PDA

View Full Version : Point of no Return



a1call
2015-Jun-18, 06:44 PM
In a faster than light expanding universe, is it possible to travel fast and far enough such that you are never able to return to your departure point or to even communicate with at the speed of light?
Thanks in advance.

antoniseb
2015-Jun-18, 06:59 PM
Rate of expansion is not a speed. The units are different. What do you mean by "a faster than light expanding universe"? If you give us a clear explanation of what you want to know someone can probably give a meaningful answer.

a1call
2015-Jun-18, 09:37 PM
Please let me rephrase the question:
Is it possible to travel fast and far enough such that you are never able to return to your departure point or to even communicate with it at the speed of light?
In particular in a universe where there are at least 2 points which are separated from each other at a rate faster than the speed of light due to expansion of the space in between them? If I am not mistaking our universe is one such universe.
Thanks in advance.

a1call
2015-Jun-18, 10:16 PM
A Wikipedia entry:


The expansion of the universe (https://en.wikipedia.org/wiki/Universe) causes distant galaxies to recede from us faster than the speed of light,
...


There are many galaxies visible in telescopes with red shift (https://en.wikipedia.org/wiki/Red_shift) numbers of 1.4 or higher. All of these are currently traveling away from us at speeds greater than the speed of light. Because the Hubble parameter (https://en.wikipedia.org/wiki/Hubble%27s_law#Interpretation) is decreasing with time, there can actually be cases where a galaxy that is receding from us faster than light does manage to emit a signal which reaches us eventually.
...

However, because the expansion of the universe is accelerating (https://en.wikipedia.org/wiki/Accelerating_universe), it is projected that most galaxies will eventually cross a type of cosmological event horizon (https://en.wikipedia.org/wiki/Event_horizon) where any light they emit past that point will never be able to reach us at any time in the infinite future,[25] (https://en.wikipedia.org/wiki/Faster-than-light#cite_note-25) because the light never reaches a point where its "peculiar velocity" towards us exceeds the expansion velocity away from us (these two notions of velocity are also discussed in Comoving distance#Uses of the proper distance (https://en.wikipedia.org/wiki/Comoving_distance#Uses_of_the_proper_distance)). The current distance to this cosmological event horizon is about 16 billion light-years, meaning that a signal from an event happening at present would eventually be able to reach us in the future if the event was less than 16 billion light-years away, but the signal would never reach us if the event was more than 16 billion light-years away.[ (https://en.wikipedia.org/wiki/Faster-than-light#cite_note-ly93-23)

https://en.wikipedia.org/wiki/Faster-than-light#Universal_expansion

I still do not know the answer to my question:

Is it possible to travel fast and far enough such that you are never able to return to your departure point or to even communicate with it at the speed of light?

antoniseb
2015-Jun-18, 10:25 PM
Say for example, if a photon emitted from a relatively dense part of the universe 380,000 years after the big bang (from the CMB) were reflected off some kind of microwave mirror here and now, could it ever (even trillions of years later) get back to the same matter that emitted it (now about 46 billion light years away)? My understanding is that the answer is no, and so there must be some point of no return. But this aspect of cosmology can have some counter-intuitive results, so wait for confirming or refuting answers before taking that as a fact.

a1call
2015-Jun-18, 10:40 PM
I think that makes sense and that's what my inclination is as well.
I will wait for another board heavy weight to confirm before posting a follow-up question.
Thank you for the reply Antoniseb.

WayneFrancis
2015-Jun-19, 04:43 AM
I'm not a heavy weight but yes you could head out from any point and given enough time find that you could never return do to cosmic expansion. That said if the expansion slows down, stops or reverse then all bets are off.

There are many places that in the early universe where causally connected but are not now. Look up into the sky to the west and to the east. The CMBR coming from both those points is not causally connected but was in the past.

a1call
2015-Jun-19, 05:32 AM
Thank you for the reply Wayne.
The heavy weights are generally too humble to post as heavy weights. So bad wording on my part.
Still I think we are all correct. But relativity is very unintuitive. There is time dilation and length contraction and approaching infinity when you travel. This is really going to be convincing if answered by Grant Hutchison or Ken G or their likes.
On the other hand traveling at maximum speed along any straight path that expands will eliminate possibility of returning to the departure point (barring relativity matters).
Then again in relativity maximum speed is anything under c which makes anything possible.

