Dark energy is thought (by those who think it exists at all) to operate over larger distances than normal gravity (whatever the heck that is…we still don’t have that figured out.) Just as the effect of the electromagnetic force isn’t significant at subatomic scales in comparison to the strong and weak nuclear forces, and gravity is only weakly operative at the scales at which the electromagnetic forces dominate, “dark energy” would be only very weakly operative at normal planetary scales, but can be seen to operate on larger cosmological regions.
We have zero actual evidence, in terms of direct observation, for “dark energy”. It is a convenient (but incomplete) theory which satisfies the need for the cosmological constant, a correction that Einstein fudged in and later quite famously regretted.
As for the OP (and as others have already indicated) energy can have positive or negative density, giving the effect of gravity or anti-gravity, respectively. Matter, which can be considered a subset of energy, is always positive (gravitationally attractive) so far as we know, though a negative energy density “exotic matter” has been proposed.
The book Voyage To The Great Attractor provides a pretty good overview of what we know (or rather, don’t know and are wantonly speculating) about dark matter, dark energy, and the relation thereof to the apparent accelerating expansion of the universe.
Ooops, sorry for misinterpreting your question. I still don’t understand your point, tho. Normal gravity is caused by mass, e.g. the star. Dark energy is spread out through all space, not attached to the star. So I don’t see why there should be a distance where the two cancel. But maybe I’m still misunderstanding?
At any rate, Stranger’s answer is probably the correct one: the dark energy is spread out so thinly that you only notice its effects on the scale of galaxies (or galaxy clusters, even), not individual stars.
Now, you might ask whether two galaxies could be at the perfect distance so that the force of attraction between them exactly balances the expansion of the universe. (Notice that we can ask this question regardless of dark energy’s existence.) I’m not sure what the answer is, but galaxies usually have enough “peculiar velocity” (velocity with repect to the average of all galaxies) that such a condition is unlikely to occur. Even if there is such an equilibrium point, it may not be a stable equilibrium.
Huh? This makes no sense to me (Note: I’m no expert in Theoretical High Energy Physics, which Mr. Teo claims to be). How can a radius be three times a mass? What kind of weird math do you have to do to get those units to work?
You just have to work in units where G=c=1. The formula in more usual units is
r = (2GM)/c^2. (Mr. Teo seems to have gotten the factor of 3 wrong.)
It’s certainly true that a photon warps space differently than a particle with rest mass of the same energy. However, if you had a box with perfectly reflective walls and filled it with photons, it would weigh more than the same box empty. Photons aren’t anti-gravity.
OK, thanks for fighting some ignorance. That definitely seems to work our since G is in units of m[sup]3[/sup]/kg * s[sup]2[/sup]. Thanks again. I thought either I or Mr. Teo were going nuts.
If one gram of mass was converted into anti-mass at Hiroshima, that anti-gram is now spread out over 60 light years. That is pretty thin.
I understand that this is the currently accepted answer. However, I am not certain it is considered undisputed fact, anymore than the mass of a neutrino is certain.
Please review the cosmic timeline.
When the balance of mass-energy is on the mass side, the universe expansion decreases, when the balance is on the energy side the expansion increases. Energy having anti-gravity properties seems at least as reasonable as any of the other incomplete theories proposed. It does not call on any unknown forces, just an unexpected one.
Please suspend your disbelief for a few minutes. If energy does indeed contain an infinitesimal amount of anti-gravity, how would the Universe be different than it is? How would you even measure the anti-mass of a photon?
Yes, and we don’t notice it’s effects now. I don’t think I understand your comment.
Well, I have to agree that no one has ever done the experiment I described, so you might find that the box weighs less with photons in it. But it would go against what we know about how light energy and gravity behave. Actually, the early universe is the one time when the electromagnetic mass-energy was big enough to affect things - see below.
The cosmic timeline is a little misleading. After inflation ends, and before the matter-dominated period, there is a radiation-dominated period, when the energy density in radiation is greater than the matter energy density. If your proposal were right, the universe would be expanding exponentially during this whole period. I can’t say for sure, but I’m guessing that would wreak havoc with successful Big Bang predictions, like the microwave background and the light element abundances, and with galaxy formation.
But, to be fair, I can’t think of any actual experiment that completely rules it out.
You’d measure the mass (positive or negative…lets not confuse repulsive gravity with antimatter) by how it affects and is affected by other mass. If photons were of negative (relativistic) mass, the would act in a repulsive way toward real mass, i.e. instead of the gravitational lensing effect seen when a light source passes behind a massive object we’d see light being dispersed.
I’m not certain where you get the idea of energy “contain[ing]…anti-gravity”, but all energy and matter that we can directly observe acts attractively toward mass. Why this is, whether gravity is composed of several positive and negative components (Kaluza-Klein formulation and its derivative forms) that almost cancel each other out, and whether dark energy actually exists or is just a placeholder for an even more bizarre explaination for the accelerated expansion of the universe are questions yet to be answered.
I submit we do notice its effects in the expansion of the universe. We just attribute those effects to incomplete theories – dark energy and/or vacuum energy.
You are referring to density. I am referring to total mass vs. total anti-mass. A kilogram of feathers has the same mass as a kilogram of lead. The density is quite different. This illustration from your cite shows the expansion as angles and straight lines. I would suggest that the lines of expansion are, even now, following a curved path.
In other words, even astrophysicists are at a loss for an explanation.
Consider the following analogy: Mass is like an atmospheric vortex, energy is as a bird. The bird does not possess enough force to counter the vortex. Its path curves toward the vortex, even as it attempts to resist. The anti-gravity of a photon is nowhere near as strong as your interpretation. How many photons are produced from a gram of mass?
Cite please (for the energy attracting mass). Maybe anti-gravity explains why even laser light eventually spreads; why an electron that absorbs a photon orbits in a higher shell.
Does that mean that my explanation is not bizarre enough? IANA Patent Clerk, but if all matter (known and dark) shares the property of gravity, why cannot all energy (known and dark) share the property of anti-gravity? I understand this may be a new and untested idea. It may not fit “classical” understanding, but still at least to me, it seems to fit the observations.
How could this be tested? Does a cosmic ray bend around a gravitational lens to the same degree as an infrared light ray?
Peace
rwj
P.S. You guys aren’t very good at suspending your disbelief.
But it’s true, I’m NOT very good at suspending my disbelief about things I KNOW to be true. Suppose someone came to you and said, “I’ve found a mixture of different kinds of metals that falls upward.” I don’t know about you, but my reaction would be “that’s impossible!” Even if I saw the thing float upwards, I’d suspect a trick, rather than suspecting the laws of physics. The reason is, we know an awful lot about how gravity works, and how it works on all substances the same regardless of how the bits are put together. For this claim to be true, all of that would have to be wrong.
The problem with your suggestion is similiar. If it were true, a lot of other things we know to be true about photons would go haywire. Stranger’s point about the bending of light is right on target. You see, if you’re going to come up with a theory of dark energy (0r anything else), it not only has to explain the dark energy, but it has to be consistent with everything else we already know.
Yes, that’s true. And so people are looking at some pretty crazy ideas: quintessence, branes, etc. The problem with your idea is that it’s not crazy enough!
Your analogy is confusing and misleading. If your bird is this anti-gravitational energy, any interaction with mass would cause it to bend away from, not into your “vortex”. This is contrary to our observations (see below). Reasoning from analogy is a poor method of logical analysis; analogy is best suited to explaination, relating a new concept to one already well-understood by the student, but the appropriateness of the analogy depends on the assumptions one makes about the relevence of the description.
The “mass” of a photon is dependent upon the energy of the photon, which itself is determined quanitatively by the wavelength of the photon, E=lambda*h (frequency times Planck’s Constant), and mass is, of course, m=E/c[sup]2[/sup], from Einstein’s famous relation. A photon’s rest mass is purely theoretical, as a photon is never at rest, but it does have an equivalent and very real momentum based upon that mass. Photons do not typically come “from” mass (except in rare cases, like the collision between a particle and its antiparticle), but rather from energy release of an electron falling to a lower energy state, or the release of binding energies between nuclear components during a decay or fusion reaction.
Here’s an article that explains about the gravitational lensing effect caused by light deflected by a large mass. Here is a site that has a simple explaination of the effect of gravity on light as predicted by General Relativity and confirmed by decades of careful measurements.
The divergence angle on a laser is due to diffraction at the edges of the aperature and interactions with other photons in the beam. This has absolutely nothing to to with anti-gravity; it’s a result of the wave formulation of photons. Here is more than you ever wanted to know about basic theory of lasers.
Atoms accept electrons or absorb photons in higher (non-ground state) shells because the resulting electron has too much energy to fit in a lower state, or because all lower states are occupied. Typically, a photon is then re-radiated in order to permit the atom to achieve a more stable state. Again, this has nothing whatsoever to do with antigravity.
Your question here doesn’t even make sense. Anti-gravity is a distortion (curvature) of space that is positive (repulsive) rather than negative. They aren’t two seperate properties. As for fitting observations, we have not observed any matter on the local level operating in any way that could be considered gravitationally repulsive. We infer that there is some kind of long-range (but very weak) gravitational repulsion which is causing the accelerated expansion of the universe, but our ability clearly observe distant areas of the universe is shrouded by time and distance; today’s observations are tomorrow’s mistaken interpretation.
Suspending disbelief is a useful quality when one is reading about elves and talking animals, or spaceships flitting across unimaginable distances through hyperspace to explore strange, new worlds. In considering theories claims about the natural world, however, one’s hypthesies must fit and predict natural phenomena. If you want to propose an alternative to general relativity, or claim the existance of repulsive gravity then your arguments must describe observed behaviors, and in such a way as to provide an equal or better description than existing theory. Your’s do not, and while I don’t mean to give offense, I do think you need a more solid grounding in conventional physical science and the observations and predictions thereof before shooting out hyptheses that are nonsensical.
Again, I ask for a cite. Dark energy-gravity behaves as negative-positive pressure.
Pressure gradients do not behave as you suggest. The infinitesimally small repulsive pressure of a quanta of dark (any?) energy cannot resist, much less repel, the unimaginably larger attractive pressure of a gravitational lens.
Around any great mass (planet; star; black hole; galaxy cluster) you will find a cyclone of matter spiraling into the vortex that is gravity.
Would you believe the interaction is more like helium filled balloon caught in an atmospheric vortex?
And I thought you were poor at suspending your disbelief.
Your responses indicate to me that you have little concept how small a quanta of dark energy is, how pressures behave, or how surprising the Universe can turn out to be. They also indicate you have little or no understanding of what I do or do not know of physics.
I was hoping for an intelligent discussion. I would suggest you get a more solid grounding in current observations before responding further.
However, if you wish to remain civil, I will be glad to compare understandings.
