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Noob question about relativity


Akolaad
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If someone went to the surface of that planet his time perception would be very different from the time perception of someone on earth.

That is false, clocks tick at the same rate all over the universe: 1 second per second. You simply repeat the same misconceptions over and over in a slightly different way.

 

 

 

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?

 

Yes, all observers agree on the age of the universe. This is the second time you are being told that. You seem to have a hard time accepting it.

Edited by xyzt
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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?

And that introduces some new wrinkles, because unlike with the time dilation we've been talking about that arises as a consequence of special relativity, gravitational time dilation is not reciprocal because a gravitational reference frame is non-inertial. If you are at the bottom of a very strong gravity well, you can tell that you are and that you are the one that time is dilating for. You wonMt see the rest of the universe also being dilated like they see you. In fact, you'll see the outside universe going by in a sort of fast-forward.

 

A bit loathe to use movie physics in an example, but they got this part more or less right so: On the planet, it had only been a few minutes or hours since the first person landed when they arrived despite years going by in the outside universe. So if the person had still been alive on the surface, they would have been able to look up and watch all the years pass in that brief amount of time.

 

So while fourteen billion years may not have passed on the planet, a person on the planet would still be able to see that much time as having passed in the universe at large.

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!

Moderator Note

xyzt, enough. Seriously. Instantly berating people for asking questions is helpful to nobody except perhaps your own ego. If you actually want to assist in someone's learning of a subject, you could do with being less condescending and plain rude when their understanding is faulty. On the other hand, if your aim is simply to insult and make others feel stupid, as it seems to be, I would suggest not posting at all.

If you think something is crank nonsense, you know how to use the report system. Do not then go out of your way to create hostility within the thread as you have done here (and in many other threads). Any more posts of this nature will be immediately removed from the forum.

Akolaad, though we understand your frustration, responding to xyzt by insulting him was not the appropriate course of action here. Please use the report post feature in the future.

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If you are at the bottom of a very strong gravity well, you can tell that you are and that you are the one that time is dilating for.

 

Everything else that you posted is correct, except the above. You cannot tell that you are in a gravity well. Your clock ticks at the same rate, of 1 second per second. Clocks do NOT 'slow down" in gravity wells, this is an unfortunate, widespread misconception. Clock ALWAYS tick at their native rate, otherwise they would be worthless as clocks. What happens in reality is that when clocks are observed by a remote observer they appear to "slow down" (to an observer "higher" that the clock in the gravity well or to "speed up" , for an observer "lower" that the clock. This is the main reason why the atomic clocks mounted on the GPS satellites need to have their frequency adjusted down at launch: such that their time stamps line up with the time of the Earth bound clocks.

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I'm surprised no-one has pointed this out yet, so allow me...

 

Comparing speeds and times across differing frames of reference is a mistake every newcomer to relativity makes. I'm sure even xyzt confused frames when he first started studying the subject.

It does not matter which of Bill or Bob is moving at near light speed, they are in different frames, and exactly thirty years will elapse for either/both, as measured in their OWN FRAME.

What you are trying to do is determine the elapsed time in Bill's frame from Bob's frame. That can only be done once there is an acceleration involved, and both Bill and Bob are back together in the same frame.

 

This applies to translational ( moving ) frames and gravitational frames.

 

You would do well to take some of xyzt's advice, and pick up an introductory textbook on special relativity. The math is not very complex, and it should answer most of your questions.

Just remember that there is no absolute or preferred frame, so you can always pick a frame where you are at 'rest' compared to everything else, so speeds and elapsed times will also be unique to your 'rest' frame and different from other frames.

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What you are trying to do is determine the elapsed time in Bill's frame from Bob's frame. That can only be done once there is an acceleration involved, and both Bill and Bob are back together in the same frame.

Actually both Delta and I have pointed that out. Repeatedly.

 

 

 

 

This applies to translational ( moving ) frames and gravitational frames.

 

Correct.

 

 

 

You would do well to take some of xyzt's advice, and pick up an introductory textbook on special relativity. The math is not very complex, and it should answer most of your questions.

 

Thank you. Apparently the moderators found my well-intentioned and appropriate suggestion insulting. This forum has turned into a battle between mainstream people and cranks, with a very heavy dose of threads that are fringe. I will be hard pressed to name a thread that was not started by a fringe set of claims.

Edited by xyzt
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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?

 

Special Relativity (1905) was before General Relativity (1915).

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Everything else that you posted is correct, except the above. You cannot tell that you are in a gravity well. Your clock ticks at the same rate, of 1 second per second. Clocks do NOT 'slow down" in gravity wells, this is an unfortunate, widespread misconception. Clock ALWAYS tick at their native rate, otherwise they would be worthless as clocks. What happens in reality is that when clocks are observed by a remote observer they appear to "slow down" (to an observer "higher" that the clock in the gravity well or to "speed up" , for an observer "lower" that the clock. This is the main reason why the atomic clocks mounted on the GPS satellites need to have their frequency adjusted down at launch: such that their time stamps line up with the time of the Earth bound clocks.

This is, of course, what I meant, but it's good to be specific, so thank you for making that distinction clearer than I did.

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This is, of course, what I meant, but it's good to be specific, so thank you for making that distinction clearer than I did.

This is the best example of a measured, rational, answer. No attempts at diversion, further elaboration, introduction of further complications, etc. You got my +1 for that. Too bad that there aren't more like you in this forum.