To reference my follow up question it's about entanglement and deductions about un-observable part of universe.

WayneFrancis
2015-Jun-19, 06:22 AM
I'm with you a1call, all the moderators, Ken G, Shaula, Jeff Root, Reality, Grant, Jens, cjameshuff, the list goes on and on heavy weights. I'm just an overly informed layperson.

That said don't confuse the red shift due to cosmic expansion as actual time dilation.
Take the following thought experiment.

Observer A & B are in the same frame of reference. They wait billions of years while cosmic expansion separates them. During this time they observe each others clocks running slower.
Stop expansion and they can then sync their clocks and see that the same amount of time has passed for them. There was never point where the clocks were really time dilated like the case were one of them had really accelerated away from the other.

Cougar
2015-Jun-19, 11:35 AM
In a faster than light expanding universe, is it possible to travel fast and far enough such that you are never able to return to your departure point or to even communicate with at the speed of light?


Superluminal expansion only occurred during inflation. During that epoch two points very quickly separated outside each other's light horizon, so no way to communicate (and they didn't have to "travel" at all - it's the space between them that expands). However, after inflation the expansion was slowing for 7 or 8 billion years, and if I understand it correctly, some objects formerly outside our light horizon would come back into view. But after that, the expansion started to accelerate, and objects formerly inside our light horizon would be carried outside it.

a1call
2015-Jun-19, 02:15 PM
The question is not about traveling FTL, or traveling using the expansion.
It's about traveling using thrust or even gravity assists at sub-c and being or not-being able to ever return at sub-c to the departure point, due to the space expansion along the path.

a1call
2015-Jun-19, 05:30 PM
Here is a repeating of the same question that might make it easier to answer:

Is it possible for vessel X to travel at sub-c away from a point A and end up far enough at point B such that light signals from vessel X at point B can never reach point A due to the expansion of the space between points A and B during the travel?

mkline55
2015-Jun-19, 05:55 PM
Here is a repeating of the same question that might make it easier to answer:

Is it possible for vessel X to travel at sub-c away from a point A and end up far enough at point B such that light signals from vessel X at point B can never reach point A due to the expansion of the space between points A and B during the travel?

I suspect your intuition is correct in theory. If the vessel were emitting photons back to point A at say 1-second intervals, then the photons would arrive less and less frequently and more and more red-shifted, until the interval between them became immeasurable - having seen the last one ever capable of crossing the distance. Replace 'vessel' with 'cosmic ray' and it might be more practical, except that whatever is at point A would have to wait a few billion years to confirm that it does not see the cosmic ray collide with anything.

a1call
2015-Jun-19, 07:36 PM
Hi Mkline,
That's a good way of thinking about this. Red shift not withstanding the pulses would arrive at larger and larger intervals. But would they ever stop? Why should they?

mkline55
2015-Jun-19, 07:58 PM
According to theory . . . In addition to what its velocity is doing, the distance between the origin and the vessel is expanding as the universe expands. At some great distance, say 16 Gly or so, the pulse of light from the vessel will have to travel the 16 GLY + any expansion that occurs over that 16 billion years. After the first billion years, the pulse has more than 16 GLY still left to travel. A few billion years more, and the pulse still has to go even farther, say 20 GLY. It's losing ground. I expect too, that the vessel will benefit from the expansion as it increases its distance from origin. Now I fully expect someone to come back and say, no, that's not right at all, but at least that's how I understand it.

Grey
2015-Jun-19, 08:50 PM
Here is a repeating of the same question that might make it easier to answer:

Is it possible for vessel X to travel at sub-c away from a point A and end up far enough at point B such that light signals from vessel X at point B can never reach point A due to the expansion of the space between points A and B during the travel?Yes. And actually, since the expansion is accelerating, I think that any speed is sufficient in principle, as long as it's at least high enough to avoid being gravitationally bound. Obviously, this would only happen on cosmological time scales.

a1call
2015-Jun-19, 08:57 PM
Thank you Grey,
You are the heavy weight I was waiting for.
I will formulate my follow-up question.
I recall posting a similar question in the past. Will have to look it up.

a1call
2015-Jun-20, 12:02 AM
For the record here is the old thread:
http://cosmoquest.org/forum/showthread.php?108258-Hey-look!-It%92s-a-bird-it%92s-a-plane%85-no-it%92s-the-unobservable-universe

The limit on our observation being the age of the universe, the question is, Can we play a sort of a battleship game with the universe beyond our observation limit and map it in part. For example use principles such as entanglement of particles created during the big bang to deduce information about particles/bodies beyond observation by observing their observable siblings. This a question about theoretical possibilities. Can we observe a photon and it's polarity and deduce about an entangled photon beyond the observable universe and it's polarity?

