Space Probes Too Slow - Scientists Ask "Why?"

Rudolf writes "Newsweek has an article this week, available here, about NASA calculating that space probes, such as Pioneer 10, 11, and Ulysses, are slowing down more than they should. A team of astronomers and physicists couldn't figure it out, so they published their findings in Physical Review Letters to generate discussion. Several possible causes of the slowing have been discussed, but nothing that completely solves the puzzle. Anyone care to rethink gravity and time?" Update: 09/29 09:00 by H :Thanks to Mark for his link to the original citation.

Re: truman show? or just a game

Haha, maybe when the probes get that far they'll find a package, whose written contents say, "very good, you humans. Now go on to level 2", and suddenly we're back as cavemen on a different planet, perhaps to a gas giant, to make things more difficult.

now where is that secret warp factor? I hope the head boss doesn't manifest himself just yet... We haven't even saved our game!

Re:General Relativity

Contrary to mettw's post, the speed of outer stars in a galaxy does not in any way constitute a discrepency in any theory of gravity.

[This is mostly background to redirect many of the wild posts regarding dark matter attached to this article]

When you measure the velocity at which stars orbit the center of a spiral galaxy (mostly external galaxies like Andromeda, but also in more difficult work on our own galaxy), you see that the stars near the center orbit in a pattern which mimics the pattern of stars we see in that galaxy. That is, if we count the stars in the center and calculate what their mass should be, it matches with the velocity at which the stars orbit (that is, gravity is balanced by centripedal force: called Keplerian rotation). But further out from the center of many galaxies, stars orbit faster than what you would predict by counting all the mass from all the stars you can see. If they are going faster than the gravity must be stronger than what you originally predicted. If all the stars you can see can't make enough gravity, then what? Throw out the theory of gravity which has proved so very successful in the past, or postulate that there must be some matter which you can't see, that is, dark matter.

Most astronomers believe the latter. They think there is not enough eveidence to toss the whole theory. Instead, they assume that galaxies are more complicated that we first thought. What causes this dark matter? Well, it must be something that gives off less light than stars. Some folks have suggested that "ordinary" matter (planets and brown dwarfs) make up the difference. Others suggest "strange" matter (stuff not discovered yet). Finally, the flippant sort of people commonly attribute the extra mass to interstellar Volkswagens (i.e. they don't care what it is just yet; they just want to measure the effect for now).

Why am I saying all of this? I just want people to be a little more informed when the term "dark matter" gets thrown into the discussion.

BTW, the "1/r^2" that the previous poster refers to is the pattern of density of dark matter needed in many galaxies to explain the patter of stellar orbits you see. That is, as you go further out from the center of a galaxy, the density of dark matter decreases by the square of the distance. This is NOT a factor thrown into a gravitational equation. It is a feature added to a density model of a galaxy that helps to explain its rotation curve. It's like needing to account for the mass of the passengers when computing the acceleration of a car. It's NOT a feature of physics.

Article and comments from the web

For those interested, the original articles and articles that cite (and comment) it can be found from the web: gr-qc/9808081 [lanl.gov]

Dust & debris

We don't have precise estimates yet on how many dust and small debris is floating out there. Maybe the impacts of big amounts of small objects slows probes down. (The debris in itself would be moving towards the sun, due to it's gravitation)

Re:'Extra' gravity, dark matter?

>>Like 'ether', dark matter is not real at all.

I don't believe the 'ether' concept is totally false. I do agree that 'dark matter' is probably not real at all though. Quantum theory has come across the ZPE (Zero point energy) concept. If you carry the concept out a little more, then wouldn't SPACE HAVE MASS. Which loosely ties into the 'ether' concept. Relativity is not 100% correct or the unified field theory would have been solved by now. So perhaps light's constant nature is only contant while moving though space and time. Meaning, if light can propogate through a void, it could be interacting with space itself which gives it the speed properties of 'C'.

We are visually driven in our research and I believe that limits us somewhat.

Warped space maybe?

Physics majors are welcome to correct me here (I'm a bit outside of my field), but I seem to remember reading an article a while back discussing the possibility that gravity warps time/space (Scientific American or Popular Science...I don't remember which). The theory held that large gravitational wells, and especially large rotating gravitational wells, literally pull at the fabric of time/space and stretch it out near the center of the affected area. To prove/disprove the theory, the scientists involved were going to put a satellite into orbit and set a high mass object into a rapid spin. If the theory was correct, spacetime distortion would be detectable around the outer edges of the rotating test object.

Could this be what we're seeing, only on a much larger scale? Perhaps the Sun, with its massive gravity well, has caused time/space to stretch within our solar system, and what we're seeing here is the effect of the probes re-entering "normal" space. From our perspective within the "stretched" area, it would appear that the craft was slowing down.

I would love it if someone could provide more info on this theory, and fill me in on whether or not it could possibly apply here.

There is no void

What you mean is a consequence of Heisenberg's uncertainty principle [aip.org] that holds too for the pair energy and time and Einsteins' equivalence of energy and matter [aip.org].

So while over long times the energy at a given empty volume in space is zero, for short times you are not sure. In fact for very short times it is unsure enough to allow the creation of virtual particles, like an electron positron pair, that "borrow" their energy from the vacuum, and annihilate after a short time, giving back the energy.

Zero Point Energy

Nope, that term describes the fact, that the lowest possible energy state for a harmonic oscillator in quantumn mechanics is non zero.

