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The Speed of Light

Onyx

Active Member
Premium Member
I read up on relativity today, and am wondering if there is extra theory behind why the speed of light in a vacuum is 299,792,458 m/s and not some other value? Or is this simply a measured phenomena with no further explanation right now? Thanks!
 

idav

Being
Premium Member
I'm thinking it has something to do with the principles regarding potential.
I think it's something like what nowhere man implies here. Simply put it would take every inkling of energy to propel something to go the speed of light, at which point there appears to be some sort of max potential but I'm not entirely sure.
 

Polymath257

Think & Care
Staff member
Premium Member
I read up on relativity today, and am wondering if there is extra theory behind why the speed of light in a vacuum is 299,792,458 m/s and not some other value? Or is this simply a measured phenomena with no further explanation right now? Thanks!

I'm not sure there is a ready answer to your question. The specific number, 299,792,458 is how we *define* the concept of a meter: by defining the speed of light to be this many meters per second, and from having a definition of a second, we get how long a meter is. We use the oscillations of a particular wavelength of light from Cesium atoms to define a second.

So, in one sense, the value is determined solely from our definitions.

On the other hand, we *could* define our notions of meter and second some other way and then ask why the speed of light turns out to have the specific value it has. For example, we might define the second as 1/86400 of a day and a meter as 1/10,000,000 of the distance from the equator of the Earth to the pole (both definitions have been used in the past). The problem is, of course, that those definitions don't get to any fundamental aspect of reality, as opposed to accidental aspects of this specific planet we call the Earth.

So, a different tact: define the meter in terms of, say, the distance across a hydrogen atom and a second as, say, the time it takes to go across some number of hydrogen atoms. But, if you think about that, this is just again defining a specific number as the speed of light.

The problem here is that by changing our definition of meter or second, we change the numerical value of the speed of light. What we really need to do is find a number that *doesn't* change simply because of our definitions of silly things like meters and seconds.

And there *is* a good number for that: it compares the speed of light and Planck's constant and so relates the speed of light and the size of quantum events. it is called the fine structure constant and has a value of 1/137 (approximately) no matter how meters and seconds are defined. it is *this* number, more than the speed of light, which defines the structure of things like atoms and the relative sizes of things.

So why is the fine structure constant what it is? Good question. Nobody knows and it isn't even clear that it *could* be a different value.

It turns out that most of physics is based on a very few 'universal constants' like the fine structure constant that then determine the relative scale of everything else. Why they have the values they do is anyone's guess. Whether they even *could* be different values is anyone's guess. Since they seem to be fixed, we don't know how or if they could be different than they are.
 

Polymath257

Think & Care
Staff member
Premium Member

It should be pointed out that many of these modified gravity proposals were recently ruled out because they disagree with the observations made of gravitational waves. In particular, the recent waves from a neutron star merger together with the observation of the same event with light shows that the speed of light and the speed of gravity are much closer than these theories allow. The difference in time was less than 3 hours and some of these theories predicted the difference would be up to a few years.
 

james dixon

Well-Known Member
Premium Member
Question~~~~

If I am standing in the train conductor's front wheel house & I point a flashlight in the direction the train is going while the train is traveling 100 mph down the track, IS THE BEAM I AM SHINNING AHEAD TRAVELING THE SPEED OF LIGHT + THE SPEED OF THE TRAIN, exceeding the speed of light-?
 

Polymath257

Think & Care
Staff member
Premium Member
Question~~~~

If I am standing in the train conductor's front wheel house & I point a flashlight in the direction the train is going while the train is traveling 100 mph down the track, IS THE BEAM I AM SHINNING AHEAD TRAVELING THE SPEED OF LIGHT + THE SPEED OF THE TRAIN-?

Ans: no. This is basic in the theory of relativity.

Both you (on the train) and someone on the ground (off the train) measure the light to be going the same speed.
 

james dixon

Well-Known Member
Premium Member
Ans: no. This is basic in the theory of relativity.

Both you (on the train) and someone on the ground (off the train) measure the light to be going the same speed.

I am ignorant to the theory of relativity. Can you lay this out in layman’s terms.,., explaining why the flashlight beam is not traveling faster than light-?

Thanks in advance
:)-
 

Grandliseur

Well-Known Member
I read up on relativity today, and am wondering if there is extra theory behind why the speed of light in a vacuum is 299,792,458 m/s and not some other value? Or is this simply a measured phenomena with no further explanation right now? Thanks!
Obviously, I have no clue.