Thanks in advance.

Grey
2015-Jun-20, 02:03 AM
This a question about theoretical possibilities. Can we observe a photon and it's polarity and deduce about an entangled photon beyond the observable universe and it's polarity?For this particular case, I don't think so. Generally speaking, to know that a photon is entangled with another photon somewhere requires a fair bit of information about the way it was produced in the first place; we generally don't have that kind of information about photons we observe from far away in the universe, and it's not clear that it would be possible even in principle.

It is true that there could be stuff which is now beyond the observable horizon which once was not beyond the observable horizon. Something like that could in principle have had some kind of measurable effect on objects that are still within the observable universe (even though anything that happens with this distant object now or in the future cannot have any effect that we'll ever see). But in practice, I don't think you could deduce anything meaningful about such hypothetical distant objects. It would be like trying to determine how many people were at a party last night by the air currents moving around in the house today.

In general, grant's responses to the questions in the previous thread were excellent.

a1call
2015-Jun-20, 07:43 PM
Hi Grey and thank you for your reply.
This being a follow up question please consider the following scenario:
Please keep in mind this is a theoretical question about possibility rather than practicality. In vicinity of earth a particle accelerator produces continuous opposing beams of entangled particles and photons. One beam is directed towards outer reaches of the universe while the other is preserved and kept close enough to be observed in future. After the passage of cosmological intervals observations of one beam will yield information about the the other which per previous question has passed the point of no return and is part of the un-observable universe. this might seem insignificant but as a theoretical principle opens the door to the possibility of more complex deductions about the un-observable universe.
IMHO

Grey
2015-Jun-20, 08:35 PM
Perhaps, but I'm not sure what information you really gain by that. Remember that the measurement on one particle of an entangled pair is completely unexciting. It's only when you look at the measurements on both particles that you find interesting correlations. Yes, in principle, measurements on one of a pair of entangled particle might constrain possible measurements on its partner, but note that you have to know that the partner exists in the first place. That is, if we came upon a collection of particles in carefully preserved states* from some alien experiment, no measurements on the particles themselves would give us the information that they had entangled partners somewhere. It's only if the alien records or apparatus tell you about their experiment that you'd even know that it involved entanglement. Nothing about the particle itself will give that away.

In principle, the same situation works even without the entanglement. If you launch a spaceship that eventually travels beyond the cosmic horizon, you now know something about the universe beyond the horizon (that it contains a spaceship, or at least the materials comprising the spaceship; it might have been destroyed after you can no longer see it). But if you were trying to figure out whether there were such a spaceship or not, when you didn't know about it in the first place, I don't think there's any realistic way to do that. Maybe possible in principle, in the same way that you might in principle be able to recreate the sounds of Bach playing the harpsichord from the motion of all the molecules in the atmosphere today, but not necessarily possible given the limits on information that we can gather.

I know that you're stressing possibility rather than practicality, but I think that's more of a continuum than you think. In some cases, it's possible to accurately determine the events of the past (and that's what the process you're proposing essentially is: detecting some past influence from an object no longer present) by examining the current state of things, but in other cases, there's simply not enough information available. In Newton's day, one might have said that it's possible to measure the position and motion of all particles with arbitrarily high precision, and that would (in principle only, of course) allow us to reconstruct the past and predict the future with equally high precision. But quantum mechanics puts definite limits on how much information we can have about the current state of the universe, and that puts definite constraints (not just practical constraints, but hard theoretical limits) on how accurate such a reconstruction can be.

