stupid physics questions

[disciple]

We pretty much covered this on page 4.

A photon, for an extreme example, travels from source to destination in zero time... from its point of view.

To wit you expressed surprise

Fuck!!!

Shit! Yes.

And I notice I then asked the question I was about to ask which is if light travels in zero-time does it consist of particles which don't actually exist (in time)!

I'll go back and reread, thanks all for answering my rubbish question.

[disciple]

Is there an end to the universe then? As in a spacial limit sort of thing..

[gravy]

Go and get that Brian "BRILLIANT" Cox book Why Does E=MC2, it's really accessible and a great read.

[gilesgoat]

... sooo .. the Earth rotates .. the gravitational force times eneregy square .. the speed of light ... ..

... so what time is in UK now ?

[Absolute_Zero]

Is there an end to the universe then? As in a spacial limit sort of thing..

Gawd... this could be infinite, in and of itself. I'll chuck out a few pointers.

If anyone says they know the answer, beyond theo-retical/logical commitment, they don't. Any apparent psuedo-certainty comes straight from the bluffers guide... no matter the credentials of the source. Anyway, it will depend in large part by what you mean by the universe.

For a ruling against a particular kind of infinite universe, Olbers paradox is worth a look. Personally, I question the logic.

Plasma cosmology argues for a much older/larger, possibly infinite, universe. Not wildly popular but at least its chief proponent, Prof. Lerner, looks like a properly mad scientist.

OTOH, on the limited side: "the" universe could be resident within the event horizon of a stupendously mega black hole... or maybe it's the obverse. Try this from Luboš Motl: The analogy between the Universe and a black hole, though I fear you'll return muttering about distant galaxies receding faster than light.

Then there's the Holographic Universe and dozens of other exotic ideas.

I kind of hope there's something to the holographic notion because [unusually for far-out, speculative cosmology] we could know quite soon.

Me? 42, what was the question again? It's probably meaningless.

[Baggypants]

... sooo .. the Earth rotates .. the gravitational force times eneregy square .. the speed of light ... ..

... so what time is in UK now ?

Depends if the clocks are set to British Slaughter Time or Greenwich Murder Time

[Darkflame]

Is there an end to the universe then? As in a spacial limit sort of thing..

Gawd... this could be infinite, in and of itself. I'll chuck out a few pointers.

If anyone says they know the answer, beyond theo-retical/logical commitment, they don't. Any apparent psuedo-certainty comes straight from the bluffers guide... no matter the credentials of the source. Anyway, it will depend in large part by what you mean by the universe.

For a ruling against a particular kind of infinite universe, Olbers paradox is worth a look. Personally, I question the logic.

Plasma cosmology argues for a much older/larger, possibly infinite, universe. Not wildly popular but at least its chief proponent, Prof. Lerner, looks like a properly mad scientist.

OTOH, on the limited side: "the" universe could be resident within the event horizon of a stupendously mega black hole... or maybe it's the obverse. Try this from Luboš Motl: The analogy between the Universe and a black hole, though I fear you'll return muttering about distant galaxies receding faster than light.

Then there's the Holographic Universe and dozens of other exotic ideas.

I kind of hope there's something to the holographic notion because [unusually for far-out, speculative cosmology] we could know quite soon.

Me? 42, what was the question again? It's probably meaningless.

It could be argued perhaps that the universe is a kinda spindle shape...as that's the shape governed by the speed of light going into both the past and future with you at the center.
\/
/\
And so, in theory, that covers all the things that could influence you and all the things you could influence. Outside that shape nothing could ever bare any relevance to you.

[zip accident]

Sorry, can't sleep, maybe this is going to seem very silly tomorrow, but:

Gravity affects light.

Therefore, because for every action there is a reaction, how does light affect gravity?

There was that anomalous acceleration of a spaceship near Earth, it was expected to get an effect from Earth's gravity, it got a bit more (or was it less?), maybe it went through the Earth's shadow and the gravity was different because light wasn't affecting it. Maybe dust clouds in galaxies by stopping light, allow more gravity effect, and that's the explanation for dark energy?

[pogo]

The 'for every action there is a reaction' describes physical actions between physical objects. You push on an object and you feel it pushing back. But light is non-physical, i.e. it has no mass, so can be pushed or pulled without a physical reaction. It's not really a force, the effect of gravity on light. It's often described as the universe being curved, so light goes where it does because of that curvature, induced by gravity. But however you describe it with light having zero mass there's no way it can exert gravitational force on other objects.

[zip accident]

The 'for every action there is a reaction' describes physical actions between physical objects. You push on an object and you feel it pushing back. But light is non-physical, i.e. it has no mass, so can be pushed or pulled without a physical reaction. It's not really a force, the effect of gravity on light. It's often described as the universe being curved, so light goes where it does because of that curvature, induced by gravity. But however you describe it with light having zero mass there's no way it can exert gravitational force on other objects.

