# Is the universe a causal system?

1. Mar 31, 2014

### HamzahA

A simple question. I might ask this as well: Is the universe causal?

The reason I'm asking this is that today I've ran into two guys having an argument about this, so I want to know.

Thank you.

2. Mar 31, 2014

### SteamKing

Staff Emeritus
And you want to jump into the middle of this argument because ...?

In any event, this question seems to fall more within the realm of philosophy than physics, and PF doesn't do philosophy according to its rules.

3. Mar 31, 2014

### phinds

In classical mechanics, yes. In GR, you need to be very clear what you mean by causal.

4. Mar 31, 2014

### HamzahA

I'm not jumping to the argument, it's just that those who were arguing seemed not to have any scientific background related. I'm simply curious.

I'll define causality like this: Causal System: A system whose output(s) depend on the current/past input(s).

5. Mar 31, 2014

### phinds

Yes, that is the standard definition and in no way changes my original answer. As I understand it, that standard definition is too vague in relation to GR and QM.

6. Apr 1, 2014

### HamzahA

Thank you for your response, but can you elaborate why is it too vague?

7. Apr 1, 2014

### phinds

Unfortunately, I cannot. This is something that I heard once, thought was a bit weird and so briefly checked out on a couple of reputable sites and since it seemed to make sense, I just left it at that simple fact, since I did not want to delve further.

I assume you should be able to find stuff on the internet. Try Goggling "QM and causality"

8. Apr 5, 2014

### oneamp

what I want to know is whether or not the universe is memoryless

9. Apr 5, 2014

### Staff: Mentor

10. Apr 5, 2014

### phinds

I have no idea what oneamp is talking about, but what problem do you have with the OP's question? It seems perfectly reasonable to me, as do our interchanges. That is, the thread seems perfectly reasonable up until oneamp's interjection.

11. Apr 5, 2014

### AlephZero

Even in special relativity, there is no concept of "universal absolute time". So there is no simple concept of "event A happens before event B". One observer might claim that A happens first, while another observer (moving relative to the first one) claims B happens first, and both of them are right.

if you can't even be sure in what order "stuff happens", the idea of "causation" is rather hard to nail down.

If you really want to get your head around this, you need to do a course on special relativity. Warning: pop-science books and websites may be seriously misleading, or just plain wrong.

12. Apr 6, 2014

### Staff: Mentor

Perhaps, but I believe the order of events that an observer goes through will be seen the same by all other observers, right? For example, if I get up at 9 am my time and eat breakfast one hour later, all observers will agree that I woke up before I ate breakfast. Is that correct?

13. Apr 6, 2014

### Chronos

Causality is invariant under SR. Since FTL travel is forbidden in both SR and GR, in no reference frame will you ever see a caused event precede its causative event. You need not worry about waking up full, Drakkith.

Last edited: Apr 6, 2014
14. Apr 6, 2014

### Staff: Mentor

So no chance of waking up with the eggs already made then. Got it.

15. Apr 14, 2014

### Tobychev

As I recall Einsteins General Relativity admits solutions that form closed time loops, so strict adherence to causality must be imposed by some other means.

And within the realm of Quantum Mechanics there is the eternal quest to inject determinism into (for example) the decays of states, thus giving a casual response to question of type "why did this particle decay at t=10?" and "why did the fourth Hydrogen atom spin-flip first?"

As for oneamps question, I know I have a memory in my computer (I remember buying it) so the universe is certainly not totally void of memories. If you can manage a more well defined version of that question you might get a better answer.

16. Apr 14, 2014

### Agrasin

Uh, this might not be the right place, but I have a question for all of you PhysicsForums veterans with thousands and thousands of posts. I've just been incredibly impressed by the level of discussion here and I want to get where you are.

What are you educations? What did you focus on in college? How long have you been physics enthusiasts?

17. Apr 14, 2014

### Staff: Mentor

This is best asked in the General Discussion forum. Otherwise we'll have to sic Phinds on you! (He's more bark than bite, but his slobber is the real threat)