# The Naked Scientists Forum

### Author Topic: Why does light travel at the speed of light? (in a vacuum).  (Read 16433 times)

#### Mr. Data

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##### Why does light travel at the speed of light? (in a vacuum).
« Reply #25 on: 06/07/2011 21:16:29 »
And how do you explain the numerical values of those parameters without knowing light' speed? Any measure of them can be traced to a light' speed measure, actually.

L-Arrow - are you sure about that?  Whilst most measurements do use the relationship through maxwell's equations - there is no requirement that this is the case.  Nowdays both permitivity and permeability are defined by their relation with the speed of light BUT the wonder of Maxwell's equations is that they can also be calculated without reference to the speed of light
You know tha μ0 is defined from the Ampère (μ0 = 4π*10-7), so it remains ε0. How can you measure it? You shoul for example measure the coulombian force between two charges at a specific distance because of:

F = (1/4πε0) q1*q2/r2

I wonder if it's possible to measure the charges q1 and q2 in Coulomb, without knowing ε0.

Well isn't it apart of the equation, so ask yourself if you pull out the same value.

#### Heikki Rinnemaa

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##### Why does light travel at the speed of light? (in a vacuum).
« Reply #26 on: 22/07/2011 11:49:14 »
Those all vibration cannot exist pure matterless place,,,

Sure they can.

Pure matterless space,,or "room" cannot exist any kind of matter,,
therefore there cannot exist any kind of matter-vibration,,waves,,
and this means that space is full of matter because there can travell radiowaves,,etc.

We known that example space-vehicles can stay on the space one point,,,or we can control motion-direction,,,,why,,because round of this vehicle is matter.

So called ether is,,but what kind is it,,i dont know,,and atom-theory cannot give that answer,,

Of cource i can be wrong,,,:)

#### JP

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##### Why does light travel at the speed of light? (in a vacuum).
« Reply #27 on: 22/07/2011 13:41:35 »
But light doesn't need an aether to propagate through.  The Michelson-Morely experiment in 1887 showed this, and then Einstein later came along and gave us good theoretical reasons why aether doesn't have to exist.  Further experiments have verified both of these results and also shown that while space does have properties that allow light waves to travel through it, these properties are nothing like matter.

#### peppercorn

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##### Why does light travel at the speed of light? (in a vacuum).
« Reply #28 on: 22/07/2011 20:20:32 »
Further experiments have verified both of these results and also shown that while space does have properties that allow light waves to travel through it, these properties are nothing like matter.

That's an interestingly placed 'while' JP. ... It got me thinking: Does matter actually have a property allowing light waves to travel through it?
I mean matter interacts with light-waves, but that is a fundamentally different mechanism isn't it?   In fact, only empty space has that ability  ... Or am I being too philosophical?

#### JP

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##### Why does light travel at the speed of light? (in a vacuum).
« Reply #29 on: 23/07/2011 14:23:18 »
That's an interesting question.  I suppose it comes back to what the definition of a wave in matter is.  Look at another wave: sound.  If sound is traveling through matter, we say it's carried by pressure waves.  But pressure is caused by the interaction of neighboring molecules via the EM force, which transmits even through the empty spaces between molecules.  So in a sense, even sound waves have some component traveling through a vacuum.  Of course, we can look at the big picture and still model a sound wave via pressure variations without worrying about the fine details of the EM forces involved.

I think for a wave in matter, what we're really looking for is some property of the matter (which need not be a fundamental one) so that you can describe the propagation of energy waves through the matter by a "waving" of this property.  In the case of light in a dielectric, I believe this is polarization of the material.  (I haven't worked through all the details, but I think it plays the same role here as pressure does for sound.)

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#### MikeS

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##### Why does light travel at the speed of light? (in a vacuum).
« Reply #30 on: 23/07/2011 17:28:06 »
Shrunk
Why does light travel at the speed of light? (in a vacuum).

Photons travel instantaneously, they exist in only the now.  Another way of looking at this is they travel at infinite speed.

How can a photons infinite speed be measured to approximately 300,000 Km per second in a vacuum.  Surely, infinity is infinity?

