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tommac
2009-Jan-25, 10:50 PM
Could powerful lasers be used to create a gravitational field below a spaceship to emulate earth like gravitation aboard a spacecraft in deep space.

novaderrik
2009-Jan-25, 10:58 PM
probably not, but if they had some Pink Floyd MP3's on board, they could have one hell of a laser light show..

IsaacKuo
2009-Jan-25, 11:01 PM
No. It's impossible.

If you had sufficient power, you could use lasers to power a rocket which accelerates the spaceship by 1 gee. This will cause anyone inside to experience Earth-like acceleration.

tommac
2009-Jan-26, 12:12 AM
No. It's impossible.

If you had sufficient power, you could use lasers to power a rocket which accelerates the spaceship by 1 gee. This will cause anyone inside to experience Earth-like acceleration.

No I am talking about shining and reflecting enough laser power to create a signifigant gravitational field.

In theory you could create a laser powerful enough to create a black hole. Right?

IsaacKuo
2009-Jan-26, 01:03 AM
In theory you could create a black hole, but this will not cause the people inside the ship to feel any gravitational effect. The ship will fall at the same rate as the ship. For the same reason people on the ISS feel like it's zero gee, the people on your ship will feel zero gee.

Of course, creating such a black hole requires unbelievable amounts of power and energy. But that's really besides the point, since creating such a black hole won't create the desired effect anyway.

Siguy
2009-Jan-26, 02:29 AM
Even if it was possible, what would be the point? A rotating toroidal section, or constant acceleration, is much easier. Constant acceleration of around 1g would be needed for quick trips to Mars ever to be a reality. (probably with some sort of high-thrust fission fragment rocket variation)

Ara Pacis
2009-Jan-26, 02:57 AM
You could create 1g for the spaceship if you dragged along earth with the rocketship.

Nick Theodorakis
2009-Jan-26, 04:19 AM
I guess if you had that much power you could just move the whole planet, or even the whole system.

Nick

nauthiz
2009-Jan-26, 05:00 AM
You could create 1g for the spaceship if you dragged along earth with the rocketship.

I suspect that this approach would also be more energy efficient than using lasers to produce a similar amount of mass in photons.

cjameshuff
2009-Jan-26, 05:18 AM
I suspect that this approach would also be more energy efficient than using lasers to produce a similar amount of mass in photons.

Indeed. If you attempted it anyway, your power plant would have a surface gravity of 1g long before the laser's output did.

nauthiz
2009-Jan-26, 06:13 AM
Indeed. If you attempted it anyway, your power plant would have a surface gravity of 1g long before the laser's output did.

Quite a long time, by my reckoning.

Our own sun produces many orders of magnitude less light than would be needed to create that kind of gravity, and it's not even saddled with having to produce all that light within a tiny area.

JustAFriend
2009-Jan-26, 02:21 PM
No I am talking about shining and reflecting enough laser power to create a signifigant gravitational field.

In theory you could create a laser powerful enough to create a black hole. Right?


Lasers don't make gravity.

You could heat up a kettle for a nice cup of tea, though....

nauthiz
2009-Jan-26, 04:35 PM
Lasers don't make gravity.

Photons exert some gravitational pull. I think that's what the OP is getting at - if you can collect together enough photons at once, then you can great a gravitational pull as strong as what we experience on the surface of Earth. If the collection of photons is about 1km away from the ship's crew cabin, for example, then by my calculations you'd need ~1.3*1018 kilograms' worth of light. (About 1/4,500,000 of the mass of Earth.)

I'm fuzzy on exactly how much light the sun emits, but I believe that's at least 100 times as much as it produces in a whole year.

IsaacKuo
2009-Jan-26, 05:06 PM
But the crew would NOT feel 1 gee of gravity, because the rest of the ship's cabin would be accelerating in freefall by the same amount as they do. They'd feel zero gee, just like someone inside the ISS or the "vomit comet". If the room around you is falling at the same rate you are, then you don't "feel" the gravity.

nauthiz
2009-Jan-26, 05:10 PM
But the crew would NOT feel 1 gee of gravity, because the rest of the ship's cabin would be accelerating in freefall by the same amount as they do. They'd feel zero gee, just like someone inside the ISS or the "vomit comet". If the room around you is falling at the same rate you are, then you don't "feel" the gravity.

