Physicists Clarify Exotic Force

Did I miss something? by markild · 2005-06-20 06:37 · Score: 1

I know this is going to get a lot of RTFM answers (even though I did), but is this just a fancy way of saying "everything as usual"?

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Re:Did I miss something? by Nos. · 2005-06-20 06:49 · Score: 4, Interesting

Well, this is the first time I've heard of the "Casimir force" force, but reading the article linked to in the article (how's that for RTFM) explains that pretty well. Now, the prevailing "ToE" is to the best of my knowledge, string theory. This theory was developed to explain the inconsistancies with Newton's and Einstein's laws when you got down to a sub-atomic level. This study is basically saying that string theory is wrong, and that Newton and Einstein (for the most part) were right.

Now, the interesting there here is the "Casimir force" which basically, is the force of photons striking an object. We touched on this actually in high school physics. We were experimenting to find out if light was a wave of a particle. (its a wave of particles). I started to ask questions like, if that's true, wouldn't most stationary objects eventually gain mass due to a build up of photons. We never quite got into that... probably a little advanced for most people in high school physics. Sorry, back on topic. This force, becomes very powerful (comparatively) at sub-atomic levels. The force of a particle travelling at the speed of light can become very significant. In fact, it becomes more significant than gravity. So, everything as usualy, I'm not sure I'd agree, but it hopefully does get us one step closer to the ToE.
Re:Did I miss something? by TripMaster+Monkey · 2005-06-20 06:50 · Score: 3, Insightful

Yes and no...what's noteworthy about this experiment is what they didn't find. Much like the Michelson-Morley experiment in 1887, which set out measure the 'aether', and instead failed utterly to detect any such thing, this experiment was devised to detect exceptions to the behavior of gravity on a quantum scale, and found no such exceptions.

Ephraim's not giving up yet, though...he plans on developing another experimental apparatus that is a million times more sensitive than the one that was used in this experiment. Also, even though this experiment was nominally a 'failure', the fringe benefit of clarifying the Casmir force is a big success.

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Re:Did I miss something? by HerbieTMac · 2005-06-20 07:35 · Score: 1

No, no, no, no. String theory is attempting to explain the observations of angular momentum of hadrons. Specifically that they appeared to be directly proportional to the square of their energies.
Casimir force has nothing, let me repeat, nothing to do with the force of photons striking an object. You are undoubtedly confused and I can't even begin to guess from where you gleaned this information. The Casimir force arises from the relative density of quantum vacuum fluctuations of the space between two very close objects and the space outside of the objects. As you said, quite advanced for a high school class, but don't let that prevent you from spreading disinformation in /. posts.
Re:Did I miss something? by Nos. · 2005-06-20 07:58 · Score: 3, Informative

You are undoubtedly confused and I can't even begin to guess from where you gleaned this information

Well, as I said, I read the article about Casimir force linked to in the original article ( http://news.uns.purdue.edu/UNS/html4ever/030811.F ischbach.casimir.html) which contains this paragraph:
The Casimir force has to do with the minute pressure that real and virtual photons of light exert when they bump against an object. High quantities of photons are constantly striking you from all directions, emitted by everything from your stovetop to distant stars.
Re:Did I miss something? by HerbieTMac · 2005-06-20 08:16 · Score: 1

Sorry for jumping on you like that. Fishbach must have been misquoted by the journalist because that is a blatantly false statement. Although the Casimir effect calculations above absolute 0 do take into account radiation, that is a correcting factor, not the effect itself. For a lay explanation of the Casimir effect, see this article on Physics Web.
Re:Did I miss something? by DrLudicrous · 2005-06-20 13:17 · Score: 1

We were experimenting to find out if light was a wave [or] a particle. (its a wave of particles).
Light is definitely not a wave of particles. In fact, light is the propogation of two orthogonal (perpendicular) oscillating fields, one electric, and the other magnetic, through space, with the oscillations being centered about a point traveling at speed c.

Common sence by Leroy_Brown242 · 2005-06-20 06:38 · Score: 1

This sound slike common sence. But hey, I'm just a computer geek.

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Re:Common sence by bluGill · 2005-06-20 07:48 · Score: 1

The problem is there are two different common senses in effect. One the one hand we know gravity exists, so common sense says it should exists everywhere. On the other hand there is no explination to account for gravity (We can measure it, but cannot explain it), and the some theories that when looked at alone seem to imply that gravity doesn't apply.

So they have proved one side right, but only at the expense of making some otherwise well tested theories fail.

Gravity is easy for the uneducated masses to understand. (by which I mean though without a degree in physics) It makes not sense to those who try to understand it more than the 'uneducated masses'.
Re:Common sence by Anonymous Coward · 2005-06-20 08:09 · Score: 0

Well, it confused me a little...
but I read it as "erotic force"....

Gravity at small length scales by dr.+loser · 2005-06-20 06:54 · Score: 5, Informative

IAAP (I am a physicist), and here's the deal:

There are suggestions out there that one way to test for the existence of extra "compactified" spatial dimensions (the kind of stuff needed in string theories) is to look for deviations from Newton's 1/r^2 gravity at small distance scales. See, for example, here.

The problem is, it's very hard to measure just the gravitational interaction between two objects separated at micron scales. Gravity is incredibly weak compared to common forces like electrostatics and magnetic interactions, and even more exotic things like Casimir forces (related to the van der Waals interaction).

The Purdue team has shown that the measured Casimir force in their experiment acts just as expected, setting a new limit on how screwy gravity can be at these distance scales.

For what it's worth, there are two other big efforts in this area. The one at Stanford is led by Aharon Kapitulnik, and is so sensitive that their apparatus can detect the different forces on Au and Si in the earth's magnetic field due to diamagnetism (!). The one at Washington is reportedly even more sensitive, and there are rumors circulating that they may have seen something exciting.

The really cool thing here is how table-top solid state experiments may have something profound to say about high energy physics, without any big accelerators.

Re:Gravity at small length scales by exp(pi*sqrt(163)) · 2005-06-20 07:32 · Score: 2, Interesting

Well **** me! I was going to call your bluff on the van der Waals force being related to the Casimir force but wisely I did a bit of web searching first and found that they are related and that this has been known since 1955. On the one hand I've studied quantum field theory and read papers on the Casimir force, and on the other hand I've worked with computational chemists who put the vdW force into their models all the time. But I had no clue these things were related. I had merely assumed that the vdW force was simply what you got when you summed together a few electrostatic potentials for the kinds of dipoles you might expect to find in atoms. Is there a good source to read up on this? (Go easy, it's years since I actually did any physical calculations.)
Sometimes science can get a little too compartmentalized. the vdW force is more in the domain of chemists than of physicists so physicists don't get taught it.