I think you mean "material" not "physical", physics is exactly what I'm talking about.

I think it would be expected that light would have an effect on gravity, otherwise it seems to me that there's a symmetry being broken.

I'm not talking about light having mass, though apparently it does, because it goes so fast that it crosses the line between massive and massless.

Having thought further since first posting, I'm talking about light affecting the strength of the gravitational field between objects that the light passes between. I think I'm going for gravity being attenuated by light.

Obviously, it's not a strong effect on the surface of the Earth, or we'd have noticed. Most of the gravitational field between the Earth and objects on it is not interfered with by light, because most of the gravitational attraction of the Earth comes through the material of the Earth itself. When ships are attracted by tall cliffs, does that effect get stronger at night? It's a slight pull at the best of times, so it might well not have been noticed if it does.

I'm not saying that shining a weak torch between two objects will give you anti-gravity, again we'd have noticed by now if that was the case, you can get a huge amount of light between two moderately separated objects, but the effect would be detectable at smaller levels if you looked for it carefully with precise enough instruments (obviously, precise enough is a getout clause if one looks at it that way, but if that aberrant spacecraft was caused by this effect, our current instruments should be good enough).

My first thought was that to break the gravity between two objects you would need to fill the space between them with light, which would mean a hideous amount of energy, but that doesn't exactly make sense, for less massive and more distant objects you would need more light, whereas it would be more like a physical process to require a certain flux of light per mass.

[gravy]

I think it would be expected that light would have an effect on gravity, otherwise it seems to me that there's a symmetry being broken.

Would there? Light travels in a straight line through spacetime. It's just that what is a straightline is spacetime can be curved. It's not like spacetime nudging the light to go where it doesn't want to go in anyway.

[pogo]

No symmetry is broken. If you do the math the force due to gravity between two objects is



But this is zero when one of the objects is light as it has zero mass. So it's more useful to look at acceleration. The acceleration due to one of the masses, e.g. m1, is



So if one object is massive and another is massless, only the massless one is accelerated by the mass of the other: the equations are symmetric but the zero mass of one means it has zero effect on the other.

(equations from here)

[zip accident]

I think it would be expected that light would have an effect on gravity, otherwise it seems to me that there's a symmetry being broken.

Would there? Light travels in a straight line through spacetime. It's just that what is a straightline is spacetime can be curved. It's not like spacetime nudging the light to go where it doesn't want to go in anyway.

So why can't the passage of light through spacetime affect the bending? the passage of a train affects the rails.

No symmetry is broken. If you do the math the force due to gravity between two objects is

I don't do the maths. Sorry, I had a problem at the age of twelve and forgot how to do long division, which I could do up to that point.

But this is zero when one of the objects is light as it has zero mass. So it's more useful to look at acceleration. The acceleration due to one of the masses, e.g. m1, is



So if one object is massive and another is massless, only the massless one is accelerated by the mass of the other: the equations are symmetric but the zero mass of one means it has zero effect on the other.

(equations from here)

However, light has energy, and thus, mass, otherwise lightsails wouldn't be expected to work, lasers wouldn't be potentially harmful, a microwave oven couldn't cook your lunch.

I'm not talking about light affecting either of the two bodies, I'm talking about it affecting the attraction between them due to those masses, interfering if you like.

[pogo]

The fact is that light, which has no mass, does not exert a gravitational force. One way to understand it is geometrically, as gravy suggests. Another way is mathematically, especially as the symmetry is there in the maths, once you allow for the masses.

Photons of light do have momentum, which accounts for their action on lightsails. There is no contradiction as light is travelling at the speed of light. If you took a massive object and accelerated it to the speed of light you would need infinite energy and so it would gain infinite momentum. Only a massless particle can have finite momentum at lightspeed. Again this all makes sense if you do the maths.

[zip accident]

The fact is that light, which has no mass, does not exert a gravitational force.

I am not saying that it does.

The fact that light has mass, suggests it might exert a gravitational force, though given that that mass is very small, the result per photon is presumably also very small.

I am saying that light has an effect on the force exerted by gravity. My first thought was something like "gravity affects light, does light affect gravity?", to which my eventual answer was it must, you can't have an action without a reaction. Put it another way, "the curvature of space affects light, does light affect the curvature of space?" I admit I don't find the curvature of space description of gravity 100% satisfying, but even so, accepting that, could light be expected to affect it? I feel that it could.

Photons of light do have momentum, which accounts for their action on lightsails. There is no contradiction as light is travelling at the speed of light. If you took a massive object and accelerated it to the speed of light you would need infinite energy and so it would gain infinite momentum. Only a massless particle can have finite momentum at lightspeed.

Except that photons only exist at the speed of light. So they always have non-null mass, related to their frequency.

Again this all makes sense if you do the maths.

Take the piss why don't you.

If it only makes sense in maths, not in spoken language, I would suggest that the maths is wrong.