They would feel something around 1g if the mass were being held at a constant distance from the ship. It's only freefall if you're falling freely.

IsaacKuo
2009-Jan-26, 05:22 PM
No, they wouldn't. They would only feel 1g if the ship's cabin were accelerated by 1g...which can be done regardless of whether there's some massive concentration of energy underneath.

Otherwise, the ship's cabin falls at the same rate as the crew within, and they enjoy zero gee freefall.

Think of it this way. SOMETHING needs to accelerate the ship's cabin in some way different from the ship's crew.

nauthiz
2009-Jan-26, 05:39 PM
No, they wouldn't. They would only feel 1g if the ship's cabin were accelerated by 1g...which can be done regardless of whether there's some massive concentration of energy underneath.

Otherwise, the ship's cabin falls at the same rate as the crew within, and they enjoy zero gee freefall.

Think of it this way. SOMETHING needs to accelerate the ship's cabin in some way different from the ship's crew.

They would feel 1g if the ship's cabin were accelerated by 1g. However, that is not the only way to produce that effect. The force we feel from our planet, for example, is not because its surface is being accelerated outward in all directions at 1g. It's because there's a large concentration of mass below us but something is preventing us from falling toward it.

Or to take an analogy that involves a vehicle as well as a large mass so the analogy is closer to what I believe tommac is proposing: the thing that keeps me against the seat of a car is not that the car is accelerating toward the sky at 9.8m/s2. It's that it is not accelerating toward the center of the earth at 9.8m/s2.

tommac
2009-Jan-26, 05:55 PM
Say for example that you had a level of the ship below where all of the people are. In there you were able to use say ... antimatter to create lasers. these lasers could be reflected around the chamber as to conserve energy.

Say that these lasers were 30 meters below the main deck and attached to the ship.



In theory you could create a black hole, but this will not cause the people inside the ship to feel any gravitational effect. The ship will fall at the same rate as the ship. For the same reason people on the ISS feel like it's zero gee, the people on your ship will feel zero gee.

Of course, creating such a black hole requires unbelievable amounts of power and energy. But that's really besides the point, since creating such a black hole won't create the desired effect anyway.

tommac
2009-Jan-26, 05:56 PM
Even if it was possible, what would be the point? A rotating toroidal section, or constant acceleration, is much easier. Constant acceleration of around 1g would be needed for quick trips to Mars ever to be a reality. (probably with some sort of high-thrust fission fragment rocket variation)

For how long could you continue to accelerate at 1g before using up lots of energy?

Rotation would work but I believe has its own problems.

tommac
2009-Jan-26, 05:58 PM
I guess if you had that much power you could just move the whole planet, or even the whole system.

Nick

I dont think you would need a whole planet of mass to produce the gravitational effect for an area as small as a rocketship. Remember that the earths gravity has to go in all directions including into the earth.

Here we would just need the energy in a small area. So it would be about a concentration of energy.

tommac
2009-Jan-26, 05:59 PM
Indeed. If you attempted it anyway, your power plant would have a surface gravity of 1g long before the laser's output did.

Use antimater as your fuel.

tommac
2009-Jan-26, 06:00 PM
Lasers don't make gravity.

You could heat up a kettle for a nice cup of tea, though....

lasers do make gravity. remember e=mc^2 plug in the amount of energy in a laser and you can see the amount of mass it would take to have an equivalent gravitational force.

nauthiz
2009-Jan-26, 06:03 PM
Say that these lasers were 30 meters below the main deck and attached to the ship.

Then you'd need a bit under a thousandth of what got assuming 1km.

