How Does Light Travel So Fast?

[Mintaka]

A rocket needs fuel and an engine, we need legs and food. Where does a photon get its energy from to enable it to travel so fast??

 

If we could watch a light beam from earth shooting across space, would it seem to move like a line across space?

 

Or can we only see the 'source' of the light, e.g a star? And not the beam itself moving across space?

[joekgamer]

F = M A

 

A = M / F

 

A photon has 0 mass (M).

 

It takes no force to accelerate a photon.

 

A photon does not need energy.

 

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As for the second part, could you clarify as to what you mean? Is the observer staring directly at the light beam, which is headed right for them, or are they standing off to the side?

[Mintaka]

F = M A

 

A = M / F

 

A photon has 0 mass (M).

 

It takes no force to accelerate a photon.

 

A photon does not need energy.

 

------------------------------------------

 

As for the second part, could you clarify as to what you mean? Is the observer staring directly at the light beam, which is headed right for them, or are they standing off to the side?

 

 

 

I mean, i'm trying to get my head around the idea of a beam of light 'travelling'.

 

When I shine a torch at night across the lawn, I see the solid beam of light.

 

If there were two planets at some distance off to our side which are visible to us, at some 3 light hours distance from each other, and someone on planet A shone a super-powerful torch in the direction of planet B, would we see that "beam" as it crawled on it's 3 hour journey to that neighbouring planet, as I would see a snail's trail that crawled across my garden?

 

I mean, light travels too fast for us to appreciate it as a "moving" thing here on earth, because there is nowhere far enough for us to "see" the effect of it moving, unlike an aural echo, which takes several seconds to come backto us and we feel the effect of its delay.

 

I have a vivid imagination, and I am trying to visualize a 'beam' of light as it travels across space , taking for example 4 years to reach our nearest star. Is it realistic to imagine it 'travelling' in the same way that we imagine , say, a meteorite travelling? Don't most things dissipate their energy the further they travel, and lose their momentum and slow down and then finally stop when the energy has run out?

 

What is so special or different about photons? Do they keep a constant velocity and momentum as they head out in space from one source to another? Why does light have this capacity to 'keep going' and not run out of energy? Would a beam of light keep going forever in space?

 

SOrry, I appreciate it if you have the patience to answer such idle witterings.

[CraigD]

A brief intro to modern physics, with some sloppy rules of thumb

A rocket needs fuel and an engine, we need legs and food. Where does a photon get its energy from to enable it to travel so fast??

Like rocket and people walking, photons - particles of light - get their energy from something that loses energy.

 

Big, complicated things like rockets and animals get energy via complicated series of events, burning chemical fuel in the first case, many biochemical and biomechanical processes in the second.

 

Photons, arguably the smallest and simplest things that exists, usually get their energy from one kind of simple event: an electron in an atom changing – “quantum jumping” from a higher orbit to a lower one. The photon gets exactly the energy that the electron loses.

 

This process is reversible. An electron can absorb a photon. The photon ceases to exist, and the electron jumps from a lower to a higher orbit, those orbits exactly the same as those it would have jumped from and to emit a photon of exactly the same energy it absorbed.

 

Because specific elements emit and absorb photons of specific energy, this is very handy for astronomers, because they can tell what elements are in distant light emitting or obscuring matter.

 

All this is really worth at least a chapter in a classroom science text, but you might get an intuitive feel for of it by reading articles like the wikipedia article hydrogen spectral series

 

If we could watch a light beam from earth shooting across space, would it seem to move like a line across space?

 

Or can we only see the 'source' of the light, e.g a star? And not the beam itself moving across space?

Ask yourself if you get wet standing beside a stream of water, or stuck by bullets if a machine gun is fired past you.

 

The answer is “usually not, but maybe.” If something interferes with the stream - makes the water splatter, or the bullets ricochet - your might.

 

The same is true with streams of photons. If something in their path absorbs some of them, then re-emits them, you might “see the beam”, but as space is mostly transparent, this is unusual, but, then again, well-collated (line up) beams of light are, too. Most light radiates in many directions from its source.

 

When I shine a torch at night across the lawn, I see the solid beam of light.

