Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Do we live in 3 or 4 dimensions?

  1. May 20, 2013 #1
    pleese don't critical on my little knowledge of this. a few of us have bull sessions sometimes and my cousin points front side up and says we live in 3 dimencions but the whole universe is 4 dimencions. this other guy always says my cousin is full of bull and einstine just used them for calculations. who is right?

    also i'm new and read the rules for this forums. what did greg mean by you can't bring up ether theory?
    Last edited by a moderator: May 20, 2013
  2. jcsd
  3. May 20, 2013 #2


    User Avatar
    Gold Member

    spacetime is a 4 vector, so while we DO live in 3 SPATIAL dimensions, we also do live in 4 dimensions so they are both right. If one of them is saying we live in 4 dimension of space, then he misunderstands Einstein.

    "Ether theory" is a theory you can find on Google. It was discredited something like 100 years ago and is not discussed on this forum.
    Last edited: May 20, 2013
  4. May 20, 2013 #3
    sorry I don't understand when you first say we also do live in 4 dimensions and then if one of them is saying we live in 4 dimension of space he misunderstands Einstein.
  5. May 20, 2013 #4
    What phinds is saying is that we live in a 4 dimensional world where 3 dimensions are spatial, and the fourth is time. It is incorrect to say that we live in 4 spatial dimensions.
  6. May 20, 2013 #5
    ok. thank you. I'm looking at some other threads and there is one by a guitar name about us moving through the 4 dimensional universe as fast as light. Does that mean that the universe is 4 dimensions (space and time) and we are just 3 dimensional moving through the 4 dimensions of the universe? So things like rocks and cars and people are three dimensions and the universe is 4 dimensions?
  7. May 20, 2013 #6


    User Avatar
    Science Advisor

    Physics deals with "events"- things that happen at a given point in space at a given time. In any coordinate system we need 3 numbers to specify the point and one to specify the time. That's what "four dimensional" means.
  8. May 20, 2013 #7


    Staff: Mentor

    We get a lot of threads here about that. It's not wrong, exactly, but it can easily lead to misconceptions. The basic physical fact underlying the statement is that every object (more precisely, every object with nonzero rest mass) has a 4-velocity vector whose length is the speed of light.

    On the view I just described, where an "object" has a single 4-velocity vector, the object is idealized as point-like, so its path through 4-dimensional spacetime is a 1-dimensional line, called its "worldline". A real object, which is extended in space, would be modeled as a whole family of worldlines that stay together; this is sometimes called a "world-tube". But if individual parts of the extended object are moving with respect to each other, they will not all have the same 4-velocity, so you can't assign a single 4-velocity vector to the entire object. (Often you can assign a sort of "average" 4-velocity to the object because all of its parts are moving really slowly with respect to each other, compared to the speed of light. But that's an approximation.)

    However, it's important to note that in all these models, the "object" (whether its an idealized point-like object or an object extended in space) is best viewed as its worldline or world tube. This is because of the relativity of simultaneity; different observers in different states of motion will "slice" 3-dimensional sections out of an extended object's world tube in different ways. So you can't really view the object as a 3-dimensional thing that moves through time; you have to look at its entire 4-dimensional world tube. When your cousin said that we live in 3 dimensions, he apparently wasn't taking that into account.
  9. May 20, 2013 #8
  10. May 20, 2013 #9
    Boy! You are really getting down to it. This is getting interesting even for a dumb guy like me. I took a course in physics in junior college and at the end of the semester he told us about relativity but never really talked about what you are saying, he did use some equations though. My cousin has had more college courses in physics and we all talk about it sometimes. Then do you mean the the objects you talk about are really 4 dimensional just like space (and time)? Are things really 4 dimensions and we just don't see all of the dimensions?
  11. May 20, 2013 #10


    Staff: Mentor

    Just like *spacetime*, yes. (Neither space nor time by themselves are 4-dimensional; only spacetime is.)

    What makes you think we don't see all the dimensions? The 4th dimension is time; we "see" the time dimension by observing that objects don't just exist for an instant; they exist for some length of time.
  12. May 21, 2013 #11
    Strickland asks:

    Here are two famous quotes that address your question in a humorous but interesting way:

    and I think it might have been Richard Feynman who said

    When you learned Newtonian physics the implicit assumptions used three dimensions of space, say (x,y,z) and an independent parameter time, t, which ticks along the same steady rate for everyone. And the assumed infinite speed of light allowed things to happen instantaneously.....so it took no time for sunlight to get from the sun to us. Everybody measures the same distances and times. All this works pretty well when relative speeds are slow......like orbiting planets.

    Einstein figured out that space and time are actually relative and depend on each other. 'Space' [distances] that appear fixed in our everyday slow speed existence can be different for different high speed observers. That is, the speed of light is finite and, crazy as it seems, everybody measures the same speed for light no matter their own local speed. So space and time vary by observer, related to their speeds, while the speed of light is finite and fixed.

