stupid physics questions

[pogo]

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.

Light, which is made up of photons, has no mass. We know this mostly as it goes at the speed of light, and by relativity it would take an infinite amount of energy to accelerate any mass to the speed of light.

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

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.

It's very difficult to understand without the maths. You can describe it in words but the concepts are too abstract for everyday language, so the words are those from maths, or theoretical physics. For example special relativity takes place in a four-dimensional non-Euclidian space: it's only really possible to specify it exactly with maths, from which you can derive special relativity using maths. Once you do that the spoken language makes sense.

[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.

Light, which is made up of photons, has no mass. We know this mostly as it goes at the speed of light, and by relativity it would take an infinite amount of energy to accelerate any mass to the speed of light.

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

From that page:

The photon is currently believed to be strictly massless, but this is an experimental question. If the photon is not a strictly massless particle, it would not move at the exact speed of light in vacuum, c. Its speed would be lower and depend on its frequency. Relativity would be unaffected by this; the so-called speed of light, c, would then not be the actual speed at which light moves, but a constant of nature which is the maximum speed that any object could theoretically attain in space-time.[19] Thus, it would still be the speed of space-time ripples (gravitational waves and gravitons), but it would not be the speed of photons.

Anyhow, moving on.

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.

It's very difficult to understand without the maths. For example special relativity takes place in a four-dimensional non-Euclidian space: it's only really possible to specify it exactly with maths, from which you can derive special relativity using maths. Once you do that the spoken language makes sense.

Four dimensional? I know what you mean, but are there really three dimensions of space, not one unified volume? it might just be that trying to reduce volume to three linear dimensions is an oversimplification. I mean, you've got dimensions x, y and z, but what is their orientation in the real world, their assignation is completely arbitrary.

However, light affects gravity, unless someone comes up with a reason why it doesn't. It seems to me this is a testable hypothesis, which may well have already been tested accidentally when someone took measurements of something or other for entirely other reasons.

[lazerwalker]

Light will affect the gravitational field, however I believe the effect under almost any circumstance will be tiny compared with any masses in the vicinity. The reason is that Einstein's field equations (ie general relativity) depend on the so-called stress-energy tensor which is determined by the matter and energy present, and light has a corresponding amount of energy. You can't use Newtonian equations to argue the opposite - the whole necessity of GR comes from the failure of Newtonian gravity.

[pogo]

Four dimensional? I know what you mean, but are there really three dimensions of space, not one unified volume? it might just be that trying to reduce volume to three linear dimensions is an oversimplification. I mean, you've got dimensions x, y and z, but what is their orientation in the real world, their assignation is completely arbitrary.

However, light affects gravity, unless someone comes up with a reason why it doesn't. It seems to me this is a testable hypothesis, which may well have already been tested accidentally when someone took measurements of something or other for entirely other reasons.

Special relativity says there are four dimensions, i.e. we live in a four-dimensional space, called spacetime. This made of three space and one time dimensions, but the space and time dimensions are not fixed: they depend on which inertial frame we are in, and someone in a different frame (so moving with a different velocity to us) will have a different time axis and different space axis. This leads to time dilation, where clocks run at different speeds for different frames, something which has been confirmed experimentally. Similar considerations of the space axes lead to length contraction, where things moving at relativistic speeds shorten in the direction of travel.

People have tried to measure the mass of a photon. So far they have come up with zero, up the limits of the accuracy of their experiments. Which is why the article I linked to first says "the photon is massless". This also corresponds well with theory: if the mass were non-zero a lot of what people think of as well-established theory would be invalidated. But people still try to measure its mass, just to be sure and just in case (and anyone who actually found the mass of a photon would be famous).

http://math.ucr.edu/home/baez/physics/P ... _mass.html

[Baggypants]

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.

now try and explain Space Giraffe using only pictures. After all, a picture is worth 1000 words.

[zip accident]

Light will affect the gravitational field, however I believe the effect under almost any circumstance will be tiny compared with any masses in the vicinity. The reason is that Einstein's field equations (ie general relativity) depend on the so-called stress-energy tensor which is determined by the matter and energy present, and light has a corresponding amount of energy. You can't use Newtonian equations to argue the opposite - the whole necessity of GR comes from the failure of Newtonian gravity.

Thanks for that affirmation.

now try and explain Space Giraffe using only pictures. After all, a picture is worth 1000 words.

[zip accident]

About waves, gravity waves, tsunami, light and to a lesser extent sound.

Until I saw this no longer available video:

http://www.yakyak.org/viewtopic.php?f=2 ... i#p1501284

I thought tsunami were like a normal sea wave, but bigger. The difference seems to be the frequency is much, much lower, on the order of 0.0001 Hz or something like.

