Author Topic: Question about the speed of light  (Read 635 times)

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Online One Above All

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Question about the speed of light
« on: June 01, 2012, 03:52:54 PM »
I've been told that the speed of light is invariant - that it is always the same, regardless of the frame of reference. Why is it that way? Is it a side-effect of time dilation and/or spacial contraction? Is it something else, possibly related to the fact that photons are massless?
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Offline inveni0

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Re: Question about the speed of light
« Reply #1 on: June 01, 2012, 06:36:17 PM »
You're being told something that we can't actually measure.  It's just a best guess based on our observations.  We don't have any way of telling if light slows down after traveling 14 billion light years.  It could go from 180k per second to 179.8k per second.  There's just no telling.  We assume it's constant based on how we observe its behavior in the cosmos.

At least that's my understanding.
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Re: Question about the speed of light
« Reply #2 on: June 02, 2012, 03:54:07 AM »
...I meant that even if you could reach the speed of light, light would still appear to be going at the speed of light from your perspective. How did you get that?
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Offline inveni0

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Re: Question about the speed of light
« Reply #3 on: June 02, 2012, 09:01:27 AM »
Ahhh, I get ya.  Yes, it's because time is relative.  That's what all that general theory of relativity is about.  If you Google it, you should get some decent layman's terms reading material to help understand how it works.  The problem, though, is that it doesn't ALWAYS work.  Take a black hole, for instance.  Its mass is so great that it distorts space and time to a point at which light can not travel outward.  It continually folds in on itself.  If the speed of light was constant, then a massive distortion in time wouldn't affect it.

But that brings me back to my original statement.  The general theory of relativity is general.  It's based on what we observe to be common states of occurrences based on our frame of reference.  Just like Newton's laws.
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Offline RNS

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Re: Question about the speed of light
« Reply #4 on: June 02, 2012, 09:05:41 AM »
So does light not slow down when it goes through different media? e.g. from space through the atmosphere or from the air into water?
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Re: Question about the speed of light
« Reply #5 on: June 02, 2012, 09:09:02 AM »
Ahhh, I get ya.  Yes, it's because time is relative.

From what I understand, this only happens with light. If you're traveling at 30 kph and a car next to you is traveling at 30 kph, you should be standing still relative to each other, just like the people inside said cars are standing still relative to their respective cars.

So does light not slow down when it goes through different media? e.g. from space through the atmosphere or from the air into water?

It does. Quite a bit, actually, depending on the medium. I was referring to the speed of light in vacuum, which is invariant.
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Offline inveni0

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Re: Question about the speed of light
« Reply #6 on: June 02, 2012, 08:11:51 PM »
So does light not slow down when it goes through different media? e.g. from space through the atmosphere or from the air into water?

http://en.wikipedia.org/wiki/Speed_of_light
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Offline Grogs

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Re: Question about the speed of light
« Reply #7 on: June 02, 2012, 08:50:51 PM »
I've been told that the speed of light is invariant - that it is always the same, regardless of the frame of reference. Why is it that way? Is it a side-effect of time dilation and/or spacial contraction? Is it something else, possibly related to the fact that photons are massless?

You've got a couple of different concepts rolled into one there.

First is the idea that nothing can move faster than the speed of light. I'm not sure we can really answer why that is. The idea came about, initially, by observation. If we give a particle X amount of energy and it goes half of the speed of light, we would expect (classically) that if we gave it 4X as much energy that it would go the speed of light. What we actually found is that 4X as much energy for maybe 80% of the speed of light, 100X got 99% the speed of light, 1000X got 99.9% of the speed of light, etc. Einstein explained this phenomenon by making the assumption that nothing could ever go faster than the speed of light. All of the effects such as time dilation fall out of the math that results from making this assumption.

The second concept is that the speed of light (in a vacuum) is the same at all times and places in the universe. There is nothing that I know of that strictly requires this to be true, but it certainly makes sense that it's so in terms of simplicity. Since the speed of light affects a great many phenomenon, there actually are quite a few tests we could perform to see if it has changed in the past. For example, since E=mc2, if the speed of light had been higher in the past, then if two electrons annihilated it would have produced higher energy gamma rays than the 511 keV ones we see today. Similarly, some atoms which are stable at our current speed of light would become unstable with a higher speed of light. To date, no effects like those have been observed.

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Re: Question about the speed of light
« Reply #8 on: June 03, 2012, 03:17:20 AM »
First is the idea that nothing can move faster than the speed of light. I'm not sure we can really answer why that is.
<snip>

We can. Light is composed of massless "particles" called photons. "Massless" being the key word there.

The second concept is that the speed of light (in a vacuum) is the same at all times and places in the universe.
<snip>

That's not the second concept. The second concept is that if you could move at 99% of the speed of light, to you, it would appear as if light was still moving at 300,000 kph. This is what appears illogical to me (and about half the things that happen when you go fast enough or small enough, but let's put those aside for now). How can light still appear to be traveling at the same speed as when you were standing still if you're going faster?
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Offline joebbowers

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Re: Question about the speed of light
« Reply #9 on: June 03, 2012, 05:30:17 AM »
What is typically referred to is the speed of light in a vacuum. It can and does slow down or speed up depending on what it is going through.
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Re: Question about the speed of light
« Reply #10 on: June 03, 2012, 05:31:06 AM »
300,000 kph.

