Does a laser have weight?

I have the lightest piece of paper you ever heard of.  I also have one of those common pointing lasers.

Does the light make an impact on the paper?  Does it move?

one of my physics professors had a demonstration where he would push a metal block across an air cushioned rail using a laser

It has mass.  Light (LASER) is affected by gravity.

934, could be too much rum tonight.

Technically it does,the light particals have mass so there must be so technical weight.

After posting that I'm thinking about eye surgery and glass cutting.

Or is that different?

I'm not an eye surgeon or a glass cutter, but I would have to say yes, eye surgery and glass cutting are different.

No buck_nay_kid, the "particles" (photons) do not have mass.

But yes, the light has momentum and it exerts a pressure (and hence a force).

The radiation pressure is given by the intensity divided by c, the speed of light.

The intensity is proportional to the power.

Here's an example from my old physics book:
stranded in space a distance of 20 meters from a ship.  you have a 1000 W laser.  total mass is 95 kg.  How long does it take to get pushed back to the ship if the laser is pointed directly away?

9.38 hours.

So if light (photons) has/have mass, how can it/they go the speed of light?  I thought that was prohibited by relativity (anything approaching the speed of light acquires infinite mass)?!  This question has bugged me for a long time.

MustangMan

So, you're saying that home lasik is out of the question?

it appears quantum theory is superseding both the general and special theories of relativity in the same way that relativity superseded newtonian physics.  remember, at quantum level, normal physics don't apply.

[b]MustangMan[/b], photons do [b]not[/b] have mass.  You are correct in your assertion--if photons did have mass then according to special relativity, they would appear to have infinite mass.  Despite having zero mass, photons [b]do[/b] have momentum.  The energy of a photon is given by E = hf, where h is planck's constant (6.022 x 10^-34 Joules per meter if I remember correctly) and f is the frequency (in meters).  The relation between energy and momentum is given by E = (p^2)/(2m), where p is momentum and m is mass.  It is this change in momentum which causes the force (related to radiation pressure).


No, light exerting force is predicted by special relativity, there is no contradiction here.

And on an unrelated note, the relationship between quantum theory and general relativity is not that one supersedes the other--it is that they are valid in completely different domains.  Quantum theory comes into play on the atomic level, while general relativity comes into play on the solar level.  Some physicists are attempting to "unite" the two theories (quantum gravitation, etc.).

But if light has no mass, how is it then affected by gravity?

-FOTBR

Part of the problem with arguments about the physical characteristics of light (as photons) is the continuing debate about the behaviour of photons en masse - do they travel as "waves" (like on water,) or as "rays" (like string of tennis balls from a launcher.)

This is compounded by the fact that light displays traits of BOTH forms, as proven experimentally.

As far as light being affected by gravity and gravitic disturbance (cf. Black Holes,) bear in mind that the "speed of light" is usually given in relation to certain media - "in vacuum," "in air," "in glass," et. al.  Therefore, the medium through which light travels has a clear effect on HOW and HOW FAST it travels - one need only look up "refraction" to understand this effect - it is what makes my spectacles work.

Now, why does light bend around a singularity?  When light is travelling through a "vacuum" in space, it is important to remember how "space" is considered.  Think of a bedsheet stretched taut around the edges - no sagging, now!

When you have that sheet straight out, you now have "space."  This is space being acted upon by nothing.  If you take a ping-pong ball and roll it across the sheet, it will take a (relatively) straight path.

Put a good-sized ball bearing on the sheet.  It will make a dimple ("gravity well") in the "fabric" of "space" (This is where the metaphor "fabric of space-time" comes from.)  Roll your ping-pong ball past the bearing, and you will see it change course as it passes thru the depression.  You are witnessing a material variation of "gravitic refraction."

Now use a bowling ball in place of the ball bearing.  This will represent your "singularity" - the centre of the black hole.  While a true "black hole" is theoretically a literal "hole" in "space," this will serve for our purposes.

