What is light?

[ramparts]

But light has mass, otherwise light wouldnt be effected by gravity unless gravity and light are within the same family of energy, light might be the weaker force, with gravity being the stronger force, otherwise if light was the stronger force, then gravity would not effect light in the manner that it does.

Light - or, to be specific, the photon - does not have mass. One of the consequences of Einstein's theory of gravity - general relativity - as opposed to Newtonian gravity is that, since gravity is a result of spacetime curvature, anything that moves follows that curvature, massive or not. The Newtonian view of gravity as a force between two masses is an approximation that works in many cases, but isn't generally true. The case of light moving under gravity is one example of that

 

[dangineer]

Momentum is actually very closely related to energy. In Newtonian mechanics, it is easy to see that liniear momentum is closely connected to linear kinetic energy (kinetic energy is the integral of momentum with respect to velocity). In electrodynamics, Maxwell's equations suggest that electromagnetic fields carry momentum (many experiments have proven this). Quantum mechanics further proved this and then provided a more detailed descirption of a photon, showing that a photon's energy is proportional to its frequency (the higher the frequency, the higher the energy).

This makes sense if you compare a photon's frequency to a rope wiggling. If you fix a rope at one end and shake the other, it makes waves. If you want to increase the frequancy, you have to wiggle rope faster (at least this is the way I like to think of it).

Now according to General Relativity, light is affected by gravity because it's moving, not because it has mass (which it does not have). The presence of energy causes spacetime itself to curve. Thus the shortest distance something travels is a curved path, instead of a straight one. So anything moving through a gravitational field tends to moved in curved paths regardless of whether it has mass.

Just as ramparts explained above.

 

[yevaud]

Thus the shortest distance something travels is a curved path, instead of a straight one.

A Geodesic.

 

[Whitespliff]

Now according to General Relativity, light is affected by gravity because it's moving, not because it has mass (which it does not have). The presence of energy causes spacetime itself to curve.

First time here so forgive me if it's a stupid question:

I once saw a documentary where they focused an intense beam of light (not sure if it was laser) on a thin sheet of aluminium foil (or it looked like that).
The sheet was bended by the beam, isn't this b/c light has mass?
If not, what caused the foil to bend?

Tnx

 

[ramparts]

Now according to General Relativity, light is affected by gravity because it's moving, not because it has mass (which it does not have). The presence of energy causes spacetime itself to curve.

First time here so forgive me if it's a stupid question:

I once saw a documentary where they focused an intense beam of light (not sure if it was laser) on a thin sheet of aluminium foil (or it looked like that).
The sheet was bended by the beam, isn't this b/c light has mass?
If not, what caused the foil to bend?

Tnx

Not sure what experiment you're referring to (but that's mostly my fault), but in general this is possible. Because light has energy (even if it doesn't have mass), it also carries momentum, and when things collide, it's momentum that gets transferred. So photons - particles of light - are massless but because they carry momentum can still "pack a punch" when they collide

 

[emperor_of_localgroup]

But wouldn't also make light a particle as well? If light emits a visible field of energy then it would also have to be a particle
maybe even a particle wave where both function of each operation is occurring at the same time.

Not necessary. I think the reason we like to treat photon as light particle is in our every day experience we observe energy can not travel all by itself, it requires a carrier. An object or a particle is energy carrier. In my opinion light energy is different from energy carried by a traveling particle. Because in case of light it can deliver all its energy and vanishes. On the other hand a traveling particle can deliver all its energy but does not vanish. It also seems light can deliver its energy to other matter much more easily than a particle can. This will be possible if light is a pure form of energy, not a particle.

Yes, I understand de Broglie's matter waves is another reason to consider light as particle because in that case light modeled as wave/particle nicely blends with other theories. If someone starts a thread on De Broglie waves, we can discuss because I have some questions about its nature.

Btw, when I say 'in my opinion', I mean in my shaky opinion, because my opinion may be changed by any person's convincing arguments.

 

[ramparts]

Not necessary. I think the reason we like to treat photon as light particle is in our every day experience we observe energy can not travel all by itself, it requires a carrier. An object or a particle is energy carrier. In my opinion light energy is different from energy carried by a traveling particle. Because in case of light it can deliver all its energy and vanishes. On the other hand a traveling particle can deliver all its energy but does not vanish. It also seems light can deliver its energy to other matter much more easily than a particle can. This will be possible if light is a pure form of energy, not a particle.

Yes, I understand de Broglie's matter waves is another reason to consider light as particle because in that case light modeled as wave/particle nicely blends with other theories. If someone starts a thread on De Broglie waves, we can discuss because I have some questions about its nature.

Btw, when I say 'in my opinion', I mean in my shaky opinion, because my opinion may be changed by any person's convincing arguments.

Here's the thing: particles aren't little billiard balls which always travel around unchanged. A photon, just like any other particle, can be created or annihilated (with other things being created/annihilated in turn, so we conserve mass, energy, etc.). In the case you're talking about, something like this happens. An atom can absorb a photon, and in turn its energy goes up (for example, it's common for an electron to be "pushed" to a higher energy level by a colliding photon). This certainly isn't evidence against light's particle nature

There are a great many convincing experiments that light is quantized - or, in other words, comes in discrete chunks. Photons, if you will I would recommend you read up on the photoelectric effect. Even Wikipedia should have a pretty decent explanation of what that effect means for the nature of light.

 

[csmyth3025]

dryson,

You said:

"But light has mass, otherwise light wouldn't be effected by gravity unless gravity and light are within the same family of energy, light might be the weaker force, with gravity being the stronger force, otherwise if light was the stronger force, then gravity would not effect light in the manner that it does."

Currently held theory states that light has no rest mass. Nevertheless, light is a form of energy and - this may sound strange but it's a consequence of General Relativity - energy can both generate and respond to a distortion in space-time.

In simplest terms, light (electromagnetic radiation) absorbed by a black hole will increase the strength of its gravity.

Light always travels in a straight line. The trouble that most people have with this concept is that what they think is a straight line depends on where they are in the universe. A gravitational field distorts space-time. A very strong gravitational field can make a beam of light's "straight line" in space curve because the space it's traveling through curves. An extremely strong gravitational field can make a "straight line" traveled by a beam of light curve so much that it becomes a circle. This effect creates what's called the photon sphere just outside the event horizon of a black hole.

In a sense, light and gravity are, indeed, within the same family of energy. They constitute two of the four known fundamental forces - the other two being the strong nuclear force and the electroweak force.

The relative strength of gravity vs light is one of those disparities that scientists have mulled over for many years. The currently held relationship is that if the force carried by a graviton (a yet to be discovered carrier of the force of gravity) were set at one, the force carried by the photon would be 10^36. That's one followed by 36 zeros! Light is by far the much stronger force. It's only when huge amounts of matter are lumped together that the gravity - the distortion of space-time - of all that "stuff" becomes noticeable to a beam of light.

Just remember - gravity doesn't bend light, it bends space. Light always travels in straight lines through space - but if the space that the light is traveling through is curved, the "straight line" that the light follows will also be curved.

Chris

 

[dangineer]

Quick correction:

The four forces are gravity, electromagnetism, stong nuclear force, and weak nuclear force. "Electroweak" is the name of the theory that unifies electromagnetism and the weak nuclear force.

Otherwise, that was a good description.

 

[ramparts]

One more thing to add to dangineer's (very true) correction: light isn't a force, it's a carrier for the electromagnetic force.

 

[Mars_Unit]

No one can ever actually see a Photon because it is invisible! For example: Radio Waves are made of Photons and cannot be seen! The Infrared photons from your TV sets remote control is invisible. Same with X-Rays and Gamma Rays.

You can however see the impacts of thousands of Photons in your eyes.

Radio Waves come off a 50,000 Watt radio tower and none are ever seen. Some people near an antenna can pick up the AM Radio waves in their toilet bowls. They hear news, sport and weather.

 

[csmyth3025]

Thanks for the correction dangineer and ramparts. You are, of course, right that the four known fundamental forces are the strong nuclear, the weak nuclear, the electromagnetic, and gravity. As you pointed out, (visible) light is a small portion of the electromagnetic spectrum.

From what I've read there seems to be no upper boundary to the energy carried by photons (particles, waves, wave packets, or wavilcles - take your pick). The estimated temperatures immediately after the Big Bang seem to be telling me that. If there is, in fact, some theoretical limit to the amount of energy a photon can carry, based on the laws of physics as we know them, please let me know.

Conversely, is there a lower boundary for the amount of energy a photon can carry? The Cosmic Microwave Background is, as I understand it, equivalent to a temperature of about 2.7 degrees above absolute zero. What temperature would, say, a common AM radio wave represent? To an even greater extreme, what temperature would the electromagnetic radiation given off by an ordinary 60 Hz electrical transmission line represent? I think the wavelength would be ([~300,000 km/sec] x 1000 m/km)/(60/sec) = ~5,000,000 meters

I found the following information on page 5 of this website:
http://pvcdrom.pveducation.org/main.html

The graphic displayed on page 5 indicates that the wavelength of microwaves is on the order of 10^-2 meters and that the wavelength of radio waves is on the order of 10 meters. Since 10 meters is 1000 times larger than 1/100 meter does this mean that a black body emitting radio waves would be at a temperature of 0.0027 degrees above absolute zero? If this is so, wouldn't the Cosmic Microwave Background prevent such an object (if it was out in space somewhere) from getting below 2.7 degrees?

Chris

 

[derekmcd]

Now according to General Relativity, light is affected by gravity because it's moving, not because it has mass (which it does not have). The presence of energy causes spacetime itself to curve. Thus the shortest distance something travels is a curved path, instead of a straight one. So anything moving through a gravitational field tends to moved in curved paths regardless of whether it has mass.

Just a technical nitpick here:

It's not a "curved path". It's a straight line in curved space. Only in Euclidean space is it explained as the shortest path. In General Relativity, you have the 4th dimension of time in which case the geodesic is described as the straightest path between two events that has the longest amount of proper time. In other words, in the context of this discussion, you might say it is actually the longest path.

 

[darkmatter4brains]

Isn't the Higg's particle supposed to have mass? So, if the Higgs particle "gives" other particles mass via interacting with it, who gives the Higg's its mass?

 

[dangineer]

The non-zero vacuum expectation value of the Higgs field gives the Higgs boson, and all other massive particles, mass. The Higgs boson is the "quantum" of the Higgs field. In quantum mechanics, fields are "quantized" into discreet parts called quanta (or singular quantum).

 

[Mars_Unit]

All of this Higgs Vector Boson Theory seems like someone is attempting to revive the Luminiferous Ether Theory which died when Einstein killed it.

You cannot see a single Photon of Light! It take thousands or millions of Photonic impacts to set off a cone or rod in your retina.

When a beam of light enters your home or even a church, you are not actually seeing the light. You are seeing Light bounce off of dust or air molecules in the air. If on the Moon, the lack of air makes Sunbeam shafts impossible.

I like to think of any light emitter like a light bulb or LED as like a sandblaster. Sand particles are shot out by compressed air to sand off old paint from a machine. You can change the amplitude or frequency of the sandblaster, also.

Sometime I wonder if Electrons are made of Photons, but that is not the case.

 

[killium]

1. I have a small problem with light's path bending by gravity. Understood it's space that is curved and light is following a straight path in this curved space. The problem is: why does curved space accelerates stuff ? If i use the trampoline/bowling ball analogy to explain this, i don't find it satisfactory cause it needs gravity to explain gravity. What i mean is that if there is no gravity under that trampoline, a particle placed on it wouldn't change velocity. It wouldn't move towards the bowling ball. The curved fabric curves the particle's path cause that particle is attracted to the ground and the trampoline's sheets blocks it.

2. Why isn't the light's speed infinite ? If vaccum of space is "nothing", then what is slowing light down ? I would risk: in total void, light speed would be infinite. So empty space is not really empty. And what we call light is a (electric and magnetic) deformation of the fabric. So the spacetime fabric itself is made of electric and magnetic potential. Light appears as a wave when it disturbs the fabric (and it's this deformation that travels at a finite speed, from one planck steps to the next). When we try to observe it using "tools", it appears as solid.

3. What is solid anyway ? If everything is made of waves, solidity is only a macrosopic manifestation of electromagnetism repulsion of opposite waves' phases. It's an illusion to us, macroscopic beeing...

