# Relativity and the speed of time

1. Apr 24, 2013

### spaceanon

Ok I have some general questions about Einstein's theory of relativity.
1) Is he asserting that the faster you move (the closer you get to the speed of light), the slower time moves from perspective of somebody in a slower state?
2) If this is indeed what he is saying, does it really mean that someone at the speed of light may only feel a few minutes have passed when somebody on earth has had years pass and become much older than the person in the ship moving at lightspeed?
3) Has this ever been proven or is it still a theory? Would somebody moving very quickly essentially age slower?

2. Apr 24, 2013

### phinds

Special Relativity has been verified experimentally. It says that fast moving things appear to age slower from the point of view of the things relative to which they are moving fast.

SO ... If I'm in a spaceship moving REALLY fast relative to Earth, and you are on Earth, you will see me age more slowly than you and I will see you age more slowly than me and we will both be right because it's symmetrical.

If we break the symmetry by having me turn the ship around and come back to Earth, then I will have aged less than you when we get back together because I was the one that was accelerating.

For me, time will have passed at the rate of one second per second and for you the time will have passed at the rate of one second per second. SO ... how can we possibly have aged differently? Well, I took a different path through space-time than you did. You moved more through time and less through space than I did and I moved more through space and less through time than you did.

Yep, it's confusing.

3. Apr 24, 2013

### Mentz114

There is no such thing as 'the faster you move'. You are only moving with respect to other things.
What SR says is that between two relatively moving clocks, they will each convert the others time coordinates to a smaller number.

Two clocks ( or people) who meet, then travel on different paths might have aged differently when the meet up again.

It has been experimentally tested. See the sticky thread above.

4. Apr 24, 2013

### spaceanon

So this phenomenon has been verified? I know that special relativity has to do without gravity and is linear...right? And General relativity combines SR with gravity and different directions?

Therefore, it is true that spacetime exists?

They are inter-related?

5. Apr 24, 2013

### phinds

Both Special Relativity and General Relativity have been verified experimentally. Special Relativity has absolutely NOTHING to do with gravity. In fact, gravity is not considered in SR. General Relativity is the one that has to do with gravity and space-time, and yes, of course space-time exists otherwise you wouldn't. Space and time are not "inter-related" so much as they are just different aspects of the same thing.

EDIT: by the way, if reference to your original question, the topic of "time dilation" has been discussed here approximately 19,000 times, so just do a forum search to get some interesting discussions.

6. Apr 24, 2013

### spaceanon

So that is the big discovery by Einstein? That space and time are tightly knit?
It's just hard to believe that if someone traveled at the speed of light, turned around and came back, years would have passed for me when only minutes would have for him. And that is the foundation of the relativity theory? That is why it is so intriguing? Very interesting... sorry but I'm new to this and want to learn more about it.

7. Apr 24, 2013

### phinds

You will find it helpful as you progress in this if you will get rid of your Newtonian notion that space and time are separate. They are not "tightly knit", they are part of the same single construct, which we call space-time.

Nothing with mass travels AT the speed of light, but if you replace that with "close to" then yes, that's what happens.

Quantum Mechanics (the very small) and cosmology (the very large) defy human "logic" and "common sense" and "intuition" because all of those things evolved within a VERY small range of physical existence and do not apply well, if at all, to things outside that small range. Nature does not care what we find hard to believe, it just does what it does.

8. Apr 25, 2013

### spaceanon

Yeah I understand they are the same thing, it's just, like you said, I currently have a more "Newtonian" mindset. I think I understand... its kind of the same way that light is light, but may have different aspects like amplitude and wavelength. (Space and time) are aspects of "Space-time."

I just never that that relativity was proven right because I thought it conflicted with quantum mechanics...but I guess both are right in their own regard, and that is one of the greatest questions scientists have (how do they combine into one theory).

On a side note, I thought light was an electromagnetic radiation, or a quanta of photons traveling (I guess). So I guess anything with a mass greater than light itself can't travel at that speed?

Oh well, I guess this seems to only lead to more and more questions rather than the opposite.

