General Relativity

[Wictorian]

I am no physicist but I find it hard to believe that time slows down when you approach the speed of light or with immense gravity. I know its about human psychology but is it 100% true or is there a chance it is wrong?

[Drama Lama]

7 minutes ago, Wictorian said:

I am no physician but I find it hard to believe that time slows down when you approach the speed of light or with immense gravity. I know its about human psychology but is it 100% true or is there a chance it is wrong?

the theory is true and it has to be accounted for when building sattelit based geolocation systems

7 minutes ago, Wictorian said:

I know its about human psychology

no it's physics

Edited February 18, 2021 by Drama Lama

[Wictorian]

58 minutes ago, Drama Lama said:

 

no it's physics

I mean it sounds fake because of human psychology. I believe it consists of many things so is the slowing time thing 100% correct? Like Newtonian physics is correct but sometimes wrong. (This sentence sounds funny but I hope you get it)

[tikker]

14 hours ago, Wictorian said:

I know its about human psychology but is it 100% true or is there a chance it is wrong?

@Drama Lama is right. It has nothing to do with psychology. Relativity is strange and very difficult and unintuitive to grasp. When I followed courses in them they were real mind benders.

 

By saying the theory is true we say that we have not yet found a theory that works better. One aspect that makes a good theory is its predictive power.

 

Of course like for any theory there is a chance GR is wrong, but it is likely an extremely small one. GR has so far survived the test of time in an amazing way. Not only does it agree with measurements that we can now make, it accurately predicts things: the orbital precession of Mercury, light getting bent by the Sun and recently gravitational waves.

13 hours ago, Wictorian said:

Like Newtonian physics is correct but sometimes wrong.

Newtonian physics can be seen as a "weak field limit" of GR. That means it works perfectly fine for normal everyday circumstances (weak gravitational fields and velocities significantly lower than the speed of light) and is by all means correct for those. Even for simulations you can in a lot of cases just take Newtonian physics and, if needed, add a few more terms if you need to (approximately) correct for GR effects. See for example this question about GR in Universe Sandbox.

14 hours ago, Wictorian said:

I am no physician but I find it hard to believe that time slows down when you approach the speed of light or with immense gravity

First of all, there are two theories aspects to relativity:

1. Special relativity -- effects from speed
2. General relativity -- effects due to gravity

Time dilation due to travelling close to the speed of light is a result of special relativity, not general relativity and is a consequence of the fact that the speed of light is constant. This is often explained with a light clock that has light bouncing between two mirrors.

http://www.pitt.edu/~jdnorton/teaching/HPS_0410/chapters/Special_relativity_clocks_rods/figures/light_clock_anim_2.gif

On the left is you with a stationary clock. Next to it is an identical clock, but it's moving with respect to you. The wiggly arrows depicts light, traveling at the speed of light. You can see that the moving clock "ticks" much slower and that the light has to travel a longer distance before ending up at the bottom of the moving clock. So as seen from your point of view the moving clock runs slower.

14 hours ago, Drama Lama said:

the theory is true and it has to be accounted for when building sattelit based geolocation systems

Funnily enough satellite clocks run slower due to special relativity from our point of view (they move), but faster due to general relativity (Earth's gravitational field strenght is weaker up there). I recently learned that at roughly 1.5 Earth radii (if Earth was a sphere) both cancel out and there would be no time difference between us and the satellite!

[Dash Lambda]

24 minutes ago, Wictorian said:

I mean it sounds fake because of human psychology. I believe it consists of many things so is the slowing time thing 100% correct? Like Newtonian physics is correct but sometimes wrong. (This sentence sounds funny but I hope you get it)

Think of it like this: You're always traveling at a constant speed relative to anything around you through spacetime. If you're standing 'still' relative to something else then all of your speed is pointed in the time direction, and if you're traveling at the speed of light relative to something then all your speed is in a space direction. The math actually works out that way, that visualization was presented to me as the "spacetime speedometer."

 

Newtonian mechanics isn't sometimes wrong, it is wrong. But it is an excellent approximation of GR at very low relative speeds, so close that there's really no reason to go through the more intensive GR calculations unless you're working over very long length or time scales or with speeds that are a significant fraction of the speed of light.

 

The fact that there is a relationship between time and space is 100% correct, and GR has so far proven to be an excellent model of that relationship. But, while it has given us extremely accurate predictions, you should never consider a model 100% accurate -our understanding of how things work is constantly evolving, and our models are too. Currently there are still questions about the relationship between GR and QM, and some of their predictions differ; neither GR nor QM are likely the final form of our models. We're probably going to see them turn into bigger, more powerful, more descriptive models down the line.