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Re:Gravity at small length scales by dpilot · 2005-06-20 08:22 · Score: 1

I was under the impression that under string theory, gravity didn't deviate significantly from classical until you got down near the Planck scale. So if I interpret the article right, they have ruled out some TOEs. But if I've interpreted what I've read about strings correctly, this experiment probably can't rule them out.

Or am I all wet?

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Re:Gravity at small length scales by dr.+loser · 2005-06-20 08:29 · Score: 2, Informative

Try this, particularly the external link to the 1999 hep-th paper.

In short, when you assume "action at a distance" and calculate the instantaneous forces between fluctuating dipoles, you get the van der Waals interaction. When you do full local treatment of the quantum EM fields, including retardation effects, you get the Casimir force.
Re:Gravity at small length scales by thermopile · 2005-06-20 08:41 · Score: 4, Informative

IAAAP (I Am Also A Physicist), and let me (humbly -- your explanation was really good) add some more meat to your description.
Physicists have a really, really hard time explaining *why* gravity is 10^42 times weaker than all other forces. (If you really want to split hairs, it's about 10^38 times weaker than the Weak Force, but what's an order of magnitude among friends?) Gravity appears to be a completely different manifestation than the electromagnetic, weak, and strong forces of nature. This irks many, and they try to rectify that by a Grand Unifying Theory (GUT).

One recent shot at explaining all this was well laid out in this article in Physics Today (subscription required, sorry) from 2002. In short, it theorized that gravity exists in 11 dimensions, not just 3, over short distances. Over some distance, the force known as gravity would "collapse" back down to our traditional 3. The fact that it acted over 11 dimensions, not 3, made gravity drop off as something like 1/r^10. This could help explain the apparent weakness of gravity.

IIRC, the authors predicted that gravity would get measurably stronger at small distances, as it was acting in many dimensions at once. Towards the upper end of their estimates, they predicted that gravity could be measurably stronger at distances around 3-5 millimeters.

As I read this latest discovery, it appears to throw water on that attempt to unify gravity with everything else. Back to the drawing board.

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Re:Gravity at small length scales by lawpoop · 2005-06-20 15:34 · Score: 1

"they predicted that gravity could be measurably stronger at distances around 3-5 millimeters"
Wouldn't gravity changing at 3-5 millimeters mean that people would notice its effects in everyday life?

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Re:Gravity at small length scales by cmsavage · 2005-06-20 15:52 · Score: 1

As far as I can tell, this result rules out extra dimensions larger than about the width of a atom. While string theory allows extra dimensions on this scale, the "natural" scale is still fifteen orders of magnitude smaller. This result doesn't seem to rule out string theory, just an enticing (but unnecessary) possibility- "large" extra dimensions.
Can anyone familiar with this experiment elaborate?
Re:Gravity at small length scales by Anonymous Coward · 2005-06-21 03:39 · Score: 1, Interesting

Nope. The only gravity you notice in everyday life is from the Earth, with the Moon and Sun causing tides but not otherwise making any noticable difference. All these things are huge, and their centres (where the net total of all their gravity comes from) are far away. Gravity from nearby stuff is pretty insignificant - you can go next to 10000 kilograms of lead and you don't end up standing at a weird angle (although a sensitive scientific instrument can measure the gravity from that much mass fairly easily, it was done over 100 years ago IIRC).
Anything less than 5mm away has to be tiny, only a few cubic mm, and therefore with virtually no mass or associated gravity. So a human certainly wouldn't ever notice.

Yes. . .everythings normal BUT. . . by robotkid · 2005-06-20 07:00 · Score: 3, Insightful

The reason why this is interesting is not because "oh look, gravity still works". Most physicists have no doubt that whenever you test it, gravity will still work. In that sense, it is kindof what another poster said, "everything's normal, big deal!"

But what's eating all the theorists is that they have absolutely no idea why. The venerable laws of gravitation are empirical, in the sense that noone knows where it comes from other than the fact that it is associated with mass. All the other forces of nature have a quantum explanation, and have a particle that transmits them (most notably electromagnetism and photons). Noone has been able to satisfactorily reconcile gravity with any fundamental (quantum mechanical) nature of a particle.

It's almost scary that we know more about what binds subatomic particles together than what keeps the moon orbiting the earth. It's also ironic that most people's only introduction to physics is newtonian physics which is presented in textbooks as complete and understood. It's true we have the math to predict the effect of gravity to arbitrary precision, but I'm sure engineers can back me up that just because something has a robust empirical law doesn't mean anyone really understands how it works.

Re:Yes. . .everythings normal BUT. . . by YetAnotherAnonymousC · 2005-06-20 07:12 · Score: 1

It's always seemed to me that we won't understand gravity until we understand mass and inertia. And we won't understand mass an inertia until we understand time.

That's my non-insightful insight for the day...
Re:Yes. . .everythings normal BUT. . . by rylin · 2005-06-20 07:34 · Score: 1

For those of you scoffing at the notion of Time being an unknown, I'd like to point out the following strangeness about Time:
1) Between 8am and 5pm, Time is slower than between 5pm and 8am.
2) Friday, 5m - Monday 8am goes by a lightning speed compared to "Normal Time"
3) One year passes faster than 365.4 days
4) Right now, it's 9:30pm, and I'm wasting precious Time on this useless post.
4a) So I *seriously* hope you are too!
Re:Yes. . .everythings normal BUT. . . by jthayden · 2005-06-20 08:40 · Score: 1

Bravo, the more I learn, the more I am annoyed with the way things were taught to me in school. There are so many things that are taught as a complete and proven body of knowledge as opposed to pointing out the very interesting holes where we don't understand what is going on. Maybe students would be more interested if they understood not everything has been done.
Re:Yes. . .everythings normal BUT. . . by robotkid · 2005-06-20 09:14 · Score: 1

I wholeheartedly agree. I think in gradeschool the emphasis is always on learning facts instead of concepts especially where science is concerned. . . I'm certain we'd have alot more interest in science if we could convey just how much we don't know instead of giving the impression that everything's been worked out and it's all unimportant minutiae you could always look up in the encyclopedia if you were really desperate. . .
I was floored on my first day of intro physics when the professor told us it's just an assumption that intertial mass was the same as gravitational mass. It's been shown to 10 decimal places or so but not formally proven (I think it stems from the same inability to describe gravity that the original post was about). My college profs were always very good about pointing out what we do and don't know. my only beef there is that only the physics majors get the good profs who tell us this stuff. . someone who was taking physics for engineers or physics for biologists probably ended up doing alot of arithmetic about bouncing balls without ever being told the parts that are actually interesting.

