Why does light travel so fast?

[questions4all]

Why does light travel so fast?

I want to know about the speed at which light travels. Why can light travel so fast? is it the weight of the photons or what? And what accelerates the matter to that speed. Everything is propelled by some force so what is lights force? Thank you.

Questions4all

 

[pallidin]

Well, a photon does not accelerate to c. It travels at c from the very instant it is a photon.
It goes from zero to c in zero time.

 

[pallidin]

As for "why", I'm not sure if anyone knows. We know that a photon has no rest mass.

A link which might be helpful is here: http://en.wikipedia.org/wiki/Photon

 

[stevebd1]

Also, I think that one of the big question in respect of general relativity is not 'why does light travel so fast' but 'what exactly is mass', you'll find that there are about ten definitions of mass in GR while (to my knowledge) there is only one of light. You could say that there is nothing out of the ordinary about c (which is in a state of m=0, t=0) and the real strangeness is mass (which is in a state of m>0, t>0). There are people here who could probably explain this better but it's worth looking at it from a different angle.

 

[pallidin]

...You could say that there is nothing out of the ordinary about c (which is in a state of m=0, t=0) and the real strangeness is mass (which is in a state of m>0, t>0).

Indeed. It is rather curious that anything without rest mass somehow naturally and instantaneously propagates at c.
Too be sure, as you said, the real strangeness is mass, and appears to be interrelated.

 

[Gnosis]

I want to know about the speed at which light travels. Why can light travel so fast? is it the weight of the photons or what? And what accelerates the matter to that speed. Everything is propelled by some force so what is lights force? Thank you.

Questions4all

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...

 

[Nick89]

I want to know about the speed at which light travels. 1) Why can light travel so fast? 2) is it the weight of the photons or what? 3) And what accelerates the matter to that speed. 4) Everything is propelled by some force so what is lights force? Thank you.

Questions4all

I would have expected people to correct your many false ideas but since nobody has explicitly done that I'm going to give it a go.

1) Why wouldn't it? See 4) also.

2) Photons have no mass, and thus no weight (at least not in the classical sense, they might have something that can be compared to weight in special relativity but I don't know much about that)

3) Photons are not matter. They are also not accelerated. A photon always travels at c, end of discussion. It cannot stop, it cannot slow down, it cannot accelerate.

4) As said, photons do not accelerate, hence no force is required to move them (they simply do).

 

[justwondering]

Like why does light travel at the speed that it does? Does it have to have a 'memory' to do this every time or is there some 'physical' restraint that we don't/can't think about that causes it to travel at a certain speed? What 'fixes' its speed? It MUST be something!

Yes, light does travel at a very slow rate considering the size of the 'territory' that it has to move along in.

 

[harvellt]

Well, a photon does not accelerate to c. It travels at c from the very instant it is a photon.
It goes from zero to c in zero time.

Do photons have a mass? We see that if they hit something it moves so they must have a mass. No matter how small they are wouldn't an instant acceleration to c take an infinite force?

 

[Nick89]

Do photons have a mass? We see that if they hit something it moves so they must have a mass. No matter how small they are wouldn't an instant acceleration to c take an infinite force?

Photons do not have mass. The force they exert on objects they hit is caused by the momentum. Photons do have momentum, but it is not equal to mv like it is for massive particles.

 

[jtbell]

Just to avoid confusion, when we say photons are massless, we're referring to the "rest mass" (also called "invariant mass"), not what is often called "relativistic mass." The general relationship between (rest) mass, energy and momentum is

E^2 = (pc)^2 + (mc^2)^2

It's possible for something to have zero mass but nonzero energy and momentum, in which case E = pc.

 

[epenguin]

A lot of people seem to think light travels fast. Nonsense! It travels painfully slow! Only to get to the other end of this Galaxy would take you 100,000 years at the speed of light. It is a good job the Universe is so old or we wouldn't know much about it.

There is another very recent thread called "reason for value of c". There you'll see you have to up-end questions like this for them to make sense.

 

[franznietzsche]

Do photons have a mass? We see that if they hit something it moves so they must have a mass. No matter how small they are wouldn't an instant acceleration to c take an infinite force?

Be careful with statements like these. You are trying to apply an old model (classical mechanics) to explain new observations. Observations always trump models.

Observation: Photons always travel at c regardless of the motion of the observer.

We don't know why. But we know that it is true, as far as we have ever been able to measure.

Newton's laws of motion, \vec F = m\frac{d\vec p}{dt}, is a model. Observations ALWAYS trump models.

Your questions make sense in classical mechanics. But classical mechanics is wrong, it makes assumptions that are not true. As others have said, it turns out that you don't need mass to have momentum, and momentum is what matters in terms of impact kinematics.

Of course, there are flaws in relativity as well (it doesn't work with very small things, just like classical mechanics doesn't work with very fast things).

 

[Dabonez]

3) Photons are not matter. They are also not accelerated. A photon always travels at c, end of discussion. It cannot stop, it cannot slow down, it cannot accelerate.

I thought light could be slowed down? I've read a couple articles about people that managed to even stop light and "restart" it.

http://www.wired.com/science/discoveries/magazine/15-11/st_alphageek

 

[Nick89]

I thought light could be slowed down? I've read a couple articles about people that managed to even stop light and "restart" it.

http://www.wired.com/science/discoveries/magazine/15-11/st_alphageek

Light and individual photons are two different things. The speed of light can be slowed down, but the speed of the individual photons is always c.

 

[Shackleford]

A lot of people seem to think light travels fast. Nonsense! It travels painfully slow! Only to get to the other end of this Galaxy would take you 100,000 years at the speed of light. It is a good job the Universe is so old or we wouldn't know much about it.

There is another very recent thread called "reason for value of c". There you'll see you have to up-end questions like this for them to make sense.

Well, 100,000 years in our reference frame.

 

[DaveC426913]

The speed of light is the universe's way of not having everything happen all at once.



Seriously though, better questions might be:

Why does EMR* travel so slow?
Why does matter travel at a glacial pace compared to EMR?


(*electro-magnetic radiation, of which light is a tiny sliver)

 

[Shackleford]

The speed of light is the universe's way of not having everything happen all at once.



Seriously though, better questions might be:

Why does EMR* travel so slow?
Why does matter travel at a glacial pace compared to EMR?


(*electro-magnetic radiation, of which light is a tiny sliver)

To the second question, noting pair production, actualized inertia for matter?

 

[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?


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.

 

[mikelepore]

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.

 

[simpleton]

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 traveling 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.

 

[DaveC426913]

The point is, light's speed at any inertial frame is 299792458m/s, so yah.

If you are on a ship traveling 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.

 

[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"?

 

[DaveC426913]

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.

 

[Doc Al]

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.)

 

[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.

 

[DaveC426913]

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?"

 

[Dale]

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.

 

[YupHio]

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