# Is it true? Is the universe flat?

Discussion in 'Physics & Math' started by camilus, Dec 6, 2010.

1. ### chingluValued Senior Member

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1,637
Here is the statement prove it.

"Besides, setting yourself to be at rest wrt to a particular huge set of photons is no different to setting yourself to be at rest wrt to anything else."

3. ### chingluValued Senior Member

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1,637

That means I do not understand anyting.

"Besides, setting yourself to be at rest wrt to a particular huge set of photons is no different to setting yourself to be at rest wrt to anything else."

Can you help me with this Pete?

I cannot make this true with my limited intelligence?

I really need a lot of help. :shrug:

Many thanks

5. ### James RJust this guy, you know?Staff Member

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30,356
AlphaNumeric already explained it to you, but I'll explain it one more time for you, ok?

Take any collection of objects. Find the average the velocities of those objects. Now, to "set yourself to be at rest wrt" those objects, take the velocity of any object of interest and subtract off the average velocity of the objects you're setting yourself at rest with respect to. You have thereby moved to a reference frame in which the average velocity of the reference objects is zero.

None of the above changes with the particular objects. So, if the objects are a set of cars of a freeway, or a set of photons in the cosmic microwave background, it makes no difference. The procedure is the same.

Got it?

7. ### DeeCeeValued Senior Member

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1,793

Is it true? Is the universe flat?

Nope.

A mate of mine fell of a cliff and died.
Can't do that in a flat universe.

Or have you science types stopped speaking English while I was away?

Nice to see your still shining light into dark corners James.
Dee Cee

8. ### chingluValued Senior Member

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1,637
How do you do this for accelerating frames?

Since to be at rest with CMB you must be at rest with acceleration, and AN and you claim this is the same as being at rest with intertial frames, how do you do this?

With inertial frames, I look for a constant frequency between the frames to measure whether I am at rest with another.

How do you do this for CMB?

If you answer this you will get caught up into absolute rest or absolute motion.

9. ### James RJust this guy, you know?Staff Member

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30,356
Nothing in my previous post mentioned acceleration. That post gave a procedure for putting yourself (instantaneously) at rest with respect to a set of photons, as requested by chinglu.

10. ### James RJust this guy, you know?Staff Member

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30,356
**** Moderator note: chinglu has been banned for 7 days for trolling in another thread.

11. ### PeteIt's not rocket surgeryModerator

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10,166
Yes, that's what AlphaNumeric said. Yes, it's correct. What do you think it proves?

Once again, in post 200 you said:
This is wrong - you're misrepresenting what AlphaNumeric said. In the post you replied to, he clearly said that finding a local inertial reference frame with no CMB dipole is within the realm of SR.

You keep saying this, and you keep being corrected.
Why do you keep saying something that's wrong?

12. ### AlphaNumericFully ionizedModerator

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6,697
No, it doesn't. The expansion of the universe will not alter the fact if you observe no dipole in the CMB now then in the future you won't.

Two people at different places who set themselves moving so they each see no dipole will be see their relative motion increase in time due to the expansion but that's a different thing.

This is the distinction between what one person sees and what two people see. A distinction I've explained several time but you've not grasped.

I repeatedly explained I was referring to an instantaneous case. Even if an object is accelerating you're able to set your velocity equal to it at any given moment.

More formally if the trajectory of an object is $x(t)$ such that $\ddot{x}(t) \neq 0$ you can still consider $\dot{x}(t)$ and at a given moment $t = t_{0}$ set your velocity to equal $\dot{x}(t_{0})$. For that instant you'll be at rest wrt the object.

That's simply a flat out trolling lie. I've addressed it at least 4 times, none of which you're retorted with anything other than "Nuh uh".

You're a troll, don't let the door hit you on the way out.

13. ### lambda orionisRegistered Member

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57
i believe that what we understand in our minds as "our universe" is 4D 3 of space and 1 of time. the last of which we have yet to understand. we dont know what a day is just because the sun comes up every morning doesnt mean its a new day in time, because we dont know what time is time is relative to us and we are just going on what we think we know. clearly we are living in a 3D world(earth) with other 3D planets in our solar system so the space around which we occupy has to be 4D. saying that the universe is flat is like saying we live inside of a plane which is not possible because we would all be 2D like what a pancake is on a griddle.

14. ### AlexGLike nailing Jello to a treeValued Senior Member

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4,304
Flat space does not mean what you think it means. It means that the geometry of space is Euclidean, i.e. the angles of a triangle add up to 180 degrees and parallel lines never meet. If space is curved, then geometry is non-Euclidean.