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# Random question about light and gravity. watch

1. (Original post by Audrey Hepburn)
It's already been explained to you why gravitational fields affect protons, it's due to their effects warping space-time and not because they effect the actual protons themselves. Also, fyi, [E^2 = (p^2)(c^2) + (m^2)(c^4)] in special relativity, then because photons have no mass this reduces to E=pc - and bear in mind that its momentum is NOT based on its mass but rather its wavelength (I think even at A level you should have studied deBroglie - I'm assuming you're not at degree level because otherwise you would have covered all of this before).

As for the universe expanding stuff, let me just point out that all evidence points to the fact that the rate of expansion of the universe is ACCELERATING. How could this be possible if it was expanding at the speed of light? It couldn't. It is indeed 'space' expanding that defines the expansion of the universe (the space between objects created in it though, I'm not trying to argue that there is an absolute space... though tbh our current system of newtonian mechanics does rely on this).
I think the universe can expand faster than the speed of light (although it's not right now), the limit applies to objects not spacetime itself.

Also, should just point out that you meant photons not protons, but I'm certain that was a typo.
2. (Original post by TableChair)
I think the universe can expand faster than the speed of light (although it's not right now), the limit applies to objects not spacetime itself.

Also, should just point out that you meant photons not protons, but I'm certain that was a typo.
Even if it can, then that means that it is indeed spacetime that is expanding and not light running away that is making it bigger. I was just trying to point out that IF the universe's expansion WAS determined by light running away as the op describes then its expansion couldn't possibly be accelerating.

Oh, and well duh with the protons/photons thing
3. (Original post by Audrey Hepburn)
Even if it can, then that means that it is indeed spacetime that is expanding and not light running away that is making it bigger. I was just trying to point out that IF the universe's expansion WAS determined by light running away as the op describes then its expansion couldn't possibly be accelerating.

Oh, and well duh with the protons/photons thing
Oh right sorry, I missed your point, should have read the post you were quoting
4. (Original post by Audrey Hepburn)
It's already been explained to you why gravitational fields affect protons, it's due to their effects warping space-time and not because they effect the actual protons themselves. Also, fyi, [E^2 = (p^2)(c^2) + (m^2)(c^4)] in special relativity, then because photons have no mass this reduces to E=pc - and bear in mind that its momentum is NOT based on its mass but rather its wavelength (I think even at A level you should have studied deBroglie - I'm assuming you're not at degree level because otherwise you would have covered all of this before).
And i already replied, saying that the warping of space-time affects massive objects (in fact, that's what a gravitational field is, no?, and that a massless floating object would not be curved by a black hole so would not be moving as the photon does. On a basic level, if light has energy, it must have mass surely? I'm not trying to argue that i'm correct, i just haven't been convinced so far.

The sources quoted at me so far say; "Some sources also refer to the "relativistic mass" concept, which is just the energy scaled to units of mass. For a photon with wavelength λ or energy E, this is h/λc or E/c2. This usage for the term "mass" is no longer common in scientific literature" and "Photons are traditionally said to be massless. This is a figure of speech that physicists use ..." - not supportive of photons being independent of gravitational fields.

And lol i am not pretending to know much, and i do not possess a physics degree!

(Original post by Audrey Hepburn)
As for the universe expanding stuff, let me just point out that all evidence points to the fact that the rate of expansion of the universe is ACCELERATING. How could this be possible if it was expanding at the speed of light? It couldn't. It is indeed 'space' expanding that defines the expansion of the universe (the space between objects created in it though, I'm not trying to argue that there is an absolute space... though tbh our current system of newtonian mechanics does rely on this).
Accelerating? Strange and interesting! - source? Do you agree that the bounds of the universe are where the furthest 'object' has reached to date, though?

So your answer would be that a photon shot from a heavy mass would continue at c in a straight line forever?
5. (Original post by nexttime)
And i already replied, saying that the warping of space-time affects massive objects,
No, massive objects warp space-time.

and that a massless floating object would not be curved by a black hole so would not be moving as the photon does.
Yes it would.

On a basic level, if light has energy, it must have mass surely? I'm not trying to argue that i'm correct, i just haven't been convinced so far.
No it mustn't. That energy can be CONVERTED to mass (or vice versa), but something having energy does not mean that it has mass - e.g. an electron and a positron colliding will convert their mass to electromagnetic waves, and the reverse is also possible and has been done experimentally. Think of light as being energy rather than having energy might be an easier way to get your head around it.

The sources quoted at me so far say; "Some sources also refer to the "relativistic mass" concept, which is just the energy scaled to units of mass. For a photon with wavelength λ or energy E, this is h/λc or E/c2. This usage for the term "mass" is no longer common in scientific literature" and "Photons are traditionally said to be massless. This is a figure of speech that physicists use ..." - not supportive of photons being independent of gravitational fields.
We use this as a figure of speech because it is true...

And lol i am not pretending to know much, and i do not possess a physics degree!
Ah, just noticed your sig, BLOODY MEDICS!!! And at Merton too, where the fun goes to die...

Accelerating? Strange and interesting! - source? Do you agree that the bounds of the universe are where the furthest 'object' has reached to date, though?
Red shift in the universe, and any astrophysics that I have studied or heard of since gcse... it's not really strange or interesting, I thought it was common knowledge really...

and bounds-of-the-universe-wise, as a physicist I'm kind of forced right now to go down the Newtonian route and say that there is a substantival space that exists independently of the objects within it, but as a philosopher I am much more of a relativist.

So your answer would be that a photon shot from a heavy mass would continue at c in a straight line forever?
Erm, dunno really, probably depends on how massive the object, is it massive enough to warp space-time, if so then the photon might go on a curved path or something, I'm no astrophysicist though.
6. Argh this just seems to be dragging on but there are so many things you just said that i thought were otherwise...

(Original post by Audrey Hepburn)
No, massive objects warp space-time.
I was under the impression that any mass bends space-time a little, with curvature being proportional to mass, and that the reason gravity acts in the way it does is due to this i.e. planets in orbit are actually moving in a straight line in 4D space.

(Original post by Audrey Hepburn)
Yes it would.
So ANYTHING is affected by gravity? Even a body with no mass will fall to the floor in a vacuum?

(Original post by Audrey Hepburn)
No it mustn't. That energy can be CONVERTED to mass (or vice versa), but something having energy does not mean that it has mass
Now that just goes against EVERYTHING i know - E=mc(2) etc - i thought one of the main reasons photons were devised was to explain the difference in mass between a nucleus and its component parts and observations in nuclear fission - your electron/positron explanation answers nothing - they both have mass, they annihilate, converting mass to EM waves... (again, don't claim to be correct...)

(Original post by Audrey Hepburn)
Ah, just noticed your sig, BLOODY MEDICS!!! And at Merton too, where the fun goes to die...
Oi - maybe be lack physics discussions (evidently) but sometimes we even get to leave our rooms after 9pm...

(Original post by Audrey Hepburn)
Red shift in the universe, and any astrophysics that I have studied or heard of since gcse... it's not really strange or interesting, I thought it was common knowledge really...
I thought that was evidence of an expanding universe, not acceleration.

(Original post by Audrey Hepburn)
Erm, dunno really, probably depends on how massive the object, is it massive enough to warp space-time, if so then the photon might go on a curved path or something, I'm no astrophysicist though.
Does it matter how massive it is? Surely the amount of curvature is continuous, not a discrete quantity.
7. (Original post by nexttime)
Argh this just seems to be dragging on but there are so many things you just said that i thought were otherwise...

I was under the impression that any mass bends space-time a little, with curvature being proportional to mass, and that the reason gravity acts in the way it does is due to this i.e. planets in orbit are actually moving in a straight line in 4D space.
Yes, kind of, (although I was under the impression that the warping of space time is not exactly the same thing as gravity, please someone who is further on in their physics degree feel free to interject) e-m waves (such as light) are affected by that curvature and follow it.

So ANYTHING is affected by gravity? Even a body with no mass will fall to the floor in a vacuum?
I'd rather not call it gravity, but if space-time was curved that way...
also I wouldn't say everything, neutrinos probs wouldn't, but that's because they bounce around through other dimensions too (or so many think...)

