# Does the speed of light change if?

1. Jun 6, 2011

### lawnmower man

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. Jun 6, 2011

### Cosmo Novice

It is impossible for anything with mass to travel faster than light. For an object to travel c it would gain infinite density and therefore require infinite energy to move. The sort of movement you are talking about (kinematic motion) is by and large irrelevant when talking about the speed of light. Light speed c is always constant in a vacuum regardless of observer movement.

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

3. Jun 6, 2011

### lawnmower man

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. Jun 6, 2011

### lawnmower man

if light is always the same speed no matter how fast the speed 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

5. Jun 6, 2011

6. Jun 6, 2011

### lawnmower man

hi read the piece on light cones and will probably read through it a couple more times to make sure i did not miss anything, thanks! but i still cant get my head round light speed always being constant maybe if i explain it this way.it will make more sence lets say if we take two planets planet a and planet b. planet a is traveling through time at the same speed as earth so 24 hour on planet a is same as 24 hour on earth and planet b is traveling trough time at half the speed of the earth and planet a so 24 hours on planet b would be 48 hours on earth and planet a so if light speed is going to remain constant with how it is on earth then the light speed would have to be halved for anyone on planet b to be veiwing light speed at 299 792 458 if this is so then light speed surely cant be constant if it was constant and did not move with time then the light speed on planet b be would have to be double to 599 584 916 to make it constant

Last edited: Jun 6, 2011
7. Jun 6, 2011

### DaveC426913

I will try to help you through this but I cannot read what you wrote. Can you please rewrite using sentences and punctuation?

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 time-dilated. 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. Jun 6, 2011

### Integral

Staff Emeritus
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. Jun 7, 2011

### mdmaaz

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. Jun 7, 2011

### Staff: Mentor

I agree with all of this except for the last bit, we can measure the speed of light relative to other things but it always measures the same.

11. Jun 7, 2011

### DaveC426913

As ryan points out, most of what you said is true, except this last bit.

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. Jun 7, 2011

### lawnmower man

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. Jun 7, 2011

### Nik_2213

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 white-dwarf star...

As it escapes from the gravity well, the light does not change speed, but it changes frequency....

14. Jun 7, 2011

### phinds

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. Jun 8, 2011

### lawnmower man

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. Jun 8, 2011

### phinds

No, you completely mis-understand the whole thing. Each of those people would have to run at exactly the same speed to run a 4-minute 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. Jun 8, 2011

### DaveC426913

No. Planet A and Earth are moving at approx .9c with respect to each other. Each sees the other as moving slower by 50%. Both are right. Neither has a privileged point of view.

B and Earth are at rest wrt each other.
C and Earth are moving at approx .9c with respect to each other. Each sees the other as moving slower by 50%. Both are right. Neither has a privileged point of view.

No. Dilation is never experienced, it is only observed. Each sees the others as moving slower ( and always slower, never faster).

18. Jun 16, 2011