# The Naked Scientists Forum

### Author Topic: Why does time slow down at speed?  (Read 19085 times)

#### Des

• Guest
##### Why does time slow down at speed?
« on: 09/05/2011 02:30:06 »

Hi there,

this may be well off the mark, but, is the reason why time slows down more and more as you travel closer to the speed of light because time and space are part of each other ie "spacetime".

Space is expanding and therefore as you travel faster you are catching up with that expansion and time itself. And so time travels more slowly as you catch up to it?

Kind regards
Des Enright

What do you think?
« Last Edit: 09/05/2011 02:30:06 by _system »

#### yor_on

• Naked Science Forum GOD!
• Posts: 11993
• Thanked: 4 times
• (Ah, yes:) *a table is always good to hide under*
##### Why does time slow down at speed?
« Reply #1 on: 09/05/2011 04:44:35 »
You're right in that they are intimately connected.

Think of two mirrors at rest relative yourself, then let a light-corn bounce between them. As it does you will find the path that light-corn to take being a straight path, back and forth, a little like a pendulum.

Now let the mirrors move with you staying at earth, it won't matter if you look at them while accelerating, or after as they start to 'coast' in space. From your point of view the light-corn suddenly will move slower, due to you finding that light-corn having a longer path between the mirrors, that as it has to traverse more 'space' as the mirrors it bounce between constantly moves away from you. Also you will find the path it takes to be  a diagonal one as it 'tags' after the mirrors in a zigzag motion relative you, being still.
==

You being still falls under SR but the mirror when accelerating falls under GR,and so as a 'system' you both belong to GR there. After the acceleration when it just 'coasts' you will both belong to SR.

Now imagine that you would move with those mirrors, being 'at rest' with them.
First of all, when will you be able to prove that you are moving?

The acceleration right?

How do you think that light-corn will behave then? Will it start to zigzag? According to you, being 'at rest' with the mirrors :) That just means that you and the mirrors now are traveling together, being still relative each other.

Contrast that to when you've stopped accelerating, instead uniformly moving (coasting).
Do you expect to see the light-corn zigzag? You still being at rest relative the mirrors?

Do you think there is a difference between the acceleration and later uniform motion?
==

And as a over course.

For the last one, imagine each scenario of the two above. Accelerating and uniformly moving. The only difference being that you now are enclosed inside a very large black box. It's really, really black in there as the engineers forgot all about the windows. Will the lightcorn behave differently?

So, in which scenario are you able to say that you are 'moving' inside that black box.
==

When you've done those, you will know what I think I know :)
And you're right, the mirrors becomes the light clock. And 'space' is a geometry that is intimately connected to 'time' expressed in 'gravity'. Gravity is what bends, distort and 'deform' space. And there is nowhere I know that you will find a 'space' without gravity. Even where you can't measure it, as in a free fall, it is still there. It's just you coming to be 'at rest' momentarily relative it.

So SpaceTime, not Space & Time.

#### Soul Surfer

• Neilep Level Member
• Posts: 3345
• keep banging the rocks together
##### Why does time slow down at speed?
« Reply #2 on: 09/05/2011 08:27:55 »
The important thing always to remember is that time only flows more slowly for outside observers observing you.  You would not notice any difference in the flow of time around you in things moving with you, although you would see big changes in the layout, appearance and behaviour of distant objects.

#### MikeS

• Neilep Level Member
• Posts: 1044
##### Why does time slow down at speed?
« Reply #3 on: 16/05/2011 07:54:44 »
As you approach the speed of light it requires more and more energy to carry on accelerating up to the speed of light.  More energy is equivalent to more mass.  More mass is equivalent to a slower passage of time (gravitational time dilation).

The speed of light is a constant.
Speed is distance divided by time.
Therefore, either distance has to shrink or the passage of time has to contract to maintain that constant.

#### JP

• Neilep Level Member
• Posts: 3366
• Thanked: 2 times
##### Why does time slow down at speed?
« Reply #4 on: 16/05/2011 09:24:07 »
As you approach the speed of light it requires more and more energy to carry on accelerating up to the speed of light.  More energy is equivalent to more mass.  More mass is equivalent to a slower passage of time (gravitational time dilation).

That isn't true.  The kind of mass you gain by moving fast isn't the same as gravitational mass.  There are two kinds of mass used in relativity, and you're confusing relativistic mass (which goes up when you move fast) with rest mass (which doesn't).

#### Airthumbs

• Hero Member
• Posts: 958
• Personal Text
##### Why does time slow down at speed?
« Reply #5 on: 16/05/2011 13:37:08 »
Is speed related to mass?  If so much energy is required to move at the speed of light that it can't be done, then how does light do it?

And we know light does do exactly that, it travels at the speed of light, so what energy causes the light to travel at such high speed?