I do realize, however, that's is much harder to teach a subject to an auditorium full of people taking the subject for a requirement than a small group of nerds who already have proven commitment given that of their own free will they signed up for masochistic science course 666. I wish schools would try and find professors who are good at teaching non-majors instead of relegating that duty to whichever disgruntled professor is on the department's naughty list. There actually was a chemistry 666 in my school, it was advanced synthetic organic chemistry I think :-)
Re:Yes. . .everythings normal BUT. . . by technoextreme · 2005-06-20 12:40 · Score: 1

was floored on my first day of intro physics when the professor told us it's just an assumption that intertial mass was the same as gravitational mass. It's been shown to 10 decimal places or so but not formally proven (I think it stems from the same inability to describe gravity that the original post was about). My college profs were always very good about pointing out what we do and don't know. my only beef there is that only the physics majors get the good profs who tell us this stuff. . someone who was taking physics for engineers or physics for biologists probably ended up doing alot of arithmetic about bouncing balls without ever being told the parts that are actually interesting.
You want to know why engineers aren't taught about this stuff. They don't care. They don't care. They don't care. It's a practicality issue. They can't muse over the issues that a physicist does. Ten decimal places is more than enough for an engineer not to care. No offense intended. It's just like trying to compare apples to oranges. Every situation is differnt and requires differing amounts of information. Personally, I find physics to be one of the hardest fields and I commend anyone who does become a physicist.
I wish schools would try and find professors who are good at teaching non-majors instead of relegating that duty to whichever disgruntled professor is on the department's naughty list. There actually was a chemistry 666 in my school, it was advanced synthetic organic chemistry I think :-)
Ehhhhhh... I go by the theory of teach only as much as the major needs.

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Re:Yes. . .everythings normal BUT. . . by robotkid · 2005-06-20 18:26 · Score: 1

I certainly agree everyone shouldn't be taught as if they were physicists. And engineers do care about completely different things than physicists, granted. But they way it is now, they don't even speak each other's language.
I guess my problem is that physicists, especially in academia, are encouraged to pursue things with no obvious practical use. Excessive attention to practicality is viewed as pandering for funding. Engineers, on the other hand, are taught the complete opposite. . .practicality is king.

Some of the worst examples of this come from NASA. Engineers trying to design space probes to specs written by scientists who have no idea how to write specs. . the engineers do their job and optimize something that completely erases the scientific value of the data. Of course, add 3 layers of management separating every scientist from an engineer and you've got absolute bedlam. And people wonder why they go over budget.

As for "teaching only as much as a major needs", that's the exact problem right there. How does a junior physics professor who studies general relativity have any idea at all what type of physics a neurobiologist is going to find useful? He doesn't, so they learn how balls bounce instead of how an MRI works. That's a big disconnect. I also never understood why future doctors need to know organic synthesis. But that's what they get taught. And it perpetuates itself, the pigeonholing of academic interests because then you can have a whole field "all to yourself" so you can get tenure. Great.

I once heard Richard Ernst (nobel laureate) speak about how it is a crime when academia sees their only real educational duties as the nurturing of future academics to someday replace them. I couldn't agree more.
Re:Yes. . .everythings normal BUT. . . by technoextreme · 2005-06-21 03:47 · Score: 1

As for "teaching only as much as a major needs", that's the exact problem right there. How does a junior physics professor who studies general relativity have any idea at all what type of physics a neurobiologist is going to find useful? He doesn't, so they learn how balls bounce instead of how an MRI works.
Obviously any school worth your money is going to try and place the right professors for the right job. Look at the factulty that the my school has for biomedical physics. Doctors in addition to regular physics professors at my school. http://www.physics.neu.edu/Department/Vone/Site/Up dated/Biomed/Biomed.htm

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Re:Yes. . .everythings normal BUT. . . by Doc+Ruby · 2005-06-21 09:59 · Score: 1

I was lucky to start school in the 1970s, and continue in schools (mostly public) which kept trying to develop students' ability to find our own facts, compare them, and compare them to our own experience. Teaching us to learn. So I find that I have learned to remember many facts, but that I'm even better at finding new ones, or rediscovering forgotten ones. With the Internet, and lots of other informed people with whom to discuss, the problem is not so much one of memory anymore, but a good BS detector. And understanding how to name things, by which they can be searched.

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Re:Yes. . .everythings normal BUT. . . by rajafarian · 2005-06-21 15:50 · Score: 1

It's almost scary that we know more about what binds subatomic particles together than what keeps the moon orbiting the earth.

If you find that almost scary then do you almost cream your pants considering that we know more about what binds subatomic particles together than the nature of our own mind? I mean we do "know" that there is a certain relationship between physical phenomena and psychical events, but yet we don't know what thoughts are and how it is even possible that the "I" can create them.

I myself have a BS in Physics but I lost interest around the time that I read this story:

A man was walking home late one night when he saw the Mulla Nasrudin searching under a street light on hands and knees for something on the ground. "Mulla, what have you lost?" he asked. "The key to my house," Nasrudin said. "I'll help you look," the man said. Soon, both men were down on their knees, looking for the key. After a number of minutes, the man asked, "Where exactly did you drop it?" Nasrudin waved his arm back toward the darkness. "Over there, in my house." The first man jumped up. "Then why are you looking for it here?" "Because there is more light here than inside my house." ( best version I could find)

My point? Well, I think that if we are seriously, honestly interested in finding a ToE, we should consider Eastern thinking a little and Buddhist philosophy a bunch. Like here: a Google search

In your face Einstein! by Anonymous Coward · 2005-06-20 07:04 · Score: 0

gravity behaves exactly as Isaac Newton predicted,

So much for your so-called "theory of relativity". Newton had it right all along!

Business as usual; gotta keep looking closer by bornyesterday · 2005-06-20 07:10 · Score: 1

In laymans terms: It's been a big topic in physics as to exactly how small a scale you have to go to before the quantum forces take over from gravity. All this experiment showed is that it's smaller than where these folks were looking.

So no big breakthrough, but it is nice that they narrowed down the field of search a bit.

Re:Business as usual; gotta keep looking closer by mattpalmer1086 · 2005-06-21 02:15 · Score: 2, Informative

Nope - this is nothing to do with when quantum effects "take over" from gravity. The reverse, in fact - those forces already swamp the measurement of such a tiny force as gravity - they had to find ways to rule them out to reveal the gravitational influence.

The experiment is looking for evidence that gravity does not follow Newton's law at very small scales. This is predicted by some theories (notably string theory). Confirmation that gravity behaves "normally" up to these atomic scales rules out some theories which require larger extra dimensions. As a side benefit, they managed to measure the casimir force really accurately too.