Note that your ping-pong ball must pass a good distance away from the bowling ball to make it past.  The closest approach to the bowling ball that the ping-pong ball can escape from would be the "Event Horizon" (properly, Schwarzchild Radius) of the black hole.  ANYTHING which passes the Event Horizon will disappear - even light.

I hope this make a little more sense to you, it is difficult to express quantum mechanics in purely physical terms...

FFZ

Here are some links to articles about using lasers to "push" things.

[url]www.usatoday.com/news/science/stuffworks/2001-02-16-light-propel.htm[/url]
[url]www.jracademy.com/~harrisa/laser.html[/url]
[url]www.planetary.org/solarsail/missions/sailing_tothe_stars.htm[/url]
[url]www.space.com/businesstechnology/technology/laser_propulsion_000705.html[/url]

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the explanation i get from my pet physicist (who is twice published in JCP), is that quantum can account for physical behavior at or above quantum level, in other words: it is a whole system of physics, as opposed to newtonian physics, which has a 'floor'.  and einstein himself pointed out several incompatabilities between relativity and quantum theory, not the least of which is the concept of locality.  dont forget EPR.

mmmmmmm.spagettification....

What about surgery on a glass eye?

basically

LOL...I wouldnt lose too much sleep over it. REAL scientists can't even explain how gravity works. Sure, they know it's there and can measure it, but they still don't know why.

Light DOES have mass, that's why it's path is affected by a gravitational field.

Remember the formula E = m c2?

I used to know from one of my physics classes that:

If one knows the amount of energy from sunlight hits the earth in one day, it can be calculated that approximately 4 kilos(I think that is right) of light will hit in a 24 hour period.

Good question--the most qualitative (non-mathematical) explanation is given by Einstein's [b]principle of equivalence[/b] which states that a gravitational field is completely indistinguishable from an accelerated reference frame.  That is, something inside an elevator in the middle of space (no external gravitational field) that was accelerating at 9.8 m/s^2 would experience effects that are the exact same as something sitting on the earth's surface (being acted on by a gravitational field of the same magnitude as the elevator's acceleration).  A light beam shining in a hole through the side of the elevator would appear to bend because the elevator would have moved before the light made it from the entrance to the other side.  The same effect is seen when light travels next to a mass.  I will try to find a link which explains it better.


I think you have misunderstood.  Quantum mechanics does not explain gravity.  This is why many physicists are working on "quantum gravitation" which would remedy this (if quantum mechanics explains gravitation, you'd better let all the physicists wasting their time on quantum gravitation and superstring theory know).


Exactly, that is precisely my point.  There are disagreements between quantum mechanics and general relativity.  The former is valid on microscopic scales, the latter is valid on macroscopic scales.  Quantum mechanics is no more a "theory of everything" than general relativity is.  They are valid only on mutually exclusive domains.  The attempts to unify the two fall under fields I have already mentioned (QG, string theory, etc.).


This is generally true.  Scientists spend most of their time figuring out [b]what[/b] will happen due to various phenomena--but they usually have little idea of why it works.  Figuring out how the force of gravity is applied ("action at a distance") is giving them enough of a challenge, there is little hope of ever figuring out what causes it to exist in the first place.



[b]THIS IS COMPLETELY FALSE.[/b]  Light does [b]not[/b] have mass, look it up.  I am a physics major, so I really ought to know this.



edit:
here is one link--[url]http://newton.dep.anl.gov/askasci/phy99/phy99415.htm[/url]

It is similar to what [b]FreeFireZone[/b] said.

I remember a story I read in a book (may have been Hawking, or Thorne, or any number of others) about Astrophysists in the 50s or 60s studying whether or not 'light' (electromagnetic radiation, all wavelengths) could be used to move dust clouds or other nebula out in deep space.

Well seems they had been debating this alot like we have here 'it is massless it cant do that' etc etc etc.  