LOL

9. Apr 25, 2013

### Staff: Mentor

Yes, it has been verified. At the top of this forum there's a sticky thread on experimental confirmation of relativity; take a look at it for details.

SR works in "different directions" too. The difference between the two is that SR only works for the special (that's why it's called "special"!) case in which there is zero gravity. GR works for the more general case in which gravity may be non-zero - but of course GR makes the same predictions in the zero-gravity case as does SR, so it's best to think of SR as a subset of GR.

10. Apr 25, 2013

### rbj

perhaps i might put it differently. but something like that.

i would say that this is a result or consequence of the foundation and not the foundation itself.

the foundation of special relativity is that for every inertial observer, every law of nature, both qualitatively and quantitatively, applies identically. no inertial observer can claim that their frame of reference is special vis a vis the other inertial observers that may be moving relative to the first. every inertial observer has equal claim to being "at rest" as any other inertial observer, even if they are moving relative to each other.

that might seem reasonable and unremarkable (and it is the former) since there was already a notion of this kind of equivalence of inertial frames of reference before Einstein, but they weren't entirely consistent about it and Einstein had the insight to be completely consistent about the notion of the relativity of motion.

but to be completely consistent about the notion of the relativity of motion, one must insist that all laws of physics as well as the quantitative parameters in those laws (the "constants of nature") must be the same for every inertial observer. this means that every inertial observer (one not being accelerated) must have the same speed of light in the physical law that governs what they observe. so two inertial observers looking at the very same ray of light must see it move at the same speed c, even if these two observers are moving relative to each other. the only way for that to happen is for each observer to observe the other's clock to tick slower than there own.

Last edited by a moderator: Apr 25, 2013
11. Apr 25, 2013

### spaceanon

Ok so now I have a new question. Let's say that someone on earth drove at 60mph for 50 years (it's hypothetical) and somebody sat for 50 years.... is it stupid to say that the person in the car would age slower at an extreamely small margin? Maybe .000000000000000001 seconds or something. Is that a stupid question? Haha

12. Apr 25, 2013

### Staff: Mentor

The age difference would be something around .000015 seconds, but yes, there would be an age difference.

13. Apr 25, 2013

### spaceanon

Ok here is where I get a little confused. If the general theory of relativity states that either party (the person in the car or the person standing still) are right in both thinking they are still while everything around them is in motion, who in fact gets older? Couldn't the person in the car picture himself as being motionless while the earth and the person in the chair is orbiting around him? Then wouldn't the person in the car become the one who got essentially "older" by .000015 seconds? If the speeds were exaggerated who would come out with the gray hair first?

14. Apr 25, 2013

### phinds

You have stumbled onto the "circular twin paradox" (that's just what I call it --- I don't know that it has a specific name). If the traveler flies away from earth and then turns around and comes back, the symmetry is broken at the turnaround because the traveler is at that time no longer in an inertial frame of reference and the result is that the traveler is younger when they meet up.

In the scenario you describe, the traveler is ALWAYS out of symmetry with the stay-at-home, it's just that the acceleration is around the Earth instead of happening all at once at the turnaround and once again, the traveler is younger than the stay-at-home when they meet up.

EDIT: by the way, this threw me when I first encountered it, exactly as it did you. I started what ended up being a moderately long, as I recall, and very enlightening, thread.

15. Apr 25, 2013

### nitsuj

refer to phinds first post where he/she mentions symmetry. Acceleration is asymmetrical motion.

Orbiting & being motionless are mutually exclusive.

If the car & driver are motionless (inertial) then what you describe is symmetrical.

If the car & driver are orbiting whatever then what you describe is non-inertial, the car & driver are accelerating.

phinds beat me to it.

16. Apr 25, 2013

### Staff: Mentor

As nitsuj and phinds have pointed out, you've just discovered the twin paradox, extensively discussed both here and on the web. Depending on how deeply you want dig into it, there's the quick answer you've already been given: Any experiment in which the twins separate and then rejoin must necessarily involve some acceleration (which unlike there speed, is NOT relative); if the accelerations they experience are different then so will the amount of time they experience, and we can use the accelerations to calculate by exactly how much. If you want to dig deeper, look for some of the references in some of the 83-bazillion threads we already have on the "twin paradox".

