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OfflineJohnRainy
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Does somebody understand relativity? * 1
    #26248882 - 10/13/19 02:19 AM (4 years, 7 months ago)

Time dilation.

If I take off in a spaceship and travel at some percentage the speed of light relative to the earth, if you could see my clock, it would be running slow, and if I could see yours, it would be running fast, right?

Edited by JohnRainy (10/13/19 03:05 AM)

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OfflineLogicaL ChaosM
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Re: Does somebody understand relativity? [Re: JohnRainy]
    #26249010 - 10/13/19 04:53 AM (4 years, 7 months ago)

Correct.

The time dilation is relative to the observer. So to an outsider, the spaceship has travelled a long time but to the person in the spaceship, only a small amount of time has gone by. Basically the space-ship creates a distorted space-time bubble around it, or at least that's how I understand it. The spaceship compresses space-time and thus time and space within the space-ship is compressed.

Heres a metaphor from Albert Einstein that describes it:

Quote:

When you are courting a nice girl an hour seems like a second. When you sit on a red-hot cinder a second seems like an hour. That's relativity. - Albert Einstein



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Offlinechibiabos
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Re: Does somebody understand relativity? [Re: LogicaL Chaos]
    #26249018 - 10/13/19 05:03 AM (4 years, 7 months ago)

Quote:

LogicaL Chaos said:
Correct.



No.  Absolutely not correct.  To the other observer, either clock runs slower compared to their own.

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OfflineLogicaL ChaosM
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Re: Does somebody understand relativity? [Re: JohnRainy]
    #26249022 - 10/13/19 05:05 AM (4 years, 7 months ago)


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OfflineLogicaL ChaosM
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Re: Does somebody understand relativity? [Re: chibiabos]
    #26249027 - 10/13/19 05:10 AM (4 years, 7 months ago)

Quote:

chibiabos said:
Quote:

LogicaL Chaos said:
Correct.



No.  Absolutely not correct.  To the other observer, either clock runs slower compared to their own.




Can you elaborate on the second sentence.

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OfflineJohnRainy
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Re: Does somebody understand relativity? [Re: chibiabos]
    #26249028 - 10/13/19 05:12 AM (4 years, 7 months ago)

Quote:

To the other observer, either clock runs slower compared to their own.




OK.

If there was a twin on Earth, when the traveler got back, the twin who stayed on earth would be older, right?

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OfflineLogicaL ChaosM
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Re: Does somebody understand relativity? [Re: JohnRainy] * 1
    #26249776 - 10/13/19 01:01 PM (4 years, 7 months ago)

So yeah i had it partially correct, partially wrong.

So if youre traveling from the Earth to another star near the speed of light and u had a clock on the outside of the spaceship, that clock would appear slow moving to Earth because the ship is approaching the speed of light and light is slowly reaching Earth. And from the spaceship, if u could see a clock on Earth, it would appear slow as well because light is slowly reaching your ship due to the ship's near speed of light velocity.

A real-world observation related to this is Redshift: http://hyperphysics.phy-astr.gsu.edu/hbase/Astro/redshf.html

Redshift is caused by the expansion of the galaxies approaching a speed near the speed of light. Because galaxies in our Universe are moving away from each other at this speed, it causes light to shift towards Red since Red has a lower energy as the light we observe from other galaxies is being "stretched" by the expansion.

An easier way to look at time dilation is reading about this awesome twin astronaut study done by NASA: https://www.space.com/33411-astronaut-scott-kelly-relativity-twin-brother-ages.html

Quote:

Albert Einstein's theory of special relativity holds that time moves more slowly for objects in motion compared to a stationary observer, and experiments have borne out this prediction. This "time dilation" is most dramatic and noticeable at relativistic speeds, but the effects manifest even at the much lower velocities experienced by bodies in Earth orbit.

"So, where[as] I used to be just 6 minutes older, now I am 6 minutes and 5 milliseconds older," Mark Kelly said Tuesday (July 12) during a panel discussion at the ISS Research & Development 2016 conference in San Diego.



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OfflineJohnRainy
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Re: Does somebody understand relativity? [Re: LogicaL Chaos] * 2
    #26249807 - 10/13/19 01:21 PM (4 years, 7 months ago)

Light slowly reaching the ship or Earth?

The speed of light(in a vacuum) is constant no matter who observes it, no matter what speed they are traveling, no matter what.  All observers agree on the speed of light.

That's not exactly expressed in your post, maybe it's still assumed but it doesn't really read like it to me.

Also, the clocks really do run at different speeds, they don't just 'appear' to.  Time really does run at different rates for things moving at different speeds relative to each other.

Maybe that's assumed too but I get the impression one could interpret the above post as saying time dilation is an illusion.

