# Why does light travel so fast???

by questions4all
Tags: light, travel
 P: 41 could the constant speed of light in a vaccume be a property of space? in a remedial way of thinking about this could the "material" that space is made of be the cause of the constant c? Off topic, but, i am simply interrested in physics. i have no educational background in the subject i am simply facinated by it so please excuse my ignorance when it presents itself.
P: 567
 Quote by tkav1980 could the constant speed of light in a vaccume be a property of space? in a remedial way of thinking about this could the "material" that space is made of be the cause of the constant c?
The value of c is a combination of the delay for empty space to respond to a changing electric field (due to the "permittivity of free space") and the delay for empty space to respond to a changing magnetic field (due to the "permeability of free space"). c can be calculated from those other two physical constants.

Maxwell used that fact to calculate, if an electromagnetic wave did exist, which they didn't know at the time, then that's the speed that it would have. When people looked at that calculated number, they noticed that it was the same as the measured speed of light. That's how people found out that light is an electromagnetic wave.
P: 59
 Quote by truhaht Oops, wrong! There is nothing fixed about the distance or time because it depends on the observer's uh Lorentz frame (or something). Yet the assessment of light-speed is ever c anyway. If we can't pinpoint the exact "why" of that truism, then I figure we can't profess to distinguish light-speed from infinite speed. (All observers agree when something is infinite, ie immeasurate) No, not particularly high among my priorities. Life's a gas
The point is, light's speed at any inertial frame is 299792458m/s, so yah.

If you are on a ship travelling near light speed pass Earth, and you see someone shooting a light beam at the moon, if you measure the time, I think the distance between the source of light and the moon surface will be shortened, so if you divide the distance by time, you still get the same speed.
P: 15,294
 Quote by simpleton The point is, light's speed at any inertial frame is 299792458m/s, so yah. If you are on a ship travelling near light speed pass Earth, and you see someone shooting a light beam at the moon, if you measure the time, I think the distance between the source of light and the moon surface will be shortened, so if you divide the distance by time, you still get the same speed.
It's not that the distance is shortened , it is that your time is compressed.

A more clean example is turniong on a flashlight inside the relativistic spaceship. Observers on Earth and observers in the spaceship both measure the speed of the beam as c. The reason it is not a paradox is that, in the relativistc spaceship, time is dilated (relative to Earth), including their stopwatches.
 P: 59 Pardon me if I get it wrong, but doesn't dilation mean time taken is longer? So it should get "stretched" instead of being "compressed"?
P: 15,294
 Quote by simpleton Pardon me if I get it wrong, but doesn't dilation mean time taken is longer? So it should get "stretched" instead of being "compressed"?
I thought you were saying that it's because distance is shortened, not because time is lengthened. That seems a bit misleading. Distances are only shortened along the line of travel. So if the spaceship observed the laser beam passing from Earth to Moon across the spaceship's path (perpendicular to the spaceship's trajectory) there would be no distance foreshortening of the lasers' path.