You may have noticed that I said the answer would be "something around" .000015 seconds after fifty years at 60 mph, not "exactly". That was a very rough calculation, just trying to get the decimal point into the right zip code. The important point is that the effect is well and thoroughly unnoticeably small at the speeds that we're used to.

17. Apr 25, 2013

### spaceanon

Ok so in this case, the person who ages slower is the person who experienced acceleration and then stopped or changed symmetry? If the person in the car stopped, he would have experienced a change in force or acceleration and if the person in the chair stopped along with the earth they would have experienced a change in acceleration? Therefore either one could end up being younger depending on who experiences the change in acceleration? Is that what you guys are saying?

18. Apr 25, 2013

### phinds

No, that is not what we are saying. What we are saying is that the traveler has been accelerating all along. It isn't his stopping that breaks the symmetry, THERE NEVER WAS any symmetry in the case of the "circular twin paradox". If is different in that way from the standard twin paradox.

19. Apr 25, 2013

### spaceanon

Ok so I think I understand... but I have another question that may seem a little silly. Does this mean that if I were traveling at .99c, I would effectively age slow? I mean, I would have a clock with me that may tick for 1000 years and I feel the true length of that time (impossible for someone on Earth), but may still be alive? Even if I don't even realize that I am traveling at such a speed? Basically would I feel like I am living a much longer life than someone on Earth would even if I were unaware of any other observers?

20. Apr 25, 2013

### Staff: Mentor

You are always stationary in your own reference frame, so you and your clock don't appear to you to to be experiencing anything unusual. But if you come back to earth after what you think are just a couple of years, you may find 1000 have passed on earth.

21. Apr 25, 2013

### spaceanon

I understand that, but even though in your own reference frame you appear stationary, if you are traveling close to the speed of light wouldn't you live a longer life? Even without a second observer wouldn't your body decay at a slower rate than somebody on Earth? So I would feel like I have lived maybe 2000 years when that is impossible to someone not traveling at such a high velocity?

22. Apr 25, 2013

### phinds

Absolutely not. Your time always passes at one second per second and you would live a normal life-span regardless of any travel speed.

23. Apr 25, 2013

### Mentz114

(My bold)

You obviously did not bother to read my post#3. Without a second observer, how do you define a velocity ? You are at rest in your frame as you've said. You still think there is an absolute ( observer independent velocity). There isn't, velocity is relative.

24. Apr 26, 2013

### nitsuj

I think it maybe a good idea to read about causation.

It could be that because you're human you find it remarkable that two spatially separated observers with relative motion would observe the other to have dilated time, simultaneously. This apparent paradox is of no physical consequence. (beyond the fact both observer the others time as dilated)

Consider that somewhere out in spacetime there is something moving at a relativistic speed relative to you. You observe it to be time dilated, it observers your to be time dilated. Ignore the individual observer, focus on the spacetime between them.

So there is already a frame of reference where you will appear to live for 2,000yrs, and I'm sure you know now that you cannot "feel" time dilation, there is no physical consequence to you, just because something else observes your time as dilated...and here is the punch line; comparatively.

You will only ever live at the rate of one second per second, proper time specifically. (funny, I see phinds already pointed this out)

Because the situation is symmetrical there is no difference between either observer.

Last edited: Apr 26, 2013
25. Apr 26, 2013

### Staff: Mentor

Relative to what you are traveling close to the speed of light from (earth), yes. Relative to anything traveling slower or not moving with respect to you (such as your clock), no.

Remember, it isn't just clocks that are slowed, it is time itself. Since clocks measure time, there can be no discrepancy between what clocks measure and other processes that happen over time. In a way, the ~100 year lifespan of a human is a type of clock, so it of course agrees with a regular clock.
There must be a second observer/reference frame, otherwise there is nothing to measure speed against. In this case, that other observer is on Earth.
Relative to them and their clock, yes. Relative to your own clock....doesn't make any sense.
Compared to someone on Earth, yes, but not as measured by your own clock.

I realize it can be tough to accept, but this is a reality. The way you are describing it is actually making it more complicated than it really is.

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