# Noob question about relativity

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Hey Guys! First time poster here. I've had this question for a while now and I can't seem to find the answer online.

So whenever relativity and time dilation is explained they start out by saying that there is no absolute motion, objects only move in relation to each other. The very next thing you see if that "one person is standing still, while another is moving close to the speed of light..." which directly contradicts the first point. If one person is "standing still" and another is "traveling near light speed" but there is no absolute motion, only motion is relation to each other, then it is just as valid to say that the person who was "standing still" was actually traveling near light speed in relation to the other person.

I often hear something like: "When Bill gets into a spaceship and flies around near light speed for only a few minutes, he comes home to find his twin brother Bob has aged many years."

But here's my question: WHY couldn't we say that Bill was "standing still" while Bob and the Earth were "moving around near light speed" in relation to Bill and therefore BILL is the older twin now????

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This is not something that can be answered fully without a great deal of explanation and building of ground work.

But the main gist is that while motion is relative and we can't say which of the two are moving at any given moment, acceleration is not. The trick is that Bill has to "come home" he has to undergo an acceleration in order for Bob and him to meet up again. This is what ends up determining which the two has aged more. (If on the other hand, if Bob had jumped into a rocket and set off after Bill, when he catch up to him, he will be the one that has aged less.

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But if there is no absolute speed, only speed relative to another body, wouldn't that mean that acceleration is relative as well? If Bill "accelerated" away from Bob, couldn't it be said that Bob "accelerated" away from Bill?

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But if there is no absolute speed, only speed relative to another body, wouldn't that mean that acceleration is relative as well? If Bill "accelerated" away from Bob, couldn't it be said that Bob "accelerated" away from Bill?

No, acceleration (proper acceleration , more correctly said) is absolute. You can measure it with an accelerometer, so you know exactly who's accelerating and who's not.

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So whenever relativity and time dilation is explained they start out by saying that there is no absolute motion, objects only move in relation to each other. The very next thing you see if that "one person is standing still, while another is moving close to the speed of light..." which directly contradicts the first point. If one person is "standing still" and another is "traveling near light speed" but there is no absolute motion, only motion is relation to each other, then it is just as valid to say that the person who was "standing still" was actually traveling near light speed in relation to the other person.

This part is correct. Either observer can be thought of as being still, so it's not a preferred frame. "Still" in relative to the one that is moving — an observer is considered to be at rest in his/her own frame.

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Just to add to what has already been said. you can tell if you are accelerating, but you can't tell if you are moving. Imagine you wake up in a train car with the windows black out. This is a specially designed train that runs so smoothly that you can cannot feel nor hear the wheels on the track. Neither is there anyway for you to tell the front from the back of the train.

There is no way for you to tell if the train is moving or not. If you feel a force pushing to one end of the train or the other, you know that the train has changed speed, but you won't know how. was the train already moving at 30 kph and slowed to 25, or did it speed up to 35, or maybe it was at rest and started moving at 5 kph? You know that you are moving at a different speed than you were before you accelerated, but not what your starting and ending speeds were.

You could have an entire lab of test equipment with you and it would not be able to detect any difference between before and after acceleration, but they would register a difference between accelerating and not accelerating.

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OK yes that actually does make sense to me, I should have thought about the force felt from acceleration. So speed is never absolute, only relative, but acceleration is real.

Still, i'm confused about time dilation. Einstein FIRST came up with general relativity, which correct me if I'm wrong, only has to do with speed, NOT acceleration. It wasn't until special relativity that he addressed changing speeds. Right?

So lets ignore the acceleration part for a minute. Imagine a woman is in labor on a spaceship going 95% light speed. She gives birth to baby Bill right as they pass earth. There is another woman on earth who gives birth to baby Bob right as the spaceship passes overhead. The spaceship continues on at 95% light speed (relative to the earth of course).

30 years passes on earth and Bob is now 30 years old. How old is Bill?

According to explanations I have seen, Bill would be only a few minutes or hours or perhaps days old, because time would be much slower for him traveling near light speed. I always see diagrams showing that as light hits off the console of the spaceship and moves towards Bill's eyes it has to travel a great distance because the console and Bill are moving laterally, so the light is taking a very long hypotenuse route between objects. It makes sense, but only if that movement i.e. speed, is real.