Anyways, what about the twin on Earth?  Would rocketman be younger than his sibling when he got back?

Edited by JohnRainy (10/13/19 01:23 PM)

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InvisibleDividedQuantumM
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Re: Does somebody understand relativity? [Re: JohnRainy] * 2
    #26249847 - 10/13/19 01:42 PM (4 years, 7 months ago)

Consider two different reference frames that are moving at high relative velocity to each other, let's call them A and B. For A, their clock is running at normal speed. It appears, however, that time has slowed for B, given the change in relative distance. For B, the clock will be running normally, but A's clock will appear slower. They are complimentary. That's why it's called relativity. Time dilation is not an illusion, it's merely not an absolute.

As far as the twins paradox, you have to invoke general relativity, not just special relativity. If you think about it, you're rocketing away and your time will be slower for those on Earth, BUT as you look back, they will be running away from YOU at the same relative speed, so it will appear that you are the one getting older. In special relativity there is no acceleration. But of course a rocket ship has to accelerate. It is these accelerations, treated by general relativity, that result in the difference in age -- not the difference in velocity.


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Vi Veri Universum Vivus Vici

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Offlinechibiabos
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Re: Does somebody understand relativity? [Re: DividedQuantum]
    #26249967 - 10/13/19 02:47 PM (4 years, 7 months ago)

It's hard to talk about without getting into the weeds with geometry, but basically what happens with the Twin Paradox (which isn't even really a paradox) is that one observer has a force exerted on them while the other one doesn't.  Forces define accelerations, accelerations are treated as equivalent to gravity because there's no way to really tell them apart and gravity defines the transformation between observers' coordinate systems.  As long as the motion of your observers is constant their trajectories can be considered as straight lines in a flat spacetime, and figuring out how to transform between each observer's time isn't too far off from grammar school trigonometry.  Aside from the fact that the observers might disagree about which direction is forward, the transformations from one coordinate system to another is basically the same.  When you have a force exerted on one observer and only on that observer then that's no longer true because there is now a curvature to the world line of the accelerating observer to take into account that doesn't bestow the same sort of symmetry that you'd find with straight lines in a flat space.  The reason that Galillean relativity doesn't come up with results like time dilation and length contraction basically boils down to the difference between treating time as domain that is completely removed from space versus a direction of some sort of a different kind of space (that's called spacetime to drive the point home).

Also, it might be helpful to think about special relativity as being something like an instantaneous rate of change of a curve.  There's no reason why you can't use it to describe aspects of a system where there is gravity, but you might also like to be able to make some guesses about what would happen if the geometry was flat.

Edited to take out some shit that was obviously wrong.

Edited by chibiabos (10/13/19 03:54 PM)

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Offlinechibiabos
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Re: Does somebody understand relativity? [Re: chibiabos]
    #26250002 - 10/13/19 02:58 PM (4 years, 7 months ago)


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OfflineJohnRainy
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Re: Does somebody understand relativity? [Re: chibiabos]
    #26250013 - 10/13/19 03:04 PM (4 years, 7 months ago)

What Im getting at here is...

If the person on the rocket sees the person on earth passing more slowly through time, how then could rocketman get back and find their twin is older?  If someone passes more slowly through time, they should be younger.

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Offlinechibiabos
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Re: Does somebody understand relativity? [Re: JohnRainy]
    #26250109 - 10/13/19 04:03 PM (4 years, 7 months ago)

Rocketman's twin shouldn't be younger.  It's not that time dilation only applies to constant motion, but you're making the same sort of error that you'd be making if you tried to use the equations for a triangle on a flat plane to describe a triangle where two legs are the same, and have the same angle between them, but the third leg is being drawn on a bulge the shape of a football.

edit: typo

Edited by chibiabos (10/14/19 03:20 PM)

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OfflineJohnRainy
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Re: Does somebody understand relativity? [Re: chibiabos] * 1
    #26250506 - 10/13/19 07:39 PM (4 years, 7 months ago)

I think I found what I was looking for here.

https://www.scientificamerican.com/article/how-does-relativity-theor/

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OfflineLogicaL ChaosM
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Re: Does somebody understand relativity? [Re: JohnRainy]
    #26250521 - 10/13/19 07:49 PM (4 years, 7 months ago)

Quote:

JohnRainy said:
Anyways, what about the twin on Earth?  Would rocketman be younger than his sibling when he got back?




So to answer this question, I was use the NASA experiment as an example: https://www.space.com/33411-astronaut-scott-kelly-relativity-twin-brother-ages.html

"So, where[as] I used to be just 6 minutes older, now I am 6 minutes and 5 milliseconds older," Mark Kelly said Tuesday (July 12) during a panel discussion at the ISS Research & Development 2016 conference in San Diego."