I thought this was solved (links)

I first saw this story in New Scientist a year ago [newscientist.com]. I think it was reported in Slashdot at the time.

A month later New Scientist published this story [newscientist.com], suggesting that the slowing was due to the reaction from heat radiated from the probes RTG power plant.

They still appear to be arguing over whether this effect is big enough. Measurements involving heat are notoriously difficult, as the cold fusion debacle showed.

Probably not dust

I don't think it's dust, or hydrogen, or heat radiation. If you read the original paper, it refers to an "anomalous, constant acceleration (my boldies).
I initially balked at the idea of 'new physics' to explain this, but when you consider how narrow our field of vision is, there cannot but be more than is dream't of in our philosophy.
These probes have travelled farther than pretty much anything else created by man; anything funky with gravity may only begin to manifest itself over billions of klicks.
My own wild and unsupported theory must go unpublished lest the drooling masses call to my door armed with pitchforks and flaming torches.

'Extra' gravity, dark matter?

One of the discrepancies that drives the dark matter theories is that the universe appears to contain much more mass than we can see. Dark matter is just a theoretical substance that has not been proven to exist; many people would say that it's a fake thing made up to explain weird phenomena (like the ether theory 100 years ago).

Now we've got additional symptoms of the universe being more massive than it ought to be. The probes are slowing down faster than expected, as if there was 'dark matter' collected near the sun.

From these *observations* I propose 2 hypotheses---

1. If dark matter is real, it must be concentrated more densely near the sun and less densely farther away. Otherwise the distribution of dark matter would not slow the probes. This makes sense because dark matter, being massive and subject to normal physical laws, would tend to collect near stars and other massive objects. In any case, why heck can't we see it?

2. Like 'ether', dark matter is not real at all. There is an unkown phenomena manifesting itself here. If so, the unknown force(s) could very well be the same ones that caused the observations that led people to propose dark matter in the first place.

How would _you_ solve the Dirac Equation?

We've known this forever

The universe simply doesn't exist
outside the borders of our solar system.

The probes are slowing down because
they are literally hitting the "wall";

Our solar system system was created
by a diety that didn't want to go
through all the fuss that comes with
creating a complete universe.

Breaking through the borders of our
solar system will let us reach the
realm of the gods.

SEND ME ALL YOUR MONEY AND GET
A SEAT ON MY SPACESHIP TO JESUS!

They made- and apologised for- this mistake before

This BBC article yesterday [bbc.co.uk] about the discovery of yet another Kuiper Belt object [asu.edu] by Pioneer [nasa.gov], mentions at the bottom:

Earlier this year, scientists were puzzled by what was described as a mysterious force acting on the probe. It led to speculation that there was something wrong in our understanding of the force of gravity.

Eventually the effect was tracked down to the probe itself, which was unexpectedly pushing itself in one particular direction.

I expect this new theory will also be dispelled by minor impacts, leaking remainders of fuel, and the fact that space isn't a true vaccuum. I'd be delighted to be proved wrong, of course.

--

General Relativity

It should be noted that general relativity
doesn't specify a single equation for gravity,
it specifies a set of conditions that an infinite
number of equations fullfill. Einstein just
chose the simplest equation, which may infact
be the wrong choice.

There are a number of known discrepancies in the
current theory such as the speed of the outer
stars in a galaxy and even the outer planets in
our solar system. It has been shown that by
adding a 1/r^2 factor to the gravitational
equation then both of the problems above are
accounted for.

My personal beleif is that astronomers will
eventually give up on the dark matter theories.

Re:Anti-materia

Problem: Anti-Matter doesn't repulse matter. In fact, there's a very good reason for this (namely being that by Feynman and Wheeler, you can look at anti-matter as just being matter going backwards through time; so it's the same stuff, just going in a different direction). Also, imagine if something that had a repulsive effect on matter actually existed? I can't remember the exact details of the argument, but me and my physics tutor worked out that if such a substance were to exist, you'd end up with it chasing across the universe, eventually gaining near-infinite energy. FROM NOWHERE. Which violates the principle of conservation of energy (and equally, by reversing the argument, would require infinite energy to create such a particle in the first place). [this is much worse than quantum physics, which violates the conservation of energy law and allows particles to "borrow" energy, as long as they give it back before anyone looks hard enough and notices that its gone...]

Besides, gravity is caused by knots in spacetime anyway (or to be precise, it's caused by tension between two ends of a wormhole connecting a particle and an anti-particle; whereas the charge is caused by the relative angle of twist of the mouth of the wormhole with respect to the surrounding spacetime... and the reason it's twisted is because that's the minimum stable configuration for a two-mouthed wormhole to exist without collapsing in on itself).

*ahem* sorry.. I was rambling ;)

Simon

Re:'Extra' gravity, dark matter?

1. If dark matter is real, it must be concentrated more densely near the sun and less densely farther away. Otherwise the distribution of dark matter would not slow the probes.
I don't think so. If the dark matter were concentrated near the sun, we'd automatically include it in our calculations; most likely in Newton's gravitational constant.
General relativity answered the problems Newton's theory had with Mercury's orbit, and does a damn fine job with all predictions gravitational around the sun. If this is the case, dark matter has to exert a force other than gravity. And if that's the case, the Pioneers' acceleration away from the sun should increase as it moves out of the dark matter's influence.
Of course, I could be talking bollocks.