Post by: chris on 09/03/2008 09:43:31
Chris
Post by: another_someone on 09/03/2008 10:14:23
There are other related issues, such as the permittivity and permeability of free space, but whether they actually give a cause to the speed of light, or merely a codependency, is another matter.
Post by: Soul Surfer on 09/03/2008 10:54:32
Post by: lightarrow on 09/03/2008 15:00:07
Post by: syhprum on 09/03/2008 17:30:08
We could have it one meter per second but then the meter would rather inconveniently large for general use (although a nano meter would be about a foot)
Post by: chris on 06/04/2008 10:22:24
Chris
Post by: Soul Surfer on 06/04/2008 10:56:58
Post by: lightarrow on 06/04/2008 12:52:41
Sure, but irrespective of what units we apply to it, light still has a maximum speed of a certain value. Why is it that value, 300 million metres per second?
Chris
For essentially the same reason glass' index of refraction is 1.55 and so light's speed is c/1.55: glass' properties. Substitute "glass" with "void".
Post by: lyner on 06/04/2008 13:08:42
Sure, but irrespective of what units we apply to it, light still has a maximum speed of a certain value. Why is it that value, 300 million metres per second?The fatuous reply is that it has to be something and it has to be constant. The actual choices of units for time and distance was basically arbitrary so the number is nothing significant. 186000miles per second is another way of saying the same thing.
Chris
If you are asking why it is constant then you are into Relativity.
A 'reason' for c being an upper limit for speeds is that mass, length and time go whacky as they approach c. In practice, to go faster and faster requires more and more energy - out of all proportion to what you would think of as Kinetic energy. This is an effect which effects how you get particles to go really fast in an accelerator. Electrons can be accelerated to very near c, at which speed, their masses are many many times their rest mass.
Post by: Madidus_Scientia on 06/04/2008 16:50:50
Post by: lyner on 07/04/2008 10:50:13
If you look upon mass as a property that an object has which governs how it will interact with another object - rather than 'the amount of stuff inside it' (leave the classical approach behind) then it can make more sense.
You could say that quantities like Energy and Momentum are more fundamental than Mass. Momentum tends to be conserved during interactions and so does 'energy plus the mass equivalent of energy'.
Post by: EdwardATeller on 17/11/2010 22:55:04
You obviously don't need to quantize space and time to make this work, but you do need to figure out why light can't travel the next Planck length any faster than 1 Planck time to advance this theory. This is so simple, it must have been suggested already and probably discredited, but the perfect simplicity of this really grabs my imagination.
Post by: peppercorn on 18/11/2010 10:17:55
This is so simple, it must have been suggested already and probably discredited, but the perfect simplicity of this really grabs my imagination.
Hi Edward! And can I offer a warm welcome to the forum [:)]
Simple as this idea is it is beyond my piecemeal knowledge of fund. physics, but I'm sure there are some here who can tackle it, er.. properly [:-X] .....
Post by: imatfaal on 18/11/2010 10:49:08
Quote
why light can't travel the next Planck length any faster than 1 Planck timeis because that's how its defined.
For your argument to succeed you need a unit of time that has a physical meaning throughout universe and history and is not defined through use of the speed of light. you also need to show an argument which explains why space and time are related in such a way that many units of time can be utilised to travelled one unit of length; but that many units of length cannot be travelled in one unit of time.
Post by: Pikaia on 18/11/2010 15:25:12
Post by: Bored chemist on 18/11/2010 19:46:53
Those two quantities are measurable (actually, one is defined by our choice of the definition of the Ampere); if you do the maths you get about 300000000 m/s.
Post by: Pikaia on 18/11/2010 20:28:40
Post by: QuantumClue on 18/11/2010 20:32:21
What is the reason that the speed of light in a vacuum has the value that it does?
Chris
Because the density of spacetime will dictate their velocities. Technically, they will always have a constant velocity, but if you altered the density the speed of light would change
.
Post by: peppercorn on 19/11/2010 12:56:50
...density of spacetime will dictate [photon] velocity.
Can you explain what you mean by density w.r.t. spacetime?
Post by: QuantumClue on 19/11/2010 13:16:40
...density of spacetime will dictate [photon] velocity.
Can you explain what you mean by density w.r.t. spacetime?
The Vacuum contains an energy density. Some scientists like Prof. John Barrow a mathematical physicist among several other scientists have stated that it is possible light speed was atleast 50 times less than what is percieved today due to the vacuum energy variating.
Post by: peppercorn on 20/11/2010 20:31:55
Still, that doesn't answer the question, why 300 and not 200 or 500?
BC explanation seems a good place to start IMO:
Because Maxwell's equations say (IIRC) it's the reciprocal of the square root of the product of the permittivity and permeability of free space.
Those two quantities are measurable (actually, one is defined by our choice of the definition of the Ampere); if you do the maths you get about 300000000 m/s.
Beyond that try reading:
http://en.wikipedia.org/wiki/Anthropic_principle
Post by: CPT ArkAngel on 20/11/2010 20:43:46
the speed of light = 299 792 458 m / s
Post by: Bill S on 21/11/2010 18:04:48
Quote from: QC
Prof. John Barrow a mathematical physicist among several other scientists have stated that it is possible light speed was atleast 50 times less than what is percieved today
Alternatively, João Magueijo and others argue that the speed of light may have been greater in the distant past. YPYM&YTYC!
