Light is weird!?

[Neil Russell]

Turn on a light bulb and we have instant light. How instant? do photons emit from the filament, at light speed instantly ( my guess) or do the photons have to accelerate up to light speed, from rest?
If the photons had to accelerate, wouldn't that involve huge inertia?

We are told to accelerate a mass close to light speed as velocity increases the mass increases to the point of infinity at light speed.

Photons have no mass? yet we can cut a steel plate with a laser. Light beams are bent by gravity around the sun.

How come the photon is not affected by the increasing mass rule ( or the time slow down thing, too)

Truly weird.

[Jens]

From what I understand, they always travel at the speed of light. So no acceleration.

And as you say, they produce heat.

Weird little things.

But studying them can be a very enlightening exercise.

[slang]

We are told to accelerate a mass close to light speed as velocity increases the mass increases to the point of infinity at light speed.

Someone will likely phrase it better than I can, but the notion of mass increasing with speed is a bit of a shortcut way to describe what really happens (or rather, how theory describes what we think really happens). It sort of applies, but not quite, and because it's a simplified description you get in trouble if you try to understand how (we think) it really works.

Photons have no mass? yet we can cut a steel plate with a laser.

Photons have energy. The plate is cut with heat. But I think I understand what you mean: if laser cutting is possible, then how does a saw work? Apart from the obvious visible effects, what really happens to break the bonds in the material?

Light beams are bent by gravity around the sun.

According to General Relativity, gravity is the curvature of spacetime. Photons just follow the curvature they encounter. Like a train on rails, in the normal flat landscape, the rails will be straight, but when there's a lake or hill, the rails will bend around them, and the train will just follow where the rails steer it.

Truly weird.

No kidding! Too bad the universe has no obligation to us to be easily understandable.. it is what it is. :)

[Jeff Root]

Some fundamental particles have mass, and other fundamental particles
do not have mass. Just like some fundamental particles have electric
charge and others do not. The photon is a fundamental particle that has
no mass. No electric charge, either. But all particles have energy. That
means massless particles always move at a constant speed, which we
commonly refer to as "the speed of light".

In addition to considering gravity as a curvature of spacetime, it can also
be considered as a force between objects that have energy. Either way
of looking at a gravitational interaction gives the same results. Because
photons move at such a high speed, they are deflected very little in their
paths by gravity fields that would strongly deflect massive bodies moving
at much lower speeds.

You would expect something moving at extremely high speed to have a
lot of energy, wouldn't you? Photons can have a lot of energy. Their
energy is directly proportional to their frequency (inversely proportional
to their wavelength). You can pack a huge number of not-so-energetic
photons into a narrow beam to get a strong concentration of energy,
such as in a laser.

-- Jeff, in Minneapolis

[Ken G]

It might also help you to recognize that relativity says that all objects in some sense move through spacetime at rate c, it's just that most objects move partly (or entirely) through time. Massless objects do not move partly through time, they are seen (by us) to move entirely through space without aging at all. When you accelerate a particle with mass from rest, you are in effect deflecting its pre-existing motion that was entirely through time (as is your own motion) a little bit into the spatial direction. So you see, even the acceleration of a particle with mass is more like a change in direction than a change in rate of motion. Since massless particles are already "fully deflected" into the spatial direction, as soon as they come into being, you cannot deflect them any more-- you cannot accelerate them in the normal sense. You can, however, slow down and speed up their phase speed by propagating them through a medium. One way to think of that is the interaction with the medium "dresses" the photon, conveying part of its rest mass onto the photon such that the photon in the medium begins to act like it has some rest mass just because of that interaction.

[Strange]

The photon is a fundamental particle that has
no mass. No electric charge, either.

So, is the photon a particle with no mass, or the particle with no mass? In other words, are there other particles with zero mass? (I gather the neutrino is now thought to have a very small mass)

And, if so, are there any particles with zero mass that have electric charge?

[Nick Theodorakis]

I think gluons are massless; they don't carry electric charge but have color charge.

If the graviton exists, it is massless (and chargeless).

Nick

[Jeff Root]

There are no particles known to have electric charge but zero mass. That
might very well be an impossible combination, though I don't know why.

-- Jeff, in Minneapolis

[cosmocrazy]

Photons have energy. The plate is cut with heat. But I think I understand what you mean: if laser cutting is possible, then how does a saw work? Apart from the obvious visible effects, what really happens to break the bonds in the material?

In layman's terms :-
As Jeff stated using a laser you pack a lot of energy into a small area/point and as you said this laser creates a lot of heat at that point thus exciting the atoms of the plate to break the bonds of the particles. When you saw through using a harder material (saw blade) to cut a softer one you are basically just ripping chunks of the softer material off, tearing the molecules away..