# Does light have a mass?

Discussion in 'Physics & Math' started by GRO, Apr 6, 2002.

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1. ### ProsoothusRegistered Senior Member

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Enqrypzion,

You're wrong. An object can only move as fast as the speed of the force pushing(or pulling it). As the object's speed comes closer to the force's speed, the acceleration of the object would decrease. When the object's speed reaches the speed of the force, the force no longer has an effect on the object no matter how large the force may be.

Lowefly is correct. The decreasing acceleration of a particle in a particle accelerator as it approaches light speed may partially be the result of the particle's speed approaching the speed of the electromagnetic fields pushing it.

Tom

3. ### EnqrypzionRegistered Senior Member

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another thought

by the way, if photons would have a mass of their own, a cilinder with two circular inwards directed mirrors that let (some) light pass through from the outside to the inside (but not the other way around), would get heavier when it 'catches' a lot of light. Ain't it possible to prove that experimentaly?

5. ### EnqrypzionRegistered Senior Member

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How about applying the force in a 90 degree angle to the object (preferably in two directions)? That way you 'squeeze' the particle a (tiny) bit like moving air forward by clapping your hands?

7. ### James RJust this guy, you know?Staff Member

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A rocket has a constant exhaust velocity (relative to the rocket). Yet it can speed up to a much higher speed than that velocity.

8. ### ProsoothusRegistered Senior Member

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James R,

After thinking about how a rocket works, I found that you are right.

A rocket is an exception because it carries the object that produces the force (the propellent) with it .

In other words, an object can only move as fast as the force pushing it, only if the force is independent of the object.

Tom

9. ### allantVersion 1.0Registered Senior Member

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Re: another thought

Unfortunately no. One the mass is very small. And two the cyclinder will heat up and radiate photons as infrared EM.

The best evidence we have is the formation of matter from light. And this takes bigest meanest baddest set of lasers we have to make only one particle (electron or similar) ! Try out E=MC^2 with the mass of the electron and check out how many power stations you need!

10. ### On Radioactive Waveslost in the continuumRegistered Senior Member

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gosh, i havnt been here that long, and i feel like i've seen generations pass through this place when look back at this thread.

11. ### RDT2Registered Senior Member

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460
No - golf balls, etc travel faster than the clubhead.

Cheers,

Ron.

12. ### ProsoothusRegistered Senior Member

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Ron,

That's only because a clubhead has more mass than a golf ball.

If you had a clubhead that had less mass than a golf ball, there would be no way of making the golf ball go faster than the clubhead that hit it.

Tom

13. ### FluidityRegistered Senior Member

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594
Actually, Prosoothus

The Golfer has many times more mass than the golfball or the club. If I had a club of zero mass with a flexible shaft, like the shafts on real golf clubs, the ball would spring off the club at a higher velocity than I 'hit' it. When a golfball is struck, the club bends several inches, in the case of a driver. The golfer continues to apply his momentum to the ball during contact, so the spring action of the shaft is pretty forceful, and quick. The ball is resilient, so it springs off the club face being compressed only by its resistance to acceleration, caused by its own mass. So, imagine a machine swinging a very low mass club head at a heavy ball. Slow things down to visualize the moment of impact, and watch the shaft of the club bend, then spring back with greater force than the 'momentum' of the club head. In golf club design, the longer the shaft, the greater the velocity, the lighter the club head. A one-iron club head is about one fourth the weight of a 9 iron, and not very much heavier than a golfball! In fact, the differential in velocity of the ball over club speed is much higher with a one iron than it is with a nine iron. There is actually in inverse relationship to mass and distance in golf club head speed--it's the resilence of the shafts and the speed they generate. Now, if the shaft did not bend, or the ball had no resilience, consider the following.

No matter how heavy an object is, it can't 'push' anything faster than its going.

Almost all solid materials have some resilience. This causes energy to be stored in the object during impact. The longer the stationary object stays in contact with the moving object, and the more resilient it is (they are), the more kinetic energy can be released into the stationary object's mass. Of course, the faster the club head is moving the harder the ball has to be to take advantage of the applied force. And, a heavier ball of the same resilience will fly farther, as long as the physics of the club remain proportional, even if the speed of impact is the same, because the moment of inertia will be longer.

Last edited: Feb 20, 2003
14. ### ProsoothusRegistered Senior Member

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Fluidity,

Well, I see from your post that you have a good grasp of mechanics. Please answer the following questions for me:

Let's say you have an object, and a force that is trying to move it. Let's say that the force is a fundamental interaction, and therefore has a limited speed of c (like the electromagnetic interaction). Can a force, that has a speed of c, accelerate the object to a speed higher than c?? As the speed of the object approaches the speed of the force, won't the acceleration of the mass decrease making it appear, to a layman, that the mass of the object is increasing?

Tom

15. ### GundamWingRegistered Senior Member

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lots of stuff to think about.