Post by: AnswerIs42 on 07/10/2012 19:43:01
In our current universe, which is always changing due to the observed expansion of the universe, the speed of light is what it is because it is the point at which any rest energy a particle has (i.e. it's mass) as been fully converted to energy. Once you no longer have mass to "push" their is nothing left of "matter" to measure a velocity on.
Now e=mc2 shows that as a particle's speed increases so does its mass and that as this continues you need more and more energy to "push" an increasing mass which is why a very small amount of mass is equivalent to orders of magnitude in energy. But if you can reach the point at which all rest energy is used - like in an matter anti-matter interaction, all that was matter and being "pushed" is now pure energy. Since "speed" is a measure of an "objects" velocity it means we can't measure energy's speed since we have no "object".
My last point is on the "current" speed of light. Note my opening remark on the "current" universe. I know we have two camps as it relates to universe inflation but my thinking is that the speed of light has not and is not always constant as we want it to be in the "standard model". Since my answer is that the "current" speed of light is set by the point at which all rest energy is converted to energy it then follows - why does this point always result in the speed of light we see today?
In other words, why does 'e' equal mc2. Was the current amount of rest energy a partical has always the same? I think not, as this to me is a function of the universe we have at any moment in time and we know the universe is in constant expansion. All current models acknowledge this expansion but still treat its fact as "non-existent" or of "zero-effect". I think this is very short sighted. If the universe is now larger than a second ago then something "is" different. Is their more/less dark energy and/or more/less dark matter in what is now a larger space?
Either way it can not be the same and this means the amount of rest energy for a given amount of mass to reach 'c' however so slight is not the same. Basically, in a perfect void of space of a given volume you have to have a limit on the total amount of 'stuff' (energy/matter both light and dark) that it can contain. If that volume is increased or decreased the 'total-stuff' limit has changed. I like to think of this 'stuff' in total as the 'cosmic pressure' or 'zero energy' of the universe. So any release of rest energy from the particle into space has to 'fight' against this ever present cosmic pressure which would affect in the end the speed of light as the universe changes in size.
Post by: Phractality on 08/10/2012 08:11:16
Most technically, these days, because we have now gone and defined the metre in relation to the speed of light and the second, so we now have the situations that the standards body have actually defined what the speed of light must be, and all we can do is change our measure of the metre rather than our measure of the speed of light.
Yes; that is the correct answer. To clarify a bit, we originally defined the meter as 1/10,000,000 of the distance along a line of longitude at sea level from the equator to the North Pole. And we defined the second as 1/86,400 of a sidereal day. Later, we measured how many seconds it takes for light to go a certain number of meters. Then we discovered that the speed of light is a more reliable constant than the physical parameters of planet Earth, and it is also easier to measure; so we redefined the meter and second in terms of the speed of light, which had already been measured. Now that the speed of light defines the meter and second, it is possible for the size of Earth in meters to vary, and the length of the sidereal day may also vary.
There are other related issues, such as the permittivity and permeability of free space, but whether they actually give a cause to the speed of light, or merely a codependency, is another matter.
This is a popular misconception. The permittivity and permeability of free space are defined in terms of the speed of light. They are what they are because the speed of light is what it is. There is no physical explanation of how permeability and permittivity cause light to propagate at that speed.
We have an excellent understanding of how acoustic waves propagate in solids (http://en.wikipedia.org/wiki/Speed_of_sound#Speed_of_sound_in_solids). In some aether theories, the speed of light can be explained by the formula for the speed of shear waves in a solid medium. If the aether is a solid, we might expect its inertial density and shear modulus to determine the speed of light, according to the acoustic formula. That can tell is the ratio of the aether's density to its shear modulus, but it can't tell us how great either of those parameters is. Conceivably, permittivity and permeability of free space are analogous to the density and shear modulus of the aether. (The existence of aether has not been disproven; it has only been shown that it is irrelevant as long as there are no faster-than-light phenomena.)
Post by: Guthers on 08/10/2012 18:19:47
Sure, but irrespective of what units we apply to it, light still has a maximum speed of a certain value. Why is it that value, 300 million metres per second?I think this is the same as asking why the speed of light is finite at all, since any numerical value we assign to it is purely arbitrary, depending on our perception of whatever time is. Other than that I like AI42's answer, that it can't go any faster because there's no fuel left to push it.
Chris
Post by: LetoII on 08/10/2012 20:03:49
about the speed of light, i think that's covered by theories that the speed of light is constant in respect to the size of the universe, it can be higher / lower in a bigger / smaller universe.
Post by: yor_on on 14/10/2012 17:36:55
You could make a clock ticking faster and define a length scale other than what we use, but using that clock and that measuring stick you would still find it to be a constant for all frames of reference (in the modern interpretation)
Post by: simplified on 19/10/2012 19:56:28
Post by: simplified on 07/11/2012 16:35:25
Post by: simplified on 10/11/2012 13:32:20
mv^2/2=mc^2(g^2-1)/2g^2
But the energy in the mass needs energy for own motion
(g-1)mc^2 - mc^2((g^2-1)/2g^2
g - gamma factor
m - mass
c - light speed
v - speed