Back on earth, light moves from a desk surface to Bob's eyes, but there is no lateral movement of Bob or Desk at any significant percentage of light speed. Again, this makes sense to me except that relative to Bill, Bob and the desk ARE going 95% light speed. So when we take into account "there is no speed but relative speed" it seems like Bob would age quickly from Bill's perspective, and Bill would age quickly from Bob's perspective. Which is just silly because that means that When Bob is 30 he thinks Bill is only minutes old, and when Bill is 30 he thinks Bob is only minutes old.

Again, it all makes quite a lot of sense to me if we admit that one person is ACTUALLY moving near light speed and the other is ACTUALLY motionless. But if speed is only relative then I don't understand time dilation.

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Still, i'm confused about time dilation. Einstein FIRST came up with general relativity, which correct me if I'm wrong, only has to do with speed, NOT acceleration. It wasn't until special relativity that he addressed changing speeds. Right?

Wrong. Have you tried actually studying the subject?

So lets ignore the acceleration part for a minute. Imagine a woman is in labor on a spaceship going 95% light speed. She gives birth to baby Bill right as they pass earth. There is another woman on earth who gives birth to baby Bob right as the spaceship passes overhead. The spaceship continues on at 95% light speed (relative to the earth of course).

30 years passes on earth and Bob is now 30 years old. How old is Bill?

According to explanations I have seen, Bill would be only a few minutes or hours or perhaps days old, because time would be much slower for him traveling near light speed.

Wrong again. Bill is also 30 years old. Look, you are grappling with the so-called "Twin Paradox". Understanding it requires a fairly good understanding of special relativity. You do not have that.

So when we take into account "there is no speed but relative speed" it seems like Bob would age quickly from Bill's perspective, and Bill would age quickly from Bob's perspective. Which is just silly because that means that When Bob is 30 he thinks Bill is only minutes old, and when Bill is 30 he thinks Bob is only minutes old.

Here you are struggling with mutual time dilation, or the so-called "Dingle paradox". This is not a paradox , it stems from the lack of understanding of basic SR.

But if speed is only relative then I don't understand time dilation.

The twins "paradox" that you are grappling with has NOTHING to do with time dilation. You have way too many misconceptions about relativity, you should consider a class. Start slowly, from the basics, only AFTER that you can tackle the more advanced subjects.

Edited by xyzt
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That was annoyingly arrogant, incredibly rude, and profoundly unhelpful. It was arrogant to act so high and mighty and "look how smart I am! I know the name of this paradox." It was rude to keep reiterating how little I know about this subject with no compassion or helpfulness to temper your remarks. I can forgive those. I know from experience that sometimes when you talk to people who know more than you do you encounter arrogance. I can deal with that. I have a thick skin and I can handle people being rude to me. Not a problem at all. But the 3rd sin in really unforgivable in a forum like this. Why even come here to be rude and arrogant if you aren't going to offer something helpful as well?

When I first posted my question I got some great answers and I really appreciated them. I'd like to hear more from those people.

Remember, I admitted in the beginning that I don't know much about these things. It doesn't help to point that out to me.

I'm not trying to be clever and trap anyone here, I'm asking sincere questions. The fact that these parodoxes have names simply means that other people have had these questions before me and it would be great if someone would explain them to me rather than tell me how stupid I am for questioning them.

I was mistaken about the difference between special relativity and general relativity, I got them mixed up. (and I said "correct me if I'm wrong") But rather than politely pointing that out I was treated like I was stupid and I had to go find that out elsewhere.

But my questions remain.

If someone is traveling at light speed relative to earth (but not accelerating) does he age slower than someone on earth? If so, why?

I don't want to know the name of the paradox, and I don't want to be told I should study the subject. I am studying the subject, and I'm asking questions of people who I assume know a lot more about it than I do. If you are interested in helping me understand please do so, if you're only interested in being rude, and in showing off how smart you are and what paradoxes you can name, feel free to just move along, you're neither needed nor wanted here.