#### JP

• Neilep Level Member
• Posts: 3366
• Thanked: 2 times
##### Why does time slow down at speed?
« Reply #6 on: 17/05/2011 00:51:25 »
You have three questions there:

Quote
Is speed related to mass?
There are two kinds of mass in special relativity: relativistic mass and invariant mass.  Invariant mass is the more useful quantity for figuring out gravitational force, etc.  Relativistic mass is only useful because it makes E=mc2 hold.  If you use invariant mass, you need to fix up the formula to read:

E2=m02c4+p2c2,

where that m0 is rest mass.

Quote
If so much energy is required to move at the speed of light that it can't be done, then how does light do it?
Only objects with zero rest mass can move at the speed of light.  However, those objects are always moving at the speed of light, no matter what.  Light doesn't need to speed up, since it is never moving (in a vacuum) at any speed other than the speed of light.

Quote
And we know light does do exactly that, it travels at the speed of light, so what energy causes the light to travel at such high speed?
That seems to be one of the fundamental properties of the universe.  Maybe some future theory will come along and answer why light speed is the limit and why massless things move at that speed, but for now it's just an assumption in relativity that happens to match very well with what we see experimentally.

#### MikeS

• Neilep Level Member
• Posts: 1044
##### Why does time slow down at speed?
« Reply #7 on: 17/05/2011 07:16:04 »
As you approach the speed of light it requires more and more energy to carry on accelerating up to the speed of light.  More energy is equivalent to more mass.  More mass is equivalent to a slower passage of time (gravitational time dilation).

That isn't true.  The kind of mass you gain by moving fast isn't the same as gravitational mass.  There are two kinds of mass used in relativity, and you're confusing relativistic mass (which goes up when you move fast) with rest mass (which doesn't).

I do understand the difference in rest mass and relativistic mass but what specifically isn't true?

#### JP

• Neilep Level Member
• Posts: 3366
• Thanked: 2 times
##### Why does time slow down at speed?
« Reply #8 on: 17/05/2011 07:23:23 »
More mass is equivalent to a slower passage of time (gravitational time dilation).

Which type of mass are you talking about here?

#### MikeS

• Neilep Level Member
• Posts: 1044
##### Why does time slow down at speed?
« Reply #9 on: 17/05/2011 07:39:25 »
More mass is equivalent to a slower passage of time (gravitational time dilation).

Which type of mass are you talking about here?

Relativistic mass

#### JP

• Neilep Level Member
• Posts: 3366
• Thanked: 2 times
##### Why does time slow down at speed?
« Reply #10 on: 17/05/2011 07:57:56 »
More mass is equivalent to a slower passage of time (gravitational time dilation).

Which type of mass are you talking about here?

Relativistic mass

That's the problem.  Relativistic mass is not equivalent to gravitational time dilation.  Invariant mass does.

If your mass is moving and you want to compute it's effect on space-time, you need to use the stress-energy tensor, which accounts for the energy and momentum of motion.  This is definitely not the relativistic mass either.

#### MikeS

• Neilep Level Member
• Posts: 1044
##### Why does time slow down at speed?
« Reply #11 on: 17/05/2011 08:08:23 »
More mass is equivalent to a slower passage of time (gravitational time dilation).

Which type of mass are you talking about here?

Relativistic mass

That's the problem.  Relativistic mass is not equivalent to gravitational time dilation.  Invariant mass does.

If your mass is moving and you want to compute it's effect on space-time, you need to use the stress-energy tensor, which accounts for the energy and momentum of motion.  This is definitely not the relativistic mass either.

What does cause the passage of time to dilate when travelling at near to the speed of light?

#### JP

• Neilep Level Member
• Posts: 3366
• Thanked: 2 times
##### Why does time slow down at speed?
« Reply #12 on: 17/05/2011 08:28:33 »
It's a consequence of special relativity: that everyone in an inertial reference frame measures the speed of light to be constant.

Also, it's important to remember that time dilation is only apparent when two observers in different reference frames compare clocks.  If I'm traveling near the speed of light, I don't notice anything funny happening on board my spaceship, since it's all in the same reference frame as me so all the clocks run the same as mine.

#### CPT ArkAngel

• Hero Member
• Posts: 584
• Thanked: 3 times
##### Why does time slow down at speed?
« Reply #13 on: 17/05/2011 10:17:34 »
There is a direct correlation between time dilation and relativistic mass, they both have the same relativistic dilation for any unique observer.

That being said, there is a corresponding length contraction or space contraction. If you look at matter's wave counterpart, space contraction corresponds to relativistic frequency red (or blue) shift (only the relativistic part)...

