Trying to understand what "time" is (layperson)

[Josh S]

TL;DR Summary

I've been trying to get the right conceptual understanding of what time is according to relativity, and I was hoping that some of the very educated people here could lmk if my understanding is correct.

Hi Everyone,

Just a disclaimer, I'm not a physicist or a physics student, just a layperson with an interest in physics. So please forgive any lack of understanding on my part :).

Anyway, I recently started trying to gain a better understand of what time is in the real world, because some folks I've talked to always bring up sci-fi concepts like FTL travel or FTL communication as if these things will one day inevitably happen. Basically the idea that we will one day have long distance interstellar travel that isn't subject to time dilation. After thinking about time for a bit, I'm honestly not so sure, and I wanted to see if any of the very educated people here could let me know if my understanding is on the right track.

To explain where I'm coming from, imagine there is an alien world 100LY away from earth. People will often say that were you to look at this world through a telescope, you would be observing it 100 years in the past. But since time is relative, you have to ask, 100 years in the past relative to WHO? It's almost impossible to answer this question, because the "100 years in the past" phrase kind of implies an objective reference to time that just doesn't exist. For example, you can't say that the people on that alien world are actually experiencing time 100 years in the "future" from what you're observing 100LY away, because that implies some kind of universal time.

And if you try to push this idea further, imagine if you COULD just teleport instantly to this alien world 100LY away, and arrive instantly in the time frame that you have observed from earth. Well if you did that, then if you were to observe earth from the alien world, it would appear to be 100 years in the past, relative to when you left it...so if you left earth in 2000AD, you would look back at earth through the telescope and it would be 1900AD. Further, if you then instantly traveled back to earth, you would literally be time traveling to the past, and this to me, just seems to break any idea of internal logic that the universe, so I don't think this idea of instant travel, or travel that somehow avoids time dilation is possible.

I also think that our idea of "time" as a fundamental thing that exists and gets "bent or distorted" may be flawed. It seems more intuitive that the only thing that really exists is objects interacting with one another, and the speed of light as some kind of "cap" on how quickly these interactions can occur. For example, even the time on a clock is just the result of objects interacting with one another, the gears inside moving, etc. And all time is, is a concept that we humans invented to help us keep track of how many of these interactions have occurred.

Does this understanding sound like it's relatively (lol) in line conceptually with general relativity?

 

[Dale]

There are two distinct concepts of time in physics.

Proper time: this is the time measured on a clock, traditionally labeled $\tau$. It is the physical time that describes the progression of any physical process. In relativity it is described mathematically as the spacetime interval along the worldline of a clock. It is not defined off of the worldline.

Coordinate time: this is the coordinate that is traditionally labeled $t$. This is nothing physical and only exists in a mathematical coordinate system. It is defined throughout a coordinate chart and is used to define the notion of simultaneity.

 

[Josh S]

There are two distinct concepts of time in physics.

Proper time: this is the time measured on a clock, traditionally labeled $\tau$. It is the physical time that describes the progression of any physical process. In relativity it is described mathematically as the spacetime interval along the worldline of a clock. It is not defined off of the worldline.

Coordinate time: this is the coordinate that is traditionally labeled $t$. This is nothing physical and only exists in a mathematical coordinate system. It is defined throughout a coordinate chart and is used to define the notion of simultaneity.

Thanks Dale! This terminology is actually really helpful for me to try to explain what I’m trying to ask better. So if you wouldn’t mind indulging me further, I would like to express my question a bit better with two scenarios…

scenario 1: Bob and Mary both live in the same neighborhood. When we talk about the worldline for proper time here, both Bob and Mary are on the worldline that represents earth’s rotation around the sun. So if Bob were to bite into a burger at precisely 12:30pm, and Mary were to bite into a steak at the same exact time, then most people would agree that Bob and Mary are both simultaneously having the EXPERIENCE of eating at the same exact time. Not just that Bob can observe Mary eating at the same time he is, but they are both having the internal experience of eating at the same time.

Scenario 2: Bob lives on earth, and Mary lives on Arrakis, a planet 100LY away from earth. When we talk about proper time here, Bob and Mary are on completely different world lines. Now let’s say that here, Bob is watching Mary through the live feed of an incredibly powerful telescope. And he takes a bite of his burger at the same moment that he observes Mary taking a bite of her steak. In this scenario though, could you really say that Mary experienced biting into the steak at the same time that Bob bit into his burger? I feel like you could say that Bob observed Mary biting into her steak at the same time, but can you really say that they both experienced eating at the same time? I mean does that statement even make sense with two subjects on different world lines?