In my "actual studying" of this issue I just came across this and it made me laugh. I suppose I'm in good company with scorn and ridicule:

"Dingle’s Argument And Question About Special Relativity

I especially like that last bit: "But no answer was received." I feel ya Dingle!

Dingles Question, pretty much verbatim to my question: “According to the theory, if you have two exactly similar clocks, A and B, and one is moving with respect to the other, they must work at different rates (a more detailed, but equally simple, statement is given on pp. 45-6, but this gives the full essence of the matter), i.e. one works more slowly than the other. But the theory also requires that you cannot distinguish which clock is the 'moving' one; it is equally true to say that A rests while B moves and that B rests while A moves. The question therefore arises: how does one determine, consistently with the theory, which clock works the more slowly?Unless this question is answerable, the theory unavoidably requires that A works more slowly than B and B more slowly than A --which it requires no super-intelligence to see is impossible. Now, clearly, a theory that requires an impossibility cannot be true, and scientific integrity requires, therefore, either that the question just posed shall be answered, or else that the theory shall be acknowledged to be false. But, as I have said, more than 13 years of continuous effort have failed to produce either response.”

So yes, I am asking "the ol' Dingle Question" apparently. Thank you Mr. Rude and Arrogant and Unhelpful for leading me to that terminology.

Now let me restate the entire purpose of this thread:

Hey guys, I had this question about Special Relativity and I recently found out it was once a big deal to a guy named Dingle as well. Anyone here got a good explanation to the ol' Dingle Question? Thanks in advance!

Let me draw a distinction though between myself and Mr. Dingle: In the wording of his question he seems to be trying to say that Special Relativity may not be true. I don't want to put words in his mouth but that's what it looks like to me.

I on the other hand, am operating under the assumption that Special Relativity IS true.

I'm not asking my question as an ultimatum:

"If no one can answer this then I won't believe in SR!"

Instead I'm saying :

"Hey science community of people way smarter than I, this seems like a great question to me and I assume it has an answer. Does anyone have any insight? I would love to understand it better."

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That was annoyingly arrogant, incredibly rude, and profoundly unhelpful.

I simply pointed out that you lack the basics for tackling more advanced issues. <shrug>

If someone is traveling at light speed relative to earth (but not accelerating) does he age slower than someone on earth? If so, why?

No one can travel at light speed.

Even if that person did, by absurd, he or she would NOT "age slower".

In my "actual studying" of this issue I just came across this and it made me laugh. I suppose I'm in good company with scorn and ridicule:

"Dingle’s Argument And Question About Special Relativity

I especially like that last bit: "But no answer was received." I feel ya Dingle!

Dingles Question, pretty much verbatim to my question: “According to the theory, if you have two exactly similar clocks, A and B, and one is moving with respect to the other, they must work at different rates (a more detailed, but equally simple, statement is given on pp. 45-6, but this gives the full essence of the matter), i.e. one works more slowly than the other. But the theory also requires that you cannot distinguish which clock is the 'moving' one; it is equally true to say that A rests while B moves and that B rests while A moves. The question therefore arises: how does one determine, consistently with the theory, which clock works the more slowly?Unless this question is answerable, the theory unavoidably requires that A works more slowly than B and B more slowly than A --which it requires no super-intelligence to see is impossible. Now, clearly, a theory that requires an impossibility cannot be true, and scientific integrity requires, therefore, either that the question just posed shall be answered, or else that the theory shall be acknowledged to be false. But, as I have said, more than 13 years of continuous effort have failed to produce either response.”

So yes, I am asking "the ol' Dingle Question" apparently. Thank you Mr. Rude and Arrogant and Unhelpful for leading me to that terminology.

Now let me restate the entire purpose of this thread:

Hey guys, I had this question about Special Relativity and I recently found out it was once a big deal to a guy named Dingle as well. Anyone here got a good explanation to the ol' Dingle Question? Thanks in advance!

Your quote come from a crackpot website, written by a relativity denier, a well known crank called Nick Percival. You will not learn anything useful from that website.

Edited by xyzt
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That was annoyingly arrogant, incredibly rude, and profoundly unhelpful.

Don't mind xyzt, he is like that with everyone. But he does know his stuff....