I think the bigger problem here is that, even if we assume the other technical problems can be overcome (an energy source that isn't the size of a star and whatnot), what you end up with is a very complicated device for generating a large amount of mass that has the downside that the mass is always going to be leaking away. It probably would be much easier and way more efficient to just strap a trillion or so metric tons of lead to the bottom of the spaceship.

tommac
2009-Jan-26, 06:03 PM
Photons exert some gravitational pull. I think that's what the OP is getting at - if you can collect together enough photons at once, then you can great a gravitational pull as strong as what we experience on the surface of Earth. If the collection of photons is about 1km away from the ship's crew cabin, for example, then by my calculations you'd need ~1.3*1018 kilograms' worth of light. (About 1/4,500,000 of the mass of Earth.)

I'm fuzzy on exactly how much light the sun emits, but I believe that's at least 100 times as much as it produces in a whole year.

I mean like 30 meters away ... so much less. Also the lasers can be reflected to that they continue to exert gravity until they are absorbed. The point is that if you were able to compact a large amount of energy in a very small area you should be able to create a gravitational field, Since we are very close to the field ( compared to the center of the earth for example ) we dont need as much energy as the mass of the earth.

tommac
2009-Jan-26, 06:05 PM
But the crew would NOT feel 1 gee of gravity, because the rest of the ship's cabin would be accelerating in freefall by the same amount as they do. They'd feel zero gee, just like someone inside the ISS or the "vomit comet". If the room around you is falling at the same rate you are, then you don't "feel" the gravity.

The lasers are attached to the spaceship. maybe they go right through the center of the spaceship for example.

tommac
2009-Jan-26, 06:08 PM
Then you'd need a bit under a thousandth of what got assuming 1km.

I think the bigger problem here is that, even if we assume the other technical problems can be overcome (an energy source that isn't the size of a star and whatnot), what you end up with is a very complicated device for generating a large amount of mass that has the downside that the mass is always going to be leaking away. It probably would be much easier and way more efficient to just strap a trillion or so metric tons of lead to the bottom of the spaceship.

I agree there are some problems ... I am asking if it would be possible or plausible. I am happy with the direction of this thread. I am learning. Some very good points are being made here.

IsaacKuo
2009-Jan-26, 06:10 PM
Say for example that you had a level of the ship below where all of the people are. In there you were able to use say ... antimatter to create lasers. these lasers could be reflected around the chamber as to conserve energy.

Say that these lasers were 30 meters below the main deck and attached to the ship.
The laser light would quickly be converted into heat of the chamber. In any case, the mass of the chamber greatly overwhelms the mass of the laser light. So you really don't need the lasers at all. Instead, you just have a huge chunk of dumb mass in the chamber. Using normal matter, this "chamber" would be several thousand kilometers in radius and have a mass on the order of 6e24 kilograms.

If you had access to more exotic forms of matter, you could reduce the required amount of mass. Good luck "attaching" anything to it, though!

nauthiz
2009-Jan-26, 06:11 PM
I mean like 30 meters away ... so much less. Also the lasers can be reflected to that they continue to exert gravity until they are absorbed. The point is that if you were able to compact a large amount of energy in a very small area you should be able to create a gravitational field, Since we are very close to the field ( compared to the center of the earth for example ) we dont need as much energy as the mass of the earth.

Yeah, I had done 1km just because I assumed we'd be working with a really big spaceship if we're considering a plan like this.

Another engineering problem to consider is, if you're packing that much energy in the form of light into such a small area, you're probably not too far away from creating the conditions that heavy elements are born in. Keeping all that light safely contained just might not be possible. I have to say, I don't think I'd want my squishy little body to be anywhere close to 1km away from something like that, let alone 30m.

cjameshuff
2009-Jan-26, 07:02 PM
Use antimater as your fuel.

The antimatter and the matter you annihilate it with will provide exactly as much gravity as the light produced by their annihilation. You're better off piling up matter to reach a surface gravity of 1g...you're back to hauling a planet along with your spaceship, but you don't need constant refueling with planet-sized quantities of matter and antimatter, and you don't need an unobtainium cavity that'll withstand the photon pressure and heat and be near perfectly reflective. Never mind the radiators to dump the waste heat, the power plants to run the lasers, the lasers themselves, the risk of destroying a good chunk of the solar system if something goes wrong...

nauthiz
2009-Jan-26, 07:08 PM
The antimatter and the matter you annihilate it with will provide exactly as much gravity as the light produced by their annihilation.