Now that just goes against EVERYTHING i know - E=mc(2) etc - i thought one of the main reasons photons were devised was to explain the difference in mass between a nucleus and its component parts and observations in nuclear fission - your electron/positron explanation answers nothing - they both have mass, they annihilate, converting mass to EM waves... (again, don't claim to be correct...)
Yes, photons explain the mass difference because some of the mass is CONVERTED into energy in the form of photons (the photons HAVE NO MASS, THEY HAVE ENERGY, WHICH IS NOT MASS [well, it kind of is, but not in the sense that you're on about at all and I don't want to go into philosophy of physics right now] - I really can't emphasise this enough, it's not a matter of debate, it's not 'just a saying', it's true, it's bloody true!! - It's like me going around saying that we don't have any organs, we're just full of pink fluff and refusing to accept it when you tell me it's not right, please stop stressing me out and accept that PHOTONS HAVE NO MASS)... wow, sorry, got a bit ranty there...

I thought that was evidence of an expanding universe, not acceleration.
red shift is greater for galaxies further away, therefore expanding faster the further it is away, the further away they get the faster they go blablabla acceleration.

Does it matter how massive it is? Surely the amount of curvature is continuous, not a discrete quantity.
As I said, I don't know. It probably will matter how massive it is, at least that's what my instincts tell me, but I'd rather not get into a debate in an area where I don't know the answer and I'm not even sure I'm right.

PLEASE CAN WE STOP THIS NOW! It's making me hate physics even more than I already do - which is a lot!
8. (Original post by Audrey Hepburn)
No it mustn't. That energy can be CONVERTED to mass (or vice versa), but something having energy does not mean that it has mass - e.g. an electron and a positron colliding will convert their mass to electromagnetic waves, and the reverse is also possible and has been done experimentally.
I've always wondered about the sense in which one can say that energy was converted to mass, or mass to energy. I mean, energy (unlike mass) isn't Lorentz invariant, so it surely can't have the same ontological status mass has. And the mass of an object obviously isn't just classically how much matter it has. How can they interchangeable? Wouldn't it be more sensible to treat mass as inertia, and view energy as contributing to the inertia of a system by E = mc^2? (Since you seem to be answering everyone else's questions on this thread-) Is there something to this?
9. (Original post by shamrock92)
I've always wondered about the sense in which one can say that energy was converted to mass, or mass to energy. I mean, energy (unlike mass) isn't Lorentz invariant, so it surely can't have the same ontological status mass has. And the mass of an object obviously isn't just classically how much matter it has. How can they interchangeable? Wouldn't it be more sensible to treat mass as inertia, and view energy as contributing to the inertia of a system by E = mc^2? (Since you seem to be answering everyone else's questions on this thread-) Is there something to this?
Well tbh I've never thought about it before and haven't ever been told why the whole mass-energy conversion thing happens. I think that the best way to look at it is that mass is a form of energy. Also, rest mass is lorentz invariant but 'mass' in general isn't (when talking about particles with mass that is) as the lorentz equations for energy relates it to momentum. I don't really see how treating mass as inertia would help at all as that means you're now talking about motion for some reason... this confuses me... I hate physics
10. (Original post by Audrey Hepburn)
Well tbh I've never thought about it before and haven't ever been told why the whole mass-energy conversion thing happens. I think that the best way to look at it is that mass is a form of energy. Also, rest mass is lorentz invariant but 'mass' in general isn't (when talking about particles with mass that is) as the lorentz equations for energy relates it to momentum.
"Mass" in general (i.e. relativistic mass), though, is a bit of a non-concept, isn't it - I mean, it's pretty much invented just to placate the equations - and wouldn't have any influence on the proper concept of mass.

(Original post by Audrey Hepburn)
I don't really see how treating mass as inertia would help at all as that means you're now talking about motion for some reason... this confuses me... I hate physics
Well, mass can't be thought of as "the amount of matter in an object", clearly... I guess inertia is the next best thing. (It also seems to cohere with this "Higgs Field" stuff...)
11. (Original post by shamrock92)
"Mass" in general (i.e. relativistic mass), though, is a bit of a non-concept, isn't it - I mean, it's pretty much invented just to placate the equations - and wouldn't have any influence on the proper concept of mass.
No, I don't think so, I'm pretty sure it does influence the proper concept of mass... I mean, physics would suck quite badly if its equations didn't correspond to actual things.