 

[Dale]

both Bob and Mary are on the worldline that represents earth’s rotation around the sun

Sure, as long as we are modeling the earth as a point.

A worldline is just relativity terminology for the line in spacetime formed by a point particle’s position as a function of time. It doesn’t require a whole world, but it can refer to a whole world if you are treating the world as a point.

In this scenario though, could you really say that Mary experienced biting into the steak at the same time that Bob bit into his burger?

Such a statement would be a statement about coordinate time, not proper time. So you could make it, but in order to make it you would have to specify the coordinates. Most of the problems with relativity “paradoxes” come from making such statements without specifying the coordinates.

 

[Josh S]

Sure, as long as we are modeling the earth as a point.

Such a statement would be a statement about coordinate time, not proper time. So you could make it, but in order to make it you would have to specify the coordinates. Most of the problems with relativity “paradoxes” come from making such statements without specifying the coordinates.

Hmmm but if we’re talking about coordinate time, we’re only talking about an abstract mathematical concept correct? So statements like “Bob and Mary both experienced biting into their food at the same time” don’t really make any sense, because here, we’re talking about two separate people experiencing something at the same time, which is more a statement about reality as opposed to just a mathematical model.

Anyway, my overall point is that I don’t feel like it’s possible to have a shared frame of reference regarding time for two entities that are very far apart. And this is why it seems like trying to say that Bob and Mary are both doing something simultaneously when they are light years away seems nonsensical.

 

[PeterDonis]

Bob and Mary both live in the same neighborhood.

That's not sufficient if they are going to have the same worldline. They need to be at exactly the same place, for all time.

When we talk about the worldline for proper time here, both Bob and Mary are on the worldline that represents earth’s rotation around the sun.

Unless you treat the whole Earth as a point, as @Dale said, this won't work. But treating the whole Earth as a point makes all questions about whether Bob and Mary do or experience anything "at the same time" meaningless--they are by definition at exactly the same place for all time, and therefore anything that happens at that place happens to both of them at the same instant, by definition.

In other words, if you construct your model this way, you are by definition ruling out any interesting questions of the sort you appear to want to ask. So this model is not a good one for you to choose.

I think the model you actually have in mind for your scenario #1 is more like this: Bob and Mary live in the same neighborhood on Earth, but they are not always exactly at the same place. If Bob takes a bite of a burger in his house at 12 noon exactly by his clock, and Mary takes a bite of a burger in her house at 12 noon exactly by her clock, then assuming their clocks are synchronized in the way that clocks in Earth people's homes both in the same time zone usually are, it is fine to say that Bob and Mary took their bites at "the same time" according to the coordinate system that defines the behavior of Earth clocks in their time zone. But this is still a coordinate time, not a proper time. It is impossible to say that two things happen "at the same" time purely using proper time unless the things happen at exactly the same place, i.e., on exactly the same worldline. That is the general answer to questions of the sort you are trying to ask.

However, there are cases where we can give a definite negative answer to questions about things happening "at the same time" without having to choose any coordinates. For example:

Bob is watching Mary through the live feed of an incredibly powerful telescope. And he takes a bite of his burger at the same moment that he observes Mary taking a bite of her steak. In this scenario though, could you really say that Mary experienced biting into the steak at the same time that Bob bit into his burger?

No. Why not? Because the two events in question, Mary taking her bite and Bob taking his bite, are connected by a light ray. That means they are null or lightlike separated, not spacelike separated. And in order for there to be any coordinates in which two events can be said to happen "at the same time", the two events must be spacelike separated.

In the case you describe here, the event of Mary taking her bite is in the past--the causal past--of the event of Bob taking his bite. That is true regardless of how you choose coordinates. The causal structure of spacetime is invariant, independent of any choice of coordinates.

statements like “Bob and Mary both experienced biting into their food at the same time” don’t really make any sense

Sure they do. However, they make sense as a matter of convention, not a matter of physics. In other words, if I tell you that Bob did X "at the same time" as Mary did Y, where Bob and Mary are not in the same place, I am actually telling you nothing about the physics of the situation. All I am actually telling you anything about is my choice of coordinates, which is a convention. That doesn't mean my statement is meaningless; it just means it's a statement about coordinates, not about physics.

we’re talking about two separate people experiencing something at the same time, which is more a statement about reality

No, it is not. That is the whole point. No statement about two separate people, not in the same place, experiencing something at the same time, is a statement about "reality". All such statements are only statements about coordinates, i.e., about an abstract mathematical convention adopted by humans.

my overall point is that I don’t feel like it’s possible to have a shared frame of reference regarding time for two entities that are very far apart.

