# Can energy exist without mass?

This is something that I have always been puzzled over.

A current running theory is that when the universe was born, there was so much energy coming out of this one point, that some of it got converted into mass.

However, I don't see how energy, in any form, can exist without mass. Yes, some of it would be converted into mass, which then energy can take effect on, but that implies that the energy that created the mass existed before mass.

So, can energy exist without mass?

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Dale
Mentor
Yes. Every photon is evidence of that.

Yes. Every photon is evidence of that.
Let me rephrase my question: Can energy exist without any particles or physical bodies?

I don't know why I didn't ask it like that the first time, sorry.

russ_watters
Mentor
When the universe was created, it was all energy, no matter. So yes.

Drakkith
Staff Emeritus
Let me rephrase my question: Can energy exist without any particles or physical bodies?

I don't know why I didn't ask it like that the first time, sorry.
Lets take a look at a hydrogen atom. The proton and electron have LESS mass when bound together than a proton and an electron do when they are free in space. Where did that energy go? It was binding energy that must be expended to rip the electron away from the proton. It was carried away most likely as a photon when the electron bound with the proton initially, or turned into kinetic energy of the particle.

Energy is the ability of something to change something else. Energy in and of itself is NOT something. You must have something physical to interact with something else in order for energy to be transferred or used or whatever.

Andrew Mason
Homework Helper
This is something that I have always been puzzled over.

A current running theory is that when the universe was born, there was so much energy coming out of this one point, that some of it got converted into mass.

However, I don't see how energy, in any form, can exist without mass. Yes, some of it would be converted into mass, which then energy can take effect on, but that implies that the energy that created the mass existed before mass.

So, can energy exist without mass?
Energy only has meaning in relation to mass. A photon's existence can be determined only when it affects matter. Whether energy can exist without matter is the kind of question that probably has no answer. But energy without matter would be meaningless and undetectable.

AM

Well if heat death is the fate of the universe then at the last gasp all that should be left is an ever attenuating sea of photons, so such a condition will have happened twice.

Drakkith
Staff Emeritus
Well if heat death is the fate of the universe then at the last gasp all that should be left is an ever attenuating sea of photons, so such a condition will have happened twice.
You sure? I thought everything could simply end up as cold matter at near 0 kelvin. Maybe a background sea of very very low level radiation, also near 0 kelvin.

You sure? I thought everything could simply end up as cold matter at near 0 kelvin. Maybe a background sea of very very low level radiation, also near 0 kelvin.
Well mostly photons and leptons to be precise with some cold matter, by this time most matter would of decayed into its constituent parts even the long lived protons. Its fairly contentious what will happen at the very end, personally I think even a photon sea is not all that far fetched as even time begins to wind down approaching nothing. Depends on your view point I guess, no one really agrees.

Thanks guys, I get it now.

but if e=mc2, wouldn't that mean matter and energy were interchangeable, and if they are interchangeable then technically they are the same?

ZapperZ
Staff Emeritus
That makes no sense. I can interchange my $1 bill into food. By your logic, I could have eaten that$1 bill since "technically they are the same".

Zz.

Drakkith
Staff Emeritus
but if e=mc2, wouldn't that mean matter and energy were interchangeable, and if they are interchangeable then technically they are the same?
Nope. You always find energy with mass and vice versa, but they are not the same thing.

Nope. You always find energy with mass and vice versa, but they are not the same thing.
You always find mass with energy, but NOT vise versa.

From what I understand, mass is a form of extremely compressed (dense) energy. One of the components in the stress energy tensor is the energy density, which is analogous to mass.

Drakkith
Staff Emeritus
You always find mass with energy, but NOT vise versa.

From what I understand, mass is a form of extremely compressed (dense) energy. One of the components in the stress energy tensor is the energy density, which is analogous to mass.
Well, I stand corrected then.

Well, I stand corrected then.
If what you said before stood, a photon would have mass :)

Drakkith
Staff Emeritus
If what you said before stood, a photon would have mass :)
It doesn't have rest mass, no. And while I definately don't understand it all, I did see this on wikipedia and have heard before or read somewhere that light still has a type of mass, just not rest mass.

Energy also has mass according to the principle of mass–energy equivalence. This equivalence is exemplified in a large number of physical processes including pair production, nuclear fusion, and the gravitational bending of light. Pair production and nuclear fusion are processes through which measurable amounts of mass and energy are converted into each other. In the gravitational bending of light, photons of pure energy are shown to exhibit a behavior similar to passive gravitational mass.

It doesn't have rest mass, no. And while I definately don't understand it all, I did see this on wikipedia and have heard before or read somewhere that light still has a type of mass, just not rest mass.
I would argue that the type of mass that the photon has is the similar to stating the amount of energy it contains!

Because the energy density of an object is essentially the mass, I guess it would be viable to say that the photon does in fact have an intrinsic mass, though because the energy is not compacted into particles, the "mass" is practically immeasurable.

I would love for someone who knows about this subject more than I to properly explain this.

Drakkith
Staff Emeritus
I would argue that the type of mass that the photon has is the similar to stating the amount of energy it contains!

Because the energy density of an object is essentially the mass, I guess it would be viable to say that the photon does in fact have an intrinsic mass, though because the energy is not compacted into particles, the "mass" is practically immeasurable.

I would love for someone who knows about this subject more than I to properly explain this.
Sounds like we are talking about momentum!

Perhaps I am wrong... I thought that the attribute of mass is not at all well understood.

Sounds like we are talking about momentum!
Could be! Though it is quite weird to think of something having momentum but no mass, yet light is a constant (no pun intended :P) reminder.

The origin of mass is not well understood, but the effects are.

Dale
Mentor
It doesn't have rest mass, no. And while I definately don't understand it all, I did see this on wikipedia and have heard before or read somewhere that light still has a type of mass, just not rest mass.
In relativity there are two kinds of mass. One is called "invariant mass" or " rest mass". For a photon that mass is 0. The other kind is "relativistic mass" which increases as your speed increases. For a photon that mass is non-0. The concept of relativistic mass is almost never used in modern physics because it is a synonym for total energy, but you will still see it in older papers and textbooks sometimes.

ZapperZ
Staff Emeritus
It is strange that we keep seeing people using "E=mc^2" to justify the argument that photons must have mass (which is wrong), but we don't see people using, say Hooke's Law F=kx to justify that force must have distance. After all, mathematically, these two equations are identical in structure. We also don't hear people arguing that potential difference is equivalent to current based on V=IR. Or better yet, why not say that energy and frequency of oscillation are the "same thing" (E=hf).

At SOME point, people need to look at the physics of the mathematical equation, rather than injecting one's own interpretation. That "m" in the infamous equation is REST MASS. That is how it was defined, and that is how it is defined! Light has this rest mass being zero as far as we know and as far as we have verified. The full relativistic equation has been described in the PF FAQ in the General Physics forum.

Just because something can be converted into something else doesn't mean that the original entity contains that final entity. A neutron can decay into a proton, an electron, and an anti-electron neutrino. Would anyone like to argue that a neutron actually contains ALL those three initially? A photon can be converted into mass (such as in pair production), but do you seriously think that a photon of, say, 1.022 MeV contains an electron and a positron, thus having the a rest mass, with BOTH of them moving together at speed c?

There's a huge amount of inconsistencies here, not to mention, several contradictory observations.

Zz.

Energy is the Behaviour of mass!!:)

Drakkith
Staff Emeritus