Accumulation of photons by jd · 2005-06-20 07:17 · Score: 4, Interesting

Actually, that is a really important point and is partially answered by the photoelectric effect predicted by Einstein. (Photons have mass, they strike electrons, this generates current, the photon is not absorbed but now has much lower energy)

Particle/wave duality is not fully explained by thinking of light as a wave of particles, as this conflicts with observations of diffraction gratings at extremely low light intensities. It is my understanding that a "refinement" is to describe light as a single photon that exists with varying probabilities across the wave. (The wave is then a probability wave.)

QM allows objects to exist at multiple points or in multiple states simultaneously, until directly observed. If you do try to directly observe a photon, you do indeed see a single packet of energy. But if you look only at the results, you see a wave.

By looking at light as a probability wave, a lot of apparent paradoxes don't "go away" but do fit a lot better with other known apparent paradoxes, which (to me) indicates the phenomena are related and not distinct.

Getting back to gravity, we could be in for an interesting dilema here. With no variations so far detected, the theory of gravity being an exchange of particles seems less likely. Einstein's model of a distorted space/time would seem to be the more probable, at this point.

This is important, as the predicted QM model for gravity could not be compatible with Enstein's model of gravity. They could not coexist, one had to be wrong. At this point, it seems likely that the particle-exchange model is the one that is wrong, which means QM in its eventual form will likely not be 100% particle based. It may need to be a heterogenius model.

As an aside, let us assume gravity does bend space/time. Since information cannot travel infinitely fast, and as no two events can occur simultaneously, when a massive object moves, space cannot restore itself the moment the object has left. Thus, there must be something analogous to a restoring force within space/time, and therefore some parallel to Hooke's Law.

By implication, an object moving fast enough should leave a trail, where the effect of gravity on space/time is apparent, even though there is no longer any source of that gravity present. A massive-enough object may even leave some sort of "wake", similar to that of a boat, only in gravity rather than in water.

Hooke predicts an upper limit to expansion, though. Something stretched beyond a certain point cannot be restored to its original dimensions, but will rather be restored to some other state, with a much lower restoring force existing.

By implication, a sufficiently massive black hole should result in a region of permanently deformed space/time, as the expansion would exceed the Universe's ability to restore.

As far as I know, no such "massless holes" have been found, but the more the Einstinian model is verified, the more certain I am that such a thing must exist.

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Re:Accumulation of photons by Nos. · 2005-06-20 07:51 · Score: 2, Interesting

Wow, loved that comment. I had never thought of the "wake" effect of a massive object moving (or disappearing) and space/time not being able to completely restore itself afterwards. Even though my understanding of physics is quite limited (high school level plus some reading and disucssions since then), I do tend to grasp most concepts, and the idea that gravity was a force travelling as a particle never quite felt right. However, the idea that a massive object could bend space/time did.

I'm part way through Hawking's "The Universe In A Nutshell". I started reading it on a trip my wife and I took to Hawaii (she grabs popular fiction, I buy Hawking's to read on the beach - I think she's starting to question my sanity). For the most part I really enjoy the book, and its definitely given me something to think about, though I'm starting to disagree with parts of it. It seems that he, and most other physicists are picturing the universe as a closed system. Given some of the things he says in that book, I'm starting to wonder if it is. Of course that just begs the question, If its not, what's outside it?

Anyways, thanks for pointing out some ideas I hadn't come across yet, and it looks like I'm finally going to have to start reading more on Quantum mechanics. I just don't seem to be getting much farther in my understanding of the universe without going down that road.
Re:Accumulation of photons by ACPosterChild · 2005-06-28 09:05 · Score: 1

Hawking currently favors the idea of "brane"s. His analogy is that our universe may be like a bubble in boiling water. Other bubbles/universes are out there in the 'water', whatever that may be. I got to hear him give a presentation a couple years ago and it sounded as if he thought universes may interact in some way (collisions, etc.).

Correction to the above.... by dr.+loser · 2005-06-20 07:32 · Score: 4, Informative

I succeeded in tracking down the actual paper from the Purdue folks. What they've really done is come up with a clever experimental scheme that measures the gravitational interaction independent of the Casimir force - basically it's a background-free measurement. Very slick.

Explaining Gravity by Squiffy · 2005-06-20 07:33 · Score: 2, Interesting

Slightly off-topic:

So they still haven't observed the graviton and are still having trouble explaining why.

What I'd like to know is, why aren't physicists trying harder to explain gravity as a "pseudo-force" like the centrifugal "force" and the Coriolis effect? That's not just a rhetorical question. What makes physicists so sure that the graviton even exists? I trust that there must be some deep reasoning involved -- what is it?

Re:Explaining Gravity by Anonymous Coward · 2005-06-20 08:47 · Score: 1, Informative

Maybe because they move too fast.

The minimum observed limit on the speed of gravity is >= 2*10^10c.

http://www.ldolphin.org/vanFlandern/gravityspeed.h tml

Yes, 20 billion times the speed of light.
Re:Explaining Gravity by jpflip · 2005-06-20 08:48 · Score: 4, Informative

You're right that we've never observed a graviton. However, most physicists would say that this is hardly a surprise. There's no trouble explaining why - any effects of quantum gravity (any behavior where you'd have to know about gravitons and not just about general relativity) probably shouldn't kick in until the Planck energy scale (the energy scale associated with the observed strength of the gravitational force), which is something like 10^16 times greater than any energy ever achieved in an accelerator. Some theorists have come up with ways in which quantum gravity effects become manifest at lower energies (such as the extra-dimension theories the experiments in this post are designed to test), but your naive guess would be that we shouldn't have seen quantum gravity yet.

What you describe (gravity as pseudoforce) is actually something like the way gravity works in general relativity. In that theory, mass warps the fabric of spacetime. Objects travel in the straightest lines they can in this curved space, and we perceive the bends in those paths as being because of a "force" between masses. This theory has been extremely successful in explaining all sorts of large-scale phenomena (not to mention the fact that it is very theoretically beautiful).

The problem is that general relativity and quantum field theory (the theoretical framework of "particles" being exchanged that works so well for the other forces) seem to be fundamentally incompatible. General relativity is fundamentally a theory of the way the geometry of spacetime changes. Field theory is formulated on a pre-existing, static background spacetime. You get into mathematical trouble however you try to get these together.

You can continue in (at least) two ways. Particle physicists are usually more inclined to think that the field theory point of view is fundamental, and that whole geometry thing is just the way things look on large scales. This leads to string theory and the usual discussion of gravitons. If you treat the geometric point of view as more fundamental, you try quantizing spacetime and get loop quantum gravity. String theory is more popular, but no one knows what the right answer is (both may even be different points of view on the same thing!).
Re:Explaining Gravity by Squiffy · 2005-06-20 12:24 · Score: 1

What about this? These people seem to think gravity might be described as a side effect of electromagnetism.

exotic force? by BortQ · 2005-06-20 07:44 · Score: 0, Offtopic

And here I thought the article would be talking about the exotic force of latin women...