Sure it is. A "shared frame of reference" is just a system of coordinates that both entities agree to use. There is nothing preventing them from making such an agreement.

And this is why it seems like trying to say that Bob and Mary are both doing something simultaneously when they are light years away seems nonsensical.

"Nonsensical" is the wrong word. See above.

 

[Dale]

Hmmm but if we’re talking about coordinate time, we’re only talking about an abstract mathematical concept correct?

Yes.

So statements like “Bob and Mary both experienced biting into their food at the same time” don’t really make any sense

They make sense, but they are mathematical conventions, not physical facts.

we’re talking about two separate people experiencing something at the same time, which is more a statement about reality as opposed to just a mathematical model.

That is incorrect. Whenever you talk about “at the same time” for spatially separated events you are talking about a mathematical convention, not anything physical. The only physical “at the same time” is also “at the same place”.

For example a collision. If they were in different vehicles and both bit into their respective meals and collided while doing so, then that would be a physical “at the same time”.

If they were spatially separated while eating then there is no physical meaning to it. It is purely a mathematical model. No physical experiment could be performed whose outcome could only be explained by a mathematical model where the experiences were simultaneous.

Anyway, my overall point is that I don’t feel like it’s possible to have a shared frame of reference regarding time for two entities that are very far apart. And this is why it seems like trying to say that Bob and Mary are both doing something simultaneously when they are light years away seems nonsensical.

They can choose to use a shared mathematical model. They just are not required to do so by physics.

 

[Josh S]

That's not sufficient if they are going to have the same worldline. They need to be at exactly the same place, for all time.Unless you treat the whole Earth as a point, as @Dale said, this won't work. But treating the whole Earth as a point makes all questions about whether Bob and Mary do or experience anything "at the same time" meaningless--they are by definition at exactly the same place for all time, and therefore anything that happens at that place happens to both of them at the same instant, by definition.

In other words, if you construct your model this way, you are by definition ruling out any interesting questions of the sort you appear to want to ask. So this model is not a good one for you to choose.

I think the model you actually have in mind for your scenario #1 is more like this: Bob and Mary live in the same neighborhood on Earth, but they are not always exactly at the same place. If Bob takes a bite of a burger in his house at 12 noon exactly by his clock, and Mary takes a bite of a burger in her house at 12 noon exactly by her clock, then assuming their clocks are synchronized in the way that clocks in Earth people's homes both in the same time zone usually are, it is fine to say that Bob and Mary took their bites at "the same time" according to the coordinate system that defines the behavior of Earth clocks in their time zone. But this is still a coordinate time, not a proper time. It is impossible to say that two things happen "at the same" time purely using proper time unless the things happen at exactly the same place, i.e., on exactly the same worldline. That is the general answer to questions of the sort you are trying to ask.

However, there are cases where we can give a definite negative answer to questions about things happening "at the same time" without having to choose any coordinates. For example:No. Why not? Because the two events in question, Mary taking her bite and Bob taking his bite, are connected by a light ray. That means they are null or lightlike separated, not spacelike separated. And in order for there to be any coordinates in which two events can be said to happen "at the same time", the two events must be spacelike separated.

In the case you describe here, the event of Mary taking her bite is in the past--the causal past--of the event of Bob taking his bite. That is true regardless of how you choose coordinates. The causal structure of spacetime is invariant, independent of any choice of coordinates.Sure they do. However, they make sense as a matter of convention, not a matter of physics. In other words, if I tell you that Bob did X "at the same time" as Mary did Y, where Bob and Mary are not in the same place, I am actually telling you nothing about the physics of the situation. All I am actually telling you anything about is my choice of coordinates, which is a convention. That doesn't mean my statement is meaningless; it just means it's a statement about coordinates, not about physics.No, it is not. That is the whole point. No statement about two separate people, not in the same place, experiencing something at the same time, is a statement about "reality". All such statements are only statements about coordinates, i.e., about an abstract mathematical convention adopted by humans.Sure it is. A "shared frame of reference" is just a system of coordinates that both entities agree to use. There is nothing preventing them from making such an agreement."Nonsensical" is the wrong word. See above.