In this post, i'm only speculating with an intuitive view that I have about light which doesn't go against (I don't think) the observed behavior and descriptions that physicists have used over the years. But, keep a pound of salt well in hand and realize that this is MY personal view and not the general view.

...also, this is extremely hard for me to explain in text but i'll do the best i can...

The way I see it "electromagnetic waves" are not 'separate' from space and time, they are somehow 'waves' that exist ON 'space-time'.

It's like you are 'pinching' the 'rubber sheet'. That is, you cannot have a electric charge or a magnetic field dipole that is independent of space or time -- it's like calling a water wave different from the water. This is why things fundamentally cannot accelerate to the speed of light, because the speed of light somehow has to do with the 'rate' at which space and time interact and 'spread'. So, you can't "beat the medium" so to speak. If you can "beat" the medium, you must be travelling in some "alternate" medium relative to the original medium.

From this point of view, the momentum that a photon carries is what is observed as the 'force' that 'pushes' matter -- really its just a transfer of momentum.

Photons have no mass because as I see it they don't "exist" in 4 dimensions:

(1) from the viewpoint of the photon -- they exist in 3 dim -- that is 2 space and 1 time direction. Of course, from the perspective of the photon, time doesn't ever change -- it sort of 'freezes' since there is no passage of 'time' at 'c' (i.e., the hypothetical clocks slows to a standstill).

(2) from the viewpoint of something external -- they exist in 3 dim -- 2 space and 1 space.

One spatial direction is the electric potential vector, the other is the magnetic moment vector, and the third is the change of these two vectors over time or the 'position' along the direction of travel, if we look at the 'total path' of the lightwave from where it starts to its target location.

Iif this analogy corresponds to how they physically behave, then can two photons which are spatially perpendicular to one another produce a perceptible interaction? or would they just zip right past one another? So, a photon traveling in 'one' direction cannot 'see' the other 'two directions' -- looking at a photon sideways -- you would see nothing. It's like looking at a sheet of paper sideways -- with zero thickness.

So, if photons are 'embedded' into the 'fabric' of the surrounding spacetime which physical objects exist in, then it makes sense how massless objects can exert 'force' on objects with mass -- they don't really exert a force, but they transfer the momentum through the medium of spacetime to the object. Does this mean that photons are 'momentum' carriers in some sense? (i.e., similar to how 'gluons' are 'force' carriers?)

... SOooo, that concludes my 'intuitive' ramble.

Last edited: Feb 23, 2003
16. ### chrootCrackpot killerRegistered Senior Member

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2,350
Photons have momentum, but not mass. Relativistically, momentum and mass are separate things. When photons hit you, they push you. They do not, however, have mass. Done.

- Warren

17. ### GundamWingRegistered Senior Member

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1. Don't ruin an innocent mind with your lack of understanding. Atoms "recoil" because of the law of conservation of momentum. But since you haven't had any physics, you wouldn't know this.

2. True: photons are influenced by gravitational fields (given that einstein's theory PREDICTED this, you are using proof from a theory that you say is wrong to make your point... good logic.) And NO: photons 'follow' space-time because they are 'part of' space-time and not 'independent of' space-time; not because they have 'mass'. You seem to get the physical "idea" that laymen have about "mass" confused with the concept of "mass" as a mathematical abstraction. You are right -- there really is NO SUCH THING as mass -- its simply a quantity used to explain the difference between two objects that accelerate at the same rate, yet produce 'different' effects. Mass has nothing to do with anything you can 'experience' -- just because your common sense tells you that mass has something to do with your bathroom scale, doesn't mean you understand 'mass' for what it is. Think about it some more and you'll get what i'm saying here. The best analogy is "why do the yellow lines on a highway seem to follow the highway?"

3. Photons do not 'exert force' -- they transfer 'momentum'. Have you heard of E = h*f ? It relates the frequency of a photon to its energy. Momentum for a photon is p = h*f / c which says, that the momentum of a photon is proportional to its frequency. So, alternately, p = E / c, which means that the energy of the photon is proportional to its momentum. If a photon strikes a surface -- it strikes at a particular angle (direction) with a particular energy. This energy is transferred along that direction, thereby appearing to 'push' the object in that direction. It is actually, more like an object that had "zero" momentum has now been imparted a "finite" momentum in a given direction without any application of force what-so-ever.

No deluded one -- most physicists who have studied relativity perform experiments to prove it WRONG, and find themselves always proving it correct. But glad to know you, the perpetual man in red underwear and cape to boot, run around physicists neighborhood's to peek under their bed in the name of untruth, injustice, and the pseudo-scientists way.

What you fail to understand is that the photon never started "at rest" it ALWAYS has velocity 'c' from 'birth'. Of course thee measured value of 'c' varies with the medium of propagation in question. The usual value of 'c' that people refer to is "the speed of light in a vacuum".