Let’s take a brief look at time.  What are its attributes?  There is an arrow of time and it can ‘flow’ (be dilated or contracted) at different rates.  At the two extremes time can ‘flow’ infinitely fast (ultimate contraction) or it can ‘flow’ ultimately slow (ultimate dilation) and stop.  ‘Infinitely’ slow (stopped) is a finite  quantity.

Where time stops can be viewed as an absolute.  It is a very similar concept to the temperature of absolute zero.  You cannot go colder, that is where the scale ends.  Time cannot ‘flow’ slower than stopped.

To a photon, the clock has stopped.  It cannot go faster than that.  To do so would require the reversal of time.

The faster anything moves, the slower time passes (dilates).  This puts an upper limit on speed as ultimately the clock stops.  The same upper limit is reached when viewed from the perspective of energy and mass.

#### PhysBang

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##### Why does light travel at the speed of light? (in a vacuum).
« Reply #31 on: 23/07/2011 17:48:12 »
Why does light travel at the speed of light? (in a vacuum).

Photons travel instantaneously, they exist in only the now.  Another way of looking at this is they travel at infinite speed.
That's no good way to look at them, since we've known since the 1600s that this is not the case.
Quote
The faster anything moves, the slower time passes (dilates).  This puts an upper limit on speed as ultimately the clock stops.  The same upper limit is reached when viewed from the perspective of energy and mass.
This too, is incorrect as far as relativity theory (or any other plausible theory that I know of) goes. Clocks never stop, they only slow in relation to certain sets of coordinates.

#### lightarrow

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##### Why does light travel at the speed of light? (in a vacuum).
« Reply #32 on: 23/07/2011 19:33:49 »
You know tha μ0 is defined from the Ampère (μ0 = 4π*10-7), so it remains ε0. How can you measure it? You shoul for example measure the coulombian force between two charges at a specific distance because of:

F = (1/4πε0) q1*q2/r2

I wonder if it's possible to measure the charges q1 and q2 in Coulomb, without knowing ε0.
Well isn't it apart of the equation, so ask yourself if you pull out the same value.
How do you measure a charge in Coulomb?

#### Heikki Rinnemaa

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##### Why does light travel at the speed of light? (in a vacuum).
« Reply #33 on: 24/07/2011 07:01:30 »
But light doesn't need an aether to propagate through.  The Michelson-Morely experiment in 1887 showed this, and then Einstein later came along and gave us good theoretical reasons why aether doesn't have to exist.  Further experiments have verified both of these results and also shown that while space does have properties that allow light waves to travel through it, these properties are nothing like matter.

Hmm,,Michelson and Einstein make those opionion those days,,because electricity-history was backround of that thinking,,,and of-cource wave-theory,,ant thinking that all light and colors are not matter think,,only wave-vibration like alternative electricity can describe,,,that thought-path was wrong and control science wrong direction,,,many hundred years,,,

Todays we known that light is some kind matter-particle-motion,,, or thouhgt we known :)

Example of moon,,,, moon cannot stay on it's position to round earth if round of the moon dont have any kind on matter,,,basic-space-matter where all this space-planets can floation,,

Some times i have think that this light-particles from sun is that basic matter--but then i change my thoughts,,because less-space opposite moon,,shadow-side has less particle and still moon goes like goes,,

When ether=some kind basic space matter,, is,, then
- planets can floating in this matter
- space-vehicles can drive where we want,,or say one position
radiowaves can go in this matter,,,like sound goes in the air,,

- sound is matter-vibration-motion
- without matter both cannot exist

Or i can be wrong still,, still without sun life at earth-ball cannot exist,, space is unknown big area,,:)

#### damocles

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##### Why does light travel at the speed of light? (in a vacuum).
« Reply #34 on: 24/07/2011 15:44:07 »
From lightarrow:
Quote
I wonder if it's possible to measure the charges q1 and q2 in Coulomb, without knowing ε0.

In some circumstances it is, lightarrow, provided you have a reliable value for the fundamental charge in coulomb from Millikan's oil drop experiment or some similar source. You simply count ions or electrons in the particles/devices making q1 and q2. And Millikan's oil drop experiment does not rely on prior knowledge of a value for the speed of light.