* I'm not actually certain that it's possible to keep an entangled particle confined indefinitely without interfering with its entanglement. To keep it confined, it obviously has to interact in some ways with whatever is confining it, and those interactions may result in the loss of that entanglement. It may be possible, but it also may not be; I'm not familiar enough with all of the details to know for certain.

a1call
2015-Jun-20, 08:52 PM
Thank you Gray,
All I was hoping for was a theoretical possibility of existence of information about the un-observable universe, which I did not get from my last thread.
Thank you again for an unbiased and honest reply.

demeter
2015-Jun-20, 09:19 PM
In a faster than light expanding universe, is it possible to travel fast and far enough such that you are never able to return to your departure point or to even communicate with at the speed of light?
Thanks in advance.

I think you need a way to define unambiguously where the departure point is at a later time. I would propose the method antoniseb used in an earlier post.

a1call
2015-Jun-20, 09:47 PM
Please see my post number 12 for a more clear version of the question. The position of the departure point in the or actual universe is not significant for me other than when it involves a confirming example as suggested by Antoniseb.

demeter
2015-Jun-20, 10:30 PM
Please see my post number 12 for a more clear version of the question.

The place that was called "the departure point" is now called "Point A". That doesn't change the fundamental issue here.

An object is at "the departure point", and then it goes somewhere else. You are interested in whether the object can return to the departure point. Where precisely would the departure point be, after this time has passed?


The position of the departure point in the or actual universe is not significant for me other than when it involves a confirming example as suggested by Antoniseb.

If you don't have some coordinate system that tells you where the departure point is now, I don't see anyway of determining whether an object has returned to it or not.

I'd place an object there and decide that that object is "stationary", which is in effect what Antoniseb did.

a1call
2015-Jun-21, 12:34 AM
There is no universal coordinate system in the universe. Departure point can be anywhere at any inertial frame of reference. The answer to the question would be the same. Again the point A is arbitrary. The actual position is not significant and would not change the outcome unless you are inside a BH EH or something which would be an unnecessary and irrelevant complication.

demeter
2015-Jun-21, 01:08 AM
There is no universal coordinate system in the universe.

There is certainly no unique universal coordinate system in the universe, which is why some additional information is needed to determine whether one point at a particular time is the "same" as another point at some earlier time.


Departure point can be anywhere at any inertial frame of reference.

Now we have an answer, the question presumes that some inertial frame of reference (a sort of universal coordinate system) has been specified.

WayneFrancis
2015-Jun-22, 04:50 AM
The question is not about traveling FTL, or traveling using the expansion.
It's about traveling using thrust or even gravity assists at sub-c and being or not-being able to ever return at sub-c to the departure point, due to the space expansion along the path.

But when you are travelling at cosmic distances then expansion is part of your travel when you consider both your departure point and your destination. In theory you could depart Earth for a destination then during your trip there notice that both Earth and your destination are no longer within your cosmic event horizon.

WayneFrancis
2015-Jun-22, 04:57 AM
Hi Mkline,
That's a good way of thinking about this. Red shift not withstanding the pulses would arrive at larger and larger intervals. But would they ever stop? Why should they?

They would stop but as with an object falling through an event horizon of a black whole you'd never be sure that you got the last one. IE there will be a finite amount of pulses sent before the ship crosses a cosmic event horizon because of the cosmic expansion. The receiver would not be able to say what is the "last one" the last one Point A will receive. There could always have been one more at an even longer interval or there may never be another because that one was the actual last photon sent before the ship crossed the cosmic event horizon of Point A.

They will "stop" because there is a point where cosmic expansion, if it continues to accelerate or even stays constant, where the distance between the emitter and receiver are receding at a rate > c

WayneFrancis
2015-Jun-22, 05:13 AM
Hi Grey and thank you for your reply.
This being a follow up question please consider the following scenario:
Please keep in mind this is a theoretical question about possibility rather than practicality. In vicinity of earth a particle accelerator produces continuous opposing beams of entangled particles and photons. One beam is directed towards outer reaches of the universe while the other is preserved and kept close enough to be observed in future. After the passage of cosmological intervals observations of one beam will yield information about the the other which per previous question has passed the point of no return and is part of the un-observable universe. this might seem insignificant but as a theoretical principle opens the door to the possibility of more complex deductions about the un-observable universe.
IMHO