So the brother on Earth was Mark Kelly. His twin brother Scott Kelly was the one who participated in the 1-year ISS time dilation experiment. So Scott Kelly essentially travelled into the future, but only by 5 milliseconds. The reason why he is ahead in time is because he was traveling at a high speed AND was farther from the gravity well of Earth. This combination causes the time dilation.

So totally hypothetical, if his brother spent 400 years in space in the ISS, he would be 1 minute younger than when he left. This is essentially because he travelled 1 minute into the future.

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OfflineJohnRainy
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Re: Does somebody understand relativity? [Re: LogicaL Chaos]
    #26250647 - 10/13/19 09:11 PM (4 years, 7 months ago)

Is that the right way to express what happened?  He traveled into the future?

Seems he's always in the present moment right along with everything else. 

I never thought of that before right now.  If all these things are running at different times, how does the whole manage to keep everything together in the present moment?  If things are running through time at different rates, why don't some things slip into the future or past?  Everything, regardless of how fast or slow it travels through time remains in the present.


I'm not smart enough to ponder these things.

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OfflineLogicaL ChaosM
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Re: Does somebody understand relativity? [Re: JohnRainy]
    #26250676 - 10/13/19 09:36 PM (4 years, 7 months ago)

The answer to that is its all relative to the celestial body :awesome:

Note theses are very very small distortions in time. When Scott Kelly spent 2 years in space on the ISS, he only travelled 5 milliseconds into the future. Very small. Since all the celestial bodies like planets move at about the same speed and similar gravities, its "relative" to our experience of time.

Also note that you cant travel back in time using Special Relativity Time-Travel. Only into the future. Theoretically, it might be possible to travel back in time but Im not sure on the concepts behind it.

But one possibility of backwards time travel is black holes, which have gravity wells that can rip thru space-time. In the singularity of the black hole, time runs backwards because of the extreme gravity. Gravity slows down time and extreme gravity can make it stop or even make it go backwards.

Hypothetically if you have a spacecraft that can compress space-time in extreme ways using artificial gravity, you can travel from Earth to say the Pleiades (424 light-years away) in say 848 minutes (1 light-year a second hypothetical velocity) then come back. This is example of time travel because as everyone on Earth experienced time normally and 848 years had gone by but for you only 848 minutes went by for you personally while you travelled in your space craft. The reason is you travelled thru space-time in a very short amount of time and ended up at the same place again, but in the future 848 years when it felt like you travelled only 848 minutes. Does that make sense?

An article about "time travel": https://www.thoughtco.com/is-time-travel-possible-2699431

Another cool article on "time travel", black holes and worm holes: https://www.thoughtco.com/time-travel-into-the-past-3072603

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OfflineJohnRainy
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Re: Does somebody understand relativity? [Re: LogicaL Chaos]
    #26250725 - 10/13/19 10:00 PM (4 years, 7 months ago)

I thought the theory was that if you travel faster than light, you move backwards through time.

It's impossible to ever accelerate above the speed of light though, so that's always going to be just a hypothetical. 

But, due the expansion of space, some galaxies are moving away from each other at speeds greater than the speed of light if they are distant enough from each other.  Does time dilation come into play here?

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OfflineLogicaL ChaosM
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Re: Does somebody understand relativity? [Re: JohnRainy]
    #26250841 - 10/13/19 11:09 PM (4 years, 7 months ago)

That's a good point, if you went faster than light you should see light from the past.

Hmmm.....

For the expansion of the Universe, yes the space between the galaxies is having time stretch as space is being stretch by the unknown force of the Universe. So light is being "time dilated" by it being slowed down from the expansion of space that is going faster than the speed of light.

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Offlinechibiabos
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Re: Does somebody understand relativity? [Re: JohnRainy]
    #26251924 - 10/14/19 01:55 PM (4 years, 7 months ago)

Quote:

JohnRainy said:
I thought the theory was that if you travel faster than light, you move backwards through time.

It's impossible to ever accelerate above the speed of light though, so that's always going to be just a hypothetical. 

But, due the expansion of space, some galaxies are moving away from each other at speeds greater than the speed of light if they are distant enough from each other.  Does time dilation come into play here?



Observers that are moving away from each other on account of inflation, at a rate exceeding the speed of light aren't causally linked.  Unless there's a definition of four-velocity that allows for imaginary components then the observing of a particle travelling at a superluminal speed is inconsistent with the theory because it requires that events in spacetime have complex coordinates.  People have done plenty of experiments that could demonstrate violations of that theory, but so far all attempts to do so have just given results that agree with it.

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