Well they went to a conference with some other physicists in other fields and this topic came up and one of the other physicists spoke up and said 'YES, It most definitly CAN!'.  But He would not go any further.  And none of the astrophysicists questioned this gentleman because they knew once he stated it that positive direct way they knew something they didnt know before.

That Physicist worked on Hydrogen Bombs.  Something that is common knowledge now was very classified information back in the 50s, seems that in order to make a hydrogen bomb you reflect the Xrays from the 'starter' fission explosion to compress the hydrogen to pressures needed to 'fuse' the hydrogen it into helium.

And they say gun people are ignorant.  [>:/]

As I recall from little ole middle/high school/community college. Every "atom" has mass, and everything that has mass has gravity, etc etc.

I see thta there are very learned folks saying that photons have mass, and others saying they don't.  

Can there be force or energy present without mass???  You can't exactly smack something with nothing...

I can't find a good definition of photon...

MY basic question is can one of you physics fellas splain' a solar sail if light doesn't have mass...  

Bunch'a white trash, in-bred, gun-toting Neanderthal hillbillies on this board, let me tell ya! [;)]


Adam

Problem is E=mc^2 is only HALF of the equation.  

It is the simlified form of E^2 = p^2c^2 + m^2c^4

For massless objects, the Equation you want to use it E= pc (of course the only massless objects we know about are Photons).

Wow, some interesting comments here.  

Zonan, I don't pretend to be a physics expert at all and some of the math you talked about went over my head, but I did some research on this a few months ago and the site I visited said that photons DO have mass (I forget where I saw this info).  In addition, isn't momentum and mass basically the same thing when you get right down to it?  I can't explain this mathematically but I remember in some of my classes seeing this correlation.  

After thinking about this some more, I heard somewhere that the reason photons can go the speed of light (relative to environment) is because they're ALREADY going that speed and not accecelerating which doesn't mess with special relativity.  And since acceleration and momentum are related, this makes more sense to me than saying photons don't have mass.  

I know I'm probably not explaining myself very well, but any thoughts on this rambling?

Thanks.

MustangMan

this only reinforces my point.  the fact that current theory does not 'explain' gravity does not mean that it cannot.  if it can't, you might want to let all those physicists know that they're wasting their time.  that being said, my usage of 'complete' was inaccurate.  allow me to state that it is the most complete system we now posess.




this is contrary to everything that i have heard and read.  it was my understanding that, while relativity could not fully describe quantum interactions, quantum theory could be reliably applied to superquantum reactions.  are you stating that quantum cannot account for simple newtonian action/reaction?



actually, all matter exhibits characteristics of both particles and waves.  remember, mass and energy are exactly the same thing.



well, properly speaking, mass does not 'have' gravity.  mass causes gravity, and as explained earlier, gravity 'bends' the space-time continuum.  where the damn graviton fits in, i have no clue.  i'll have to defer to the physics student.  [:)]

(p.s.  this is why i love the internet.  i came to learn about ARs, and and i wind up kibbitzing about physics.  you guys rock.)

I have to meet with some people, so I will respond to some other points later, but for the time being, I found this with a quick search:
[url]http://www.mtnmath.com/whatth/node56.html[/url]

some highlights:
[i]There is no theory that combines quantum mechanics and general relativity. In quantum mechanics the greater the accuracy of a measurement of location the more uncertainty there is a
in the measurement of energy. The uncertainty principle applies not just to particles but also to empty space. Over very short intervals phantom or virtual particles can appear. The shorter
the time the more massive the particles can be. At very short intervals virtual particles will be massive enough to form black holes. One cannot extrapolate simultaneously both quantum
mechanics and general relativity to minute distances. The theories explode or diverge.[/i]

and: [i]General relativity and quantum mechanics have disjoint experimental domains. General relativity is only observable with massive objects. Quantum effects are only observable with
minute particles. Thus these incompatible theories can coexist in a temporary truce. Eventually something has to change.[/i]

and: [i]The hottest research area for extending theoretical physics is combining these theories. The experimental
domain in which such combinations could be tested is unreachable with existing and foreseeable technology. The situation is not unlike that in mathematics where fundamental research
focuses on properties of large cardinals when no infinite sets let alone large cardinals may exist. Reconciling the two fundamental physical theories is a mathematical exercise that may
be devoid of physical content.[/i]