Edited by xyzt
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This is the best example of a measured, rational, answer. No attempts at diversion, further elaboration, introduction of further complications, etc. You got my +1 for that. Too bad that there aren't more like you in this forum.

There's no skill in preaching to the choir.

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Cut it out xyzt.

 

You being nice to people just seems...weird !?!?

Well, it happens very seldom that I get a honest reply to my criticisms. Normally, I get a crank run-around.

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I'm thinking this might not be the best place for me to ask questions. Though I really appreciate some of the genuine attempts to help my understand (specifically one Delta12) I think that there might be a better science forum out there for beginners to ask questions. I think xyzt might agree with me for once when I say: this isn't the best place for my questions.

So thank you all for engaging. It has been an educational introduction to scienceforums.net. I wish you all the best and I hope there are people who find this a good forum for them. I also hope I find one more on my level elsewhere. :)

Goodbye.

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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.

How can you tell the reading on your accelerometer from proper acceleration from a reading on your accelerometer from being in a gravitational field?

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How can you tell the reading on your accelerometer from proper acceleration from a reading on your accelerometer from being in a gravitational field?

If the gravitational field is uniform and if the proper acceleration is constant, the principle of equivalence teaches us that we cannot tell one from the other.

If the gravitational field is not uniform, then we have an effect called "tidal" , i.e. there is a transverse "squeezing" of the accelerometer, an effect that cannot be mimicked with uniform acceleration.

Thank you for actually asking an interesting , intelligent question that takes this thread out of the cesspool it had descended.

Edited by xyzt
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If the gravitational field is uniform and if the proper acceleration is constant, the principle of equivalence teaches us that we cannot tell one from the other.

 

So what I quoted from you wasn't quite correct. Perhaps I should berate you now. That's how we play here? We berate people trying to learn over inaccuracies until they want to leave?

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So what I quoted from you wasn't quite correct.

Err , I am under the impression that you do not even understand my initial post. It has absolutely nothing to do with being able to tell the difference between the effects of a gravitational field and of acceleration.

When I said that acceleration is "absolute" (as opposed to velocity), that means that you can ALWAYS tell, with the help of an accelerometer, if you ARE accelerating or NOT. By contrast, you cannot tell if you are in uniform motion. I made it quite clear:

 

 

 

You can measure it with an accelerometer, so you know exactly who's accelerating and who's not.

 

 

 

Perhaps I should berate you now. That's how we play here? We berate people trying to learn over inaccuracies until they want to leave?

 

Not very becoming for a moderator. Especially in the context of clearly having misunderstood the post you are attempting to criticize.

Edited by xyzt
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So what I quoted from you wasn't quite correct.

Err , I am under the impression that you do not even understand my initial post. It has absolutely nothing to do with being able to tell the difference between the effects of a gravitational field and of acceleration.

 

Really? Let's pull the quote again:

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.

"You can measure it with an accelerometer, so you know exactly who's accelerating and who's not" has "absolutely nothing to do with being able to tell the difference between the effects of a gravitational field of acceleration."? How, exactly is that?

 

Maybe you don't even understand your initial post. Either that or you are so terrible at communicating that it's a suprise that you can even use your keyboard (Gee, isn't being a dick to people fun? It's no wonder you like to do it so much).

 

You said "you can measure it with an accelerometer SO you know exactly who is accelerating and who's not". That word "so" there is used in such a way to imply that the second clause is somehow causally dependent upon the first clause. It is precisely because we can measure acceleration with an accelerometer that we know who is and isn't moving. Yet, that's not actually true, since the reading could quite possibly be from a stationary observer in a gravitational field.

 

That's what your initial post said. Do you understand it yet?

 

 

 

 

 

 

Perhaps I should berate you now. That's how we play here? We berate people trying to learn over inaccuracies until they want to leave?

Not very becoming for a moderator.

 

Ah, so you understand that berating people for inaccuracy is inappropriate. Now, given that you've been told so many times before to stop and that you finally now appear to understand that it's inappropriate, I'm going to be heavy-handed with the warning points in the future. If I see you berating people, you'll get a warning point. If I see you berating people to the point of people wanting to leave the site, it will not be hard at all for me to get enough moderator support to send you on an involuntary vacation.

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You said "you can measure it with an accelerometer SO you know exactly who is accelerating and who's not". That word "so" there is used in such a way to imply that the second clause is somehow causally dependent upon the first clause.

 

 

 

 

 

 

 

Look,

 

You still don't get it and you are behaving in a manner that is unfitting for a moderator. Let me try to explain one more time (not for your benefit, since you won't admit to error).

 

In the twins paradox (which was the context of the discussion) , with the help of an accelerometer, the twins can tell the accelerating twin from the inertial one.

 

 

 

It is precisely because we can measure acceleration with an accelerometer that we know who is and isn't moving. Yet, that's not actually true, since the reading could quite possibly be from a stationary observer in a gravitational field.

 

Nice try but this was not the context being discussed. Bill and Bob are the twins, one is accelerating away, the other one is not. There is no "gravitational field". If the twins are stationary, then there is no separation and there is no paradox.

 

 

That's what your initial post said. Do you understand it yet?

 

I understand that you do not understand, yet you are acting in a manner that is unbecoming for a moderator. Especially since you are wrong. One more thing: don't talk down to me and don't threaten.

Edited by xyzt
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I agree, there was no need for the heavy handed intervention by ydoaPs.. I would expect an admission of not understanding the subject matter and an apology.

It is exactly the behavior he criticized me for that he exhibited (to a much greater extent) , especially in the context of his being wrong.

Edited by xyzt
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