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Author Topic: How does light remain constant when travelling through space?  (Read 2877 times)

@rathgar

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@rathgar asked the Naked Scientists:
   
When light is travelling through space that is expanding, how does it remain constant? Is it just relative?

What do you think?
« Last Edit: 30/09/2011 14:30:06 by _system »


 

Offline MikeS

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How does light remain constant when travelling through space?
« Reply #1 on: 01/10/2011 07:45:30 »
It would seem that if anything happens to potentially alter the speed of light then the passage of time (or possibly length) alters in just the right manner to keep light speed constant.
 

Offline JP

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How does light remain constant when travelling through space?
« Reply #2 on: 01/10/2011 12:38:54 »
It's one of the fundamental properties of the universe.  The speed of light is always constant in a vacuum.  Other properties of the universe come from that property.
 

Offline yor_on

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How does light remain constant when travelling through space?
« Reply #3 on: 01/10/2011 18:11:40 »
As JP said it. Also, can there be any radiation in a space that doesn't exist? Before the 'expansion' becomes measurable? And as soon as 'it' is there it has to obey the laws governing SpaceTime, as long as you accept relativity. If it didn't, Einstein should have been wrong, but I don't think he is.
 

Offline imatfaal

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How does light remain constant when travelling through space?
« Reply #4 on: 04/10/2011 10:36:21 »
@rathgar,

whilst the above are mostly correct - I am not sure they dealt with your question.  Firstly to re-iterate what JP said; light speed is always constant in a vacuum - every second it goes a whisker under 3*10^8 metres, in a year it travels a light year.  Through space in a local frame it is such a constant that much of physics is based on that fact. But if I might throw a spanner in the works...

if you look at the light emitted by the universal haze at the very beginning of the universe (the poetically named surface of last scattering);  this light was emitted by very hot hydrogen about ~13.7Gy (billion years) ago and it has been travelling at 299,792,458 m/s ever since.  This is what we now call the Cosmic Microwave Background Radiation.  To restate - the light we now see on earth as CMBR has been travelling for ~13.7Gy, so if light speed is constant can we safely assume that the matter that emitted that portion of the CMBR we are now observing is ~13.7Gly (billion light years) away?  NO.  In fact the best estimate is about 45.7Gly. 

It seems that we can do a simple v=s/t - but that would give light speed as being bigger than light speed and is wrong.  Light travels in the vacuum locally at c - no faster no slower, but the universe has expanded in the meantime.  Over long distances and time there is an illusion that light covers more ground than it should because space itself is expanding (a velocity calc needs to assume that s distance is measurable and unchanging); but in fact light always travels through local space at light speed
 

Offline Titanscape

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How does light remain constant when travelling through space?
« Reply #5 on: 04/10/2011 13:37:50 »
I wrote a similar question about light being dragged by the pull of a black hole. For example consider heat or light emitted by an object falling into a black hole. The light emitted in two directions, toward and away. It does not speed up or slow down, before or after the event horizon? Despite mass and gravity? How so?

http://www.thenakedscientists.com/forum/index.php?topic=4319.0

This thread offers the explanation of distorted space.
« Last Edit: 04/10/2011 13:47:26 by Titanscape »
 

Offline JP

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How does light remain constant when travelling through space?
« Reply #6 on: 04/10/2011 13:53:09 »
I wrote a similar question about light being dragged by the pull of a black hole. For example consider heat or light emitted by an object falling into a black hole. The light emitted in two directions, toward and away. It does not speed up or slow down, before or after the event horizon? Despite mass and gravity? How so?

It comes back to the fact that the speed of light is constant in a vacuum.  We can't explain, on a fundamental level, why this fact is true, but all our experimental and observational evidence leads us to believe this is true. 

Of course, light is still affected by gravity.  The path of a light ray can be bent due to gravity, though the speed it moves remains constant.  The wavelength of light can also change due to gravity.  This means it loses energy when moving away from a gravitational source and gains energy when moving towards it, even though its speed doesn't change.
 

Offline MikeS

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How does light remain constant when travelling through space?
« Reply #7 on: 07/10/2011 14:24:53 »
I wrote a similar question about light being dragged by the pull of a black hole. For example consider heat or light emitted by an object falling into a black hole. The light emitted in two directions, toward and away. It does not speed up or slow down, before or after the event horizon? Despite mass and gravity? How so?


After light has fallen into the event horizon we do not know what has happened to it.

How does light emitted from an object falling into a black hole remain a constant speed?  Light travelling away from the object is the same speed as light emitted towards the black hole due to different rates of time dilation.  Time dilates more the closer the object approaches the event horizon.  Whilst the object remains visible its emitted light remains at 'c'.
 

Offline Titanscape

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How does light remain constant when travelling through space?
« Reply #8 on: 15/10/2011 00:15:41 »
I wrote a similar question about light being dragged by the pull of a black hole. For example consider heat or light emitted by an object falling into a black hole. The light emitted in two directions, toward and away. It does not speed up or slow down, before or after the event horizon? Despite mass and gravity? How so?

It comes back to the fact that the speed of light is constant in a vacuum.  We can't explain, on a fundamental level, why this fact is true, but all our experimental and observational evidence leads us to believe this is true. 

Of course, light is still affected by gravity.  The path of a light ray can be bent due to gravity, though the speed it moves remains constant.  The wavelength of light can also change due to gravity.  This means it loses energy when moving away from a gravitational source and gains energy when moving towards it, even though its speed doesn't change.

So I take it that a black hole can change the wave length, instead of slowing it loses energy and has a greater wavelength, and goes from light to infra red. What is the basest infra red? Is their a limit?
« Last Edit: 15/10/2011 00:19:13 by Titanscape »
 

Offline yor_on

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How does light remain constant when travelling through space?
« Reply #9 on: 15/10/2011 02:29:19 »
Yea, nice one Titan. But there is one more thing to consider, light as a 'light quanta'. A light quanta do not change its 'energy', what changes it is the relation between 'frames of reference'. That means that any light climbing a gravity well will to you outside the well be 'red shifted' but to a imagined traveler 'at rest' with that light quanta it never change 'energy'. So in a way your, and mine too, question becomes one about what 'time' is.

Because if we look at a event horizon, we should always find a level where, relative you observing, that light has to become 'smeared out', or 'red shifted', to a level where we will find it very hard to measure. Now, the question becomes one of where did it go?

Conceptually you can always find a frame of reference from where it isn't gone, just as my example above, but as your 'universe', and 'detector' is the one telling you the truth about you relative that red shift, as in being able to measure it, the question must be a true one for you. And there the answer must be that it, from your perspective, now has joined the 'room time' existing.

And that opens for a lot of other thoughts, as there 'coexist' frames of reference from where it must be measurable. It's a 'time skewed' universe.
 

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How does light remain constant when travelling through space?
« Reply #9 on: 15/10/2011 02:29:19 »

 

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