[tikker]

1 hour ago, Dash Lambda said:

But it is an excellent approximation

Indeed. If it's good enough to take us to the Moon it's good enough for day-to-day life.

 

Even more elusive than the theory itself is how on Earth he ever thought of it. It's not just any theory it still holds even after 100 years. Every test we come up with GR is just "see, exactly as I predicted 100 years ago".

[Wictorian]

8 hours ago, tikker said:

First of all, there are two theories of relativity:

1. Special relativity -- effects from speed
2. General relativity -- effects due to gravity

Time dilation due to travelling close to the speed of light is a result of special relativity, not general relativity and is a consequence of the fact that the speed of light is constant. This is often explained with a light clock that has light bouncing between two mirrors.

http://www.pitt.edu/~jdnorton/teaching/HPS_0410/chapters/Special_relativity_clocks_rods/figures/light_clock_anim_2.gif

On the left is you with a stationary clock. Next to it is an identical clock, but it's moving with respect to you. The wiggly arrows depicts light, traveling at the speed of light. You can see that the moving clock "ticks" much slower and that the light has to travel a longer distance before ending up at the bottom of the moving clock. So as seen from your point of view the moving clock runs slower.

Funnily enough satellite clocks run slower due to special relativity from our point of view (they move), but faster due to general relativity (Earth's gravitational field strenght is weaker up there). I recently learned that at roughly 1.5 Earth radii (if Earth was a sphere) both cancel out and there would be no time difference between us and the satellite!

I don't understand how time slows down in the first place. How much time does it take light to cover a 300000 kms distance? If it takes 1 seconds how does time slow down? You said it slows down for observers, but don' we see sun 8 minutes in the past and life travels from sun to earth in 8 m,ns. so 8 = 8. Isn't it like saying time is faster in a car because you cover distance more rapidly in a smaller amount of time? Does time as a fundamental forces of physics actually exist? Then shouldn't be able to travel back and forth in time? I think if we think that time actually doesn't exist, it answers some question like why is there something instead of nothing. Also out of context but, if we accept time as the 4th dimension, then are we 4 dimensional?

[Bombastinator]

11 hours ago, Wictorian said:

I am no physician but I find it hard to believe that time slows down when you approach the speed of light or with immense gravity. I know its about human psychology but is it 100% true or is there a chance it is wrong?

Physician?  I’m suspect an autocorrect on physicist.  Relativity is wacky.  It’s pretty well tested though.

[Wictorian]

6 minutes ago, Bombastinator said:

Physician?  I’m suspect an autocorrect on physicist.  Relativity is wacky.  It’s pretty well tested though.

LOL. I am not native so.

[PCGuy_5960]

11 hours ago, tikker said:

First of all, there are two theories of relativity:

1. Special relativity -- effects from speed
2. General relativity -- effects due to gravity

Time dilation due to travelling close to the speed of light is a result of special relativity, not general relativity and is a consequence of the fact that the speed of light is constant.

Just wanted to point out that they aren't really two separate theories. Special relativity is a special case of general relativity where velocity is constant and spacetime is flat.

2 hours ago, Wictorian said:

I don't understand how time slows down in the first place.

I think this video does a good job of explaining the basic concept:

2 hours ago, Wictorian said:

Isn't it like saying time is faster in a car because you cover distance more rapidly in a smaller amount of time?

Actually you do experience the effects of relativity in a car or even when you are walking, but because the speed is almost 0 compared to the speed of light, the effects are negligible.

[LordVetinari]

3 hours ago, Wictorian said:

I don't understand how time slows down in the first place. How much time does it take light to cover a 300000 kms distance? If it takes 1 seconds how does time slow down? You said it slows down for observers, but don' we see sun 8 minutes in the past and life travels from sun to earth in 8 m,ns. so 8 = 8. Isn't it like saying time is faster in a car because you cover distance more rapidly in a smaller amount of time? Does time as a fundamental forces of physics actually exist? Then shouldn't be able to travel back and forth in time? I think if we think that time actually doesn't exist, it answers some question like why is there something instead of nothing. Also out of context but, if we accept time as the 4th dimension, then are we 4 dimensional?

Time is not a universal constant - the speed of light is. 

 

Have a read of the Hafele-Keating experiment. 