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Re:exotic force? by faqmaster · 2005-06-20 08:11 · Score: 1

These are physicists. They have only theoretical experince with women.

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Re:exotic force? by robotkid · 2005-06-20 09:46 · Score: 1

A friend of mine (whom I'm pretty sure only had theoretical knowledge of women) often liked to quip (in a yoda voice)
"social life. . heh. . .women. ..heh. . .a physicist craves not these things.. ."

I think he went on to say econ majors were the dark side of the force but he was usually drowned out by hysteric laughter at this point.

I have to tell you, though, it doesn't help that most of the women in physics I knew were completely asexual. Except for the one diamond in the rough that was an avid video gamer, RPG freak, complete sci-fi dork, and I have no doubt she making some lucky geek very happy right now :-)

What? by HerbieTMac · 2005-06-20 08:04 · Score: 4, Informative

Sorry, but you are misinformed. Gravity does not warp space-time, gravity is the warping of space-time.

So, no, you will not see a "wake" of gravity because you are an observer, you will be affected by the gravity of the object at a point. Since the object itself cannot move faster than the speed of light, the gravity well will always be able to restore faster than the object moves.

You may be thinking of frame-dragging, which is a different phenomenon.

BTW, what moderator decided that this comment was "Interesting"? What I wouldn't give for a "-1, Uninformed" mod.

Re:What? by jd · 2005-06-20 08:46 · Score: 2, Informative
In order for that to be true, you must assume certain other things to be true:
- Space/time can bend without any means of exchanging information with the object (without some exchange of information, space/time would not be affected by any mass within it). Both the exchange of information and the consequence are referred to as gravity, but it would be more correct to describe the latter as a gravitational well or gravitational field, as distinct from gravity itself.
- Two events can occur simultaneously (the motion and the change in gravitational field) - problem is, this is expressly prohibited
- That the change of shape of space/time will occur the instant the information reaches it - problem is, this requires abnormally high densities of "quantum foam", creating a "spray" of Hawking Radiation, as it would be impossible for the particles to pair up correctly. Such a spray does not exist.
- That "quantum foam" has no repulsive forces, which would slow down any restoration of space/time - so far unknown, but not impossible.
- Velocity is relative, so what would you be measuring the velocity relative to? Space itself? But the whole point of relativity is that there is no absolute space to measure against. And even if there were, you're altering its shape, so there is no consistant point of reference to relate to, even if you could!
- All of this assumes, of course, that information is being continuously exchanged between the object and space. This is unlikely, on the face of it, as QM especially does not support the notion of anything being continuous. Thus, there will be some non-zero time interval between the motion of the object and the next exchange of data.
- Without a wake, the predicted gravitational waves for co-orbiting stars would be impossible.
In other words, there are many reasons for assuming some sort of gravitational wake, and some predictions that would seem to make such a hypothesis inevitable. I see nothing in your reasoning that suggests that this is impossible, so you may want to expand on it a little.
--
It's a small world and it smells funny; I'd buy another if it wasn't for the money; Take back what I paid (SoM)
Re:What? by Anonymous Coward · 2005-06-20 09:05 · Score: 0

You forgot to say:

BTW, What I wouldn't give for a "-1, Pwned" moderation
Re:What? by HerbieTMac · 2005-06-20 09:31 · Score: 2, Informative
Sorry but you are confusing things.
- As an observer, you cannot perceive more than one point of gravity (or any other light-speed wave) at a time. In other words, you perceive the effects of gravity as a point source.
- If you want to argue that space and time are quantized, I am afraid that I cannot engage you in a discussion as you will have to assume a radical reworking of mathematical analysis to fit observational data. If you have some reason to assume this quantization, I will listen, otherwise you are engaging in a bit of intellectual masturbation.
- We are not talking about energy differentials as the gravitational energy is the result rather than the cause of space-time distortion. Without energy differentials, no quantum foam, no Hawking Radiation "spray." You need to be extremely careful when positing these sorts of causal structures.
- See above
- Velocity is measured as a fraction of the speed of light, relative to any observer. Since all observers see the speed of light as the same speed in their own reference frame, it is a useful measurement. It remains unproven that anything, including your moving object, can violate this.
- Again, you are arbitrarily quantizing space and time. No matter what the university drop-out on the cover of Wired magazine said, there is no evidence for this. Postulates without evidence mean nothing. Since all observational evidence points to time and space being continuous, your argument has no point.
- No. That is wrong. Binary star systems have no need of "wakes" to produce the observed eccentricities. Just non-instantaneous information exchange.
Since no one has yet been able to rectify a quantized space-time with observation, I would have to say that I am skeptical that this breakthrough will come in a slashdot posting.
Re:What? by Curtman · 2005-06-20 09:57 · Score: 3, Interesting

"BTW, what moderator decided that this comment was "Interesting"?"

What deity bestowed the ultimate truth and power to judge the value of opinions upon you? I found the comment interesting. This is a great forum for discussion of news items, and that is what I come here for.

What I wouldn't give for a "-5, Callous Pedantism" mod.
Re:What? by jd · 2005-06-20 10:18 · Score: 5, Interesting

Let's start with the easy one: Is space/time quantized? To test this, you must point a telescope at a great enough distance that the angle produced by the increment is clearly visible and clearly uniform. Have telescopes seen clearly visible and "unnaturally" uniform regions of space? Uh, yes. That's exactly what is seen in the early Universe, despite the obvious problems this would create (such as no way for structures to form). The solution to this problem is to say that we are seeing a small enough region of quantized space that variation is impossible, that we are not seeing the variation that must exist because we aren't looking at a continuun. Ergo, yes, we have observations that can best be explained by quantized space/time.
Now onto the rest of my post: The physicists at Warwick University, for the Einstein Celebration, considered my theories on relativity to actually be pretty good. :) I had produced a summary of the derivation of relativity and from that derived what overall physical phenomena must underpin the entire theory.
The quantization of space/time is guaranteed. Why? Because matter is quantized, and matter and energy are simply different facets of the same thing, energy must be quantized. (Matter is merely condensed energy, it is therefore the same stuff, just in a different state.)
If energy is quantized, then fields must be quantized, as fields define energy. If fields are quantized, then space and time are quantized, as fields are defined over these.
The scale of quantization is extremely small. A Higg's Particle is the smallest unit of matter definable, but in order to have energy to condense, the scale on which a photon itself exists must be smaller still. There may well be smaller particles in the "quantum foam", which is fine as they don't have to be stable. The Higg's Particle is stable and is likely the smallest object that can be stable.
What does that give us for scale, though? Without knowing even the theoretical mass of the Higg's Particle, that is hard to even guess at, but a guesstimate based on existing data would imply quantization of space at around about 10^-50 m, and something comparable for time.
No, you do not perceive gravity as a point source, because you are not a point. That is why objects in a gravity well will stretch. EACH point of you will experience gravity differently and not from "one source" but rather from the composite value.
(If you are between Earth and the moon, you will experience gravity from each. At the right point, you will be held stationary because the interference will produce no net force. If that were not the case, the Universe would be in serious trouble. As would most of physics, as a lot depends on overlapping fields.)
We are talking about energy differentials. It is a grave mistake of the first order to distinguish between phenomena that are, in fact, the same thing. All objects travel at the speed of light, at different angles to space/time. It is the angle that produces relativistic mass, reliativistic length and relativistic time.
This can be proved by simple trigonometry, and is likely where Einstein got the equations in the first place. Relativity is just a restating of Pythagoras, as all equations are based on the same formula: R' = sqrt(1 - v^2/c^2), where R' is the relativistic version of the variable of interest.
When re-written, this becomes R'^2 + V^2 = C^2, which is basically Pythagoras.
However, the consequence of this is both simple and profound. If all objects indeed travel at the speed of light, at different angles to space/time, and indeed relativity is nothing more than the projection onto the plane of interest, then gravity is a direct consequence of this motion through space/time.
(Relativistic mass, by this logic, is simply a force exerted at 90' to space. The distortion of space is the result of this. This means tha