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#### MikeS

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##### Why does light travel at the speed of light? (in a vacuum).
« Reply #35 on: 27/07/2011 06:43:35 »
Shrunk
Why does light travel at the speed of light? (in a vacuum).

Photons travel instantaneously, they exist in only the now.  Another way of looking at this is they travel at infinite speed.
That's no good way to look at them, since we've known since the 1600s that this is not the case.
Quote
The faster anything moves, the slower time passes (dilates).  This puts an upper limit on speed as ultimately the clock stops.  The same upper limit is reached when viewed from the perspective of energy and mass.
This too, is incorrect as far as relativity theory (or any other plausible theory that I know of) goes. Clocks never stop, they only slow in relation to certain sets of coordinates.

It is the way that many people look at them.  I can find no references why this should not be the case.  Would you please quote references to prove otherwise.  No need to go back as far as the 1600s.
From the coordinates of the photon the clock has stopped.  In certain aspects of relativity the clock never actually stops but the trend is the same.  An object with mass can never actually reach the speed of light but the the statement "The faster anything moves, the slower time passes (dilates)" is correct.

"Photons travel instantaneously.
The faster anything moves, the slower time passes (dilates)."

These statements are both true.  Put them together and it explains (as I have done) why the speed of light is what it is and why nothing can go faster than that.

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#### imatfaal

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##### Why does light travel at the speed of light? (in a vacuum).
« Reply #36 on: 27/07/2011 10:06:05 »
Shrunk
Mike - please keep this to new theories.

#### Airthumbs

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##### Why does light travel at the speed of light? (in a vacuum).
« Reply #37 on: 29/07/2011 02:31:50 »
I have to honestly say I have read all the responses and I only really understand about 50% of it!!  One thing I might have picked up which has only raised another question is this.....
If one were able to ride on a beam of light would time stand still in relation to everything else?  We use time as our universal clock but is light itself not subjected to time because of the speed it travels at?

Can someone also please try to explain in English, without funky letters or equations for which I am completely unfamiliar with, why light travels at the speed it does through a vacuum?

PS. I am sure if I asked the same kind of question about the speed of sound through our atmosphere at sea level I would get a response for which I could clearly get my head round but this it seems is something else?

#### MikeS

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##### Why does light travel at the speed of light? (in a vacuum).
« Reply #38 on: 29/07/2011 05:51:05 »
I have to honestly say I have read all the responses and I only really understand about 50% of it!!  One thing I might have picked up which has only raised another question is this.....
If one were able to ride on a beam of light would time stand still in relation to everything else?  We use time as our universal clock but is light itself not subjected to time because of the speed it travels at?

Can someone also please try to explain in English, without funky letters or equations for which I am completely unfamiliar with, why light travels at the speed it does through a vacuum?

PS. I am sure if I asked the same kind of question about the speed of sound through our atmosphere at sea level I would get a response for which I could clearly get my head round but this it seems is something else?

Yes, to both

I think I would be right in saying the mainstream answer is "it just does".
If you find that unsatisfactory you can read my explanation here in new theories.

http://www.thenakedscientists.com/forum/index.php?topic=40322.msg363362#msg363362

#### lightarrow

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##### Why does light travel at the speed of light? (in a vacuum).
« Reply #39 on: 29/07/2011 08:02:33 »
If one were able to ride on a beam of light would time stand still in relation to everything else?
...and time would stand still for everything else in relation to light. So it doesn't count...

Quote
We use time as our universal clock but is light itself not subjected to time because of the speed it travels at?
Define "subjected to time", because for what I wrote up, it's not clear.

#### MikeS

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##### Why does light travel at the speed of light? (in a vacuum).
« Reply #40 on: 29/07/2011 12:58:55 »
If one were able to ride on a beam of light would time stand still in relation to everything else?
..]and time would stand still for everything else in relation to light. So it doesn't count...