[tikker]

20 minutes ago, PCGuy_5960 said:

Just wanted to point out that they aren't really two separate theories. Special relativity is a special case of general relativity where velocity is constant and spacetime is flat.

Ah yes true. I should probably have said there area two aspects to relativity.

3 hours ago, Wictorian said:

I don't understand how time slows down in the first place. How much time does it take light to cover a 300000 kms distance? If it takes 1 seconds how does time slow down? You said it slows down for observers, but don' we see sun 8 minutes in the past and life travels from sun to earth in 8 m,ns. so 8 = 8. Isn't it like saying time is faster in a car because you cover distance more rapidly in a smaller amount of time? Does time as a fundamental forces of physics actually exist? Then shouldn't be able to travel back and forth in time? I think if we think that time actually doesn't exist, it answers some question like why is there something instead of nothing. Also out of context but, if we accept time as the 4th dimension, then are we 4 dimensional?

The thing is that time is relative. It depends on who is observing who. The concept is that we see things happening from our point of view and we interpret them in our reference frame as we call it. If we stand still and see something move near light speed, from our point of view their clock runs slower. However, in their reference frame we area the ones moving and thus they see our clock running slower. At the same time for each of you time flows "normally" to you.

 

The fixed speed of light in every not-accelerating reference frame is crucial. That is one of the reasons why the math leads to time dilation as in the example of the light clock.

[Wictorian]

57 minutes ago, PCGuy_5960 said:

Just wanted to point out that they aren't really two separate theories. Special relativity is a special case of general relativity where velocity is constant and spacetime is flat.

I think this video does a good job of explaining the basic concept:

Actually you do experience the effects of relativity in a car or even when you are walking, but because the speed is almost 0 compared to the speed of light, the effects are negligible.

Imagine you are in a bus with the speed of 50 mph and you start running at 10 mph. Are you running at 60 mph? Congragulations you have surpassed Usain Bolt. This video rather confused me more. I think light can travel at its speed in a metro that is travelling at the speed of light. I feel like a flat-earther but I really don't believe that time slows down like that. 

[Eigenvektor]

14 hours ago, Wictorian said:

Like Newtonian physics is correct but sometimes wrong. (This sentence sounds funny but I hope you get it)

Newtonian physics isn't "wrong" as such, it simply isn't precise enough to describe certain phenomena that occur at high velocities. For most everyday things it is still "good enough". You may want to read this essay: Relativity of Wrong

 

You could use General Relativity for everything, but you'd quickly discover that the answer you get is essentially identical to Newtonian physics in 99% of all cases. Meaning you don't gain anything useful from using the more complex formula in these cases.

 

Of course we already know that General Relativity isn't "correct" either, because there are some things in the universe it can't explain either (Quantum Mechanics). If you want to become famous, find a way to unify General Relativity and Quantum Mechanics into a new theory that can be used to describe everything at once.

[LordVetinari]

32 minutes ago, Eigenvektor said:

Newtonian physics isn't "wrong" as such, it simply isn't precise enough to describe certain phenomena that occur at high velocities. For most everyday things it is still "good enough". You may want to read this essay: Relativity of Wrong

 

You could use General Relativity for everything, but you'd quickly discover that the answer you get is essentially identical to Newtonian physics in 99% of all cases. Meaning you don't gain anything useful from using the more complex formula in these cases.

 

Of course we already know that General Relativity isn't "correct" either, because there are some things in the universe it can't explain either (Quantum Mechanics). If you want to become famous, find a way to unify General Relativity and Quantum Mechanics into a new theory that can be used to describe everything at once.

I haven't done much reading recently on unified field theories but I thought the current consensus was that time was a quantum entanglement problem which sort of 'works' for both Quantum and Relativity. 

 

*  hears OPs mind explode * 

[PCGuy_5960]

52 minutes ago, Wictorian said:

Imagine you are in a bus with the speed of 50 mph and you start running at 10 mph. Are you running at 60 mph? Congragulations you have surpassed Usain Bolt.

In the reference frame of the bus (which is moving at 50mph), no, you'd still be running at 10mph. In the reference frame of someone standing still at a bus stop, yes, you'd effectively be running at 60mph. This doesn't even have anything to do with special relativity actually, it's just regular reference frames. If you accounted for relativity, to an observer standing still, you'd be running at ever so slightly less than 60mph.

1 hour ago, Wictorian said:

I think light can travel at its speed in a metro that is travelling at the speed of light.