So, in water waves, as the frequency rises, the energy decreases, more or less. With electro magnetic radiation, light for example, the opposite is true, as the frequency rises, the energy increases. To me, thinking about it, this implies that the amplitude of EM radiation is constrained to always be the same in the same medium. That we don't yet know what that amplitude is, implies that it varies between media, though it is barely possible that nobody has bothered to try to work out what it is.

From these musings, I came to the idea that just maybe, we're looking for the wrong gravity waves, the gravitational equivalent of gamma rays, but if gravity waves perform like water waves such as tsunami, it would be the low frequency waves which were energetic, and it's not clear that a small (hundreds of metres long at most) detector could pick up really low frequency waves.

[zip accident]

Unrelated to anything else in this thread except the generous title; is it possible that the number of dimensions in the universe declines with scale? This would allow the twelve or whatever at atomic scales, might allow four geometic dimensions at the size of bacteria, and might allow the extreme distance to become the hologram that some hypothesise is the boundary of the universe, with the implication that at bigger scales than that, it would all fold down to a line, and beyond that to nothing at all. All a bit over cozmic really, I don't know what I was thinking about (erm), but is there any chance the idea might be useful?

[Istar]

The general consensus is that higher dimensions are wrapped-up so tightly we can't see (or detect) them. Ergo, at nanoscopic and smaller scales, they may begin to become evident. There is some thought that quantum tunnelling may be related to a dimensional edge-condition.

[zip accident]

The general consensus is that higher dimensions are wrapped-up so tightly we can't see (or detect) them. Ergo, at nanoscopic and smaller scales, they may begin to become evident. There is some thought that quantum tunnelling may be related to a dimensional edge-condition.

Yeah, I was generalising from that, taking that hologram gravity thing into the mix. It may not be coherent, but I don't see why it's incoherent so far.

[Darkflame]

Not sure that would work though, a dimension is either there or it isnt. You cant have half a dimension, and I cant picture one "just stopping" at some scale.

The idea that they curve in on themselves with different internal size's I guess could work though.

[pogo]

Pretty much all of the laws of physics only make sense with the number of dimensions we have, i.e. four, on all scales we deal with. It can be argued that our 4D spacetime is embedded in a higher dimensional space, such as one with five dimensions, curved in a way that matches the curvature of space that we can measure. But physics still takes place in 4D, down to the subatomic scale.

I'm fairly sceptical of the theories that there are another half-dozen dimensions, but wrapped up so small that we can't detect them. The size of algebraic structures associated with each dimension doubles with each dimension, so by the time you get to ten or eleven dimensions these are so complex that all sorts can be derived from them. I.e. the extra dimensions would seem to be for mathematical convenience, not because they've been observed or justified by observations.

[Dio]

Missed this earlier in the year...

So, in water waves, as the frequency rises, the energy decreases, more or less.

No. The two variables are completely independent in water and sound waves.

What decreases with increasing frequency is amplitude. For a given energy, a low-frequency wave has a larger amplitude than a high-frequency wave. This is what causes the wave breaking effect; the wave slows as it approaches the shore, lowering its frequency and raising its amplitude, because the energy has to remain constant.

A tsunami starts out with a relatively low amplitude at low frequency, but as the pulse is compressed coming onshore the amplitude rises greatly (the energy disperses by a standard inverse-square law from the point of origin).

[Darkflame]

Pretty much all of the laws of physics only make sense with the number of dimensions we have, i.e. four, on all scales we deal with. It can be argued that our 4D spacetime is embedded in a higher dimensional space, such as one with five dimensions, curved in a way that matches the curvature of space that we can measure. But physics still takes place in 4D, down to the subatomic scale.

I think I once read that one possibly explanation for Dark Matter was its just normal matter displaced over an axis we cant see (ie, a 5th dimension, or a 4th non-time dimension, however your counting).
The fundimental forces like gravity wouild work evenly in all dimensions, so even though we couldnt see this matter, on very large scales it would still effect galaxy formation.

It seemed to be a nice idea to me.
Of course, it would raise the possibility of very weird (from our viewpoint) effects in matter displaced this way came near us. But given the vaste and mostly emptyness of space on small scales it would be relatively unlikely.

[zip accident]

Missed this earlier in the year...

So, in water waves, as the frequency rises, the energy decreases, more or less.

No. The two variables are completely independent in water and sound waves.

Hm, that's new to me.

What decreases with increasing frequency is amplitude. For a given energy, a low-frequency wave has a larger amplitude than a high-frequency wave.

This may also be about right. Do higher energy waves travel faster?

This is what causes the wave breaking effect; the wave slows as it approaches the shore, lowering its frequency and raising its amplitude, because the energy has to remain constant.

This isn't quite right, though it may be a terminology thing rather than a misunderstanding of the underlying physics. What happens is that as the speed slows, the frequency (the time betweeen wavecrests) stays the same but the wavelength (the distance between wavecrests) shortens, so the amplitude has to go up, and the wave breaks. The speed is related to the depth of the water, I don't remember whether I ever knew what the exact relationship is.