Yes, it seems all things propagate as waves and interact as particles.  Evidence:  Electron microscopes, for example, use the wave property of electrons (it allows for such detail because the wavelength is so much shorter than that of visible light).


Yes, thanks for the good discussion

what about good old F=ma?

in order to exert force an object must have mass.

That is Newtonian.  Special Relativity threw it out.

[url]http://www.physlink.com/Education/AskExperts/ae180.cfm[/url]

So no explanation of a solar sail????

Photons have mass by virtue of thier velocity. They are massless and cease to exist if you slow them down. Then they simply become absorbed by the reipient matter and if aaaaathey are absorbed in sufficient quantity, cause a fluctuation in the orbits of the absorbing atoms electrons. Thye solar wind is not just photons, it it also charged plasma and gas, so yes the sail would work, not only from light pressure if designed to take advatage of that, but also from energetic particles stiking the sail. Does that help?

No. Antimatter is antimatter because of its' spin. There is no such thing as a negative energy, and mass is mass, antimatter or not.

Not really(as in doesn't yet help), what the hell has charged plasma and gas have to do with it??? Wouldn't it be the regular ole every action equals....type thing?

I had a teacher, and this was in MIDDLE SCHOOL who evidently felt very strongly about light having mass...He was an older guy an a DR. of something...don't remember what...

Anyways, he had this "vacume bulb" with a post in it and a butterfly lookin thing on said post.  When he opened a few strips of the window blinds it would slowly begin to spin.  When he closed the blinds, it would eventually stop spinning...About same thing as a solar sail as far as I can tell, but I haven't taken a real physics course, and you fellas are above my head, but if photons are particles, it seems that they would have some sort of mass, even if very very little.

I beleive deeply in my soul(not really) that light has mass, even if I can't sound all genius like in my arguments[:D]

Ok the way a light sail works, if I can expain it.  When a light reflects off something it doesnt 'bounce' like a billard ball.  It is absorbed.  And then emitted (in various forms and over various times).  Both of these actions give momentum to the object.  The more effcient the 'reflection' (the emission of photons very close to their original wavelength in a uniform direction) the higher the gain in momentum.  In the Bulb the white side 'reflects' better than the black side so it causes the windmill looking panels to spin.  


If you really want to fry your brain try this:

Stop thinking of photons as little 'bullets' flying around.  

Think of them more as 'strings' that connect two events in space-time.  Even if the events take place billions of years apart to us, to the 'photon' they take place at the same time.  

Photons are KE, and they live outside of time.

Well, we don't know that for sure.  There's a whole lot of things that are theoretically possible.

Hey Silence, it seems that some would disagree that photons are only KE, I'm not sure that light might be absorbed but haven't thought much on that. It is certainly reflected better from some surfaces than others...

By absorbed do you mean attenuated and changed to heat or dispersed?  This might be a good point, when you puit the flashlight to your hand, does it absorb the light not passed throuhg the hand? Do the remaining photons get absorbed into your hand, or just reflected or dispersed at much lower energy levels ?

I don't think anyone has proven either.  It seems that there is plenty of disagreement over the definition of photons too.

In the little searchin on the web I did at various ".edu"s there is still much disagreement.

Possible and probable are not two sides of the same coin. That being said, you are right, I should have been a little more precise in what I said. negative energy has never been proven to exist, but then we may not have the proper method and apparatus necessary to detect it.

