SPEED OF LIGHT-follow up thread

Lonesomedave · #1 10-31-2011, 10:23 AM

so...i was thinking about this discussion we had this weekend, and i was not completely sure i understood the answer, so i wrote it out on photoshop elements and i went over to belmont university in nashville and looked up a physics professor.

his name was doctor hawley, and he is, as i said, at the physics department of belmont university.

here is the problem i raised and the questions i had.

see the pic below for a setup of the problem and of my questions:

I raised 3 questions:

1. If we start the objects instantly to .8c and then instantly stop them exactly one second later, how far apart would they be? and i speculated this would be (186,000 x .8) x 2 miles apart or 297,600 miles

he agreed with this. that is the distance they would be apart.

2. I asked, does this not mean that their combined speed of recession is 160% of c?

he did NOT agree with this. he explained it (as well as he could in 5 minutes) but the bottom line is, their combined speed of recession of something other than 160% of c. i did not really understand his explanation, and I don't know what it is, and I don't know how to square this with the fact that the objects are 297,600 miles apart after one second, only passing on what the professor said....

3. i asked, if, instead of stopping instantly, they instead kept going, and after 1/2 second, one of them started emitting light in all directions, would the light from that object ever reach the second object?

this was the only question i felt i knew the answer to, after thinking about it all weekend. i believed that it would, because the light does not care what the motion of it's emitter is, it simply starts out at 186,000 miles/second, and it's is therefore tracking down an object only going 148,800 miles/second

he agreed, but pointed out that it would be considerably red shifted- that is the effect that the speed of it's emitter has, to red shift it, but not to affect it's speed.

anyway, i found this interesting and i thought i would pass it along.

seventhson · #2 10-31-2011, 10:27 AM

From the boxes perspective, they are 297,600 miles apart from each other after--get this, not one sec--but more than one sec. You see, time moves more slowly for them.

Lonesomedave · #3 10-31-2011, 10:32 AM

Quote:

Originally Posted by seventhson

From the boxes perspective, they are 297,600 miles apart from each other after--get this, not one sec--but more than one sec. You see, time moves more slowly for them.

^ ^ ^

you are right... we did not discuss this because i was only interested in the perspective of the stationary observer... but i have finally gotten this concept through me head...however, are you sure it is MORE than 1 second? is it not LESS than one second?.... just asking, because i am really fuzzy on this.

hbarcat · #4 10-31-2011, 02:13 PM

Your main problem in understanding the situation is your defining the speed of the objects in terms of your own position relative to the objects and then using that reference point to deduce the speed of the objects relative to one another.

That works fine for ordinary objects in our everyday world but it doesn't work that way for objects moving close to the speed of light.

You MUST define the problem from a specified inertial reference frame (this also means you must include acceleration, which you neglected in assuming the objects could stop instantly) and then calculate from there using the Lorentz transformation equations for relativity (not the classical Newtonian equations).

Technically, this problem involves general relativity, which is much more math intensive than the much easier to calculate special relativity but that's probably overkill for what you're trying to solve.

hbarcat · #5 10-31-2011, 02:19 PM

Quote:

Originally Posted by Lonesomedave

^ ^ ^

you are right... we did not discuss this because i was only interested in the perspective of the stationary observer... but i have finally gotten this concept through me head...however, are you sure it is MORE than 1 second? is it not LESS than one second?.... just asking, because i am really fuzzy on this.

The answer depends on which perspective. It will be different for the stationary observer than for either of the moving objects.

Both answers are correct, but are only valid for their own inertial reference frame. This is the central idea of special relativity (and is also critical to general relativity) and it's what you need to get your head around if you want to really understand it.

**bassybill** · #6 10-31-2011, 02:27 PM

297, 600 miles apart as measured by whom? Stationary relative to what?

For example, if you were travelling on a photon at c from one to the other, in one sense they'd be zero distance apart. Nobody on either cube would observe your clock tick at all when travelling from one to the other, so your presence at both would be instantaneous in YOUR time as measured by the observer - but in their own time frame, the journey would have a definite duration.

Does that help?

DerHoggz · #7 10-31-2011, 02:34 PM

You couldn't even spell check it before you showed it to a physics professor?

Jay2U · #8 10-31-2011, 02:46 PM

Further to this discussion, you may try the following as an experiment to understand the concept of the speed of light: Express velocity as seconds required for a certain distance. So, miles per second becomes seconds per mile. If you then would be travelling at the speed of light, it would take you 0 seconds to do a mile. Your 'clock' would come to a standstill. The (distant) observer, however, sees you going from A to B and draws the conclusion from his measurements, that you were pretty damn close to the speed of light. This also emphasizes that it all depends on the frame you are in. It's relative!

**bassybill** · #9 10-31-2011, 02:52 PM

Quote:

Originally Posted by Jay2U

Further to this discussion, you may try the following as an experiment to understand the concept of the speed of light: Express velocity as seconds required for a certain distance. So, miles per second becomes seconds per mile. If you then would be travelling at the speed of light, it would take you 0 seconds to do a mile. Your 'clock' would come to a standstill. The (distant) observer, however, sees you going from A to B and draws the conclusion from his measurements, that you were pretty damn close to the speed of light. This also emphasizes that it all depends on the frame you are in. It's relative!

Your clock is only at a standstill when observed by someone else, to whom you are relatively travelling at the speed of light. To you, it would keep right on ticking, and that would also be true as observed by someone who is maintaining an equal distance from you (as measured by you and them). Your clock would go at the same rate as theirs.

warnergt · #10 10-31-2011, 03:05 PM

Question: If you travel at the speed of light, will you stop
seeing your image in a mirror? Heh.

**bassybill** · #11 10-31-2011, 03:08 PM

You'd be able to shave just fine. Someone watching you go past would see you stop.

Schlyder · #12 10-31-2011, 03:12 PM

I have a question about the speed of light, the big bang, and the speed of the expansion of the universe.

At the moment of the big bang, when light, matter, and space were created(theoretically). And if light is the fastest thing in the universe, and being that light is a particle and wave, it has to occupy space. So how did the space get there before the light?

**bassybill** · #13 10-31-2011, 03:15 PM

Quote:

Originally Posted by Schlyder

I have a question about the speed of light, the big bang, and the speed of the expansion of the universe.

At the moment of the big bang, when light, matter, and space were created(theoretically). And if light is the fastest thing in the universe, and being that light is a particle and wave, it has to occupy space. So how did the space get there before the light?

Welcome . Department of Physics & Engineering Physics . College of Arts and Science . University of Saskatchewan

Schlyder · #14 10-31-2011, 03:21 PM

Quote:

Originally Posted by bassybill

Welcome . Department of Physics & Engineering Physics . College of Arts and Science . University of Saskatchewan

Does that mean you don't know?

I am guessing that light isn't the fastest thing in the universe.

Jay2U · #15 10-31-2011, 03:25 PM

Quote:

Originally Posted by bassybill

Your clock is only at a standstill when observed by someone else, to whom you are relatively travelling at the speed of light. To you, it would keep right on ticking, and that would also be true as observed by someone who is maintaining an equal distance from you (as measured by you and them). Your clock would go at the same rate as theirs.

You are right! I expressed myself quite sloppy.

Lonesomedave · #16 10-31-2011, 03:43 PM

Quote:

Originally Posted by DerHoggz

You couldn't even spell check it before you showed it to a physics professor?

how the heck do you spell check in photoshop elements text?

just asking because i don't know how to do it; fortunately he wasn't anal about it ....

DerHoggz · #17 10-31-2011, 04:04 PM

Quote:

Originally Posted by Lonesomedave

how the heck do you spell check in photoshop elements text?

just asking because i don't know how to do it; fortunately he wasn't anal about it ....

Sorry, I was in a bit of a pissed off mood from writing up a bunch of lab reports and being hungry. I have no clue about Photoshop Elements, but still follow proper English.