If someone is traveling at light speed relative to earth (but not accelerating) does he age slower than someone on earth? If so, why?

So, if we ignore acceleration (and therefore never bring the spaceship back to Earth) then Bill will see Bob age more slowly that him, and Bob will see Bill age more slowly.

The theory of relativity s about relative observations / measurements. This is what Dingle fails to understand: his "paradox" is only a paradox if there is some absolute time that we can use to determine who "really" ages more slowly. (No one does.)

And if you do bring Bill back to Earth, then he will have experienced less time and will be younger than his "twin" on Earth. This is a bit subtle but the Wikipedia page has quite a good explanation.

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Don't mind xyzt, he is like that with everyone. But he does know his stuff....

So, if we ignore acceleration (and therefore never bring the spaceship back to Earth) then Bill will see Bob age more slowly that him, and Bob will see Bill age more slowly.

This is not quite right. Bob measures an em ray sent by Bill as redshifted (Doppler). Bill measures an em ray sent by Bob redshifted by the same amount. Nothing to do with total elapsed proper time. Proper elapsed proper time omes into play only when the twins are reunited via some mechanism and they can compare clocks side by side.

Dingle mixed the mutual time dilation with the twins paradox. The two are two DIFFERENT effects of SR.

And if you do bring Bill back to Earth, then he will have experienced less time and will be younger than his "twin" on Earth. This is a bit subtle but the Wikipedia page has quite a good explanation

Correct. I wrote a big chunk on that page.

Edited by xyzt
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xyzt is pretty knowledgeable and generally knows what he's talking about but isn't always the most polite about it. Don't take it personally.

OK yes that actually does make sense to me, I should have thought about the force felt from acceleration. So speed is never absolute, only relative, but acceleration is real.

Still, i'm confused about time dilation. Einstein FIRST came up with general relativity, which correct me if I'm wrong, only has to do with speed, NOT acceleration. It wasn't until special relativity that he addressed changing speeds. Right?

So lets ignore the acceleration part for a minute. Imagine a woman is in labor on a spaceship going 95% light speed. She gives birth to baby Bill right as they pass earth. There is another woman on earth who gives birth to baby Bob right as the spaceship passes overhead. The spaceship continues on at 95% light speed (relative to the earth of course).

30 years passes on earth and Bob is now 30 years old. How old is Bill?

According to explanations I have seen, Bill would be only a few minutes or hours or perhaps days old, because time would be much slower for him traveling near light speed. I always see diagrams showing that as light hits off the console of the spaceship and moves towards Bill's eyes it has to travel a great distance because the console and Bill are moving laterally, so the light is taking a very long hypotenuse route between objects. It makes sense, but only if that movement i.e. speed, is real.

Back on earth, light moves from a desk surface to Bob's eyes, but there is no lateral movement of Bob or Desk at any significant percentage of light speed. Again, this makes sense to me except that relative to Bill, Bob and the desk ARE going 95% light speed. So when we take into account "there is no speed but relative speed" it seems like Bob would age quickly from Bill's perspective, and Bill would age quickly from Bob's perspective. Which is just silly because that means that When Bob is 30 he thinks Bill is only minutes old, and when Bill is 30 he thinks Bob is only minutes old.

Again, it all makes quite a lot of sense to me if we admit that one person is ACTUALLY moving near light speed and the other is ACTUALLY motionless. But if speed is only relative then I don't understand time dilation.

For the sake of allowing me to skip having to actually do a lot of math, mind if we use 86% of light speed rather than 95%? The numbers come out a little rounder and I already know what they are that way.

The answer to your question, and ultimately the trick to the problem you are having with the rest of this, is that it depends on whose frame you are looking at this from.

From Bob's perspective on Earth, when he is 30 years old, Bill is 15 and everything you just described applies.

From Bill's perspective on the spaceship, you can look at the spaceship as being stationary while the Earth flies by at 86% the speed of light. From Bill's frame of reference, when he is 30 years old, Bob is 15. Everything that you just described as the experience on the spaceship from Earth's perspective is what the spaceship sees as happening on Earth from the spaceship's perspective.

Now, how can Bill be 15 when Bob is 30 and Bob be 15 while Bill is 30? The answer to that is something called relativity of simultaneity. It turns out that not only is speed relative to your frame of reference, but so is what events you measure as being simultaneous.

Observers in different frames will not necessarily agree on whether distant events happen simultaneously, or what order certain events happen in. This requires a bit more math, but it does work out so that nobody is going to observe causally linked events happening in the wrong order, like someone dying before they are born. But it does mean that Bill and Bob can't agree on a "same moment" at which to compare their ages.

Let's say that after one year on Earth, when Bob's parents see Bill as being 6 months old, they send a signal to the spaceship. From there perspective, when Bob is 1 year old, the spaceship is is 0.86 light years away. So they send the signal and, since the spaceship continues to move away at 86% the speed of light, it takes an additional 6 years (and change) to catch up to the spaceship, during which time Bill has aged another 3 years.

So Earth sees Bill receive the signal sent on Bob's 1st birthday (when they saw Bill as being 6 months old) when Bill is a little over 3 and a half.

From the spaceship's perspective, Earth still sends the signal on Bob's first birthday, but Bill is two. The Earth, having been receding from the spaceship at 86% of the speed of light for two years at this point, is a little over a light year and a half away. So it takes a bit over a year and a half from the time the Earth sends the signal to reach the spaceship (according to the spaceship's frame) and it was sent when Bill was two (again, according to the spaceship), so Bill receives it when he is a bit over 3 and a half years old.

So everyone agrees on all of the events (a signal was sent from Earth on Bob's first birthday and arrives at the spaceship when Bill is just over 3 1/2), but they disagree on what moment was simultaneous with each event in the other's frame and how long the light signal took to reach its destination from the time it was sent.

Earth thinks that they sent the message when Bill was six months old, and that it took six years for the signal to get there. The spaceship thinks that the signal was sent when Bill was two and that it took a year and a half.

The only way to reconcile the discrepancies is to bring the frames back together, but to do that, either the Earth or the spaceship have to turn around in order to come back to the same location and be in the same frame as the other, and that requires someone to accelerate, breaking the symmetry and making one or the other "right" in their interpretation.

Edit: started this before other people posted.

Edited by Delta1212
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Thank you Delta12. That actually was really helpful and I think I get it. Without acceleration for either party then their speeds will be a matter or perspective and their TIME will be a matter of perspective as well.

I watched this documentary with Brian Greene once where he showed all of spacetime, all events past present and future, as a loaf of bread. We think of "the present" as being a slice of bread, where all events that are happening "right now" constitute "the present" but then he showed that depending on perspective, you don't always have "slices" that are parallel to each other.

In the case of Bob and Bill, traveling away from each other relatively, their "slice" of time constituting "the present" will be angled slightly. So that for Bob "the present" means he is x years old and Bill is x/2 years old, but Bill's slice of "the present" involved himself being x years old and Bob being x/2 years old.

Does that sound right?

A follow up question I have: Here on earth we perceive that the Universe is 13.82 billion years old. Assuming that we find a habitable planet somewhere in the universe, and assuming we were able to teleport there instantaneously, once we arrived on that planet, could we measure the age of the universe and find that it is much older or much younger for that planet? Might there possibly be planets in this universe that are presently existing only 1 billion years after the big bang? Might there be planets that are presently existing 200 billion years after the Big Bang?

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A follow up question I have: Here on earth we perceive that the Universe is 13.82 billion years old. Assuming that we find a habitable planet somewhere in the universe, and assuming we were able to teleport there instantaneously,

You cannot teleport "instantaneously". You cannot even travel at the speed of light.

once we arrived on that planet, could we measure the age of the universe and find that it is much older or much younger for that planet?

Neither, if you understood what was explained to you, you would have understood that the the age is still 13.82, speed has no effect.

Might there possibly be planets in this universe that are presently existing only 1 billion years after the big bang? Might there be planets that are presently existing 200 billion years after the Big Bang?

No.

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I clearly did not ask if it were possibly to teleport instantaneously.

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I clearly did not ask if it were possibly to teleport instantaneously.

You didn't ask, you asserted. Amongst other fringe claims.

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I clearly did not assert that it were possible to teleport instantaneously.

