and these events are "time-like" separated if you could do it below the speed of light and "light-like" or "null" separated if you could only get from one to the other at the speed of light). The order of events that are far enough apart that you can't get from one to the other ("space-like" separated) even at the speed of light is a matter of choice.

Of course, two events that are space-like separated from you now may not be space-like separated from each other. Their relative order is fixed - but whether either or both happened before "now" is a matter of choice.

 

[e-pie]

Am I saying right?

Time like separated if i can proceed from one event to another at speed below c. That means in this dimension. Should we use speed or velocity? Events have space-time coordinates.

Light Like if I can go at speed of light.

Space Like if they are widely separated.

How can two events be space like separated from me and not from themselves?

 

[e-pie]

Their relative order is fixed - but whether either or both happened before "now" is a matter of choice

Say I map the entire timeline of the universe into the number line, starting from t=0 to infinity. Then each point will refer to a particular time in the whole timeline. Say we are at the 1000th position at this instant. Then taking that position as T=0 and as the present instant I measure the time that passed(in positions) in writing this post. Say I arrive at 1002th position. Now the choice “now“ is not abstract. We are referring to a particular point in the whole timeline.

Is this feasible?

 

[jbriggs444]

Is this feasible?

When you mark down time 1000 at sunrise on August 10 over here, what event over there in the Andromeda galaxy are you considering as occurring at time 1000?

You have a wide variety of choices available.

 

[Ibix]

Time like separated if i can proceed from one event to another at speed below c.

Yes.

That means in this dimension.

No. Spacetime is four dimensional, meaning, roughly speaking, that there are four basic directions - forward/backward, left/right, up/down, future/past. You seem to be using it in the science fiction sense of "a different dimension".

Light Like if I can go at speed of light.

Two events are light-like separated if the only thing that could get from one to the other is something traveling at the speed of light. You cannot do this.

Space Like if they are widely separated.

Two events are light-like separated if the distance between them is more than the speed of light times the time between them.

How can two events be space like separated from me and not from themselves?

They're close to each other but not to you. For example, the Sun is eight light minutes away. But a couple of spaceships near the Sun and a few hundred metres apart from each other don't care that you are eight light minutes away.

Say I map the entire timeline of the universe into the number line

You can do this unambiguously for an individual person or a clock or something. And you can do it unambiguously for another clock. But you can't unambiguously specify that both clocks read 1000 at the same time - this is the key difference between relativity and Newtonian physics. Time is not an absolute quantity that everyone agrees on.

Now the choice “now“ is not abstract. We are referring to a particular point in the whole timeline.

You are referring to a point on a timeline. But time is not a line - it's a direction in spacetime. And you have a lot of freedom to choose which direction you call "time", and you don't have to use the same direction here as someone else does elsewhere. Hence you have a lot of freedom to define what you call "all of space now".

There are natural choices of what you mean by "all of space now" in many circumstances, but there's never only one possible choice.

 

[e-pie]

Should we use speed or velocity? Events have space-time coordinates.

What about this?

 

[e-pie]

When you mark down time 1000 at sunrise on August 10 over here, what event over there in the Andromeda galaxy are you considering as occurring at time 1000?

If second is the base unit. Then. Some formula like. 1 Earth seconds=... Other galaxy seconds that can be used to scale the earth-timeline.

 

[e-pie]

You are referring to a point on a timeline. But time is not a line -

Maybe a directed line segment.

 

[e-pie]

You seem to be using it in the science fiction sense of "a different dimension".

Excuse my choice of word. Maybe the classical/Newtonian world.

 

[e-pie]

But you can't unambiguously specify that both clocks read 1000 at the same time - this is the key difference between relativity and Newtonian physics.

What about synchronization and simultaneity? I am referring to SR.

What if I use t=o for both clocks at the start of the universe as base.

Why 'a' timeline? Why can't I scale the whole galaxy into one timeline?

 

[jbriggs444]

If second is the base unit. Then. Some formula like. 1 Earth seconds=... Other galaxy seconds that can be used to scale the earth-timeline.

No. You are not getting it. This is a relativity of simultaneity thing. Not a scaling thing.

 

[e-pie]

Not a scaling thing

Please explain why?

 

[e-pie]

Off topic

Why is each post duplicating?Happening to me also.

 

[fresh_42]

Off topic

Why is each post duplicating?Happening to me also.

This is usually due to low response times and a doubled send command because of it.

Please return to the topic.

 

[Ibix]

If second is the base unit. Then. Some formula like. 1 Earth seconds=... Other galaxy seconds that can be used to scale the earth-timeline.

You are missing the point. Time is not an absolute in relativity any more than left and right are in our every day experience - you can use different meanings depending on which way you are facing. It doesn't matter what units you use. There are no scratches in spacetime saying "time is exactly this direction and no other", any more than there are scratches in space saying "this direction is always to be called forwards". You have freedom to choose how you slice spacetime into space and time, just as you have freedom to choose how to slice space into infront and behind you.

Maybe a directed line segment.

No. Time is a dimension, a direction in spacetime. It's different from the three spatial dimensions, but just as you can rotate coordinate axes and change the direction you call x, you can rotate them and change the direction you call t.

What about synchronization and simultaneity? I am referring to SR.

That's exactly what I'm talking about. Different frames synchronise clocks by the same process and come to a different idea of simultaneity. Whether something happened now, in the past, or in the future depends on your definition of now - i.e. your simultaneity convention.