... and you are WHO? What about all those people who say that "everything tastes like chicken"? Are you going to argue with them too? :bugeye:

For a MUCH better understanding of photons and so on... I suggest this website:

http://www.wikipedia.org/wiki/Photon

Last edited: Feb 22, 2003
18. ### GundamWingRegistered Senior Member

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Besides all the comments already given I think this whole thing has been beaten to death. Let's look at interesting APPLICATIONS now...

This is the coolest thing to hit reality I think... Check this out!

http://www.lightcrafttechnologies.com/

they have nice movies of working models. I can't wait to be able to ride one of these things one day.

19. ### On Radioactive Waveslost in the continuumRegistered Senior Member

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I took a quick look. Interesting that the latest news I saw there was from the end of 2000.

20. ### GundamWingRegistered Senior Member

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ya it is perhaps outdated... but the physics makes sense, the technology is real and demonstrated (see the movies they provide).... and in 15-30 years... i'll be hopping around in a light craft hopefully. It's like a couple steps above solar-sail concepts. It appeared once in a popular science issue as the cover -- I forget which issue.

Last edited: Feb 22, 2003
21. ### one_ravenGod is a Chinese WhisperValued Senior Member

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13,406
Some help?

I have been reading this thread for a few days, breaking when I needed more information to stay along with the various trains of thought.

I am up to about page 22 now, but I feel I need more information on a few things to catch up with myself and my mind.

I was hoping you could help so that when I am done and possible ready to reply, I would have a better understanding of what I am talking about.

1.) The two slits experiment:
Can someone point me to more information/analysis of what happend in this scenario?
Not 1500 mathematical equasions, but the practical relaities of it, how it was conducted, how it was measured, conclusions etc.
I don't believe that we have any apparatus that can "see" a single photon of light. let alone follow it's path and trace it's movements, therefore how is it we know that a sibgle photon of light followed two seperate paths at the same time?
I would like to understand this better, and have not found a site oir reference that explains it logically.
I hace read, from Hawking and others, only:
There is a photon, it is directed at two slits, that one photon traveles through both slits at the same time.
My question is how do we know?

2.) People keep saying that light is made up of photons that are particles that behave like waves.
Can someone please elaborate on that at least just a little?
How do we know that photons are particles?
For example, are photons considered particles because the math of relativity says they are, or are they know to be particles and the math of relativity is supported by that fact?
What wave-like properties do they exhibit?
What particle-like properties do they exhibit?

3.) What is the assumption that c is the universal speed limit based on (other than relativity math)?
I understand the experiments that support tha c is constant, but what experiments have been done to verify teh validity of c being the limit for anything to travel?

4.) I have read in a couple of places that light used to be thought of as a wave the propagates through some medium (aether or some other medium) rather than particles actually traveling.
Why was that idea laid to the side?
I read somewhere about an experiemnt with a beam of light that did not display the results of speed change that would have been expected if the aether theories were correct.
Can someone elaborate on that just a bit?
Were there any other experiments regarding this belief of light being a wave rather than sperate particles done?
What were they?

5.) I read somewhere on this forum (perhaps this post, but I am not sure) plus I recall something in "The universe in a Nutshell" about one of the experiments that were done with the atamic clock/plane setup.
The plabe flew East the clock lost time.
BUT when it then flew West it gained time.
(or it may have been vice versa)
Does anyone know any more about this and why it is considered to support the claims of relativity rather than refute them?
It seems to me that if all motion is relative than it shouldn't matter if the plane is flying East or West the clock (according to relativity) should slow down regardless.
Doesn't teh clock speeding up in the other directionn seem to point at some other reason for the percieved time changes (such as patterns and movements of gravitaional filed lines of flux, perhaps, or some other fprm of intereference?

That is it for now.
That should give me enough to at least finish reading this debate and have some level of confidence in the opinion I form.

Thanks a lot.

22. ### GundamWingRegistered Senior Member

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367
Re: Some help?

It's hard to explain these without pictures, so i've found some websites that do some justice to this. I think you should look at the following links in order and get the 'gist' of it.

waves vs particles
>> http://www.qmw.ac.uk/~zgap118/

two-slit experiment (w/ JAVA animation for illustration)
>> http://members.tripod.com/~vsg/interfer.htm

photoelectric effect
>> http://lectureonline.cl.msu.edu/~mmp/kap28/PhotoEffect/photo.htm

summary of everything
>> http://hyperphysics.phy-astr.gsu.edu/hbase/mod1.html

Hope that helps.

If you have further questions, do ask.

23. ### GundamWingRegistered Senior Member

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367
Re: Some help?

It's not an assumption -- it's a proven fact.

See the Michaelson-Morley Experiment
>> http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/morley.html

Also see...
>> http://www.reenigne.org/maths/relativity.html

Also see... (more math required here)