- #1

- 13

- 0

Thanks,

Adam

You are using an out of date browser. It may not display this or other websites correctly.

You should upgrade or use an alternative browser.

You should upgrade or use an alternative browser.

- Thread starter skycastlefish
- Start date

- #1

- 13

- 0

Thanks,

Adam

- #2

- 5,607

- 40

I don't understand why the speed of light measures constant if you are moving toward light that is also moving toward you?

no one understands that..it does NOT make intuitive sense..nor do many things in quantum mechanics, nodbody understands that either...but it turns out to be experimentally verifiable so we accept it as "fact"...

everyone measures the speed of light as "c"...like it or not it appears to be correct!!!

It's no crazier than space and time being variables...how can THAT be????

- #3

JesseM

Science Advisor

- 8,518

- 15

Precisely because he synchronizes his clocks differently--that's what "relativity of simultaneity" means! The Einstein clock synchronization convention is that each observer defines what it means for two clocks at different locations to be "synchronized" using theNext is the part I don't understand. Imagine the same space ship scenario as above only this time, in addition to the two light beams fired by the ships, a light beam is also fired at the space ships from directly ahead and in the line of travel. So now the moving ship is chasing a light beam and moving to meet a light beam that is also racing toward him. How can he not measure the light racing toward him as faster than the one he's chasing?

But now think about how this would look in your frame where the ship is moving. If you have synchronized all your clocks in such a way as to guarantee that both beams travel at the same speed in

For a numerical example of how it all works out, here's something I came up with on an older thread:

Say there's a ruler that's 50 light-seconds long in its own rest frame, moving at 0.6c in my frame. In this case the relativistic gamma-factor (which determines the amount of length contraction and time dilation) is 1.25, so in my frame its length is 50/1.25 = 40 light seconds long. At the front and back of the ruler are clocks which are synchronized in the ruler's rest frame; because of the relativity of simultaneity, this means that in my frame they are out-of-sync, with the front clock's time being behind the back clock's time by vx/c^2 = (0.6c)(50 light-seconds)/c^2 = 30 seconds.

Now, when the back end of the moving ruler is lined up with the 0-light-seconds mark of my own ruler (with my own ruler at rest relative to me), I set up a light flash at that position. Let's say at this moment the clock at the back of the moving ruler reads a time of 0 seconds, and since the clock at the front is always behind it by 30 seconds in my frame, then in my frame the clock at the front must read -30 seconds at that moment. 100 seconds later in my frame, the back end will have moved (100 seconds)*(0.6c) = 60 light-seconds along my ruler, and since the ruler is 40 light-seconds long in my frame, this means the front end will be lined up with the 100-light-seconds mark on my ruler. Since 100 seconds have passed, if the light beam is moving at c in my frame it must have moved 100 light-seconds in that time, so it will also be at the 100-light-seconds mark on my ruler, just having caught up with the front end of the moving ruler.

Since 100 seconds passed in my frame, this means 100/1.25 = 80 seconds have passed on the clocks at the front and back of the moving ruler. Since the clock at the back read 0 seconds when the flash was set off, it now reads 80 seconds; and since the clock at the front read -30 seconds, it now reads 50 seconds. And remember, the ruler was 50 light-seconds long in its own rest frame! So in its frame, where the clock at the front is synchronized with the clock at the back, the light flash was set off at the back when the clock there read 0 seconds, and the light beam passed the clock at the front when its time read 50 seconds, so since the ruler is 50-light-seconds long, the beam must have been moving at 50 light-seconds/50 seconds = c as well! So you can see that everything works out--if I measure distances and times with rulers and clocks at rest in my frame, I conclude the light beam moved at 1 c, and if a moving observer measures distance and times with rulers and clocks at rest in his frame, he also concludes the same light beam moved at 1 c.

- #4

- 13

- 0

- #5

JesseM

Science Advisor

- 8,518

- 15

You've pretty much got it, I think. The one thing I'd quibble with slightly is your comment that "an event cannot happen in a reference frame (have a cause-effect) until the light from the event has reached that reference frame". You have to distinguish the time that information about a particular event reaches a given inertial observer, and the time the event is reckoned to have happened in that observer's own inertial reference frame. If the event happens far from the observer, the observer will

- #6

- 13

- 0

For example, if in 2010 the observer looks through his telescope and sees an explosion 2 light-years away (as measured by a ruler at rest relative to the observer), he'll retroactively assign that event a time-coordinate of 2008. And if there had been a clock at the end of his 2-light-year-ruler, which was synchronized with his own clock, then when he saw the explosion through his telescope he'd see that the clock would indeed be showing a time of 2008 as the explosion was happening right next to it (and he'd just be seeing the light from that reading now because it took 2 years to reach him).

Say there's an explosion 2 light-years away (as measured by a ruler at rest relative to the observer) that is so powerful it could destroy the earth and our solar system. Imagine that the expanding explosion is traveling at a significant fraction of the speed of light. Anything, even the expansion of the explosion, is forbidden to travel faster than the speed of light, correct? Therefore, the explosion (cause) could not damage our reference frame (effect), nor could it be observed until it was right on top of us? Does this mean that an extreme event like an atomic explosion in space could happen 10,000,000 light years away from us (determining our fate) and not effect us at all (at all!) until 10,000,000 years after the event when the light reaches us -- followed closely by the explosion itself? In other words, If its agreed that an object is 2 light years away from our reference frame, then any physical event happening there cannot have a physical effect on our reference frame any sooner than its light could reach us, right?

Share: