I am right now studying to a physics test I have tomorrow in special relativity, basic quantum mechanics and astronomy. Among other thing I have learned to calculate the momentum of a photon (p = h/wavelength). Now, with this you can easily calculate the "supposed" mass of a photon (m = p/c). However this should not be possible since light does not interact with gravity and are "weightless" (have no mass). I have calculated that a photon with a wavelength of 400 nanometers should have a mass of 5,526 * 10^-36 kg, a VERY small number, but indeed not 0, which is the mass photons are supposed to have.

Can anybody who understood what I just described and/or have perhaps even studied this tell me if this means something or is just a nonsense answer?

This is a serious question. If you think I just made up some sci-fi shit here you should brush up on your physics a bit.

You'll find an attitude here to do your own homework.

However you'll also find that the m=p/c is only an approximation which doesn't work well close to the speed of light. Use E^2 = p^2c^2 + m^2c^4 (or E=pc for a particle of no mass) instead.

It was not asking for help with the homework, I just stumbled over this and wanted to ask why I got an answer when there should not be one. Anyway thanks for that, I found that equation here in the book when I looked closer.

Edit: Any mod can lock the thread now if they want to, I got the answer I was looking for.

Fun subjects anyway, though the topic probably isn't Symposium-worthy to start with.

It's a common error, using a formula when it does not apply. Of course here it gave an obviously incorrect answer, so it didn't do much harm.

Well if it doesn't suit the Symposium I don't think it suits anything here. I thought about what happened if I tried to put in that answer in the equation you gave me, I'll see what happens just for fun.

The "quick question about" part is really contrary to the idea of "serious general discussion", but would do well in the free for all thread. Just saying.

On an unrelated note (well, really only related to you mentioning basic quantum mechanics), the one photon at a time approach to the double-slit experiment is some of the coolest stuff ever.

Moved to Tavern for obvious reasons.

Corimngul, do the Voyagers show any signs of the so-called Pioneer anomaly? I didn't find anything with my feeble net searching skills, and I wonder if anything of the kind has been observed, since it's an interesting topic, and I think those things are far enough for it to show already if the effect is not particularly Pioneer-related.

EDIT: Never mind, I found the answer already...

Well there you go...

Hopefully the planned ESA missions will answer some of these gravitation-linked questions. LISA and the gravitational waves come first, however.

I can't wait to take physics next year...

Question. If light has no mass, and you say that it can't be affected by gravity, how do black holes work?

"The gravity of a black hole is so strong that not even light can escape" Therefore, light must have some mass, right?

Good luck finding an answer to that question that makes sense. My friends and I have been looking for a clear answer to that exact question for about a year and a half. Two teachers avoided the question and one mentioned the dual properties of light. The idea is that, light can act as a wave (and have no mass) or as a particle (and have a mass). I don't know how it does this or why and understanding this phenomenon requires a knowledge of physics that is beyond my own. Hell, the teacher that told us admitted that they, themselves, only had a slim understanding of it.

It's one of those things where receiving an answer, just raised further and more complicated questions. You can go off and research light's dual properties on your own if you like, no promises that you'll understand what you find though. You may know more about physics than I do (I only have year 11 physics under my belt, no high end stuff) and be able to comprehend your findings. If you do, try and explain it to me.

Of course, anyone on the board who understands all this is free to clear up the confusion that I have spawned with this post.

This is off of my rudimentary physics knowledge, but even when considering the particle property of light, isn't it still considered to be a massless particle?

Ugh, I love physics but damn, there's no way we're ever going to work everything out about it.

I did not know that a massless particle could exist (although, now that I think of if, some sub-atomic particles are massless, right?). I can only go by the information that my physics teacher gave me and they were under the impression that light had a mass, as a particle. I'm not saying that is correct, it may very well be wrong, but that is what I was told. If that's wrong, then I'm back to square one; how the hell does light get sucked into a black hole without a mass??!?!

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

"Mass: 0"

Gravity bends the space-time around a massive body so therefore the light is bent, or completely pulled in by the black hole (or so goes my first year of university understanding). It's a little like putting a weight on a piece of fabric suspended by its corners and rolling marbles past it, they tend to curve towards the weight.

A quick Google search brought this up:
http://van.physics.uiuc.edu/qa/listing.php?id=2044

Who says massless particles aren't affected by gravity?

This is explained in general relativity by, as Foxx say, bent space time. A black hole is essentially a lot of mass in a small volume, which bends space time so much that all possible paths through space time eventually lead to the center of this volume. Thus both particles with mass and radiation are pulled in by the gravity.

You basically take the view of gravity that Foxx proposed and then bend it out of all proportions - strange things do after all happen nearby singularities.

And I had learned about photons, just before summer break. Extended periods of doing nothing and listening to massive amounts of metal have erased my once grand knowledge of science.



Yes, I remember my biology teacher (also a qualified physics teacher) telling my something like that briefly. I thought it was interesting but once again left me clueless on some aspects. Really, the only thing I don't understand there is; what space time is. I'd like to hear and explanation, if it's not too much trouble.



Nice find, Balth. Does help clear things up a bit.

e = MC2

Hope this helps.

Your teacher seems confused. There are two definitions of mass (in special relativity). The ordinary mass which we usually calls mass is also known by the name of rest mass or invariant mass. And no, light never has any of this.

The other concept, which isn't used much these days because it's quite misleading, is relativistic mass. It's essentially a measure of the total energy in the system and of course, photons often have energy. I find it a lot easier and more accurate to call this energy though, instead of mass. This doesn't mean much though, not in this issue, except that your teacher needs to take one or two university level relativity courses.



Assuming you worded that correctly, a black hole without mass? Not really feasible. There are micro black holes but they work rather through quantum effects than through gravity.

She probably was. She is a very good teacher but admitted that she could not recall all that she had been taught at university, in regards to that subject. Thanks for the information, it did help make things clearer.




You assumed wrong!

I should have said; "how the hell does light get sucked into a black hole, if it has no mass??!?!". So, I was asking how light, if it has no mass, is effected by the gravity of a black hole. However, you and other posters have answered that question, largely by dispelling the misconception that particles with no mass can be effected by gravity, because of the whole, "bending space time" concept.

What I'd like to know now is, what is space time? And is light therefore effected by all gravity? So, our sun's gravity, for example. Or even the tiny gravitational forces produced by people or a brick? I'm aware that even if it is effected, the effects will be so small you may as well say they aren't there, but I'm asking for theory, rather than practical.

Ah, that question. Basically, it's an abstract construct, uniting space and time. It's not much harder to model mathematically than the ordinary euclidean space, but of vital importance in relativity. You take our ordinary (euclidean) space - length, width, height and add a fourth dimension - time. Yes, you really need to bring these together. In say Newtonian mechanics we look at the space and keep time separate. Here, they are merged into this four-dimensional spacetime.

Usually we treat time as something simple, only going in one direction at a fairly constant speed. That's not the case in relativity. The rate of time depends on your velocity relative to the speed of light according to an observer and on mass concentrations nearby. And this is why it's vital to actually put them all together.

So well, spacetime is not all that complicated in itself. What's complicated is the properties of it in the different theories of relativity or quantum physics. The problem is, to get any real understanding or intuition from it one needs to understand bits of the actual theories.



Well, yeah. All mass concentrations would bend the spacetime, only not as much as the extreme cases known as black holes. Actually one of the first pieces of evidence for this came with a solar eclipse, where it was proven that light was deflected. Not much obviously. In this case the light came from a distant star, passed by the sun and was deflected less than 1/2000th of a degree. A brick would not do much then, but it's still a concentration of mass and should thus curve spacetime a little bit aka deflect light.

Thank you very much, Corimngul. You proved both very knowledgeable and informative.

What I'd like to know now is, what is space time? And is light therefore effected by all gravity? So, our sun's gravity, for example. Or even the tiny gravitational forces produced by people or a brick? I'm aware that even if it is effected, the effects will be so small you may as well say they aren't there, but I'm asking for theory, rather than practical.[/quote]

Spacetime is basically the three dimensions (x,y,z) added with time. Well, it's a bit more complex than that.

http://en.wikipedia.org/wiki/Spacetime

Light and Gravity. Yes light is effected by gravity. During a total solar eclipse, stars near the sun (percieved positions in the sky, not actual) appear to be closer than when the sun was not in front of them. This means that the sun's gravity was bending light from stars behind. This basically proved that Einstein was right.

http://www.firstscience.com/site/articles/coles.asp

Ready for something that will blow your mind?

We might be living in a hologram.

http://www.newscientist.com/article/mg2 ... ogram.html

So, we are theoretically creations of light from a distant part of the universe, where all our actions and sentience actually being mimicry.

At the same time, if projection blur is much larger, and we are the projection, wouldn't electron microscopes start being able to pick up on this within our own bodies if it occurs at a size as 'large' as 10^-6 meters?

Also, if we are even semi-perfect projections, why would our projector be experimenting with this unless they are a projection as well, such that we are mimicking it? The only explanation for this I can think of is that if we are a hologram, we are made up of bits of data defining a reflection passed on by light (in the same way that computers use light to pass data), and somewhere along the path from the projector, the light is being corrupted such that we act differently.

I guess we could also be the projection of a projection (mirror reflecting a mirror) though, but an alteration of behavior would still have to happen at some point.

Possibly explaining a 'multiverse' or infinite universes?

This might sound like an ignorant question but I would like to know how it is answered pertaining to physics.
If you are standing in the middle of the south pole, why would you feel just as upright as if you were standing in the middle of the north pole? Wouldn't you feel like you were hanging upside down?

Unless some knowledge of magnetic fields escape me, you wouldn't feel like your hanging upside down. If you did, that would mean that gravity is trying to draw you towards the sky, which is something that gravity does not do. By the same logic, all Australians should fall off the bottom of the world.

Yeah, we're not stuck to the Earth because of magnetism, it's all gravity and gravity is the same all over the planet (it changes with height of course, but that's a very small difference.

Remember: Aside from the miniscule amounts of iron present in our blood, HUMANS ARE NOT MAGNETIC.