Light and gravity

• neh4pres
No. Gravity curves spacetime. Light, like mass, follows the geodeisc of this curved space. It is not necessary for light to have mass in order to do this.
I don't know why so many people here are allergic to specifying which mass they are talking about, which causes all kinds of disagreements over terms and formulae where there should be none.

Light has no rest mass. It has gravitational mass. In a gravitational field, it has a weight. If you take lights dp/dt in gravitational field, you get a non-zero value. That's a force. That's weight. It's an inertial force, yes, but so is the weight of any other object.

Naty1
OK I think light has mass because it's affected by gravity

Not likely in the sense you mean it...but light does have relativistic mass if that's what you mean. See post # 4 in this thread.

From E = mc2 we know energy and mass are two sides of the same entity...Einstein's work showed that in either form, mass or energy, gravity has an influence. It turns out pressure also has gravitational effects.

Mueiz
Even Newtonian gravity does not require an object to have mass in order to be affected by gravity:

F=GMm/r²
ma=GMm/r²
a=GM/r²

So the acceleration is independent of the mass of the object.

No.. Newtonian gravity requires an object to have mass in order to be affected by gravity
that is clear from the law . if we put m=0 then f=0 so acceleration = o according to Newton's first law.
What is independent of mass is the magnitude of acceleration and not the existence of acceleration
In GR yes .. because acceleration is a result of space-time curvature (caused by M) which is independent of m
This is one of the differences between GR and Newtonian theory

Mentor
if we put m=0 then f=0 so acceleration = o according to Newton's first law.
How much force does it take to produce a finite acceleration of a particle of 0 mass?

Fast77
Even Newtonian gravity does not require an object to have mass in order to be affected by gravity:

F=GMm/r²
ma=GMm/r²
a=GM/r²

So the acceleration is independent of the mass of the object.

Oh Yes. I never accually thought of the acceleration and it's true, but it's still affected from something. I think if a substance is not affected by gravity then it would have no problem passing the light barrier. 1000 years ago people never dreamed of cars or planes, but we can now beak the sound barrier. Why not the light barrier, if we found a substance that is not affected by time, space or gravity.

Thanks for pointing the acceleration.

Masih,13

Fast77
Even Newtonian gravity does not require an object to have mass in order to be affected by gravity:

F=GMm/r²
ma=GMm/r²
a=GM/r²

So the acceleration is independent of the mass of the object.

This thread is two years old.

No. Gravity curves spacetime. Light, like mass, follows the geodeisc of this curved space. It is not necessary for light to have mass in order to do this.

Stay in school. Learn.

Please I thought you were man of logics. Why is it that humans are so stupid, especially adults. They kill, they waste and all they want is power, but kids are smarter and yet because of jealousy and power all adults do to kids it make them have a bad life. By the way I can't imagine you being a 13 year and thinking the way I have and done and what have you discovered all your life with your stupid education. Yes I know education is good but what's more important is the way you think.

Masih,13

Mentor
if we found a substance that is not affected by time, space or gravity.
That's a pretty big "if". But yes, something like that would clearly not behave according to current physical laws.

Thanks for pointing the acceleration.
You are very welcome!

Last edited:
Fast77
That's a pretty big "if". But yes, something like that would clearly not behave according to current physical laws.

You are very welcome!

Non Newtonian Fluids behave differntly then regular fluids. Even antimatter, so there is a possibility that just mabey Dark Matter might be the key to breaking the light barrier or reach that speed. That is what I believe and think. Another soloution might be to make a spaceship from space itself. I know it sounds stupid and insane but our universe is believed to be living in a ball of space.

Masih,13

Mentor
there is a possibility that just mabey Dark Matter might be the key to breaking the light barrier or reach that speed. That is what I believe and think. ...

Masih,13
Look Masih, I hope when my kids are 13 that they will be as excited about physics as you, but you need to be careful. The rule against overly speculative posts is strongly enforced here.

Fast77
Look Masih, I hope when my kids are 13 that they will be as excited about physics as you, but you need to be careful. The rule against overly speculative posts is strongly enforced here.

Yes I understand and I shall take your advice, but all I am doing is including the possiblities that are not being presented. Thanks for the that.

By the way is that a rule in this website?

Masih

Mentor
Yes I understand and I shall take your advice, but all I am doing is including the possiblities that are not being presented. Thanks for the that.
No problem, I hope you will continue to enjoy physics in general and PF in particular. It can be a great resource.

By the way is that a rule in this website?
Yes, there is a link to the rules at the top of each page, rught under the Physics Forums banner. It is under the bold heading "Overly Speculative Posts". I do realize that speculating can be a lot of fun, but that it the purpose of friendly chats and science fiction novels, not this site. There is plenty of enjoyable stuff to discuss that is well understood and not speculative.