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I clearly did not assert that it were possible to teleport instantaneously.

"Assuming that we could teleport instantaneously....". This just only one of the fringe things you posted.

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"Assuming that we could teleport instantaneously....". This just only one of the fringe things you posted.

Assume = Pretend.

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xyzt, It's called a hypothetical situation, and it's used in something called a thought experiment. Yes it's like pretending. Pretend people could live for 1000 years, what might we accomplish? That doesn't mean I'm asking if people could live for 1000 years. It certainly doesn't mean I'm CLAIMING people can live 1000 years. It's hypothetical situation used for a thought experiment.

You're now straining at things to be rude about. You're a complete asshole. And what are you even doing on this forum? Are you so insecure in your social life that you need to get online and show complete strangers how smart you are at physics by being arrogant, rude, and unhelpful? I seriously don't understand.

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xyzt, It's called a hypothetical situation, and it's used in something called a thought experiment. Yes it's like pretending.

It is called GiGo (Garbage In , Garbage Out)

You're a complete asshole.

You must be talking to your mirror again.

And what are you even doing on this forum?

Keep crank posts to a minimum.

Edited by xyzt
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Complete asshole.

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Thank you Delta12. That actually was really helpful and I think I get it. Without acceleration for either party then their speeds will be a matter or perspective and their TIME will be a matter of perspective as well.

I watched this documentary with Brian Greene once where he showed all of spacetime, all events past present and future, as a loaf of bread. We think of "the present" as being a slice of bread, where all events that are happening "right now" constitute "the present" but then he showed that depending on perspective, you don't always have "slices" that are parallel to each other.

In the case of Bob and Bill, traveling away from each other relatively, their "slice" of time constituting "the present" will be angled slightly. So that for Bob "the present" means he is x years old and Bill is x/2 years old, but Bill's slice of "the present" involved himself being x years old and Bob being x/2 years old.

Does that sound right?

Yes, that sounds about right.

A follow up question I have: Here on earth we perceive that the Universe is 13.82 billion years old. Assuming that we find a habitable planet somewhere in the universe, and assuming we were able to teleport there instantaneously, once we arrived on that planet, could we measure the age of the universe and find that it is much older or much younger for that planet? Might there possibly be planets in this universe that are presently existing only 1 billion years after the big bang? Might there be planets that are presently existing 200 billion years after the Big Bang?

While I realize that xyzt again doesn't always present things in strictly the most helpful fashion, he is right about the problem that trying to imagine a scenario with instantaneous travel presents.

Let's imagine that we're on Planet Bob, and the planet that we're traveling to is Planet Bill. We leave Planet Bob when it is one year old and instantly travel to Planet Bill in order to measure the age of the universe as seen by Planet Bill. Well, on Planet Bob, we measure Planet Bill as being six months old when our Planet Bob is 1 year old. So when we arrive there, it will still be 6 months old and the universe will appear six months younger right?

Well, from Planet Bill's perspective, Planet Bob is one year old when Planet Bill is two years old. So now when the people of Planet Bill determine that Planet Bob is 1 year old and we are then teleporting to Bill, we should arrive when Planet Bill is two, and the universe is older than it was from the place we left.

To find out what is going on "right now" at a place, you have to be there. To be there, you have to physically travel there. "Right now" loses its meaning over significant distances, and if you cut out the travel time to try to determine what is going on "right now" at another place, you can get literally any answer you want depending on how you set up the problem. Since you can't actually do it, you can't pick one of those set ups and say that is the correct one. They are all equally valid (or invalid).That's why you can't really do that.

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Ah, I see. Point taken. Ok so venturing once again into GENERAL RELATIVITY, let's say there was a planet with a great deal of force acting on it, such as the planet orbiting the black hole in "interstellar" (hated the movie for a million reasons, but from what I've read, the time dilation between that planet and earth was at least somewhat accurate). If someone went to the surface of that planet his time perception would be very different from the time perception of someone on earth. Let's say that Matthew Mcconaughey's character had set up a lab while he was there, with a nice telescope and all the staff and instruments necessary to measure the age of the universe, would he have discovered that it was 13.82 billion years old?

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