--
It's a small world and it smells funny; I'd buy another if it wasn't for the money; Take back what I paid (SoM)
Re:What? by HerbieTMac · 2005-06-20 12:55 · Score: 0, Flamebait

Sorry there, little dude. You are obviously more creative than most cranks but you still score pretty high on Baez's 17pt scale. Let's see if we can't point you in the right direction.
Starting with some algebra. If R'=sqrt(1-v^2/c^2), then (R'^2)*c^2+v^2=c^2. You forgot a term. Remember this sequence: simple math FIRST, then general relativity. The other way around just needlessly complicates things.

Second, your formula doesn't even support your statement that all objects travel at the speed of light. This is a nonsensical statement and I suppose it only figures that it needs nonsensical math to back it up, but wow you take it to extremes.

You cannot derive gravity or any other aspect of general relativity using Trigonometry and Newton. Sorry. I wish you could. Grading papers would a cinch, instead of worrying about Levi-Civita connectors, holonomic coordinate bases and Reimannian manifolds.

In all honesty, I am glad you are interested in physics. You should really pursue it. Portland State has some decent courses that I'm certain they would let you audit. Just remember to start with the basics before that GR class; I recommend analysis and topology as a pair of definite prerequisites.
Re:What? by HerbieTMac · 2005-06-20 13:32 · Score: 1

Yeah, you're right man. That last comment was uncalled for. Consider it withdrawn.
Re:What? by Curtman · 2005-06-20 18:36 · Score: 1

I know how hard it is to keep from getting caught up in these discussions sometimes. I do it far too often myself. Thanks for clarifying, Let me know if I step out of line too please. :)
Re:What? by mattpalmer1086 · 2005-06-20 23:31 · Score: 2, Interesting

Hmmm... just finished reading an excellent book about cosmology (The Fabric of the Cosmos, by Brian Greene). In it, the author clearly states that all objects are constantly moving at the speed of light through space-time.

If you are stationary in space, then you are "moving" at the speed of light through time. Any motion through space reduces your velocity through time - but it always adds up to the speed of light.

I must admit, that idea made me stop and re-read that section of the book a couple of times, as I'd never heard relativity expressed that way before. I may have not entirely presented his arguments correctly (I am not a physicist), but that was the essence of it.
Re:What? by AKAImBatman · 2005-06-21 02:13 · Score: 2, Insightful

If you are stationary in space, then you are "moving" at the speed of light through time. Any motion through space reduces your velocity through time - but it always adds up to the speed of light.

That's more or less what relativity says. It's not so much that your velocity always adds up to the speed of light, however, as it is that we are travelling on a four dimensional vector. i.e. Just as a car traveling in a diagonal path has a slower southward velocity than a car travelling at the same speed but heading due south, so is our apparent velocity through time affected as we "turn" more into the three dimensions that we know.

I must admit, that idea made me stop and re-read that section of the book a couple of times, as I'd never heard relativity expressed that way before.

One important thing to remember is that there are two theories: The General Theory of Relativity (i.e. Gravity) and The Special Theory of Relativity (i.e. Space-Time Warps). Most people are referring to the former when they speak of relativity, thus causing no end of confusion. :-)

--
Javascript + Nintendo DSi = DSiCade
Re:What? by zCyl · 2005-06-22 04:33 · Score: 1

Second, your formula doesn't even support your statement that all objects travel at the speed of light. This is a nonsensical statement and I suppose it only figures that it needs nonsensical math to back it up, but wow you take it to extremes.

Actually, this has become a fairly standard and accepted way of formulating SR from a simple geometric viewpoint.

It's okay to criticize people's theories, but if you're going to insult them and call them a "crank", at least make sure you're completely correct first.
Re:What? by duffahtolla · 2005-06-23 02:35 · Score: 1

Yeah, he blew the description by getting all Alex Chiuy on us.

30 second version.

Einstein postulated that the speed of light is the same for all oberservers. With this and the Pythagorean formula you can find the time dialation.

Dude sits in a box that has a device that emits a single photon from the floor towards the top. From his perspective the box is C*Tdude in height. Speed-o-light times the time it took for a single photon to travel from the bottom to the top of the box as mesured by the dudes clock. (consider this to be the verticle part of a right triangle).

Another dude (outside the box) sees this box fly by horizontaly at speed V. He notes the same photon and also notes the horizontal distance traveled by the box during the photons transit from floor to roof. Consider this to be the horizontal (base) of the right triangle. So the length of the base is V*Tobsvr. Where Tobsvr is the time messured by the Observer.

To the outside observer the path taken by the photon is a diagnol. Consider this to be the Hypotenuese (sp?) of our right triangle. Its length is C*Tobsvr. Again Speed-o-light times that time to make the trip as seen by the observer.

Pythagorean Theorm:
(C*Tdude)^2 + (V*Tobsvr)^2 = (C*Tobsvr)^2

Solve for Tdude:
Tdude = sqrt(1-v^2/c^2)*Tobsvr

This factor is (I think) called the gamma factor and pops up everywhere in relativity.

So there you have it. Velocity slows time. And no trig was used.

When v=0 the paths are the same so the times are the same.

When V is approx C then the gamma factor is approx 0. So no matter how much time passes for the observer, the dude experiences approx no time.