Yes, but if space and time didn't "change", an outside observer would effectively see the light travelling at 2x the speed of light, which is impossible because the speed of light is constant. Think of it this way, velocity is just distance/time, right? For a constant distance, you can decrease the velocity by increasing time and likewise for a constant amount of time, you can decrease the velocity by decreasing the distance. Hence, time dilation and space contraction. I think this is as simple as I could possibly describe it. ¯\_(ツ)_/¯

[lewdicrous]

This might help.

 

[Wictorian]

37 minutes ago, PCGuy_5960 said:

In the reference frame of the bus (which is moving at 50mph), no, you'd still be running at 10mph. In the reference frame of someone standing still at a bus stop, yes, you'd effectively be running at 60mph. This doesn't even have anything to do with special relativity actually, it's just regular reference frames. If you accounted for relativity, to an observer standing still, you'd be running at ever so slightly less than 60mph.

Yes, but if space and time didn't "change", an outside observer would effectively see the light travelling at 2x the speed of light, which is impossible because the speed of light is constant. Think of it this way, velocity is just distance/time, right? For a constant distance, you can decrease the velocity by increasing time and likewise for a constant amount of time, you can decrease the velocity by decreasing the distance. Hence, time dilation and space contraction. I think this is as simple as I could possibly describe it. ¯\_(ツ)_/¯

Why is speed of the light constant? If velocity is distance/time and time isn't constant why can't velocity be relative? Why is it impossible for observers to see light travelling 2x the speed of light?

[PCGuy_5960]

21 minutes ago, Wictorian said:

Why is speed of the light constant?

This is far more complicated to explain, but it has to do with Maxwell's equations and experimental data backing up relativity.

21 minutes ago, Wictorian said:

If velocity is distance/time and time isn't constant why can't velocity be relative? Why is it impossible for observers to see light travelling 2x the speed of light?

Velocity is relative, the speed of light isn't. As for why it isn't, the theories say so and the experimental data backs them up. 

[tikker]

2 hours ago, Eigenvektor said:

Of course we already know that General Relativity isn't "correct" either, because there are some things in the universe it can't explain either (Quantum Mechanics). If you want to become famous, find a way to unify General Relativity and Quantum Mechanics into a new theory that can be used to describe everything at once.

Now that would be winning life. You've found the ultimate theory that describes everything in the Universe, nothing left to do, roll credits.

1 hour ago, Wictorian said:

Why is speed of the light constant?

The unsatisfactory answer is pretty much, because the speed of light being constant is a core assumption of relativity. It's constant because Einstein said it is constant. He didn't pull it completely out of thin air however. As @PCGuy_5960 says Maxwell's equations gave good support to such an assumption. Went reading for a bit (to also refresh my memory again) and found this to be a nice explanation:

Quote

The key logic behind Special Relativity was that Maxwell's equations for electromagnetism looked like exact, universal laws of physics, and their solution gives light waves with a universal speed. Now it was logically possible that those laws were only true in one special reference frame, but by 1905 no experiment (including the famous attempt by Michelson and Morley) provided any evidence that they failed to work in any inertial frame. Einstein showed that there was a logical, consistent framework (Special Relativity) in which Maxwell's equations worked in all inertial frames, and Newton's laws also almost worked for any objects moving slowly with respect to a frame.
https://van.physics.illinois.edu/QA/listing.php?id=2605&t=why-constant-speed-of-light

The bold part is the important bit. Assuming a constant speed of light turns Maxwell's equations into powerful general laws.

[Wictorian]

4 hours ago, tikker said:

Now that would be winning life. You've found the ultimate theory that describes everything in the Universe, nothing left to do, roll credits.

The unsatisfactory answer is pretty much, because the speed of light being constant is a core assumption of relativity. It's constant because Einstein said it is constant. He didn't pull it completely out of thin air however. As @PCGuy_5960 says Maxwell's equations gave good support to such an assumption. Went reading for a bit (to also refresh my memory again) and found this to be a nice explanation:

The bold part is the important bit. Assuming a constant speed of light turns Maxwell's equations into powerful general laws.

I would say maybe speed of light isnt constant if the theory of general relativity is not complete. However as far as I understand it's one of the key concepts so it should be like that and otherwise the theory wouldn't work at all. I still think however that light can travel at the speed of the light in a metro that is travelling at the speed of the light. If that was the case for instance, would it hurt the theory?

[Rohith_Kumar_Sp]

on a related note, no one can really measure the speed of light 

 

 

 

[tikker]

1 minute ago, Wictorian said:

I still think however that light can travel at the speed of the light in a metro that is travelling at the speed of the light. If that was the case for instance, would it hurt the theory?

No, this is exactly what is in the theory: light always travels at the speed of light. Even in a hypotethical metro travelling at the speed of light, light from a source travelling on the metro wil still travel at the speed of light to any observer whether they are on the metro or not.

[Wictorian]

21 minutes ago, tikker said:

No, this is exactly what is in the theory: light always travels at the speed of light. Even in a hypotethical metro travelling at the speed of light, light from a source travelling on the metro wil still travel at the speed of light to any observer whether they are on the metro or not.

Let's say you are travelling from A to B at the speed of light, and light is travelling from B to A. At the moment you meet, what do you see? Is it the same thing? You just see it at the speed of light and time slows down. 
 

Also if an astronaut aged 5 years when we aged more, did he feel 5 years is past or more?

 

3 minutes ago, Dash Lambda said:

  

Light travels at the speed of light relative to everything -so when we observe light, to us it travels exactly 299,792,458 m/s. If it's coming out of a moving flashlight, no matter how fast the flashlight is moving, the light is always going at the speed of light. That's why we get red/blue-shifting, because the wave gets stretched out or scrunched up.

 

Now, if you were to think about a photon's perspective, no time would pass. Photons don't change or evolve, they don't change direction (light bending around planets and stuff is technically it following a straight line, but that's GR being weird), they just appear and disappear. There's an effect that comes out of special relativity called length contraction, where the faster you're going the shorter distances appear to be, and due to length contraction, a photon would perceive the universe as totally flat, with no distance between where the photon begins and ends.

 

Physicists don't like referring to a photon's perspective, though. As I understand it, anything traveling at the speed of light doesn't actually have an inertial reference frame, so it can't be an 'observer' in the same way a slow-moving object can.

 

So back to the bus thing, if you have a bus going 99% the speed of light and you shoot a laser towards the front, the photons will still be going at the speed of light from every perspective. To someone outside the bus the frequency of the light is shifted super high, but to an observer on the bus the frequency is 'normal'. You can connect these two perspectives with length contraction -if you think about the light as a sine wave, then both observers see the same number of ups and downs, but the external observer literally sees a shorter bus, squeezing those waves into a shorter length.

 

If you extend that behavior to a bus traveling at the speed of light, then the bus would have no length. That means the external observer would see the photon and the bus travel together, at the same speed.

What makes light so special? (the pun) I can travel at different sppeds, why can't light?

 

What if you travel at the speed of the light for 1 hour (relative to us) and then come back to earth and slow down, will you think no time has passed and it will be like you fell asleep and woke up?

[Dash Lambda]

  

27 minutes ago, Wictorian said:

I would say maybe speed of light isnt constant if the theory of general relativity is not complete. However as far as I understand it's one of the key concepts so it should be like that and otherwise the theory wouldn't work at all. I still think however that light can travel at the speed of the light in a metro that is travelling at the speed of the light. If that was the case for instance, would it hurt the theory?

Light travels at the speed of light relative to everything -so when we observe light, to us it travels exactly 299,792,458 m/s. If it's coming out of a moving flashlight, no matter how fast the flashlight is moving, the light is always going at the speed of light. That's why we get red/blue-shifting, because the wave gets stretched out or scrunched up.

 

Now, if you were to think about a photon's perspective, no time would pass. Photons don't change or evolve, they don't change direction (light bending around planets and stuff is technically it following a straight line, but that's GR being weird), they just appear and disappear. There's an effect that comes out of special relativity called length contraction, where the faster you're going the shorter distances appear to be, and due to length contraction, a photon would perceive the universe as totally flat, with no distance between where the photon begins and ends.

 

Physicists don't like referring to a photon's perspective, though. As I understand it, anything traveling at the speed of light doesn't actually have an inertial reference frame, so it can't be an 'observer' in the same way a slow-moving object can.

 

So back to the bus thing, if you have a bus going 99% the speed of light and you shoot a laser towards the front, the photons will still be going at the speed of light from every perspective. To someone outside the bus the frequency of the light is shifted super high, but to an observer on the bus the frequency is 'normal'. You can connect these two perspectives with length contraction -if you think about the light as a sine wave, then both observers see the same number of ups and downs, but the external observer literally sees a shorter bus, squeezing those waves into a shorter length.

 

If you extend that behavior to a bus traveling at the speed of light, then the bus would have no length. That means the external observer would see the photon and the bus travel together, at the same speed.