I think that's the root of the problem. Assuming everything is 100% efficient, you'd need to carry an amount of fuel equal to the mass of the Earth. But if the thing leaks at all, the amount of fuel you need will weigh much, much more than our planet.

eburacum45
2009-Jan-26, 07:45 PM
Let's try to imagine a gravity generator. It might consist of a plate of neutronium, the stuff that neutron stars are made of. That's fine, except neutronium is probably a fluid, so it would quickly flow into a ball. Hmm; you could try spinning it, so that it becomes an oblate spheroid; spin it enough, and it would resemble a disk.

Now try to imagine the gravity field around such an object...

Ara Pacis
2009-Jan-26, 09:11 PM
Let's try to imagine a gravity generator. It might consist of a plate of neutronium, the stuff that neutron stars are made of. That's fine, except neutronium is probably a fluid, so it would quickly flow into a ball. Hmm; you could try spinning it, so that it becomes an oblate spheroid; spin it enough, and it would resemble a disk.

Now try to imagine the gravity field around such an object...

Wouldn't Neutronium revert back to non-neutronium as soon as it leaves the gravitational field responsible for forcing it into that degenerate state? Or might lasers be used maintain that pressure... but I suspect we'd be back to the bigger-than-the-sun problem.

Now, maybe it would be better to research some method of directly generating gravitons artificially, if gravitons exist.

Or we can go back to rotation, despite all its "problems", as if it's within multiple orders of magnitude of the fantasies being discussed.

eburacum45
2009-Jan-26, 09:12 PM
If we can't use neutronium (which probably needs a mimimum amount of pressure to remain stable) we could try arranging a flat grid of microscopic black holes under the floor. If we have enough of them and somehow stop them from attracting each other into a dance of mutual death, we could end up with a gravity generator, which could produce a flattish disk-shaped region of space with a 1 gee environment.
Actually there would be two such regions, as the disk would have two sides.

This region of artificially produced gravity would be like nothing we are familiar with; the gravity would twist round at the edges of the disk untill it faced the other way, and there would be a very strong decrease in the gravity with distance from the generator- so much so that your feet might be in a noticably stronger gravity field than your head.

I don't like the implications of such an artificial gravity field - the idea of walking round in it makes me feel very uncomfortable. Feet like lead, head much lighter, and terrible twists in the vertical near the edge of the disk.

Additionally, a rocketship with such an artificial field inside would be an absolute nightmare to dock with - don't forget, the gravity field in such a situation would extend well outside the ship in all directions. It's not like Star Trek, where the gravity field is only found inside the ship, like the atmosphere. Ship's hulls are transparent to gravity.

Ara Pacis
2009-Jan-26, 09:41 PM
Isn't the mass of a blackhole relative to its size? Wouldn't you still need a blackhole with a mass equivalent to earth in order to generate 1gee?

cjameshuff
2009-Jan-26, 09:54 PM
Isn't the mass of a blackhole relative to its size? Wouldn't you still need a blackhole with a mass equivalent to earth in order to generate 1gee?

At a distance of 1 Earth radius, yes. At half an Earth radius, you would need 1/4 the mass. Tidal effects would be higher, though. On a 30 m sphere with 1g surface gravity, your head would experience about 0.9 g.

That sphere would mass 1.3e14 kg, about 0.02 billionths the mass of Earth itself, but would require an average density of about 1200 kg/cm^3.

nauthiz
2009-Jan-26, 10:00 PM
Hey, that's only 60 times the density of gold!

cjameshuff
2009-Jan-26, 10:15 PM
Hey, that's only 60 times the density of gold!

kg/cm^3, not g/cm^3.

ravens_cry
2009-Jan-27, 12:41 AM
So. . .60,000 times the density of gold?