This should be kps[1]. Sorry about that.
 1. Kilometers per second.
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Offline Grogs

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Re: Question about the speed of light
« Reply #11 on: June 03, 2012, 06:58:08 AM »
We can. Light is composed of massless "particles" called photons. "Massless" being the key word there.

That doesn't answer the questions of why does the universe have a speed of light, and why is it the particular value it is? That's what I thought you were getting at.

That's not the second concept. The second concept is that if you could move at 99% of the speed of light, to you, it would appear as if light was still moving at 300,000 kph. This is what appears illogical to me (and about half the things that happen when you go fast enough or small enough, but let's put those aside for now). How can light still appear to be traveling at the same speed as when you were standing still if you're going faster?

OK, let's say you're in a space ship going about 99.5% of the speed of light. That value gives you a time dilation factor of about 10:1. That is, it would take the space ship just over 10 years to travel to a star 10 light years away as observed from Earth, but to the occupants of the space ship the journey would only take 1 year. The natural question is, "How is that possible?" Shouldn't the crew members observe that they're traveling 10x the speed of light since they travel 10 light years in only one year of time from their perspective? The answer is no. If the crew members measure the distance to the star when they begin the trip, they will find that the length has contracted (because of their speed) and that the star only appears to be one light year away from them now.

So, person on Earth sends out a photon, and he observes that it travels a distance of 10 light years in 10 years.

A person on the space ship sends out a photon and observes that it travels one light year in one year.

Although those two observations are vastly different, the one constant between them is the speed of light. It's the other effects (time/length dilation) that allow that to be so.

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Re: Question about the speed of light
« Reply #12 on: June 03, 2012, 07:43:43 AM »
That doesn't answer the questions of why does the universe have a speed of light, and why is it the particular value it is? That's what I thought you were getting at.

Not at all. As far as I know, the answer to that question is "random chance".

<snip>

Thanks for the answer.
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Offline inveni0

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Re: Question about the speed of light
« Reply #13 on: June 05, 2012, 06:27:42 PM »
That's not the second concept. The second concept is that if you could move at 99% of the speed of light, to you, it would appear as if light was still moving at 300,000 kph. This is what appears illogical to me (and about half the things that happen when you go fast enough or small enough, but let's put those aside for now). How can light still appear to be traveling at the same speed as when you were standing still if you're going faster?

You're thinking of this in the wrong way.  If you start traveling from Earth away from the sun at the speed of light, then those light rays emitted from the sun will appear to be moving slower than the speed of light.  However, if you then light a match as you're traveling, the light from the match will appear to be traveling at the speed of light.  This is because time is relative.  People standing on Earth would measure the light from your match as moving at 2x the speed of light.  Is it really traveling at that speed?  No.  Because it's speed can only accurately be measured according to the space through which it's traveling.
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Re: Question about the speed of light
« Reply #14 on: June 06, 2012, 01:50:45 AM »
That is not what "invariant" means. Regardless of the frame of reference (note the "regardless"), light will always appear to be traveling at ~300,000 kps (which I will refer to as "c"), even if the photon had been emitted by a source already traveling at 0.999c or even c.
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Offline Avatar Of Belial

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Re: Question about the speed of light
« Reply #15 on: June 06, 2012, 02:07:49 AM »
So does light not slow down when it goes through different media? e.g. from space through the atmosphere or from the air into water?
What is typically referred to is the speed of light in a vacuum. It can and does slow down or speed up depending on what it is going through.

To both answer the first, and correct the second.

No photon ever goes slower than the speed of light. However, light gives the illusion of going slower when passing through various materials, because as the photons pass through the material and impact on the molecules within, they get absorbed. After a short delay another photon is released with the same - or at least a similar[1] trajectory. Thus the speed of light does not change - even when light is "slower".
 1. Refraction
« Last Edit: June 06, 2012, 02:09:44 AM by Avatar Of Belial »
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Offline Cyberia

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Re: Question about the speed of light
« Reply #16 on: June 06, 2012, 01:51:05 PM »
People standing on Earth would measure the light from your match as moving at 2x the speed of light.

No they wouldn't.  They'd see it moving at c.

rate = distance/time

c is a rate, a constant rate, which REQUIRES that distance and time are VARIABLE!  Admittedly this is counter-intuitive to humans, but that means what to the universe?  The variability of time and space manifest as Lorenz Contraction and Time Dilation, and c stays fixed.
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Offline inveni0

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Re: Question about the speed of light
« Reply #17 on: June 07, 2012, 07:52:26 AM »
People standing on Earth would measure the light from your match as moving at 2x the speed of light.

No they wouldn't.  They'd see it moving at c.

rate = distance/time

c is a rate, a constant rate, which REQUIRES that distance and time are VARIABLE!  Admittedly this is counter-intuitive to humans, but that means what to the universe?  The variability of time and space manifest as Lorenz Contraction and Time Dilation, and c stays fixed.

I actually worded that in a (really) bad way.  I'm trying to decipher it, but it came out so wrong I can't figure out what I was trying to say.  I need to start proof-reading.
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