What you’re seeing is photons absorbed and reemitted by electrons in atoms in the air.

 

If the night was very dry and dust free, you wouldn’t see the beam of light.

 

Your title question is “how does light travel so fast?” This question is worth a whole introductory class in modern physics, so hard to sum up in a short post, but here goes, anyway, in the form of a few “modern physics rule of thumb”:

• Things with non-zero rest mass (eg: the protons, neutrons, and electrons that make up ordinary matter) require an infinite amount of energy to move at the speed of light (usually abbreviated "c"), so always move slower than c.
  
• Things with zero rest mass (eg: photons of light) can only move at c, never slower.
  

You can think of this, using terribly muddy math, as something like:

• Non-zero times infinity = infinity, so never happens (the case with protons, neutrons, and electrons)
  
• Zero times less than infinity = zero, so doesn’t exist (the case of photons moving slower than c)
  
• Zero time infinity = something finite, so happens (the case with photons moving at c)
  

 

This is a lot to absorb - most people who understand it do from hours of science classwork spread across years in various schools, and people who don't, don't because they didn't have this school experience. If you're willing to self-educate, through "informal classrooms" like hypography and websites and books you find on your own, you can, but keep in mind it's a lot to absorb!

[Qfwfq]

The answer is “usually not, but maybe.” If something interferes with the stream - makes the water splatter, or the bullets ricochet - your might.

Great illustration! :)

 

In order to avoid confusion, I think it is worthwhile to make a slight correction:

...well-collated (line up) beams of light are, too.

I suspect Craig means collimated which is the term for indicating when a beam is of fairly parallel rays and so doesn't quickly spread wider and wider with distance travelled.

 

Moreover I think the essential is to say that, by light that reaches your eyes, you see either the source when it comes directly from it or else whatever is scattering the light. We do not see all the light which is constantly going through space between stars, where there is nothing to scatter it.

 

We see clouds and also the blue colour in the sky in this manner, just as we see the surface of solid objects in an illuminated place. Some nights, if a searchlight is pointed skyward, we only see a blob high in the sky, due to a layer of haze or cloud, and no beam between it and the lamp. Some astronomy photos of things outside of our galaxy show a cloud of dust or gas lit up by stars in or around it. If you imagine there is a flashing star at the edge of such a cloud that is wide enough for the period of flashing and not too thick in our line of sight, we could expect to see the effect you ask about, like onion layers, each moving away from the source.

[Little Bang]

A simple answer. If an electron loses energy at A the photon created (from it's frame of reference) instantaneously appears at B. You see the beam shinning across your lawn only if it is foggy, smokey or with dust in the air.

[joekgamer]

A simple answer. If an electron loses energy at A the photon created (from it's frame of reference) instantaneously appears at B. You see the beam shinning across your lawn only if it is foggy, smokey or with dust in the air.

A photon does not have an infinite velocity. It does not move accross your lawn instantaneously, but much too quickly for us to observe. A photon moves about 300000000 meters every second.

[Little Bang]

If an observer travels at c his clock is stopped and arrives at his destination instantly. The same is true of the photon.

I'll make a prediction. The speed of light slows as the Universe expands and has been slowing since the BB. The less dense (mass per cubic meter) the Universe becomes the faster it's clocks will run hence the slowing of c.

[joekgamer]

Yes, but that is only from the photon's perspective.

[Qfwfq]

The less dense (mass per cubic meter) the Universe becomes the faster it's clocks will run hence the slowing of c.

Compared to what?

[Mintaka]

Compared to what?

 

Thank you, you've given thorough explanations of the physics of light, but I still feel my question hasn't been understood in the way I meant it.

 

About the two planets off to our side, and one person shining a super-powerful-multi-mirror-super-intense-light-beam focussing TORCH over to it's neighbouring planet which is 4 light-hours away in distance,

 

and we are off to the side of these two planets so we can see them side-on, would we see that super -intense beam of light TRAVEL across space like a slowly extending line?

 

Ok, let's assume there are a lot of dust and particles in the space between these two planets. Is there any scenario in which it might actually be possible to see a light beam as it 'travels' through space from one inter-galactic object to another?

