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Does the speed of light change if? 
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#1
Jun611, 09:57 AM

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obviouly the speed of light is constant when we are measuring it here on earth however as mater speeds up time slows down. so if we were to measure the speed of light standing on a planet moving at the speed of light. what speed would we then be viewing light at? oh i do know that it is looking impossible at the moment for anything to move at the speed of light. i also know that it will be possible to work out any speed variations even though it is not possible for anyone to travel at light speed and see it through there owns eyes at the moment. however i just dont know how to do it myself and i am guessing that somebody else has already worked it out anyway!



#2
Jun611, 10:08 AM

P: 366

I am sure someone can explain this a little more elaborately, or point out if I have made any mistakes. 


#3
Jun611, 10:28 AM

P: 6

if light is always the same speed no matter how fast the speed of time is running at. then does this mean that light and time have an unbreakable bond that cant be broken, to a point that it would be possible to state that light and time are the same thing?



#4
Jun611, 10:51 AM

P: 6

Does the speed of light change if?



#5
Jun611, 11:01 AM

Mentor
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The speed of light is constant in all reference frames.
Time and light are not the same. Time is a dimension alongside the three spatial dimensions (creating spacetime). Light is single or groups of photons. You might be interested to read about light cones. The relationship between time and light are best described in this manner. 


#6
Jun611, 11:44 AM

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#7
Jun611, 12:31 PM

P: 15,319

In a nutshell, time dilation between frames of reference that have different velocoties will cause both frames of reference to observe the speed of light to be constant. A common way to imagine this is to examine a spaceship flying past Earth at .99c while the pilot on the spaceship shines a flashlight out the front window. Both the pilot and an observer on Earth will measure the propogation of the beam of light to be c, even though the pilot is chasing down the beam of light at .99c. How is this possible? Compared to Earth, the pilot is relativistically timedilated. He is moving slowly. By the time a full second has passed for him, he will see the beam of light 300,000 km ahead of him. Whereas on Earth, ten seconds have now passed. In both cases the distance light is observed to travel from its point of origin works out to 300,000km/s. 


#8
Jun611, 02:46 PM

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Keep in mind that with respect to distant objects in the universe the Earth is moving at a signifianct fraction of c.
So one could argue that all of our measurements of the speed of light are made from a platform moving at nearly c. 


#9
Jun711, 01:14 AM

P: 41

Every speed is measured relative to something else. For example, if you walk across the aisle in a moving bus. You will think you're walking at 3 miles per hour. Someone on the ground will think you're walking at 53 miles per hour (since the bus is moving), an astronaut on the moon will see you walking at 20 miles per second (since the Earth moves). But the speed of light is special. It is the same everywhere. Light moves at the same speed relative to everything. If we travel in a spacecraft flying at 99.999% the speed of light, and shoot a gun in the direction of our flight, the bullet will not exceed the speed of light. Because the speed of light is not measured relative to anything.



#10
Jun711, 04:24 AM

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#11
Jun711, 08:17 AM

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c is measured relative to the observer, it's just that every observer, regardless of ther own velocity relative to anythin else, measures it to be the same. 


#12
Jun711, 04:43 PM

P: 6

hi thanks to everybody for the replys so far. I think I almost understand everything about light speed now.
There is one thing I'm still not sure about though. I will try to write everything down in a way thats easy to understand. I understand that light speed is always constant with the speed of time that anyone is experiecing, however if this is so. For light speed to remain consant in all time frames then light is going to have to change speed as time expands and decreases for anyone measuring time in all these time frames to get the same result. so if anyone was to measure the speed of light whilst travelling just under the speed of light, time would be expanded to such a great length that the speed of light would have to be tiny for it to still have the same light speed as we have on earth. So how can it take infinite mass and infinite energy for matter to reach a speed that just gets slower the faster you go? 


#13
Jun711, 07:03 PM

P: 217

Slightly OT: http://en.wikipedia.org/wiki/Gravitational_redshift
IIRC, this effect was used ~2005 by Hubble space telescope to weigh Sirius_B, probably our nearest whitedwarf star... As it escapes from the gravity well, the light does not change speed, but it changes frequency.... 


#14
Jun711, 07:43 PM

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lawnmowerman, I think you don't get the fundamental fact that NO object is moving relative to itself and light relative to itself moves at "the speed of light" (= "c"). A separate observer may see the object move at close to the speed of light but if the mover turns on a flashlight and shines it directly ahead relative to its motion BOTH of them will see the light beam move at a speed of c.
Yes, I KNOW it doesn't seem to make "sense" but that's because our brains grow up experiencing ADDITIVE motion. You walking plus the speed of the bus make your speed relative to the ground different than the buss's speed or your speed, because they add. If you shine a flashlight on the bus, you see the light moving at c and so does an observer not on the bus REGARDLESS of how fast he thinks YOU are moving. Light moves at c. Just keep saying that over and over. Light moves at c. Period. 


#15
Jun811, 03:01 PM

P: 6

hi again, thanks to anyone who has replied to me so far .this is about My last question that i posted. I dont think I had phrased it right because both the replys I received seem to be answers to a different question than i thought i had asked. So i will try phrasing the question without using the speed of light in it. to see if it helps. right there are 3 planets and 3 poeple standing on them.
planet A is traveling through time twice as fast as earth. Planet B is traveling through time at the same speed as earth. Planet C is traveling through time at half the speed of earth on planet A anna runs a 4 minute mile with time travel at twice the speed as on earth on planet B bob runs a 4 minute mile with time traveling at the same speed as on earth on planet c clare runs a 4 minute mile with time traveling at half the speed as on earth so if we now measure how fast each runner would had to of run to achieve a 4 minute mile using the time speed of planet Bs to measure all 3 then anna would have to run at 7.5 miles per hour bob would have to run at 15 miles per hour clare would have to run at 30 mile per hour so my question is? is this correct? 


#16
Jun811, 04:39 PM

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No, you completely misunderstand the whole thing. Each of those people would have to run at exactly the same speed to run a 4minute mile; 15mph. It does not matter in the least to them what motion their planet has relative to anything else, and your whole concept of "traveling through time" is just messed up. Sorry if I sound rude, I'm just trying to be correct. You need to study RELATIVE MOTION AT RELATIVISTIC SPEEDS for any of this to make any sense to you.
No one sees time dilation happening TO THEM, it's something they see happening to others moveing at relativistic speeds relative to them. If two people move past each other at relativistic speeds, it does not matter at all who is "moving" it only matters to each of them that the other is moving relative to him. EACH one of them sees the other's clock ticking slower than their own. This can be hard to get your head around. 


#17
Jun811, 09:39 PM

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#18
Jun1611, 07:49 PM

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