« Last Edit: 17/05/2011 11:12:11 by CPT ArkAngel »

#### simplified

• Sr. Member
• Posts: 428
##### Why does time slow down at speed?
« Reply #14 on: 17/05/2011 10:33:12 »
More mass is equivalent to a slower passage of time (gravitational time dilation).

Which type of mass are you talking about here?

Relativistic mass

That's the problem.  Relativistic mass is not equivalent to gravitational time dilation.  Invariant mass does.

If your mass is moving and you want to compute it's effect on space-time, you need to use the stress-energy tensor, which accounts for the energy and momentum of motion.  This is definitely not the relativistic mass either.

What does cause the passage of time to dilate when travelling at near to the speed of light?
The relativity denies an ether, therefore only field communication can slow down time.

#### MikeS

• Neilep Level Member
• Posts: 1044
##### Why does time slow down at speed?
« Reply #15 on: 17/05/2011 11:02:20 »
It's a consequence of special relativity: that everyone in an inertial reference frame measures the speed of light to be constant.

Also, it's important to remember that time dilation is only apparent when two observers in different reference frames compare clocks.  If I'm traveling near the speed of light, I don't notice anything funny happening on board my spaceship, since it's all in the same reference frame as me so all the clocks run the same as mine.

Does the speed of the passage of time in the spacecraft actually slow down or not?  From the crews point of view it obviously would not.  They are part of that frame of reference and wouldn't notice anything amiss, even if the spaceship accelerated up to the speed of light, in which case they would be frozen in time.  So can we actually say that mass travelling at the speed of light experiences the passage of time?

#### JP

• Neilep Level Member
• Posts: 3366
• Thanked: 2 times
##### Why does time slow down at speed?
« Reply #16 on: 17/05/2011 11:05:58 »
They are part of that frame of reference and wouldn't notice anything amiss, even if the spaceship accelerated up to the speed of light, in which case they would be frozen in time.  So can we actually say that mass travelling at the speed of light experiences the passage of time?

Mass can't move at the speed of light, so that question can't be answered.  Trying to answer it by plugging v=c into the Lorentz factor leads to nonsensical results because the entire theory was derived under the assumption that the case where v=c cannot be handled for objects with mass.

#### JP

• Neilep Level Member
• Posts: 3366
• Thanked: 2 times
##### Why does time slow down at speed?
« Reply #17 on: 17/05/2011 11:09:25 »
There is a direct correlation between time dilation and relativistic mass, they both have the same relativistic dilation for any unique observer.

Energy and time transform in similar ways when you change inertial reference frames.  Relativistic mass is just a fancy word for energy.  The factor you're talking about is hardly unique to energy and time, since it's part of the equations that define how space and time transform.  Any quantities which live in space and time are going to transform in ways where that same factor shows up.  It's called the Lorentz factor.

http://en.wikipedia.org/wiki/Lorentz_factor

#### CPT ArkAngel

• Hero Member
• Posts: 584
• Thanked: 3 times
##### Why does time slow down at speed?
« Reply #18 on: 17/05/2011 11:32:38 »
yes, but it is not so fancy if you believe in Einstein's Equivalence Principle.

#### MikeS

• Neilep Level Member
• Posts: 1044
##### Why does time slow down at speed?
« Reply #19 on: 17/05/2011 11:39:06 »
They are part of that frame of reference and wouldn't notice anything amiss, even if the spaceship accelerated up to the speed of light, in which case they would be frozen in time.  So can we actually say that mass travelling at the speed of light experiences the passage of time?

Mass can't move at the speed of light, so that question can't be answered.  Trying to answer it by plugging v=c into the Lorentz factor leads to nonsensical results because the entire theory was derived under the assumption that the case where v=c cannot be handled for objects with mass.

I was simply continuing the example you gave.  The trend being nothing that travels at the speed of light experiences any passage of time.

yes, but it is not so fancy if you believe in Einstein's Equivalence Principle.

At last.

#### JP

• Neilep Level Member
• Posts: 3366
• Thanked: 2 times
##### Why does time slow down at speed?
« Reply #20 on: 17/05/2011 11:59:49 »
They are part of that frame of reference and wouldn't notice anything amiss, even if the spaceship accelerated up to the speed of light, in which case they would be frozen in time.  So can we actually say that mass travelling at the speed of light experiences the passage of time?

Mass can't move at the speed of light, so that question can't be answered.  Trying to answer it by plugging v=c into the Lorentz factor leads to nonsensical results because the entire theory was derived under the assumption that the case where v=c cannot be handled for objects with mass.

I was simply continuing the example you gave.  The trend being nothing that travels at the speed of light experiences any passage of time.
I never said that.  In fact, I've told you a few times that you can't apply special relativity to v=c.  In fact, one of those places was right here:
http://www.thenakedscientists.com/forum/index.php?topic=39022.msg354305#msg354305

Quote from: MikeS
yes, but it is not so fancy if you believe in Einstein's Equivalence Principle.

