# Gravity warps space and time

1. Apr 16, 2005

### LizardKing23

this is just a thought ive been toying around in my head for the past 6 minutes after reading one of the posts.

gravity warps space and time. commonly recognized fact. but, this feature of gravity is exclusive to gravity. the strong and weak forces, and electromagnetism do not display this characteristic. in a unified theory, wouldnt these forces, at some point, have to share this characteristic? and if this is true, wouldnt a unified theory be impossible UNLESS gravity was thought of as a completely separate entity? is gravity something more than just the fourth force? i am not suggesting deistic influence or advance civilizations messing with us, im just wondering if we're thinking about gravity all the wrong way.

food for thought.
enjoy.

2. Apr 16, 2005

### misskitty

Yummy...I love food for the brain.

There must be some other place in the universe that exhibits the gravity thing your talking about. It's interesting that strong, weak, and EM do not display this. There must be some reason we haven't found or discussed yet as to why this is. I think you might be on to something with the impossible unified theory IF it were true and gravity were a separet entity all together. However, what would gravity be a separet entity from? This poses a new query to throw into the mix. As well as how would this affect the current calculations we use to learn about the universe? I would think some of the theories concerning gravity may have to be adapted to compensate for gravity being a separted entity.

3. Apr 17, 2005

### quasar987

The relativity gurus of this forum will tell you it is not gravity that bends space, it is not even matter, it is the energy stress-tensor (or something like that)... in other words, it is energy.

But it is well known that a photon curves space-time as well, for it carries an energy hf... But a photon is but an electromagnetic wave. Hence the energy contained in the oscillating electric and magnetic fields curves space-time. So the feature of curving space is not exclusive to gravity (or MATTER).

4. Apr 17, 2005

### inquire4more

Well, ok, but a photon does possess some mass. I agree that a stress tensor gives the measure of the, well, the stress, on a region of space-time due to other, surrounding regions, affecting curvature, but it still seems to me that it is matter that warps space-time. If one of the relativity gurus is around, I'd be interested to know their thoughts. How does the stress tensor differ from this relativistic stress-energy tensor?

Last edited: Apr 17, 2005
5. Apr 17, 2005

### quasar987

Photons have no mass.

As a demonstration, consider the following two relations derived directly from the Lorentz transformations:

$$\vec{p}c^2 = \vec{v}E$$

$$E^2 - (pc)^2 = (mc^2)^2$$

v = c in the first one gives pc = E. This last result in the second equation give

$$0 = (mc^2)^2 \Leftrightarrow m = 0$$

6. Apr 17, 2005

### AntonVrba

yes that is food for thought, gravity might be something completely deifferent than what we thought up till now- but then many new theories need to be developed - a long slow progress.

7. Apr 17, 2005

### inquire4more

Hmmm...I cannot argue with the math. I'm going to have to read a little on this and give it some thought, and you have, most definitely, given me something to think about, quasar.

8. Apr 17, 2005

### inquire4more

$$\vec{p}c^2 = \vec{v}E$$

Here then, $$\vec{p}$$ represents momentum? Or did I miss by a mile here? If so, does this differ in some way from momentum as it is treated classically?

9. Apr 17, 2005

### quasar987

I was hoping you wouldn't ask. :tongue2:

The relativistic momentum is defined as

$$\vec{p} = \frac{m\vec{v}}{\sqrt{1-(v/c)^2}}$$

But for a photon, this relationship explodes because v = c makes the denominator 0.

However, it had been discovered by Einstein a few months prior to the publication of his first paper on special relativity that the energy of a photon is directly proportionnal to its frequency: $E = h\nu$. So this, together with $pc^2 = vE$, allows us to give a special definition of momentum to photons:

$$p = \frac{h\nu}{c}$$

10. Apr 18, 2005

### inquire4more

Sorry to be a bother about this quasar, but do you know where I could go to find out exactly how Einstein arrived at $E = h\nu$? I'm not disputing the validity, but it might help me to understand the why, if only I can see the how.

11. Apr 18, 2005

### Crosson

In General Relativity, gravity is not a force. That is, it does not even remotely resemble a force in highly relativistic situations.

12. Apr 18, 2005

### quasar987

I don't know where to go about finding this but I can tell you that he figured that out in trying to explain the photoelectric effect. The photoelectric effect is the observation that when a photon collides with an elektron, the photon is "absorbed" and the elektron's kinetic energy is augmented by an amount exactly equal to.....you guessed it, $h\nu$.

Last edited: Apr 18, 2005
13. Apr 19, 2005

### Sempiternity

"Force is an illusion, a by-product of warped space-time." - Michio Kaku

14. Apr 19, 2005

### misskitty

Quasar, how does a photon curve space-time ? I don't know much about them because I have just started to study them.

15. Apr 19, 2005

### misskitty

Being a high school student who does not yet posess the required mathematical background to comprehend what you just typed....what does this mean?

16. Apr 19, 2005

### misskitty

Not to sound like a complete moron , but my GR background is like my knowledge of photons. How is it that gravity is not a force? Gravity is the attracting force between matter. How does that work?

17. Apr 19, 2005

### misskitty

This might help; I believe the paper concerning this was published with the math in 1920. That might narrow down your search. I don't remember who published it though, sorry.

18. Apr 19, 2005

### quasar987

I can't give a better explanation than what I have said earlier. Energy curves space. Photons have energy. Hence, they curve space.

19. Apr 19, 2005

### quasar987

I start from two equations of relativity to show that the mass of a particle moving at speed c (i.e. a photon) has no mass.

20. Apr 19, 2005

### misskitty

Ah, fair enough. I think we need to find one of the relativity gurus ...lol. I can accept that explanation. So if energy curve space, does energy curve everything, in small amounts, or just space?