Thanks for your detailed response!

I know that my language is clumsy here, but I think I did get the answer I was looking for, which is basically that FTL travel, like we see in sci-fi media where people travel halfway across the galaxy and then back with no apparent time dilation, just as if they were flying from New York to Houston, is impossible. Not just impossible with our current technology, just impossible period.

 

[PeterDonis]

FTL travel, like we see in sci-fi media where people travel halfway across the galaxy and then back with no apparent time dilation, just as if they were flying from New York to Houston, is impossible

More precisely, if our current theory of relativity in its entirety is correct, including the claim that the principle of relativity holds for all physical phenomena, then "FTL travel" would have to allow closed causal curves, and closed causal curves are generally believed to be impossible.

It's worth noting, however, that not all physicists agree that closed causal curves are impossible. Kip Thorne and Igor Novikov are two physicists who have published papers on the topic; Googling should find them. Thorne has a fairly good layman's discussion in one of the later chapters of his book Black Holes and Time Warps. The basic idea is that as long as every actual event is fixed and well defined (i.e., what happens at that event is the same no matter how many times a given worldline, such as your own, passes through it), there is actually no problem (though there might be serious constraints on things like human free will that we generally find implausible--but "implausible" is not the same as "physically impossible"). For a classic fictional description of such a scenario, see Heinlein's story By His Bootstraps.

 

[Josh S]

More precisely, if our current theory of relativity in its entirety is correct, including the claim that the principle of relativity holds for all physical phenomena, then "FTL travel" would have to allow closed causal curves, and closed causal curves are generally believed to be impossible.

It's worth noting, however, that not all physicists agree that closed causal curves are impossible. Kip Thorne and Igor Novikov are two physicists who have published papers on the topic; Googling should find them. Thorne has a fairly good layman's discussion in one of the later chapters of his book Black Holes and Time Warps. The basic idea is that as long as every actual event is fixed and well defined (i.e., what happens at that event is the same no matter how many times a given worldline, such as your own, passes through it), there is actually no problem (though there might be serious constraints on things like human free will that we generally find implausible--but "implausible" is not the same as "physically impossible"). For a classic fictional description of such a scenario, see Heinlein's story By His Bootstraps.

Interesting! Also, I'm just curious, because this is what's really throwing me for a loop. If you were able to travel to a distant planet nearly instantly using some kind unknown method, "when" relative to the world you traveled to, would you arrive there?

Because I feel like in order to define that "when" there has to be some kind of shared frame of reference between where you are coming from and where you are going, and this is what I was getting at before.

 

[PeterDonis]

If you were able to travel to a distant planet nearly instantly using some kind unknown method, "when" relative to the world you traveled to, would you arrive there?

That would depend on your choice of coordinates.

in order to define that "when" there has to be some kind of shared frame of reference between where you are coming from and where you are going

Any valid coordinate chart will work as a "shared frame of reference" as long as both places choose to adopt it.

 

[phinds]

@Josh S I think that it is possible that some of your confusion is that you haven't quite got the concept of "frame of reference". For example, and I may be wrong here, it seems you might believe that I, sitting in my chair typing this, cannot be in the same frame of reference as a person sitting in a plane flying over my house.

That would not be correct. I am in that person's frame of reference and he is in mine. In his frame of reference, I am moving under him at 300 mph and in my frame of reference, he is moving above me at 300mph.

We are both, at the same time, in the frame of reference of someone up in the space station and they are in each of ours. In all three of these rest frames, the motion of the three of us will be described differently, but that does not imply that we are not in each others reference frames.

 

[PeterDonis]

To add to what @phinds posted in #12, there is also no requirement that everyone must use a frame of reference in which they are at rest. For example, @phinds, the pilot of the plane flying over his house, and the astronaut in the space station, for many purposes would typically use a common frame of reference in which the Earth is at rest. Depending on whether this frame was an Earth-centered inertial frame or an Earth-centered rotating frame (i.e., rotating with the actual Earth), either none of them might be at rest in the frame, or only @phinds would be (at least as long as he is sitting at his desk rather than moving around his house or going out somewhere).

 

[Josh S]

@Josh S I think that it is possible that some of your confusion is that you haven't quite got the concept of "frame of reference". For example, and I may be wrong here, it seems you might believe that I, sitting in my chair typing this, cannot be in the same frame of reference as a person sitting in a plane flying over my house.