Did you know that the speed of light you quote is the unfettered speed. Light has been slowed down in several experiments to a fair crawl.

I would speculate that when we have about 10 or 11 dimensions of space once we go into the scientific theories, string, and all those quarks that have to keep interacting to hold together our matter, and the quantum void of probability particles in the space-time field, that perhaps the speed has an upper limit pertaining to these matters. Light does need to travel through the space-time field in which these interactions occur. Since things are taking place there, I see light as a ship sailing through water which has resistance, and the faster you go, the harder the going gets. With light, a balance would then happen to create a speed limit when passing through this space-time field.

Ups! Just my ruminations. Nothing but assumptions. :p
 

Onyx

Active Member
Premium Member
If my understanding is correct, the maximum speed of light is independent of physical motion or frames of reference, therefore the velocity of the train would have nothing to do with it. There is the velocity of the train, and the velocity of light, separately. Did I get that right?
 

Revoltingest

Pragmatic Libertarian
Premium Member
I'm not sure there is a ready answer to your question. The specific number, 299,792,458 is how we *define* the concept of a meter: by defining the speed of light to be this many meters per second, and from having a definition of a second, we get how long a meter is. We use the oscillations of a particular wavelength of light from Cesium atoms to define a second.

So, in one sense, the value is determined solely from our definitions.

On the other hand, we *could* define our notions of meter and second some other way and then ask why the speed of light turns out to have the specific value it has. For example, we might define the second as 1/86400 of a day and a meter as 1/10,000,000 of the distance from the equator of the Earth to the pole (both definitions have been used in the past). The problem is, of course, that those definitions don't get to any fundamental aspect of reality, as opposed to accidental aspects of this specific planet we call the Earth.

So, a different tact: define the meter in terms of, say, the distance across a hydrogen atom and a second as, say, the time it takes to go across some number of hydrogen atoms. But, if you think about that, this is just again defining a specific number as the speed of light.

The problem here is that by changing our definition of meter or second, we change the numerical value of the speed of light. What we really need to do is find a number that *doesn't* change simply because of our definitions of silly things like meters and seconds.

And there *is* a good number for that: it compares the speed of light and Planck's constant and so relates the speed of light and the size of quantum events. it is called the fine structure constant and has a value of 1/137 (approximately) no matter how meters and seconds are defined. it is *this* number, more than the speed of light, which defines the structure of things like atoms and the relative sizes of things.

So why is the fine structure constant what it is? Good question. Nobody knows and it isn't even clear that it *could* be a different value.

It turns out that most of physics is based on a very few 'universal constants' like the fine structure constant that then determine the relative scale of everything else. Why they have the values they do is anyone's guess. Whether they even *could* be different values is anyone's guess. Since they seem to be fixed, we don't know how or if they could be different than they are.
Arghchgch!
It's "tack", not "tact"!


I feel better now.....
Pedantic urge subsiding.....
Even if you give me a well deserved slap.
 

Polymath257

Think & Care
Staff member
Premium Member
It does not seem to be unfettered because it has limitations, unless the vacuum of space itself exerts some form of resistance against it.


And there is a sense in which it does. Light is an electromagnetic wave, so electric and magnetic properties affect it. There are a couple of constants, called the permittivity of free space and the magnetic permeability of free space, which together determine the speed of light.

Vacuum permittivity - Wikipedia
 

Brickjectivity

Turned to Stone. Now I stretch daily.
Staff member
Premium Member
I am ignorant to the theory of relativity. Can you lay this out in layman’s terms.,., explaining why the flashlight beam is not traveling faster than light-?

Thanks in advance
:)-
This is from Physics 200 level: Experiments indicate it is due to a time adjustment. Velocity is how far you travel in an amount of time, but experience of time changes when you reach higher velocities. As you approach light speed your own time is slowing compared to that of slower objects (as the theory suggests). This is true for photons, too. So when you are on that vehicle shining your flashlight, the time that the photons experience adjusts so that the photons from both your flashlight and a stationary flashlight will only travel at the same maximum velocity.

Suppose you run, really, really fast. Your personal time slows. Normally it doesn't slow enough for you to notice, so your watch still matches everyone elses. If you could run 80% of the speed of light though, that would not be the case. Everyone else's watches would run faster than yours while you were running. You could miss days, weeks or years while only experiencing minutes. Photons never exceed light speed, because a faster particle experiences slower time.
 

