Light and Gravity

anantchowdhary

Light is energy.Also light has photons as energy carriers.So,if large amount of light is concentrated on a small area to an extent that the energy has effect of significantly large mass,would it create gravity as energy also warps space-time?

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Hootenanny

Staff Emeritus
Gold Member
Yes, photons do produce curvature in space-time. As you correctly say, energy curves space-time. However, as I have said in previous threads I strongly disagree that photons have 'mass' so much in as I strongly disagree with the use of 'relativistic mass' which is not real. So I would prefer if we do not discuss that photons 'in effect' have mass.

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disregardthat

No, at least not the way I see it. The photon have 0 mass, and there is no number you can muliply it with to get more than 0.
Anyway, the 'evidense' that the photon has zero mass, is because it moves at einsteins constant 'c', I think.

anantchowdhary

Hmm.Mass and energy have the same effects if im not mistaken!

disregardthat

This makes me confused, if energy equals mass, how can zero mass be energy at all :S. the photon have energy all right, but I thought it was in a 'preserved medium' that it didn't have any mass. Guess it was wrong...

anantchowdhary

Sorry.Mass is a form of energy.When mass energy is converted to any other form of energy E=mc^2 can be useful in calculating only the AMOUNT of energy.A photon has effects of mass as it caries energy

anantchowdhary

@Hootenanny

Why do forms of energy differ at all?

Hootenanny

Staff Emeritus
Gold Member
Jarle: What is a preserved medium?

To all: This is why I dislike the use of relativistic mass, it only results in confusion.

Hootenanny

Staff Emeritus
Gold Member
Why do forms of energy differ at all?
They differ in the way they are defined. For example gravitational potential energy is defined as the potential energy per unit mass of an object in a gravitational field. Thermal energy is defined as internal energy of a system.

cristo

Staff Emeritus
I agree with Hootenanny: using the relativistic mass leads to confusion. The relativistic mass is not "real" in the sense that, with the correct choice of inertial frame, the relativisitic mass of a particle will be zero.

However, the mass of a particle is invariant under transformations between inertial frames, and thus this is a "real" quantity.

anantchowdhary

Then why are their effects different in nature?

Hootenanny

Staff Emeritus
Gold Member
Then why are their effects different in nature?
What do you mean by 'their effects'?

Gold Member
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anantchowdhary

heat energy has the heating effect,electrical energy causes motion of electrons.Why isnt it say,the other way round?

cristo

Staff Emeritus
@Hootenanny

Why do forms of energy differ at all?

Anantchowdhary: I recall a few days ago, you asking the same question, to which russ_waters replied and gave you a link to a site describing different forms of energies. Here is the link to that thread: https://www.physicsforums.com/showthread.php?t=150002

anantchowdhary

Yea surely i did as the thread was made by me.But still i havent got any clear answer.Zap was wrong as what he was trying to say was lik saying if u stack up shoes ull get a house.That was not what i meant

ranger

Gold Member
heat energy has the heating effect,electrical energy causes motion of electrons.Why isnt it say,the other way round?
Electrical energy doesnt bring about the motion of electrons, rather its the energy made available by the flow of electric charge in a conductor.

Hootenanny

Staff Emeritus
Gold Member
Then what do you mean, because I for one am pretty confused as to what you're asking if that previous thread didn't answer your question.

cristo

Staff Emeritus
Yea surely i did as the thread was made by me.But still i havent got any clear answer.Zap was wrong as what he was trying to say was lik saying if u stack up shoes ull get a house.That was not what i meant
With all due respect, I suggest you read up on the fundamentals of energy. Then come back with a specific question with respect to a point that you are not clear on, since, at present, you are just waffling!

notknowing

Yes, photons do produce curvature in space-time. As you correctly say, energy curves space-time. However, as I have said in previous threads I strongly disagree that photons have 'mass' so much in as I strongly disagree with the use of 'relativistic mass' which is not real. So I would prefer if we do not discuss that photons 'in effect' have mass.
It seems that a lot of people get quit emotional about relativistic mass :rofl:. The concept of relativistic mass is a DEFINITION which can be looked up in many books, so the concept is real enough (even if some dislike it).

Consider however the following thought experiment. Take a box with perfect mirrors on the inside and weigh the box on a very precise balance (for instance at the surface of the earth). Then, inject a large number of photons into this box (which continue bouncing inside the box since we have assumed perfect mirrors). I'm very sure that the photon-filled box will weigh now more then the empty box. This proves that photons have mass (while the invariant mass is of course zero).

Chris Hillman

Say "kinetic energy", not "relativistic mass"

The concept of relativistic mass is a DEFINITION which can be looked up in many books, so the concept is real enough (even if some dislike it).
This is just a (very misleading) synonym for kinetic energy (in relativistic kinematics).

I'm very sure that the photon-filled box will weigh now more then the empty box. This proves that photons have mass (while the invariant mass is of course zero).
Photons do not have mass. But they carry energy, which gravitates.

Here's an easier conundrum: suppose you have an isolated object in deep space and you someone heat it up. Does it now have a larger gravitational mass?

Littlepig

Here's an easier conundrum: suppose you have an isolated object in deep space and you someone heat it up. Does it now have a larger gravitational mass?[/QUOTE]

larger gravitacional mass don't think so. but who am I? but what happens to its spacetime curvature?? increases????

Hootenanny

Staff Emeritus
Gold Member
Chris Hillman has answered the technical points of your post, so I won't repeat what has already been said. However, I would still like to comment on something
It seems that a lot of people get quit emotional about relativistic mass :rofl:. The concept of relativistic mass is a DEFINITION which can be looked up in many books, so the concept is real enough (even if some dislike it).
True, I do disagree with the notion of relativistic mass (as Einstein*), since the actual definition, is somewhat misleading and as Chris Hillman said, it is much better to consider it kinetic energy in SR. However, another reason I disagree with its use is that it usually only serves to complicate problems (especially conceptual ones), as this thread exemplifies. For example, a common 'definition' of 'relativistic mass' is, as I'm sure you know;

$$m=\gamma m_{0} = \frac{m_{0}}{\sqrt{1-\beta}}$$

Using this definition, how can you claim that a photon has relativistic mass?

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*And before the replies come flying in, no I am not comparing myself to Einstein.

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jtbell

Mentor
I'm very sure that the photon-filled box will weigh now more then the empty box. This proves that photons have mass (while the invariant mass is of course zero).
Only if you require that mass be an additive quantity.