That would not be correct. I am in that person's frame of reference and he is in mine. In his frame of reference, I am moving under him at 300 mph and in my frame of reference, he is moving above me at 300mph.

We are both, at the same time, in the frame of reference of someone up in the space station and they are in each of ours. In all three of these rest frames, the motion of the three of us will be described differently, but that does not imply that we are not in each others reference frames.

Thanks for the reply! What you're saying makes total sense, and I think I've been kind of clumsy trying to explain my confusion/question here. Where all of this came from was FTL travel in scifi media, so let me try to use a hypothetical from that to better explain my concern.

Assume that Han Solo is on Tatooine, and Lando is on Bespin, Tatooine is 1LY from Bespin. In the morning, Han decides to visit Lando, he hops in the Millenium Falcon, engages hyperdrive and arrives on Bespin in a few minutes. They chat for a while, then Han remembers he forgot to put his clothes in the dryer, tells Lando he's going to do that and be back in an hour. Han then uses his hyperdrive to go back to Tatooine, does his laundry, then comes back to Bespin in exactly an hour, where Lando is waiting for him...one hour passing for each of them.

What I'm describing here adheres to the internal logic of just about every piece of popular scifi media. And it makes sense that it does because it reflects how all of we humans experience life and time on the earth. In Star Wars, the Empire is moving the Death Star to Endor at the same time that the Rebellion is flying its fleet there, even though they are several light years away. Just like how in the real world, Germany is planning a defense of its beaches, at the same time that the allies are preparing to launch D-Day. In both of these examples, events are assume to happen de facto simultaneously.

Now at least to me, the biggest theoretical problem with something like the FTL we see in scifi media existing in the real world is that they all have an implicit assumption that there is a "universal" timeline (ie frame of reference) that is shared between everyone. This is how they explain Han Solo being able to travel multiple light years to another planet, but still arrive in minutes to the perspective of the people living on that planet. Because no matter how far apart things are, they still have an effectively shared temporal experience, just like we experience on earth.

But according to relativity, this can't be the case. And even if there were some super technology that allowed you to traverse space incredibly quickly, you still couldn't "escape" time dilation like they do in scifi media, because there is no objective, shared timeline that everyone in the universe is "really" on. If you were to travel to a very distant planet "instantly," that instantly is just completely relative to you...not the people on that planet, so you could never do something like I described in my first hypothetical about Han visiting Lando.

This is basically what I'm trying to get at. That FTL travel, as portrayed in scifi media, is impossible because it implicitly assumes a shared/objective timeline throughout the entire universe.

 

[phinds]

This is basically what I'm trying to get at. That FTL travel, as portrayed in scifi media, is impossible because it implicitly assumes a shared/objective timeline throughout the entire universe.

Well, yeah, that's one reason, but it's really more fundamental than that. Nothing can travel faster than light.

 

[PeterDonis]

That FTL travel, as portrayed in scifi media, is impossible because it implicitly assumes a shared/objective timeline throughout the entire universe.

No, it doesn't. It just assumes that all of the FTL travel devices work in a particular way, such that no closed causal curves or paradoxical situations are ever created. That doesn't require "a shared/objective timeline throughout the entire universe". It might, however, violate the principle of relativity, depending on the details of how the FTL travel devices implement the restriction described above. But of course none of the scifi media you reference ever go into that level of detail. They're just trying to tell a good story; they're not trying to construct a theory of physics.

 

[PeterDonis]

Nothing can travel faster than light.

More precisely, we have no evidence that anything can travel faster than light, and the obvious way to implement that in a relativistic theory is to required that all causal curves must be either null or timelike.

However, there have been some theoretical explorations of what would be implied if we relaxed that restriction. I referenced a few in a previous post. The classic tachyon papers also fall into this category. So does Bell's classic "tachyonic antitelephone" paper.

 

[Josh S]

Well, yeah, that's one reason, but it's really more fundamental than that. Nothing can travel faster than light.

True, but I think that a lot of people believe that there may be ways to "cheat" this, like finding some way to get form A to B without actually traversing space (wormhole or something), and most scifi goes in this direction (oh we are folding space). I guess what I'm saying is that the issue is bigger than that, the issue is that the conception that there is a shared "now" between distant worlds is incorrect. So even if you could instantly travel to a distant world, there would be no defined "now" for you to arrive there.