WalrusLike
2009-Jan-27, 01:03 AM
Photons exert some gravitational pull....

Um... I am way out of my depth here. But is the above correct? I don't think so.

I know that photons respond to a gravitational well to the extent that it bends spacetime and the path the photons travel is therefore also bent. But I thought that relativistic particles are massless.... so no gravity. Zero restmass. (does that mean non zero while moving? Feeling unsure of my case now.... )

So I am happy to be corrected, but I think the premise may be wrong.

But in any case TANSTAAFL applies. So even if I am wrong cjameshuff has pointed out you might as well use same amount of ordinary mass... same result. And if you do then you have just built the heaviest and slowest spaceship ever conceived. :doh:

nauthiz
2009-Jan-27, 01:33 AM
Zero restmass. (does that mean non zero while moving? Feeling unsure of my case now.... )

Zero rest mass, but they can still have relativistic mass on account of their energy. (Energy and mass are equivalent according to relativity - E=mc2, conversely m=E/c2.) Photons are never at rest, so they always have relativistic mass.

nauthiz
2009-Jan-27, 01:33 AM
kg/cm^3, not g/cm^3.

:doh:

Gandalf223
2009-Jan-27, 01:50 AM
It probably would be much easier and way more efficient to just strap a trillion or so metric tons of lead to the bottom of the spaceship.
I agree there are some problems ... I am asking if it would be possible or plausible. I am happy with the direction of this thread. I am learning. Some very good points are being made here.

By the time you have enough laser power to create the desired gravity, you might already have a trillion metric tons in the laser's power supply...

WalrusLike
2009-Jan-27, 01:54 AM
...(Energy and mass are equivalent according to relativity... ...Photons are never at rest, so they always have relativistic mass.

I suspect you are correct, but to ask a little further...

I think that would imply that gravitons are emitted and absorbed by photons as they travel. Can that be true? Or are we being misled by the term 'mass' in 'relativistic mass'

Does anyone have a link to show that a light beam exerts a gravitational field?

ravens_cry
2009-Jan-27, 02:43 AM
No idea how one would do this, but if Higgs bosons are real, and there was a way of manipulating them so they had a greater or lesser effect, you could not only have artificial gravity, but near light speed starships.

nauthiz
2009-Jan-27, 04:22 AM
I think that would imply that gravitons are emitted and absorbed by photons as they travel. Can that be true? Or are we being misled by the term 'mass' in 'relativistic mass'

I think that's just another example of a way that relativity and quantum mechanics don't fit together very well.

That said, the term "relativistic mass" is a bit misleading; it would be more correct to say that photons have momentum but no mass. They have mass according to the version of mass that's used in E=mc2, but that equation uses a nonstandard definition and the more correct (though less iconic) version of the equation is E2=m2c2+p2c2, where in this case m is the intrinsic mass - 0 for a photon - and p is the momentum.

Does anyone have a link to show that a light beam exerts a gravitational field?
Here:

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

tommac
2009-Jan-27, 04:13 PM
Isn't the mass of a blackhole relative to its size? Wouldn't you still need a blackhole with a mass equivalent to earth in order to generate 1gee?

I think you could get much closer to it. So it would be a stronger effect.

The earths mass generates a massive gravitational field. We dont need such a large area/volume

tommac
2009-Jan-27, 04:16 PM
I suspect you are correct, but to ask a little further...

I think that would imply that gravitons are emitted and absorbed by photons as they travel. Can that be true? Or are we being misled by the term 'mass' in 'relativistic mass'

Does anyone have a link to show that a light beam exerts a gravitational field?


here is one ... maybe not the best:
http://adsabs.harvard.edu/abs/2008GReGr.tmp..120B

earch this board for it.

mugaliens
2009-Jan-27, 09:46 PM
...we could end up with a gravity generator...

Already got one of those.

You're standing on it.

slang
2009-Jan-27, 10:41 PM
I'm confused. How much gravity does a single foton exert? How many of them would you need in one location to create a gravitational field that causes a 1g acceleration aboard the ship? How would you keep all those fotons in one place?