I may be getting stuff wrong here so take it with a grain of salt.
As your entangled particles shoot off in opposite directions while they are entangled there is no way to use such entanglement as a way to communicate to the location where the other particle now resides. All we can say is what the entire state of the particles is in. We can't alter one of the particles then from the other side "detect" that change and use it to transfer information. When ever I watch videos explaining why the entangled particles can't be used to communicate it makes sense but my brain dumps the details away leaving me unable to explain it well but the memory that it made sense at the time.
So I'm not sure how they could be used to transmit data over these long distances.

a1call
2015-Jun-22, 05:56 AM
Where did you see anything about communication using entanglement in this thread, that made you reply that it can't Be done?

glappkaeft
2015-Jun-23, 06:32 PM
Wayne might be using knowledge not present in this thread. His is correct though, see for instance https://en.wikipedia.org/wiki/No-communication_theorem

One simplistic way of illustrating entanglement can be made using a pair of shoes. A pair of shoes has the property that one shoe is the left and the other is the right so in a sense the shoes are entangled (if one shoe is the left the other must be the right).

You put the shoes in different boxes and mail one to New York (or if we want to be fanciful to the Andromeda Galaxy) and the other to Sydney. If the person in Sidney knows the shoes are entangled (both come from the same pair) and that the other package got sent to New York (or the Andromeda galaxy) the instant he opens the package and sees the left shoe he will then know that the person in New York (or the Andromeda galaxy) got the right shoe. This however cannot be used to transmit information.

Grey
2015-Jun-23, 06:57 PM
Wayne might be using knowledge not present in this thread. His is correct though, see for instance https://en.wikipedia.org/wiki/No-communication_theorem

One simplistic way of illustrating entanglement can be made using a pair of shoes. A pair of shoes has the property that one shoe is the left and the other is the right so in a sense the shoes are entangled (if one shoe is the left the other must be the right).

You put the shoes in different boxes and mail one to New York (or if we want to be fanciful to the Andromeda Galaxy) and the other to Sydney. If the person in Sidney knows the shoes are entangled (both come from the same pair) and that the other package got sent to New York (or the Andromeda galaxy) the instant he opens the package and sees the left shoe he will then know that the person in New York (or the Andromeda galaxy) got the right shoe. This however cannot be used to transmit information.This is pretty tangential to the thread, but I want to point out that this is a really uninteresting aspect of entanglement. In the case of the shoes as you describe it, it's a case of classical ignorance, and can be explained easily by assuming that the shoes have real properties, but we don't know what they are. The interesting bit about entanglement is that you can have situations where assuming that particles have definite real (but unknown) properties doesn't work to explain what we see. I wrote up an example (with shoes! ;)) way back in this thread (http://cosmoquest.org/forum/showthread.php?33993-Absolute-Truth&p=588171#post588171) to help illustrate the difference.

a1call
2015-Jun-23, 09:33 PM
Wayne might be using knowledge not present in this thread. His is correct though, see for instance https://en.wikipedia.org/wiki/No-communication_theorem

Correcting an error that is not present in the quoted text nor anywhere else in the entire thread and has nothing to do with subject of thread is posting Off-Topic. I generally don't mind wondering off in parallel but as a starter of this thread I find this subject unnecessarily clouding the original concept which has nothing to do with FTL travel nor communication.

Reality Check
2015-Jun-24, 12:36 AM
Hi a1call. Elsewhere (in this entire thread :D) is

...For example use principles such as entanglement of particles created during the big bang to deduce information about particles/bodies beyond observation by observing their observable siblings. This a question about theoretical possibilities. Can we observe a photon and it's polarity and deduce about an entangled photon beyond the observable universe and it's polarity?

So no derail - just attempts to answer the question you asked.

a1call
2015-Jun-24, 12:57 AM
Hi Reality Check,
How is observing an entangled photon and it's polarity, constituting communication?
There is no mention of this by me anywhere in this thread before being brought up by others.
This thread deals with something which I find very complicated as is and has not settled for me yet.
I wonder off in parallel myself all the time. But please, if anyone would like to discuss entanglement and communication then start your own thread on the subject.
Thanks.

