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How fast we travel?by cryptist
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#1
Jan1713, 04:09 PM

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If earth is moving with the speed of 800 km/s in universe, then do we travel at 1/375 of the speed of light according to a hypothetical static point in universe?



#2
Jan1713, 04:29 PM

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You posted this in "Special and General Relativity" so surely you know that speed can only be given relative to some specific point. So I have no idea where you got the figure "800 km/s in universe". You say "according to" (relative to) some "hypothetical static point" but since speed is relative there is NO independently "static" point.



#3
Jan1813, 09:46 AM

P: 70

So the speed of an object is dependant on the reference frame i.e. Imagine that I live on the equator, me and my dad are both sitting in the living room, in this reference frame his velocity is 0. At the equator, the circumference of the Earth is 40,070 kilometers, and the day is 24 hours long (approximately) so the speed is 1670 kilometers/hour ( 1070 miles/hr), hence my dad is moving at 1670km/h, but the orbital speed of the Earth averages about 29.8 km/s (107,000 km/h), so is he not moving at 29.8 km/s. The truth is that there are reference frames in which my dad is moving 0km/s, there are reference frames in which he is moving at 30km/s and hypothetically there are reference frame in which he is moving at the speed of light. If by hypothetical static point in the universe you mean. If the earth is moving with velocity x,are we not also moving at velocity x, then yes we are. But if you walk towards the front of a flying plane you don't say I'm walking at 570 (+ 3)mph you don't look like your walking at 573 mph to someone on the plane. Someone on the floor however will see the plane and assuming they can see you, it will look like you are travelling at the 573mph (assuming the plane is travelling at 570 mph and you walk at approximately 3mph.) Does this make sense... does it go anyway to answering your question? 


#4
Jan1813, 10:52 AM

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How fast we travel?



#5
Jan1813, 11:34 AM

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#6
Jan1813, 12:04 PM

P: 70




#7
Jan1813, 12:30 PM

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The reason you've never noticed this is because c^{2} is a very large number, so at any of the speeds that we've ever experienced, the difference between the two formulas is not noticeable. When you're talking about things moving at an appreciable fraction of the speed of light, it starts to matter. 


#8
Jan1813, 01:24 PM

P: 70

Ahhh, right. Nice one, good knowledge I'll remember that! So in fact the observed speed would be: v = c/(1+0.25).... i.e. 0.8c. Cool. I have to admit the idea of being able to observe something appearing to travel faster than light did give me pause for thought lol. 


#9
Jan2013, 10:34 AM

P: 109

My question is not badly worded actually. I guess we are not talking about the same thing.
Earth is moving with a velocity, solar system is moving with a velocity, milky way is moving with a velocity. I couldn't find where I got this number but, somewhere it says earth is in total moving with a velocity 800 km/s. (If this number is wrong, please feel free to write the right one..) Since we are on earth, we are also moving with that velocity. So if that number is right, we are actually travelling at 1/375 of the speed of light. Well, I know of course, its all relative, and that's why I called it as hypothetical static point. If it was real, I would not write hypothetical. Imagine yourself outside of our spacetime. Then, you are a static point. I'm talking about that point.. 


#10
Jan2013, 10:51 AM

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#11
Jan2013, 10:53 AM

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#12
Jan2013, 11:06 AM

P: 109

Actually it says "1 second after, earth will be 800km far away from this point" So I conclude that earth is moving with 800km/s, of course as you all said, relative to some reference point.



#13
Jan2013, 04:37 PM

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#14
Jan2113, 02:58 AM

P: 181

Our local group of galaxies is moving at 600 km/s w.r.t CMBR, so that is the approximate velocity of the Milky Way too. Very 'back of the envelope' calculation  add another 220 km/s as the Sun's velocity around the Milky Way center, and you get something roughly around 800 km/s. 


#15
Jan2313, 10:25 AM

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#16
Jan2313, 11:00 AM

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wikipedia: http://en.wikipedia.org/wiki/Cosmic_...ole_anisotropy



#17
Jan2313, 11:13 AM

P: 579

Thanks Naty



#18
Jan2313, 12:30 PM

P: 181




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