In our every day lives, we believe that here is a shared now because it reflects our lived experience, but really, this is kind a special case because we happen to live so close to one another...and even then, it's just a "de facto" shared now.

 

[PeterDonis]

I think that a lot of people believe that there may be ways to "cheat" this

And the vast majority of things people say and believe along these lines are off topic here, because they are not actual peer-reviewed physics papers, they're either fiction or uninformed speculation. The only valid basis for discussion here is what is in the actual peer-reviewed literature; I have referenced some relevant papers in prior posts.

I guess what I'm saying is that the issue is bigger than that, the issue is that the conception that there is a shared "now" between distant worlds is incorrect.

While the statement that "now" at spatially separated locations has no physical meaning in a relativistic theory is correct, it is not a relevant issue in the actual theoretical explorations that have been made of FTL travel. So your stated concern here is misplaced. I have tried to state what the actual issues are in previous posts.

 

[PeroK]

This is basically what I'm trying to get at. That FTL travel, as portrayed in scifi media, is impossible because it implicitly assumes a shared/objective timeline throughout the entire universe.

It's impossible in the same way that Middle Earth, elves and hobbies are impossible given the geological and anthropological history of the Earth. And whether Gandalf operates within the laws of physics is another moot point.

 

[Josh S]

It's impossible in the same way that Middle Earth, elves and hobbies are impossible given the geological and anthropological history of the Earth. And whether Gandalf operates within the laws of physics is another moot point.

Of course whether fictional characters act within the laws of physics is a moot point, but that’s not really what I’m saying. I could be wrong, but I’m pretty sure that most people view interstellar travel, as portrayed in sci-fi media, as something that one day could happen.

There’s even a famous paradox about why we haven’t encountered interstellar aliens yet (the Fermi paradox). There’s an organization dedicated to searching for extraterrestrial life. There was just a congressional hearing about “UFO’s.” People think that the possibility of interstellar aliens isn’t just possible, it’s relatively plausible.

But what I’m saying is that realizing that there is likely flat out no way to go jump around the galaxy like they do in scifi, and you’re always going to be subject to time dilation, makes interstellar aliens (including even future humans) incredibly implausible. I mean consider that there’s about 10 star systems within 10LY of earth, and it’s probably really unlikely that one of them houses super intelligent aliens.

So realistically, if there were aliens that lived 10LY away from earth, going to earth and back even with super technology, would make very little sense. Even if they could travel to earth near instantaneously relative to them, by the time they got back to their home world, a minimum of 20 years would have passed there. So any need that they were trying to fulfill for their home world would have likely long expired, considering at minimum, it would be a 20 year journey.

The point is that relative time just makes interstellar travel pretty useless. What’s the point in going to a world 10LY away, when you know you won’t even be able to communicate with your home world, and even if you return, decades would have passed there.

 

[PeterDonis]

I’m pretty sure that most people view interstellar travel, as portrayed in sci-fi media, as something that one day could happen.

This is (a) not supported by any evidence, and (b) irrelevant to this thread.

There’s even a famous paradox about why we haven’t encountered interstellar aliens yet (the Fermi paradox).

Which has nothing whatever to do with anything we're discussing in this thread. Or even with the other (off topic) issues you raise in your post.

realizing that there is likely flat out no way to go jump around the galaxy like they do in scifi, and you’re always going to be subject to time dilation, makes interstellar aliens (including even future humans) incredibly implausible.

Nonsense. An alien intelligence could be billions of years older than we are. That is plenty of time to explore an entire galaxy even at sub-light speeds. There is plenty of discussion of this case in the literature. It's even enough time for aliens to come from some other galaxy to ours.

I mean consider that there’s about 10 star systems within 10LY of earth, and it’s probably really unlikely that one of them houses super intelligent aliens.

You do realize that our galaxy is a lot bigger than 10 LY, right? And that the universe as a whole is a lot bigger than that?

realistically, if there were aliens that lived 10LY away from earth, going to earth and back even with super technology, would make very little sense.

That depends a great deal on the aliens. If they had very long life spans compared to the 20 year round trip travel time, it might make a great deal of sense to make such trips.

In any case, all of this is speculative and off topic for this thread.

The point is that relative time just makes interstellar travel pretty useless.

This is your unsupported (and IMO uninformed) opinion, and in any case is off topic for this thread.

The actual thread topic has been discussed sufficiently at this point, and the thread is veering off into off topic speculation. Therefore, this thread is closed.