I can't access the article you link to, and don't understand the preview part of it. A search for the filename on the forum yielded no results. How does this article support 'a gravitational field below a spaceship to emulate earth like gravitation'?

nauthiz
2009-Jan-28, 12:37 AM
How much gravity does a single foton exert?
Very, very little. A photon with a wavelength of 400nm (way out on the red edge of the range of human vision) has an energy of about 3 eV. An electron masses about 500,000 eV. So at a given distance, the pull from an electron with no energy aside from its mass would be about 166,666 times as strong as the pull from a photon.


How many of them would you need in one location to create a gravitational field that causes a 1g acceleration aboard the ship?
The number of discrete photons you'd need would vary quite wildly depending on the wavelength of light we're talking about. But the answer will invariably be a lot.

bodycode
2009-Jan-28, 11:30 AM
Gravity is defined as objects being pulled by a curvature of space-time in the direction of the center of mass of that curvature, due to the presence of mass. That's it. No other fact is relevant. Gravitons are nonsense. Gravity is distortion which is percieved as a force. But it's not really a force, it's really only a kind of falling, or, a kind of "bending" within the distortion.

There are two and only two doable ways to emulate gravity that will in fact, prevent people from dying in space from bone loss and muscle wasting.

The first, is a temporary solution, and that has been mentioned here, and that is, acceleration at 1 gee, as mentioned by another smart person here :) But that's only good for long journeys, and, only good if we've advanced sufficiently to sustain massive propulsion.

The second, is a "permanent" solution, is simple and doable now, with our current tech level. That is centripital force, or, being continuously thrown against a bulkhead in a rotating toroid.

bodycode

Starfury
2009-Jan-28, 02:33 PM
In his book The Case for Mars Robert Zubrin describes a nifty way of generating artificial gravity for astronauts traveling to the Red Planet.

He proposes that the crew habitat be joined by a long tether to the spent upper stage booster of the liftoff vehicle, which would serve as the counterweight. According to Zubrin, a 345 meter long tether, with the whole apparatus turning at 1 rpm, would generate Mars-equivalent gravity.

nauthiz
2009-Jan-28, 03:01 PM
Gravity is defined as objects being pulled by a curvature of space-time in the direction of the center of mass of that curvature, due to the presence of mass. That's it. No other fact is relevant. Gravitons are nonsense. Gravity is distortion which is percieved as a force. But it's not really a force, it's really only a kind of falling, or, a kind of "bending" within the distortion.

What causes the falling within the distortion?

cjameshuff
2009-Jan-28, 05:45 PM
Very, very little. A photon with a wavelength of 400nm (way out on the red edge of the range of human vision) has an energy of about 3 eV. An electron masses about 500,000 eV. So at a given distance, the pull from an electron with no energy aside from its mass would be about 166,666 times as strong as the pull from a photon.

The number of discrete photons you'd need would vary quite wildly depending on the wavelength of light we're talking about. But the answer will invariably be a lot.

Counting photons is doing it the hard way. Half the photons of double the energy will have the same gravitational effect. What matters is total energy content of the chamber, and thus the power of the laser and size/reflectivity of the chamber. Say the photons typically bounce for one second before being absorbed and radiated away. A 1 petawatt laser would then fill the chamber with 1 petajoule of energy, equivalent to a bit over 11 g. For the light to be trapped for one second, a chamber 60 m across would have to be reflective enough for the light to bounce on average 5 million times before being absorbed.

The total power output of the sun in that 30-m-radius, 1-second chamber would give 3.846e26 J, equivalent to a more impressive 4.3 million tonnes, with a surface gravity of nearly 1 cm/s^2, a good 0.001 g. Dealing with the resulting 385 billion petawatts of waste heat is left as an exercise for the reader...

mugaliens
2009-Jan-28, 09:22 PM
What causes the falling within the distortion?