Please discuss.....

 

[MeteorWayne]

1. I have a small problem with light's path bending by gravity. Understood it's space that is curved and light is following a straight path in this curved space. The problem is: why does curved space accelerates stuff ? If i use the trampoline/bowling ball analogy to explain this, i don't find it satisfactory cause it needs gravity to explain gravity. What i mean is that if there is no gravity under that trampoline, a particle placed on it wouldn't change velocity. It wouldn't move towards the bowling ball. The curved fabric curves the particle's path cause that particle is attracted to the ground and the trampoline's sheets blocks it.

2. Why isn't the light's speed infinite ? If vaccum of space is "nothing", then what is slowing light down ? I would risk: in total void, light speed would be infinite. So empty space is not really empty. And what we call light is a (electric and magnetic) deformation of the fabric. So the spacetime fabric itself is made of electric and magnetic potential. Light appears as a wave when it disturbs the fabric (and it's this deformation that travels at a finite speed, from one planck steps to the next). When we try to observe it using "tools", it appears as solid.

3. What is solid anyway ? If everything is made of waves, solidity is only a macrosopic manifestation of electromagnetism repulsion of opposite waves' phases. It's an illusion to us, macroscopic beeing...

Please discuss.....

1. The rubber sheet representation is an analogy, not reality. It is a way to help us visualize the concept, it is not the mathematical underpinnings of the theory.

2. The speed of light (in a vacuum) is not infinite, because it is a fundemental feature of our Universe. There is no why, it is just a fact. Everything else in space-time falls out of that fact.
The ideas you are throwing out have been investigated and rejected by experiment. There is no medium, the vacuum speed of light is one of the most fundamental facts of our Universe.

3. A solid is something that hurts if you hit it hard enough with your fist

 

[ramparts]

To elaborate on point 1, light (or anything under the influence of gravity) doesn't accelerate, strictly speaking. The paths objects take aren't straight lines in curved space, they're straight lines in curved spacetime. Everything maintains a constant velocity through spacetime - the speed of light

 

[killium]

"2. The speed of light (in a vacuum) is not infinite, because it is a fundemental feature of our Universe. There is no why, it is just a fact."

That is what i call a dogma.

I want to know the "why", i'm sorry, there HAS to be one, don't you think ?

 

[ramparts]

Hi killium - there is a reason, but I'm afraid it's pretty mathematical The structure of spacetime encodes the speed of light in a pretty special way. We often talk about four-dimensional "spacetime". Well, if you think about it (for a nanosecond), time clearly is different from space - for one thing, they have different units. So we multiply our time coordinate by a speed (a time multiplied by a speed gives a distance) to make it "fit" with our dimensions, and that speed happens to be the speed of light. The rest of it - like the speed of light being a fundamental limit - falls out of that, again in a pretty mathy way.

Now, why that speed happens to be the speed of electromagnetic waves, well, that's something entirely more mysterious

 

[EarthlingX]

Excuse me for being a little off topic, but i got this questions reading posts so far:
- Is energy curving the time space ?
- If so, how much would momentum of stars in galaxy curve time-space around it ?
- Is hot object heavier than cold ?
- Would light exiting stars also curve a time space ?

 

[ramparts]

Excuse me for being a little off topic, but i got this questions reading posts so far:

We're an internet forum, we specialize in thread drift

- Is energy curving the time space ?

Sure is!

- If so, how much would momentum of stars in galaxy curve time-space around it ?

Well, what do you mean by "how much"? A lot, is the answer, but I don't think there's a number that you'd find very meaningful.

- Is hot object heavier than cold ?

Not unless it's reeeeeeeeeeeeeeeaalllllllyyy hot

- Would light exiting stars also curve a time space ?

Negligibly, but technically, yeah.

 

[EarthlingX]

Thanks

Well, what do you mean by "how much"? A lot, is the answer, but I don't think there's a number that you'd find very meaningful.

Thank you for a warning Maybe something relative to visible orbiting mass, like 10e-6 or something ?

 

[ramparts]

I'm still confused. 10e-6 what? You're asking for a measure of curvature... right?