# Do particles without a rest mass bend spacetime?

• ZirkMan
In summary: Sun.The rotational speed of a star is determined by conservation of angular momentum combined with its moment of inertia. Neither of these is changed signficantly by emission of radiation.

#### ZirkMan

Hi guys,

simple question I have:

Do particles without a rest mass (including EM radiation) cause spacetime to bend? Or only those with a rest mass have gravity?

Yes, all forms of energy contribute to the curvature of spacetime.

Although the role of vacuum energy is still under debate ;)

xepma said:
Yes, all forms of energy contribute to the curvature of spacetime.

Although the role of vacuum energy is still under debate ;)

Ok, so, the higher Energy the higher local spacetime curvature. Therefore gamma rays bend spacetime more than i.e. radio waves, right?

If so, then there is a lot of energy out there radiated in the course of the Universe's history. Is there any estimate of how much energy is out there in form of radiation versus matter? Just wondering if this has been included in the "Dark matter" problem explanations? If a large fraction of Universe's energy is out there in form of invisible radiation and it still has a gravitation effect it could account for a lot of the "too much gravity" problem.

DaleSpam said:
Yes. These solutions are called pp-wave spacetimes:
http://en.wikipedia.org/wiki/Pp-wave_spacetime

Thanks, I see that this also explained my problem from my other post where an object could become a black hole if you passed fast enough close to it :).

Although, I still cannot buy that gravity waves phenomenon. To me existence of gravity waves implies that the energy from a gravity field can vary with a distance (in a wave-like pattern) and therefore not obey the strenght to be measured inversely proportional to the square of the distance rule.

ZirkMan said:
Although, I still cannot buy that gravity waves phenomenon.

The universe does not consider our likes and dislikes.

I should point out that one can apply your exact same objection to electromagnetic waves.

The universe does not consider our likes and dislikes.

I should point out that one can apply your exact same objection to electromagnetic waves.

But unlike electromagnetic waves, the gravity waves have never been observed. And that's where a personal objection can sneak in ;)

That's a different objection. Furthermore, you have to qualify it still further, in that gravity waves have not been directly observed. The rate of energy loss seen in the orbital decay of PSR B1913+16 matches exactly one's expectations from gravitational radiation.

xepma said:
Yes, all forms of energy contribute to the curvature of spacetime.

More precisely, the curvature of spacetime at a point is determined by the stress-energy tensor at that point, whose components depends on both energy and momentum.

There was a time when the universe was dominated by radiation rather than matter. You can pretty much get this from simple scaling arguments: http://www.lightandmatter.com/html_books/genrel/ch04/ch04.html#Section4.2 [Broken] See section 4.2.2, example 5.

Last edited by a moderator:
bcrowell said:
There was a time when the universe was dominated by radiation rather than matter. You can pretty much get this from simple scaling arguments: http://www.lightandmatter.com/html_books/genrel/ch04/ch04.html#Section4.2 [Broken] See section 4.2.2, example 5.
Yes, according to the Big Bang theory that was until the CMBR event. But before that was most of the radiation absorbed and re-emited again. So more specifically I would be interested in a matter/radiation estimate after the CMBR event. I guess there is a lot of "flying mass" since just our Sun according to https://www.physicsforums.com/showthread.php?t=238006 looses 4,200,000,000 kilograms of mass to energy every second!

And this mass is traveling (and bending spacetime) to all directions from every star there is. I would certainly love to see a model of a galaxy that takes this energy distribution into account and see what effect it has on rotational speed of stars in the galaxy.

Last edited by a moderator:
ZirkMan said:
Yes, according to the Big Bang theory that was until the CMBR event.
You're mixing up two different things here: (1) the time when the universe became transparent to radiation, and (2) the time when radiation stopped being the dominant form of mass-energy.

I would certainly love to see a model of a galaxy that takes this energy distribution into account and
http://relativity.livingreviews.org/Articles/lrr-2001-1/ [Broken]

see what effect it has on rotational speed of stars in the galaxy.
Virtually none. The rotational speed of a star is determined by conservation of angular momentum combined with its moment of inertia. Neither of these is changed signficantly by emission of radiation.

Last edited by a moderator:
bcrowell said:
You're mixing up two different things here: (1) the time when the universe became transparent to radiation, and (2) the time when radiation stopped being the dominant form of mass-energy.

I thought both were equal. So when did the (2) happen?

bcrowell said:
Virtually none. The rotational speed of a star is determined by conservation of angular momentum combined with its moment of inertia. Neither of these is changed signficantly by emission of radiation.

Sorry, I meant the orbiting speed of stars around the centre of their galaxy. You know, the current problem is that unlike the planets in our solar system, the stars (or any observable objects like hydrogen clouds) in most of galaxies do not reduce their galaxy orbiting speed with distance from their galaxy centre. Instead it stays more or less the same from core to the outer edges. To explain this requires existence of the gravitational pull of the dark matter. The idea I am exploring is if some of the "dark matter effect" is not due to the gravity of energy in form of radiation coming from the galaxy?

## 1. What are particles without rest mass?

Particles without rest mass, also known as massless particles, are particles that do not have any rest mass or intrinsic mass. They only have energy and momentum, and travel at the speed of light. Examples of massless particles include photons, gluons, and gravitons.

## 2. How do particles without rest mass bend spacetime?

According to Einstein's theory of general relativity, mass and energy are equivalent and can cause spacetime to curve. Since particles without rest mass have energy, they can also contribute to the curvature of spacetime. This means that they can bend spacetime just like particles with mass.

## 3. Can particles without rest mass create gravitational fields?

Yes, particles without rest mass can create gravitational fields. Since they can bend spacetime, they can also create gravitational fields that can affect the motion of other particles, including particles with rest mass.

## 4. How do particles without rest mass interact with matter?

Particles without rest mass interact with matter through the fundamental forces of nature. For example, photons, which are massless particles, interact with matter through the electromagnetic force. Gravitons, which are also massless particles, interact with matter through the gravitational force.

## 5. Why is the study of particles without rest mass important?

The study of particles without rest mass is important because it helps us understand the fundamental forces and interactions in the universe. It also plays a crucial role in the development of theories such as general relativity and the Standard Model of particle physics. Furthermore, massless particles have many practical applications, such as in communication technology (photons in fiber optics) and medical imaging (gamma rays in PET scans).