At last.

Can you explain what you mean by this?  If you're invoking the equivalence principle of general relativity on top of special relativity, you now have 4 masses in play: inertial mass, gravitational mass, relativistic mass and rest mass.  And relativistic mass still isn't equivalent to inertial or gravitational mass.
« Last Edit: 17/05/2011 12:01:34 by JP »

#### JP

• Neilep Level Member
• Posts: 3366
• Thanked: 2 times
##### Why does time slow down at speed?
« Reply #21 on: 17/05/2011 12:06:23 »
There is also a fairly simple thought experiment that shows that relativistic mass is not the mass you want to think about as causing gravity.  Say I have a rocket ship traveling by the earth incredibly fast.  An observer on the earth will calculate the rocket ship's relativistic mass and say, "Wow!  That has so much mass it will have to become a black hole!"  The observer on the ship, of course, sees his mass as the rest mass and doesn't turn into a black hole.

It would be somewhat absurd for this ship to be a black hole and not a black hole at the same time.

The resolution is that you can't just plug relativistic mass (or invariant mass, for that matter) into the equations of general relativity by itself.  You need to actually use the entire stress-energy tensor which has 16 terms to it, specifying the energy, momentum as well as flow of energy and momentum in space and time.

The only time where you can just plug in mass and nothing else is when things are perfectly stationary.  And of course in this case, relativistic and rest mass agree.

#### CPT ArkAngel

• Hero Member
• Posts: 584
• Thanked: 3 times
##### Why does time slow down at speed?
« Reply #22 on: 17/05/2011 22:26:36 »
And relativistic mass still isn't equivalent to inertial or gravitational mass.

Yes it is JP, I have read an article of a physicist working at CERN explaining that the relativistic mass of 2 protons colliding in the LHC have each a maximum of about 7500*m0 (m0 being the proton's rest mass). The total energy produced by the collision is 15000*m0. He said that they never observed any discrepancy between relativistic mass, inertial mass and gravitational mass. He was answering to the question of the difference between gravitational mass and relativistic mass. Each proton is perceiving the other proton's mass increase as a true mass and they are much smaller in size, which makes the collisions more difficult to obtain.

E = (m0^2*c^4+p^2*c^2)^1/2 = m*c^2 where m is the relativistic mass.

If i find the article, i will post it.

« Last Edit: 17/05/2011 22:49:33 by CPT ArkAngel »

#### yor_on

• Naked Science Forum GOD!
• Posts: 11993
• Thanked: 4 times
• (Ah, yes:) *a table is always good to hide under*
##### Why does time slow down at speed?
« Reply #23 on: 17/05/2011 23:48:21 »
You better do that CPT :)

As far as I know the difference between relativistic mass and invariant (matter) is that both equivalently are able to translate into 'Energy' but only the invariant mass will be there in any experiment measuring the mass in that 'frame of reference'. That is, for example, you being at rest versus whatever you want to measure a mass from/on.

Relativistic mass is 'real' in the same manner as 'potential energy' is real. What that means is that you by choosing your possible frame of 'interaction' aka a collision will get different results on what this relativistic mass or potential energy is.

A simple proof of that is all 'uniform motion' where, according to the equivalence principle, it won't matter what you expect your 'speed' to be (relative some origin, for example Earth). You won't be able to define that speed in a black box scenario from any experiments and so you will be able to define your speed from zero to any value relative what you measure against.

With no possibility to define a speed your relativistic mass also will be questionable. What's not expected to be questionable though is that invariant mass, expected to be the same in any frame thought up.

Relativistic mass and potential energy are both definition using comparisons between 'frames of reference' and as you can have several different frames simultaneously to compare between, like several spacecrafts at 'different accelerations (as well as uniformly moving) you will find yourself to have a different potential energy. As for your relativistic mass it is in fact so that you either will measure it against some frame of origin, or possibly CBM or blue shift and from there calculate a possible 'interaction' with the energy released in a collision. But, as far as I know, it will not be present in the atoms 'jiggling' in those ships moving relative some frame of definition.

What you seem to state there is that there is?
Show the proofs, I'm interested  :)

#### CPT ArkAngel

• Hero Member
• Posts: 584
• Thanked: 3 times
##### Why does time slow down at speed?
« Reply #24 on: 18/05/2011 00:15:21 »
my point of view is the same as yours, i don't understand what you mean. Invariant mass is rest mass, inertial mass and gravitational mass are relativistic mass and are frame dependent, obviously. Gravitational mass is often wrongly associated to invariant mass but they are not the same in GR.

#### The Naked Scientists Forum

##### Why does time slow down at speed?
« Reply #24 on: 18/05/2011 00:15:21 »