I think it was Carl Sagan who said that if the greeks had only known that the speed of light was constant, that maybe we would have had the theory of relativity over two thousand years ago.

Neat thought.
Re:What? by Anonymous Coward · 2005-06-23 04:42 · Score: 0

I thought if you were calling them a crank, you should just be sure that they were mostly wrong.

Cavendish: It takes Big Balls to measure Gravity! by antispam_ben · 2005-06-20 08:15 · Score: 1

http://kossi.physics.hmc.edu/Courses/p23a/Experime nts/Cavendish.html
http://www.fourmilab.com/gravitation/

--
Tag lost or not installed.

Free Link by Anonymous Coward · 2005-06-20 08:22 · Score: 1, Informative

Physical Review Letters is pricey. Here's the free copy off arXiv. Anonymous for no karma whoring.

Photons have mass? by Nasarius · 2005-06-20 08:26 · Score: 4, Informative

Photons have mass

No! Photons have momentum. This does not imply that they have mass.

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Re:Photons have mass? by Moekandu · 2005-06-20 09:20 · Score: 1, Informative

Photons have mass
No! Photons have momentum. This does not imply that they have mass.

Actually, it more than simply implies that they have mass. You cannot have momentum without mass. You can have velocity without mass (in the case of neutrinos, I think), but not momentum. In fact, this is how you calculate momentum:

p = mv

p = momentum
m = mass
v = velocity

--
Mediocrity knows nothing higher than itself; but talent instantly recognizes genius. -- Sir Arthur Conan Doyle
Re:Photons have mass? by Nos. · 2005-06-20 09:23 · Score: 1

Maybe it doesn't imply, but experimentation and observation suggests they do. For example, blackholes create such a powerful gravity well, that light cannot escape. If photons do not have mass, why are they affected by gravity. And its not just blackholes, light can be seen to "bend" around large objects like planets. Unless I'm mistaken, the general belief is that a photon does have mass.
Re:Photons have mass? by qbwiz · 2005-06-20 09:41 · Score: 1

Firstly, neutrinos have mass. Secondly, you're using Newtonian calculations for relativistic speeds, which doesn't always work.

--
Ewige Blumenkraft.
Re:Photons have mass? by Nasarius · 2005-06-20 09:45 · Score: 1

You're about 100 years behind. Newton's equations were amended by Einstein, Planck, and others.
See Ask A Scientist.

--
LOAD "SIG",8,1
Re:Photons have mass? by WaterBreath · 2005-06-20 09:46 · Score: 4, Informative

In fact, this is how you calculate momentum...
Not for photons.

This is how you calculate momentum for photons:

p = h / lambda, where lambda is wavelength.

Alternatively:

p = hf / c, where E is energy, and f is frequency.

More info here:
http://scienceworld.wolfram.com/physics/Photon.htm l

And here:
http://scienceworld.wolfram.com/physics/Energy.htm l

You can "back-calculate" a supposed mass for a photon, once you know its momentum, by using the p = mv equation. But this often called a "fictional" mass, because it is purely relativistic. If you took away a photon's speed, it would have neither mass nor momentum, and would essentially cease to exist. Mass as an fundamental physical quantity exists even in the absence of velocity. This cannot happen with a photon...

Unless you subscribe to the view that photons do not always travel at c in vacuum. But I will not argue that here. Not enough space, and I don't want to be in a flamewar.
Re:Photons have mass? by Admiral+Ackbar+8 · 2005-06-20 09:50 · Score: 1

They are affected because gravity is the warping of space. A black hole is a sign of very strongly curved space, therefore photons are affected by gravity.
Re:Photons have mass? by doppe1 · 2005-06-20 09:55 · Score: 3, Informative

It is not generally accepted that photons have mass, they are generally believed to have no mass.
The bending of light around large objects is not due to the planet excerting a force due to gravity on the photon, but instead the presence of the planet bending the space-time around the planet, then the photon travels in a straight line through this curved space-time.

This means that the photon does not need to have mass to be bent by light.
Re:Photons have mass? by WaterBreath · 2005-06-20 09:56 · Score: 2, Informative

Unless I'm mistaken, the general belief is that a photon does have mass.
No offense intended, but you are mistaken.

If photons do not have mass, why are they affected by gravity?

According to relativity, gravity bends space. It doesn't act directly on other mass. Rather space acts on mass, by telling "how to move", which is along paths called "geodesics". A geodesic is a path demarking the "shape" of spacetime in a region. Light moves along geodesics, which is basically a way of saying that it perceives itself to move straight through local space, though that space may not appear "flat" externally. It's similar to how driving on a straight road on the surface, we do not directly perceive the curvature of the planet. Mass bends space, which causes the local geodesics to be curved relative to distant space that is differently curved. So from here on earth, we perceive the light to bend, but in a local context, the light is travelling in a straight line.
Re:Photons have mass? by WaterBreath · 2005-06-20 09:59 · Score: 1

Er... ignore the "where E is energy" part.
Re:Photons have mass? by Anonymous Coward · 2005-06-20 12:06 · Score: 0

I thought when you took away a photon's speed, its mass increased towards infinity? i.e. light is heavier the slower you can make it go? Isn't that how the two-lasers-in-a-ring time machine works?
Re:Photons have mass? by DavidTC · 2005-06-20 14:19 · Score: 1

Talking about what photons do when you take away their speed is silly, as you cannot do that.
And it would take an infinite amount of energy if you could. So it's not absurd to say it would end up with an infinite mass...that energy would have to end up somewhere. ;)

(No, I'm not talking about outside of a vacuum. That isn't really taking away their speed.)

--
If corporations are people, aren't stockholders guilty of slavery?
Re:Photons have mass? by Khashishi · 2005-06-21 14:16 · Score: 2, Interesting

Photons have no rest mass. Rest mass is the magnitude or length of the energy-momentum pseudotensor. Whether it has mass depends on how you define mass. If you use a semi-classical definition of mass as that which resists motion and creates gravity, then mass is energy, which photons have. But a lot of physicists use mass to mean rest mass, which is 0 for a photon. If you try to decelerate a photon, it keeps moving at c, but it decreases in frequency. If you try to bring it to a rest, you will red-shift it into oblivion.
Re:Photons have mass? by Anonymous Coward · 2005-07-02 12:54 · Score: 0

In fact, this is how you calculate momentum... Not for photons. This is how you calculate momentum for photons: p = h / lambda, where lambda is wavelength. Alternatively: p = hf / c, where E is energy, and f is frequency. More info here: http://scienceworld.wolfram.com/physics/Photon.htm l [wolfram.com] And here: http://scienceworld.wolfram.com/physics/Energy.htm l [wolfram.com] You can "back-calculate" a supposed mass for a photon, once you know its momentum, by using the p = mv equation. But this often called a "fictional" mass, because it is purely relativistic. If you took away a photon's speed, it would have neither mass nor momentum, and would essentially cease to exist. Mass as an fundamental physical quantity exists even in the absence of velocity. This cannot happen with a photon... Unless you subscribe to the view that photons do not always travel at c in vacuum. But I will not argue that here. Not enough space, and I don't want to be in a flamewar. -- Copyright 2005, all rights reserved. Duplication of this post or sig, without express permission, will be prosecuted. Oh, oops, did I duplicate that? Sorry bout that.