What if I use t=o for both clocks at the start of the universe as base.

This is going beyond SR. Assuming that the clocks both see the cosmic microwave background as isotropic, then this is the definition of co-moving time, and is the scheme usually used to quote the age of the universe. It's a sensible choice, but it's still a choice - others can be made. Someone who decides to use clocks that do not see the CMB as isotropic (i.e. are moving with respect to the first set) won't agree about elapsed time or simultaneity.

Why 'a' timeline? Why can't I scale the whole galaxy into one timeline?

Measuring time in spacetime is like measuring a length in everyday experience. You can measure your height, for example, and I can measure mine. But answering the question "is my head higher than yours" needs us to agree whether our floors are at the same height, and what we mean by vertical. You can't do that with a meter rule unless we're in the same place. Similarly, defining "at the same time" can't be done with a clock unless we're in the same place. We have to agree a meaning, and there's quite a lot of flexibility.

 

[e-pie]

This is usually due to low response times and a doubled send command because of it.

Thanks.

Please return to the topic.

My last request stands.

 

[Ibix]

Please explain why?

Newtonian physics thinks of time as a totally separate thing from space. But relativity regards time and space as aspects of one whole, called spacetime. How you choose to slice spacetime to separate out time is largely up to you. You can make different choices.

 

[e-pie]

Though cannot be condensed in a nutshell,
But,

The way I look at time(meaning how I define/measure it) depends on my choice of how I try to define each. As each choice is unique, we won't get any particular choice to be the standard/base.

Right?

 

[e-pie]

I mean I can try to give finiteness to time?Human thoughts cannot perceive infinite time just as we cannot perceive 1D or 2D(Euclidean space).

 

[e-pie]

Someone who decides to use clocks that do not see the CMB as isotropic (i.e. are moving with respect to the first set) won't agree about elapsed time or simultaneity.

Who is that someone if we are choosing the very start of the universe?

Biological life hasn't started in our universe yet and if we say that other universe exists and people from them is that someone we neither can prove nor disprove the fact that someone was there to measure with a clock during big bang(I hold this model out of many).

Even if someone was there, organisms from this universe would not perceive it as true. Therefore for us it is not a fact but a fiction or a theorem (edit:proposition) that needs proof. And if we proceed to a proof, there will not be any examples in support(according to our knowledge), therefore such "life" never existed?

 

[Ibix]

I don't understand whst you are trying to say. This has nothing to do with finite or infinite.

The point is this: time is a direction in spacetime. But there's some freedom to choose which direction you call time, just as there's some freedom to choose which direction you call "forwards". Depending on the choice you make, you get a different meaning for "sinultaneous".

Look up Einstein's train thought experiment. It's really simple and shows how two people can disagree whether two things happened at the same time or not.

 

[e-pie]

Yes I understand. I was asking another question that came up in my mind.

What about post 25?Is it correct? And my argument in post 27?Thank you for your time.

 

[Ibix]

Who is that someone if we are choosing the very start of the universe?

We choose to regard time zero as the start of the universe. If you get into the guts of your phone it has a clock that simply records time in seconds since midnight on January 1st 1970. Did your phone have to be there in 1970 for its clock to measure time since then? Of course not. We just start its clock ticking already showing about 120 million seconds.

General relativity tells us how the universe looks as it gets older. Right now it looks as if imaginary clocks that started ticking at the start of time and always saw the universe as isotropic would show 13.9bn years.

Regarding post 25 - as I said, no, this has nothing to do with finite or infinite. Look up Einstein's train.

 

[e-pie]

Did your phone have to be there in 1970 for its clock to measure time since then?

But the hardware/software that calculates the time had to be there before that time.

Regarding post 25 - as I said, no, this has nothing to do with finite or infinite. Look up Einstein's train.

Except my "finiteness" argument. Is it correct?
I am editing it. Maybe it should be on a separate thread.

 

[jbriggs444]

But the hardware/software that calculates the time had to be there before that time.

You are trying to say that because VAX/VMS happens to use November 17, 1858 (it is a 64 bit integer measured in 100 nanosecond ticks) as its base date that there must have been a VAX back then? Not so.

Edit: Before we dive further into the relevance of epoch times in operating system timekeeping, it is past time for you to Google Einstein's train though experiment and get a handle on the relativity of simultaneity.

 

[e-pie]

Okay got the point. @jbriggs444 and @Ibix. Thanks to both.

And the new post 25?

 

[jbriggs444]

And the new post 25?

i.e.

The way I look at time(meaning how I define/measure it) depends on my choice of how I try to define each. As each choice is unique, we won't get any particular choice to be the standard/base.

The time coordinate in a coordinate system depends on the choice of coordinate system, yes.
The laws of physics fail to pick out a preferred coordinate system, yes.

 

[e-pie]

it is past time for you to Google Einstein's train though experiment and get a handle on the relativity of simultaneity

Almost midnight here. I will do it later. Let is stop the discussion here. I will start another thread/continue after I read Einstein's train experiment and clear my head a bit.I mean I tried but I am feeling sleepy and Wikipedia is not helping.Thanks.A Request: I think that, if this discussion is restricted between us three, it will not create any confusion(to me/any future viewers). Multiple opinions on same topic do tend to get messy and different commentaries can be hard to read(for me).And I am getting positive feedback from both.
Thanks. Logging of for now.