Oh and Silence, your gut is right. Gravatational lensing is a proven effect, so light does have proven mass.

This is the way it was described to me, or at least the way I understood it.

When photons strike electrons they never 'bounce' like a billard ball against the rail, they are absorbed into the electron.   Think of it this way the Billard ball hits the rail and is absorbed, everytime.  Immediately, the Rail then emits a nearly identical billard ball in a certain direction, hence a 'reflection'.  Or the rail 'grows' (virtually) a bit and then emits lots of tiny tiny billiard balls over time in every direction, aka radiating 'heat', as it returns to its original 'size'.  (btw dont forget that light really isnt a billard ball, and an electron isnt really a rail)

The reason why a 'mirror' makes a better 'light sail' is that the sail gains momentum with the 'absorbtion' of the photon, then it gains momentum with the emission of the photon(s).  A 'refection' or emission of a nearly identical photons in a uniform direction gives a better momentum in a specific direction is better to provide 'thrust', over a uniform and directionless radiation of many smaller photons in every direction via the 'heat' idea. Dont forget that 'heat' is 'light', just different wavelengths.

As to photon mass, most physists agree it is massless (rest mass that is), but they cannot 'prove' it, they can only prove that there is an upper limit to its mass.  I think that is now something like 1x10^-17, or 1x10^-27 (I cant remember off the top of my head), EV (electron volts).  The EV is the 'mass' of relativity.  Compare the upper limit of the Photons mass to the mass of an electron (1x10^6 EV), a difference of 23, or 33, 'zeros'.  

As to the definition of photons yes the can be defined several ways, and have the definitions be right.  Freaky huh?

the hand-

Some photons are absorbed and radiated as heat. Some pass entirely though the hand (more of the 'red' wavelengths than others) and do not react with any part of the hand.  

Huh?

How does gravitational lensing prove light has mass?

Damn Zonan. Beat me 2 it. I like spewing my BS when these posts come up.
Your a sharp one.

[:D] Bongs and particle physics don't mix [:D]

Because garvity is an interaction between two or more masses. Since the "beam" of light shifted in its' path when passing next to a massive body, that means photons have mass. get it?

Jesus H. Christ, I love this place!

Sorry Bign!  I hope to respond to some posts tonight.  I was up until 4am doing work last night so I didn't get around to this....

There are two main variants of "solar sails."  I am not familiar with the exact designs, but the most commonly referred to is pushed by the radiation pressure (the same effect mentioned in other posts).  It is also theoretically possible to create a magnetic sail which is pushed by the "solar wind" (charged particles from the sun).  I haven't studied either, but I imagine the magnetic sail would give more flexibility regarding steering, because of the nature of the magnetic force (F = qv x B, which means the force is perpendicular to both the velocity of the particles and the magnetic field)--by changing the direction of the magnetic field, one could easily change the direction in which the force was exerted.

This seems to be a good link about solar sails.  I haven't read it closely, but it looks like they have a decent introduction available:
[url]http://www.ugcs.caltech.edu/~diedrich/solarsails/[/url]




Everyone reading this thread may be interested in this article:
[url]http://www.newscientist.com/news/news.jsp?id=ns99992901[/url]
It tells about one person's idea of using radio waves (which are the same thing as light, just a different wavelength, so all of these radiation pressure effects are still valid) to assemble things in space from the ground.

Nope. That only proves that massive bodies warp spacetime, not that photons have mass.  In order for photons to 'interact' graviticly they would have to to have non-virtual mass, which has not been proven.

Photons always travel in straight lines, from the photons point of view.  Remember time is meaningless to a photon, they live outside of it, and without time spacetime has no meaning, and without spacetime, space has no meaning.  

galaxies, singularities, stars, planets, you, me, all these things warp spacetime on a greater or lesser scale.  That is what causes the 'beam' to appear like it is 'bending' to an outside observor (us).  Even though it is not, to the photon.