I know... by ectotherm · 2005-06-20 08:30 · Score: 0

Midichlorians...

--
"Nature bats last..."

Re:Explaining Gravity Clarified by robotkid · 2005-06-20 09:31 · Score: 1

I think the terminology is throwing you off.

Centrifugal "force" and the Coriolis "force" are not real forces because in both cases you would see it's just inertia and nothing more if you were observing from the proper inertial frame . It just looks like an extra force when we make the assumption that our rotating Earth is an inertial frame or that the merry-go-round we are on is an inertial frame (both cases are rotating which means they are being accellerated and are therefore improper frames). So the short answer is, we understand those "forces" as much as we understand inertia. . the "pseudo" part is just an artifact of its a real pain in the ass to have to deal with the earth's rotation in every little computation we do - so like the lazy bums we are, we just ignore it if we can get away with it.

Everytime gravity stops sucking... by ignatz72 · 2005-06-20 10:26 · Score: 2, Funny

Brian Greene loses a "brane" cell...

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

$\alpha \leq 10^{12}$? Why so large? by menscher · 2005-06-20 13:07 · Score: 2, Insightful

IAAP, and just skimmed the PRL on this. I'm a bit surprised to see they have only found (for $\lambda\approx 200nm$) that $\alpha\leq 10^{12}$. Here it's defined through $V(r)=V_N(r)[1+\alpha e^{-r/\lambda}]$ where V_N(r) is the expected Newtonian gravity.

So, as I see it, they've shown that this "other" interaction is less than a million million times stronger than Newtonian gravity, right? Until $\alpha \approx 1$, I wouldn't say they've "shown that gravity behaves exactly as Isaac Newton predicted". This is interesting, of course, but there's a long way to go. Fortunately they conclude their Letter by saying they expect to be able to get limits on $\alpha$ down to 10^6 with $\lambda\approx 100nm$. We'll be looking forward to it.

As a side note, it'd be really nice if /. learned to render TeX for any non-physicists who might be reading this.

String theory by Pseudonym · 2005-06-20 13:45 · Score: 2, Interesting

Nobody has ever adequately explained to me why string theory is popular. It isn't actually a "theory", since it hasn't made any testable predictions. There is no problem which it has (yet) solved. Its desirable features (e.g. supersymmetry) are not known to be useful in the first place. Even its motivating examples don't seem to fit the theory. (Hadrons look like strings, but no known string models look anything like hadrons.)

As far as I can tell, the argument seems to be that high-energy physicists need to be doing something other than sitting around twiddling their thumbs, and until someone comes up with an alternative, string theory is that something.

--
sub f{($f)=@_;print"$f(q{$f});";}f(q{sub f{($f)=@_;print"$f(q{$f});";}f});

Re:String theory by jpflip · 2005-06-21 02:29 · Score: 2, Interesting

There is definitely a certain amount of what you describe. Particle theorists need something to do, and string theory is the best game in town. It has essentially no testable predictions currently, and it was motivated originally by hadron physics (which the current implementations have nothing to do with).

I think the thing that really got people excited about string theory was the fact that it's a quantum theory of gravity that works at all. That's pretty powerful, since people had been trying crazy tricks for decades to get particle physics and gravity to go together. This time they had come up with a particle physics theory via an entirely different road and later noticed that it happened to solve the gravity problem! This framework also has lots of attractive features. I believe it was the first framework for quantum gravity that was renormalizable (free of the "bad" infinities that screw up quantum gravity), and I think some current implementations are thought to be finite (no infinities at all!). It necessarily contains supersymmetry, a proposed symmetry of nature that helps with many issues of particle physics (i.e. the stability of the mass of the Higgs boson). It also has a very mathematically rich structure - even if it had no relation to the real world, mathematicians would still love to study its intricacies. It hasn't produced any testable real-world predictions, but it has been used for a few useful calculations - in particular, it has been used to account for the details of Hawking radiation and black hole entropy.

I think that string theory is a bit too popular, and that the general public and funding agencies have the idea that it just has to be right. It's very attractive, but still very speculative and far-removed from the real world. There are far too many posts on the slashdot boards, say, to the effect of "Q: What does this recent physics experiment say about string theory? A: Nothing."

Re:$\alpha \leq 10^{12}$? Why so large? by dr.+loser · 2005-06-20 14:33 · Score: 1

The clever bit is that they've come up with an experimental technique that let's them avoid having Casimir forces contribute a background to their measurements of G.

You're right about TeX. I suppose we can wait for /. to start supporting MathML....

Theory of Everything by batquux · 2005-06-21 01:20 · Score: 1

42

Don't photons have energy? by jgoemat · 2005-06-21 20:14 · Score: 1

I thought that mass and energy were interchangeable. E=MC^2? Wouldn't the energy of a photon curve space like a mass would? Granted it would be small...

Re:Don't photons have energy? by luna69 · 2005-06-23 03:32 · Score: 1

Yes. Photons do, in fact, "gravitate" in the sense you're speaking of.

--
No gods, no demons, and no masters. Secular Humanism!
Re:Don't photons have energy? by doppe1 · 2005-06-23 11:18 · Score: 1

No, the equation you have quoted you have misinterpreted.
The mass defined here is the effective mass, the full equation is E^2=(pc)^2 + mc^2. Where m = m_0 {sqrt[1-(v/c)^2]}^(-1), and m_0 is the rest mass, the mass a particle has when it is not moving. So for a particle with mass, at rest, then the equation is E=m_0 c^2. But for a photon, which doesn't have mass, m_0=0, and the second term is zero, all its energy comes from the first term, so E=pc, momentum times the speed of light.

If a photon had mass, and as photons always travel at the speed of light, then the term in {} would be 0, and hence the mass infinite, so the only solution of the equation is for the photon to have zero rest mass.

So therefore, anything which travels at the speed of light, eg photons, cannot have mass, and vice-versa, anything with mass cannot travel at the speed of light, as its effective mass becomes infinite, and hence needs an infinite amount of energy to accelerate it to the speed of light.
Re:Don't photons have energy? by doppe1 · 2005-06-23 11:32 · Score: 1

Oops the equation is meant to read
E^2=(pc)^2 + (mc^2)^2

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