Polymath257

Think & Care
Staff member
Premium Member
I am ignorant to the theory of relativity. Can you lay this out in layman’s terms.,., explaining why the flashlight beam is not traveling faster than light-?

Thanks in advance
:)-


Why is a tricky question in this context. It is a fundamental aspect of our universe that the speed of light in a vacuum is always the same.

This was first discovered by Michelson and Morley. They were attempting to measure the speed of the Earth through space by carefully measuring the speed of light with respect to the Earth in different directions. They expected to see the speed to be slightly less in the direction of the Earth's motion and a bit more in the opposite direction. Instead, they saw no difference. This is one of the most important null-results in the history of science.

The basic effect is that when two observers are moving with respect to each other, each sees the clocks of the other moving slightly slower and the distances of the other to be slightly shortened. The effect is larger as the relative speeds get close to the speed of light.

So, the train sees the light moving away at the speed of light. But, for those on the ground, the clocks and the measuring rods of the train are slightly changed to product that result. The ground *also* sees the light as moving away at the speed of light (the same speed always).

This is a fair intro:
Special relativity - Wikipedia
 
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Polymath257

Think & Care
Staff member
Premium Member
This is from Physics 200 level: Experiments indicate it is due to a time adjustment. Velocity is how far you travel in an amount of time, but experience of time changes when you reach higher velocities. As you approach light speed your own time is slowing compared to that of slower objects (as the theory suggests). This is true for photons, too. So when you are on that vehicle shining your flashlight, the time that the photons experience adjusts so that the photons from both your flashlight and a stationary flashlight will only travel at the same maximum velocity. This velocity is always the same, so that if you could stand on a car traveling at 1/2 the speed of light, and there were a stationary flashlight nearby, the speed of the photons would be the same. Not only that, but your car would never exceed the speed of light, as time would adjust precisely enough to prevent that. It would effectively stop for you, perhaps. Currently we have every reason to think that there is no way for matter to go faster than light.

One caution here: it appears from your description that there is a well-defined speed for each object. This is false. The only speeds that make sense are *relative* speeds: how fast is one thing going as measured by another?

So, if I am going past you at half the speed of light, I will see you as going past *me* at half the speed of light in the other direction. Part of relativity is that there is no way to determine an absolute speed: all speeds are relative.

Another aspect of this is that if I am going past you at half the speed of light, you will see
my clocks as going slower and my measuring rods as shorter. But, since I see *you* as going past me, I will see *your* clocks as going slower and *your* measuring rods as shorter. This seems like it is contradictory, but that difficulty is resolved in the Lorentz transformations.
 
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Thief

Rogue Theologian
Time does not exist
It is a means of measure
a cognitive device created by Man to serve Man

If you could approach the speed of light
any light traveling beside you would appear as a blurred photo

any light moving behind you (with you) will never catch up
pitch black would be the view

any light coming toward you would be C doubled
the combined energy (head on collision ) would be ......firm

MOTION is relative

time is not a force or substance
and has nothing to do with it
 
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Grandliseur

Well-Known Member
Ref 13
It does not seem to be unfettered because it has limitations, unless the vacuum of space itself exerts some form of resistance against it.
I was trying to describe its normal speed that doesn't cause a loss of speed. I already mentioned that since it has to traverse the space-time field that this would most likely have its inherent control, a balance of sorts.

I have never seen, though I have looked for it, an explanation that tells us how light propagates, moves, through the space-time field, and without this knowledge which the speed certainly is dependent upon, the answer to this question (post ref #1) cannot be known.

I have read a lot about this, and forgotten much. However, I have never seen an explanation - that I remember - telling us how and what the space-time field is. It doesn't seem to be an electric field?! or, a gravitational field?! So, what kind of field is it?
 
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sayak83

Veteran Member
Staff member
Premium Member
Obviously, I have no clue.

Did you know that the speed of light you quote is the unfettered speed. Light has been slowed down in several experiments to a fair crawl.

I would speculate that when we have about 10 or 11 dimensions of space once we go into the scientific theories, string, and all those quarks that have to keep interacting to hold together our matter, and the quantum void of probability particles in the space-time field, that perhaps the speed has an upper limit pertaining to these matters. Light does need to travel through the space-time field in which these interactions occur. Since things are taking place there, I see light as a ship sailing through water which has resistance, and the faster you go, the harder the going gets. With light, a balance would then happen to create a speed limit when passing through this space-time field.

Ups! Just my ruminations. Nothing but assumptions. :p
You should have stopped with the first sentence. :);):D
 
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