# Time Dilated Acceleration

1. Mar 8, 2014

### God Plays Dice

At v = c, time (hence physics) stands still.
You can never reach c, but lets say near enough.

Therefore if you accelerate at an extremely high rate, from 0 - c in perhaps a fraction of a second, wouldn't the slowing rate of physics prevent you from being crushed?

ie, is there a way of changing speed and preventing the g force via time dilation?

2. Mar 8, 2014

### Staff: Mentor

No, it doesn't. This is a common misconception, but it is not correct.

No. Time dilation does not affect proper acceleration, which is the acceleration you actually feel.

3. Mar 8, 2014

### phinds

Putting acceleration aside, Gods, just in case you haven't been aware of it, right now as you read this you already ARE traveling at .9999999% the speed of light from the frame of reference of an accelerated particle at CERN. Do you feel any different? Peter's point, is that time dilation is something that is observed by a remote observer as a difference between his time rate and yours. It has nothing to do with your actual time rate.

4. Mar 8, 2014

### God Plays Dice

I guess what I mean is, if you accelerated to ~c relative to earth and it was near instantaneous, then the same deceleration near say mars, would time dilation prevent your craft from crushing itself.... as the time you would experience for this sequence of events is near negligible...

5. Mar 8, 2014

### God Plays Dice

A stargate basically. Or does physics say you would be crushed no matter what

6. Mar 8, 2014

### Staff: Mentor

No, you would still be crushed. The time you would experience doesn't affect the acceleration you feel.

7. Mar 8, 2014

### Staff: Mentor

That's not what you were describing; you were describing a method of travel that accelerates and then decelerates you, not something like a wormhole that warps spacetime itself. The latter type of scenario brings in a lot of complications that you shouldn't try to tackle until you've got the basics down.

8. Mar 8, 2014

### ghwellsjr

By your logic, the safest way to decelerate at mars would be to crash into it since that would take the least amount of time, correct?

9. Mar 8, 2014

### Staff: Mentor

Actually, if you were in a super-rocket traveling at near the speed of light (relative to Mars), crashing into Mars might not decelerate you that quickly, compared to using the super-rocket's engines, which as I understand the OP, are supposed to be designed to be able to bring you from near light speed to relative rest in a very short time. Crashing into Mars might just dig a very, very deep hole, meaning much slower deceleration than the super-rocket.

(Of course, this is not to say the super-rocket's engine firing would not crush you; I'm just pointing out that, if we take the OP's hypothesis at face value, it would crush you even *more* effectively than crashing into Mars at near light speed. )

10. Mar 9, 2014

### sshai45

I'm curious about this: if this is not correct, then what is the correct conception? As, for example, I've heard of the deal with "neutrinos" changing type as they fly, which is said to imply they have mass, because if they had no mass, and so traveled at "c", they could not undergo any internal changes, suggesting that time is stopped at "c".

11. Mar 9, 2014

### Staff: Mentor

That objects that move at $c$ are fundamentally different, physically, from objects that move slower than $c$. The concept of "elapsed time" does not apply to the former type of object. Saying that "time stops" for the former type of object implies that the concept of elapsed time *does* apply, which is incorrect.

Which basically repeats the same misconception. It's true that the fact that neutrinos changing type means that at least some neutrinos have mass, but that has nothing to do with "time stopping" at $c$; it has to do with how quantum mechanics works. Quantum mechanically, if neutrinos can change type, it means that neutrinos of different types have wave functions that oscillate at different rates, which means neutrinos of different types have to have *different* masses (since the mass determines the rate at which the wave function oscillates). So at least two of the three neutrino types must have nonzero mass if neutrinos can change type. Technically, one of the three types could still have zero mass, just on the basis of neutrino oscillations, but other considerations lead theorists to give nonzero mass to all three types.

12. Mar 14, 2014

### sshai45

So would it be compatible with relativity for an object traveling at "c" to undergo some kind of change or evolution as seen by an outside observer, like how an object traveling slower can? If the answer is "no", then why -- since the reason can't be that "time stops" at "c", since, as you mention, that is wrong.

Last edited: Mar 14, 2014
13. Mar 14, 2014

### phinds

You can't see anything that is traveling at c except right at the moment it impinges on your inner eye, nor can instruments see electromagnetic radiation until it hits their detector so there is no "outside observer".

14. Mar 14, 2014

### ghwellsjr

Time does not apply to objects traveling at "c" so there can be no change or evolution for such objects. It's not like a radioactive particle that can decay as defined by a half life, particles traveling at "c" cannot have any characteristic that is related to time in any way, not even a half life.

It basically has to do with the fact that time has to be measurable in order to be meaningful. And in order to measure it, you need a clock. And a clock needs to have parts in it the can move with respect to each other in order for it to "tick".

Think about a light clock which, in its simplest idealized form, is a device with two mirrors and a photon bouncing between them. If you consider the two mirrors to be traveling at "c", one ahead of the other, then the photon could never propagate from the trailing mirror to the leading mirror because all three, the two mirrors and the photon are traveling at the same speed. This problem applies even if you could build the mirrors out of photons or some other massless particle.

That's why it's incorrect to say that "time stops" at "c", not because the clock ceases to tick at "c", but because the concept of a clock ceases to exist at "c" and so the concept of time doesn't have meaning at "c".

15. Mar 14, 2014

### PAllen

Note that the relation between acceleration of some body observed in an inertial frame where it has high speed, versus measured in an instantaneously comoving inertial frame (which is the same as proper acceleration, the acceleration measured by an accelerometer attached to the object), is the opposite of the OP's expectation. For the co-linear case (acceleration in the same or opposite direction to the velocity), the acceleration felt by the object is greater by the cube of the time dilation factor compared to the acceleration observed in the frame where the object is moving near c. Thus, for an object moving such that it's time rate is found to be slowed by a factor of 10 in (say, earth/mars frame), and observed to be slowing with deceleration g, the acceleration felt on the body would be 1000g. As a result (very loosely speaking), despite time dilation, there would be plenty of time to be crushed. Time is only slowed by a factor of 10, while the acceleration/deceleration is increased by a thousand.

16. Mar 18, 2014

### sshai45

So then, do I have this right:

1. Time does not exist at "c", as opposed to merely being "stopped", because it is impossible to construct anything which measures it which moves at "c",

2. BUT, this has some of the same effects as time being "stopped" -- e.g. nothing which flies at "c" can change or evolve.

?