[modest]

About the two planets off to our side, and one person shining a super-powerful-multi-mirror-super-intense-light-beam focussing TORCH over to it's neighbouring planet which is 4 light-hours away in distance,

 

and we are off to the side of these two planets so we can see them side-on, would we see that super -intense beam of light TRAVEL across space like a slowly extending line?

 

Ok, let's assume there are a lot of dust and particles in the space between these two planets. Is there any scenario in which it might actually be possible to see a light beam as it 'travels' through space from one inter-galactic object to another?

 

Yes. If there is dust then we'd see it make its progress from the source to the target at the speed of light. 4 hours after we see the beam leave the first planet we'd see it arrive at the second.

 

~modest

[Mintaka]

Yes. If there is dust then we'd see it make its progress from the source to the target at the speed of light. 4 hours after we see the beam leave the first planet we'd see it arrive at the second.

 

~modest

 

Thank you. :)

[pjlueck]

Hi! I just joined...and felt the need to spout verse...

 

(hopefully, related verse!!)

 

Travel Light

 

They are born travelers...

these little photons,

zipping away from their creation.

 

Weighing nothing, it takes nothing

to speed them on their way

 

They don't dally, they don't dither.

Choosing the most likely path,

Guided by quantum probability.

 

Straight across the universe,

pausing only for an observation,

they carry past and future within.

 

Oh, little photons...

If only we could follow you...

see your trajectory.

 

To you, your universe wide trip

is just a "timeless blip,"

no time, no wait, just zip!

 

 

The great mystery.

The photon begs the question:

 

What creates the cosmic speed limit?

What universal traffic cop

waves you through at such

a constant rate?

[joekgamer]

The "Cosmic Speed Limit" is the fastest something can go before it becomes 'nonexistant' (not completely literally, but in a way, at least). At the speed of light © (about 3 x 10 ^ 8 meters per second [this is defined, not measured, by saying that a meter is the distance that light travels in 1 / c seconds]), time has slowed down to nothing for you, so past the speed of light, time is going backwards for you. So, to the observer, even if they were using a camera that can slow down what it is seeing to an infinitly slow video, the moment you pass the speed of light you dissappear, simply because you do not exist in that frame of space-time anymore, even if you are in the same spot as before you dissappered. Therefore, because the universe is defined as one frame of the time component of space-time, you would not exist in the universe if you were to go past the spped of light. Also, if you were to go past the speed of light, then your spepd would actually be negative, because V = d / t, so for negative time (going backwards in time), V is negative, so you have a negative speed, which would become smaller (larger absolute value, but still negative) a you speed up.

[CraigD]

The "Cosmic Speed Limit" is the fastest something can go before it becomes 'nonexistant' (not completely literally, but in a way, at least). At the speed of light © (about 3 x 10 ^ 8 meters per second [this is defined, not measured, by saying that a meter is the distance that light travels in 1 / c seconds]), time has slowed down to nothing for you, so past the speed of light, time is going backwards for you.

Though you hear the idea that an object traveling faster than c would travel backward in time quite a bit in science fiction and pop science, it’s not a consequence of any usual physics.

 

The formula for time dilation is [math]\frac{t'}{t}=\sqrt{1 - \left( \frac{v}{c} \right)^2}[/math], so when speed [math]v[/math] exceeds the speed of light [math]c[/math], time [math]\frac{t'}{t}[/math] becomes not negative, but imaginary. For example, an object moving at speed 2c would be going not backwards (ie: -1), but at [math]\sqrt{-1}[/math] (conventionally written using the symbol "i"), in time.

 

What this would mean, physically, is unclear. Some sci-fi types have decided it means moving “at right angles to reality” (this is actually a pretty sensible interpretation, mathematically), then gone on to suggest that “right angles to reality” means something like “into other/alternate realities”.

 

Since there’s no evidence or theory that it’s in any real way possible, there’s not a lot of scientific or hard SF concern about interpreting what “imaginary time” means. We can be certain, though, it isn’t simply a synonym for “negative time”.

[joekgamer]

Thanks for clearing that up.