I mean, I know what you're saying. I'm just not sure the message is going to be clear to truhaht.
Mentor
P: 40,261
 Quote by simpleton Pardon me if I get it wrong, but doesn't dilation mean time taken is longer? So it should get "stretched" instead of being "compressed"?
Moving clocks are observed to tick slower, thus a "second" measured on a moving clock is observed to be stretched out or dilated compared to a "second" measured on the observing frame's clock according to that observing frame. (But take care to not get hung up on semantics.)
 P: 2,141 c = 1 in natural units. And in natural units, it is natural to consider c to be dimensionless. It is legitimate to assign Time and Length the same dimensions, so that speeds become dimensionless numbers. In principle, you could do that in any unit system, but the fundamental physics equations strongly suggest that this should be done in natural units. So, we can say that c = 1 and that 1 is not a large number at all. The reason why c is large in SI units is because of the way we decided to define the meter and second. Note that it doesn't make sense to consder a dimensionful number to be large or small. So, if you say that c is large in SI units, what you mean is that: c* second/meter is large Now, if we evaluate this in natural units in which c = 1 (and dimensionless), this tells you that the second is huge compared to the meter. So, relative to a consistent definition of the unit of time relative to the unit of spatial distance, we have decided to use inconsistent units for time and spatial distances. For historic reasons we decided to define units so that the older definitions would still be approximately valid. And a long time ago the smallest units for lengths and time intervals that were used a lot in practice were the smallest quantities that were still relevant for humans. This means that the reason why c is large in SI units is because we are very slow. We can only perceive changes that happen on extremely long time scales compared to our size. If things happen too fast we perceive that as in instant change, we don't see that the change in fact happened gradually.
P: 15,294
 Quote by Count Iblis c = 1 in natural units. And in natural units, it is natural to consider c to be dimensionless. It is legitimate to assign Time and Length the same dimensions, so that speeds become dimensionless numbers. In principle, you could do that in any unit system, but the fundamental physics equations strongly suggest that this should be done in natural units. So, we can say that c = 1 and that 1 is not a large number at all. The reason why c is large in SI units is because of the way we decided to define the meter and second. Note that it doesn't make sense to consder a dimensionful number to be large or small. So, if you say that c is large in SI units, what you mean is that: c* second/meter is large Now, if we evaluate this in natural units in which c = 1 (and dimensionless), this tells you that the second is huge compared to the meter. So, relative to a consistent definition of the unit of time relative to the unit of spatial distance, we have decided to use inconsistent units for time and spatial distances. For historic reasons we decided to define units so that the older definitions would still be approximately valid. And a long time ago the smallest units for lengths and time intervals that were used a lot in practice were the smallest quantities that were still relevant for humans. This means that the reason why c is large in SI units is because we are very slow. We can only perceive changes that happen on extremely long time scales compared to our size. If things happen too fast we perceive that as in instant change, we don't see that the change in fact happened gradually.
This is what I'm sayin' in post 21.

"Why does matter travel at a glacial pace compared to EMR?"
Mentor
P: 15,576
 Quote by Count Iblis The reason why c is large in SI units is because of the way we decided to define the meter and second. Note that it doesn't make sense to consder a dimensionful number to be large or small.
Well said. To follow-up on that I would just like to point out that the value of any dimensionful constant depends on the choice of units, and it is only the dimensionless constants which really have any independent importance. In fact, if c were to double in such a way that none of the dimensionless constants changed (e.g. the fine structure constant) then we would not even be able to detect the change in c. Only dimensionless constants have physical meaning beyond the choice of units.
P: 15
 Quote by Gnosis I view this matter from a different perspective. I realize that light-speed (c = 299,792,458 m/s) is roughly 900,000 times faster than that of sound however, compared to the colossal distances that light will traverse through the great expanse of the universe, the speed of light is comparable to the hour hand of an intermittent clock. For instance, point a powerful laser out into deep space and it still won't have arrived in some star systems even after traveling for the next 100 billion years. Light is painfully slow compared to the great expanse that it must traverse simply to reveal its ancient light information to us about the distant cosmos. So rhetorically speaking, my question would be; "Why isn't light MUCH faster than it is in reality? Why is it so comparatively slow compared to the unimaginable vastness of the universe that it actually traverses?" I have a theory as to why light only equals 'c' in a vacuum, but I think perhaps this is not yet the time for such a discussion. I will say this however, from this perspective, it would appear there is much potential for objects to travel far faster than 'c'. After all, there is more than enough starting and stopping distance available for such velocities. In any case, this is a classic example where the terms "fast" and "slow" are relative to one's perspective. I tend to think in the expanded mode...
Speed is not only relitive to distance but also to time. By slow, you mean it takes time that seems long to you to travel a certain distance. To a 3 billion year old being, it might seem very fast because a year is a fraction of a second relative to his experienced age. So light isn't really fast or slow, that's all relative and there is really no answer to the posts question. But I'm a sucker for rhetorical questions so that's my \$0.02

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