Well, mass can't be thought of as "the amount of matter in an object", clearly... I guess inertia is the next best thing. (It also seems to cohere with this "Higgs Field" stuff...)
Why? and why? It just seems completely ridiculous to me.
12. (Original post by Audrey Hepburn)
No, I don't think so, I'm pretty sure it does influence the proper concept of mass... I mean, physics would suck quite badly if its equations didn't correspond to actual things.
Hmm. One's invariant and the other isn't... but they're the same?

(Original post by Audrey Hepburn)
Why? and why? It just seems completely ridiculous to me.
If mass were just the amount of matter in an object, then you'd have thought that the mass of a body composed of two smaller bodies would just be the masses of the two smaller bodies added together. But it isn't.
13. (Original post by shamrock92)
Hmm. One's invariant and the other isn't... but they're the same?
Well one's the thing when it's not moving, and the other is it when it is moving.

If mass were just the amount of matter in an object, then you'd have thought that the mass of a body composed of two smaller bodies would just be the masses of the two smaller bodies added together. But it isn't.
This can be explained using energy-mass conversion though.
14. (Original post by Audrey Hepburn)
Well one's the thing when it's not moving, and the other is it when it is moving.

This can be explained using energy-mass conversion though.
I don't mean Planck's thing about how binding energy decreases the mass... I mean the kinetic energies of the particles making up a body affect its mass (so mass isn't additive, which suggests that mass isn't a measure of matter, as measure would be additive).

Spoiler:
Show
This proof is quite cool... In the frame at rest relative to the body as a whole, gamma=1, so the mass of the body as a whole

.

Then for a constituent of the large body with energy

and then you can expand that to give

.

If we ignore the higher terms (if we included them that would just make my point even better), then, substituting this expression for the energies of the individual components, the mass of the object as a whole

i.e. the mass of an object is greater than the mere sum of its constituents' masses.
15. (Original post by shamrock92)
I don't mean Planck's thing about how binding energy decreases the mass... I mean the kinetic energies of the particles making up a body affect its mass (so mass isn't additive, which suggests that mass isn't a measure of matter, as measure would be additive).

Spoiler:
Show
This proof is quite cool... In the frame at rest relative to the body as a whole, gamma=1, so the mass of the body as a whole

.

Then for a constituent of the large body with energy

and then you can expand that to give

.

If we ignore the higher terms (if we included them that would just make my point even better), then, substituting this expression for the energies of the individual components, the mass of the object as a whole

i.e. the mass of an object is greater than the mere sum of its constituents' masses.
Still don't get why this can't be explained with mass-energy conversion...

I probably can't help you with any problem you have with whatever it is, I don't quite understand why you have it other than to just make life difficult for yourself.
16. (Original post by shamrock92)
I don't mean Planck's thing about how binding energy decreases the mass... I mean the kinetic energies of the particles making up a body affect its mass (so mass isn't additive, which suggests that mass isn't a measure of matter, as measure would be additive).
As an interested non-physicist i always thought mass was made of energy, that as the universe expanded reducing the energy-density the energy condensed into particles, so mass and matter essentially is energy, as Audrey said:

(Original post by Audrey Hepburn)
I think that the best way to look at it is that mass is a form of energy.
However, she seems to be arguing against this being true, insisting some distinction is necessary. I am confused .
17. (Original post by ashy)
But it can have momentum. Try to sort that one out in your head
Light had many different forms, it acts like a wave but also acts like a particle. It may also have more forms
18. (Original post by imhiya)
Light had many different forms, it acts like a wave but also acts like a particle. It may also have more forms
It has both wave like and particle like properties, yes... What other properties would you like it to have?
19. (Original post by ashy)
It has both wave like and particle like properties, yes... What other properties would you like it to have?
Cake?
20. (Original post by Pulkpull)
Cake?
That would be friggin awesome, actually. Just think... interference creates more and more CAKES.

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