WayneFrancis
2015-Jun-24, 03:40 AM
Where did you see anything about communication using entanglement in this thread, that made you reply that it can't Be done?

well if you are trying to get information space where one of the 2 entangled particles is and relay that information to the second particle then that is a transfer of information and thus communications. Where I got that it can't be done is every single description of entangled particles when that questions is asked.

https://en.wikipedia.org/wiki/No-communication_theorem

Am I missing something? I thought you wanted to know if you could use entangled particles to probe areas of space that are beyond an event horizon from this post (bold mine)


For the record here is the old thread:
http://cosmoquest.org/forum/showthread.php?108258-Hey-look!-It%92s-a-bird-it%92s-a-plane%85-no-it%92s-the-unobservable-universe

The limit on our observation being the age of the universe, the question is, Can we play a sort of a battleship game with the universe beyond our observation limit and map it in part. For example use principles such as entanglement of particles created during the big bang to deduce information about particles/bodies beyond observation by observing their observable siblings. This a question about theoretical possibilities. Can we observe a photon and it's polarity and deduce about an entangled photon beyond the observable universe and it's polarity?

Thanks in advance.

WayneFrancis
2015-Jun-24, 03:49 AM
Hi Reality Check,
How is observing an entangled photon and it's polarity, constituting communication?
There is no mention of this by me anywhere in this thread before being brought up by others.
This thread deals with something which I find very complicated as is and has not settled for me yet.
I wonder off in parallel myself all the time. But please, if anyone would like to discuss entanglement and communication then start your own thread on the subject.
Thanks.

Then why are we concerned if it is entangled or not? Why do we care where the other particle is? I'm a bit lost now as to what you are actually asking.
Can we observe an entangled particle regardless of where its counter part is? Yes, sure, so what? I'm not sure what type of information you are trying to get from it and I guess that is my question.

a1call
2015-Jun-24, 04:29 AM
Getting information about the polarity and spin off the particles and photons after they have passed the point of no return by observing the random spins and polarities of their entangled counterpart without influencing these parameters so no communications. Communication was never stated nor implied by me anywher on this thread and it is an error on your part to correct me on something I have not stated nor implied.

WayneFrancis
2015-Jun-24, 05:02 AM
Getting information about the polarity and spin off the particles and photons after they have passed the point of no return by observing the random spins and polarities of their entangled counterpart without influencing these parameters so no communications. Communication was never stated nor implied by me anywher on this thread and it is an error on your part to correct me on something I have not stated nor implied.

I did say that I may be missing something. So why do you want to get information about the polarity and spin of an entangled particle? What information are you trying to deduce beyond that there is an entangled particle out there some where that you can tell what some of its properties are?

a1call
2015-Jun-24, 05:12 AM
The whole concept of this thread is to discuss the possibility of existence of information about the un-observable universe in the observable universe. As I pointed out earlier observing the random parameters of entangled particles and photons might seem insignificant but if indeed possible it would open the door to the possibility of more Complex deductions about the un-observable universe. This is because this would prove that deductions about the un-observable universe are not impossible as a rule of nature.
ETA BTW I still have my doubts about the possibility of reaching the point of no return. I need to have time to wrap my head around all the different horizons involved. I really wished Dr. H. would make an exception and weigh in here.

WayneFrancis
2015-Jun-25, 01:36 AM
The whole concept of this thread is to discuss the possibility of existence of information about the un-observable universe in the observable universe. As I pointed out earlier observing the random parameters of entangled particles and photons might seem insignificant but if indeed possible it would open the door to the possibility of more Complex deductions about the un-observable universe. This is because this would prove that deductions about the un-observable universe are not impossible as a rule of nature.
ETA BTW I still have my doubts about the possibility of reaching the point of no return. I need to have time to wrap my head around all the different horizons involved. I really wished Dr. H. would make an exception and weigh in here.

How do you even know where the other entangled particle is? If it is within or outside our cosmic event horizon? I'm really trying to get my head around what you want to do.
If you want to get "information about the un-observable universe in the observable universe" and you want to use "entangled particles" then if you aren't talking about communication then we are left with you just wanting to know the state of an entangled pair. This is doable regardless of where the particles are but you can not tell where the other particle is. I'm not even sure you can tell it is an entangled particle. Without knowing the particle was entangled in the first place and not having access to the second particle. I'm not sure what can be achieved. Are you just saying there might be science we don't know about that could give us access to information to things we currently believe are, in principal using current science, unknowable?