Why doesn't it count?
« Last Edit: 29/07/2011 13:00:54 by MikeS »

#### lightarrow

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##### Why does light travel at the speed of light? (in a vacuum).
« Reply #41 on: 29/07/2011 13:17:10 »
Why doesn't it count?
Because you can't say which clock goes slower than the other, since *both* are seen going slower, from the other: a clock on Earth is measured going slower, from a fast spaceship's frame; the spaceship's clock is measured going slower from the Eart's frame.
I know you are talking of light instead of a spaceship, but you shouldn't, if you want to know what you would see or measure from a very fast frame.

The question:
"If one were able to ride on a beam of light would time stand still in relation to everything else?"

since it's not possible *even in principle* to ride on a beam of light because a frame of reference in which light (in the void) is stationary doesn't exist,
should be interpreted in this way:
"If one were inside a spaceship traveling at almost light's speed with respect (e.g.) to Earth, as near to c as one want, would time go slower for him with respect to Earth?"
The answer is: yes, and time would go slower for Earth with respect to the astronaut.

Twin's "paradox" comes into existence only when one of the two (the spaceship or the Earth) goes back (I make it simple, this is just an example) and they meet again. The one who comes back then is younger than the other.
« Last Edit: 29/07/2011 13:31:06 by lightarrow »

#### MikeS

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##### Why does light travel at the speed of light? (in a vacuum).
« Reply #42 on: 29/07/2011 14:48:56 »
If one were able to ride on a beam of light would time stand still in relation to everything else?
...and time would stand still for everything else in relation to light. So it doesn't count...

I think the sentence is a little misleading as it stands.  If you had written
"from the 'perspective (point of view, reference frame etc.)' of light, time would stand still for everything else in relation to light"  then I would agree it doesn't count.

#### Airthumbs

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##### Why does light travel at the speed of light? (in a vacuum).
« Reply #43 on: 29/07/2011 14:52:54 »
Define "subjected to time", because for what I wrote up, it's not clear.
[/color]

Sorry I don't think I can...

#### lightarrow

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##### Why does light travel at the speed of light? (in a vacuum).
« Reply #44 on: 29/07/2011 14:54:23 »
If one were able to ride on a beam of light would time stand still in relation to everything else?
...and time would stand still for everything else in relation to light. So it doesn't count...
I think the sentence is a little misleading as it stands.  If you had written
"from the 'perspective (point of view, reference frame etc.)' of light, time would stand still for everything else in relation to light"  then I would agree it doesn't count.

It was what I intended. (But you are answering to me, here, or to Airthumbs?)
« Last Edit: 29/07/2011 14:58:33 by lightarrow »

#### MikeS

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##### Why does light travel at the speed of light? (in a vacuum).
« Reply #45 on: 30/07/2011 06:40:29 »
If one were able to ride on a beam of light would time stand still in relation to everything else?
...and time would stand still for everything else in relation to light. So it doesn't count...
I think the sentence is a little misleading as it stands.  If you had written
"from the 'perspective (point of view, reference frame etc.)' of light, time would stand still for everything else in relation to light"  then I would agree it doesn't count.

It was what I intended. (But you are answering to me, here, or to Airthumbs?)

I was answering to you.  "and time would stand still for everything else in relation to light.  So it doesn't matter"

#### damocles

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##### Why does light travel at the speed of light? (in a vacuum).
« Reply #46 on: 30/07/2011 07:21:51 »
One of the main direct verifications of Special Relativity is that unstable particles moving close to the speed of light appear to us to decay slower than they should.

Perhaps a photon has a really short decay lifetime in its own framework, but for us, its time is stopped, so it never decays!

(totally whimsical speculation).

#### MikeS

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##### Why does light travel at the speed of light? (in a vacuum).
« Reply #47 on: 30/07/2011 09:13:54 »
One of the main direct verifications of Special Relativity is that unstable particles moving close to the speed of light appear to us to decay slower than they should.

Perhaps a photon has a really short decay lifetime in its own framework, but for us, its time is stopped, so it never decays!

(totally whimsical speculation).
As far as we know a photon never decays in any time frame, it just 'splats'.

#### The Naked Scientists Forum

##### Why does light travel at the speed of light? (in a vacuum).
« Reply #47 on: 30/07/2011 09:13:54 »