Ah... Now that's the question of the millennium, isn't it? Certainly worth enough to spend billions of dollars, searching for an elusive Higgs boson, to help explain how massless elementary particles can cause matter to have mass...


The total power output of the sun in that 30-m-radius, 1-second chamber would give 3.846e26 J, equivalent to a more impressive 4.3 million tonnes, with a surface gravity of nearly 1 cm/s^2, a good 0.001 g. Dealing with the resulting 385 billion petawatts of waste heat is left as an exercise for the reader...

Well, it's at least abundantly clear that the creation of gravity is a darn sight more practicle on the m side of E=mc2 than the E side. But I guess that's the difference between 1 and 89,875,517,873,681,764 m2/s2

nauthiz
2009-Jan-28, 11:02 PM
Ah... Now that's the question of the millennium, isn't it? Certainly worth enough to spend billions of dollars, searching for an elusive Higgs boson, to help explain how massless elementary particles can cause matter to have mass...

True, but I was more wondering from a purely relativistic stanpoint. As in, what is it about the metric tensor that causes objects to move toward nearby concentrations of energy? Is it like for classical gravity where the reason masses tend to accelerate along the gradient of the gravitational field is "just because"?

WalrusLike
2009-Jan-29, 01:00 AM
I must thank you tommac for asking this, and thereby creating a discussion which has informed me of something that I had not heard of before... photons exerting a gravitational attraction. I am not totally convinced yet, but am willing to accept it as fact for the moment.

But as someone above mentioned... it seems to be the hard side of the equation to get a gravitational field from. I dont reckon your original idea is ever going to fly. :cry:

I wish it would... I am still waiting for my anti-grav belt and my flying car. :)

nauthiz
2009-Jan-29, 01:45 AM
I must thank you tommac for asking this, and thereby creating a discussion which has informed me of something that I had not heard of before... photons exerting a gravitational attraction. I am not totally convinced yet, but am willing to accept it as fact for the moment

Maybe go at it another way: We have observed from things like gravitational lensing that massive objects can gravitationally influence light.

In that case:
Newton's 3rd law tells us that if this is so, then the converse must also be true.
Or alternatively, if the light's momentum is going to change then the massive object's momentum must also change in order to avoid violating conservation of momentum.

WalrusLike
2009-Jan-29, 02:14 AM
Maybe go at it another way...

Um... nauthiz you do not need to try and convince me.... My cursory examination of the couple of links that were given do seem to indicate that the photons exert gravitational force. I accept that as 'fact' with the two provisos:
- that I havent seen a huge amount of evidence... (maybe this interpretation is not mainstream but I will assume good faith and take it that it is mainstream)
- and that, as usual, the theory could be wrong or incomplete

So I will assume from now on that it does exert a gravitational effect unless someone shows otherwise. Thanks for letting me know about it because it is a new thing to me... never thought about it before.

Having said that... I am not sure that your latest example shows that momentum is involved... the light bends because spacetime is curved by the gravity well... the path is straight as far as the light is concerned. (I think... I could be wrong because I am out of my depth here...)

mugaliens
2009-Jan-29, 07:26 PM
True, but I was more wondering from a purely relativistic stanpoint. As in, what is it about the metric tensor that causes objects to move toward nearby concentrations of energy? Is it like for classical gravity where the reason masses tend to accelerate along the gradient of the gravitational field is "just because"?

It's due to the mass-equivalence of light by means of it's momentum, frequency, and quantum physics.

tommac
2009-Jan-30, 03:14 AM
Dealing with the resulting 385 billion petawatts of waste heat is left as an exercise for the reader...

I think it is obvious that energy would be used to bring the spaceship to warp speeds.

Ara Pacis
2009-Jan-30, 03:49 AM
I think it is obvious that energy would be used to bring the spaceship to warp speeds.

If you put some of that energy into going forward, you won't need gravitational acceleration because you'd have linear acceleration. Then coast once you get to a significant fraction of light speed and then worry about generating gravity.

Now, if only we could find a way to manipulate gravity, by blocking or enhancing, then we might use it as a star drive.