# I Why does time link gravity and masslessness?

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1. Dec 15, 2015

### toastercombo

Can you tie this together for me?

My understanding:

If something does not have mass, it does not interact with time. If something has mass it interacts with spacetime. When mass interacts with spacetime, and creates gravity, it may travel through n+1 dimensions.

My questions:

A.Time is considered a dimension, right?
B.If so, does light only "travel through"/"interact with" 3 dimensions (x,y,z)?
C.Does mass "travel through"/"interact with" one more dimension than that?(x,y,z,t)?
D.Is it possible that gravity completes the pattern and "travels through"/"interact with" one more dimension(n+1)?

2. Dec 15, 2015

### Orodruin

Staff Emeritus
I do not know where you heard this or where you have interpreted this from or what you even would mean by "interact with time". This is not standard physics nomenclature.

This is not what happens in general relativity. Everything which has energy and momentum will be a source of gravity in GR, regardless of whether it has a mass or not.

3. Dec 15, 2015

### Staff: Mentor

As far as I can interpret what you are saying, this is not correct.

When we use the word "interact" we usually mean that the object in question has some property which shows up in the corresponding field equations. So an electron interacts with the EM field because it has charge, which shows up in Maxwells equations.

So, time is not a field, but it would be part of spacetime. The field equations governing spacetime are called the Einstein field equations. The corresponding property for interacting is called stress-energy, which includes energy, momentum, and stress.

Massless objects still have energy and momentum, and so they have a non zero stress energy tensor. Therefore, they do interact with spacetime.

4. Dec 16, 2015

### Fuinne

I am sorry if I'm not supposed to be writing here, but I think what's he's trying to say is that since space and time are in one 4 dimensional spacetime, an object with mass causes curvature on spacetime. Since an object with mass effects spacetime by warping it, it interacts with both space and time, but a massless object such as a photon would not interact with space or time since it has no mass to cause any curvature. So I think what he's saying is that because mass effects the curvature of spacetime, it interacts and experiences time, however a massless particle would not interact with space and time.

5. Dec 16, 2015

### Ibix

But, as Dale said in his last paragraph, massless objects do cause curvature. The source of gravity in GR is the stress-energy tensor, of which mass is merely one component. Furthermore, gravitational lensing, where light paths are curved by gravitational fields, has been observed many times. So light clearly does interact with space time.

6. Dec 16, 2015

### Fuinne

Well if light does interact with space time, and photons experience momentum, that would mean photons do experience time. So they do interact with space, but isn't it that as soon as a photon is emmited it is absorbed?

7. Dec 16, 2015

### Orodruin

Staff Emeritus
The path of a light signal being a null geodesic does not mean that it experiences zero time or that the universe is flat to it. It is also unclear what you mean by "interact with space". Space-time is a background in special relativity. In GR, there is an interaction between the electromagnetic field and the geometry of space-time. You cannot separate space and time in relativity.

8. Dec 16, 2015

### Fuinne

So assuming one were on a ship traveling at light speed, it's theorized that at such a speed, time would stop relative to whoever is on that ship. If I'm on that ship, I wouldn't be experiencing time compared to anyone else. For me, my clock would be clicking normally, but if someone were to see me in the ship, my clock would stop. Would I be experiencing time in that case? I would be in space, as that is all existence, but would I be interacting with time? If not, I would only be living in a 3 dimensional space time, right?

9. Dec 16, 2015

### Orodruin

Staff Emeritus
You are not allowed to make this assumption. A ship is a massive object and cannot travel at the speed of light.

10. Dec 16, 2015

### Fuinne

Yes, I know that. I'm saying if it could theoretically happen.

11. Dec 16, 2015

### Orodruin

Staff Emeritus
So what you are saying essentially boils down to "I know that this is not allowed by the theory, but what does the theory predict when it happens?"

12. Dec 16, 2015

### Fuinne

Yes.

13. Dec 16, 2015

### rootone

It can't happen because as the ship approaches light speed it requires ever more energy to accelerate it further, and to actually get to light speed the amount of energy needed goes towards infinity.

14. Dec 16, 2015

### Fuinne

Yes. I know it's impossible in reality. I know that. I am talking theoretical. If you could THEORETICALLY do that, I am asking what would happen.

15. Dec 16, 2015

### Janus

Staff Emeritus
Which is a non-nonsensical question. If you suppose a scenario which is forbidden by a theory, then you are saying that the theory doesn't hold true. If the theory doesn't hold true, then it is useless for making predictions.

16. Dec 16, 2015

### Janus

Staff Emeritus
The point is that it isn't even 'theoretically' possible. For it to be theoretically possible,you would first have to have a theory that allows it. Since we know of no such theory that matches our present observations of the universe, there is no answer to the question.

17. Dec 16, 2015

### Orodruin

Staff Emeritus
But it is completely pointless to ask such a question. Of course the theory cannot have anything to say about what happens in a situation which is impossible according to the theory - apart from that it cannot occur.

18. Dec 16, 2015

### Fuinne

I understand where you're coming from, but I don't neccasarily want to admit that a question can be pointless. There is no such thing as a pointless question. A new question adds a new explanation.

I understand that such an event is impossible. I am aware of the physical limitations that block that scenerio, however when it comes to the brain there are no physical limitations imagining a theoretical scenerio such as so.

Again, I am aware of what you're saying. But let's just ignore the mass. Let's assume you're riding on a photon.

19. Dec 16, 2015

### Staff: Mentor

"Theoretical" means "according to theory". If there were a theory that allowed travel at the speed of light, that theory would be a good starting point for working out what theoretically would happen when we did. But a theory that doesn't allow lightspeed travel can't answer questions about about what would happen if we were to move at that speed, for about the same reasons that biology won't tell us much about how the metabolism of herbivorous flying walruses develops the energy needed for sustained flight.

20. Dec 16, 2015

### Fuinne

Then if that's how it's going to be, let's just assume you're a photon. You're a middle aged photon named Darrel and you have a nice wife, house, and kids but when you're going to work, you just walk past your car and run as fast as you can to work. By the time you get to your job, would any time have passed, since you're Darrel the photon and can travel at light speed?

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