    In fact it was Einstein's college teacher, Minkowski, who realized that Einstein's early work meant that space and time should be treated equally, that events take place in four-dimensional space-time. Space and time were no longer to be considered separate, independent entities! So the three 'dimensions' of Newton (x,y,z) became four dimensions [spacetime] of relativity: (t,x,y,z).

    edit: If I have stated things correctly here, it should all be consistent with what PeterDonis has posted. The inexpensive book FABRIC OF THE COSMOS by Brian Greene describes 'moving through spacetime at the speed of light' and a lot more without any math. I found that book to be fascinating reading when I started reviewing relativity.
    Last edited: May 21, 2013
  13. May 21, 2013 #12
    You are blowing me away with these ideas Dr. Donis. My mind is spinning. But I think Im begining to catch on some.

    Now I think Im begining to get it. I really liked taking computer drawing in jr college. With this program called I-Deas you could sketch a 2 dimensional surface and then just extend it into the 3rd dimension for whatever length you choose. So maybe relativity is a little like starting with a 3 dimensional thing and then extending it along the 4th dimension. Along a path you called a world tube? So if I see a rock on the ground I could imagine extending it along its world tube? This is heavy stuff. You are about the smartest physicist here. How could anyone ever figure out that was what is going on?
  14. May 21, 2013 #13
    I think this is just what Dr. Donis was saying. But I am going to the book store today and look for that book. And I have to think hard about how the 4 dimensions are not just space but what you and Dr. Donis call spacetime. When I think about the way I did drafting I can picture a rock extending into the 4th dimension but I have to think some more about how that could be something new called spacetime. Is it some kind of combination of space and time that is too hard for us to imagine what it is or do you and Dr. Donis know what it is?
  15. May 21, 2013 #14


    User Avatar
    Gold Member

    Your statement of the question "what is it?" is not helpful. "WHAT" something is is just a name we give it. In this case we give it the name "spacetime". There is no other answer to WHAT it is. Science is about describing the properties of the things we give names to and the properties of spacetime are discussed in general relativity.

    A really excellent example of this terminology issue is the confusion that was experienced early in the days of quantum mechanics when people insisted on calling a photon either a "wave" or a "particle" because they were hung up on WHAT it is. In reality, a photon is NEITHER a wave nor a particle but rather a "quantum object" that sometimes has the properties of a wave and sometimes has the properties of a particle.

    So "spacetime" is not exactly "space" added to "time", it is its own thing called spacetime and it has characteristics that can be discussed although discussing them in English leads to problems. The proper language of science is math and if you want to REALLY describe spacetime you're going to have to learn a lot of math.
  16. May 21, 2013 #15


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    A car is 3-dimensional, but its entire existence from when it's built to when it's destroyed, is 4-dimensional. If you want to understand this better, you should learn about spacetime diagrams (also called Minkowski diagrams).

    Relativity doesn't say anything like that.
  17. May 21, 2013 #16
    Yes. It was supposed to be the same....Getting different descriptions, even though equivalent, is often needed to gain perspective.....

    I'll give you a rough analogy for space and time being considered 'spacetime' :

    If you consider the ocean, say, one, and air the other, they seem pretty distinct, right? ....especially when it's flat calm.... Easy to tell them apart....Then the wind picks up and waves are seen....where did the air go?? Now it's 'curved' too just like the surface of the water....[Space, time and spacetime also 'curves' by the way] then at around 50 or 60 mph, the surface of the water is picked up in a 'spume'....like rain droplets driven from the surface of the water...and it may extend three feet or more above the surface....you can no longer, perhaps, see the waves....'air and water' now have no clear boundary....they 'merge' into each other as if they maybe were a single entity....I've seen this a number of times when boating....and reading physics....

    Well, in an analogous way, 'flexible' space and time also change from one to the other based on frames of reference [different observer perspectives]. Space and time 'conspire' together in such a mathematical form as to keep the speed of light the same for all observers.

    By the way, if you start buying books, see Amazon Books or other online sources for less costly used copies.....
  18. May 21, 2013 #17
    Strickland posts

    BRAVO!!! I forgot to complement you on doing some searches..... just about everything has been discussed numerous times. The trick is to figure out who knows what they are doing...the 'experts' I call them...and then you can just read those posts, because some threads go on and on and on.....Few if anybody, I think, knows it ALL....
  19. May 21, 2013 #18
    Thank you Mr. phinds. I can handle it not knowing what spacetime really is as long as no one really knows, I just thought maybe everone here knows what spacetime is. I won't study on that one any more.
  20. May 21, 2013 #19
    I wanted to come back to this because it really puts your finger on something really cool. In my drafting class we would picture a solid and then draw a line across it with arrows pointing in the direction for a cross-section view. We would do this for two or three different cross sections and label them Sec A-A, Sec B-B, Sec C-C, and so on. So you are telling me that we could imagine my rock extended into the 4th dimension of spacetime (not space, I'm getting that part now) and different people would see different cross sections of the 4 dimensional rock? What determines what their cross section view will be?
  21. May 21, 2013 #20
    First you said a cars existence is 4 dimensional just like Dr. Donis said. But then why did you say relativity doesnt say anything like that?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook